transform boundary
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Abstract The 1972 Mw 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 ∼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 Mw = 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.

The paper seeks to explain in new ways the change in the geodynamic regime at the eastern margin of the Eurasian paleocontinent in the Early Cretaceous from the convergent type of plate boundary to the transform boundary. Certain global geodynamic characteristics were analyzed in the 200-65 Ma interval, which reflect the processes both at depth and on the Earth’s surface and are seemingly unrelated directly to the formation of the transform boundary in the east of Asia. The change of geodynamic regimes is found to occur within the 115 to 110 million years ago time range.

SUMMARY The 280-km-long San Cristobal Trough (SCT), created by the tearing of the Australia plate as it subducts under the Pacific Plate near the Solomon and Vanuatu subduction zones, has hosted strike-slip earthquake sequences in 1993 and 2015. Both sequences, which likely represent a complete seismic cycle, began along the oldest section of the SCT—the portion farthest from the tear that has experienced the most cumulative displacement—and migrated to the younger sections closer to the tear. The SCT's abundant seismicity allows us to study transform boundary development—a process rarely observed along a single fault system—through observations of earthquake rupture properties. Using the spectral ratio method based on empirical Green's functions (EGFs), we calculate the corner frequencies of three Mw ∼7 2015 earthquakes and colocated smaller earthquakes. We utilize two different spectral ratio stacking methods and fit both Brune and Boatwright models to the stacked spectral ratios. Regardless of stacking methods and spectral models, we find that the corner frequencies of the 2015 Mw ∼7 earthquakes decrease slightly with distance from the tear. Assuming a constant rupture velocity and an omega-square spectral model, this corner frequency decrease may be due to an increase in rupture length with distance from the tear. The spectrum of the 2015 earthquake farthest from the tear also deviates from the omega-square model, which may indicate rupture complexity. Stress drop estimates from the corner frequencies of the 2015 Mw ∼7 earthquakes range between 1 and 7 MPa, whereas stress drop estimates of their EGFs range from ∼0.05 to 10 MPa with most values between 0.1 and 1 MPa. Independent evidence from a second moments analysis of the 2015 earthquake sequence also indicates a possible increase in rupture length with distance from the tear, confirming the results from the spectral ratio analysis. We also observe an increase in normalized centroid time-delay values, a first-order proxy for rupture behaviour, with distance from the tear for the 2015 sequence. A similar trend for the 1993 sequence suggests that earthquake rupture varies systematically along the SCT. Since distance from the tear corresponds to cumulative fault displacement, these along-strike rupture variations may be due to a displacement-driven fault maturation process.