Surface faults on Montague Island associated with the 1964 Alaska earthquake

Abstract

Two reverse faults on southwestern Montague Island in Prince William Sound were reactivated during the earthquake of March 27, 1964. New fault scarps, fissures, cracks, and flexures appeared in bedrock and unconsolidated surficial deposits along or near the fault traces. Average strike of the faults is between N. 37° E. and N. 47° E.; they dip northwest at angles ranging from 50° to 85°. The dominant motion was dip slip; the blocks northwest of the reactivated faults were relatively upthrown, and both blocks were upthrown relative to sea level. No other earthquake faults have been found on land. The Patton Bay fault on land is a complex system of en echelon strands marked by a series of spectacular landslides along the scarp and (or) by a zone of fissures and flexures on the upthrown block that locally is as much as 3,000 feet wide. The fault can be traced on land for 22 miles, and it has been mapped on the sea floor to the southwest of Montague Island an additional 17 miles. The maximum measured vertical component of slip is 20 to 23 feet and the maximum indicated dip slip is about 26 feet. A left-lateral strike-slip component of less than 2 feet occurs near the southern end of the fault on land where its strike changes from northeast to north. Indirect evidence from the seismic sea waves and aftershocks associated with the earthquake, and from the distribution of submarine scarps, suggests that the faulting on and near Montague Island occurred at the northeastern end of a reactivated submarine fault system that may extend discontinuously for more than 300 miles from Montague Island to the area offshore of the southeast coast of Kodiak Island. The Hanning Bay fault is a minor rupture only 4 miles long that is marked by an exceptionally well defined almost continuous scarp. The maximum measured vertical component of slip is 16⅓ feet near the midpoint, and the indicated dip slip is about 20 feet. There is a maximum left-lateral strike-slip component of one-half foot near the southern end of the scarp. Warping and extension cracking occurred in bedrock near the midpoint on the upthrown block within about 1,000 feet of the fault scarp. The reverse faults on Montague Island and their postulated submarine extensions lie within a tectonically important narrow zone of crustal attenuation and maximum uplift associated with the earthquake. However, there are no significant lithologic differences in the rock sequences across these faults to suggest that they form major tectonic boundaries. Their spatial distribution relative to the regional uplift associated with the earthquake, the earthquake focal region, and the epicenter of the main shock suggest that they are probably subsidiary features rather than the causative faults along which the earthquake originated. Approximately 70 percent of the new breakage along the Patton Bay and the Hanning Bay faults on Montague Island was along obvious preexisting active fault traces. The estimated ages of undisturbed trees on and near the fault trace indicate that no major disc placement had occurred on these faults for at least 150 to 300 years before the 1964 earthquake.