Modelling In-Slab Subduction Earthquakes in Psha: Current Practice and Challenges for the Future

Abstract eng:
The seismic threat that active subduction zones present to populations located within the fore- and back-arc regions is well recognised by scientists and policy makers. Recent megathrust earthquakes such as the 2004 Sumatra earthquake and the 2011 Tohoku event illustrated just how catastrophic the impacts of large subduction events, both local and global, can be. By virtue of their deeper sources and typically smaller magnitudes, however, intra-slab earthquakes are often seen as less consequential in probabilistic models of seismic hazard. Yet events such as the 2009 Padang, Indonesia earthquake (M 7.6) demonstrate that their potential impacts on cities is far from negligible. A review of national and regional probabilistic seismic hazard models for subduction regions reveal an absence of consensus on the approaches taken to characterize earthquakes originating within the Benioff zone. Amongst the different approaches are smoothed seismicity, the adoption of uniform volumetric zones, or in some cases a series of characteristic faulting planes. Adding further complexity to the problem is the representation of subduction sources within current ground motion prediction equations created for application to subduction regions. As a result of these potential inconsistencies, some recently published seismic hazard models have retained point-source representations, whilst others based on finiterupture provide little indication as to how to scale the rupture surface in a manner that is consistent with the physical characteristics of the ruptures within the subducted slab. The current study outlines several requirements for seismogenic source characterisation for in-slab earthquakes in probabilistic seismic hazard analysis (PSHA). These include the constraints that the slab geometry place upon both the source location and the finite rupture extent of seismicity and flexibility to account for changes in rupture properties (such as orientation and style of faulting) within the Benioff zone. We present a new in-slab source modelling methodology using in the OpenQuake-engine software for PSHA, which aims to satisfy these requirements in a clear, flexible and efficient manner. The methodology is compared against existing techniques for characterising in-slab sources in PSHA using a branchmark test for in-slab seismicity proposed as part of the Pacific Earthquake Engineering Research Center (PEER) tests for PSHA software verification. This exploration may help to guide future revisions to national and regional hazard models and it may help identifying those elements of the subduction system upon which efforts to better constrain its physical properties should be placed in order to model more consistently, and perhaps reduce, the uncertainty in future seismic hazard studies.

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