The determination of source mechanisms of small earthquakes and revised models of local crustal structure by moment tensor inversion

We describe a procedure for retrieving the source mechanism of small earthquakes and a revised model of crustal structure in heterogeneous media based on interactive inversion of high frequency seismograms and demonstrate its application to three local earthquakes. The earthquakes are located along the Rukwa rift, south-western Tanzania, and were recorded by a local network in the rift. Due to strong lateral crustal heterogeneity in the area, the procedure involves a single-station interactive inversion for moment tensor solution and modelling of the crustal structure along the propagation path between the source and the station. Based on this source solution and the a priori crustal structure, synthetic seismograms are generated and compared with observed displacement seismograms at other stations where the event has been recorded. Deviations between the calculated and observed seismograms are indicated by time shifts and amplitude mismatches on prominent phases; these are employed to correct the phase arrivals used to invert for the partial multistation source solution. The moment tensor inversion of high frequency seismogram data in this study is based on a point source approach. The process includes generation of synthetic seismograms using the frequency-wavenumber integration technique for an elastic, horizontally-layered, medium. Moment tensor inversion is carried out by minimizing the L2 norm of the difference between synthetic and observed displacement data under a deviatoric condition. Results for the three small earthquakes show good agreement between single station and multistation source solutions. The inversions for these events indicate dominantly dip-slip mechanisms with seismic moment magnitudes in the range 2.0 ≤Mw≤ 3.3.