Geophysical Studies of the Subsurface Structure of the Castle Mountain Fault System, Upper Cook Inlet, Alaska

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The Castle Mountain fault system (CMFS) lies less than 50 km from Anchorage and represents an important seismic hazard to the Anchorage-Matanuska-Susitna Valley region. The character of seismicity varies along the CMFS. Although its western strand (west of 149.5 W) has produced 4 magnitude > 7 earthquakes within the past 2700 years, it has been seismically quiet over the past 40-50 years. In contrast, the eastern strand is associated with considerable seismicity including the Mw 5.7 1984 Sutton earthquake. We have used gravity and magnetic data, constrained with results of previous seismic reflection and seismic tomography studies, to model subsurface structure of the CMFS from 149 to 151 W. Our models are consistent with high angle reverse faulting (dip~80 degrees) along the CMFS. At the western and eastern ends of our study area the fault system appears to lie at the contact between Cretaceous igneous rock and late Mesozoic sedimentary rock, while in the middle region the fault system appears to have offset late Mesozoic sedimentary rock 1000-2000 m over early Tertiary sedimentary rocks. Comparison of our subsurface models to background seismicity suggest that both the bedrock geology and location of the southwestern edge of the Yakutat microplate influence the current seismic behavior of the CMFS.

Presenting Author: Diane I. Doser

Authors