Slow slip events from multi-decadal deformation records: The 1966 - 1981 Banyak Islands slow slip event in the Sumatran subduction zone

Abstract:

Previous studies had not identified any slow slip events along the Sumatran subduction zone, other than afterslip following major earthquakes. In the Banyak Islands of Sumatra, coral microatolls reveal a 15-year-long reversal of interseismic vertical displacement from subsidence to uplift between 1966 to 1981. We have examined various oceanographic and tectonic processes that could potentially explain the coral data, and our results show that a long-duration slow slip event is the most likely explanation. Based upon coral records of sea level, we have modeled a range of locations and sizes for this event. Our models suggest that this slow slip event occurred on the portion of the megathrust under the Banyak Islands, within the downdip zone of transitional friction properties from locked to creeping behavior. It had a cumulative moment magnitude of ~M_w 7.3.

To the best of our knowledge, this is the longest-lasting slow slip event known. Its 15-year duration suggests that with increasingly long geodetic records, we will continue to make new discoveries about the varied spectrum of megathrust slip behavior, highlighting both the importance of maintaining multi-decadal GPS networks and the value of paleogeodetic data in extending deformation records further back in time.