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Prayoedhie S.,Indonesian Agency for Meteorology | Fujii Y.,International Institute of Earthquake Engineering and Seismology | Shibazaki B.,International Institute of Earthquake Engineering and Seismology
Bulletin of the International Institute of Seismology and Earthquake Engineering

We conducted numerical simulations to forecast near-field tsunami at ocean bottom tsunami meter (OBTM) and GPS buoy stations along the Sumatra Island. We used modeling codes of TUNAMI-N2 (Tohoku University's Numerical Analysis Model for Investigation of Near-Field Tsunami, No.2) and TUNAMI-F1 (Far-Field Tsunami, No.l) developed by the Disaster Control Research Center (DCRC), Tohoku University, Japan in order to estimate the tsunami height and arrival time using proposed scenarios. For numerical simulation, we used three different scenario earthquakes which are considered along the Sunda trench. The source parameters for scenario earthquakes are based on the coral-reef study by Natawidjaja et al. (2006) for the earthquakes of 1797 (Mw 8.7) and 1833 (Mw 8.6 and 8.9). For the 1797 event (scenario 2), we obtained the tsunami height of 1.547 m with the arrival time of 0.8 min at TM2 offshore output point located in the uplift area, while 0.109 m and 3.9 min for the tsunami height and arrival time, respectively, at TM3 offshore output point located to the south from the source. Based on a simulation result for three scenarios using different bathymetry data (1 arc-minute and 30 arc-second), we found that the tsunami waves reach the coastal area in less than 40 min for the output point located in Padang city (OP2). By using detailed bathymetry data, we can estimate more accurate on tsunami arrival time and tsunami height. Source

Fatchurochman I.,Indonesian Agency for Meteorology | Yagi Y.,University of Tsukuba
Bulletin of the International Institute of Seismology and Earthquake Engineering

We investigated the source process of the 2010 Mentawai earthquake by joint inversion method using near source and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their relative weights on the observed data using ABIC criterion. The teleseismic waveforms were windowed for 150 sec, band-passed between 0.001 and 1.0 Hz, and then integrated into displacement. The strong motion data were windowed for 250 sec, band-passed between 0.005 and 0.5 Hz, and then integrated into velocity. The re-sampling rate for both data set to be 0.25 sec. We estimated the fault area to be 190 × 70 km 2. The main source parameters are as follows: (strike, dip, rake) < (324°, 10°, 94.6°); the depth of hypocenter 13.5 km; the seismic moment 5.814×10 20 Nm (Mw 7.8); source duration 102 sec; and the maximum slip amounts to 3.9 m. The source rupture process obtained probably can be divided into 2 stages. At stage 1, the rupture nucleated near the hypocenter and then propagated to the southwestward and broke the first asperity centering at 14 km from the epicenter. At stage 2, the rupture propagated to the northwestward and broke the second asperity which was centered about 78 km from the epicenter. Our total slip distribution is well consistent with the result of tsunami waveform analysis by Satake et al. (2011). This earthquake was categorized as a tsunami earthquake due to the long rupture duration and the generation of tsunami much larger than expected for its magnitude. Source

Ginanjar G.,Indonesian Agency for Meteorology | Tanioka Y.,Hokkaido University
Bulletin of the International Institute of Seismology and Earthquake Engineering

The source process of the 2010 Mentawai Tsunami is estimated by using tsunami waveforms at a DART station and tide gauge stations around the Indian Ocean. The inversion results show that the major slip region for the October 25 2010 Mentawai earthquake is located near epicenter. The slip amount near the epicenter is about 1.3 to 2.5 m, and the slip amount around the trench is about 1.8 m.. The seismic moment calculated from the slip distribution is 1.07 × 10 21 Nm (moment magnitude Mw= 8.0), larger than that obtained from seismic inversion. However, comparison of the measured tsunami heights at Mentawai islands region with the calculated maximum coastal tsunami heights from slip distribution shows that the simulated tsunami heights is underestimated the measured tsunami heights at Mentawai islands region. The distribution of aftershocks and the area of dominant slip in the slip distribution model from tsunami inversion suggest that the event ruptured up-dip from the point of nucleation (hypocenter) and very near the trench. Such near-trench rupture supports that this earthquake was a "tsunami earthquake". The fringes calculated from InSAR data show one cycle of color corresponding to 2 pi radians phase change. The one color cycle of 2 pi radians phase change would correspond to 2.8 cm of displacement along the radar line of sight (LOS) for South Pagai island of Mentawai islands region. The amount of displacement in the direction of the line of sight calculated from tsunami inversion is also estimated to be one color cycle of 2 pi radians phase change. Source

Zhang L.-F.,Institute of Seismology | Zhang L.-F.,China Earthquake Administration | Fatchurochman I.,Indonesian Agency for Meteorology | Yao Y.-S.,Institute of Seismology | And 4 more authors.
Dizhen Dizhi

On 13, April, 2010, a great earthquake of MW7.0 occurred in Yushu County, Qinghai Province, which is another big one in China since 2008 Wenchuan earthquake. And with seismic wave data, InSAR data and field investigations, many researchers studied the focal mechanism and source rupture process of this earthquake and many valuable results were obtained. However, there are some arguments on the rupture velocity. Some think that this earthquake is a super-shear rupture event, and some insist on opposite opinion. In order to explore whether it is a super-shear rupture event or not, this study chooses the teleseismic wave data recorded by 33 seismic stations with epicentral distances between 30~90 degrees, good azimuth coverage and high signal-noise ratio to reexamine the rupture process using Yagi's program. By comparison of different given rupture velocities in the range of 2.5~5.5 km/s, it is found that rupture velocity of 4.7km/s yields the smallest normalized misfit between the observed and synthetic waveforms. And the inversion result is more in accordance with field observation. The relationship between subfault dimension, rise time and rupture velocity is discussed, which shows that the rupture velocity is not so dependent on the two parameters. And by teleseisemic analyses using an envelope deconvolution method with an empirical Green's function, the location and timing of the high-frequency event also show a rupture velocity of 4.7 to 5.8 km/s, which is apparently greater than the shear wave velocity in this region. By comprehensive analyses, it can be concluded that the super-shear rupture exists in this earthquake. According to our inversion result, the strike, dip, and rake angle of this earthquake separately is 300, 88 and 4. Beach ball shows the seismogenic fault is of strike-slip type, which is consistent with the Ganzi-Yushu Fault. And the rupture extended to the surface on the northwest and southeast segments of the Yushu Fault with the length of 19 km and 31 km. Due to the existence of pull-apart Longbao Basin, the central part where the epicenter is did not rupture. By comprehensive analysis, super shear rupture is one of the main reasons that caused serious damage to Yushu County. Source

Horspool N.,Geoscience Australia | Horspool N.,Institute of Geological & Nuclear Sciences | Pranantyo I.,Bandung Institute of Technology | Griffin J.,Geoscience Australia | And 7 more authors.
Natural Hazards and Earth System Sciences

Probabilistic hazard assessments are a fundamental tool for assessing the threats posed by hazards to communities and are important for underpinning evidence-based decision-making regarding risk mitigation activities. Indonesia has been the focus of intense tsunami risk mitigation efforts following the 2004 Indian Ocean tsunami, but this has been largely concentrated on the Sunda Arc with little attention to other tsunami prone areas of the country such as eastern Indonesia. We present the first nationally consistent probabilistic tsunami hazard assessment (PTHA) for Indonesia. This assessment produces time-independent forecasts of tsunami hazards at the coast using data from tsunami generated by local, regional and distant earthquake sources. The methodology is based on the established monte carlo approach to probabilistic seismic hazard assessment (PSHA) and has been adapted to tsunami. We account for sources of epistemic and aleatory uncertainty in the analysis through the use of logic trees and sampling probability density functions. For short return periods (100 years) the highest tsunami hazard is the west coast of Sumatra, south coast of Java and the north coast of Papua. For longer return periods (500-2500 years), the tsunami hazard is highest along the Sunda Arc, reflecting the larger maximum magnitudes. The annual probability of experiencing a tsunami with a height of > 0.5 m at the coast is greater than 10% for Sumatra, Java, the Sunda islands (Bali, Lombok, Flores, Sumba) and north Papua. The annual probability of experiencing a tsunami with a height of > 3.0 m, which would cause significant inundation and fatalities, is 1-10% in Sumatra, Java, Bali, Lombok and north Papua, and 0.1-1% for north Sulawesi, Seram and Flores. The results of this national-scale hazard assessment provide evidence for disaster managers to prioritise regions for risk mitigation activities and/or more detailed hazard or risk assessment. © Author(s) 2014. Source

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