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Bulletin of the Seismological Society of America, Vol. 76, No. 3, pp. 733-755, June 1986

EVIDENCE OF TECTONIC RELEASE IN LONG-PERIOD S WAVES FROM UNDERGROUND NUCLEAR EXPLOSIONS AT THE NOVAYA

ZEMLYA TEST SITES

BY R. W. BURGER, T. LAY, TERRY C. WALLACE, AND L. J. BURDICK

ABSTRACT

Evidence of tectonic release produced by underground nuclear explosions at Novaya Zemlya is present in teleseismic, long-period S waves. SH waveforms from representative events at the Southern and Northern Novaya Zemlya test sites were modeled to determine the SH radiation patterns. Long-peded P and SV waves were also investigated to further constrain the equivalent double- couple orientation of the tectonic release. Events within each test site have similar faulting mechanisms, but there are definite differences in the tectonic release orientation between the two sites. The southem test site double-couple orientation is either vertical strike slip or 45°-dipping thrust, while the northem test site has an oblique-normal dip slip orientation. The limited resolution of SH signals and complexity of the observed SV waveforms, along with uncertainties in the absolute explosion source strengths and t*, prevent unique solutions for both test sites. Assuming a strike slip orientation, the 27 October 1973 Southem Novaya Zemlya event has the largest tectonic release moment (1.6 x 1024 dyne- cm) and 14 October 1969 northern event has the smallest moment (0.9 x 10" dyne-cm) of the 12 events studied. The ratio of tectonic release moment to explosion strength varies by as much as a factor of 3 at the northem test site and by a factor of 2 at the southern site. The variability of the tectonic release orientation and strength at the two test sites allows us to address the question of whether tectonic release affects yield estimation.

INTRODUCTION

It is a common observation that many underground nuclear explosions produce substantial nonisotropic radiation (e.g., Press and Archambeau, 1962; Toks6z et al., 1965). The preferred explanation for this source asymmetry is release of preexisting tectonic strain near the explosion (tectonic release). Two basic models of tectonic strain release have been proposed: triggered movement on a nearby fault (Brune and Pomeroy, 1963; Aki et al., 1969; Aki and Tsai, 1972) or stress relaxation of the highly fractured zone created by the explosion (Archambeau and Sammis, 1970; Press and Archambeau, 1972; Archambeau, 1972; Lambert et al., 1972). The long- period, teleseismic radiation for either model can be represented by an equivalent double-couple source. Constraining the orientation of the double couple and determining its effects on the explosion signals are important problems in estimating the yield of nuclear explosions.

Surface wave observations provide the principal evidence for source asymmetry of nuclear explosions. However, recent studies demonstrate that tectonic release also has a significant long-period body-wave signature. Wallace et al. (1983) showed that tectonic release has an effect on long-period P waves at upper mantle distances for some Pahute Mesa explosions. Nuttli (1969), Hirasawa (1971), and Wallace et al. (1985) analyzed long-period S H waves to study the tectonic release from NTS explosions. Other studies (Murphy et al., 1983; Lay et al., 1984a; Burger et al., 1985) have suggested that tectonic release may influence short-period P waves, possibly biasing mb and yield estimates.

The present study is concerned with determining the orientation and moment of 733

734 R . W . BURGER, T. LAY, T. C. WALLACE, AND L. J. BURDICK

tectonic release for underground nuclear explosions at the Novaya Zemlya test site in the Soviet Union. The largest nuclear tests conducted by the Soviet Union, some with estimated yields of more than 3 megatons (Dahlman and Israelson, 1977), took place at Novaya Zemlya, an island north of the Arctic circle in the Barents Sea. Figure 1 shows Novaya Zemlya and the two main subsites, which are separated by about 300 kin.

Long-period SH waves are commonly observed at teleseismic distances from the larger Novaya Zemlya underground explosions. The SH observations enable us to constrain the SH radiation pattern, but there is an intrinsic ambiguity in using SH alone to determine the focal mechanism. The P and S V observations from Southern

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FIG. 1. Base map showing the Novaya Zemlya testing areas. The northern and southern test sites are separated by about 300 kin.

Novaya Zemlya require that the double-couple orientation is either vertical strike slip or 45°-dipping thrust. The Northern Novaya Zemlya test site tectonic release is predominantly dip slip, with an oblique-normal faulting orientation. Using synthetic calculations, seismic moments are estimated for representative events from the two test sites. Relative SH amplitudes are used to estimate the seismic moments of the other events in each test site. The evidence for and implications of changing orientation and strength of the tectonic release are discussed.

BODY-WAVE OBSERVATIONS

Many underground nuclear explosions at the Novaya Zemlya test sites radiated long-period tangential energy to teleseismic distances. Three events from Southern Novaya Zemlya (SNZ) and nine events from Northern Novaya Zemlya (NNZ) produced clear SH body waves. Figure 2 shows naturally rotated SH component

EVIDENCE OF TECTONIC RELEASE AT NOVAYA ZEMLYA 735

observations from a Northern and a Southern Novaya Zemlya event at station JER (A = 43 °, Az = 204°). The different first motions and waveforms at a common station indicate that the faulting mechanisms are different at the two sites. Recog- nizing that there is clear evidence of tectonic release at the Novaya Zemlya test sites and that it is different at each site, we will seek to determine the orientation and strength of the tectonic release mechanisms using body-wave observations. Table 1 gives the epicenters and origin times of the 12 events analyzed. The standard magnitudes determined by Burger et al. (1985) and yield estimates from Dahlman and Israelson (1977) are included in Table 1.

Southern Novaya Zemlya. The 2 November 1974 SNZ event produced a large number of teleseismic SH observations. Some representative waveforms of this event are shown in Figure 3. The entire set of observed SH first motions is shown

NOVAYA ZEMLYA TESTS JER SH COMPONENT

Northern Novaya Zemlya 8/28/72

Southern Novoya Zemlya 11/2/74

FIG. 2. Naturally rotated SH components at station JER for the 28 August 1972 Northern Novaya Zemlya and 2 November 1974 Southern Novaya Zemlya events. These data are E-W WWSSN long- period analog records.

on a lower hemisphere, equal-area projection. There are clear polarity changes at azimuths near 60 °, 150 °, 240 °, and 330 °, with the SH nodal planes being nearly vertical. Both vertical strike slip and 45 ° dip slip mechanisms (either normal or thrust faulting) are characterized by four-lobed SH radiation patterns with vertical nodal planes. Wallace et al. (1985) demonstrate that these fault orientations are indistinguishable on the basis of SH alone, but the moment estimate will be doubled and the strike azimuth will be rotated 45 ° for the dip-slip solutions.

The moments in Figure 3 are for the strike-slip solution (strike, ¢ = 287°; dip, = 90°; rake, X = 180°). The synthetics are computed for a point source dislocation at a depth of 2 kin, with a triangular source function of 0.6-sec duration and t~* of 3.0 sec. A half-space model with a velocity model with a shear velocity of 2.4 kin/ sec and compressional velocity of 4.0 km/sec was used, thus only direct S and the surface reflection (sS) are included in the synthetics. The average seismic moment

736 R. W. BURGER, T. LAY, T. C. WALLACE, AND L. J. BURDICE

is 8.5 x 1023 dyne-cm, which was computed by matching the observed peak-to-peak amplitudes with synthetics for 20 nonnodal observations. The source depth is not well-resolved, but does appear to be less than 3 km. For source durations less than about 5 sec, the synthetic seismograms are insensitive to source duration. However, if a longer period time function is used, say a 3.0-sec triangle (1.5, 1.5), the moment increases by about 10 per cent. The short-duration time function was motivated by

TABLE 1

EVENT INFORMATION*

Date UTC Latitude (°N) Longitude ('E) rnb Yield (kt)

Southern Novaya Zemlya Events

27 Oct. 1973 06:59:57 70.78 54.18 6.98 3200 2 Nov. 1974 04:59:57 70.82 54.18 6.72 1600

18 Oct. 1975 08:59:56 70.84 53.69 6.47 1400

Northern Novaya Zemlya Events

27 Oct. 1966 05:57:58 73.44 54.75 6.37 770 14 Oct. 1969 07:00:06 73.40 54.81 5.97 340 14 Oct. 1970 05:59:57 73.31 55.15 6.72 2100 27 Sept. 197! 05:59:55 73.39 55.10 6.53 770 28 Aug. 1972 05:59:57 73.34 55.08 6.25 690 12 Sept. 1973 06:59:54 73.30 55.16 6.85 2700 29 Aug. 1974 09:59:58 73:37 55.09 6.39 870 23 Aug. 1975 08:59:58 73.37 54.64 6.37 550 21 Oct. 1975 11:59:57 73.35 55.08 6.35 700

* Locations and yields are from Dahlman and Israelson (1977). Magnitudes are from Burger et al. (1985).

11/2/74 SH

BLA MBC INK

IST JER NIL

FIG. 3. Long-period S H observations and synthetics for the 2 November 1974 SNZ event. Numbers are the moments required to match the observed amplitude at the station (in 1023 dyne-cm). The lower hemisphere, equal-area projection shows all the observed S H first motions (filled circles are clockwise) and the S H radiation nodes for the vertical strike-slip mechanism.


the results of Wallace et al. (1983), which show that tectonic release time functions for Pahute Mesa events have durations of about 0.6 sec.

In order to reduce the ambiguity of the tectonic release focal mechanism, S V signals were analyzed. Figure 4 displays radial S V observations for the 2 November 1974 event. The numbers shown are the peak-to-peak amplitudes in millimicrons after applying a geometrical spreading correction (from Langston and Helmberger, 1975) to equalize the amplitudes to a common distance of 50 °. The S V signals are comprised of the tectonic release S V arrival and the explosion pS pulse. The pS pulse should have a downward (toward the source) first motion with no azimuthal variation in waveform. However, the observed S V waveforms clearly show a two- lobed amplitude pattern, with strong arrivals near 240 ° and nodal arrivals near 150 ° and 330 ° . We interpret this azimuthal pattern as the result of interference between the tectonic release S V and explosion pS signals. Note that the similarity of the waveforms at opposite azimuths, such as JER and INK, MAT and VAL, and KBL

11/2/74 SV

.% OGD ti A 477,. 668 8 / 2 6 / 1 7 9 5 ~ , MAT

- V W '£220 lS29 ~ 7~7 ~ ~10 t~ "r~'

MAL STU IST

FIG. 4. Long-period S V observations for the 2 November 1974 SNZ event. The numbers are the observed peak-to-peak amplitudes in millimicrons corrected for geometrical spreading to a distance of 50". The lower hemisphere, equal-area projection shows the location of each observation and the S V radiation nodes for the strike-slip mechanism.

and BLA. This indicates a very regular interference pattern, although the typically complex S V waveforms are difficult to model using standard techniques.

Figure 5 shows the azimuthal variation of SH and S V amplitudes for the 2 November 1974 and 27 October 1973 SNZ events, along with synthetic amplitude calculations for the strike-slip mechanism at a distance of 50 °. The observations are corrected for geometric spreading to a distance of 50 °, but are not corrected for ray parameter differences at the source, so the scatter is slightly greater than for individual station synthetics. The moment for the 27 October 1973 event is 1.6 × 1024 dyne-cm, which was computed for 27 nonnodal SH observations. This moment is 1.9 times larger than for the 2 November 1974 event, which is consistent with the amplitude ratios measured at common stations for SH (1.91 on 26 observations), SV (1.92 on 22 observations), and long-period P (1.88 on 43 observations). The SH synthetic curves closely match the observed azimuthal patterns, both polarity

738 R. W. BURGER, T. LAY, T. C. WALLACE, AND L. J. BURDICK

changes and amplitudes. Furthermore, the two events have almost identical SH and S V amplitude patterns and waveforms, indicating the similarity of their tectonic release orientations. The SV amplitude variations are unusually coherent.

The SV radiation pattern for a vertical strike-slip dislocation has a sin 28 azimuthal variation, while for both 45 ° dip-slip mechanisms, the radiation varies as (1 + sin28). The stereographic projection in Figure 4 shows the SV nodal planes for the vertical strike-slip mechanism that fits the SH data. For this mechanism, the quadrants to the northeast and southwest should have constructive interference between the tectonic release SV and explosion pS signals, while the other two quadrants should have destructive interference. This is consistent with the observations. The normal fault orientation would produce constructive interference at all azimuths, while the thrust orientation predicts destructive interference at all azimuths. Thus, we must know the relative strength of the explosion and tectonic release SV signals to discriminate between the mechanisms. Ideally, we could model the SV waveforms in Figure 4 to obtain the best solution, however, even for

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Fie. 5. Observed and synthetic SH and SV amplitude patterns for the 27 October 1973 and 2 November 1974 SNZ events. The source model for the synthetic amplitudes is the explosion plus strike- slip tectonic release model.

teleseismic distances the propagation effects on SV signals are very complicated (Baag and Langston, 1985), and require detailed knowledge of the source and receiver structures. Lacking such information, we sought models that predict the observed SV amplitude behavior, as well as the SH- and P-wave observations. To do this, we assume each of the three possible faulting mechanisms as constrained by the SH observations. Then we explore their implications for the SV- and P- wave observations, rejecting any models which clearly contradict the data.

If we assume that the tectonic release mechanism is vertical strike slip, we can infer from the lack of polarity reversals and the presence of SV nodes that the explosion and tectonic release signals have nearly the same peak amplitudes. Stations near nodes of the S V strike-slip radiation should record only the explosion pS arrival. For the 2 November 1974 event, we measure an average pS amplitude of 1000 m# from nine stations at the assumed strike-slip nodes (such as INK, COL, JER, and TRN in Figure 4). From this amplitude, we predict the long-period level


of the explosion source excitation, ~ , to be 1.8 × 1011 c m 3 for the 2 November 1974 event and 3.5 × 1011 c m 3 for the 27 October 1973 event. These values are in close agreement with the estimates given by Burger et al. (1985) (1.7 × 1011 cm 3 and 3.4 x 1011 cm 3) from analysis of short-period P waves. This may be fortuitous because of the uncertainties in attenuation and source velocity structure for both estimates. The SV synthetic amplitude patterns in Figure 5 include the vertical strike-slip tectonic release S V and explosion pS signals. The sin 20 S V radiation for the strike- slip mechanism produces the azimuthal variations, which closely match the amplitude data. We assumed that the tectonic release and explosion arrivals coincide in time, a reasonable assumption using long-period S data. The lack of a source ray parameter correction in the SV data in Figure 5 probably accentuates the scatter, but the basic pattern is well-modeled. Slightly different ~® values would simply introduce a baseline shift in the absolute amplitudes without significantly changing the azimuthal pattern. This modeling does not closely match the observed SV waveforms, probably due to the complex SV receiver interactions which we have not included. The strike-slip mechanism will produce weak effects on teleseismic P waves.

If we assume that the tectonic release orientation is the normal fault solution that matches the SH amplitude pattern (~ = 332 °, ~ = 45 °, X -- - 9 0 °, M0 -- 1.7 × 1024 dyne-era for 2 November 1974), then the explosion pS signal, and hence ~®, must be negligible. This is because the tectonic release and explosion signals interfere constructively at all azimuths. The tectonic release amplitudes alone range from 690 to 2400 m/z, with the smaller amplitudes at azimuths of 332 ° and 152 °. Any explosion pS signal will enhance the amplitudes uniformly, thus violating the observation of clear nodes in the data (see Figures 4 and 5). This mechanism predicts destructive interference in teleseismic P waves, assuming that the explosion and tectonic release arrivals are coincident.

The thrust solution (~ = 242 °, 5 = 45 °, X = +90 °, M0 = 1.7 × 1024 dyne-cm for 2 November 1974) requires a large ~ (3.8 x 1011 cm 3) because the SV amplitude pattern must be produced by destructive interference at all azimuths. The SV amplitudes of the tectonic release signals range from 690 to 2400 m/z, with small amplitudes at azimuths of 242 °, and 62 °, thus the explosion pS amplitude will he 2300 m/z. This mechanism predicts constructive interference in teleseismic P waves.

We now consider the observed long-period P-wave amplitude and waveform variations for the SNZ events to seek evidence of tectonic release arrivals. Figure 6 shows the observed peak-to-peak amplitudes of the long-period P waves (corrected to 50 °) for the 2 November 1974 and 27 October 1973 events. The amplitude data for the latter event have been divided by a factor of 1.88 to put the two data sets on a common scale. The average amplitude for the 2 November 1974 event is 2686 m/z (51 observations), and the average amplitude for the 27 October 1973 event is 5049 m/z (55 observations). In general, the relative amplitudes at common stations are very similar, as are their waveforms. No azimuthal pattern is apparent in the data.

Representative waveforms for the 2 November 1974 event are also shown in Figure 6 along with synthetics for various possible tectonic release parameters. The basic P-wave observations that we attempted to model were the average amplitudes of the short- and long-period P waves, as well as the dominant period of the long- period signals. The models were constrained to have parameters matching the long- period S waves. There are many uncertain quantities in this modeling. These include the value of t,* and any possible frequency dependence of attenuation (we chose 0.5 to 0.75 sec as a reasonable range), the explosion overshoot parameter B


(we chose 0.1 and 1.0 as end members), and the relative and near-source crustal reverberations (not included), and the relative timihg and frequency content of the tectonic release and explosion sources (a range of triangular source functions was used for the tectonic release, and the explosion source parameters are from Burger et al., 1985). While we use the Futterman (1962) attenuation model, we do allow for variation in t* between the long- and short-period bands.

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A Z I M U T H , deg

Az=14 11/2/74 Explosion + Tectonic Release CO-'~\ r"'J~. I /~ A =41 Vertical 45 ° Thrust

v Strike- Slip 4 5 = Thrust Short - Period

84 Az = 242 332 242 332 242 332 MAT j ~ 8=,0 t'=05 ~/~86 ~ 5 9 _/L~.06 _/L~S j~90~089

(0 3,0 3) 131 SN~ (o3 B t'=O5 =01 O3) j ~ 8 _ ~ 3 7 ~ 3 9 ] ~ 8 ~ 2 5 1 ~ 38

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347 T U ~ / ~ s 1"=0.75 =o.i . . ~ 5 j ~ 7

(10, i0)

. ~ 0 5 ~ 2 3

IOsec' ~ ~ FIG. 6. Long-period P-wave amplitudes for the 2 November 1974 and 27 October 1973 SNZ events.

Also shown are selected observations for the 2 November 1974 event. Synthetic explosion plus tectonic release P waves are computed for different choices of B, t , and tectonic release source function.

Even with these uncertainties, the P-wave modeling provides important information. It proved impossible to match the long-period P waves with normal faulting tectonic release given the low ~b® value required by the S V amplitudes. The strike- slip model with t* = 0.5, a dislocation source function of 0.6-sec duration and B = 1 matches the observed amplitudes for both short-period (2239 m#) and long-period (2686 m/~) P waves. The tectonic release contribution is negligible for either case, so these results do not depend on our choice of time function. The only failing of


this model is that the synthetic long-period P waves are much higher frequency than observed. Given the difficulty of accurately digitizing the signals and the lack of any near-source velocity information, this may not be too serious. Including crustal reverberations would broaden the synthetic waveforms (Douglas and Hud- son, 1983). The dominant period can also be decreased by raising t* or by decreasing B, but both actions result in models that underpredict the observed amplitudes. However, if the explosion pS amplitudes are affected by nonlinearity near the source, the ¢® value predicted by the SV amplitudes could be underestimated. The ¢~ needed to match the P amplitudes for both B = 0.1 and t* = 0.75 is 4.5 × 1011 cm 3, which requires a factor of 2.1 correction to the pS-¢~ scaling relation.

The thrust mechanism predicts a greater effect on the teleseismic P waves. The 0.6-sec duration source function predicts strong P arrivals, comparable in size to the explosion arrivals. For t* values less than 0.75 sec, the predicted short- and long-period P amplitudes are too large for any value of B. Adopting a longer duration dislocation source function, say 2.0 sec, substantially reduces the tectonic release contribution, and the observations can be matched using a range of t* of 0.5 to 0.75. For the 2.0-sec dislocation duration and B = 0.1, a short-period t* of 0.6 and a long- period t* of 0.75 match the amplitudes. The tectonic release contribution broadens the long-period P waves, more accurately matching the dominant period of the observations than the strike-slip models. The major problem with the thrust model is that explosion pS amplitude scaling between the two test sites does not track the short-period P amplitudes, although differences in t* or crustal structure could be responsible.

Thus, the body-wave analysis of the Southern Novaya Zemlya events does not presently unambiguously resolve the tectonic release orientation. Either the 45 ° thrust or the vertical strike-slip mechanism can explain the P and S wave observations given the uncertainty in the earth and source parameters. There is no obvious tectonic release contribution in either the short- or long-period P waves, and models that predict little effect on the explosion P waves are generally successful. Thus, for these events, tectonic release appears to have only minor effects on the short-period P waves and will not bias yield estimates based on rob.

Northern Novaya Zemlya. Figure 7 presents SH observations for the 12 Septem- ber 1973 NNZ event. There are clear polarity reversals between azimuths 165 ° and 183 ° (QUE and SHI) and between 264 ° and 322 ° (ESK and BLA). The entire set of SH first motions are projected onto the focal sphere, showing that the nodal planes are well constrained. Several dislocation orientations with fault planes dipping between 75 ° and 47 ° are found to have this SH radiation pattern. Table 2 gives the full range of possible orientations and resultant moment estimates. The synthetics shown in Figure 7 are for a fault plane with dip = 55 °, rake = -41 °, and strike = -20 °. The depth is 3 km, t~* is 3.0, and a triangular time function with a 0.6-sec duration is used. The moment is 11.4 × 1023 dyne-cm and was computed by matching the observed peak-to-peak amplitudes with synthetics at 23 stations.

Figure 8 shows SV observations for the 12 September 1973 NNZ event. Unlike for the 2 November 1974 event, there are no obvious nodal stations. There is, however, evidence for a reversal of first motion of the SV waveforms between azimuths near 200 ° (EIL) and 320 ° (BLA). The positive first motion (outwards from the source) at BLA and SHA requires a strong tectonic release component that flips the negative first motion of pS from the explosion. All of the solutions given in Table 2, except for the mechanism dipping 47 °, have a strong positive SV at 320 °. For the 2 November 1974 event, stations at opposite azimuths had similar


waveforms; bu t this is no t t rue for the 12 S e p t e m b e r 1973 event . For example, E I L and COL, and E S K a nd M A T have very d i f ferent signals. Th i s conf i rms t h a t an oblique t ec ton ic release m e c h a n i s m is p resen t a t N N Z .

Figure 9 p resen t s the ent i re set o f SH a nd SV ampl i tudes for the 12 Sep t ember 1973 even t equal ized to a d is tance o f 50 °, and syn the t i c ampl i tude p a t t e r n s for the

9 / 1 2 / 7 5 SH

LON

BLA ~, ,~ A,kA, . ~ i . . . - " mr', vv col

A. / MAT

SHI QUE

FIG. 7. Long-period SH observations and synthetics for the 12 September 1973 NNZ event. Synthetics are for an oblique-normal double-couple dipping 55 ° . Numbers are the moments necessary to match the observed amplitude at the station (in 1023 cl))ne-em). The lower hemisphere, equal-area projection shows all the observed SH first motions and the SH radiation nodes for the tectonic release mechanism.

SEISMIC MOMENT

TABLE 2

ESTIMATES FOR THE 12 SEPTEMBER 1973 NNZ EVENT*

Dip Strike Rake Mo (dyne-cm)

75 -12 -22 10.4 x 1023 70 -14 -24 10.5 x 10 ~ 65 -14 -30 10.3 x 1023 60 -18 -35 10.4 × 1023 55 -20 -41 11.4 × 1023 50 -35 -58 13.2 X 10 ~ 47 -37 -65 17.1 x 1023

* Moment estimates are for a point source dislocation. Source depth is 3 km, shear velocity is 2.4 km/sec, triangular source-time duration is 0.6 sec, and t*~ is 3.0 sec.

focal m e c h a n i s m d ipp ing 55 ° (¢ = 340 °, X = - 4 1 °, Mo = 11.4 x 1023 dyne-cm) . T h e SH rad ia t ion is wel l -matched. T h e syn the t i c SV ampl i tude p a t t e r n includes the explos ion pS and tec ton ic release SV. T h e lower SV syn the t i c ampl i tude curve is for a tec tonic release o r i en ta t ion d ipp ing 55 ° and a pS explos ion magn i tude o f 1585 mu. T h e 12 S e p t e m b e r 1973 pS ampl i tude is based on scal ing relat ive to the 2


November 1974 SNZ event, assuming the relative ~b® estimates from Burger et al. {1985) and a pS amplitude for 2 November 1974 event of 1000 m~, appropriate for the strike-slip orientation at SNZ. This curve matches the observations in an average sense. The upper curve is for a double-couple dipping 75 ° and a pS explosion

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FIG. 8. Long-periodSVobservationsforthe12September1973NNZevent. Numbers are the observed peak-to-peak amplitudes in millimicrons equalized to a distance of 50". The lower hemisphere, equal- area projection shows the location of each observation.

9/12/73 SH 9/12/75 SV 4000 i ol __ i " ' I /

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AZIMUTH, deg

FIG. 9. Observed and synthetic SH and SV amplitude patterns for the 12 September 1973 NNZ event. The synthetic SH curve is for a dislocation mechanism dipping 55 °. The lower SV synthetic curve is for an explosion pS amplitude of 1585 m# and a mechanism dipping 55 °. The upper SV synthetic curve is for an explosion pS amplitude of 3340 m# and a mechanism dipping 75 °.

amplitude of 3340 m#, based on scaling relative to the 2 November 1974 pS amplitude of 2300 m#, appropriate for the thrust mechanism. When the larger pS amplitude is adopted for the SNZ event, the NNZ SV amplitudes are overestimated. If the upper SV curve was recomputed for a double-couple dipping 47 °, the ampli-

744 R. W. B U R G E R , T. LAY, T. C. W ALLACE, AND L. J . B U R D I C K

tudes would be even larger because the SV waves from the normal faulting mechanism are constructive with the explosion pS. Also, for oblique mechanisms, SV radiation is strongly dependent on take-off angle. Thus, much of the SV amplitude scatter in Figure 9 is attributed to the effect of ray parameter as well as receiver complexities. A more complete waveform modeling of SV signals including receiver

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315

Fro. 10. Long-period P-wave ampl i tudes and waveforms for the 12 September 1973 N N Z event. Synthet ic explosion plus tectonic release P waves are computed for the fault or ienta t ions dipping 75 °, 55 ° , and 47 ° .

interactions and source complexities may help to uniquely determine the tectonic release double couple at Northern Novaya Zemlya.

Figure 10 shows the long-period P-wave amplitudes and selected waveforms for the 12 September 1973 NNZ event. The average peak-to-peak amplitude is 4482 m# (49 observations). There are no clear azimuthal amplitude or waveform variations, indicating that the tectonic release signature is subtle. Also shown are synthetic P waves for various explosion plus tectonic release models. Each of the

EV IDE NC E OF T E C T O N I C R E L E A S E AT NOVAYA ZEMLYA 745

fault orientations predict destructive interference with the explosion P wave. We have limited the values of B and t* to be 0.1 and 0.6, respectively. The low value of B was chosen such that the period of the synthetics agrees as well as possible with the period of the observations (see Figure 6). Slightly larger values of t* do not greatly affect the frequency of the waveforms, but reduce the amplitudes significantly. The value of ~® for the 12 September 1973 event is based on scaling relative to the value of ~® which matches the P-wave amplitudes for the 2 November 1974 event. For both the strike-slip and the thrust orientations, the 2 November 1974 ~® is 4.5 x 1011 cm 3, giving a ~b® value of 7.3 x 1011 c m 3 for the 12 September 1973 event. These choices of ~b®, B, and t* are found to predict the P-wave observations quite well.

The tectonic release models which produce the least teleseismic P energy fit the data best. This is because they predict little azimuthal variation in the amplitude and waveform data and the overall average amplitude. Thus, the tectonic release mechanisms with dips greater than 55 ° are favored. The double-couple orientation dipping 47 ° (~ = 323 °, k -- -65 °, M0 = 17.1 x 1033 dyne-cm) decreases the explosion amplitude by over 20 per cent and produces a noticeable azimuthal waveform variation. However, the many trade-offs affecting the P-wave synthetics limit any strict conclusion about the tectonic release orientation. We, therefore, maintain only that any tectonic release signature in the long period P waves is subtle, and models that predict little effect are consistent with the data. The more vertically oriented mechanisms for NNZ predict very little effect on the short-period P waves, which is consistent with the observations. The short-period amplitude (3020 m~) is also matched with these tectonic release plus explosion models.

The data above demonstrate that the tectonic release focal mechanisms are different for events from SNZ and NNZ. Figure 11 summarizes the SH first motions and focal mechanisms for the representative events from Northern and Southern Novaya Zemlya. Also shown are the P-wave radiation patterns for the preferred tectonic release orientations for each event. The SNZ tectonic release is either vertical strike slip or 45 ° thrust, and the NNZ tectonic release is oblique-normal dip slip. The extreme orientations for tectonic release at NNZ are also shown in Figure 11.

To summarize the waveform modeling results, we find that for the 12 September 1973 NNZ event, orientations dipping between 75 ° and 47 ° fit the SH wave observations, but the more vertically dipping models fit the SV and P waves better. The 12 September 1973 explosion ~® is 7.3 x 1011 cm 3 with a B of 0.1. For the 2 November 1974 SNZ event, both the thrust and strike-slip orientations can match the overall observations. Each of the SNZ models have slight discrepancies; but in either case, viable explanations are available. However, note that the strike-slip solution for SNZ has similar tension axes to the NNZ mechanism (Figure 11), suggesting that it may be the appropriate mechanism.

We now compare the tectonic release orientations within the two subsites separately. As indicated in Figure 5, for Southern Novaya Zemlya, the SH and SV radiation patterns and waveforms are very similar from event to event, suggesting that the double-couple orientation is similar for events within that test site. Figure 12 shows long-period, vertical P, radial SV, and tangential SH for the three SNZ events at stations JER and COL. The seismograms are plotted at the same amplitude scale as the P wave for each event. The waveforms and relative amplitudes are very consistent for the 2 November 1974 and 27 October 1973 events, but differ somewhat for the 18 October 1975 event. Burger et al. (1985) found that the 18 October 1975


event has a double nuclear event, based on the split pulses seen in long-period P waveforms. The S waves have slightly different waveforms as well, but we cannot resolve a difference in tectonic release character for this event. However, the proportion of tectonic release is on average a factor of 2 lower for the 18 October 1975 event than for the other SNZ events.

Figure 13 shows long-period, three-component seismograms for seven NNZ events at station MBC. The SH waveforms and radiation patterns are similar for each

SOUTHERN NORTHERN

NOVAYA ZEMLYA NOVAYA ZEMLYA

N N

E W E

)K Clockwise o Counterclockwise

N N

IN -E IN

s 5

~ E

Vertical Strike Slip/Thrust Oblique Normal

FIG. 11. The lower hemisphere equal-area projections show the SH- and P-wave radiation patterns for the SNZ and NNZ tectonic release mechanisms. SNZ tectonic release is either vertical strike-slip or 45 °-dipping thrust and NNZ tectonic release is oblique-normal. The SNZ vertical strike-slip mechanism is oriented with a dilatational first motion ot the northwest, and the thrust mechanism is compression in the center of the focal sphere. The extreme orientations for the NNZ double couple are also shown. The NNZ mechanisms have dilatational first motions to the northwest.

event, suggesting that the tectonic release orientation does not vary dramatically at NNZ. However, there is clear evidence for varying proportions of tectonic release. The 27 October 1966 and 29 August 1974 events have nearly identical mb'S, but the peak-to-peak SH amplitude is a factor of 4 larger for the 27 October 1966 event. The differences in the P waveforms are attributed to changes in the explosion source and possibly, variable strength of tectonic release. The SV signals are very complex, due to the nature of SV propagation and the complicated interaction of explosion pS and tectonic release SV. There is no strong evidence for a reversal of the S V first motion.

There is evidence for small changes in tectonic release orientation at the NNZ test site. Figure 14 shows SH waveforms at stations NDI (Az = 153°), SHI (Az =


183°), and HLW (Az = 210°). The S-wave arrival time is shown on each seismogram. The SH waves at NDI and HLW are similar from event to event, and the first motion polarity reversal indicates that there is a nodal plane between NDI and HLW. But the waveforms at SHI show substantial variations in the strength of the first upswing. The 27 October 1966, 27 September 1971, and the 28 August 1972 events show a strong tangential negative first motion (counterclockwise), but the 14 October 1970 and 12 September 1973 events show weak negative first motion. This observation requires slight variations of the SH nodal plane near azimuth 183 °

JER P SV SH

~ 8 3231 2257 10/27/73 W ~ rob= 6.98

. ~ 8 1476 1426 ll/02/74 ~ V ~ ~ . ~ , r m b = 6.72

. ~ 5 274 446 ,oi,8175

rob= 6.47

I 5 0 sec I

COL

10/27/75

I 1/02/74

10/18/75

5608

3028

6

1941 1740

860 828

FIG. 12. Comparison of three-component seismograms for three SNZ events at JER and COL. Numbers are the observed peak-to-peak amplitudes in millimicrons equalized to a distance of 50".

(SHI), without disrupting the waveforms at azimuths 153 ° (NDI) and 210" (HLW). This is consistent with the focal mechanism given in Figure 7 in which changes in the strike or rake on the order of 5 ° to 10" are allowed. The SH wave radiation at NDI is shown to be nodal for direct S, and these small changes in orientation will still result in the dominance of sS in the waveform. But at SHI, direct S is stronger, and small rotations in the focal mechanism change the importance of S relative to sS. Thus, for 27 October 1966, 27 September 1971, and 28 August 1972 events, the strong counterclockwise first motion indicates a strong direct S pulse and the nodal plane shifting towards the southeast (towards NDI),


RELATIVE F-FACTORS

Numerous studies have performed inversion of Rayleigh wave amplitude and Love/Rayleigh wave amplitude ratios to determine the relative size of tectonic release to explosion strength. The parameter used to measure the ratio of nonisotropic to isotropic component is given by the F-factor (ToksSz e t al., 1965),

F = 1.5 (Mo"VMo~"), (1)

MBC P SV SH

10/27/66 rob= 6.37

I0/14/70 rob= 6.72

3497

--'-,vv ~ 7 5

1274 8/28/72 m b = 6.25

9/12/73 rob= 6.84

8/29/74 v ~ 037

m b = 6.39

8/23/75 ~ 4 1

rob= 6.37

10/21/75 ~ 3 0

rob= 6.35

1488 1786

1733 2340

711 816

1590 x ~ , ~ . 4 2 0

839 315

1142 1072

612 1020

I I 50 sec

FIG. 13. Comparison of three-component seismograms for seven NNZ events at MBC. Numbers are the observed peak-to-peak amplitudes in millimicrons equalized to a distance of 50 °.

where the moment of the explosion is given by Aki et al. (1974) as

Mo exp = 4rpa2¢~o. (2)

Using our source strength estimates and assumed values for p and a (2.7 gm/cm 3 and 4.0 km/sec), the F-factor for the 2 November 1974 SNZ event (assuming the strike-slip fault orientation) is 0.52, and the F-factor for the 27 October 1973 SNZ event is 0.49. The F-factors, assuming the thrust mechanism, are 1.03 for 2 November 1974 and 0.98 for 27 October 1973. These F-factors are for B = 0.1, with larger values of B increasing the values. The long-period P-wave modeling and the


S V amplitude analysis indicate that these F-factors are at least approximately correct. Following the same assumptions, the F-factor for the 12 September 1973 NNZ event for a fault orientation with a dip of 55 ° is 0.43.

Wallace et al. (1983, 1985) computed the seismic moments for a set of Pahute Mesa explosions using long-period P and SH waves, and Lay (1985) computed ~b= values for the same events using short-period P waves. Adopting those results and equations (1) and (2), we estimated the F-factors for the Pahute Mesa events. For example, Greeley, a high-tectonic release event, has an F-factor of about 6. Handley, a low-tectonic release explosion, has an F-factor of about 2. It appears that the low

SH HLW SHI NDI

Az = 210

1 0 / 2 7 / 6 6

1 0 1 1 4 1 7 0

9 / 2 7 1 7 1

8 1 2 8 1 7 2

9 1 1 2 / 7 3

77e I

:A 33e8 I

AZ = 183

I I ~ 8 5 4

5 9 7

362 1790

I I 5 0 s e c

AZ = 153

' 7 5 4

I I 849 ',< I 6 4 3

I I 372

I 2112

I

FIG. 14. Comparison of SH waveforms at HLW, SHI, and NDI for five NNZ events. The dashed lines indicate the S arrival time. Notice tha t the waveforms are similar between events at HLW and NDI, but change slightly at SHI.

tectonic release events at Pahute Mesa have larger relative components of tectonic release than do the large Novaya Zemlya events.

The F-factors given above are subject to several large baseline uncertainties. Foremost of these are the average t* and source velocity structure. Although the absolute values of the explosion moment and tectonic release moment are not well- resolved, their relative values are considered quite accurate. Burger et al. (1985) give the relative explosion ~= estimates for the Novaya Zemlya events determined from short-period P waves. To determine the relative F-factors of these events requires relative seismic moments. For each subsite, relative S H amplitudes were measured and used to determine the relative seismic moments. Peak-to-peak S H amplitudes were found relative to the 2 November 1974 event for SNZ and relative to the 12 September 1973 event for NNZ. We chose to use relative amplitudes to obtain

750 R . W . BURGER, T. LAY, T. C. WALLACE, AND L. J. BURDICK

10/27/73

11/2/74

10/18/75

JER SH

INK SH 3026 /~ 4540

,o,~ ~, ~ ~ : ~ - - " - ~ ~ J " V ~

1112174

417 1252

10/18/75

a

10/14/69

10/14/70

9/27/71

8 / 2 8 / 7 2

9 / 1 2 / 7 3

b

HLW SH

51 481

i • 50 se¢

FIG. 15. (a) Comparison of SH (S and SS) waveforms at JER and INK for three SNZ events. (b) Comparison of SH (S and SS) waveforms at HLW for five NNZ events.

EV IDE NC E OF T E C T O N I C R E L E A S E AT NOVAYA ZEMLYA 751

seismic moments in order to eliminate the bias caused by using different stations in each moment estimate. The validity of this procedure was established by the results presented in Figure 5 for the SNZ events of 2 November 1974 and 27 October 1973, which have enough observations to measure absolute moments as well as amplitude ratios. The tectonic release orientation, source depth, and source-time history is assumed to be constant within each test site.

For the relative amplitude analysis, both tangential S and S S peak-to-peak amplitudes were used. Figure 15a shows examples of S and S S waveforms for the three SNZ events, and Figure 15b shows S and S S waveforms for five NNZ events. The S S waveforms are similar from event to event, as are the S waveforms. The use of S S enabled us to increase the number of relative amplitude measurements, which is important for the lower tectonic release events for which only high-gain or high-Q stations recorded S H waves. One NNZ event in particular (14 October 1969) is poorly recorded at most stations but displays clear S and S S waveforms at HL W (Figure 15b). Furthermore, at many stations, such as at INK for SNZ events, S S is larger than S, thus it is more easily and reliably measured.

Table 3 gives the seismic moments (>f the three SNZ events relative to the 2 November 1974 event. The relative seismic moments are based on at least 13 and

T A B L E 3

RELATIVE TECTONIC RELEASE PARAMETERS FOR SOUTHERN NOVAYA ZEMLYA

Relative Relative Event Mo F*

Double-Couple Moment Explosion Size

27 Oct. 1973 1.9 16.2 )< 1023 1.98 0.96 2 Nov. 1974 1.0 8.5 x 1023 1.00 1.00

18 Oct. 1975 0.38 3.2 × 1023 0.78 0.49

* T he relative double-couple m o m e n t and relative explosion size are relative to the 2 November 1974 event.

as many as 26 relative S and S S amplitude ratios. The amplitude ratios vary by, at most, a factor of 2. Burger et al. (1985) determined the explosion size estimates of the SNZ events relative to the 2 November 1974 SNZ event given in Table 3. The relative source strengths are used to estimate the relative F-factors (F*). F* is defined as

F * = (Mo<~IMo%,)I( ¢®I¢®~D, (3)

where ~®/¢~74 is the relative explosion source strength. F* for the 2 November 1974 event is, by definition, 1.0. Note that the range in F* at SNZ is 0.49 to 1.0, only a factor of 2. The absolute double-couple moment and explosion ~b® estimates are subject to intrinsic baseline uncertainties, but relative values are free of this problem. Thus, relative F-factors are considered reliable and may be used to compute absolute F-factors once more reliable baselines are determined. The absolute F-factor may be obtained from

fabsolute = F'FIll2~74. (4)

The absolute F-factor of the 2 November 1974 SNZ event is 0.52 assuming the B = 1 explosion model and the vertical strike-slip tectonic release orientation.

Table 4 gives the seismic moments and F-factors of the nine NNZ events relative

752 R. W. B U R G E R , T. LAY, T. C. W A L L A C E , AND L. J . B U R D I C K

to the 12 September 1973 event. The relative moments are based on from 5 to 23 amplitude ratios in each case. The 12 September 1973 moment is 11.4 × 1023 dyne- cm with an absolute F-factor of 0.43. The range in F* at NNZ is larger than it was at SNZ, ranging from 0.42 to 1.25, spanning a factor of 3.

D I S C U S S I O N

One of the principal motivations for detailed analysis of tectonic release is to develop corrections for its effects on seismic yield estimates. It is well known that these corrections can be significant for yield estimates utilizing Ms measurements or waveforms of long-period surface waves (e.g., von Seggern, 1970; ToksSz and Kehrer, 1972; Aki and Tsai, 1972). Long-period body-wave signals such as P or S V can also be strongly affected by tectonic release (Wallace et al., 1983). It is particularly important to establish the tectonic release orientation when using the explosion generated long-period pS (SV) signal to estimate yield. The simplest correction procedure is to identify stations with nodal S V radiation from the tectonic release, thus directly determining the explosion ~b~ from the isolated pS arrival. For

T A B L E 4

RELATIVE TECTONIC RELEASE PARAMETERS FOR NORTHERN NOVAYA ZEMLYA

Relative Relative Event Mo F* Double-Couple Moment Explosion Size

27 Oct. 1966 0.42 4.8 x l0 g 0.33 1.25 14 Oct. 1969 0.081 0.9 × 10 ~3 0.13 0.62 14 Oct. 1970 0.53 6.2 × 1023 0.68 0.80 27 Sept. 1971 0.45 5.2 × l0 S 0.46 0.98 28 Aug. 1972 0.21 2.4 × 1023 0.25 0.84 12 Sept. 1973 1.00 11.4 × l0 g 1.00 1.0 29 Aug. 1974 0.14 1.6 × l0 S 0.34 0.42 23 Aug. 1975 0.26 2.9 × l0 S 0.31 0.82 21 Oct. 1975 0.21 2.4 × l0 S 0.32 0.66

* T he relative double-couple m o m e n t and relative explosion size are relative to the 12 September 1973 event.

example, Lay et al. (1984b) found that S V was a good estimator of explosion source strength for the Amchitka tests, which had low F-factors. Helmberger (1985) obtained accurate yield estimates for Pahute Mesa explosions from S V signals at stations near known nodes of the tectonic release S V radiation. There is less certainty that tectonic release corrections are needed for yield estimates based on short-period waveforms or mb measurements. There is still no unambiguous evidence of tectonic release effects on short-period teleseismic P waves or regional phases, although some observations may be accounted for by plausible tectonic release models (Murphy et al., 1983; Lay et al., 1984a; Burger et al., 1985).

Sykes and Wiggins (1985) utilized mb and Ms measurements to estimate yields of the Novaya Zemlya explosions. The oblique-normal faulting mechanism for the northern test site probably enhances some of their average Ms measurements, with some scatter resulting from variable F-factors. Assuming that good azimuthal coverage is available, the Ms values for the southern site may be unaffected if the strike-slip solution is correct, or diminished if the thrust solution is correct. Any


such biases due to tectonic release are probably not large given the moderate F- factors, and Sykes and Wiggins (1985) did not observe any mb -- Ms anomaly for large F-factor events. Future detailed surface wave analysis may help to resolve the tectonic release parameters for these events as well as quantifying the Ms biases.

The teleseismic long-period P waves in this study did not show any clear effect of tectonic release contamination, but this modeling established that even strong tectonic release arrivals may have subtle manifestations. The S V signals do show clear interference effects, but these can be qualitatively matched by simple explosion plus tectonic release models. Assuming that the vertical strike-slip orientation is correct for the southern site, the stations with nodal tectonic release S V radiation provide ¢~ estimates consistent with short-period determinations. However, it is clear that deterministic waveform modeling, accounting for the propagation and interference effects, is required to extract more information from the S V signals. The relative excitation of shear-coupled PL phases may prove diagnostic of tectonic release orientation for the southern test site.

There is no direct evidence for tectonic release effects on the short-period P waves from the Novaya Zemlya events, although again such effects can be very difficult to detect. There are large differences in the average azimuthal amplitude patterns between NNZ and SNZ (Butler and Ruff, 1980; Burger et al., 1985), which could be due either to structural variations or to differences in tectonic release orientation. Burger et al. (1985) explore the latter possibility, showing that the tectonic release models determined above may contribute to the relative patterns. Waveform variations at common stations for different F-factor events are observed to be quite subtle, and it is difficult to distinguish structural effects from those of tectonic release. This is also the case for the Pahute Mesa tests (Lay et al., 1984a).

CONCLUSIONS

The tectonic release from underground nuclear explosions at the Novaya Zemlya test sites in the Soviet Union has been represented with equivalent double-couple mechanisms. S H waves from Southern and Northern Novaya Zemlya were modeled to obtain the S H radiation pattern. P and S V waves were investigated to further constrain the mechanism. The tectonic release orientations were found to be similar for each event within SNZ and NNZ separately, but the mechanisms are different between the two test sites. The SNZ double couple is either vertical strike slip or 45°-dipping thrust, and the NNZ double couple is oblique-normal. There is uncertainty in the exact orientations due to uncertainty in the absolute explosion source strength, uncertainty in t* and near-source structure, the ambiguity of SH, and the complexity of S V signals. Furthermore, evidence was found for small rotations of the double-couple orientation at NNZ. The strength of tectonic release relative to explosion size was found to vary by as much as a factor of 3. The use of P and S V signals not only provided a constraint on the tectonic release orientation at SNZ, but also gave reliable estimates of the explosion strength consistent with short- period determinations.

ACKNOWLEDGMENTS

The authors thank Jeff Barker and the anonymous reviewer for reviewing this manuscript. We thank Cindy Arvesen for drafting the figures and Linda Burger for typing the manuscript. T. L. acknowledges support from the Michigan Memorial-Phoenix Project. This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Office of Scientific Research under Contract F19628-85-C-0036.


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von Seggern, D. (1970). The effects of radiation pattern on magnitude estimates, Bull. Seism. Soc. Am. 60, 503-516.

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Wallace, T. C., D. V. Helmberger, and G. R. Engen (1985). Evidence of tectonic release from underground nuclear explosions in long-period S waves, Bull. Seism. Soc. Am. 75, 157-174.

WOODWARD-CLYDE CONSULTANTS 566 EL DORADO STREET PASADENA, CALIFORNIA 91101 (R.W.B., L.J.B.)

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF MICHIGAN ANN ARBOR, MICHIGAN 48109 (T.L.)

DEPARTMENT OF GEOSCIENCES

UNIVERSITY OF ARIZONA TUCSON, ARIZONA 85721 (T.C.W.)

Manuscript received 29 October 1985

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