Time functions of deep earthquakes from broadband and short-period stacks

To constrain dynamic source properties of deep earthquakes, we have systema tically constructed broadband time functions of deep earthquakes by stackin g and scaling teleseismic P waves from U.S. National Seismic Network, TERRA scope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1 , 1992 and July 31, 1995. To directly compare time functions, or to group t hem by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short-period stacks of P waves rec orded by west coast regional arrays. The comparison of broadband with short -period stacks yields a considerable advantage, enabling more reliable meas urement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time-function shape with moment, indica ting successful scaling and minimal effects of attenuation at the periods c onsidered here. The average shape of the envelopes of the short-period stac ks is very similar to the average broadband time function. The main variati ons seen with depth are (1) a mild decrease in duration with increasing dep th, (2) greater asymmetry in the time functions of intermediate events comp ared to deep ones, and (3) unexpected complexity and late moment release fo r events between 350 and 550 km, with seven of the eight events in that dep th interval displaying markedly more complicated time functions with more m oment release late in the rupture than most events above or below. The firs t two results are broadly consistent with our previous studies, while the t hird is reported here for the first time. The greater complexity between 35 0 and 550 km suggests greater heterogeneity in the failure process in that depth range.