Isolating along‐strike variations in the depth extent of shallow creep and fault locking on the northern Great Sumatran Fault

Abstract

The Great Sumatran Fault system in Indonesia is a major right‐lateral trench‐parallel system that can be divided into several segments, most of which have ruptured within the last century. This study focuses on the northern portion of the fault system which contains a 200‐km‐long segment that has not experienced a major earthquake in at least 170 years. In 2005, we established the Aceh GPS Network for the Sumatran Fault System (AGNeSS) across this segment. AGNeSS observes large displacements which include significant postseismic deformation from recent large megathrust earthquakes as well as interseismic deformation due to continued elastic loading of both the megathrust and the strike slip system. We parameterize the displacements due to afterslip on the megathrust using a model based on a rate‐ and state‐dependent friction formalism. Using this approach, we are able to separate afterslip from other contributions. We remove predicted deformation due to afterslip from the observations, and use these corrected time series to infer the depth of shallow aseismic creep and deeper locked segments for the Great Sumatran Fault. In the northern portion of this fault segment, we infer aseismic creep down to 7.3 ± 4.8 km depth at a rate of 2.0 ± 0.6 cm/year. In the southwestern portion of the segment, we estimate a locking depth of 14.8 ± 3.4 km with a downdip slip rate of 1.6 ± 0.6 cm/year. This portion of the fault is capable of producing a magnitude 7.0 earthquake.

Co-seismic offsets due to two earthquakes (M w 6.1) along the Sumatran fault system derived from GNSS measurements

“…The estimated stress drop in the current work was relatively small compared to those found in other studies (e.g., Allmann and Shearer 2009). In this region, Ito et al (2012) observed that the creep region, which is close to the broken segment, may be relatively weaker than other regions. Our estimated low stress drop may thus be consistent with that reported by Ito et al (2012).…”

“…In this region, Ito et al (2012) observed that the creep region, which is close to the broken segment, may be relatively weaker than other regions. Our estimated low stress drop may thus be consistent with that reported by Ito et al (2012).…”

“…We included the permanent International GNSS Service (IGS) sites (KUNM, PIMO, HYDE, and COCO), the IGS final ephemeris, earth rotation parameters, ionosphere model parameters, and differential code biases for satellites and receivers. The AGNeSS observed the post-seismic deformation exceeding 90 cm for 9 years following the 2004 Sumatra-Andaman earthquake (Ito et al 2012). Figure 1c shows the time series of displacement at the Aceh GNSS site.…”

“…In March 2005, we established the Aceh GNSS Network for the Sumatran Fault System (AGNeSS) in the northwestern part of the Sumatran island (Ito et al 2012;Gunawan et al 2014). The AGNeSS consisted of seven continuous and 17 campaign GNSS sites spanning the northwestern segment of the GSF system.…”

“…We estimated the co-seismic offset at the AGNeSS site using a correction for post-seismic deformation due to the 2004 earthquake. In order to remove the deformation at the campaign GNSS site, we applied post-seismic deformation modeling based on Ito et al (2012). After correction, we estimated the co-seismic offset due to the event.…”