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  • Microearthquakes near Grybów, Poland, Western Carpathians Pawe³ Wiejacz (pwiejacz@igf.edu.pl), Wojciech Dêbski (debski@igf.edu.pl), Artur Kotwicki (artur@igf.edu.pl),

    £ukasz Rudziñski (rudzin@igf.edu.pl). Institute of Geophysics PAS ul. Ksiêcia Janusza 64 Warszawa, Poland

    The occurrence of small earthquakes in the Outer Western Carpathians on September 24 - 25, 2007 near the town of Grybów has gone almost unnoticed, as the earthquakes have not been felt. Although weak seismicity of the Western Carpathians is a known fact, the events have been of some surprise to seismologists as there have been no previous reports of earthquake activity in this immediate area. In fact, the microearthquakes have been identified thanks to the setting up in 2004 of a local seismic station at Stebnicka Huta (STHS) by the Geophysical Institute of Slovak Academy of Sciences. Altogether on September 24 and 25 there have occurred 13 microearthquakes. The events are not a main shock - aftershock sequence, in fact the biggest of them was not the first one but the eleventh.

    No.

    Date-time

    Stations that recorded

    Magnitude

    MW

    Location

    1

    Sep.24 10:13:00.1

    STHS, NIE, CRVS, OJC

    2.23

    as event 11

    2

    Sep.24 11:12:48.4

    STHS, NIE, CRVS, OJC

    2.17

    as event 11

    3

    Sep.24 11:14:03.9

    STHS, NIE

    1.83

    as event 11

    4

    Sep.24 12:16:52.5

    STHS

    N/a (weak)

    as event 11

    5

    Sep.24 15:13:16.4

    STHS, NIE, CRVS, OJC

    2.17

    as event 11

    6

    Sep.24 15:31:43.0

    STHS

    1.75

    as event 11

    7

    Sep.24 15:32:21.6

    STHS, NIE, CRVS, OJC

    1.95

    as event 11

    8

    Sep.25 16:27:25.3

    STHS, NIE, CRVS, OJC, KOLS

    2.33

    9

    Sep.25 19:03:52.6

    STHS

    1.82

    as event 11

    10

    Sep.25 19:05:10.2

    STHS

    1.51

    as event 11

    11

    Sep.25 21:32:13.2

    STHS, NIE, LIKS, CRVS, OJC, KOLS, VYHS, OKC, KWP

    2.59

    12

    Sep.25 21:36:00.1

    STHS, NIE

    1.71

    as event 11

    13

    Sep.25 21:40.03.1

    STHS

    1.45

    as event 11

    49.471N (±0.5 km) 20.728 E (±1.1 km)

    49.588N (±0.4 km), 20.914E (±0.9 km)

    Table 1. List of the Grybów area microearthquakes, Sep.24-25, 2007. Origin times for events 8 and 11 are calculated, for others estimated from phase arrival time at STHS and the difference between arrival time and calculated origin time at STHS for event no. 11.

    Figure 1. Map of southern Poland and northern Slovakia showing the l o c a t i o n s o f t h e G r y b ó w microearthquakes of September 24- 25, 2007 (star). The location of event no. 8 is denoted by circle with no. 8 in it. Map shows the seismic stations used for locating the events (triangles).

    Location of the events is possible only for the largest 6, namely no. 1,2,5,7,8 and 11, however only the location of the largest event no. 11 is well resolved. Location of the events has been performed by the probabilistic method utilizing the Metropolis Algorithm. (Dêbski, 1996, Dêbski et al., 2008). Scarcity of the data did not allow to resolve for depth, source depth has been fixed to 10 km in the program. Location attempts with source depth at 5 km depth have resulted with greater errors of location result, while in case 15 km was assumed, the wave velocities to the different stations vary abnormally broadly. Therefore the 10 km depth appears to be a fair educated guess. Locations of the events no. 1,2,5 and 7 fall very close to the location of the event no. 11 and the differences may be attributed to phase pick accuracy. In this situation, similarity of the records of all events except no. 8 to the signal of the greatest event no. 11 has prompted us to assume the location of the event no. 11 as the location of all the events except no. 8. Results have been included in Table 1. The two locations of the event no. 11 and the event no.8 are shown on map in Fig.1 and Fig. 3.

    Figure 2. Map of uncertanities for event 11( cartesian coordinate system). Maximum likelihood point ( X = 3 8 7 0 . 5 ; Y = 4 8 6 6 . 4 ) corresponds to 49.588 N and 20.914 E in geographic coordinate system.

    Figure 3. Map of southern Poland (enlargement of Grybow and its surroundings) showing the locations of the Grybów microearthquakes of September 24-27, 2007. The location of event no. 11 is denoted by star, and event no. 8 is denoted by circle with no. 8 in it.

    Striking is the similarity of records of all the events especially on the nearest station STHS. Figure 4 shows the seismograms of 10 biggest events recorded at STHS with maximum amplitudes scaled to the same size. Except for event no. 8 the records are identical, if - for smaller events - not count the background noise.

    Figure 4. Seismograms of the 9 largest September 24-25 Grybów area microearthquakes, recorded at SHTS. One can see the records practically identical except for event no.8 which is different from all the others. Seismograms have been scaled so their maximum amplitudes are the same. The largest event of all is no. 11.

    Spectra and consequently spectral parameters may be calculated except for the two smallest events no. 4 and 13. However, for smaller events the results can be obtained only from one station, STHS. The spectral analysis has been done using the method used by Domañski (2007), assuming the Brune model of approximation of far field spectra, details of the method have been given by Andrews (1986) and Snoke(1987).

    12 13 Spectral analysis for the largest event no.11 evidences fairly consistent results for seismic moment (from 5.8·10 to 1.2·10 N·m) and other parameters, thus justifying the averaging out over stations. Noteful may be the lower seismic moment values obtained from S wave at CRVS and from P wave at OJC. However, the corresponding P wave values at CRVS and S wave values at OJC are consistent with the average and the values appropriate to the different-origin event no. 8 do not show such discrepancy.

    Seismic

    moment Energy

    Source

    radius

    Stress

    drop Slip [Nm] [J] [m] [Pa] [mm]

    1 2.42

    10

    12 1.1 10

    5 326 3.1 10 4

    1.3 10 3 2.9 0.2

    2 1.84 1012 9.5 104 281 3.6 104 1.5 103 3.2 0.3

    3 0.57 10 12

    1.1 10 4 258 1.5 10

    4 5.9 10

    2 3.3 0.1

    5 1.86 1012 9.5 104 285 3.5 104 1.5 103 3.1 0.2

    6 0.44 1012 3.9 103 314 6.1 103 2.6 102 2.8 < 0.1

    7 2.34 10 12

    7.6 10 4 332 2.8 10

    4 9.6 10

    2 2.7 0.2

    8 3.30 10 12

    2.8 10 5 353 3.3 10

    4 2.5 10

    3 2.9 0.3

    9 0.53 1012 4.6 103 416 3.2 103 2.6 102 2.1 < 0.1

    10 0.18 1012 5.2 102 348 1.9 103 83 2.6 < 0.1

    11 8.75 1012 1.5 106 374 7.3 104 5.0 103 2.5 0.7

    12 0.23 1012 4.1 102 276 4.7 103 6.1 102 3.1 < 0.1

    No.

    Apparent

    stress

    [Pa]

    Corner

    Freq.

    [Hz]

    Table 2. Results of spectral analysis of the Grybów area microearthquakes, September 24-27, 2007. Events no. 4 and 13 were too small for spectral analysis.

    It is difficult to speculate on the cause of the 2007 Grybów area microearthquakes since the location accuracy is limited, there is no macroseismic data and the number of seismic stations that have recorded the events is too low to allow any form of source mechanism determination. The events except event no. 8 seem to have come from one source in the vicinity of the town of Grybów and based on the similarity of seismic records observed at STHS are likely to have a common nature. There is not known any active fault structure in the area. There does exist in the area a local water reservoir with 36 m high dam but it is unlikely that the events were connected with it. Source location error of event no. 11 is much less than the distance from the source to the dam, while event no.8 is still more far away.

    Spectral analysis evidences of considerable source size - 258 to 416 meter source radii, unusually high for events of this size. What is more, whereas generally source radii are bigger for larger events, neither this dependence is clear nor the differences in size big. Similarity of the seismograms and no clear dependence of source radius on event size seem to favor the idea that all the events have come from the same source and the differences in source radii result from numerical data processing and background noise. If so, then all the events except event no.8 could be just 12 episodes of one faulting process. However, following such interpretation, this slip pertains only to the slip that has taken place in form of seismic events and it is likely there might have been a plastic flow process in between the microearthquakes.

    Event no. 8 seems to have origined at a different location closer to NIE station. The seismic station at NIE observes similar size local microearthquakes several times a year (NIE station semiannual reports; unpublished) and having the event no. 8 within the Grybów area microearthquake sequence may have been a pure coincidence.

    CONCLUSIONS:

    Figure 5. Examples of P-wave (left) and S -wave (right) displacement spectra for event 11 made with the use of the FOCI software (G. Kwiatek, PAS Warsaw).

    REFERENCES

    Andrews, D.J., 1986, Objective determination of source parameters and similarity of earthquakes of different size, In: S.Das,

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