Ground Motion and Hazard Context for the Anchorage Earthquake

Date: 4/26/2019

Time: 01:30 PM

Room: Cascade II

The 2018 Anchorage earthquake was the most significant earthquake in Alaska in a half century. While neither the source nor the magnitude are tectonically unusual, the close proximity to urban areas make this earthquake exceptional. Compared to most quakes along the Alaska-Aleutian arc, this event caused far more damage and triggered the first federal disaster declaration for an earthquake in Alaska since 1964. However, the earthquake also occurred in one of the most densely instrumented parts of Alaska. As a result, there are excellent ground motion records of the mainshock and the prodigious aftershocks (likely to eclipse 10,000 by the time of the SSA meeting). The combination of good ground motion records and well-documented damage from the earthquake make for a dataset without precedent in Alaska. This presentation will examine a few of the issues and topics raised thus far. The hypocenter was just 12 km north of Anchorage, but was 47 km below the surface. The area was spared the most violent shaking that would have accompanied a similar earthquake at shallow depth. But the depth, together with predominant soil types and topography, created a notably large area of strong shaking. Mercalli Intensities of 7 or higher spanned more than 150 km. Though this earthquake is likely to stimulate considerable discussion of early warning systems, the depth and proximity to Anchorage make this a particularly challenging test case. From a hazards perspective, the earthquake highlights the difficulty of quantifying future intraslab earthquakes. The absence of geologic and geodetic constraints remove some of the primary tools for seismic hazard evaluation. The statistical seismic record is a valid tool but is compromised by the short historical record. A final issue highlighted by this event is the very different styles of ground motion (frequency content and duration) from proximal earthquakes and the much larger but more distant great earthquakes on subduction interface.

Presenting Author: Michael E. West

Authors