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Seismological Evidence for a Low-YieldNuclear Test on 12 May 2010 in North Koreaby Miao Zhang and Lianxing Wen

Online Material: Location uncertainty estimation; figures ofwaveform comparison, location maps, and Pg/Lg spectral ratios;tables of earthquake parameters and Lg-wave amplitude ratios.

INTRODUCTION

Three nuclear tests (in 2006, 2009, and 2013) conducted bythe Democratic People’s Republic of Korea (North Korea) areall detected and confirmed by many governmental andinternational agencies (e.g., the U. S. Geological Survey[USGS] and the Comprehensive Nuclear-Test-Ban TreatyOrganization [CTBTO]). The locations and yields of thesetests have also been extensively studied by many researchgroups (e.g., Richards and Kim, 2007; Koper et al., 2008; Zhaoet al., 2008, 2012, 2014; Murphy et al., 2010; Wen and Long,2010; Chun et al., 2011; Zhang and Wen, 2013). However, itis under intensive debate among the governmental agenciesand research groups whether North Korea has conducted othersmall nuclear tests. In particular, De Geer (2012) reported thedetection of xenon and xenon daughter radionuclides between13 and 23 May 2010 in four atmospheric radionuclide surveil-lance stations, located in South Korea, Japan, and the RussianFederation. He suggested the presence of barium-140 can beexplained only by a sudden nuclear event, with the correspond-ing trinitrotoluene equivalent in a range of 50–200 t and theestimated time-zero at 6:00! 18 hr= − 30 hr UTC on 11May 2010 (De Geer, 2012; see also Brumfiel, 2012). The fissilematerial of the possible mid-May 2010 nuclear test is indicatedas uranium-235 rather than the plutonium-239 inferred fromthe radioxenon signal detected at Geojin in South Korea (DeGeer, 2012, 2013), althoughWright (2013) suggested they can-not be clearly discriminated from atmospheric transport mod-eling of the observed radionuclides. Other studies also obtainedsimilar findings based on the detected types and ratios ofisotopes (De Geer, 2013; Ihantola et al., 2013; Wotawa,2013; Wright, 2013). Based on 2 hr time slices, a more accuratetime-zero is estimated to be 16:00 UTC on 12 May 2010 byIhantola et al. (2013), which is limited between 9:00 UTCon 11 May 2010 and 13:00 UTC on 13 May 2010 based onits 95% uncertainty. Wright (2013) concluded that the mostlikely origin of the radionuclides is close to North Korea’s nu-clear test site (NKTS). Coincidentally, on 12 May 2010, theNorthKorean official dailymorning newspaperRodong Sinmun

reported that North Korea succeeded in nuclear fusion on theDay of the Sun (http://www.kcna.co.jp/item/2010/201005/news12/20100512‑05ee.html; last accessed July 2014), althoughthe report was ridiculed by the South Korean andWestern me-dia (Brumfiel, 2012).

However, without seismic data or on-the-ground inspec-tions to support the radioisotope data, it is impossible to verifywhere the isotopes come from (Brumfiel, 2012). The study ofDe Geer (2012) stirred a serious controversy about which rep-resentatives of the U.S. government and CTBTO refused tocomment (Brumfiel, 2012; De Geer, 2013). So far, the firstand only attempt to search for seismic signal of possible nuclearevents in the reported period turned out to be negative (Schaffet al., 2012). In that study, they tried to detect possible smallnuclear tests in the NKTS by applying the three-componentcross-correlation method on seismic waveforms recorded bystation MDJ, using the 2006 and 2009 nuclear tests as the tem-plate. They failed to find any evidence for any undergroundexplosion detectable at that station in the five specific days(14–16 April and 10–11 May) suggested by De Geer (2012).Thus, there has not been any seismological evidence to supportthe above radionuclide findings so far.

In the study of Schaff et al. (2012), only the seismic datafrom one station (MDJ) are used, and MDJ is at a distance of370 km from NKTS. Their study does not exclude the possibil-ity that a low-yield event may have escaped detection because ofits weak signal at large distance or occurrence at other un-checked time windows. In this study, we search for possibleevents using regional seismic data recorded in April andMay 2010 in northeast China, within 200 km of NKTS, anda newly developed event detection method called the match-and-locate (M&L) method (M. Zhang and L. Wen, unpub-lished manuscript, 2014). The M&L method is an effectivemethod for small event detection by stacking cross correlo-grams between waveforms of the template events and potentialsmall event signals in the continuous waveforms over multiplestations and components. Unlike the traditional match filtermethod, which assumes that the template event and slave eventare sufficiently close that they generate effectively the samewaveform (apart from a scale factor) and effectively the sametime shift at all recording stations, the M&Lmethod scans overpotential small event locations around the template by makingrelative travel-time corrections based on the relative locations

doi: 10.1785/02201401170 Seismological Research Letters Volume 86, Number 1 January/February 2015 1

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▴ Figure 1. (a) Location of North Korea’s nuclear test site (NKTS, red star), seven seismic stations (red triangles) within 200 km of the testsite, and three nearby earthquakes (blue stars) used in event type comparison. (Inset) A regional map of eastern Asia in which the blackrectangle indicates the study area. (b) Maximal values of the stacked cross correlograms for every 0.001 s time interval (red dots) from 1April 2010 to 31 May 2010 (only the values greater than 0.2 are plotted) and a detected event at 00:08:45.067 UTC on 12 May 2010 (dotlabeled by the event origin time). Gray dashed line stands for the mean CC threshold of 0.25. Gray area indicates the time window of datagap (from 16:00 UTC on 15 May 2010 to 16:00 UTC on 16 May 2010).

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of the template event and the potential small event beforestacking (M. Zhang and L. Wen, unpublished manuscript,2014). It makes event detection more efficient and at the sametime relocates the detected event with high precision. We de-tect a low-yield nuclear test on 12 May 2010 from the seismicdata using the M&L method. We present details of the detec-tion and location of the event; we further discriminate theevent to be a nuclear test using the Pg/Lg spectral ratio andestimate its yield.

DETECTION AND LOCATION OF A LOW-YIELDNUCLEAR TEST ON 12 MAY 2010

We use the M&L method to detect and locate possible nucleartests through the seismic data recorded by the nearest seventhree-component stations (except for the vertical componentof station MJT, which had been changed over time) in Jilinprovince of the People’s Republic of China from 1 April 2010to 31 May 2010 (Fig. 1a). These stations are within 200 km ofthe test site and also recorded the seismic waves from NorthKorea’s 2009 and 2013 nuclear tests. We convert the seismicdata to ground velocity by removing the instrument responseand then band-pass filter the ground velocity from 1 to 6 Hz.To improve the detection ability, we combine the 2009 and2013 events as our templates (Table 1). For each potentiallocation and occurring time, we shift the correlograms basedon the predicted travel-time differences then stack the crosscorrelograms over all channels and average for both templates.Because source depth trades off event origin time in M&L de-tection when using a limited number of stations, we fix thedepth to be zero and search for the potential event location inan area of 0.08° in latitude and 0.08° in longitude centered atthe 2009 event, with an interval of 0.0002°. Waveforms of thePg wave are used, and the Pg phase is adopted in the relocationprocedure based on the IASP91 model (Kennett and Engdahl,1991). A 4 s time window (1 s before and 3 s after the predictedPg-wave arrival) is used as the template waveform window.Except for one origin time (00:08:45.067 UTC, 12 May 2010)that has a mean cross-correlation coefficient (CC) value of

0.291, all other origin times have mean CC values less than0.25 (Fig. 1b). We regard the mean CC value of 0.291 as pos-itive detection of an event, as it is significantly larger than thebackground mean CC values, satisfying the detection criterionsimilar to those set for event detection in many other studies(e.g., Peng and Zhao, 2009; M. Zhang and L. Wen, unpublishedmanuscript, 2014). The location is inferred based on the loca-tion of maximal mean CC value at 41.2863° N, 129.0790° E(Fig. 2b and Ⓔ Fig. S1b, available in the electronic supplementto this article). The detected 2010 event is located at 227 m westand 844 m south of the 2009 nuclear test and 227 m east and499 m south of the 2013 nuclear test (Fig. 2b andⒺ Fig. S1b).The location uncertainty is determined to be 350 m, based onthe analysis of the scaled-down seismic signals of past nucleartests in the region (Fig. 3; Ⓔ see the analysis in the Contextssection of the electronic supplement).

The corresponding mean CC of the detected 2010 eventexhibits a distinct impulse in the stacked cross correlogram(Fig. 2a), indicating the seismic signal is coherent in time inthe individual channels. The maximal mean CC value is welllocalized in a small region in the mean CC distribution over allpotential locations (Fig. 3a), indicating reliable detection andhigh-resolution relocation of the detected event. Waveformmatching between the detected event and the template eventis good and consistent for both template events, as illustratedby almost all positive CC values over all channels for both tem-plate events (Fig. 2c and Ⓔ Fig. S1c).

Both Pg and Lg waves are clearly recorded in all sevenstations for the 2009 and 2013 nuclear tests (Fig. 4a,b). Thesetwo phases are also clearly visible in the recordings of moststations for the 2010 event (Fig. 4c and Ⓔ Fig. S2). The Lgphases of the 2010 event are observed distinctly at stationsSMT and CBS in a relatively broad frequency band from 1 to20 Hz (Fig. 4c) and at stations MJT, ZXT, FST, and YNB inthe frequency band from 1 to 5 Hz (Ⓔ Fig. S2a). The Pgphases of the 2010 event are clearly visible at stations CBTandSMT over 5 Hz and stations YNB and ZXT in higher fre-quency bands (5–10 Hz and 15–20 Hz for YNB and5–15 Hz for ZXT) (Ⓔ Fig. S2). Further detailed analysis

Table 1Location, Time, and Yield of North Korea’s Nuclear Tests

Test Date (yyyy/mm/dd) Latitude (°N) Longitude (°E) Origin Time (hh:mm:ss.sss) Magnitude Yield2006 2006/10/09 41.2874* 129.1083* 01:35:28.000† 3.93‡ 0.48 kt‡

2009 2009/05/25 41.2939§ 129.0817§ 00:54:43.180§ 4.53±0.12‖ 7.0±1.9 kt#

2010 2010/05/12 41.2863** 129.0790** 00:08:45.067** 1.44±0.13** 2.9±0.8 t**2013 2013/02/12 41.2908# 129.0763# 02:57:51.331# 4.89±0.14# 12.2±3.8 kt#

*Satellite images.†U.S. Geological Survey.‡Zhao et al. (2008).§Wen and Long (2010).‖Zhao et al. (2012).#Zhang and Wen (2013).**This study.

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▴ Figure 2. (a) Stacked correlogram for the detected event at 00:08:45.067 UTC on 12 May 2010, (b) locations of the determined 12 May2010 event (black star) and two template nuclear tests (red circle, 25 May 2009; gray circle, 12 February 2013), and (c) the template nucleartest waveforms (red traces, 25 May 2009) plotted in comparison with the continuous waveforms (black traces) along the predicted arrivaltimes of the detected event on 12 May 2010. The gray dashed line in (a) indicates the mean CC threshold (0.25) of detection. In (c), eachtrace is labeled with station name on the left and CC value on the right, and the total mean cross-correlation coefficient (CC) computed inmatch and locate (M&L) is labeled under the subtitle.Ⓔ The seismogram comparison between the other template (12 February 2013) andthe continuous waveforms is shown in Figure S1.


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for the seismic recordings at borehole station SMT suggeststhat, although the relative amplitudes of Pg and Lg waves aresimilar at various frequency bands between the 2009 and 2013tests, the Lg wave for the 2010 test has relatively larger ampli-tudes at the frequencies below 10 Hz (Fig. 4d). These wave-form characteristics may be due to these reasons: (1) the localbackground noise may affect the waveform characteristics dif-ferently at different frequency bands, (2) the explosion char-acteristics of Pg/Lg behavior shift toward higher frequenciesfor a smaller explosion, and (3) the 2010 test may be a de-coupled event. We will further elaborate the last two possiblereasons later in this paper.

DISCRIMINATION OF THE 12 MAY 2010 NUCLEARTEST BASED ON PG/LG SPECTRAL RATIO

P/S-type spectral ratios of regional phases (e.g., Pg/Lg, Pn/Lg,Pn/Sn) are usually used in event discrimination of explosionsfrom earthquakes (Walter et al., 1995; Xie, 2002; Richardsand Kim, 2007; Zhao et al., 2008, 2014). For example, Richardsand Kim (2007) showed that the Pg/Lg spectral ratios of theNorth Korea’s 2006 nuclear test are very different from earth-quakes. We analyze the Pg/Lg spectral ratio of the vertical seis-mograms recorded by the borehole short-period station SMT(0.5–50 Hz), which is the station that possesses the highest sig-nal-to-noise ratio (SNR) above 1 Hz (Fig. 4d and Ⓔ Fig. S2).Three nearby earthquake waveforms are also collected and an-alyzed for spectral comparison (Fig. 1a andⒺ Table S1). A 7 s

time window (2 s before and 5 s after the predicted Pg-wavearrival) is used as the Pg window, and we use group velocitiesof 3:50–2:60 km=s to pick the Lg waves (Chun et al., 2009)(Fig. 4). A 20% cosine taper is used for both Pg and Lg phases.We calculate displacement Fourier spectra of these six eventsafter removing instrument responses (Ⓔ Fig. S3). Only thespectral estimates with SNR > 2 are used in smoothed spectralratios, following the approach of Zhao et al. (2008). The spec-tral SNR is defined as the ratio of power spectral density of Pg orLg with respect to that of pre-P noise (Xie, 2002).Here,we haveignored distance corrections because distance differences fromthese events are small and attenuation in the region is low(Kim and Richards, 2007; Richards and Kim, 2007; Zhao et al.,2013). The smoothed spectral ratios for these six events clearlyseparate the 2009 and 2013 explosions from the earthquakes atfrequencies above 2 Hz and the 2010 event above 5 Hz (Fig. 5).The separation of the 2010 event spectrum at a higher frequencyis consistent with the conclusion that the frequency range of thespectral ratio of explosion separation from the earthquake shiftshigher for smaller explosions (Xie, 2002; Fisk, 2006). In addi-tion, the Pg/Lg spectral ratio would also be affected by cavitycoupling and burial depth (Murphy et al., 1997; Fisk, 2006).

YIELD ESTIMATION OF THE 12 MAY 2010NUCLEAR TEST

We follow the procedure in Zhang and Wen (2013) to esti-mate the yield of the 2010 test. We first calculate the Lg-wave

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▴ Figure 3. (a) Best-fitting location of the 12 May 2010 nuclear test (star labeled as 2010/05/12, which corresponds to the maximal meanCC value in M&L detection) relative to the locations of template 2009 and 2013 tests (stars labeled as 2009/05/25 and 2013/02/12), plottedcentered at the location of 2006 test (star labeled as 2006/10/09). The mean CC values in the neighboring locations of the inferred 2010 testsite are plotted in color (only the regions with mean CC > 0:24 are presented). The black ellipse represents the confidence level of the 2010test location within 94.3% of the maximal mean CC (Ⓔ see the analysis in the electronic supplement). (b) Locations (circles), origin times(labeled red), and yields (labeled blue) of the 2006, 2009, 2010, and 2013 nuclear tests, plotted on a Google Earth map (image on 23 January2013) of the area shown in (a), centered at the 2006 test site identified by the satellite images. The sizes of the 2009 and 2013 symbols areproportional to their yields. The event parameters for North Korea’s four nuclear tests are shown in Table 1.


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magnitude using the amplitude ratios of the Lg waves observedbetween the 2009 and 2010 tests; we then estimate the yield ofthe 2010 test based on a modified empirical Lg-magnitude–yield–depth relationship using the calculated Lg magnitudeand the burial depth inferred from satellite imagery. The Lgmag-nitude of the 2009 test was estimated after correcting the pathand station effects (Zhao et al., 2012). Because the separation ofthe 2009 and 2010 nuclear tests is just 873 m, the path effect andstation corrections are the same for the same station between thetwo tests. The Lg waves recorded at the borehole station SMTare chosen to estimate the Lg magnitude and yield, because oftheir high SNR in each frequency band (Ⓔ Fig. S2).

We first deconvolve the instrument response from the ob-served vertical component of station SMT and then convolve

the seismograms with the World-Wide Standardized Seismo-graph Network instrument response. Three different methodsare used to measure their relative amplitude ratios: integratedenvelope, third-peak amplitude, and the root mean square am-plitude (Zhang and Wen, 2013). Based on the Lg-wave ampli-tude ratio (Ⓔ Table S2), the Lg magnitude of North Korea’s2010 nuclear test is inferred to be mb"Lg# $ 1:44% 0:13, in-cluding the uncertainty of %0:12 inherited from Zhao et al.(2012) and an uncertainty of %0:04 from the variation of es-timation of relative Lg-amplitude ratio between the two tests.

A modified empirical Lg-magnitude–yield–depth rela-tionship mb $ 1:0125 log"Y # − 0:7875 log"h# ! 5:887 hadbeen adopted by Zhang and Wen (2013), in which mb isLg magnitude, h is burial depth, and Y is yield, which included

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▴ Figure 4. (a)–(c) Displacement seismograms (1–20 Hz) of the template (a) 2009 and (b) 2013 tests and (c) the detected 2010 test re-corded at the nearest seven stations within 200 km of the test site shown in Figure 1a. Station names are labeled on the left side of eachtrace. (d) Comparisons of displacement seismograms are shown in different frequency bands (labeled on the left) for the 2009, 2013, and2010 tests recorded at borehole station SMT. The predicted template Pg-phase arrival times are marked with red lines, and the predictedLg-wave time windows are marked by two blue lines corresponding to the group velocities of 3.50 and 2:60 km=s, respectively.


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the traditional magnitude–yield relationship in NKTS (Bowerset al., 2001; Zhao et al., 2008, 2012, 2014; Schaff et al., 2012)and the depth correction proposed by Patton and Taylor(2011). The burial depth is estimated from the difference ofthe surface elevation between the associated tunnel entranceand the identified test location. Based on the location of the2010 nuclear test, we regard “the west portal” identified byPabian and Hecker (2012) as the most likely tunnel entranceto the test. The surface elevations of the west portal and theidentified test location of the 2010 event are 1400 and 1630 m,respectively (Ⓔ Fig. S6). We thus inferred the burial depth ofthe 2010 nuclear test to be 230 m, that is, the elevation differ-ence between the tunnel entrance and the test site. By applyingthe above modified empirical Lg-magnitude–yield–depth rela-tionship, the yield of the 2010 test is estimated to be2:9% 0:8 t, based on a burial depth of 230 m.

DISCUSSION

De Geer (2012) suggested there might be another low-yieldnuclear test in mid-April 2010, carried out in the same cham-ber of the mid-May event. We did not detect any potentialevent in the seismic data in mid-April (Fig. 1b). Although wecould not exclude the possibility that the magnitude of thepostulated event was too small to be detected, it is also possiblethat the event did not occur. The existence of a mid-April eventwas proposed to explain the disagreements of the xenon ratiobetween the data and De Geer’s model. In later studies, thexenon signatures are also explained by an underground nuclearexplosion in mid-May 2010 without postulating an early event(De Geer, 2013; Wright, 2013).

The origin time we have determined is within the timewindow based on the analysis of radionuclide isotope ratios(Ihantola et al., 2013). However, the yield of the 2010 test es-

timate based on the seismic data (2:9% 0:8 t) is much smallerthan the 50–200 t suggested by De Geer (2012) or a few hun-dred tons inferred based on radionuclide activity calculations(Wright, 2013). If the inferred yield based on radioisotopesholds true, the large difference in yield estimates may suggestthat the 2010 test is at least partially decoupled, consistent withthe indication from the noble gases as suggested by De Geer(2013). Our study demonstrates the scientific capability ofmonitoring low-yield nuclear tests by combining seismic andradionuclide isotope data.

CONCLUSION

We detect and locate a low-yield nuclear test conducted on 12May 2010 by North Korea. We apply the M&L method andsearch for potential events in the continuous seismic data re-corded in seven stations within 200 km of North Korea’s testsite from 1 April 2010 to 31 May 2010, using North Korea’s2009 and 2013 tests as templates. A detectable event occurredat 00:08:45.067 UTC on 12 May 2010, located at 41.2863° N129.0790° E with a geographic precision of 350 m. The de-tected 2010 event is about 227 m west and 844 m south of thelocation of North Korea’s 2009 nuclear test; about 227 m eastand 499 m south of the location of its 2013 nuclear test. Pg/Lgspectral ratios of the event further indicate it is explosive innature. We estimate the yield of the event to be 2:9% 0:8 t,based on the Lg-wave amplitude ratio between the 2009 and2010 tests and the burial depth inferred from satellite imagery.Our study provides seismological evidence for a low-yieldnuclear test in North Korea on 12 May 2010, supporting theradionuclide isotope observations, and demonstrates the scien-tific capability of monitoring low-yield nuclear tests by com-bining seismic and radionuclide isotope data.

ACKNOWLEDGMENTS

We thank the China Earthquake Networks Center for provid-ing seismic data. This paper benefited significantly from thesuggestions and comments of Paul Richards, two anonymousreviewers, and Editor Zhigang Peng. This work was supportedby the National Natural Science Foundation of Chinaunder Grant NSFC41130311 and the Chinese Academy ofSciences and State Administration of Foreign Experts AffairsInternational Partnership Program for Creative ResearchTeams.

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Miao ZhangLaboratory of Seismology and Physics of Earth’s Interior

School of Earth and Space SciencesUniversity of Science and Technology of China

Hefei, Anhui 230026People’s Republic of China

[email protected]

Lianxing Wen1

Department of GeosciencesState University of New York at Stony Brook

Stony Brook, New York 11794 U.S.A.

Published Online 19 November 2014

1 Also at Laboratory of Seismology and Physics of Earth’s Interior; Schoolof Earth and Space Sciences, University of Science and Technology ofChina, Hefei, Anhui 230026, People’s Republic of China.


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