Gempa Besar Mengamcam Padang II

Regional Seismic Hazard Posed by the Mentawai Segment of the Sumatran Megathrust

Kusnowidjaja Megawati and Tso-Chien Pan

Several lines of evidence have indicated that the Mentawai segment of the Sumatran megathrust is very likely to rupture within the next few decades.

The present study is to investigate seismic hazard and risk levels at major cities in Sumatra, Java, Singapore, and the Malay Peninsula caused by the potential giant earthquakes. Three scenarios are considered. The first one is an Mw 8.6 earthquake rupturing the 280 km segment that has been locked since 1797; in the second scenario, rupture occurs along a 600 km segment covering the combined rupture areas of the 1797 and 1833 historical events, producing an Mw 9.0 earthquake; and the third scenario has the same rupture area as the second scenario but with doubled slip amplitude, resulting in an Mw 9.2 earthquake. Simulation results indicate that ground motions produced by the hypothetical scenarios are strong enough to cause yielding to medium- and high-rise buildings in many major cities in Sumatra. It is vital to ensure that the overall strength, stiffness, and integrity of the structures are maintained throughout the entire duration of shaking. However, the ductile detailing in current practice is formulated based on an assumption that ground motions would last from 20 to 40 sec. This has not been tested for longer durations of 3–5 min, expected from giant earthquakes. In Singapore and Kuala Lumpur, only medium- and high-rise buildings, especially those located on soft-soil sites, are at risk. Given that seismic design has not been required in either city, and thus the resulting structures are relatively brittle, it is crucial to investigate their performance under moderate-amplitude, long-duration, ground motions. The present study also points out that shifting the response spectrum toward a longer period range becomes significant for sites located far from potential seismic sources, which should be carefully considered in formulation of future seismic codes.

Gempa Besar Mengamcam Padang I

Tsunami threat in the Indian Ocean from a future megathrust earthquake west of Sumatra

John McCloskey^, Andrea Antonioli,etc

Abstract

Several independent indicators imply a high probability of a great (M > 8) earthquake rupture of the subduction megathrust under the Mentawai Islands of West Sumatra.

The human consequences of such an event depend crucially on its tsunamigenic potential, which in turn depends on unpredictable details of slip distribution on the megathrust and how resulting seafloor movements and the propagating tsunami waves interact with bathymetry. Here we address the forward problem by modelling about 1000 possible complex earthquake ruptures and calculating the seafloor displacements and tsunami wave height distributions that would result from the most likely 100 or so, as judged by reference to paleogeodetic data. Additionally we carry out a systematic study of the importance of the location of maximum slip with respect to the morphology of the fore-arc complex. Our results indicate a generally smaller regional tsunami hazard than was realised in Aceh during the December 2004 event, though more than 20% of simulations result in tsunami wave heights of more than 5 m for the southern Sumatran cities of Padang and Bengkulu. The extreme events in these simulations produce results which are consistent with recent deterministic studies. The study confirms the sensitivity of predicted wave heights to the distribution of slip even for events with similar moment and reproduces Plafker's rule of thumb. Additionally we show that the maximum wave height observed at a single location scales with the magnitude though data for all magnitudes exhibit extreme variability. Finally, we show that for any coastal location in the near field of the earthquake, despite the complexity of the earthquake rupture simulations and the large range of magnitudes modelled, the timing of inundation is constant to first order and the maximum height of the modelled waves is directly proportional to the vertical coseismic displacement experienced at that point. These results may assist in developing tsunami preparedness strategies around the Indian Ocean and in particular along the coasts of western Sumatra.