To analysis of source mechanism of the 26 December 2004 Indian Ocean tsunami

1. Introduction

[2] It is well known that 26 December 2004 and 28 March 2005 in the Indian Ocean there occurred two earthquakes: the first one, with magnitude 9.2, produced a largest tsunami which caused almost 300.000 deaths; and the second one, with magnitude 8.8, led to significant damages due to ground shaking but not produced a noticeable tsunami [for review, see, e.g., Lay et al., 2005]. Both earthquakes occurred in the Indian Ocean west of the Sumatra Island and Thailand and they are related to the same part of Philippine and Sunda island arcs. Which cause is for so different results of earthquakes at so close earthquake magnitude? And though the cause of absence of strong tsunami due to 28 March, 2005 earthquake is not clear up to now, then it can be supposed that the difference is connected with features of seafloor displacements in the earthquake source.

[3] As known, the formation of tsunami depends on character and dynamics of displacements in earthquake source zone, i.e. on the initial seafloor displacements. As a rule, under computations of tsunami wave generation there are used the seismic data which indicate the rupture orientation in the source and the energy of tsunami. Then, the static hydrodynamical problem on the recount of seafloor displacement distribution to the ocean surface shape is considered. Further, the obtained displacements of the water surface with fixed length and height of the wave are taken as initial conditions and then it is performed the numerical simulation of wave propagation in given basin with taking into account the real bathymetry. In present time, there are a number of numerical models and program complexes [see, e.g., Goto et al., 1997; Titov et al., 2005; etc.], which permits to perform accurately enough computations of tsunami wave propagation up to the coast. After the Indian Ocean tsunami the accuracy of such computations can be estimated by comparison of 3D-section in the Indian Ocean with satellite data on the water surface displacement at tsunami propagation [see, e.g., Kulikov et al., 2005]. However, the question on the adequateness of the source model used at such simulations remains to be open. The features of tsunami generation, its parameters, initial velocity, characteristics of the coast (especially in the near-field zone) directly depend on the numerical model used to determine the initial movements of the seafloor in the earthquake source. Large uncertainty in these calculations stems from often poorly-defined seafloor displacements. Typically for the modeling purposes, the rupture orientation and associated displacement discontinuity is presupposed. Then, the distribution of the sea-bottom displacements is inferred from the static solution for a dislocation in the elastic half-space [Okada, 1992]. Such approach does not take into account the real structure of the Earth crust and lithosphere, and the initial stress-strain distribution in the zone of earthquake preparation [Garagash and Ermakov, 2001; Garagash and Lobkovsky, 2006]. In addition, the static solution does not allow to study the dynamic process of formation of sea-bottom displacements. The length of tsunami wave and its amplitude depend on all of the factors listed above. Development of an adequate numerical model to predict the sea-bottom movements at the moment of the earthquake will raise the accuracy of the situational modelling of a tsunami and its influence on the shore.

Figure 1

Figure 2

[4] In present time, it is elaborated a mechanism of strong earthquakes in subduction zones [Lobkovsky, 1988; Lobkovsky et al., 2004]. It is known that narrow seismic belts of the Earth are connected with contact conditions on the boundaries of large lithosphere plates. Interaction of plates in subduction zone is responsible for seismic process in island arcs and active continental margins. The strongest earthquakes occur in subduction zones in the vicinity of gentle plane of a contact between the base of the island-arc wedge and roof of the underthrusting plate (Figure 1). The numerous geomorphological and geology-geophysical data demonstrates that island-arc wedge consists of separated large segments formed by transcurrent faults passing up to roof of the subducted plate (Figure 2).

Figure 3

[5] For example, traces of these faults are well seen in the bathymetric map of part of Philippine and Sunda island arcs where two strongest earthquakes under consideration occur (Figure 3). The presence of transcurrent faults requires to introduce new smaller interaction elements, so-called keyboard blocks of the frontal edge of the overriding plate. It was obtained that such minimal complication of conventional subduction scheme is quite enough to account for successfully the main features of seismic process in subduction zones [Lobkovsky et al., 2004]. The characteristic size of keyboard blocks is about 100 km. Such "cutting into blocks" of frontal parts of island and continental margins determines structurally the size of strong earthquake source. Mainly, such sources are connected with keyboard blocks in the subduction zone which are deformed and "shooting" at stress release. But sometimes the source length corresponds to several adjoining blocks in which simultaneous release of accumulated elastic energy occurs. It can be proposed that in December 2004, in the Indian Ocean 8 or 10 keyboard blocks of the Sunda Island arc "shooted" almost simultaneously and this powerful "chord" produced a formation of huge source of earthquake and as a consequence appearance of giant tsunami.

[6] In this work, the simplified keyboard model with vertical displacements of blocks is analyzed. The long-term factors which determine the tectonic stress distribution in the Earth's core are the inhomogeneity of Earth's core mechanical properties and its density variations [Lobkovsky et al., 2004]. At earthquake, initial stress distribution determines essentially the character of motion in the vicinity of the earthquake source. The earthquake occurs when stress at any region of contact surface overcomes the breaking point and the motion on it is accelerated. This process depending on earthquake preparation process and the initial stress level will proceed quite differently. And at the same vertical displacements the tsunami waves generated by them will be essentially different in character [Lobkovsky et al., 2005a, 2005a]. In first part of this work (Sections 2-4) it is considered a formation of tsunami source without taking into account the initial tectonic stresses in the earthquake source. In second part of the work (Section 5) there is performed evaluation of affect of initial stress in zone of earthquake preparation.