Rupture of a seismic gap in southeastern Alaska: the 1972 Sitka earthquake (Ms 7.6)

Abstract

The Sitka, Alaska, earthquake of July 30, 1972 (Ms = 7.6) is one of the largest strike-slip events since 1963. This earthquake is thought to have ruptured the Pacific-North America plate boundary segment between the large strike-slip events in 1949 and 1958, and the Sitka earthquake was heralded as one of the first successful `forecasts' of an earthquake based on the seismic gap hypothesis. Another motivation to study this event is the availability of a strong-ground motion recording ~ 50 km from the epicenter, which is near the middle of the 180 km long plate boundary segment. The teleseismically located aftershocks occur only in the southern half of this segment. We have analyzed Rayleigh waves and P waves recorded by WWSSN long-period vertical instruments. A total of 25 very-long-period Rayleigh phases (R2-R5) define a symmetric four-lobed radiation pattern from which we can infer a nearly pure right-lateral strike-slip fault, striking 340[deg] (N20[deg]W), and a bilateral rupture. Comparison of synthetic and observed Rayleigh waves yields a seismic moment of 3 x 1027 dyn cm (MW = 7.6). The P waves are characterized by a dominant complex pulse in the first 40 s, and a succession of smaller pulses after that. The source time functions are deconvolved from 32 P waves with good azimuthal distribution to obtain the moment, depth extent, and rupture history. The mean moment of the source time functions is 5.0 +/- 2.4 x 1027 dyn cm, and the depth extent is 0-10 km. Tomographic inversion of the source time functions produces an image of the earthquake rupture process. This procedure maps the moment release of the complex pulse into two high-displacement regions (i.e., asperities): (1) the epicentral area to 40 km north, and (2) between 60 and 90 km south of the epicenter. The main asperity just north of the epicenter coincides with a quiescent zone of microaftershocks. While the overall average seismic displacement is 6 m, the displacement at the main asperity is 7 m, the same value as the tectonic displacement accumulated since the previous earthquake in 1848. The fault averaged stress drop is 100 bars, while the asperity stress drop is 130 bars. Tomographic imaging of the source time functions thus resolves several features of the rupture process, notably the bilateral propagation and rupture initiation at the largest asperity. This is in contrast to the large 1976 Guatemala strike-slip earthquake, where the largest displacements occurred away from rupture initiation.