Tectonic model of the Pacific‐North American Plate Boundary in the Gulf of Alaska from broadband analysis of the 1979 St. Elias, Alaska, earthquake and its aftershocks

Abstract

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 9.4x1019N-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 1.3x102ø 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 bound...

Mechanism of the 1992 Erzincan earthquake and its aftershocks, tectonics of the Erzincan Basin and decoupling on the North Anatolian Fault

“…Errors are one standard deviation, and they are not multiplied by a factor unlike those ofEstabrook et al (1992).…”

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“…The leading, structurally lowest thrust is active, based on observations from the 1979 St. Elias earthquake (e.g., Estabrook et al, 1992) and from global positioning system (GPS) observations (Freymueller et al, 2008;Elliott et al, 2010;Elliott, 2011). Bruns and Schwab (1983) referred to this leading thrust as the Malaspina fault.…”

“…The only subsurface constraints are our analysis of the Chaix Hills #1 well and our assumption that the active thrust décollement projects downward through the aftershock zone to the primary rupture zone of the 1979 earthquake. Our depth estimate for the main thrust is shallower than that of Estabrook et al (1992) because we have used a revised velocity model and associated earthquake relocations based on the STEEP seismic array (G. Pavlis, 2010, personal commun. ).…”

Relocations of Earthquakes (1899–1917) in South-Central Alaska

“…2) (ESTABROOK et al, 1992). Two great earthquakes (M S > 7.8) occurred in the Yakutat region on 4 and 10 September, 1899 (events 1899b and 1899d) along with over five aftershocks/ foreshocks of M S > 6.8.…”

“…2). ESTABROOK et al (1992) and PE´REZ and JACOB (1980) suggest that the Pamplona fault zone represents the North Figure 2 Yakutat study area. Black dots are relocated events, triangles are original locations.…”