Depth of faulting during the 1968 Meckering, Australia, Earthquake sequence determined from waveform analysis of local seismograms

Abstract

Waveforms for 11 foreshocks and 48 aftershocks of the Ms 6.8 Meckering earthquake recorded at the World‐Wide Standard Seismograph Network station Mundaring are analyzed to determine the depth distribtion of faulting during this unusual intraplate earthquake sequence. Clear depth phases including sP and Rg are seen in the local Seismograms at distances of 60–95 km and are studied using synthetic seismograms computed using generalized ray theory and wave number integration techniques. Large Rg/S ratios seen on the vertical component short‐period data for many events imply source depths less than 2 km. The short‐period P waveform contains the best depth estimator in the form of sP so that depth can be estimated to within an uncertainty of about 1 km for most events. The foreshocks cluster at less than 2 km depth, and most aftershocks occur within 1 km of the surface. A few aftershocks occur as deep as 7 km. These results are consistent with a previous teleseismic waveform study in which faulting was inferred to start near the surface at 1.5 km depth with rupture proceeding downward and not exceeding 6 km depth. These results, coupled with previous stress studies in the Australian shield and models of crustal strength, show that faulting in a cold shield area is a near‐surface phenomenon and implies that most of the crust is too strong to be fractured.