Geological and Geophysical Evaluation of the Mechanisms of the Great 1899 Yakutat Bay Earthquakes

Abstract

We have used tectonic, geologic, and seismologic observations to reevaluate the mechanisms and seismotectonic significance of the two great (M w = 8.1 and 8.2) September 1899 Yakutat Bay earthquakes. In their comprehensive study of these earthquakes between 1905 and 1910, Tarr and Martin (1912) showed that these events were accompanied by shoreline changes in Yakutat Bay that ranged from 14.4 m emergence to 2.1 m submergence, uplift of about 1 m at Yakataga 160 km west of Yakutat Bay, and by several zones of surface fissures on land. Although major earthquake faults were not found, Tarr and Martin postulated that the shoreline displacements were caused by vertical movements on a system of concealed steep normal faults and that the fissure zones on ridges were along subsidiary faults. Our geologic studies in the Yakutat Bay region indicate that: (1) the emergent shorelines along Yakutat Bay define a broad upwarp roughly 50 km x 30 km that is primarily related to reverse slip on local concealed shallowly dipping thrust faults; (2) the reported subsidence was due largely, or entirely, to nontectonic surficial submergence of unconsolidated deposits; and (3) most, if not all, of the zones of surface fractures related to the 1899 earthquakes are sackung that were probably caused by large-scale gravitational slumping of steep slopes, rather than faulting. A small number of early damped seismograms and the vertical uplift data were used to constrain the fault slippage that occurred during the two great earthquakes of 1899. Seismic moments determined from 50-s surface wave amplitudes are ∼2 x 10 21 N m for these two events, equivalent to M w 8.1. Uplift determined from raised shorelines within Yakutat Bay can be accounted for by the 10 September event alone, and these data can be fit by ∼10- to 20-m dip slippage on three dextral oblique thrust faults that dip ∼30° northeast or north. Faulting complexity is also shown by the S-wave seismogram of the 10 September shock, which lasted ∼3 min and shows at least three distinct long-period pulses. The large seismic moment of the 4 September event and uplift of ∼1 m at Yakataga suggest a 150-km westward extension of faulting along the foothills fold-and-thrust zone. Our reassessment suggests that, although some portions of the complex plate boundary zone ruptured in 1899, regional seismic hazard is currently significant. First of all, none of the potentially tsunamigenic offshore thrust faults west of the Pamplona zone slipped in 1899. It is unlikely that all of the onshore thrust faults south of the Chugach-St. Elias thrust fault system did either. Furthermore, more than 100 years of convergence at 48 mm/yr across the region has reloaded faults that slipped in 1899 and added further strains on those that did not. Matters are much less clear for the Yakutat Bay thrusts because although they slipped in 1899, we have no good constraints on the convergence rates across these faults. The most recent pre-1899 uplift event in Yakutat Bay was at least 380 ± 70 years ago.