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

Detection of weak earthquakes by small seismic array using DP/EP in the Czech Republic. Jaroslav Strunc. Outline. Shortly about Academy of Sciences Areas of interest Experimental arrays Detections – classic & correlation Implementation model for WEBNET. Academy of Sciences.

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

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  1. Detection of weak earthquakes by small seismic array using DP/EPin theCzech Republic Jaroslav Strunc

  2. Outline • Shortly about Academy of Sciences • Areas of interest • Experimental arrays • Detections – classic & correlation • Implementation model for WEBNET NORSAR

  3. 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. NORSAR

  4. Areas of Interest (1) maps.google.com NORSAR

  5. Areas of Interest (Natural Seismicity) • Western Part • swarm area • Eger Rift & Marianske Lazne Fault • MLmax = 5 • Eastern Part • rare events but „strong“ • Hronov-Porici Fault • MLmax = 3.5 NORSAR

  6. 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 NORSAR

  7. 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) NORSAR

  8. 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 NORSAR

  9. KVCA (Kvetna) NORSAR

  10. NKCA www.mapy.cz KVCA www.ig.cas.cz NORSAR

  11. Swarm Activities (1) Last Swarm Activity : October/November 2008 www.ig.cas.cz NORSAR

  12. Swarm Activities (2) www.ig.cas.cz NORSAR

  13. 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? A That‘s the purpose of my visit. NORSAR

  14. 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 NORSAR

  15. Objectives (2) • Select on array • KVCA (West Bohemia) • Perform all experiments with the data • detections • locations • Prepare automated process for the other arrays NORSAR

  16. 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 NORSAR

  17. But...? NORSAR

  18. Period of Study, KVCA • April 17 ~ August 12, 2008 • 117 days • 131 local detections by WEBNET • MLmin = -1 • Array sensors „best“ features at • freq. band 8 ~ 16 Hz • spurious resonances present NORSAR

  19. Array Transfer Function NORSAR

  20. 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 NORSAR

  21. Event Example - Resonances NORSAR

  22. Event Example - Detail ! NORSAR

  23. 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) A Data are too bad for classic DP NORSAR

  24. What about Cross-Correlation? • 131 detected events by WEBNET • from 22 near locations • 11 suitable templates • distinct phases • „weak“ enough (ML < 0) • only 7 of 9 array traces used (malfunction) A Finally: it takes time bud it works! NORSAR

  25. 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! NORSAR

  26. Whole Period of Time(117 days) NORSAR

  27. 9.3 11.8 16.8 7.6 19.8 NORSAR

  28. Detections per Day NORSAR

  29. Templates & Detections NORSAR

  30. 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 NORSAR

  31. 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 NORSAR

  32. Thank you for attention • Many thank to Johannes & Steven • and to everybody for the nice atmosphere at Norsar NORSAR

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