SOURCE SCALING OF EARTHQUAKES IN THE SHUMAGIN REGION, ALASKA - TIME-DOMAIN INVERSIONS OF REGIONAL WAVE-FORMS

The scaling of pulse duration to seismic moment is estimated for earth quakes along an interplate thrust zone, from digital waveforms recorde d by short-period and broad-band instruments of the East Aleutian (Shu magin) Seismic Network. We measure pulse duration using an empirical G reen's function technique based on damped time-domain deconvolution. F rom several thousand events, 22 earthquakes with magnitudes 3.0-7.0 an d depths 23-56 km are found to give reliable estimates of pulse durati on. Durations are also determined directly from one-parameter nonlinea r inversions, for a variety of simple functional forms of source time functions. Symmetric source pulses (boxcar or triangle shapes) fit wav eforms better than an asymmetric model [t exp(-2t/D)] for most (62 per cent) of the waveform pairs, while the asymmetric model fits best for only 8 per cent of the data. Pulse duration increases with the size o f events, from 0.1 to 10s over the seismic moment (M(0)) range of 10(1 4) to 3 x 10(19) Nm. When normalized by the cube root of seismic momen t, pulse durations show similar to 8 x variation; comparable static st ress drop estimates range from 0.2 to 135 MPa. Contrary to predictions of some laboratory and theoretical studies, earthquakes at the deepes t part of the thrust zone do not show significantly higher stress drop s than do shallower events. Rupture properties, however, show a strong dependence on earthquake size. The three largest events (M(0) > 5 x 1 0(18) Nm) have the three longest normalized durations, on average 3.8 times longer than those for smaller events. The durations require smal ler events to have 10-100 x larger static stress drops, or similar to 4 x faster rupture velocities, or some combination of the two. Possibl y, the largest events rupture both strong and weak patches while small er events just rupture strong patches on the fault surface. The charac teristic dimension that separates large from small events, 3-15 km, is comparable to characteristic wavelengths of Pacific basin bathymetry and may reflect the influence of the subducted sea-floor upon fault-zo ne heterogeneity.