Following the devastating Mw 9.2 great Sumatra earthquake in 2004, the predominant types of aftershocks varied along the length of the nearly 1,500-kilometer-long megathrust rupture. On the northern Sumatra segment, where the earthquake began, compressional (thrust) earthquakes prevailed, whereas extensional earthquakes dominated in the Andaman segment farther north.
To better understand puzzling aftershock patterns, a team of researchers created eQuakes, a computer program that helps apply basic structural geology principles to the analysis of aftershock sequences using data extracted from the Global Centroid-Moment-Tensor (CMT) database. Here Lister and Forster used the output to conduct a structural analysis of the highly seismogenic Sumatra margin.
The results indicate that the stress state along the megathrust varied widely across a short distance following the great Sumatra earthquake. In the south, the overriding crust was compressed perpendicular to the plate margin, a direction consistent with the relative motion of the adjoining tectonic plates. To the north, however, the researchers found that the overriding crust was stretched perpendicular to the plate margin, a direction that they argue is consistent with gravity-driven motion toward the gravity well that had accrued because of the earlier rollback of the Indian plate’s subducting edge.
This gravity-driven motion was relatively short-lived, however; the team’s analysis of an earthquake cluster in the Andaman Sea that occurred 15 months after the main shock shows that the margin had resumed its plate-driven motion by that time. According to the authors, this transition may be explained by fluid activity that temporarily reduced frictional constraints along the megathrust, allowing gravity to drive the motion while the fault was in a weakened condition.
These findings indicate that abrupt switches between tectonic modes are possible even within the duration of a single earthquake cycle. They also suggest that although plate tectonic–driven motion dominates during interseismic gaps, gravity tectonics may play an important postseismic role. In this case, the crust slid sideways (westward) toward the gravitational potential well caused by the rollback of the subducting edge of the Indian plate. This study has broad implications for understanding both the seismic cycle and the long-term dynamics of mountain building. (Tectonics, https://doi.org/10.1002/2017TC004708, 2018)
—Terri Cook, Freelance Writer