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Seismological study in Northern Sumatra; experiments and results

Event Type: 
Seminar
Event Date: 
15 Sep 2017
Venue: 
EOS Seminar Room (N2-01b-38)
Speaker: 
Muskin Umar
About the speaker: 

Born and raised in Aceh, Dr Muksin is now a lecturer and researcher at the Geophysics and Engineering Department of Syiah Kuala University, Banda Aceh Indonesia. He is also the coordinator of geological hazard research group at the Tsunami and Disaster Mitigation Research Centre of the University. After receiving his Physics B.S. degree from Syiah Kuala University and Physics Master degree from Institut Teknologi Bandung (ITB), he obtained his Master and Ph.D. degrees in Geophysics from the Australian National University and GFZ-Potsdam, respectively. Dr. Muksin is particularly interested in the seismological studies and hazard mitigation in Sumatra, motivated by the Great Sumatra-Andaman Earthquake in 2004.

About the event: 

In this talk I will introduce several seismic experiments in Sumatra since 2008 for different purposes. We first used the regional seismic arrivals provided by the International Seismological Center complemented by the Toba and the Australian data to characterize the subduction zone of Southeast Asia and particularly beneath the Sumatra region. We found that the subduction zone in Sumatra is less steep compared to those beneath Java.

In 2011, we installed a dense seismic network with 42 short period seismometers (1 Hz) for 10 months to study the geothermal resources in Tarutung, the Northern Sumatra Province. We recorded 2586 earthquakes which were mostly the aftershocks of the M 5.5 Tarutung earthquake. We revealed the duplex fault system within the Tarutung pull-apart basin. We used three different techniques of seismic tomography including Vp and Vps/Vs (traveltime), attenuation, and ambient noise tomography. The Vp model images the geometry of the basins with the depths of ~2.0 km. High Vp/Vs and high attenuation (low Qp) anomalies are associated with high fluid content in the Tarutung area and with a weak zone caused by volcanic activities along the Sarulla graben. Low Vp/Vs and low conductivity anomalies found is interpreted as dry, compact, and rigid granitic rock. The images suggest that the geothermal manifestations are fed by the same hot fluid below the Tarutung basin. There is no indication that the Martimbang volcano controls the geothermal system around the Tarutung basin.

We also conduct cross-correlations for the ambient noise data of the 40 seismic stations to derive a shallow crustal S-wave velocity model of Tarutung. Group velocities of the Rayleigh waves could be determined in the period range from 0.71 to 4.4 s and then 2D group velocity could be mapped at specific periods. The derived 2D group velocity maps were inverted for a 3D S-wave velocity model. The model shows a region of a strong velocity decrease which coincides with a caldera-like morphological feature. The hidden caldera could act as the heat source of the geothermal manifestations. The weak morphological expression at the surface indicates that the caldera might be older than the Tarutung pull-apart basin.

In 2014-2015, we installed a seismic network with 30 seismometers in Central Aceh for 10 months motivated by the 2013 earthquake with magnitude of M 6.1 followed by a landslide which killed 43 people and destroyed the village. The network recorded more than 1,702 local earthquakes. After simultaneous relocation and velocity determination, the seismicity delineates well several active segments including the Batee, Beutong, Mane-Geumpang, Lampahan, Tripa, and Nisam. The focal mechanisms of earthquakes (based on P polarities) along the Sumatran Fault are right lateral strike slip which is consistent with the characteristic of the Sumatran Fault. The Lampahan segment strikes NW-SE similar to that of the Sumatran fault which is consistent to the fault derived from the elevation model of earth’s surface. We proposed to study the active fault system covering all Aceh region by using seismological data.