This document discusses the remote detection of weak aftershocks from DPRK underground nuclear explosions using waveform cross-correlation. It summarizes that: 1) Five aftershocks with magnitudes between 2.9-4.1 were detected routinely using cross-correlation with explosion waveforms as templates. 2) One aftershock with a magnitude of 2.4 was only detectable using other aftershocks as templates. 3) Discrimination of DPRK explosions from aftershocks can be done accurately using Pg/Lg spectral ratios.

1. Remote detection of weak aftershocks
of the DPRK underground explosions
using waveform cross correlation
Dmitry Bobrov, Ivan Kitov, Ronan Le Bras, Mikhail
Rozhkov, PTS CTBTO, and Irina Sanina, IDG RAS

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OUTLINE
• Six DPRK explosions
• Routine detection of explosions and aftershocks
using waveform cross correlation
• Tuning of waveform cross correlation, multiple
masters
• Usage of P/S spectral amplitude ratio method
for discrimination of DPRK explosions and their
aftershocks

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SEISMIC EVENTS
AT THE DPRK TEST SITE
Aftershock SEL3 LEB REB Date Time mb
1 - + - 11.09.2016 1:50:49.86 -
2 + + + 3.09.2017 3:38: 31.88 4.12
3 - + - 23.09.2017 4:42: 59.95 -
4 + + + 23.09.2017 8:29: 16.29 3.40
5 - + - 12.10.2017 16:41: 8.11 -
Event Lat, deg Lon, deg ndef mb (IDC) Ms (IDC) ML(IDC)
DPRK1 41.312 129.02 22 4.08 3.89
DPRK2 41.311 129.05 72 4.51 3.56 4.27
DPRK3 41.301 129.07 110 4.92 3.95 4.52
DPRK4 41.304 129.05 102 4.82 3.92 4.61
DPRK5 41.299 129.05 120 5.09 4.17 4.29
DPRK6 41.32 129.03 189 6.07 4.91 5.17
DPRK TESTS
DPRK AFTERSHOCKS FOUND WITH EXPLOSION TEMPLATES

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EXPLOSION AND
AFTERSHOCK WAVEFORMS
Regional network:
IMS: KSRS and USRK
non-IMS: MDJ and SEHB
PgPn
Lg

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WAVEFORM TEMPLATES
FOR CROSS CORRELATION
SEHB, A_SHOCK2
MDJ, DPRK5

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CROSS CORRELATION
KSRS

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TUNING WAVEFORM CROSS
CORRELATION
HIGHEST KSRS SNRCC
HIGHEST USRK SNRCC
D1 D2 D3 D4 D5 D6 A1 A2 A3 A4 A5 A6
DPRK1 221 73 45 34 50 66 8.3 37 15 31 23 5.7
DPRK2 88 373 76 246 344 293 8.6 34 18 40 17 7.2
DPRK3 80 534 92 201 468 381 11 47 18 33 14 5.8
DPRK4 39 173 84 303 193 282 9.4 54 12 37 15 6.4
DPRK5 34 135 56 537 120 657 8.7 42 14 35 18 6.5
DPRK6 48 141 70 196 199 597 9.9 51 15 36 15 5.6
A_SHOCK1 5.7 9.1 7.6 7.8 9.4 6.7 107 7.3 8.8 19 12 23
A_SHOCK2 30 53 29 59 72 72 11 112 12 27 14 9.2
A_SHOCK3 19 13 13 15 13 18 9.2 10 87 144 97 8.5
A_SHOCK4 21 65 30 24 81 30 19 21 59 209 79 16
A_SHOCK5 32 20 11 14 23 20 14 12 63 85 151 15
A_SHOCK6 5.2 6.3 5.7 5.5 6.2 5.9 28 5 8.5 13 11 98
CC WINDOW 10 s to 150 s
STA = 0.1 s; LTA = 20 s
6 FBs
Detection threshold
SNRCC = 5
D2 D3 D4 D5 D6 A1 A2 A3 A4 A5 A6
DPRK2 204 50 273 248 273 14 13 14 42 40 8.2
DPRK3 228 62 182 166 182 16 18 12 34 19 8.6
DPRK4 226 34 539 301 539 23 29 9.7 23 10 11
DPRK5 274 38 291 400 291 26 25 14 16 14 13
DPRK6 252 68 212 356 212 25 27 12 21 14 15
A_SHOCK1 22 13 24 27 24 85 13 6.1 9.5 7.3 17
A_SHOCK2 47 11 71 43 71 15 160 15 51 13 8.9
A_SHOCK3 9.8 11 9.1 12 9.1 7.2 24 67 75 119 6.8
A_SHOCK4 12 11 14 22 14 12 9.2 132 148 83 10
A_SHOCK5 17 8.1 10 15 10 10 22 116 123 100 5
A_SHOCK6 9.3 6.8 11 10 11 17 9.3 7.6 16 5.3 66
LIKELY SIMILAR:
A_SHOCK3 to A_SHOCK5
LIKELY SIMILAR:
A_SHOCK1 and A_SHOCK6

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COMBINED MASTER EVENT
AFTERSHOCK 6
COMBINED MASTER AFTERSHOCK 6
SINGLE MASTERDATE 2017246 ORIGIN TIME 09:31:29.86
Distance from DPRK5 3.8 km
Station Arrival time Residual,s SNRCC SNR
SEHB 09:32:18.64 -1.13 43.6 3.4
SEHB 09:32:19.99 0.22 21.9 2.3
SEHB 09:32:20.69 0.92 6.2 3.2
SEHB 09:32:21.44 1.67 7.3 2.6
MDJ 09:32:22.59 -0.60 5.8 4.0
MDJ 09:32:23.19 0.00 5.5 3.9
MDJ 09:32:23.41 0.22 48.8 3.9
MDJ 09:32:23.53 0.35 6.1 4.0
MDJ 09:32:23.61 0.43 5.9 4.0
USRK 09:32:25.28 -0.69 15.7 4.8
USRK 09:32:25.50 -0.47 8.0 4.8
USRK 09:32:25.55 -0.42 13.0 4.8
USRK 09:32:25.68 -0.29 7.7 4.8
USRK 09:32:25.73 -0.24 9.6 4.8
USRK 09:32:25.88 -0.41 57.9 4.8
USRK 09:32:26.12 0.16 14.7 4.8
USRK 09:32:26.58 0.61 5.1 4.8
USRK 09:32:27.32 1.36 9.8 4.8
KSRS 09:32:31.00 -1.00 5.3 5.5
KSRS 09:32:31.20 -0.80 21.8 5.5
KSRS 09:32:31.35 -0.44 98.0 5.5
KSRS 09:32:31.65 -0.35 5.3 5.5
KSRS 09:32:32.25 0.25 15.5 5.5
KSRS 09:32:32.25 0.25 7.0 5.5
KSRS 09:32:32.40 0.40 13.1 5.5
DATE 2017246
ORIGIN
TIME 09:31:28.12
Distance from DPRK5 9.6 km
Station Arrival time
residua
l, s SNRCC SNR
SEHB 09:32:20.87 0.37 7.3 2.6
MDJ 09:32:22.34 0.03 6.1 4
USRK 09:32:25.98 -0.02 5.1 4.8
KSRS 09:32:32.3 -0.37 7 5.5
Combined masters provide:
• Lower detection threshold
• More reliable solution
• More reliable location
• More accurate location
Notice much higher SNRCC than
standard SNR

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Lg SIZE SCALING
EVENT USRK KSRS MDJ SEHB mean stdev relative mb
A_SHOCK1 -1.455 -1.143 -1.368 -1.193 -1.29 0.13 2.82
A_SHOCK2 0.000 0.000 0.000 0.000 0.00 0.00 4.11 - ref
A_SHOCK3 -1.201 -0.962 -1.311 -1.178 -1.16 0.13 2.95
A_SHOCK4 -0.504 -0.329 -0.639 -0.537 -0.50 0.11 3.61
A_SHOCK5 -0.927 -0.724 -0.982 -0.907 -0.89 0.10 3.23
A_SHOCK6 -1.730 -1.349 -1.638 -1.730 -1.61 0.16 2.38
LOW FREQUENCY BAND 1-2 Hz

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Pg/Lg RATIOS

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MEAN VALUE AND
STANDARD DEVIATION

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DISTANCE BETWEEN MEAN
VALUES MEASURED IN
STANDARD DEVIATIONS

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NETWORK DISCRIMINATION
Pg/Lg RATIOS
STATION DPRK2 DPRK3 DPRK4 DPRK5 DPRK6 A1 A2 A3 A4 A5
FB1 0.526 0.587 0.643 0.713 0.966 0.928 0.659 0.615 0.290 0.702
FB2 0.807 0.896 1.038 1.039 1.235 0.741 0.562 0.542 0.371 0.504
FB3 1.089 1.198 1.302 1.236 1.459 0.680 0.543 0.575 0.414 0.460
FB4 1.568 1.643 1.581 1.480 1.723 0.760 0.568 0.589 0.426 0.430
FB5 1.997 2.056 1.766 1.694 1.836 0.819 0.630 0.575 0.441 0.412
FB6 2.767 2.821 2.251 2.256 2.295 0.829 0.887 0.610 0.549 0.432
FB1 0.851 0.809 1.042 1.353 1.809 0.763 0.285 0.411 0.351 0.354
FB2 1.302 1.355 1.649 2.033 2.548 0.786 0.250 0.385 0.390 0.372
FB3 1.684 1.953 2.123 2.370 2.936 0.737 0.249 0.402 0.422 0.382
FB4 2.196 2.688 2.589 2.705 3.241 0.753 0.329 0.527 0.530 0.445
FB5 2.559 2.861 2.768 2.925 3.475 0.904 0.443 0.667 0.612 0.538
FB6 3.211 3.036 3.171 3.205 3.870 1.190 0.568 0.933 0.710 0.718
FB1 0.564 0.574 0.808 0.873 1.029 0.654 0.485 0.480 0.347 0.336
FB2 1.059 1.185 0.931 1.713 1.589 0.842 0.417 0.711 0.432 0.511
FB3 1.630 1.901 0.997 2.410 1.940 0.906 0.399 0.946 0.549 0.705
FB4 2.477 2.870 1.171 3.092 2.549 0.962 0.436 1.096 0.771 1.007
FB5 2.814 3.081 1.234 3.318 2.891 1.005 0.517 1.118 0.907 1.175
FB6 3.312 3.284 1.177 3.723 3.279 1.127 0.709 1.075 0.928 1.087
KSRSUSRKMDJ
EVENT

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NETWORK DISCRIMINATION
MAHALANOBIS DISTANCE
Short (0.1-4.2) Mahalanobis
distances within groups
Large (100-20000) Mahalanobis,
(square) MD,
distances between groups
Critical α=0.001 corresponds to
MD =16.3 for df=3

15. DISCUSSION
• Waveform cross correlation is an effective detection tool
• Five aftershocks with mb between 2.9 and 4.1 were
found in routine cross correlation processing with
explosions as master events (waveform templates)
• One aftershock with mb=2.4 can only be found with
other aftershocks as master events
• Combined master events are more effective in detection
of the smallest events
• DPRK explosions and their aftershocks can be
accurately discriminated using Pg/Lg spectral ratio
• The Mahalanobis distance serves for network
discrimination
• Two more aftershocks were found a few days ago