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Václav Vavryčuk Rosalia Daví

Václav Vavryčuk Rosalia Daví. Seismic network calibration for retrieving accurate moment tensors. Institute of Geophysics, Academy of Sciences, Praha e-mail: vv@ig.cas.cz . MOTIVATION. Non-DC components of moment tensors require:. Accurate locations Accurate velocity model

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Václav Vavryčuk Rosalia Daví

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  1. Václav Vavryčuk Rosalia Daví Seismic network calibration for retrieving accurate moment tensors Institute of Geophysics, Academy of Sciences, Praha e-mail: vv@ig.cas.cz 

  2. MOTIVATION Non-DC components of moment tensors require: • Accurate locations • Accurate velocity model • Focal sphere coverage • High-quality data - high signal-to-noise ratio - high-quality stations High-quality stations: • Unified transfer functions • Correct amplifications • Correct sensor orientations • Correct polarities • No local site effects • Network calibration • using a joint inversion • for MTs of many events

  3. INVERSION FOR SINGLE EVENT Moment tensor inversion for a single event G - Green’s function amplitudes u - amplitudes observed at N stations m - moment tensor

  4. JOINT INVERSION FOR MANY EVENTS One station with unknown amplificationC (N+1) Stations with known and unknown amplifications

  5. NUMERICAL MODELLING • Generation of datasets of 200 events with synthetic shear focal mechanisms. • Calculation of synthetic amplitudes. • Contamination of the amplitudes with random noise. • Multiplication of the noisy amplitudes by synthetic station amplifications (between 0.4 and 2.0). • Network of 10 and 22 seismic stations • Partial network calibration & complete network calibration.

  6. PARTIAL & COMPLETE NETWORK CALIBRATIONS Sparse and dense configurations 22 stations 10 stations Number of events: 200 Noise levels: up to 10%, 25% and 50% of the noise-free amplitude Number of fixed stations: 5, 10, 15

  7. NETWORK CALIBRATION: RESULTS Conditions needed for the MT inversion: • Good station coverage • Good signal-to-noise ratio • Accurate locations • Accurate velocity model Additional conditions needed for the joint inversion: • High number of events (100 or more) • Variety of focal mechanisms

  8. SENSITIVITY TO THE STATION LOCATION Inversion of P amplitudes • Stations in the proximityto the nodal lines - high errors • For a high a variety of focal mechanisms - the same accuracy for all stations

  9. COMPLETE NETWORK CALIBRATION • The complete network calibration can adjust station amplifications by including the local site effects at all stations. • The simplest way to calibrate the complete network (of N stations), is to perform the calibration in iterations. • We cannot invert for scalar seismic moments. • Average amplification of the network is fixed.

  10. ACCURACY Inversion of P-wave noisy amplitudes of 22 stations, 200 shear events Retrieved station amplifications (a) True station amplifications (b) Relative errors • The retrieved amplifications are slightly biased from the true amplifications (noise in the data). • The lowest accuracy is achieved for station KAC (in the intersection of the nodal lines). KAC

  11. MOMENT TENSORS Uncalibrated network DC component Non-DC components RMS • high RMS values • significant false non-DC noisy amplitudes, uncorrected amplifications Calibrated network • low RMS values • less scattered non-DC DC component Non-DC components RMS noisy amplitudes, corrected amplifications

  12. WEST BOHEMIA REGION • 22 short-period seismic stations Germany • Epicentres of the 2008 swarm • Depth of 7.6 to 10.8 km. • 200 microearthquakes: • high signal-to-noise ratio • highly accurate hypocenter locations Czech Republic

  13. ACCURACY • KAC station (unfavorable position). • HRED station (rather high noise level). • No station with a reversed polarity. • TRC station (an anomalous medium response). • The scatter of the amplification corrections is high: the values range from 0.62 to 1.45. Amplifications TRC Amplification errors KAC HRED

  14. MOMENT TENSORS Uncalibrated network • DC is stable DC component Non-DC components RMS • Low RMS • A compact cluster of non-DC • Non-DC: less compressive Calibrated network DC component Non-DC components RMS

  15. MOMENT TENSORS • The average CLVD changed from -16.3% to -9.7%. • The average RMS is reduced from 0.19 to 0.11 .

  16. CONCLUSIONS • New method for calibrating seismic networks is proposed. • The calibration detects: reverse polarities, incorrect orientation and amplification of sensors, anomalous local site effects at stations. • The moment tensors for calibrated networks display lower RMS than the original moment tensors. • The accuracy of the non-DC components is increased. • The method is can be used for data gathered: • in laboratory experiments, • in boreholes or in mines • in field experiments (networks consisted of variety of instruments).

  17. Thank you for your attention!

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