Continuous flux of sediments carried by the oceanic plate into subduction zone is one of the distinct features of the convergent plate boundary, giving rise to probably the most pronounced low-velocity fault zones ⎯ a factor that has largely been overlooked by the earthquake modeling community. The dominance of sediments in the subduction zone has been considered to contribute to great complexities in subduction zone earthquakes that are not seen in crustal earthquakes.
The proposed research focuses on a special class of subduction zone earthquakes: tsunami earthquakes. Numerous observations indicate that tsunami earthquakes occur at shallow depths close to the trench and are associated with the unusually long rupture duration, low rupture velocity, and/or small stress drop. These rupture characteristics have been attributed to sediments in subduction zone. Sediments in the forearc basin have also been found to affect the location of large slip asperities on the plate interface and correlate with large tsunami generation.
The underlying physics as to how sediments contribute to these processes is, however, not well understood. Why is the rupture velocity slow for tsunami earthquakes? Why do these earthquakes have long rupture duration and small stress drops, which are distinctly different from regular earthquakes? Are these features related to the anomalous tsunami generation? How significant is the role of sediments? Does the free surface play an important role in the rupture dynamics? Can sediments and free surface together give rise to large seafloor uplift and tsunamigenesis? The dynamic rupture models that we propose provide the principal theoretical means currently available for helping answer these questions.
Understanding the physical mechanism for controlling tsunami earthquakes and tsunami generation has its obvious value for reducing tsunami hazards to the human society. The proposed research will use dynamic rupture models to investigate the role of sediments in rupture dynamics of tsunami earthquakes and to better understand the physical mechanism for anomalous tsunami generation. The 2-year research proposed here will include the following activities: (i) Investigate the dynamic stress evolution on faults during tsunami earthquakes induced by both sediments and free surface, and its relations with rupture velocity and slip. (ii) Explore the effect of off-fault yielding of sediments on the rupture characteristics and seafloor deformation. (iii) Simulate the effect of forearc basin on seismic wave propagation, seafloor deformation and fault slip distribution.
It has been a long-standing puzzle to scientists that why earthquakes in shallow subudction zones produce large seafloor uplift while the plate interface is shallowly dipping and why these earthquakes radiate small-amount of high-frequency energy to cause ground shaking. Our results from this project show that material in the overriding wedge can fail extensively due to elevated pore pressure driven by up-dip earthquake rupture and initial stress states in the wedge close to failure (according to a classic theory in geology). Extensive wedge failure causes large inelastic uplift landward from trench generating tsunami and acts as a large energy sink (absorbing high-frequency energy). It also explains why these earthquakes are slow. Convetionally scientists attribute large tsunami generation to large earthquake slip on the plate interface. However, large slip on a shallowly dipping plate interface (as is the case in most subduction zones) causes predominantly horizontal seafloor displacement, which is inefficient in generating tsunami. In our new mechanism the shallow dipping plate geometry in fact enhances failure because the material in the wedge is under a low confining stress, thus has no shear strength. The extensive failure promotes large seafloor uplift. This mechanism provides a physical explaination to nearly all anamalous observations associated with shallow subduction zone earthquakes and tsunami generation. It challenges the conventional understanding of physics of these earthquakes.
