Václav Vavryčuk Rosalia Daví Institute of Geophysics, Academy of Sciences, Praha Seismic network calibration for retrieving accurate moment tensors

Accurate locations Accurate velocity model Focal sphere coverage High-quality data - high signal-to-noise ratio - high-quality stations MOTIVATION High-quality stations: Non-DC components of moment tensors require: 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

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

One station with unknown amplification C (N+1) Stations with known and unknown amplifications JOINT INVERSION FOR MANY EVENTS

Generation of datasets of 200 events with synthetic shear focal mechanisms. Calculation of synthetic amplitudes. Contamination of the amplitudes with random noise. NUMERICAL MODELLING 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.

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

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

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

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. COMPLETE NETWORK CALIBRATION

(a) (b) KAC 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). True station amplifications Retrieved station amplifications ACCURACY Relative errors Inversion of P-wave noisy amplitudes of 22 stations, 200 shear events

Uncalibrated network DC componentNon-DC componentsRMS high RMS values significant false non-DC MOMENT TENSORS DC componentNon-DC componentsRMS Calibrated network low RMS values less scattered non-DC noisy amplitudes, uncorrected amplifications noisy amplitudes, corrected amplifications

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

KAC HRED TRC 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 ACCURACY Amplifications Amplification errors

DC is stable MOMENT TENSORS Uncalibrated network DC componentNon-DC componentsRMS DC componentNon-DC componentsRMS Calibrated network Low RMS A compact cluster of non-DC Non-DC: less compressive

Dataset DC [%] CLVD [%] ISO [%]RMS Uncorrected moment tensors Corrected moment tensors The average CLVD changed from -16.3% to -9.7%. The average RMS is reduced from 0.19 to MOMENT TENSORS

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).

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