Determining the Double Couple Moment Tensor for Kamchatka Earthquakes from Regional Seismic Waveforms

The digital seismic network in Kamchatka deployed in 2006-2010 provided a fundamental possibility for calculating seismic moment tensor (SMT) of Kamchatka earthquakes from broadband waveforms recorded by regional stations. The paper describes the method for calculating SMT for the earthquakes with pure double couple (DC) mechanism based on the DC tensor decomposition and explicit partial linear inversion in two (of four) variables. This allows the least square objective function to be expressed in terms of two angles specifying the orientation of the neutral (zero) axis of the DC tensor. The inversion with respect to the angles is conducted using the Levenberg-Marquardt iterative method. This technique was used for calculating focal mechanisms, moment magnitudes, and equivalent point-source depths for 31 Kamchatka earthquakes with moment magnitudes M-w = 4.3-6.2. The comparison of the obtained estimates with the corresponding GCMT catalog data which exist for 19 events has shown fairly reasonable agreement between the results. The angle K characterizing the discrepancy between the DC mechanism calculated directly from the waveforms in our study and the best DC mechanism constructed from GCMT null-trace tensor is at most 25 degrees for 16 events of 19. The moment magnitudes are underestimated by similar to 0.1 relative to the GCMT counterparts and the depths are underestimated by 8 km on average.