draft - university of toronto t-space · draft 6 were about 700 aftershocks in the gsc catalogue...

Draft

Focal depth distribution of the 1982 Miramichi earthquake

sequence determined by modeling depth phases

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2016-0111.R2

Manuscript Type: Article

Date Submitted by the Author: 14-Nov-2016

Complete List of Authors: Ma, Shutian; Carleton University, Earth Sciences Motazedian, Dariush; Carleton University, Earth Sciences

Keyword: 1982 Miramichi earthquake, depth phases, focal depth distribution, seismogenic layers, seismotectonics

https://mc06.manuscriptcentral.com/cjes-pubs

Canadian Journal of Earth Sciences

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Focal depth distribution of the 1982 Miramichi earthquake sequence determined by

modelling depth phases

Shutian Ma and Dariush Motazedian

Department of Earth Sciences, Carleton University

1125 Colonel By Drive

Ottawa, Ontario, K1S 5B6, Canada

Corresponding author: Shutian Ma (email: [email protected]).

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Abstract

On 9 January 1982 in the Miramichi region of New Brunswick, Canada, an mb

5.7 earthquake occurred and extensive aftershocks followed. The mainshock was felt

throughout Eastern Canada and New England, USA. The mainshock and several

principal aftershocks were digitally recorded worldwide, but smaller aftershocks were

digitally recorded only at regional stations. Digital stations were not yet popular in

1982, and therefore available regional digital waveform records for modelling are

very limited. Fortunately, two Eastern Canada Telemetered Network (ECTN) stations,

EBN and KLN, produced excellent waveform records for most of the aftershocks until

their closure at the end of 1990. The waveform records can be retrieved from the

archive database at the Geological Survey of Canada (GSC). Since EBN had clear

sPmP records of the larger aftershocks (with mN ≥ 2.8), we were able to determine

focal depths for these larger events. Most of the focal depth solutions for the 113

larger aftershocks were within a depth range of three to six kilometres. The majority

of the depths were at about four and a half kilometres. Some aftershocks had depths of

about one and two kilometres. The focal depth solutions for the shallow events were

confirmed by the existence of prominent crustal Rayleigh waves. As the records for

the foreshock and the mainshock at EBN were not available, we used the records at

LMN for the foreshock and a teleseismic depth phase for the mainshock. The

teleseismic depth phase comparison shows that the mainshock and its three principal

aftershocks migrated from a depth of about seven kilometres to near the Earth’s

surface.

Key words: Miramichi earthquake, depth phases, sPmP, sPg, sP, shallow focal depth.

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Introduction

Focal depths can be used to estimate the thickness of the seismogenic layers and

are important pieces of information for understanding intraplate earthquakes and

seismotectonics. Strong earthquakes are rare in eastern North America and the 1982

Miramichi mainshock is the largest earthquake in Eastern Canada since the

Cornwall-Massena earthquake of 1944. It was also the first earthquake in Eastern

North America large enough to be recorded on modern seismograph networks

worldwide, allowing for its source parameters to be determined using many different

techniques. Excellent regional digital waveform records for most of the aftershocks

were produced by two Eastern Canada Telemetered Network (ECTN) stations, EBN

and KLN, until their closure at the end of 1990. Waveform records at the ECTN

stations LMN and GGN are also available but the shapes are complex. Figure 1 shows

the locations of these stations. The waveform-record data for the Miramichi sequence

have been saved in the archive database at GSC. Therefore these regional depth phase

records can be used to determine focal depths for the sequence through a modelling

procedure (Ma 2010).

The earthquake sequence has been studied by many scientists (for example,

Wetmiller et al. 1984; Choy 1983; Saikia and Herrmann 1985). The focal mechanism

was of thrust type, and the focal depths determined using conventional methods were

between three and seven kilometres (Wetmiller et al. 1984).

For small earthquakes, focal depths can be estimated jointly while locating

epicenters based on the arrival times of the Pg and Sg phases (P and S phases

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travelling in the upper crust) recorded at nearby stations. However, if regional

network coverage is sparse, this approach does not provide a reliable focal depth

solution, because the travel times of the Pg and Sg phases are mainly determined by

station distances, not by focal depth. As the contributions from focal depth to the

travel times are much smaller than those from distance, errors in focal depth are

usually much larger than those in epicenters. The error in the velocity model is one of

the factors that produce errors in focal depth. For example, if the velocity model used

in a conventional event-locating program is unrealistically slow, the calculated focal

depth will be too shallow. The reason is that for observed time durations between the

onsets of the S and P phases at stations surrounding an epicentre, similar time

durations of the synthetic S and P phases could be produced through shorter travel

paths than the real paths when the velocities used in the conventional locating

program are slower than the real velocities. Accordingly the determined hypocenter is

moved upwards. As such, the uncertainty in the focal depth solutions determined

using conventional methods may be too large for practical usage.

An alternative to conventional methods employs regional depth phases to

determine focal depth for moderate and small earthquakes (Ma and Atkinson 2006).

Regional depth phases, such as sPg1, sPmP

2, and sPn

3 are often detectable in certain

1sPg: an S-wave travels upward to the surface, is converted to a P-wave, which then

travels along or close to the surface towards the station. 2sPmP: an S-wave travels upward to the surface, is converted to a P-wave, which then

travels downward to the Moho, is reflected there and then travels upward to the

station. 3sPn: an S-wave travels upward to the surface, is converted to a P-wave, which then

travels along the Pn path to the station.

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geological environments.

The sketch paths of these three regional depth phases have been shown several

times (Ma and Eaton 2011). The time difference between a depth phase such as sPmP

and its reference phase such as PmP4 is not sensitive to the error in the epicentre

location (Ma 2010). Therefore the focal depth solution obtained using the time

difference between a depth phase and its reference phase is similarly insensitive to

errors in the epicentre location. The time difference does change with station distance,

but the error in the focal depth caused by the error in the epicenter is insignificant if

the sPmP phase is used (Ma 2010).

Unlike the onsets of the P and S phases, depth phases may not be observable at

some or all stations, depending on the geological environment in the epicentral region.

For example, if the sedimentary layers are thick, then the reflectivity near the surface

would be poor so that the depth phases cannot be developed. As the epicentral region

of the Miramichi earthquake sequence lies within the northern Appalachian

Mountains along the axis of the northeast trending Miramichi Anticlinorium

(Wetmiller et al. 1984), the regional depth phases sPg and sPmP recorded at stations

KLN and EBN were well developed.

The 1982 Miramichi mb 5.7 earthquake had many aftershocks. The aftershocks

continue to occur, although the mainshock occurred more than 33 years ago. Within

the latitudes of 46.88º N ~ 47.16º N and the longitudes of 66.35º W ~ 66.80º W, there

4PmP: a P-wave travels downward to the Moho, is reflected there and then travels

upward to the station.

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were about 700 aftershocks in the GSC catalogue database as of the end of 2014. The

sPg phase generated by almost all of the aftershocks was recorded at KLN, but the

workload to analyze all of the waveforms would be huge. To retrieve major features

and to reduce workload, we first retrieved waveform records for all aftershocks with

magnitude mN ≥ 2.8 (magnitude defined by Nuttli 1973). We selected mN 2.8 as our

bottom limit because if mN 3.0 is used, there are not enough earthquakes available for

our study in a region in eastern Canada. If the magnitude mN 2.5 is used, the depth

phase sPmP and especially its reference PmP, are usually hard to identify. So mN 2.8

came from a trade-off. Figure 1 shows the epicentral distribution of these aftershocks

(many epicentres are overlapped). We then performed depth phase modelling for

those earthquakes.

In this article, we outline the principle for using regional depth phases, present the

depth phase features observed at station EBN (located 135 kilometres away from the

epicentre), display some original records, and provide a modelling example. We also

confirm the shallow focal depth solutions using the Rg records at KLN, and provide

all the focal depth solutions (listed in Table 1) obtained by modelling depth phases.

To help some readers understand regional depth phases we prepared a supplement, in

which all the waveform records we analyzed at KLN, and the P wave segments at

EBN, are included.

Outline of the depth phase modelling procedure

At a specific station where a regional depth phase and its reference phase are

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identified, a sequence of synthetic waveforms can be calculated using the

epicentre-station distance, the crustal model for the epicentral region, and the focal

mechanism. Figure 2 (a) shows a sequence of synthetic vertical-component

seismograms at a distance of 135 kilometres for the Pg, PmP, sPg and sPmP phases.

The top trace was generated using a depth of 0.1 kilometres, the second trace was

generated using a depth of 0.5 kilometres, and the other traces were calculated at a

consecutive depth increment of 0.5 kilometres.

A reasonable crustal model in the epicentral region is required when synthetic

seismograms are calculated. Figure 3 shows the crustal model that was used to

generate the synthetic seismograms in Figure 2. This crustal model was a revised

version (Ma 2015) based on models by Motazedian et al. (2013).

When we generate seismograms a focal mechanism or moment tensor is also

required. The one we used in our program to generate the seismograms (Figure 2 (a))

is a default focal mechanism (Ma and Eaton 2007; Table 3, moment tensor solution

for 1990/10/19, Mont Laurier, Quebec, earthquake), which is very similar to the focal

mechanism of the Miramichi main shock. The focal mechanism does not determine

the arrival times of the seismic phases but rather the crustal structures determine the

arrival times. The crustal model is, therefore, a key factor in generating synthetic

regional depth phases. The location is also not a key factor in generating the time

difference between the sPmP phase and its reference phase PmP (Ma 2010).

As the time difference between a depth phase and its reference phase is

approximately positively proportional to focal depth, we can generate a seismogram

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sequence using several depths that bracket an expected focal depth. For example,

since earthquakes occur in the middle and upper crust in Eastern Canada, we can

generate a seismogram sequence using depths from one to 20 kilometres at increments

of one kilometre. One of the generated time differences between a depth phase and its

reference phase may be approximately equal to the observed time difference between

the same type of depth phase and its reference phase. The depth used to generate a

time difference that matches the observed time difference is the focal depth solution

for the earthquake being studied. The demonstration of the regional depth phase

modelling procedure has been provided using clearly recorded PmP and sPmP phases

in the West Quebec Seismic Zone (e.g. Ma and Eaton 2011). The following two steps

are required for each depth phase modelling:

1) For a given station where a regional depth phase and its reference phase are

recorded, the synthetic seismograms are calculated using the reflectivity

method (Randall, 1994) for a range of depths within specific increments;

2) The synthetic seismograms are visually compared with the observed

seismograms on a computer screen. A trial depth, for which the calculated

synthetic time difference between a depth phase and its reference phase best

matches the observed time difference between the depth phase and its

reference phase, is selected as the focal depth solution.

Almost all of the events in the Miramichi earthquake sequence had regional phase

records. Unfortunately, the regional phase records are not available for the main shock

and one of its principal aftershocks. Fortunately, the teleseismic depth phases sP for

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these two key events were available. The modelling procedure for the teleseismic

phases was the same as that for the regional depth phase. To generate teleseismic

depth phases, a crustal model at the source region and a focal mechanism were needed.

To facilitate the calculation, the crustal model was obtained by simplifying the model

used for regional phase calculations. The focal mechanism used was the centroid

moment tensor (CMT) obtained for the main shock by the Harvard Seismology Group

(www.globalcmt.org). The computer program used to generate the synthetic

teleseismic phases was written by Rongjiang Wang (Wang 1999). We made slight

revisions to the code to satisfy our needs.

Analysis of observed depth phases

Among the approximately 700 aftershocks, there were 154 with mN ≥ 2.8. After

analyzing the waveforms for those 154 earthquakes, we found that 107 had sPmP

phases at EBN, and 77 had sPg phases at KLN. As the crustal structure between the

epicentre and a station at shallow depth has a larger influence on the time difference

between the sPg phase and its reference phase Pg than that between the sPmP and

PmP phase, we generally did not use sPg phases to systematically estimate focal

depth in this article, only using it for some events when the sPmP phase was not

available.

Figure 4 shows selected seismograms of 15 aftershocks, which reveal the sPmP

phases and high-frequency Rg phase. It is known that if the Rg phase generated by a

small earthquake is prominent, the focal depth of the earthquake must be shallow

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(Kafka and Reiter 1987). Therefore we can use an observed Rg phase to confirm the

shallow focal depth solutions obtained by modelling a depth phase. For example, in

the middle of Figure 4, along the trace No. 9 (No. 88 in Table 1), the time difference

between the sPmP and PmP phases is small, and the Rg phase is obviously strong.

The modelling test (Ma and Eaton 2009) showed that the focal depth cannot be deeper

than three kilometres for a small earthquake of mN 2.9 that can generate a strong Rg

phase with a period of around one second. However, the time difference between the

sPmP and PmP phases along the trace No. 8 (No. 87 in Table 1), which shows no sign

of an Rg phase, is much larger than those along the traces with an Rg phase. The

modelling test by Ma and Eaton (2009) showed that if there is no sign of an Rg phase

at a frequency of around 1 Hz, the focal depth cannot be shallower than three

kilometres. However, the above statements are not applicable for moderate or strong

earthquakes.

Figure 5 shows the vertically enlarged, horizontally expanded P-wave segments

of Figure 4. The traces demonstrate that some aftershocks had similar focal depths.

For example, the third and fifth traces indicate that these two aftershocks had similar

focal depths. The focal depths can be sorted by direct waveform comparison on a

computer screen, because the time difference between a depth phase and its reference

phase directly relate to the focal depth.

The phases between sPmP and Pg in Figures 4 and 5 are a combination of the

PmP and sPg phases. Figure 2 (a) shows that, at a station distance of 135 kilometres,

the arrival times of the synthetic phases PmP and sPg generated using a depth of five

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kilometres are very close.

In Figure 5 the phases connected with a dashed line are complex. Three possible

factors generate the complexity: source process, short period seismograph, and crustal

structures. The shape of the Pg phase at station KLN clearly shows that for most

aftershocks the source process is simple (Figures S1, S2, and S3 in the supplement).

The source process could be ruled out to be a key factor to cause the complexity. The

dominant period of the short period seismographs at stations KLN and EBN may be

approximately 1-second. For P and S waves generated by the aftershocks of the

Miramichi sequence, 1-second may be wide enough to cover the frequency contents.

So the seismographs may not be a major factor to cause the complexity either.

The above analysis implies that the crustal structures in the Miramichi earthquake

sequence region are the major factor that caused the complexity of the shapes of the

sPmP phase (segment) identified. To provide a possible specific explanation for this

complexity we then created an experiment. In the experiment we revised one

previously used crustal model. We let the model have two strong interfaces: one is the

Moho, and the other is six kilometres above the Moho (Table 2). Then we changed the

thickness 6.0 kilometres, to 5.0, 4.0, 3.0, 2.0, 1.0, and 0.1 kilometres (zero kilometres

is not suitable for the computer programs we are using to generate synthetic

seismograms). We generated seven seismograms. From those seismograms we found

the depth phases generated at Moho and the other interface can be separated when the

thickness of the layer is 6.0 5.0, or 4.0 kilometres. When the thickness is 3.0

kilometres, the two depth phases were merged. When the thickness is 2.0 and 1.0

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kilometres, the sPmP phase generated at Moho comes earlier (Figure 6). This

phenomenon comes from the total travel time of the phase.

In the geological history in the region of the Miramichi earthquake sequence,

several strong earthquakes might have occurred. Even though we have not thoroughly

studied the crustal structures, we guess the complex crustal structures may exist. So

the shape complexity of the sPmP phase (sPmP segment) is very possibly related to

the crustal structures.

Regional depth phase modelling results

In the previous sections, we introduced the regional depth phase modelling

procedure and the available depth phases. In this section, we introduce the modelling

results.

Modelling of sPmP and sPg depth phases

In this study, we generated synthetic seismograms for depths 0.1 kilometres to 10

kilometres at an increment of 0.1 kilometres at a station distance of 135 kilometres

(from EBN to the epicenter of the mainshock). We then compared the synthetic

seismograms with the observed seismograms generated by the 107 aftershocks

recorded at EBN. Figure 7 shows the modelling of the sPmP phase at EBN for

aftershock No. 87 in Table 1, mN 3.0 aftershock (No. 8 in Figure 4, marked as

84Jul02/05:23:54). The modelled focal depth could be approximately 4.6 kilometres

or 5.5 kilometres. This aftershock had a relatively clear record at EBN, and many

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aftershocks had similar time differences between the sPmP and Pg/PmP phases, as

shown in Figure 5. We used this mN 3.0 aftershock as an example, because its

waveform record at EBN has clear sPmP and PmP phases.

Figure 8 shows the modelling at KLN using the sPg phase for the same

earthquake as that in Figure 7. As the top crust at the conversion point of the sPg

phase has a low-velocity layer (Fyffe et al. 1981), the recorded sPg phase was

generated by a composite effect from the surface and the interfaces above the

earthquake hypocenter. We added one low-velocity layer on the top of the crustal

model (Figure 3) used to generate synthetic seismograms at EBN, leading to an

increase in the period of the synthetic sPg compared to that generated without a

low-velocity layer.

When we perform a regional depth phase modelling, we actually compare the

time durations between a depth phase and its reference phase measured along an

observed and a synthetic seismogram. We need two time points to make the

comparison. Usually we take the maximum points.

For displacement seismograms the shapes of a depth phase and its reference

phase are relatively simple (see example in Figure 7). First we aligned the PmP phase

at the points along the vertical line A. For the sPmP phase we aligned at points B and

C (Figure 7). In this way the focal depth solution for that earthquake using the depth

phase at EBN is 5.5 kilometres. The focal depth solution obtained for the same

earthquake using sPg at KLN is about 5.5 kilometres (Figure 8). For the same

earthquake, the focal depth solution obtained from different sources should be similar,

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so we aligned the second peak of the observed sPmP with the synthetic sPmP at EBN.

In this way we could maintain consistency between solutions for the same earthquake

obtained using sPmP at EBN and sPg at KLN and make the relative error in focal

depths determined from the sPg phase more consistent with that determined from the

sPmP phase. The same philosophy was employed for other earthquakes in the

Miramichi earthquake sequence, so we could maintain the error consistency in focal

depth solutions for earthquakes that only have a record at EBN or KLN.

We compared all the P-wave segments generated by the 107 aftershocks with the

synthetic seismograms generated at EBN and obtained focal depth solutions for these

events. We also applied sPg modelling at KLN for some events without EBN records.

These focal depth solutions are listed in Table 1.

To observe the depth distribution, we plotted the number of aftershocks against

depth. It was found that most aftershocks occurred at a depth range between four and

six kilometres (Figure 9). Some were shallower than two kilometres; the deepest

events were at a depth of about 8.5 kilometres.

Focal depth of a principal aftershock sequence

In the mainshock sequence, there were three larger (principal) aftershocks with

mb ≥ 5.0 (No. 3, 10, and 44 in Table 1). The principal aftershocks have their own

aftershock sequences. Figure 10 shows the seismograms at KLN generated by the

March 31, 1982 mb 5.0 principal aftershock and two of its aftershocks. The time

durations between the sPg and Pg phases show that these three events had very

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similar focal depths.

The focal depth values obtained by modelling the sPmP phase and its reference

phase were approximately two kilometres. Choy et al. (1983) obtained a focal depth

of four kilometres for the same aftershock. Wetmiller et al. (1984) reported that the

rupture of the March 31 aftershock clearly seemed to be associated with the depth of

the upper edge of the mainshock rupture plane (on the order of two to three

kilometres).

Focal depth of the 28 November 1981, mN 3.8 foreshock

The waveform record at EBN generated by the 28 November 1981, mN 3.8

foreshock (No. 1 in Table 1) cannot be used for modelling, as the station did not

properly record the foreshock. However, the P-wave segment of the seismogram,

generated by the foreshock, recorded at LMN is clear. Due to the lack of a proper

crustal model for LMN, we determined the focal depth for the foreshock by waveform

comparison. Figure 11 shows two P-wave segments generated by the foreshock and

an aftershock that had a focal depth solution (5.2 kilometres) obtained by modelling

sPmP at EBN. Given the time differences between Pn and its following phases along

the two traces were almost equal, we assigned 5.2 kilometres as the focal depth for the

foreshock.

Focal depths of the mainshock and its three principal aftershocks

We searched for regional waveform records generated by the mainshock, but

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none could be found. We then downloaded the teleseismic waveform records from the

database at Incorporated Research Institutions for Seismology (IRIS), and found the

tele-depth phase sP is clear at stations TOL, BOCO, KONO, and GRFO (Figure 12).

Choy et al. (1983) estimated the focal depths for the mainshock and two principal

aftershocks as nine, six, and six kilometres, respectively. We believed that nine

kilometres for the mainshock may be too deep, as the dominant aftershock depth was

not as deep (Figure 9). Therefore we estimated the focal depth of the mainshock by

modelling the teleseismic depth phase sP.

We generated the P-wave segments at a depth range of five to eight kilometres at

an increment of 0.5 kilometres and compared the synthetic seismograms with

observed seismograms at station TOL. We found that the focal depth of the mainshock

was somewhere between 6.5 kilometres and seven kilometres (Figure 13). We took

6.8 kilometres as our solution. This solution was very close to the depth of seven

kilometres determined by Wetmiller et al. (1984).

To strictly compare the focal depth of the mainshock with those of the three

principal aftershocks, we plotted the teleseismic depth phases recorded at station

BOCO on the SHZ channel. Figure 14 shows the comparison. The time difference

along each trace indicated by the horizontal line with two arrows shows the focal

depth, which is proportional to the time difference. The 6.8 kilometres along the top

trace was obtained by the modelling described above. At the same station the focal

depth is approximately proportional with the sP-P time difference. The 5.5 kilometres

and 5.2 kilometres along the second and the third traces were calculated from the time

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difference printed along the same trace.

The solution of 5.2 kilometres, estimated from the sP-P time difference, for the

mb 5.4 aftershock (No. 10 in Table 1) is consistent with that obtained using the

regional depth phase. Along the bottom trace in Figure 14, the sP phase is not

observable. The absence of this phase shows that the principal aftershock of mb 5.0

was very shallow (No. 44 in Table 1). This is also consistent with the solution

obtained using the regional depth phase.

In Figure 14 from top to bottom, the four traces were arranged by the origin times

of the four events. The four focal depth values show that the four events migrated

from deeper to shallower parts.

Discussion and conclusion

When regional depth phases are used to determine focal depths for moderate and

small earthquakes, the first step in the procedure is to identify the regional depth

phases and their reference phases (Ma 2010). As the travel path of the sPg phase is

close to the surface, the crustal structure has a relatively larger influence on the time

difference between the sPg and Pg phases. If a lower velocity layer exists at the top of

the crust in the epicentral region, the period of the sPg phase is long, and the point

along the waveform record used to measure the arrival time of the sPg phase is hard

to identify. The sPn phase is generated by relatively larger earthquakes, generally

larger than mN 4. This phase is usually hard to identify, because it mixes with the

arrivals of other phases, such as the Pg phase. As such, this phase can only be used for

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some moderate earthquakes. Although the tele-depth phase pP5 is observable at some

tele-stations, the regional depth phase pPmP phase6 is not observable. Figure 2 (b)

shows the synthetic vertical-component seismograms generated at a regional station

using an explosive source, which does not directly generate S waves and enables us to

observe the behaviours of a P phase. Along those traces we can see that only Pg and

PmP phases are identifiable. The pPmP phase is not identifiable.

The sPmP and PmP phases exist in many records for the Miramichi aftershock

sequence; therefore they are the best choice for determining focal depth. The utility of

this pair of phases does not rely on a very small error in a crustal velocity model in the

modelling procedure when the focal depth is within the upper crust. This is because

their travelling paths are similar for the most part. However, if the hypocenter is in the

middle or lower crust, a well-constrained velocity model is required if a small error in

focal depth solution is expected.

Many aftershocks in the Miramichi earthquake sequence have been pegged at the

mainshock location of 47.0ºN 66.6ºW. Wetmiller et al. (1984) noted that all

aftershocks with an Sg - Pg of 17 +/- 1 s were pegged at this location, indicating a

considerable uncertainty in individual aftershock epicentres. If we assume the velocity

of Pg is Vp = 6.0 kilometres and the Vp/Vs ratio is 1.73, then the difference of 1.0 s in

Sg - Pg corresponds to about seven- to 10-kilometre differences in the epicentral

5 pP: a P phase travels upward to the surface, is reflected there, and then travels to a

tele-station. 6 pPmP phase: a P phase travels upward to the surface, is reflected there, then travels

downward to the Moho, is reflected again, and then travels upward to a regional

station.

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location.

To estimate errors in the modelled focal depth caused by errors in epicentral

location, Ma & Atkinson (2006) performed a numerical experiment, and found that in

the distance range from 1.8º to 3.0º the differential time sPmP – PmP is negligibly

affected by errors in epicentral location of up to 0.1º (11 kilometres).

In this article most station distances used in the modelling are 135 kilometres.

This distance is shorter than 1.8º (about 200 kilometres). The error in the focal depth

solution caused by the error in the station distance (from epicenter) used to generate

the synthetic seismograms may be larger than the error in the distance greater than

about 200 kilometres. To quantify this error we made the following test: we first

generated a vertical displacement seismogram using distance 135 kilometres and focal

depth 5.0 kilometres. Other parameters used are the same as those for seismograms in

Figure 2 (a). Then we generated additional vertical displacement seismograms for

distances 125 kilometres and 145 kilometres, and focal depths 4.8 kilometres, 4.9

kilometres, 5.0 kilometres, 5.1 kilometres, and 5.2 kilometres (a total of 10

seismograms). Figure 15 shows the 11 seismograms that we generated in total. We

found the time durations generated with distances and depths 125 kilometres and 4.9

kilometres; 135 kilometres and 5.0 kilometres; 145 kilometres and 5.1 kilometres, to

be very close. This comparison implies that when the error in the distance (135

kilometres) used to generate synthetic seismograms is 10 kilometres, the error in the

retrieved focal depth is about 0.1 kilometres. In another words, if the real station

distance is 145 kilometres, when the distance 135 is used to retrieve a focal depth

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from the same observed sPmP – PmP time duration, the solution is 5.0 kilometres,

rather than 5.1 kilometres. The error is 0.1 kilometres.

We could only retrieve focal depths for aftershocks up to the end of 1990, when

the EBN and KLN stations were closed. Among the 113 earthquakes that we analyzed,

most focal depths fell in the range of three to six kilometres. The dominant focal

depth was at about 4.5 kilometres. Some aftershocks had very shallow focal depths

(one to two kilometres), as represented by the 31 March 1982 mb 5.0 principal

aftershock. The shallow focal depths were confirmed by the existence of Rg phases,

which exists for small earthquakes with focal depths less than five kilometres (e.g.

Kafka and Reiter 1987).

Choy (1983) estimated a rupture length of approximately 5.5 kilometres for the

mainshock. The dominant depth range of three to six kilometres in Figure 9 may

correspond to the rupture length, which was approximately equal to 5.7 kilometres

when the fault dip angle of the mainshock was assumed to be 45º. This value is

similar to the values of 4.5 kilometres to 6.5 kilometres obtained by Wetmiller et al.

(1984) and Choy (1983).

Based on a strict teleseismic depth phase comparison for the mainshock and its

three principal aftershocks, the ruptures migrated from a depth of around seven

kilometres up to near the surface.

As the distance between station KLN and the epicenter of the mainshock was

only about 20 kilometres, this station had clear records for almost all of the 700

Miramichi aftershocks in the GSC database. We may perform further analysis to those

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21

records in the future.

Data and Resources

The seismograms used in this study were collected from the Geological Survey

of Canada (GSC) at http://www.earthquakescanada.nrcan.gc.ca and the Incorporated

Research Institutions for Seismology (IRIS) Consortium at http://www.iris.edu/hq/

(last accessed on June 30, 2015).

Acknowledgements

This research was supported by the Natural Sciences and Engineering Research

Council of Canada under the Discovery Grant program. We gratefully acknowledge

the constructive comments, suggestions, and text revisions from the two anonymous

reviewers, Editor Ali Polat and the Editorial Assistant Brenda Tryhuba. The waveform

records were processed using SAC2000, redseed, and geotool programs.

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22

References

Choy, G. L., Boatwright, J., Dewey, J. W., and Sipkin, S. A. 1983. A teleseismic

analysis of the New Brunswick Earthquake of January 9, 1982, J. Geophys. Res. 88,

2199-2212.

Fyffe, L.R., Pajari, J. E., and Cherry, M. E. 1981. The Acadian plutonic rocks of New

Brunswick, Maritime Sediments and Atlantic Geology, 17, 23–36.

Kafka, A. L., and Reiter, E. C. 1987. Dispersion of Rayleigh waves in southeastern

Maine: Evidence for lateral anisotropy in the shallow crust, Bull. Seismol. Soc. Am. 77,

925–941.

Ma, S. and Atkinson, G. M. 2006. Focal depths for small to moderate earthquakes (mN

≥ 2.8) in western Quebec, southern Ontario, and northern New York, Bull. Seismol.

Soc. Am. 96, 609–623.

Ma, S. 2010. Focal depth determination for moderate and small earthquakes by

modeling regional depth phases sPg, sPmP, and sPn, Bull. Seismol. Soc. Am. 100,

1073–1088.

Ma, S. and Eaton, D. 2007. The Western Quebec Seismic Zone (Canada): Clustered,

mid-crustal seismicity along a Mesozoic hotspot track, J. Geophys. Res. 112, B06305.

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Ma, S. and Eaton, D. 2011. Combining double-difference relocation with regional

depth-phase modelling to improve hypocentre accuracy, Geophys. J. Int. 185,

871–889, doi:10.1111/j.1365-246X.2011.04972

Ma, S. and Eaton, D. 2009. Anatomy of a small earthquake swarm in southern Ontario,

Canada. Seismol. R. Letters, 80, 214–223.

Ma, S. 2015. S-wave velocity models retrieved using Rg wave dispersive data at

shallow parts of the crust in southern New Brunswick region, report to: Natural

Resources Canada, Ottawa, contract No. 3000565835, pp 34.

Motazedian, D., Ma, S., and Crane, S. 2013. Crustal shear-wave velocity models

retrieved from Rayleigh wave dispersion data in north-eastern North America, Bull.

Seismol. Soc. Am. 103, 2266–2276, doi: 10.1785/0120120187.

Nuttli, O. W. 1973. Seismic wave attenuation and magnitude relations for eastern

North America, J. Geophys. Res. 78, 876–885. DOI: 10.1029/JB078i005p00876

Randall, G. 1994. Efficient calculation of complete differential seismograms for

laterally homogeneous earth models, Geophys. J. Int. 118, 245–254.

Saikia, C. K. and Herrmann, R. B. 1985. Application of waveform modeling to

determine focal mechanisms of four 1982 Miramichi aftershocks, Bull. Seismol. Soc.

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Am. 75, 1021–1040.

Wang, R. 1999. A simple orthonormalization method for stable and efficient

computation of Green's functions, Bull. Seismol. Soc. Am. 89, 733–741.

Wessel, P. and Smith, W. H. F. 1991. Free software helps map and display data, EOS

Earth & Space Science News, 72, 441–444.

Wetmiller, R. J., Adams, J., Anglin, F. M., Hasagawa, H. S., and Stevens, A. E. 1984.

Aftershock sequence of the 1982 Miramichi, New Brunswick, earthquake, Bull.

Seismol. Soc. Am. 74, 621–653.

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25

Figure 1. Location of the 1982 Miramichi earthquake sequence. The red solid circles

show the earthquakes with mN ≥ 2.8. Many of the epicenters overlap. The diamonds

show the locations of cities; the triangles show the selected seismic stations. This

figure was generated with GMT (e.g. Wessel and Smith 1991).

Figure 2. (a) Synthetic vertical-component seismograms generated using a dislocation

source at a station distance 135 kilometres for Pg, sPg, PmP and sPmP phases. The

number at the left of each trace is equal to 10 times of the depth used to generate the

trace. The top trace was generated using a depth of 0.1 kilometres; the bottom trace

was generated using a depth of 10.0 kilometres; and the other trace labels have similar

meanings. The time differences between sPmP and PmP, sPg and Pg increase with

depth, while the differences between depth phases sPmP and sPg remain

approximately the same when the depth changes. (b) Synthetic vertical-component

seismograms generated using an explosive source at the same distance. All other

parameters used are the same as those for (a). Only Pg and PmP phases are

identifiable.

Figure 3. The crustal velocity models used to generate the synthetic seismograms in

Figure 2. The solid lines labelled with EBN show the model used at EBN. The dashed

lines, placed here as a reference, show the crustal model used by the GSC to locate

events in Eastern Canada.

Figure 4. Vertical component short-period records at EBN (approximately 135

kilometres to the mainshock) for 15 aftershocks, aligned approximately at the onset of

the Pg phase. The time at the left side of each trace is the record start time. The down

arrows point approximately to the sPmP phase, while the up arrows point

approximately to the PmP phase. The records with an Rg phase show that the

corresponding focal depths were very shallow.

Figure 5. Detail of the vertical component short-period P-wave segments shown in

Figure 4. The jagged, dashed line connects the peak points of the sPmP phase. The

number at the right side of each trace is the index in Table 1. The time difference

between the sPmP and PmP phases along each trace, for example along the trace

numbered 87 (No. 87 in Table 1), indicated by δt, are used to determine the focal

depth. An earthquake with a deeper focal depth will generate a seismic trace with a

longer time difference between the sPmP and PmP phases.

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Figure 6. Depth phases generated at depth 5.1 kilometres and different interfaces in

assumed crustal models. Trace 5.1_6 was generated using model A in Table 2. In the

model the thickness of the fourth layer is 6 kilometres. Trace 5.1_5 was generated

using model B. The thickness of the fourth layer is five kilometres. Other traces have

a similar meaning. The depth phase generated at the bottom of the fourth layer is

called sPmP, while the depth phase generated at the upper interface is called sPmP`.

Among the seven crustal models for all seven sPmP` phases, the travel times are

theoretically the same. In contrast, the sPmP travel time becomes shorter when the

thickness of the fourth layer becomes thinner. It is noticeable that when the fourth

layer is three kilometres thick, sPmP` and sPmP are merged.

Figure 7. Regional depth phase sPmP modelling at EBN (distance approximately 135

kilometres) for the 1984-0702 mN 3.0 aftershock (No. 87 in Table 1). The top and the

bottom traces are the P wave segment records at station EBN. The synthetic trace 046

(the depth value multiplied by 10) was generated using a depth of 4.6 kilometres. The

labels of other synthetic traces have a similar meaning. The PmP phases were aligned

by the vertical dashed line indicated with letter A. The synthetic sPmP phase along

trace 055 and the observed sPmP phase were aligned at time points B and C, as well

as at E and F by the vertical lines. By fitting the time difference between the sPmP

and PmP phases, the modeled focal depth was about 5.5 kilometres, or 4.6 kilometres.

As the peak indicated by an arrow with letter E might come from another interface,

we took 5.5 kilometres as the focal depth solution for this aftershock.

Figure 8. Regional depth phase sPg modelling at KLN (distance approximately 21

kilometres) for the 1984-0702 mN 3.0 aftershock (No. 87 in Table 1). The top trace

021 was generated using a depth of 2.1 kilometres. The labels of other synthetic traces

have similar meanings. The dashed line connects the onsets of the sPg phase along

each trace. We aligned the observed (bottom) sPg phase with the synthetic sPg phase

at the onset along trace 055. The modelled focal depth was about 5.5 kilometres.

Figure 9. Focal depth distribution determined using the depth phase modelling

procedure.

Figure 10. Vertical component short-period waveform records at KLN generated by

the 31 March 1982 mb 5.0 principal aftershock and two of its own aftershocks (No. 44,

46, and 48 in Table 1). Almost equal time differences between the Pg and sPg phases

along different traces show that these three events had a very similar focal depth of

about 2.0 kilometres.

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Figure 11. Waveform comparison between the P-wave segment generated by the 28

November 1981 mN 3.8 foreshock, and the one generated by the 9 January 1982 mN

3.8 aftershock (No. 1 and 4 in Table 1). The two traces were aligned at the Pn phase.

The time differences between the Pn phase and its following phases along these two

traces are almost equal, implying that the focal depths of these two events are almost

equal.

Figure 12. Tele-depth phase sP displacement, generated by the 1982 Miramichi

mainshock, recorded at short period stations BOCO (distance ~ 43º), KONO ( ~ 45º),

TOL ( ~ 45º), and GRFO ( ~ 50º). The time at the left side of each record is the record

start time.

Figure 13. Depth phase sP modelling for the mainshock at teleseismic station TOL

(distance approximately 45º). The top trace was generated using a depth of 5.0

kilometres. The labels for the other traces have similar meanings. The fifth trace is the

P-wave segment recorded at TOL. The modelled focal depth was about 6.8

kilometres.

Figure 14. Teleseismic depth phases at station BOCO (station distance approximately

43º) on the SHZ channel and the corresponding focal depths for the mainshock and its

two principal aftershocks. The time at the left end of each trace is the record start time.

The bottom trace was generated by the 31 March 1982 principal aftershock (No. 2, 3,

10, and 44 in Table 1).

Figure 15. Comparison to the time durations between the synthetic depth phase sPmP

and its reference phase PmP, generated at station distances 125 kilometres, 135

kilometres, and 145 kilometres with focal depths 4.8 kilometres, 4.9 kilometres, 5.0

kilometres, 5.1 kilometres, and 5.2 kilometres, respectively. The top trace was

generated using distance 125 kilometres and focal depth 4.8 kilometres. Other traces

have a similar meaning. The vertical dashed line 1 runs through the peak points of the

PmP phase; the dashed line 2 approximately runs through the peak points indicated

with A, B, and C, which were generated using distances and depths 125 kilometres

and 4.9 kilometres, 135 kilometres and 5.0 kilometres, 145 kilometres and 5.1

kilometres, respectively. Other parameters used to generate these 11 traces are the

same as those for Figure 2 (a).

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Figure 1. Location of the 1982 Miramichi earthquake sequence. The red solid circles show the earthquakes with mN ≥ 2.8. Many of the epicenters overlap. The diamonds show the locations of cities; the triangles show the selected seismic stations. This figure was generated with GMT (e.g. Wessel and Smith, 1991).

168x171mm (300 x 300 DPI)

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Figure 2. (a) Synthetic vertical-component seismograms generated using a dislocation source at a station distance 135 kilometres for Pg, sPg, PmP and sPmP phases. The number at the left of each trace is equal to

10 times of the depth used to generate the trace. The top trace was generated using a depth of 0.1 kilometres; the bottom trace was generated using a depth of 10.0 kilometres; and the other trace labels

have similar meanings. The time differences between sPmP and PmP, sPg and Pg increase with depth, while the differences between depth phases sPmP and sPg remain approximately the same when the depth

changes. (b) Synthetic vertical-component seismograms generated using an explosive source at the same distance. All other parameters used are the same as those for (a). Only Pg and PmP phases are identifiable.

127x81mm (300 x 300 DPI)

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Figure 3. The crustal velocity models used to generate the synthetic seismograms in Figure 2. The solid lines labelled with EBN show the model used at EBN. The dashed lines, placed here as a reference, show the

crustal model used by the GSC to locate events in Eastern Canada.

89x75mm (300 x 300 DPI)

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Figure 4. Vertical component short-period records at EBN (approximately 135 kilometres to the mainshock) for 15 aftershocks, aligned approximately at the onset of the Pg phase. The time at the left side of each

trace is the record start time. The down arrows point approximately to the sPmP phase, while the up arrows

point approximately to the PmP phase. The records with an Rg phase show that the corresponding focal depths were very shallow.

242x384mm (300 x 300 DPI)

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Figure 5. Detail of the vertical component short-period P-wave segments shown in Figure 4. The jagged, dashed line connects the peak points of the sPmP phase. The number at the right side of each trace is the

index in Table 1. The time difference between the sPmP and PmP phases along each trace, for example

along the trace numbered 87 (No. 87 in Table 1), indicated by δt, are used to determine the focal depth. An earthquake with a deeper focal depth will generate a seismic trace with a longer time difference between the

sPmP and PmP phases.

243x636mm (300 x 300 DPI)

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Figure 6. Depth phases generated at depth 5.1 kilometres and different interfaces in assumed crustal models. Trace 5.1_6 was generated using model A in Table 2. In the model the thickness of the fourth layer

is 6 kilometres. Trace 5.1_5 was generated using model B. The thickness of the fourth layer is five kilometres. Other traces have a similar meaning. The depth phase generated at the bottom of the fourth layer is called sPmP, while the depth phase generated at the upper interface is called sPmP`. Among the seven crustal models for all seven sPmP` phases, the travel times are theoretically the same. In contrast,

the sPmP travel time becomes shorter when the thickness of the fourth layer becomes thinner. It is noticeable that when the fourth layer is three kilometres thick, sPmP` and sPmP are merged.

87x74mm (300 x 300 DPI)

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Figure 7. Regional depth phase sPmP modelling at EBN (distance approximately 135 kilometres) for the 1984-0702 mN 3.0 aftershock (No. 87 in Table 1). The top and the bottom traces are the P wave segment records at station EBN. The synthetic trace 046 (the depth value multiplied by 10) was generated using a

depth of 4.6 kilometres. The labels of other synthetic traces have a similar meaning. The PmP phases were aligned by the vertical dashed line indicated with letter A. The synthetic sPmP phase along trace 055 and the observed sPmP phase were aligned at time points B and C, as well as at E and F by the vertical lines. By fitting the time difference between the sPmP and PmP phases, the modeled focal depth was about 5.5 kilometres, or 4.6 kilometres. As the peak indicated by an arrow with letter E might come from another

interface, we took 5.5 kilometres as the focal depth solution for this aftershock.

133x164mm (300 x 300 DPI)

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Figure 8. Regional depth phase sPg modelling at KLN (distance approximately 21 kilometres) for the 1984-0702 mN 3.0 aftershock (No. 87 in Table 1). The top trace 021 was generated using a depth of 2.1

kilometres. The labels of other synthetic traces have similar meanings. The dashed line connects the onsets

of the sPg phase along each trace. We aligned the observed (bottom) sPg phase with the synthetic sPg phase at the onset along trace 055. The modelled focal depth was about 5.5 kilometres.

102x163mm (300 x 300 DPI)

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Figure 9. Focal depth distribution determined using the depth phase modelling procedure.

110x117mm (300 x 300 DPI)

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Figure 10. Vertical component short-period waveform records at KLN generated by the 31 March 1982 mb 5.0 principal aftershock and two of its own aftershocks (No. 44, 46, and 48 in Table 1). Almost equal time differences between the Pg and sPg phases along different traces show that these three events had a very

similar focal depth of about 2.0 kilometres.

67x33mm (300 x 300 DPI)

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Figure 11. Waveform comparison between the P-wave segment generated by the 28 November 1981 mN 3.8 foreshock, and the one generated by the 9 January 1982 mN 3.8 aftershock (No. 1 and 4 in Table 1). The two traces were aligned at the Pn phase. The time differences between the Pn phase and its following phases along these two traces are almost equal, implying that the focal depths of these two events are

almost equal.

55x15mm (300 x 300 DPI)

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Figure 12. Tele-depth phase sP displacement, generated by the 1982 Miramichi mainshock, recorded at short period stations BOCO (distance ~ 43º), KONO ( ~ 45º), TOL ( ~ 45º), and GRFO ( ~ 50º). The time at

the left side of each record is the record start time.

78x61mm (300 x 300 DPI)

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Figure 13. Depth phase sP modelling for the mainshock at teleseismic station TOL (distance approximately 45º). The top trace was generated using a depth of 5.0 kilometres. The labels for the other traces have similar meanings. The fifth trace is the P-wave segment recorded at TOL. The modelled focal depth was

about 6.8 kilometres.

100x127mm (300 x 300 DPI)

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Figure 14. Teleseismic depth phases at station BOCO (station distance approximately 43º) on the SHZ channel and the corresponding focal depths for the mainshock and its two principal aftershocks. The time at the left end of each trace is the record start time. The bottom trace was generated by the 31 March 1982

principal aftershock (No. 2, 3, 10, and 44 in Table 1).

80x44mm (300 x 300 DPI)

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Figure 15. Comparison to the time durations between the synthetic depth phase sPmP and its reference phase PmP, generated at station distances 125 kilometres, 135 kilometres, and 145 kilometres with focal depths 4.8 kilometres, 4.9 kilometres, 5.0 kilometres, 5.1 kilometres, and 5.2 kilometres, respectively. The top trace was generated using distance 125 kilometres and focal depth 4.8 kilometres. Other traces have a similar meaning. The vertical dashed line 1 runs through the peak points of the PmP phase; the dashed line

2 approximately runs through the peak points indicated with A, B, and C, which were generated using distances and depths 125 kilometres and 4.9 kilometres, 135 kilometres and 5.0 kilometres, 145 kilometres and 5.1 kilometres, respectively. Other parameters used to generate these 11 traces are the same as those

for Figure 2 (a).

81x78mm (300 x 300 DPI)

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Table 1. Focal depth solutions for the Miramichi earthquake sequence

No date time lat. long. H mN st

1 1981/11/28 05:12:02 46.945 -66.757 5.2 3.8

2 1982/01/09 12:53:52 47.000 -66.600 6.8 5.7mb

3 1982/01/09 16:36:44 47.000 -66.600 5.5 5.1mb

4 1982/01/09 17:27:54 47.000 -66.600 5.2 3.8 EBN

5 1982/01/09 17:37:36 47.000 -66.600 4.0 3.2 EBN

6 1982/01/09 22:13:18 47.000 -66.600 8.5 3.0 EBN

7 1982/01/09 22:45:10 47.000 -66.600 5.5 3.7 EBN

8 1982/01/09 23:12:38 47.000 -66.600 6.5 2.9 EBN

9 1982/01/10 21:12:22 47.000 -66.600 5.3 3.0 EBN

10 1982/01/11 21:41:08 47.000 -66.600 5.2 5.4mb EBN

11 1982/01/11 22:02:44 47.000 -66.600 6.8 3.1 EBN

12 1982/01/11 22:36:33 47.000 -66.600 3.6 3.4 EBN

13 1982/01/12 01:58:01 47.000 -66.600 3.6 3.5 EBN

14 1982/01/12 02:09:45 47.000 -66.600 4.3 3.1 EBN

15 1982/01/12 05:29:01 47.000 -66.600 4.6 3.1 EBN

16 1982/01/12 11:49:31 47.000 -66.600 8.5 2.9 EBN

17 1982/01/12 13:38:33 47.000 -66.600 5.6 3.3 EBN

18 1982/01/12 21:47:40 47.000 -66.600 4.3 2.9 EBN

19 1982/01/13 00:39:10 47.000 -66.600 5.5 2.9 EBN

20 1982/01/13 02:05:44 47.000 -66.600 2.6 3.0 EBN

21 1982/01/13 07:24:05 47.000 -66.600 4.8 3.0 EBN

22 1982/01/13 17:06:19 47.000 -66.600 4.7 3.1 EBN

23 1982/01/13 17:56:43 47.000 -66.600 4.6 4.0 EBN

24 1982/01/15 08:28:55 47.000 -66.600 5.3 3.0 EBN

25 1982/01/15 14:36:37 47.000 -66.600 4.8 3.2 EBN

26 1982/01/17 13:33:56 47.000 -66.600 3.8 3.6 EBN

27 1982/01/23 08:56:47 47.000 -66.600 5.4 3.2 EBN

28 1982/01/26 05:00:30 47.000 -66.600 4.0 3.3 EBN

29 1982/02/24 04:43:01 47.000 -66.600 3.1 2.9 KLN

30 1982/02/27 17:34:58 47.000 -66.600 4.0 3.4 EBN

31 1982/03/01 09:33:57 47.000 -66.600 4.3 3.4 EBN

32 1982/03/03 00:28:32 47.000 -66.600 5.3 2.8 EBN

33 1982/03/04 06:06:31 47.000 -66.600 5.5 2.9 EBN

34 1982/03/13 11:38:13 47.000 -66.600 5.2 2.9 EBN

35 1982/03/13 23:27:51 47.000 -66.600 4.3 2.9 EBN

36 1982/03/16 11:14:01 47.000 -66.600 3.1 3.5 EBN

37 1982/03/16 19:01:09 47.000 -66.600 3.0 2.8 EBN

38 1982/03/18 03:27:20 47.000 -66.600 1.2 3.2 EBN

39 1982/03/18 21:34:16 47.000 -66.600 4.4 2.9 EBN

40 1982/03/20 03:08:11 47.000 -66.600 3.5 3.0 EBN

41 1982/03/21 02:33:41 47.000 -66.600 5.1 2.9 EBN

42 1982/03/26 05:36:40 47.000 -66.600 5.3 2.8 EBN

43 1982/03/26 13:38:07 47.000 -66.600 5.1 2.9 EBN

44 1982/03/31 21:02:20 47.000 -66.600 2.0 5.0mb EBN

45 1982/03/31 21:29:19 47.000 -66.600 3.1 2.9 EBN

46 1982/04/02 13:50:12 47.000 -66.600 2.0 4.3 EBN

47 1982/04/02 19:49:45 47.000 -66.600 3.3 3.1 EBN

48 1982/04/08 04:54:34 47.000 -66.600 2.0 3.4 EBN

49 1982/04/11 18:00:53 47.000 -66.600 4.5 4.0 EBN

50 1982/04/11 18:27:19 47.000 -66.600 4.5 3.2 EBN

51 1982/04/11 20:06:59 47.000 -66.600 4.4 2.9 EBN

52 1982/04/18 22:47:21 47.000 -66.600 3.3 4.1 EBN

53 1982/04/28 06:36:02 47.000 -66.600 3.2 3.4 EBN

54 1982/05/02 01:42:44 47.000 -66.600 4.5 3.1 EBN

55 1982/05/02 23:31:37 47.000 -66.600 4.4 3.3 EBN

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56 1982/05/06 16:28:07 47.000 -66.600 4.0 4.0 EBN

57 1982/05/16 22:45:16 47.000 -66.600 5.3 2.8 EBN

58 1982/06/18 11:24:36 47.000 -66.600 4.6 3.0 EBN

59 1982/06/25 06:47:10 47.000 -66.600 4.6 2.9 EBN

60 1982/07/18 15:01:05 47.000 -66.600 4.6 2.9 EBN

61 1982/07/28 05:35:37 47.000 -66.600 5.5 3.7 EBN

62 1982/08/12 20:43:18 47.000 -66.600 4.1 3.3 EBN

63 1982/09/02 11:36:04 47.000 -66.600 4.3 2.9 EBN

64 1982/09/19 01:37:17 47.000 -66.600 5.8 3.1 EBN

65 1982/10/09 09:26:45 47.000 -66.600 2.0 2.8 EBN

66 1982/10/18 04:37:49 47.000 -66.600 4.6 3.0 EBN

67 1982/10/21 18:12:47 47.000 -66.600 4.6 2.8 EBN

68 1982/10/26 15:31:33 47.000 -66.600 3.5 3.5 EBN

69 1982/10/28 06:35:11 47.000 -66.600 5.4 2.8 EBN

70 1982/10/31 12:44:41 47.000 -66.600 4.6 2.9 EBN

71 1982/10/31 12:45:20 47.000 -66.600 4.6 2.8 EBN

72 1982/12/22 12:53:26 47.000 -66.600 4.8 3.0 EBN

73 1983/02/12 18:00:26 47.000 -66.600 4.6 2.8 EBN

74 1983/05/12 20:42:25 47.000 -66.600 4.1 3.0 EBN

75 1983/05/13 17:26:02 47.000 -66.600 4.5 3.5 EBN

76 1983/05/13 23:40:57 47.000 -66.600 4.5 3.9 EBN

77 1983/06/10 04:22:39 47.000 -66.600 4.7 3.3 EBN

78 1983/06/11 13:47:58 47.000 -66.600 4.7 3.4 EBN

79 1983/06/28 08:05:49 47.047 -66.680 2.0 3.3 EBN

80 1983/11/02 06:02:00 47.000 -66.600 2.0 2.8 EBN

81 1983/11/16 12:13:56 47.000 -66.600 3.0 3.2 EBN

82 1983/11/17 15:32:18 47.000 -66.600 4.6 3.7 EBN

83 1983/11/18 10:28:40 47.000 -66.600 1.0 3.0 EBN

84 1984/02/24 03:17:13 47.000 -66.600 4.5 3.7 EBN

85 1984/03/27 22:56:24 46.914 -66.481 4.5 3.0 EBN

86 1984/04/13 15:35:51 47.000 -66.600 4.6 3.1 EBN

87 1984/07/02 05:24:54 47.000 -66.600 5.5 3.0 EBN

88 1984/08/04 05:11:13 47.000 -66.600 1.5 2.9 EBN

89 1984/10/13 01:45:15 47.000 -66.600 2.6 3.0 EBN

90 1984/11/07 19:44:31 47.000 -66.600 4.6 2.8 EBN

91 1984/11/30 05:54:22 47.000 -66.600 4.3 3.8 EBN

92 1984/12/07 20:50:17 47.000 -66.600 5.3 3.1 KLN

93 1985/05/13 18:46:19 47.000 -66.600 1.3 2.8 EBN

94 1985/10/05 05:34:13 47.000 -66.600 5.6 3.9 EBN

95 1985/10/05 06:17:33 47.000 -66.600 5.7 2.8 EBN

96 1985/12/21 06:03:11 47.000 -66.600 1.2 3.1 EBN

97 1986/01/21 02:32:26 47.000 -66.600 4.5 3.4 EBN

98 1986/03/16 05:01:47 47.000 -66.600 4.5 2.8 EBN

99 1986/06/01 14:53:14 47.000 -66.600 4.5 3.4 EBN

100 1986/10/17 14:47:59 47.000 -66.600 4.5 4.1 EBN

101 1986/10/18 12:24:30 47.000 -66.600 4.5 2.8 EBN

102 1986/10/23 12:58:04 47.000 -66.600 4.5 3.4 EBN

103 1986/10/28 16:48:13 47.000 -66.600 4.5 3.4 EBN

104 1987/04/22 14:32:53 47.000 -66.600 3.0 2.8 EBN

105 1988/03/06 18:13:17 47.000 -66.600 1.0 3.2 EBN

106 1988/05/09 01:23:03 47.000 -66.600 4.5 3.5 EBN

107 1988/06/12 18:10:15 47.000 -66.600 4.6 2.8 EBN

108 1988/08/26 05:59:10 47.000 -66.600 4.5 3.8 EBN

109 1989/01/16 02:33:55 47.000 -66.600 4.5 3.0 EBN

110 1989/06/10 10:39:49 47.000 -66.600 4.8 2.8 EBN

111 1990/06/16 13:48:05 47.000 -66.600 4.4 3.0 KLN

112 1990/10/12 00:31:04 47.000 -66.600 4.6 2.9 KLN

113 1990/12/12 05:15:07 47.000 -66.600 4.6 3.5 KLN

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Draft

The catalogue was retrieved from the GSC database. The units for latitude and

longitude are degrees. The label st refers to station. Most of the focal depth solutions

with unit kilometres in column H were determined by modelling sPmP at EBN, some

by sPg at KLN or teleseismic phase sP at TOL. The earthquakes in bold font were

mentioned in the paper.

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Draft

Table 2. Assumed 7 crustal models used to generate depth phases.

mod ∆H Vp Vs ρ ly

A

B

C

D

E

F

G

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

6.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

5.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

4.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

3.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

2.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

1.0 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

1.0 5.69 3.28 2.30 1

8.0 6.25 3.61 2.53 2

22.0 6.50 3.75 2.63 3

0.1 7.10 4.10 2.87 4

0.0 8.00 4.62 3.23 5

All the parameters in the seven crustal models are the same except the thickness of the

fourth layer, indicated with bold text. The velocities in the third layer are obviously

slower than those in the fourth layer. The fourth layer has two interfaces, at which

depth phases can be generated.

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Draft

Supplement.

Short period waveform records at station KLN and P-wave segment records at EBN,

generated by the 1982 Miramichi aftershocks with mN ≥ 2.8. The P-wave segment

records at EBN with index from 80 to 95 are those in Figure 5.

Figure S1. Waveform records at station KLN for 30 aftershocks. The record start time

is attached at the left side of each record. The number at the right side of each record

is the index in Table 1.

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Draft




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Draft




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Draft

Figure S4. P wave segment records at station EBN for 30 aftershocks. The record start

time is attached at the left side of each record. The number at the right side of each

record is the index in Table 1.

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Draft

Figure S5. P wave segment records at station EBN for 31 aftershocks. The record start time is

attached at the left side of each record. The number at the right side of each record is the index in

Table 1.

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Draft

Figure S6. P wave segment records at station EBN for 31 aftershocks. The record

start time is attached at the left side of each record. The number at the right side of

each record is the index in Table 1.

Page 52 of 52



draft - university of toronto t-space · draft 6 were about 700 aftershocks in the gsc catalogue...

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