Creep and seismic rupture of a serpentinite-rich Sumatran fault segment

Earthquake is produced by shear dislocation of rocks across the fault, the frictional status and the area of locked/creeping patches on the fault thus govern the size and occurrence of damaging earthquakes. To better understand these fundamental earthquake physics issues, we deployed over 130 short period seismic nodal stations along the plate boundary type Sumatran fault in Aceh region to cover a segment that was reported to be creeping at various depths. We maintained the nodal array deployment from Jan 2020 to July 2021 by recharging the nodes every 35 days. A machine learning based earthquake detection algorithm was applied to the acquired dataset, which results in a high-resolution seismic catalog that has more than 8000 micro-seismic events. These events clearly delineate the subvertical creeping segment of the Sumatran fault and its Seulimeum branch to the northwest. The seismicity on the creeping segment is almost uniformly distributed from 3 to 12 km in depth, confirming the creeping nature of the fault segment as revealed by geodetic observations, but providing a much more accurate depth constraint. In contrast, the Seulimeum fault branch shows a much deeper seismicity at the depth range of 18 to 25 km, indicating the entire upper crust is fully locked. Sharp stepovers are observed along both strike (~10km) and strike-normal (~4km) directions between the seismicity on these fault segments. The creeping segment of the Sumatran fault, as defined by similar earthquake families, agrees well with the lenses of serpentinite, which has much smaller frictional coefficient that facilitates fault creep. Similar earthquake families show ~ km scale lineation along strike of the fault, where repeating earthquake pairs are identified. However, two shallow Mw6 earthquakes occurred on the creeping segment in the last 25 years. In particular, finite fault inversion of the 2013 Mw6.1 earthquake shows the rupture from 12 km to the surface. These observations suggest a partially creeping/locking or conditionally stable frictional status on the serpentinite-rich segment of the Sumatran fault, that should be considered in both single event and earthquake cycle simulations, as well as seismic hazard assessment.