AEIC Moment Tensor Solutions

The real-time earthquake detection system at AEIC is based on the Antelope software package from BRTT, Inc. Multiple additional modules have been developed to fit particular needs of the AEIC. Automatic earthquake locations are searched over a pre-calculated three-dimensional grid to find the best fit for the set of arrivals included into an event trigger packet. Once the event is located, its magnitude is calculated. Location and magnitude along with the set of associated arrivals and other information are written into the real-time earthquake database. A moment tensor inversion program is then triggered by a module that watches continuously the real-time earthquake database. When a new event above a certain magnitude level (M 4.0) has been recorded, it triggers the execution of the moment tensor inversion module. The procedure consists of several steps. First, the waveforms are extracted for the broad-band stations. If the waveforms within a certain epicentral distance are available, then the moment tensor inversion is performed. The moment-tensor inversion is based on a software package by Douglas Dreger as used at the Berkeley Seismological Laboratory for automatic moment tensor calculations. It performs a time domain inversion of three-component seismic data for the seismic moment tensor. It uses a library of precalculated Green's functions to compute synthetic seismograms for a range of source depths (from 5 to 200 km with 5 km interval). Currently, we use 3 regionalized velocity models: (1) Aleutian Islands region east of 157W longitude; (2) central Alaska region north of 62.5N latitide; (3) southern Alaska region south of 62.5N latitude and east of 157W longitude. The epicentral location is given by the automatic event location system. Synthetic seismograms are compared with the actual data and the result with the best fit is identified as the inversion output. The program generates a series of output files including postscript graphics file with the actual and synthetic waveforms and the best fit moment tensor parameters, a map with the earthquake location and the focal mechanism obtained, and an ascii file with the moment tensor parametric data. The automatic moment tensors are reviewed by the data analyst on the same or next business day and necessary adjustments are performed. The updated information replaces automatic solutions on the web.

Updated: January 2009