Determination of earthquake source mechanisms using teleseismic 30–140 s waves: The January 17, 1994, Northridge earthquake

Abstract

Complete seismic waveforms with periods between 30 and 140 s recorded at teleseismic distances (≥10°) are used to determine the source mechanism of the January 17, 1994, Northridge earthquake (Ms=6.8). An algorithm called the Earth simplifying transformation (EST) is applied to the mainshock (primary event) data to calibrate and remove propagation effects of Earth's lateral and radial heterogeneities that are not predicted by a reference Earth model. The procedure involves frequency domain division of each pair of observed and synthetic waveforms for an auxiliary event with a well‐constrained source mechanism in proximity to the primary event, with the synthetics being computed for a reference Earth model. If the auxiliary event source mechanism is correct, the differences between the data and synthetics represent the residual effects of the Earth's structure that are not predicted by the Earth model. The frequency dependent deconvolution filters are convolved with the corresponding observed signals for the primary event. The resulting waveforms are then used to determine the source mechanism of the primary event by forward modeling and by centroid‐moment‐tensor inversion, with excellent fits to the complete waveforms being achieved. The resulting solution for the Northridge event has a centroid depth of 12±1 km and a mechanism with strike, ϕ=115°, dip, δ=40°, and rake, λ=95°, with 5° uncertainty for each parameter. The EST procedure holds promise for reliable source inversion for small earthquakes in a region calibrated by large events with well‐determined source parameters.