1 / 17

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

eldora
Télécharger la présentation

Václav Vavryčuk Rosalia Daví

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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!

More Related