searching for aftershocks of underground explosions with cross correlation

International Data Centre

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

[email protected]


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

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 ω𝑁,∆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 ∆tT

The inner product of υ𝑁,∆𝑡 𝑡υ and ω𝑁,∆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 𝑡

𝑁𝑖=1

2𝑀𝑡=1

𝑁 𝑢𝑖 𝑡2𝑁

𝑖=1𝑀𝑡=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 =𝑁1𝑁1 + 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

Search for aftershocks


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

Technology

international data centre

ims data

idc data processing

archives data

raw data cont

standard idc products

crosscorrelation technique

cross correlation reb