Comparison of dynamic source parameters for earthquakes in different tectonic regions of the northern Cascadia subduction zone

Abstract

Based on characteristics of displacement amplitude spectra of near-distant S-waves from 52 earthquakes in the magnitude ( M L ) range −0.6 to 5.4, dynamic source parameters were computed and compared for three tectonic regions in the northern Cascadia subduction zone: a downgoing slab of the Juan de Fuca Plate with earthquake depths of about 30 km; the same slab further east with deeper events, about 60 km; and an overriding part of the North American Plate. No distance dependence of observed corner frequencies was obtained, nor any significant regional difference in source parameters, although there is an indication of smaller stress drop in the North American Plate for larger events. Factors which may distort a regional comparison were examined, but neither source radiation directivity nor propagation path and station site characteristics have significant influence. The structural model has influence, however, and two different density/velocity models were used for the deeper Juan de Fuca slab events. Seismic moments (M 0) range from E + 17 to 0.4 E + 24 dyn cm, source radii from 0.07 to 1.6 km, stress drops from less than 0.1 to 40 bar and average dislocations from less than 0.1 to 7 cm. In the range E + 18 to E + 21 dyn cm, in which the vast majority of the events are confined, both the stress drop and displacement increase rapidly with event size (seismic moment or magnitude) as the seismic moment increases rapidly with fault dimension. The relation between seismic moment (dyn cm) and M L magnitude computed from data from all three regions is similar to relations presented for California and reads log( M 0) = 17.65 + 1.02 M L . There are, however, differences between events in the North American Plate and the Juan de Fuca Plate, with larger M L for the former at equivalent seismic moment.