A geophysical study of the Castle Mountain Fault, southcentral Alaska

Abstract

The Castle Mountain Fault (CMF) lies less than 50 km from Alaska's most populous city of Anchorage and represents an important seismic hazard to the Anchorage-Matanuska-Susitna Valley region in southcentral Alaska. The character of seismicity varies along the CMF. Although the western CMF (between 149.5° and 151° W) shows evidence for Late Pleistocene or Holocene reverse and dextral movement, it has been seismically quiet over the past ~50 years of local seismic network operation. In contrast, the CMF east of 149.5° W is associated with considerable background seismicity, including the Mw 5.81984 strike-slip 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 CMF in an effort to determine what may control the variation in its seismic behavior. Our models are consistent with a component of high angle reverse faulting (northwest side up, dip~85°to 87°) that appears to have vertically offset Late Mesozoic rocks 2000 to 2500 m. Lack of data coverage precludes an estimate of its total strike-slip motion. Holocene movement on the CMF appears to be associated with a region where both sides of the fault consist of sedimentary rocks as defined by a gravity and magnetic low. If most of the vertical offset has occurred since the Miocene in response to collision of the Yakutat microplate with North America, then our results are comparable to recent thermochronological and paleoseismic estimates of vertical slip rates. Subduction of the edge of the Yakutat microplate appears to spatially influence seismic behavior along the CMF while the occurrence of the 1964 Great Alaska earthquake has also temporally affected local seismicity.