Source retrieval from broadband regional seismograms: Hindu Kush region

Abstract

A method of relocation and source characterization of small earthquakes using one modern regional station is developed and tested. First, we model teleseismic body waves of two events, which are used as masters. Short period depth phases, pP and sP, are used to establish the epicentral depth, and the events are relocated using calibrated stations and a mantle model derived for this region, TIP. The events moved upward by 12 and 31 km, respectively. The regional waveforms recorded at the IRIS station GAR from the best-determined source are forward modeled to establish a local crustal model. A four-layered model with a thickness of 65 km proves effective. Synthetics from this crustal model can then be compared with the data from other events where the depths are poorly known, i.e. PDE depths of 33 km. Forward modeling of the short period phases allows better estimates of depth and makes relocation possible. Next, we perform a long period inversion of whole and partial waveform data to obtain source mechanism and moment, and repeat the procedure if necessary. This technique is applied to events in the tectonically active Pamir-Hindu Kush region to test its usefulness. Five crustal events sampling various azimuths are presented as examples of relocation and determination of source mechanisms from small events. The smallest event has a moment of 5.1 × 10 22 dyn cm. For the seven events we studied, the average depth correction to PDE is 19 km, and the location correction is 11 km on average. The method can be used to identify earthquakes and thus it lowers the threshold below present m b: M S discrimination, since these events are too small to be seen teleseismically. Secondly, these events can be used as ‘masters’ in the calibration of other systems, case-based event characterizations, etc.