Searching for aftershocks of underground explosions with cross correlation

International Data Centre Page 1
Searching for aftershocks of underground explosions with
cross correlation
Kitov, Ivan
Bobrov, Dmitry
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

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.

International Data Centre 25 October 2010 Page 3
Objectives
To assess the performance of a cross-correlation technique as
applied to automatic and interactive processing of two DPRK
tests.
To search for aftershocks of the DPRK09 at a lower magnitude
threshold

Primary seismic network
Not started Under construction Installed Certified
Status as of February, 2010

Seismic-acoustic event processing at the IDC
Data Processing, Analysis, Screening, Fusion, Review, Access
Fused
Products &
Executive
Summaries
(daily)
Outputs:Outputs:
Seismic Hydro-
acoustic
Infra-
sound
Radionuclide
Standard
Event List
1
Standard
Event List
2
Standard
Event List
3
Reviewed
Event
Bulletin
Standard
& Standard
Screened
Event Bulletins
Reviewed
Radnuc.
Report
Final
Quality
Review
(done for all
reviewed
products)
Auxiliary
Seismic
Event
Screening
Analyst
Review
Fission
Product
Screening
Automatic
Radnuc.
Report
Raw Data
Analyst
Review
Standard
Screened
Radnuc.
Event
Bulletin
All
raw
data
(cont.)
Inputs:Inputs:
(accessible as soon as it arrives)

Cross correlation as an IDC technique
Intuition
1. Close events generate signals well correlated at regional
and teleseismic distances
2. The Reviewed Event Bulletin (REB) produced by the IDC
contains more than 335,000 events with the average
distance between closest events less than 50 km
3. Small events might be considered as point sources
4. Many IMS primary stations are arrays enhancing the
capability of cross correlation analysis

Cross correlation
REB events with zero depth:
yellow – a neighbor closer than 50 km; red – no neighbor at 50 km.

Cross correlation
Coherent beams for IMS station WAR: DPRK 2006 vs. DPRK 2009
DPRK 2006
DPRK 2009

Cross correlation coefficient
,
The notation ω 𝑁,∆𝑡 𝑡0 is used to denote the discrete vector of N
consecutive samples of a continuous time function ω 𝑡 , where 𝑡0 is the
time of the first sample and ∆𝑡 is the spacing between samples:
ωN,∆t t0 = ω t0 , ω t0+∆t ,…, ω t0+ N−1 ∆t
T
The inner product of υ 𝑁,∆𝑡 𝑡υ and ω 𝑁,∆𝑡 𝑡ω is defined by
υ 𝑡υ , ω 𝑡ω 𝑁,∆𝑡 = υ 𝑡υ + 𝑖∆𝑡 ω 𝑡ω + 𝑖∆𝑡
𝑁−1
𝑖=0
and the normalized cross-correlation coefficient by
𝐶𝐶 υ 𝑡υ , ω 𝑡ω =
υ 𝑡υ , ω 𝑡ω 𝑁,∆𝑡
ω 𝑡υ , ω 𝑡υ 𝑁,∆𝑡 υ 𝑡ω , υ 𝑡ω 𝑁,∆𝑡

AKASG: DPRK06 vs. DPRK09
Cross correlation: channel by channel

Cross correlation analysis
Templates: windows and filters
Phase Filter Window, s
Low (Hz) High
(Hz)
Type order Lead Signal
P 0.8 2.0 BP 3 1.0 5.5
P 1.5 3.0 BP 3 1.0 4.5
P 2.0 4.0 BP 3 1.0 3.5
P 3.0 6.0 BP 3 1.0 3.5
Pn 0.8 2.0 BP 3 1.0 10.0
Pn 1.5 3.0 BP 3 1.0 10.0
Pn 2.0 4.0 BP 3 1.0 10.0
Pn 3.0 6.0 BP 3 1.0 10.0

AKASG: DPRK09 vs. Chinese earthquake
Cross correlation with a distant event of different nature
The change in azimuth

AKASG: DPRK09 vs. a Chinese
earthquake
The change in slowness and shape

Data quality problems
Spikes, gaps, and polarity

Data quality problems
# working channels, IMS station ESDC
CC
#
BCC
#

Waveform template Waveform under analysis

An example of cross-correlation analysis. DPRK06 as a template.
Station AKAGG, filter between 0.8 Hz and 2.0Hz.
STA
LTA
SNR=STA/LTA >3.0
CC

Average CC or beam CC; BCC = ΣCCi(t)/Nch
CC1
CC20
BCC
CCi

F-probability > 0.3
The semblance of an N-element array, over an M-sample window is defined as
𝑆 =
𝑢𝑖 𝑡𝑁
𝑖=1
2𝑀
𝑡=1
𝑁 𝑢𝑖 𝑡 2𝑁
𝑖=1
𝑀
𝑡=1
Thus, S is the power on the beam divided by the average power of the channels used to form the beam,
each averaged over an M-sample time window.
F-statistic is computed as
𝐹 =
𝑆
1 − 𝑆
(𝑁 − 1)
F has a non-central distribution 𝐹(𝑁1, 𝑁2, 𝜆), where 𝑁1 = 2𝐵𝑇 and 𝑁2 = 𝑁1(𝑁 − 1) are the degrees of
freedom, 𝜆 = 2𝐵𝑇𝑅2
is the non-centrality parameter, B is the bandwidth in Hz, T is the time window (s),
and R2 is the ratio of the signal and noise power on the beam.
F-probability of non-central F-distribution can be computed as
𝑃(𝐹′
|𝑁1, 𝑁2, 𝜆) ≈ 𝑃(𝐹|𝑁1
∗
, 𝑁2)
where,
𝐹 =
𝑁1
𝑁1 + 𝜆
𝐹′
and 𝑁1
∗
=
(𝑁1 + 𝜆)2
𝑁1 + 2𝜆

FK and relative magnitude
 
 
 
 









 2
1
2
1
1
2
2
1
)(12
))((
)(
),( f
ff
J
i
f
ff
J
i
deastsdnorthsf
i
en
iFJ
efF
ssP
en
In FK analysis for seismic arrays, spectra are computed from the vertical channels in a station-dependent time window.
For each slowness vector, the FK power spectrum is calculated as:
where deasti and dnorthi are the east-west and north-south coordinates, respectively, of the ith sensor array element
relative to the reference station.
Gibbons and Ringdal (2006) introduced an amplitude scaling factor: a = x‧y/ x‧x, where x and y are the vectors of
data for the master and slave event, respectively. For two collocated events with the same source time history but
different amplitudes, the amplitude scaling factor completely defines the difference in sizes. For close events with
similar source functions, the amplitude scaling factor defines the least square solution of the equation y = ax+n.
In order to reduce the influence of the distance, we propose to use the ratio of norms ｜x｜/｜y｜instead of a. The
logarithm of the ratio,
RM = log(｜x｜/｜y｜)= log｜x｜- log｜y ｜ ,
is essentially the magnitude difference or relative magnitude

MASTER STA PHASE FILTER CC TRES
CC
AZRES
AZRES
CC
SLORES
SLORES ALPHA RM
2009 AKASG P P1530 0.666 -0.07713 -0.07 -3.00 -0.20 0.25 -0.740 -0.564
2009 ASAR P P1530 0.660 -0.15482 2.03 2.00 0.15 1.40 -0.652 -0.471
2009 GERES P P1530 0.571 0.21017 4.46 -12.50 -0.15 0.12 -0.818 -0.575
2009 MJAR Pn P2040 0.677 0.14737 -0.92 -5.90 -0.12 1.31 -0.479 -0.309
2009 MKAR P P0820 0.522 0.71556 -0.46 8.60 -2.45 2.23 -0.952 -0.670
2009 NOA P P2040 -0.757 -0.40213 8.66 -0.10 1.55 0.24 -0.602 -0.481
2009 NVAR P P1530 0.957 0.09787 -0.26 -2.00 0.47 1.08 -0.590 -0.571
2009 PDAR P P0820 0.759 0.04787 2.88 18.20 -1.52 -1.80 -0.697 -0.578
2009 SONM Pn P0820 0.617 0.55787 3.53 6.20 -0.58 0.24 -0.719 -0.509
2009 WRA P P1530 -0.903 0.24228 -0.28 1.30 0.05 0.46 -0.462 -0.417
2006 AKASG P P1530 0.674 0.0749 -0.54 -3.10 0.25 0.10 0.393 0.564
2006 ASAR P P1530 0.674 0.17759 -2.65 5.70 -0.04 0.29 0.300 0.472
2006 GERES P P1530 0.549 -0.2124 -0.42 -10.60 1.33 0.12 0.311 0.571
2006 MJAR Pn P2040 0.685 -0.1371 1.93 -4.00 -0.06 -0.47 0.145 0.309
2006 MKAR P P0820 0.517 -0.71779 3.15 6.50 -0.04 0.82 0.374 0.661
2006 NOA P P2040 -0.758 0.3999 4.08 -0.80 -0.25 0.27 0.365 0.485
2006 NVAR P P1530 0.956 -0.1001 -3.33 -5.30 -0.05 0.49 0.553 0.573
2006 PDAR P P0820 0.753 -0.0251 -29.79 -2.40 1.25 -1.25 0.455 0.578
2006 SONM Pn P0820 0.630 -0.5551 -1.21 4.80 1.39 -0.24 0.317 0.518
2006 WRA P P1530 -0.907 -0.24451 -1.03 2.10 0.05 0.37 0.374 0.416
FK analysis and RM

Page 22
Search for aftershocks
International Data Centre
1. Five days after DPRK09
2. DPRK06 and DPRK09 waveforms as templates
3. All relevant IMS primary arrays including KSRS and USRK
4. All filters
5. BCC threshold 0.2
6. SNRBCC>3.0
7. Three primary stations as Event Definition Criteria
Outcome
No aftershocks are found during the five days after the DPRK09

Discussion
• Cross correlation is a powerful technique to find (build) new
REB events using nearby master events
• IMS array stations allow for enhancement of cross correlation
analysis
• Accurate master events may reduce the magnitude threshold of
seismic monitoring
• No aftershocks are found after the DPRK09 at the level of
magnitude around 3.0. The absence of RN signals confirms the
absence of significant aftershocks

Searching for aftershocks of underground explosions with cross correlation

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