Geophysical analysis of the 30 July 1972 Sitka, Alaska, earthquake sequence

The 1972 M-w 7.6 Sitka earthquake is the largest historical event along the southeastern Alaska portion of the strike-slip Queen Charlotte fault, the transform boundary between the Pacific and North American plates. The fault is one of the fastest moving transform boundaries in the world, having accumulated enough slip since 1972 to produce an event of comparable size in the near future. Thus, understanding the controls on the rupture process of the 1972 mainshock is important for seismic hazard assessment in Alaska. Following the mainshock, the U.S. Geological Survey installed a network of portable seismographs that recorded over 200 aftershocks. These locations were never published, and the original seismograms and digital phase data were misplaced. However, we were able to scan paper copies of the phase data, convert the data to digital form, and successfully relocate 87 aftershocks. The relocations show two clusters of aftershocks along the Queen Charlotte fault, one similar to 40 km north of the mainshock epicenter and the other just south of the mainshock, both regions adjacent to portions of the fault that experienced maximum moment release in 1972. Many of the northern aftershocks locate east of the Queen Charlotte fault. This pattern is similar to aftershocks observed in the 2013 M-w = 7.5 Craig, Alaska earthquake. Recent and pre-1971 (1925-1970) seismicity indicates that the regions where aftershocks clustered remained active through time. Gravity, magnetic, and bathymetric anomalies suggest that the structural variations in both the Pacific and North American plates (e.g., age, density, rock type, and thickness) play roles in rupture nucleation and termination along the northern Queen Charlotte fault.