Waveform cross correlation is used to find aftershocks of seismic events: from weak to catastrophic

1. 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

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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
• Recovery of repeating quarry blasts
• DPRK5 aftershock

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Cross correlation as a monitoring technique
Intuition
1. The REB contains several examples of aftershock sequences with thousands of events
2. 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.
3. Almost all events have a neighbor (less than 100 km) in the REB.
4. Empirical observation: events at distances of 50 to 100 km generate signals well correlated
at regional and teleseismic distances
5. 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
6. IMS stations cover the globe and many of primary seismic stations are arrays enhancing
the capability of cross correlation analysis
7. The Reviewed Event Bulletin (REB) of the International Data Centre includes a few
spatially close underground nuclear tests measured at IMS seismic stations
8. Historical UNEs conducted within test sites and PNEs provide digital waveforms at
legacy/surrogate IMS and non-IMS stations for a broad range of test conditions: yield,
depth, geology, seismic coupling, etc.
9. Encouraging results of other studies of waveform cross correlation at teleseismic and
regional distances

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Waveform template
– high quality signal
Searching for similar (repeating) signals in
continuous waveforms
Cross correlation
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.

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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
Cross Correlation Coefficient, CC
Valid signal

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Local Association (LA)
1. 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
where i is arrival index at station j.
TTij = TTj !
2. 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.

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International Data Centre
Association with seismic events
M1
Master i
2-6 sec
PP P
Master 1
• For the XSEL, one arrival can participate only in one event hypothesis
• The XSEL includes events with 3 and more primary stations and arrivals with
accurate azimuth and slowness estimates, i.e. only REB-compatible events are
created
P
Arrival times are reduced to origin times before association
Local association (LA) with XSEL events
Origin time
Conflict resolution (CR)
winners
Origin times from 3 and more IMS stations within a few sec interval create an event hypothesis
More stations
Same number of stations
lower scattering
not associated – same station
P
not associated – out of range
P P

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Additional location grid for LA
MASTER EVENT
-10
-5
0
5
10
-10 -5 0 5 10
N
E
dtk = S · dk - travel time correction
OTk
ij = ATij - TTj + dtjk – corrected origin time
From origin time residuals to
relative location
• k nodes, rectangular of circle;
• 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
s

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Preparation of master events (ME)
Tohoku, 11 March 2011: 725 REV events
SEL3 includes 538 events with evids
from the REB. 628 SEL3 events in
the aftershock area
SEL3 and historical REB events could be used as master events
There are hundreds of historical
REB events in the aftershock
area

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Global set of real master events
7860 events with 10 and more primary stations and SNR>5
signals
Red –master events, yellow – shallow REB events Ms>7

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Historical master events
ME from the REB ME for Tohoku earthquake

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PRIMARY IMS SEISMIC STATIONS

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TOHOKU 11.03.2011
STATISTICS OF IMS SEISMIC STATIONS

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Tohoku, 11 March 2011
Historical master events
4 defining stations
1411 XSEL events
3 defining stations
2554 XSEL events

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Tohoku, 11 March 2011
Aftershocks as master events
162 ME (0.5 deg spacing) 69 ME (1 deg spacing)
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 3 defining stations
944 XSEL events 1530 XSEL events

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Tohoku, 11 March 2011
REB as master events: sparse set (69 ME)
4 defining stations 3 defining stations
980 XSEL events 1682 XSEL events

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Tohoku, 11 March 2011
Regular grid
ME from aftershocks Regular grid

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Global Grid of master events
Locations of 25,000 master events

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Tohoku, 11 March 2011
Regular grid, 29 ME
4 defining stations 3 defining stations
834 XSEL events 1522 XSEL events

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Tohoku, 11 March 2011
Grand Master, 29 Nodes x 10 GM
Grand Master from aftershocks Reproduced GM

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Tohoku, 11 March 2011
Grand Master, 29 Nodes x 10 GM
4 defining stations 3 defining stations
995 XSEL events 1520 XSEL events

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Tohoku, 11 March 2011
Comparison of ME performance
ME Set
# defining
stations
XSEL
REB TIME
(12 s)
REB 2
STA (4 s)
Tohoku aftershocks,
dense set - 162 ME
3 1530 725 654
4 944 711 621
Tohoku aftershocks,
sparse set - 69 ME
3 1682 723 660
4 980 711 615
Historical, sparse set
- 109 ME
3 1590 725 670
4 1411 721 633
Regular Grid, 29 ME
3 1522 720 611
4 834 675 538
Grand Master, 10
GM, 29 ME
3 1520 722 572
4 995 702 524

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SEL3 vs XSEL
Automatic processing
1. SEL3 is not a good start interactive analysis of the biggest
aftershock sequences (AS): missing hypotheses, many rejected
event hypotheses. Larger mislocation results in additional work.
2. XSEL is an excellent start for interactive analysis: REB events
have REB-compatible hypotheses with 3 primary stations, REB
from XSEL is more complete, the analysts’ workload is reduced
by a factor of 2.
REB
data day
REB
AS area
REB with SEL3
evids
SEL3
data day
SEL3 in AS
areas
XSEL REB
FOUND
REB NOT
FOUND
NEPAL
(2015115) 286 209 132 262 109 362 197 12
TOHOKU
(2011070) 797 725 538 887 628 1530 725 0
SUMATRA
(2004361) 724 625 497 910 526 1839 611 14

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Cross correlation distance
Earthquakes in Atlantic Ocean
Cross
correlation
coefficients,
IMS
array
TORD
Distance between events, km
Not corrected arrival time delays
Corrected arrival time delays
CC
CC

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Global Grid of real, grand master,
and synthetic master events
Primary seismic network Location of 25,000 master events
GG coverage
Finding the DPRK3 with
synthetic UNE waveforms

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Repeating mining blasts
s
Aitik
Kiruna

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Repeating mining blasts: Aitik copper mine, Sweden
1. Select historical REB events detected
by ARCES, FINES, and NOA
2. Select events with high quality
waveform templates at all stations
3. Continuous cross correlation
4. Local association of all detections
5. Creation of an automatic XSEL
6. Interactive analysis
2017051 18:02:50 2017053 18:17:54
s

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Continuous monitoring of the DPRK test site:
weak aftershock of the DPRK5
Sta Dist, km EvStaAz,
deg
Phase Arrival time tres, s CC dRM SNRCC SNR
SEHB 346 191.7 Pn 01:51:39.35 -0.21 0.25 -2.90 2.6 1.9
MDJ 367 6.7 Pn 01:51:43.35 0.26 0.28 -2.66 2.7 2.1
USRK 410 35.8 Pn 01:51:46.05 0.16 0.26 -2.94 3.1 2.8
KSRS 440 193.6 Pn 01:51:51.92 -0.21 0.20 -2.89 3.3 2.2
network
Templates and waveforms
relative location
absolute location

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Discussion