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For the first time a dense seismic network (AcehSeis) consisting of 30 short-period seismic stations was installed for 10 months from July 2014 in order to record crustal earthquakes in Central Aceh, Northern Sumatra, following the hazardous earthquake with magnitude Mw. 6.1, July 2nd, 2013. The objective of this research is to investigate the active fault characteristics derived from local earthquake data combined with a digital elevation model. The network recorded more than 1790 local events occurring along the main Great Sumatran Fault (GSF) and its secondary faults. Local earthquakes with azimuthal gap angles greater than 180° were excluded to avoid non-unique earthquake locations. Therefore from all recorded earthquakes, 1127 local earthquakes were used for further seismological analysis. A method of simultaneous earthquake relocation and 1D velocity determination was applied to obtain better location of earthquakes. The output of the first relocated seismicity was then relocated again by using a double difference relocation method. After relocation, earthquakes are concentrated in a narrow zone along the active fault, indicating the reliability of the earthquake location. We then determined the focal mechanism of earthquakes, based on the first polarity picks. The seismicity distribution correlates with the well-known GSF and its main secondary faults. Earthquake locations also clearly revealed several active segments which include the Batee, Beutong, Tangse-Geumpang, Lampahan, Aceh and Seulimeum, and Nisam faults. Also less-known faults as the Pamue and Atu Lintang-Peusangan faults are imaged. In Northern Aceh an unknown earthquake cluster associated with an active fault, the Nisam Fault was detected, which is also visible in the topography. The seismicity along GSF is offset at around 96.7 E, constrained by the nearly N-S striking Atu Lintang-Peusangan Fault confirming the segmentation of GSF at this longitude. The Aceh segment and the Seulimum Fault in the northernmost region of the network are both active and clearly imaged by the seismicity. The Pamue and the Lampahan Faults seem to be the splay of GSF as indicated by identical strike direction as the GSF. The Lampahan fault is characterized by NW-SE seismicity lineation, which is in agreement with the focal mechanisms. The Laut Tawar Lake could be a part of the Lampahan Fault that is also offset at ∼96.7 E at the line of the Atu Lintang-Peusangan fault. The Burni Geureudong Fault on the other hand strikes ENE-WSW which suggests a conjugate fault of the Lampahan Fault.

On 6 December 2016, an Mw 6.5 earthquake occurred in Pidie Jaya, Aceh, about 30 km to the north of the Sumatran fault (SF) that killed more than 100 people and destroyed ∼3000 buildings. Mainshock focal mechanism inversions using regional Agency for Meteorology, Climatology, and Geophysics of Indonesia (BMKG) broadband data and teleseismic waveform data all indicate a strike-slip event with a centroid depth of 11-15 km. The observed macrointensity data show that most of the damaged buildings are distributed along the coast, approximately perpendicular to the ruptured fault strike instead of parallel with it. The strong shaking and damage sites are primarily located on the coastal sedimentary soils, highlighting the importance of site conditions in determining risk. We used one-month data recorded by nine temporal broadband stations to locate aftershocks with gridsearch and double-difference algorithms, thereby resolving a linear trend of seismicity aligned in a northeast-southwest direction. The refined aftershock locations indicate a leftlateral rupture that is in agreement with the preliminary finite-fault slip inversion as well as geomorphic signatures of local geological structure. Using a well-located ML 4.2 aftershock for path calibration, we relocated the mainshock epicenter with regional P-wave arrivals. The refined epicenter falls within the cloud of the well-located aftershocks, whereas locations from the global and regional catalogs are located 10-20 km away. Aftershock focal mechanisms determined by the first motion reveal similar solutions as the mainshock. This earthquake sequence ruptured a previously unidentified fault that is either located at the west of the fault that produced the 1967 Mw 6.1 earthquake sequence or is actually at the same fault. The Pidie Jaya earthquake and other off-SF events suggest strong distributed crustal deformation in Aceh, highlighting the need for better understanding of active faulting and seismic hazard in this region. Accepted version

Lack of observation network in the vicinity of oceanic intraplate earthquake lead the estimation of coseismic fault slip with high uncertainty. Satriano et al. [2] and Wei et al. [3] found NNE trending left-lateral slip as the primary features. In another hand, Yue et al. [4] and Hill et al. [5] proposed WNW trending right-lateral faults structure as the main characteristic. Here, we investigate the coseismic fault model that could explain the coseismic offset both vertical and horizontal in a 3-D heterogeneous earth structure. We constructed finite element model that include three-dimensional velocity structure, topography/bathymetry, spherical-earth and subducting slab. In this study, we employed scaling slip to adjust slip amount and total seismic moment. Instead of original slip amount, we preserve seismic moment as a basis comparison. Based on vertical and horizontal observation data, WNW trending right-lateral fault could fit better than NNE trending left-lateral fault. The present study demonstrates best-fit calculation using scaling slip optimized to the horizontal or vertical observation lead the both fault model worsen the misfit of vertical or horizontal component, respectively. This result analysis indicates a trade-off between vertical and horizontal component and reflects the importance of revisiting the fault slip modeling incorporating vertical and horizontal data equally.Lack of observation network in the vicinity of oceanic intraplate earthquake lead the estimation of coseismic fault slip with high uncertainty. Satriano et al. [2] and Wei et al. [3] found NNE trending left-lateral slip as the primary features. In another hand, Yue et al. [4] and Hill et al. [5] proposed WNW trending right-lateral faults structure as the main characteristic. Here, we investigate the coseismic fault model that could explain the coseismic offset both vertical and horizontal in a 3-D heterogeneous earth structure. We constructed finite element model that include three-dimensional velocity structure, topography/bathymetry, spherical-earth and subducting slab. In this study, we employed scaling slip to adjust slip amount and total seismic moment. Instead of original slip amount, we preserve seismic moment as a basis comparison. Based on vertical and horizontal observation data, WNW trending right-lateral fault could fit better than NNE trending left-lateral fault. The present study demonstrate...

We investigate the postseismic deformation of the 2004 Sumatra–Andaman earthquake (SAE) using 5 years of Global Positioning System (GPS) data located in northern Sumatra. Continuous GPS data from northern Sumatra suggest that the relaxation time in the vertical displacement is longer than horizontal displacements. This implies that there are multiple physical mechanisms that control the postseismic deformation, which refer to afterslip and viscoelastic relaxation. In this study, we introduce an analysis strategy of postseismic deformation to simultaneously calculate multiple mechanisms of afterslip and viscoelastic relaxation. The afterslip inversion results indicate that the distribution of the afterslip and the coseismic slip are compensatory of each other. Also, afterslip has a limited contribution to vertical deformation in northern Sumatra. In our rheology model, we use a gravitational Maxwell viscoelastic response and the result indicates that the elastic layer thickness is 65 ± 5 km and the Maxwell viscosity is 8.0 ± 1.0 × 1018 Pa s. We find that afterslip plus Maxwell viscoelastic relaxation are appropriate to explain the deformation in northern Sumatra. We also find that our rheology model reproduces the long-term features of the GPS time series in Thailand. Applying our rheology model to the data in Andaman Islands our afterslip estimation is located at the down-dip part of the plate boundary. Finally, we showed that our rheology model is applicable to the GPS datasets of postseismic deformation of the 2004 SAE located in northern Sumatra, Thailand, and Andaman–Nicobar, respectively.

Since the 2004 Sumatra–Andaman earthquake (M w 9.2), the northwestern part of the Sumatran island has been a high seismicity region. To evaluate the seismic hazard along the Great Sumatran fault (GSF), we installed the Aceh GNSS network for the Sumatran fault system (AGNeSS) in March 2005. The AGNeSS observed co-seismic offsets due to the April 11, 2012 Indian Ocean earthquake (M w 8.6), which is the largest intraplate earthquake recorded in history. The largest offset at the AGNeSS site was approximately 14.9 cm. Two M w 6.1 earthquakes occurred within AGNeSS in 2013, one on January 21 and the other on July 2. We estimated the fault parameters of the two events using a Markov chain Monte Carlo method. The estimated fault parameter of the first event was a right-lateral strike-slip where the strike was oriented in approximately the same direction as the surface trace of the GSF. The estimated peak value of the probability density function for the static stress drop was approximately 0.7 MPa. On the other hand, the co-seismic displacement fields of the second event from nearby GNSS sites clearly showed a left-lateral motion on a northeast–southwest trending fault plane and supported the contention that the July 2 event broke at the conjugate fault of the GSF. We also calculated the Coulomb failure function ΔCFF caused by the first event to evaluate its effect on the second event. The results showed that the July 2 event was likely brought 0.1 MPa closer to failure by the January 21 event.

We have conducted geodetic and geomorphic observations in the northernmost part of Sumatra, Indonesia to monitor strain accumulation in the vicinity of the northern Sumatran fault. Evaluation of the earthquake generation potential in this region is highly urgent because of a large fault slip rate, absence of major earthquakes for more than 100 years and recent Coulomb stress increase on the fault due to the 2004 Sumatra-Andaman earthquake (M w 9. 2). We have deployed Aceh GPS Network for the Sumatran Fault System (AGNeSS) since 2005. The data collected have been used for estimating slip/locking distribution of the Sumatran fault and constructing a comprehensive model for postseismic deformation after the 2004 event. Tectonic geomorphic features are also important to reveal long-term slip history of the fault. We have used high-resolution stereo-paired ALOS (Advanced Land Observing Satellite) PRISM (Panchromatic Remote-sensing Instrument for Stereo Mapping) satellite images to map the surface trace of the Sumatran fault and conducted field observations to ensure the trace by geomorphic and geologic evidence. We introduce preliminary results of the fault mapping together with a brief description of the crustal deformation field detected by GPS.

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.