No abstract

We have compiled the first self‐consistent GPS‐based earthquake catalog for the Sumatran plate boundary. Using continuous daily position time series from the Sumatran GPS Array (SuGAr), we document 30 earthquakes which occurred within or outside the SuGAr network from August 2002 through the end of 2013, and we provide estimates of both vertical and horizontal coseismic offsets associated with 1 M9.2, 3 M8, 6 M7, 19 M6, and 1 M5.9 earthquakes, as well as postseismic decay amplitudes and times associated with 9 M > 7 earthquakes and 1 M6.7 earthquake. For most of the previously studied earthquakes, our geodetic catalog provides more complete coseismic displacements than those published, showing consistent patterns of motion across a large range of distances. For many of the moderate to large earthquakes, we publish their coseismic displacements for the first time, providing new constraints on their locations and slip distributions. For the postseismic time series, we have tackled the challenge of separating the signals for individual events from the overlapping effects of many other earthquakes. As a result, we have obtained either new or much longer time series than previously published. Based on our long time series, we find logarithmic decay times ranging from several days to more than 20 years, and sometimes a second decay time is needed, suggesting that when studying large to great Sumatran earthquakes, we need to consider multiple postseismic mechanisms. Our geodetic catalog provides rich spatial and temporal Sumatran earthquake cycle information for future studies of the physics and dynamics of the Sumatran plate boundary.

Geologic reconstructions of the Main Himalayan Thrust in Nepal show a laterally extensive midcrustal ramp, hypothesized to form the downdip boundary of interseismic locking. Using a recent compilation of interseismic GPS velocities and a simplified model of fault coupling, we estimate the width of coupling across Nepal using a series of two‐dimensional transects. We find that the downdip width of fault coupling increases smoothly from 70 to 90 km in eastern Nepal to 100–110 km in central Nepal, then narrows again in western Nepal. The inferred coupling transition is closely aligned with geologic reconstructions of the base of the midcrustal ramp in central and eastern Nepal, but in western Nepal, the data suggest that the location is intermediate between two proposed ramp locations. The result for western Nepal implies either an anomalous coupling transition that occurs along a shallowly dipping portion of the fault or that both ramps may be partially coupled and that a proposed crustal‐scale duplexing process may be active during the interseismic period. We also find that the models require a convergence rate of 15.5 ± 2 mm/year throughout Nepal, reducing the geodetic moment accumulation rate by up to 30% compared with earlier models, partially resolving an inferred discrepancy between geodetic and paleoseismic estimates of moment release across the Himalaya.