This document describes the design and testing of a global grid of master events for waveform cross correlation to improve seismic monitoring capabilities. The grid uses real and synthetic master events from seismic arrays and other stations. Testing in February 2013 using 134 events found 92 matches using the grid, demonstrating its potential to detect more small events through cross correlation across the global seismic network. Future versions aim to optimize the use of real data, principal components, and machine learning to further enhance monitoring.

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Global grid of master events for waveform cross correlation:
design and testing
Kitov, I., D. Bobrov, and M. Rozhkov
International Data Centre
Preparatory Commission for the Comprehensive
Nuclear-Test-Ban Treaty Organization
Provisional Technical Secretariat
Vienna International Centre
P.O. Box 1200
A-1400 Vienna
AUSTRIA
ivan.kitov@ctbto.org

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Acknowledgements
The authors are grateful to the IDC and especially to all
analysts reviewing XSEL and REB events. This presentation
has been produced with the assistance of the European Union,
EU Council Decision 2010/CFSP of 26 July 2010.
Disclaimer
The content of this presentation is the sole responsibility of the
authors and can in no way be taken to reflect the views of the
European Union and the CTBTO Preparatory Commission.

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Background
Comprehensive Nuclear-Test-Ban Treaty
The Comprehensive Nuclear-Test-Ban Treaty (CTBT) obligates each State Party
not to carry out any nuclear explosions, independently of their size and purpose.
The Technical Secretariat (TS) of the Comprehensive Nuclear-Test-Ban Treaty
Organization will carry out the verification of the CTBT. The International Data
Centre (IDC) is an integral part of the (currently Provisional) TS. It receives,
collects, processes, analyses, reports on and archives data from the International
Monitoring System (IMS).
The IDC is responsible for automatic and interactive processing of the IMS data
and for standard IDC products.
The IDC is also required by the Treaty to progressively enhance its technical
capabilities.

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Objectives
To built a global grid of master events for waveform cross
correlation
To assess the performance of waveform cross correlation as a
technique of seismic monitoring using the global grid of master
events

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Outline
1. Motivation
2. Global seismic monitoring: IMS
3. Global seismicity: IDC view
4. Cross correlation at teleseismic distances
5. Actual and grand master events
6. Machine learning and classification
7. Synthetic master events
8. Underground nuclear explosions as master events
9. Global cross correlation grid. Design
10.Testing. February 12, 2013

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Cross correlation as an IDC technique
Motivation
• Regional studies demonstrate significant improvement in
detection, location, and magnitude estimation.
At least an order of magnitude!
• Many IMS primary stations are arrays enhancing the capability
of cross correlation analysis
• For arrays, correlation distance depends on phase and its
slowness
• At teleseismic distances, high level of cross correlation is
observed for signals from events spaced by 100 km and even
more
• Remote events may have similar signals
• Small events can be considered as point sources

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IMS, seismic network
Green circles – primary arrays
Green triangles – primary 3-C stations
Small green circles – auxiliary arrays
The primary network includes many arrays

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Global seismicity: the IDC view
Waveform cross correlation relies on high quality master events
REB events with zero depth:
yellow – a neighbor closer than 50 km; red – no neighbor within 50 km
Monitoring is global.
How to populate the aseismic area with quality master event?

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Waveform cross correlation
6 s 6 s
CC
STA
LTA
SNR=STA/LTA≥3.0
Detection:
CC > CCtr
SNR_CC > SNRtr
Multichannel waveforms
Master template
Four frequency bands
Adjusted template length
Waveform quality check
CC for individual channels
Averaged CC trace
Detection
Multichannel CC-detector better sees signals from slave events close to the master

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Actual and grand masters: Sumatera 2012
-2
0
2
4
6
86 88 90 92 94 96
lat,
deg
long, deg
Regular grid
REB
Real masters
main shock
-2
0
2
4
6
86 88 90 92 94 96
lat,
deg
long, deg
1181 REB events between
April 11 and May 24 , 2012
• 7 IMS array stations
• 16 master events: actual and grand
masters
• 2763 XSEL hypotheses
• 409 (~15%) randomly chosen XSEL
events were reviewed by analysts
• 119 new REB events

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Machine learning: classification
Nsta XSEL
Tree
Bagger SVM
Naive
Bayes
0 0 1066 1406 848
1 0 489 487 613
2 0 382 253 540
3 2080 324 208 452
4 514 347 256 198
5 129 115 113 80
6 31 31 31 24
7 9 9 9 8
Total 2763 826 617 762
ASAR CMAR GERES
MKAR
SONM
WRA
ZALV

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Grand masters: Atlantic Ocean
Cross correlation of signals from remote events
931 REB events; 3 array stations with SNR>3

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Grand masters: Atlantic Ocean
Cross correlation coefficient for 931 events in seismic region 32
Events are ordered by latitude: north to south
Matrix of cross correlation coefficients (color coded)
Signals correlation does not dependent on the distance between events

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Cross correlation: explosion signals
• 100 waveforms
• 25 underground
nuclear explosions
• 6.2 > mb > 4.5
• 2015 m > H > 150 m
• 60 stations
• 16º > Δ > 100º
Towards seismic monitoring of underground nuclear explosions

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Cross correlation of explosion signals
Synthetic
seismograms:
Δ =30º, 45º, 60º, 90º
H=0.1, 0.3, 0.6, 1.0,
2.0 km
fc= 0.8 Hz to 4.8 Hz

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Cross correlation of explosion signals
Principal Component Analysis
10 best components for real and synthetic waveforms
CCs for 100 real waveforms
correlated with real PC
CCs for 100 real waveforms
correlated with synthetic PC
Synthetics demonstrate excellent performance when used for
waveforms cross correlation

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Global Cross Correlation Grid
Master: Ten primary seismic arrays at P-wave distances
(6 to 96 degrees)

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Global Cross Correlation Grid
Segment
R = 100 km

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Global Cross Correlation Grid
Testing, February 12, 2013
REB - 134 events
Grid: 25000 nodes
Group 1 = WRA, TORD, MKAR, ILAR, GERES, PDAR, CMAR, SONM, AKASG, BRTR, GEYT
Group 2 = ASAR, ZALV, YKA, ARCES, TXAR, KSRS
Group 3 = USRK, FINES, NVAR, NOA, MJAR
Defining parameters: Templates: simplest 1D synthetic waveform for all arrays, theoretical time delays
Detections: SNRmin = 0.5; SNR_CCmin=2.5; CCmin = 0.2; FKSTATmin = 2.5; AZRESmax= 20.0º;
SLORESmax = 2.0 s/º;
Events: dTorigin = 6s; NSTAmin= 3; AZGAPmax= 330º
RESULTS: Total arrivals and hypotheses: 22,900,402 arrivals; 107,969 events;
After conflict resolution:
SNR_CC>2.5 SNR_CC>3.0 SNR_CC>3.5 SNR>2
XSEL 6,141 events 766 122 2351
REB Matched 92 90 77 101
DPRK 2013: time - 02:57:50.799 , d=24.92 km, OTres=0.1s; nsta=9:
AKASG, BRTR, CMAR, GERES, GEYT, ILAR, MKAR, SONM, WRA

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Global Cross Correlation Grid
• V0.1: All master templates are synthetics same at all stations, a version of f-k
analysis
• V0.2: Master templates are station/master specific synthetics in 1D velocity
model
• V0.3: Master templates are station/master/source (e.g. explosion) specific
synthetics calculated for 2D velocity structure (e.g. ak135+CRUST 2.0)
• V1.1: Real master templates are used where possible
• V1.2: Grand master events are applied where possible
• V2.0: The set of principal components are optimized where possible as
obtained by the PCA applied to the complete set of actual and historical
data
• V3.0: Synthetic + real master templates based on principal components with
classification algorithms trained on actual data

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Discussion
• IMS array stations make possible automatic processing based on
waveform cross correlation
• Cross correlation is a powerful technique allowing to reduce the
detection threshold and relative location accuracy by an order of
magnitude, i.e. to find by 50% to 100% more (smaller) REB
events
• Grand master and synthetic master events may reduce the
magnitude threshold of seismic monitoring by 0.4 units of
magnitude
• The Global Cross Correaltion Grid is flexible (e.g. master density,
templates, number of stations, thresholds, etc.) to fulfill various
tasks including effective monitoring of UNEs