Subduction zone scenarios

Megathrusts host one of the biggest earthquakes worldwide, such as the 1960 Great Chiliean Earthquake, the 2004 Sumatra-Andaman Earthquake and the 2011 Tohoku earthquakes. Most of these events are followed by devastating tsunamis.
We use dynamic rupture simulations to increase our understanding of the earthquake source characteristics leading to tsunamis.

From a computational point of view these scenarios are particular challenging: Due to the intersection of the low dipping subduction fault with topography and bathymetry as well as the subsurface structure, many small discretizations elements are generated during the automatic mesh process.
These small elements require small time steps in order to fulfill stability conditions which quickly leads to a large increase in computational time.

In order to deal with these kind of geometries, we recently incorporated local time-stepping for the whole simulation process, including the dynamic rupture part of the software, tremendously shortening time-to-solution (Breuer et al. 2014, Uphoff et al. 2017).

The 2004 Sumatra-Andaman Earthquake

A specifically devastating megathrust earthquake is the Mw 9.1 2004 Sumatra-Andaman Earthquake and Indian Ocean Tsunami. The Sumatra earthquake ruptured the greatest fault length of any recorded earthquake and triggered a series of tsunamis, killing up to 280,000 people in 14 countries.

To gain insight into the earthquake source processes that lead to this tsunami we conducted the very first dynamic rupture scenario of the 2004 Sumatra earthquake. We resolve the full frictional sliding process in a complex fault network as well as the seismic wave field with frequency content up to 2.2 Hz in the to-date longest (500 s) and largest (1500 km) physics-based earthquake simulation.

Figure 1. Left: Tectonic setting of the Sumatra subduction zone including past earthquakes and their magnitudes adapted from P. Shearer and R. Bürgmann [2010]. Middle: Unstructured tetrahedral mesh of the modeling domain, including refinement to resolve high-frequency wavepropagation and frictional failure on the fault system. The purple box marks high-resolution (30 arc-second) topography. Blue curves are splay fault traces. Right: Complex 3D geometry in the region marked by the black dotted box in middle figure. The curved megathrust is intersecting bathymetry, as are the 3 splays: one forethrust and two backthrusts. The subsurface consists of horizontally layered continental crust and subducting layers of oceanic crust. Each layer is characterized by a different wave speed and thus requires a different mesh resolution. Red curves mark the megathrust trace in all figures.

Simulation results

The large- scale, high-resolution scenario is compared against geodetic, seismological and tsunami observations. The comparison with available GPS data (yellow) shows that our simulation results (green) fit quite well.
The output of the simulations is analyzed in terms of fault mechanical properties such as slip rate and total slip on the fault.
The modeled high-resolution seafloor displacement will serve as spatio-temporal input in tsunami simulations to study tsunami generation and propagation.

Figure 2. Rupture propagation across the megathrust and the lower backthrust splay fault and the emitted seismic wave field at 85 s simulation time. The unstructured tetrahedral mesh is refined in the vicinity of the fault and towards lower wave speeds close to the seafloor bathymetry.

For the future we plan detailed parameter studies to understand the conditions under which a megathrust earthquake can efficiently generate tsunamis.

This scenario and its optimization is fully described in the SC17 paper "Extreme scale multi-physics simulations of the tsunamigenic 2004 sumatra megathrust earthquake" by C.Uphoff, S. Rettenberger, M. Bader, E. H. Madden, T. Ulrich, S. Wollherr und A.-A. Gabriel (Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis, November 2017. Finalist for Best Paper Award.)

Press release of the Gauss Centre for Supercomputing.