Detection of weak earthquakes by small seismic array using DP/EP in the Czech Republic Jaroslav Strunc

November 4, 2009NORSAR2 Outline Shortly about Academy of Sciences Areas of interest Experimental arrays Detections – classic & correlation Implementation model for WEBNET

November 4, 2009NORSAR3 Academy of Sciences Institute of Rock Structure & Mechanics −dept. of Seismology Czech Technical University −PhD student – measurement tech. What should be done −evaluation of experimental data −implementation in the Czech Rep.

November 4, 2009NORSAR4 maps.google.com Areas of Interest (1)

November 4, 2009NORSAR5 Areas of Interest (Natural Seismicity) Western Part −swarm area −Eger Rift & Marianske Lazne Fault −M Lmax = 5 Eastern Part −rare events but „strong“ −Hronov-Porici Fault −M Lmax = 3.5

November 4, 2009NORSAR6 East Bohemia Mount Ostas, 2005 Aperture 40m 1 broadband Guralp (30s) 3-D geophones SM6 (4.5Hz) 10 km NE far from Hronov-Porici Fault Near-surface events

November 4, 2009NORSAR7 West Bohemia WEBNET (West Bohemia Network) 13 permanent stations −Guralp (30s) & SM3 (1s) −Lennartz, Nanometrics 2 experimental arrays – NKCA, KVCA −triangular shape −aperture 90m −SM6 geophones (4.5Hz)

November 4, 2009NORSAR8 NKCA & KVCA Novy Kostel (2007), Kvetna (2008) Aperture 90m 3-D geophones SM6 (4.5Hz) Hypocentre b. Novy Kostel (5~15 km) KVCA placed about 5 km SE

November 4, 2009NORSAR9 KVCA (Kvetna)

November 4, 2009NORSAR KVCA NKCA

November 4, 2009NORSAR11 Swarm Activities (1) Last Swarm Activity : October/November 2008

November 4, 2009NORSAR12 Swarm Activities (2)

November 4, 2009NORSAR13 What with the data? 3 arrays −several years of continuous data No software for data processing −go through data manually? lack of time Is the shape and place of array suitable?  That‘s the purpose of my visit.

November 4, 2009NORSAR14 Objectives (1) Get familiar with array processing tech. Evaluate −array shape −array location −used type of sensors Find out the optimal method of detection for current configuration

November 4, 2009NORSAR15 Objectives (2) Select on array −KVCA (West Bohemia) Perform all experiments with the data −detections −locations Prepare automated process for the other arrays

November 4, 2009NORSAR16 Array Equipment Sensors −geophones – velocity (SM6b, Sensor Netherland) −theoretical flat resp. in range 4.5~1000Hz −sensitivity 2600 V/(m/s) Digitizers −RUP2004 (our product) −21-bit A/D Converter −sampling 100 Hz

November 4, 2009NORSAR17 But...?

November 4, 2009NORSAR18 Period of Study, KVCA April 17 ~ August 12, 2008 −117 days 131 local detections by WEBNET −M Lmin = -1 Array sensors „best“ features at −freq. band 8 ~ 16 Hz −spurious resonances present

November 4, 2009NORSAR19 Array Transfer Function

November 4, 2009NORSAR20 Classic DP 4 freq. bands (8-10, 10-12, 12-14, 14-16) used 227 beams (horizontal, vertical) threshold 12.5 ~ 6.5 lower SNR −increased false detections rapidly caused by spurious resonances of geophones −many „visible“ events stayed undetected

November 4, 2009NORSAR21 Event Example - Resonances

November 4, 2009NORSAR22 Event Example - Detail !

November 4, 2009NORSAR23 DP Results Best Results for −vertical SNR ≥ 6.5 −horizontal SNR ≥ 10 −backazimuth 270° ~ 360° April 19, 2008 −Σ 135 events 54 false at least 15 missed (visible without beamforming)  Data are too bad for classic DP

November 4, 2009NORSAR24 What about Cross-Correlation? 131 detected events by WEBNET −from 22 near locations 11 suitable templates −distinct phases −„weak“ enough (M L < 0) −only 7 of 9 array traces used (malfunction)  Finally: it takes time bud it works!

November 4, 2009NORSAR25 CC Results – Unique Detections April 19, 2008 −Σ 103 events (including template) for CC_SNR ≥ 9.5 −9 false −2 missing? −Σ 90 events (including template) for CC_SNR ≥ 10.5 −3 false −7 (+2?) missing!

November 4, 2009NORSAR26 Whole Period of Time (117 days)

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November 4, 2009NORSAR28 Detections per Day

November 4, 2009NORSAR29 Templates & Detections

November 4, 2009NORSAR30 Conclusions Geometry −suitable for detections in Fault Zone −but: none redundancy Features of the Sensors disable to use DP −they should be replaced CC-DP increases no. of detections −template necessary −cannot fish out new events

November 4, 2009NORSAR31 Future Work Implementation of on-line DP/EP in Academy of Science −at the beginning as template based CC-DP Process data from the other arrays Prepare a short paper

November 4, 2009NORSAR32 Thank you for attention Many thank to Johannes & Steven and to everybody for the nice atmosphere at Norsar