Source analysis of the Memorial Day explosion, Kimchaek, North Korea

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L21304, doi: /2009gl040003, 2009 Source analysis of the Memorial Day explosion, Kimchaek, North Korea Sean R. Ford, 1 Douglas S. Dreger, 2 and William R. Walter 1 Received 17 July 2009; revised 5 August 2009; accepted 1 September 2009; published 7 November [1] A series of source inversions are performed for the 25 May 2009 (Memorial Day) North Korean seismic event using intermediate period (10 50 s) complete waveform modeling. An earthquake source is inconsistent with the data and the best-fit full seismic moment tensor is dominantly explosive (60%) with a moment magnitude (M W ) of 4.5. A pure explosion solution yields a scalar seismic moment of dyne-cm (M W 4.1) and fits the data almost as well as the full solution. The difference between the full and explosion solutions is the predicted fit to observed tangential displacement, which requires some type of non-isotropic (non-explosive) radiation. Possible causes of the tangential displacement are additional tectonic sources, tensile failure at depth, and anisotropic wave propagation. Similar displacements may be hidden in the noise of the 2006 event. Future analyses of this type could be used to identify and characterize non-earthquake events such as explosions and mine collapses. Citation: Ford, S. R., D. S. Dreger, and W. R. Walter (2009), Source analysis of the Memorial Day explosion, Kimchaek, North Korea, Geophys. Res. Lett., 36, L21304, doi: /2009gl Introduction [2] The Democratic People s Republic of Korea (DPRK) announced it conducted a second nuclear test on 25 May 2009 ( /05/25/world/asia/25nuke-text.html). Within hours of the test, and before the official DPRK announcement, several organizations, including the U.S. Geological Survey National Earthquake Information Center and the Comprehensive Test Ban Treaty Organization (CTBTO), reported a seismic signal in the magnitude range 4.5 to 4.7 near the vicinity of the 2006 DPRK nuclear test ( usgs.gov/eqcenter/eqinthenews/2009/us2009hbaf; Many seismologists have noted similarities between the scaled 2009 waveforms and those from 2006, and we show such a comparison in this paper, indicating the location and source type of the two events are similar ( However, the International Monitoring System of the CTBTO did not detect 1 Lawrence Livermore National Laboratory, Livermore, California, USA. 2 Berkeley Seismological Laboratory, University of California, Berkeley, California, USA. Copyright 2009 by the American Geophysical Union /09/2009GL radioactive noble gases that would confirm the test of a nuclear-device [Clery, 2009; On 15 Jun 09 the U.S. Director of National Intelligence released a statement that North Korea probably conducted an underground nuclear test... (t)he explosion yield was approximately a few kilotons (available at [3] In previous work, Ford et al. [2009] showed it was possible to identify explosions at the Nevada Test Site with a full moment tensor inversion of seismic waveforms from nearby stations in the Western US. In this paper, we extend this analysis to the 2009 Memorial Day event near Kimchaek, North Korea and show that the best-fit source is dominantly isotropic, which is consistent with an explosion. We also compare the 2009 event with the 9 October 2006 declared North Korean nuclear explosion, finding very similar results, with the 2006 source strength approximately five to six times smaller than the 2009 event. 2. Data and Method [4] Three component waveform data from Global Seismograph Network (GSN) and China Digital Seismograph Network (CDSN) stations MDJ, INCN, BJT and HIA along with station TJN from the Ocean Hemisphere Project Seismological Network (Figure 1) are instrument corrected, integrated to displacement, and band-pass filtered using an 6th order acausal Butterworth filter with corners at 0.02 and 0.10 Hz. Green s functions were computed using the MDJ2 velocity model (Table 1) for stations MDJ, INCN, and TJN, and filtered similarly to the data. The velocity model is a modification of the surface-wave-derived MDJ model [Nguyen, 1994]. The Green s functions were aligned with the data based on a location and origin time of 00:54:43.38, 25 May 09, / (D. Dodge, unpublished data, 2009). Green s functions from station TJN required a shift of 2 sec relative to the data to produce a good match, which may be due to error in the assumed velocity model along this path. Ford et al. [2009] found that shifts of less than a half-cycle still produced accurate solutions. [5] However, the MDJ2 velocity model was too fast at the farther stations, BJT and HIA (1099 and 1147 km, respectively) and required shifts greater than a half-cycle (5 s). Therefore, the velocity model was modified below the source layer to better model the waveforms at these distant stations. These changes are given in Table 1. A source depth of 600 m was initially assumed, though the solution sensitivity to this assumption will be tested. Explosion, earthquake (doublecouple or DC), deviatoric, and full moment tensor solutions were evaluated and are compared in Figure 2. The explosion and DC solutions were obtained with a grid-search to find L of5

2 Figure 1. Source region map of the Yellow Sea/Korean Peninsula with the Memorial Day explosion (star) as well as the stations used in the analysis (triangles). The region is outlined in the global inset map. the best-fitting parameters, whereas the deviatoric and full moment tensor solutions were calculated with a least-squares linear inversion. Details of the inversion are given by Minson and Dreger [2008]. 3. Results [6] The pure explosion model is able to fit the waveforms with a variance reduction of 75% and yields an isotropic moment of dyne-cm (M W 4.1; all seismic moment values are calculated with the method of Bowers and Hudson [1999]). In contrast, the pure DC earthquake solution fits the data much worse at 52% with M 0 = dyne-cm (M W 4.4). The fact that the single degree of freedom explosion model fits so much better than the four degree of freedom DC model is highly significant and indicates that such a comparison can be a useful discriminant. The strike, rake, and dip of the best-fit DC is 50, 85, and 10, respectively. Such a steep dip-slip mechanism is unusual for an earthquake. Of all sources calculated by the Global CMT Project (globalcmt.org) less than 1.6% have dips less than 10. This type of information can be used as an additional indication of anomalous sources. The differences in the fits between explosion (Best-iso) and DC (Best-DC) sources can be viewed in Figure 2a, where the DC overpredicts the Love wave amplitude on the transverse components at almost all stations and underpredicts the Rayleigh wave amplitudes, especially at station INCN. [7] Comparisons of waveforms and spectra for the 9 October 2006 nuclear test and 2009 event indicates that the 2009 event is approximately 5.7 times larger than the 2006 event. The isotropic moment for the 2006 event was found to be dyne-cm [Walter et al., 2007], which agrees with the Koper et al. [2008] value. Therefore, from the waveform comparison the 2009 event should be approximately to dyne-cm (scale factor of 5 to 7); a range that includes the pure explosion result ( dyne-cm) obtained from waveform modeling. [8] The full moment tensor inversion fits the data at 81% and yields an isotropic moment of dyne-cm, and a total moment of dyne-cm (M W 4.5). The deviatoric moment tensor inversion fits the data at 80% and a total moment of dyne-cm (M W 4.3). If the deviatoric source is decomposed to a compensated linear vector dipole (CLVD [Knopoff and Randall, 1970]) and DC sharing the same principal axes, then the source is 70% CLVD. The similarity in fits between the dominantly CLVD deviatoric source and dominantly isotropic full moment tensor shows that at shallow depths, a vertical CLVD mechanism can effectively mimic an explosion at the distances and periods analyzed here. This can be seen in the waveform comparison in Figure 2a. The full moment tensor isotropic moment is two times larger than the pure explosion indicating that the compound source of the full moment tensor solution (DC + CLVD + Isotropic) required to fit the Love waves also modifies the Rayleigh waves causing the isotropic component to increase in order to compensate. [9] The source-type parameters, e and k [Hudson et al., 1989], for each source inversion are calculated and plotted on the source-type plot in Figure 2b. e measures the ratio of deviatoric eigenvalues and plots along the horizontal axis. Here k measures the relative amount of isotropic moment and plots along the vertical axis. The axes are then transformed so as to best represent error in these parameters as a normal distribution (ellipse on the sourcetype plot). Using this plot, one can discern the relative source components allowing for a potential seismic discriminant. For example, sources that deviate from the center of the plot have a large non-dc component. The best-fitting sources are far from the center, indicating that the source is anomalously non-dc. Along with the bestfitting sources (corresponding to those shown in Figure 2a), Figure 2b plots solutions and their 95% error ellipses for explosions at the Nevada Test Site (NTS) and earthquakes in the Western US from Ford et al. [2009]. The best-fit full moment tensor plots in the same region as the explosions and away from the earthquake population. The error ellipses for the best-fit sources from this study are all smaller Table 1. MDJ2 Velocity Model a Thick (km) V a (km/s) V b (km/s) r (g/cc) Q a Q b (+0/+0) 3.09 (+0/+0) 2.57 (+0/+0) ( 0.16/ 0.31) 3.35 ( 0.09/ 0.17) 2.66 ( 0.03/ 0.04) ( 0.07/ 0.07) 3.62 ( 0.04/ 0.04) 2.78 ( 0.03/ 0.03) (+0/+0) 4.37 (+0/+0) 3.17 (+0/+0) a Shown with absolute BJT/HIA model differences. 2of5

3 Figure 3. Depth sensitivity analysis where the percent isotropic component (black circles) and total moment (gray triangles) are shown as a function of assumed source depth. The median values are shown next to an arrow at their appropriate axis. The solutions in Figure 1 are given for a depth of 600 m (black dashed line). than the symbol used to plot the solutions due to the very high signal-to-noise of the event records. [10] Unlike earthquake inversions, full moment tensor inversions of an explosive source cannot constrain the source depth by comparing fits at different depths at the frequencies examined here. Event locations put the source at less than 1 km, so the results discussed above assume a source depth of 600 m. However, the isotropic component and total moment are dependent on the assumed depth. Ford et al. [2009] showed that though the source-type is robust, the total moment trades-off with the assumed depth, and more moment goes into the vertical elements of the moment tensor as the depth decreases. Since the Green s functions for these elements vanishes in the limit as the depth of the source decreases (due to vanishing traction at the freesurface) these elements grow large and increase the total moment, thereby reducing the percentage of isotropic moment. We vary the depth between 300 and 900 m and calculate the isotropic component as well as the total moment and give the results in Figure 3. The variance reduction at each of these depths is approximately the same, so an analysis of all the solutions less than 1 km is necessary. The median isotropic component is 57% (range = 48 63%) with a median total moment of dyne-cm (range = dyne-cm) (M W 4.5; range = ). [11] As mentioned previously, some non-isotropic radiation is required to fit the source due to the observed Love waves as can be seen on the tangential component waveforms in Figure 2a. The same amount of non-isotropic energy could have been present in the recordings of the 9 October 2006 nuclear test, but were obscured due to noise. Figure 4 shows the raw waveforms at station MDJ for both the 2006 and 2009 North Korea events filtered between 10 and 50 sec. When the waveforms of the 2006 test are mag- 3of5 Figure 2. (a) Models and their respective forwardpredicted waveforms as a function of color compared with the actual waveforms (black line) all filtered between 10 and 50 sec period. The tangential (T), radial (R), and vertical (V) displacement waveforms are shown. Note the time axis is different for the first two stations and is shown by the bar. The text block to the left of the waveforms gives the station name, azimuth, epicentral distance (km), and maximum displacement (nm). The moment magnitudes of the models are also given in parentheses, and the compressional (P) and tensional (T) axes of the Best-full model is shown on the focal sphere. (b) Source-type plot with various solutions corresponding to the models given in Figure 2a and their associated variance reduction fit percent. Note that the Bestdc solution is at the center ([e, k] = [0, 0]) and the Best-dev solution is along the abscissa (k = 0). Also plotted are results for explosions at the Nevada Test Site (NTS) from Ford et al. [2009] with their associated 95% error ellipses. Error ellipses for the North Korea test are smaller than the plotted symbol. Standard sources are also noted with crosses.

4 Figure 4. Raw waveforms filtered between 10 and 50 sec for the 2006 (black line) and 2009 (gray line) North Korea events, where the amplitudes (in counts) are scaled to the vertical maximum of the 2009 event and the 2006 waveforms are magnified by a factor of 6. (a) Comparison at station MDJ. Note that the azimuth to MDJ is 6, so the eastwest (E W) and north-south (N S) components are effectively naturally rotated to the tangential (T) and radial (R) directions, respectively. Each trace is 100 sec long (the black bar is 6 sec) and begins 80 sec after their respective origin time. (b) Comparison at other stations used in this study. Each trace is 100 sec long (the black bar is 20 sec) and begins 50 sec before a 3 km/sec arrival. Waveforms of the T and R components for the 2006 event at station HIA are too noisy and are not shown. nified by a factor of six, the north-south and vertical components are very similar to the 2009 event. However, the tangential energy that is clear in the 2009 event due to the high SNR is still too small to peak above the noise in the 2006 event. [12] There are several possible causes of the energy seen on the tangential component. The compressional (P) and tensional (T) axes of the full moment tensor (shown in Figure 2b) are consistent with principal strain axes calculated for South Korea by Jin and Park [2006]. The P and T axes are also similar to those from a study by Herrmann et al. [2005] of earthquakes in the Yellow Sea/Korean Peninsula ( Tangential displacements could have been caused by tectonic release that accompanied the explosion in response to crustal stresses. Another possible source is shock-induced, deepseated tensile failure as described by Patton and Taylor [2008]. Anisotropic radiation could be created if the CLVD in their model were tilted several degrees off vertical. However, both these models cannot explain the displacements seen at all stations, where for example the DC component of the full solution fails to match the amplitude at station INCN and the CLVD component of the deviatoric solution fails to match the phase at station HIA. A third cause of displacement recorded on the tangential component is multipathing and anisotropic wave propagation through a heterogeneous crust and mantle. Pasyanos et al. [2006] imaged very thick sediment along the path to BJT in the vicinity of Bohai Bay (Figure 1), and Hong et al. [2008] note effects caused by interaction with the continental margin along the paths to stations INCN and TJN. Rodgers et al. [2008] and Kim et al. [2009] have shown that, even at the periods examined in this study, three-dimensional effects can have a large impact on observed displacements. [13] The M S :m b of the 2006 nuclear test is similar to some Eurasian earthquakes [Bonner et al., 2008, Figure 3] and dissimilar to NTS and STS explosions [Patton and Taylor, 2008, Figure 4]. Patton and Taylor [2008] suggest the greater than expected M S of the 2006 explosion is due to the lack of a tensile failure component (modeled as a CLVD). Since at periods greater than at least 10 sec the 2009 event is a scaled version of the 2006 event (Figure 4), the results presented here could be used to comment on the Patton and Taylor [2008] hypothesis regarding the M S :m b performance of the older test. However, due to the long periods analyzed here and the uncertainty in absolute depth, the relative contribution of tensile and monopole (CLVD and explosion) components cannot be constrained (Figures 2 and 3) and this study cannot support or refute the Patton and Taylor [2008] hypothesis. The results presented here do show that there is a non-isotropic component to the radiation pattern of the 2009 event, and the similarity of the waveforms suggests the same for the 2006 event. Bonner et al. [2008] comment that there is some scatter in the M S measurements of the 2006 nuclear test, which they speculate could be due to pathspecific effects or a Rayleigh-wave radiation pattern. This study shows that indeed there should be a pattern associated with the amplitudes of the 2006 event. 4. Conclusion [14] Modeling of intermediate period, regional distance waveforms identifies the Memorial Day event in Kimchaek, North Korea as decidedly non-tectonic with the best-fit model dominated by an explosion source. While the source type is well determined to be non-dc, the isotropic moment of the full moment tensor inversion has some uncertainty and the M W is between 4.4 and 4.6. Comparison of pure explosion and pure double-couple models indicate that the simpler explosion model fits the waveform data substantially better than the higher degree of freedom DC model, where the isotropic moment of the explosion model is dyne-cm (M W 4.1). However, there are Love waves observed at several stations indicating that the source must have some non-isotropic component. Possible causes of the tangential displacement are additional tectonic sources, tensile failure at depth, and anisotropic propagation. The same non-isotropic component could have been present in the previous 2006 test, but was masked by the noise. 4of5

5 [15] Acknowledgments. We are grateful for very helpful internal reviews by Artie Rodgers and Mike Pasyanos. This is LLNL contribution LLNL-JRNL and BSL contribution DD was supported by the National Nuclear Security Administration, contract DE-FC52-06NA References Bonner, J., R. B. Herrmann, D. Harkrider, and M. Pasyanos (2008), The surface wave magnitude for the 9 October 2006 North Korean nuclear explosion, Bull.Seismol.Soc.Am., 98, , doi: / Bowers, D., and J. A. Hudson (1999), Defining the scalar moment of a seismic source with a general moment tensor, Bull. Seismol. Soc. Am., 89, Clery, D. (2009), Test ban monitoring: No place to hide, Science, 325, , doi: /science.325_382. Ford, S. R., D. S. Dreger, and W. R. Walter (2009), Identifying isotropic events using a regional moment tensor inversion, J. Geophys. Res., 114, B01306, doi: /2008jb Herrmann, R., Y. Jeon, W. Walter, and M. Pasyanos (2005), Seismic source and path calibration in the Korean Peninsula, Yellow Sea and northeast China, in 27th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies [CD-ROM], pap. 1-05, Natl. Nucl. Security Admin., Washington, D. C. (Available at researchreview/2005/papers/01-05.pdf.) Hong, T.-K., C.-E. Baag, H. Choi, and D.-H. Sheen (2008), Regional seismic observations of the 9 October 2006 underground nuclear explosion in North Korea and the influence of crustal structure on regional phases, J. Geophys. Res., 113, B03305, doi: /2007jb Hudson, J. A., R. G. Pearce, and R. M. Rogers (1989), Source type plot for inversion of the moment tensor, J. Geophys. Res., 94, , doi: /jb094ib01p Jin, S., and P.-H. Park (2006), Strain accumulation in South Korea inferred from GPS measurements, Earth Planets Space, 58, Kim, A., D. S. Dreger, and S. Larsen (2009), Moderate earthquake ground motion validation in the San Francisco Bay Area, Bull. Seismol. Soc. Am., in press. Knopoff, L., and M. J. Randall (1970), The compensated linear-vector dipole: A possible mechanism for deep earthquakes, J. Geophys. Res., 75, , doi: /jb075i026p Koper, K. D., R. B. Herrmann, and H. M. Benz (2008), Overview of open seismic data from the North Korean event of 9 October 2006, Seismol. Res. Lett., 79, , doi: /gssrl Minson, S., and D. Dreger (2008), Stable inversions for complete moment tensors, Geophys. J. Int., 174, , doi: /j x x. Nguyen, B. (1994), Surface-wave study of the 22 January 1992 western Yellow Sea earthquake, Eos Trans. AGU, 75, Fall Meet. Suppl., Abstract S51D-12. Pasyanos, M. E., G. A. Franz, and A. L. Ramirez (2006), Reconciling a geophysical model to data using a Markov chain Monte Carlo algorithm: An application to the Yellow Sea Korean Peninsula region, J. Geophys. Res., 111, B03313, doi: /2005jb Patton, H. J., and S. R. Taylor (2008), Effects of shock-induced tensile failure on m b -M S discrimination: Contrasts between historic nuclear explosions and the North Korean test of 9 October 2006, Geophys. Res. Lett., 35, L14301, doi: /2008gl Rodgers, A., N. Anders-Petersson, S. Nilsson, B. Sjögreen, and K. McCandless (2008), Broadband waveform modeling of moderate earthquakes in the San Francisco Bay Area and preliminary assessment of the USGS 3D seismic velocity model, Bull. Seismol. Soc. Am., 98, , doi: / Walter, W., E. Matzel, M. Pasyanos, D. Harris, R. Gok, and S. Ford (2007), Empirical observations of earthquake-explosion discrimination using P/S ratios and implications for the sources of explosion S-waves, in 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies [CD-ROM], pap. 3-20, Natl. Nucl. Security Admin., Washington, D. C. (Available at researchreview/2007/papers/03-20.pdf.) D. S. Dreger, Berkeley Seismological Laboratory, University of California, Berkeley, CA 94720, USA. S. R. Ford and W. R. Walter, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. (sean@llnl.gov) 5of5