A huge earthquake of magnitude M 9.0 occurred at 00:58 (UT), December 26, 2004, in the sea off the west coast of northern Sumatra, Indonesia, followed by a huge tsunami that hit almost all coasts facing the Indian Ocean. We conducted a field survey in the residential area of Banda Aceh, the town of the severest damage by the tsunami, on the west coast of the northernmost point Sumatra, Sigli City, about 80 kilometers east of Banda Aceh three-four weeks after the event. In Banda Aceh, almost all houses in the residential area about 2 km from the coast were swept away, while houses more than 3 km rarely were. Inundation continued about 5 to 6 km from the shoreline. In Lhoknga and several villages on the west coast of Sumatra Island near Banda Aceh, where tsunamis 15 to 30 meters high hit coastal villages, nobody survived. Along the valley about 1 km north of the cement plant, seawater rose to a height of 34.8 m above MSL, which is the highest recorded inundation measured in our survey.
Zona subduksi di sepanjang pantai barat dan sesar aktif yang berada di pulau Sumatera merupakan sumber gempa bumi yang aktif. Upaya peningkatan mitigasi di wilayah Sumatera menjadi bagian yang penting, untuk itu diperlukan suatu studi hazard seismik dan hubungannya dengan intensitas seismik di pulau Sumatera dan sekitarnya. Studi ini menggunakan data dukung seperti data katalog gempa bumi, data sesar, data subduksi, data intensitas seismik dan data pga dari rekaman akselerograf BMKG. Berdasarkan hasil studi hazard seismik, diperoleh beberapa kota besar di Sumatra yang mempunyai hazard seismik tinggi seperti :
Majalengka is a district in West Java which is located close to local faults and subduction zone, so that made this area has a high enough risk of earthquake hazards. Engineering Seismology Team of Meteorology Climatology and Geophysics Agency has been carried out microtremor and MASW survey in Majalengka. The combination of HVSR and MASW inversion methods is applied to obtain the value of the shear wave velocity (Vs), a combination of the two methods is expected to overcome the deficiency of each method. Results of the distribution of the value of the shear wave velocity can explain the subsurface conditions related to a vulnerability in the study area to seismic hazards. The results of HVSR and MASW inversion are 1-D (Vs) profiles at each measurement point, after that interpolated to obtain a 3-D Vs30 model. The results showed that Vs30 values in Majalengka District varied around 80 - 1500 m/s. Layers with low (Vs) values were relatively thick dominant in the north and northwest regions of the study area, soil types in this area were mostly composed of medium soils, a small part was composed of soft soil layers and rock layers. Therefore, in theory, the disaster in the north and northwest regions has a relatively higher vulnerability compared to other regions in Majalengka District.
Series of strong-motion instruments are being installed as part of the seismic hazard reduction program. One of the required information for an instrument station is the geotechnical characteristics, particularly the 30 m deep weighted average of shear wave velocity, VS,30. The VS,30 values of 25 strong-motion instrument stations in western part of Java Island and western-southern part of Sumatra Island were used to evaluate the topographical information and the geomorphological information based VS,30 estimation models. The ratio of the measured VS,30 to the estimated VS,30 is evaluated, and the simple statistical parameters could not suggest the better model. No apparent geographical and/or geological factors could be identified as the affecting factor as well. Furthermore, the ratio is found to decrease with increasing the estimated values. Based on these observations, several recommendations are proposed, including to develop a new VS,30 estimation model, specifically for Indonesia
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