Tectonic Model of the Pacific-North American Plate Boundary in the Gulf of Alaska from Broad-Band Analysis of the 1979 St-Elias, Alaska, Earthquake and Its Aftershocks

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Journal of Geophysical Research-Solid Earth
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May 10
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The St. Elias, Alaska earthquake of 28 February, 1979 (M(s) 7.2) is reanalyzed using broadband teleseismic body waves and long-period surface waves because of unresolved questions about its depth, focal mechanism, seismic moment, and location in a seismic gap. Teleseismic waveforms are simultaneously inverted to determine the source mechanism, seismic moment, rupture history and centroid depth. These data are well modeled with a point source propagating in the ESE direction with an average kinematic rupture velocity of 2.5 km/s. The best-fitting source mechanism indicates underthrusting on a NE-dipping plane. The mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the Pacific-North American plate boundary. These depths are substantially different from those of earlier body wave studies and regional seismic network aftershock depth determinations but are in accord with the Harvard Centroid-Moment Tensor and International Seismological Centre determinations. The seismic moment determined from body waves is 9.4 x 10(19)N-m (M(w)7.3). The spatial and temporal distribution of moment release indicates that the St. Elias earthquake was a complex rupture consisting of two distinct subevents within 38 s of the initial onset, followed by low moment release during the next 34 s. Earlier studies indicated an unusual amount of surface wave energy at very long periods (> 200 s) that led some workers to suggest that St. Elias was a "slow" earthquake. Our broadband modeling does not require more than 34 s of additional moment release after the first two subevents. Moreover, we are able to match the phase and amplitude of 200-s Love and Rayleigh waves with a thrust fault point source of moment 1.3 x 10(20)N-m (M(w)7.4) located at the body wave centroid. The moment difference is not discernible with body waves for moment evenly distributed over 72 s. Thus, the St. Elias earthquake is not slow with respect to 200-s surface waves but is complex with regard to the broadband body waves. Upper plate structure apparently controlled the gross characteristics of rupture. The rupture direction parallels mapped upper plate faults. Rupture propagated unilaterally to the ESE, with little initial moment release, as a shallow, north-dipping thrust that later changed to more steeply NE dipping with a large right-lateral strike-slip component. The locations and source mechanisms of these subevents and locations of aftershocks define a shallow dipping surface at the eastern edge of the Pacific plate. Moreover, the component of strike-slip motion increases with time in the mainshock implying that the transition to strike-slip faulting occurs along the plate interface. The estimated nucleation point of the second subevent coincides with a large concentration of aftershocks interpreted as representing a barrier to continuous rupture associated with the northern-most boundary of the Yakutat terrane. Joint relocation of aftershocks suggests that the main plate boundary may be offset vertically by 5-10 km as a result of this structure. The southern part of the aftershock zone, while containing many aftershocks, appears not to have ruptured coseismically, but may have failed later by aseismic creep as seen in geodetic measurements.Faults associated with the Malaspina fault system (the onshore extension of the Aleutian trench) appear to be the surface expression of the underthrusting plate boundary; however, upper plate deformation is widespread because of the collision of the Yakutat terrane. The convergence direction may explain the lack of a highly active Wadati-Benioff zone downdip of the St. Elias zone. The neotectonic deformation of the Chugach-St. Elias mountains is probably related to collision and subduction of the Yakutat terrane: A terrane in the process of accreting and subducting will cause considerable upper plate deformation over a wide zone. Once subduction of a terrane has begun, deformation may then become localized.


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