Impact of Flat Slab Subduction on Earthquake Ground Motions in South Central Alaska Including the 2018 M7.0 Anchorage Inslab Earthquake

Date: 4/26/2019

Time: 11:00 AM

Room: Cascade II

The attenuation of subduction zone earthquake ground motions can be affected by flat slab subduction beneath a region as well as directivity and site conditions. Recent crustal and subduction earthquakes in south central Alaska, including the 2018 M7.0 Anchorage event, demonstrates these effects. Flat slab subduction puts oceanic crust beneath continental crust with little or no intervening hotter mantle material. Such a condition occurs beneath the Cook Inlet and Kenai Peninsula region of Alaska and allows seismic waves to propagate through less attenuating crusts to the surface causing an increase in observed ground motions. The 2015 heat flow map of Alaska shows reduced heat flow in this same region. Ground motion observations from the 2018 M7.0 Anchorage AK earthquake show significantly higher ground motions than current subduction ground motion prediction equations within 50-100 km of the epicenter. At short periods ground motions show reduced amplitudes (PGA ~0.2g, 0.2s ~0.6g) due to sediment nonlinear effects in the Anchorage area, reducing the damage potential of the earthquake. Duration of shaking was too short for widespread liquefaction effects unlike during the 1964 M9.2 earthquake. At long periods ground motions are little affected by sediment nonlinearity and remain higher than expected (up to 0.5g at 1s). Other earthquakes show similar increases in ground motions in the Cook Inlet and Kenai Peninsula region. Ground motions from the 2016 Iniskin M7.1 inslab earthquake show higher than normal ground motions in the Cook Inlet region, in part due to directivity. The 2015 Redoubt M6.3 inslab earthquake also shows increased ground motions in the Cook Inlet region without directivity effects. Crustal Q estimates from Lg waves also show less attenuation (higher Q) in south central Alaska. In the larger south central Alaska region crustal Q(f) = 336f^0.34 compared to Q(f) = 200f^0.8 in other portions of Alaska.

Presenting Author: Chris H. Cramer

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