A Tale of Two Earthquakes: Two Mw 7.1 Earthquakes in Alaska Reveal the Importance of Deep Earth Structure on Ground Motion

Date: 4/26/2019

Time: 08:30 AM

Room: Cascade II

Two Mw 7.1 intraslab earthquakes occurred in Alaska within the subducting Pacific plate in 2016 and 2018 and were well recorded on the Transportable Array broadband seismometers and accelerometers. The January 24, 2016 Mw 7.1 Iniskin earthquake occurred at 125 km depth, 250 km southwest of Anchorage. Unexpectedly, accelerations exceed 0.2 g at 250 km distance from this earthquake. The near-Anchorage earthquake of November 30, 2018 occurred at 45 km depth, 14 km northwest of Anchorage, and generated high accelerations nearby. Conventional models for ground motion captured the variation with distance of the near-Anchorage earthquake across the region, while the same models under-predict accelerations from the intermediate-depth Iniskin earthquake by at least an order of magnitude in greater Anchorage and across the Kenai peninsula. Peak amplitudes of weak-motion signals show similar amplification patterns for 227 intermediate-depth earthquakes (IDEs) near the Iniskin hypocenter (M=3-6), indicating that the acceleration pattern from the Iniskin earthquake is due to path effects and not source effects. All show a sharp transition between low amplitudes in the arc and back-arc and high amplitudes over the forearc, a hallmark of variable seismic attenuation. We measure >10x variation in Q between forearc and backarc paths from IDEs near the Iniskin earthquake hypocenter. Teleseismic body-wave attenuation varies with a similar pattern as regional earthquakes. The pattern resembles predictions from thermal models featuring a hot sub-arc mantle and a cold slab and forearc, which predict similar variations in attenuation. Thus, mantle thermal structure has a major effect on earthquake hazard.

Presenting Author: Michael Mann

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