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, [email protected]Second annual meeting Advanced Industrial Microseismic Monitoring AIM Prague 29-30.9.2011
29
Embed
Small-aperture array at Dobrá Voda: first records of local earthquakes
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, [email protected]. Second annual meeting Advanced Industrial Microseismic Monitoring AIM - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Small-aperture array at Dobrá Voda: first records of local earthquakes
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)