Small-aperture array at Dobrá Voda: first records of local earthquakes

Jiří Málek, Milan Brož

and Jaroslav Štrunc

Institute of Rock Structure and Mechanics AS CR,

Prague, Czech Republic, malek@irsm.cas.cz

Second annual meeting Advanced Industrial

Microseismic Monitoring AIM

Prague 29-30.9.2011

Contents

Small-aperture seismic arrays (SSA) for microseismic monitoring

SSA Dobrá Voda

Tests - records of distant earthquakes

Records of local earthquakes

Examples from other SSA operated by IRSM

Future plans

Small-aperture seismic array (SSA) for microseismic monitoring

Detection of very weak earthquakes: local shallow earthquakes M>-1.5 can be detected

Detection of (non-volcanic) tremors, recognition from industrial noise

Recognition of new phases on the seismograms of local earthquakes

Measurements of rotational components of seismic waves Volcanic seismology – seismic antennas

Small-aperture seismic array (SSA) for microseismic monitoring

Special features:

High frequencies (10 – 30 Hz) Small aperture (50 – 120 m) Only several stations (IRSM – 4 stations 3C)

SSA’s built by IRSM AS CR

OSTA – Eastern Bohemia – Hronov-Poříčí Fault (2005) NKCA – Western Bohemia – Seismic swarms (2007) PROA – Provadia (Bulgaria) – Induced seismicity (2009) LAZA – Western Bohemia – Seismic swarms (2011) DOVA – Little Carpathians – (2011) – AIM project

Optimal dimensions

Signals have to be coherent – small aperture Time differences between stations should be

significant – bigger aperture Our recommendation:

aperture is equal to wave-length / 4

of the typical signal

Va=8 km/s, F= 20 Hz => A=100 m

P-wave group, NKCA, shifted seismograms

Optimal azimuth (130°) and apparent velocity (15 km/s)

P-wave group, NKCA, shifted seismograms

Wrong azimuth (310°) and apparent velocity (15 km/s)

Linear and non-linear stacking of coherent signals

To determine azimuth and aparent velocity f-k analysis is normally applied

Alternatively, we use optimization of stacked seismograms in time domain.

Linear or non-linear filters can be used A new method GAS (Generalized Average

Method) was developed and applied

Generalized average of signals

Dobrá Voda Array (DOVA) was built around the DOV (MKNET) station at the main epicentral

zone of the Malé Karpaty region

DOVA consists of 3 short-period three-component stations: D1V, D2V, D3V

Distance between stations is 120 m

Central station Guralp-5s (sampling 100 Hz), other stations Le3D-1s (sampling 250 Hz)

Registration started on May 17, 2011

100 m

DOV

D1V

D3V D2V

Test 1, Fiji, h=599km, mb=5.8 PKIKP Va > 100 km/s DOVA

Test 2, Japan, mb=6.2 P DOVA

Test 2, Japan, mb=6.2 P

Rotational components ?

DOVA

New Portable Mechanical Sensor System for Rotational Seismic Motion Measurements

2nd IWGoRS Workshop, Prague, 10 – 13 October 2010

It consists of highly sensitive geophones, measuringreliably in the range 2 – 100Hz, connected to a common recording device (amplifier, transducer, etc.);

The geophones are mounted in diametrical pairs to a rigid ground-based undeformable (metal) skeleton;

The distance separating the geophones in the given pair is much smaller than but sufficiently large to allow the differential motions to be detected;

The instrument provides records of translational seismic motions together with rotations;

Rotation rate is determined by more than one geophone pair, which allows to perform 'in situ' calibration of the geophones simultaneously with the measurement.

Rotaphone - basic features:

Brokešová et al. 2010

Local Earthquake N from Piestany, 20.7.2011 18:31:00.9

48.65 N 17.89 E Depth: 2 km ML=1.8 (GFU SAV)

27km, 79°

5 km

Malé Karpaty Mt.

Povážský Inovec Mt.

Váh iver alleyr v

P

S

DOVA

Earthquake N from Piestany, 20.7.2011 18:31:00.9P wave

Difference from real backazimuth 5°

Earthquake N from Piestany, 20.7.2011 18:31:00.9S wave

Difference from real backazimuth 7° Vp/Vs = 1.92

Earthquake Dobrá Voda, 20.7.2011 23:52:14 48.61 N 17.54 E Depth: 5km ML= -0.9

Not detected by other seismic stations DOVA

Earthquake Dobrá Voda, 20.7.2011 23:52:14 P wave

High apparent velocity 23.1 km/s

Earthquake Nový Kostel, ML=1.6, Near epicenter Z

NKCA

Earthquake Nový Kostel, ML=3.3, Epic.Dist. 22 km Z

LAZA

Induced E. Provádia, ML=1.6, Near epicenter Z

PROA

Conclusions

Small-aperture seismic array DOVA started operation in May 2011

Local events ML > -1 are recognized Coherency for local earthquakes is weak comparing to

other SSAs

– DOVA is too large Rotational components could be important

– we suggest direct measurement using Rotaphone

We want to implement an automatic evaluation of DOVA data next year

Generalized average of complex numbers

p>0

Generalized average of complex numbersxi = crosses, yp = dots

Generalized average of signals 3

Phase-weighted stack (Schimmel and Paulssen, 1997)