Preparatory Commission for the Comprehensive. Nuclear-Test-Ban Treaty Organization, Provisional Technical Secretariat. Vienna International Centre. P.O. Box ...
Using waveform cross correlation for automatic recovery of aftershock sequences Ivan Kitov, Seismic-Acoustic Officer, IDC/SA/SM Dmitry Bobrov, Seismic-Acoustic Officer, IDC/SA/SM Mikhail Rozhkov, Fusion and Review Officer, IDC/OD/QMDR
International Data Centre Division Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, Provisional Technical Secretariat Vienna International Centre P.O. Box 1200, A-1400 Vienna, AUSTRIA
Outline
• Waveform cross correlation: master events and waveform templates • Detection on cross correlation traces • Local Association (LA) of cross correlation detections • Tohoku 2011 – the biggest aftershock sequence • Global Grid of real master events at the IDC
International Data Centre
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Cross correlation as a monitoring technique Intuition 1. 2. 3. 4. 5.
6. 7. 8.
9.
The REB contains several examples of aftershock sequences with thousands of events One can use as master events, i.e. as the sources of high quality waveform templates, more than 400,000 zero depth seismic events: nuclear tests, earthquakes, quarry blasts, etc. Almost all events have a neighbor (less than 100 km) in the REB. Empirical observation: events at distances of 50 to 100 km generate signals well correlated at regional and teleseismic distances Almost all low magnitude seismic events might be considered as point sources (source size lower than wavelength) with identical (δ-) source functions, but likely with different directivity IMS stations cover the globe and many of primary seismic stations are arrays enhancing the capability of cross correlation analysis The Reviewed Event Bulletin (REB) of the International Data Centre includes a few spatially close underground nuclear tests measured at IMS seismic stations Historical UNEs conducted within test sites and PNEs provide digital waveforms at IMS and non-IMS stations for a broad range of test conditions: yield, depth, geology, seismic coupling, etc. Encouraging results of other studies of waveform cross correlation at teleseismic and regional distances
International Data Centre
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Cross correlation Waveform template – high quality signal
Searching for similar (repeating) signals in continuous waveforms
Continuous multichannel cross correlation. For arrays, only vertical or 3-C channels (e.g., ARCES) are used. For 3-C stations, all three channels can be used. International Data Centre
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Cross correlation detection An example of cross correlation detection: DPRK 2006 vs. DPRK 2009. DPRK06 as a template. Station AKASG, BP filter between 0.8 Hz and 2.0Hz STA
LTA
Threshold: SNR=STA/LTA >3.0
Valid signal
Cross Correlation Coefficient, CC
International Data Centre
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Local Association (LA) For all valid arrivals at primary stations, which are found with a given master event, origin times, OTij, are calculated. The empirical travel times from the master event to the relevant primary stations, TTij, are subtracted from the arrival times, ATij :
OTij = ATij – TTij However,
TTij = TTj ! Empirical travel times from a master event to seismic stations are characterized by ZERO modelling errors and very low measurement errors. These conditions allow extremely accurate relative location. International Data Centre
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Association with seismic events Arrival times are reduced to origin times before association P
ST1
P
P
P
P
time
P
ST2 P
ST3 P
ST4 1-6 sec
M1
P P
P
not associated – out of range Origin time not enough stations
not associated – same station
• For the cross correlation bulletin, XSEL, one arrival can participate only in one event hypothesis • The XSEL includes events with detections at 3 and more primary stations, i.e. only REB-compatible events are created. These stations are called defining. International Data Centre
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Additional location grid for LA dtk = S · dk - travel time correction
10
OTkij = ATij - TTj + dtjk – corrected origin time
5
N
MAST ER 0 EVENT
-10
-5
0
5
10
From origin time residuals to relative location
-5
s -10
E
• k nodes, rectangular or circles; • grid size from 1 to 100 km; • spacing from meters to 10-15 km • Average OT and RMS OT residual are calculated in each node International Data Centre
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Conflict resolution
winners Master m
time More stations
Same number of stations lower OT scattering
No conflict different stations
Master n
When origin times at one or more stations from two or more master events are within several seconds we have to decide which master it is associate with When an event lost one or more stations it still can survive if it matches the Event Definition Criteria for the REB
International Data Centre
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Preparation of master events Tohoku, 11 March 2011 (725 aftershocks) Automatic SEL3 includes 538 events There are hundreds of historical with evids from the REB. 628 SEL3 REB events in the aftershock area events in the aftershock area
Task: outperform SEL3 and REB in automatic processing International Data Centre
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Tohoku 11.03.2011 (725 aftershocks) Statistics of IMS seismic stations
International Data Centre
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Tohoku, 11 March 2011 Recovery with historical master events 4 defining stations
3 defining stations
1411 XSEL events
2554 XSEL events
International Data Centre
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Tohoku, 11 March 2011 Recovery with aftershocks as master events
162 ME (0.5º cells)
International Data Centre
69 ME (1º cells)
Circles – ME between 0 and 40 km depth; triangles – deeper than 40 km
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Tohoku, 11 March 2011 Aftershocks as master events: dense set (162 ME) 4 defining stations
944 XSEL events International Data Centre
3 defining stations
1530 XSEL events Page 14
Tohoku, 11 March 2011 Aftershocks as master events: sparse set (69 ME) 4 defining stations
980 XSEL events International Data Centre
3 defining stations
1682 XSEL events Page 15
Tohoku, 11 March 2011 Regular grid (corrected TTj)
ME from aftershocks
International Data Centre
Regular grid of ME
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Tohoku, 11 March 2011 Regular grid, 29 ME 4 defining stations
834 XSEL events International Data Centre
3 defining stations
1522 XSEL events Page 17
Tohoku, 11 March 2011 Grand Master, 29 Nodes x 10 GMs Grand Master from aftershocks
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Grand Master
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Cross correlation distance Earthquakes in Atlantic Ocean
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CC Corrected arrival time delays
CC
Cross correlation coefficients, IMS array TORD
Distance between events, km
Not corrected arrival time delays
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Tohoku, 11 March 2011 Grand Master, 29 Nodes x 10 GM 4 defining stations
995 XSEL events International Data Centre
3 defining stations
1520 XSEL events Page 20
Tohoku, 11 March 2011 Comparison of ME performance ME Set Tohoku aftershocks, dense set - 162 ME Tohoku aftershocks, sparse set - 69 ME
Historical, sparse set - 109 ME Regular Grid, 29 ME Grand Master, 10 GM, 29 ME International Data Centre
# defining stations
3 4 3 4 3 4 3 4 3 4
REB by REB by 2 XSEL time (12 s) sta (4 s)
1530 944 1682 980 1590 1411 1522 834 1520 995
725 711 723 711 725 721 720 675 722 702
654 621 660 615 670 633 611 538 572 524 Page 21
Global set of real master events Red –master events, yellow – shallow REB events Ms>7
International Data Centre
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Global Grid of master events Locations of 25,000 master events
International Data Centre
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