The Sumatra active margin extends more than 1600 km from northwest to southeast.The almost north-south direction of subduction is normal in front of Java and oblique in front of Sumatra. The transition between the two regimes of subduction occurs south of the Sunda Strait. A structural and stratigraphical study of the forearc domain, based on data collected during a French-Indonesian programme of cooperation in oceanography, confirms that the convergent motion is partitioned into a convergent motion, more or less perpendicular to the trench, and a strike-slip motion parallel to the trench. This latter motion is taken up along two major faults, the Sumatra and the Mentawai faults. At its northern end, the Mentawai fault is attenuated and seems to terminate within the accretionary prism. It is relayed and connected to the Sumatra fault by the Batee fault. This pattern can be explained by a simple model with two sliver plates: the Mentawai and Aceh sliver plates, on the top of which the forearc basin has developed. The accretionary prism itself is moving northwestwards along the Mentawai fault. Because there is no major evidence of extension within the Mentawai plate, we conclude that the motion along the Sumatra fault may occur at a uniform rate south of latitude 3°N. Strike-slip motion along the Mentawai fault may be explained by relatively better coupling between the subducting slab and the upper plate beneath the accretionary prism compared to that beneath the forearc, perhaps because of the large oceanic structures entering the subduction zone.

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This paper presents an estimation of demand and its comparison to the capacity of the existing tsunami shelters in Padang. A combination of horizontal and vertical evacuation plans was simulated with giving priority to the horizontal ones. Demand for the existing shelters was estimated with an assumption that the population is spreading uniformly within the administrative area. It was also assumed that the number of people who entered the area is the same as those who went out of the area at the time of the anticipated earthquake occur. The coverage areas of shelters were estimated based on the distance that can be reached by people within an effective available evacuation time. The capacity of the tsunami shelters was obtained from secondary data. The study found that almost half of the existing shelters in Padang will be overloaded if all of the people in the coverage area of a shelter being evacuated to the shelters. Addition of 37 more shelters with a capacity of 2000 is needed.

This paper aims to identify factors influencing the walking speed of evacuees by conducting an evacuation drill in Padang, West Sumatera, Indonesia. A number of 18 volunteers and 6 observers, were gathered in an evacuation experiment on 3 routes with 5 segments each. The chosen routes are almost equal in terms of distance, but different in terms of the number of turning points. The experiment comprises of three cases in terms of the complexity of the route. These cases represent a simple, a medium and a complex route based on the number of turning points. The volunteers were asked to move, as if in an evacuation, to a particular place which was assumed to be a shelter. The observers were placed at some particular waypoints to record the time when an evacuee passes their location. The distance between the observers was measured using a manual distance meter. This study found that the average walking speed during the evacuation was 1.69 m/s. In addition, walking speed varied by age, gender, and walking distance. This finding significantly has an effect on the estimations of the coverage area of tsunami shelters. The data collection method is one of the novelties of this research. In this study, the walking speed was observed through an evacuation drill from a location to a shelter of 1.5 km away, whilst most of the previous studies estimate the walking speed based on an observation of a group of pedestrians crossing a road.