Subduction zones are plate margins where one tectonic plate 'subducts' or is thrust beneath another plate. These zones produce most of the seismicity on Earth. The zones can be complex, producing earthquakes at the interface or 'megathrust' fault between the plates, in the downgoing plate or 'slab', and in the deforming region at the margin of the upper, overriding plate. For hazard models produced by the GEM Secretariat, the plate interface and the subducting slab are characterized and modeled with subduction-specific tools we have developed alongside our modeling efforts, while the deformation within the upper plate is modeled as part of the active shallow crust.
Among PSHA models, various source model approaches are used to model interface seismicity. Models produced by GEM use OpenQuake complex faults (surfaces with complex geometry) to account for subduction interface seismicity, and float all possible ruptures within specified magnitude limits and have a given rupture aspect ratio across the meshed surface. In some cases, we segment the surfaces along-strike to define firm barriers to rupture or capture changes in subduction characteristics. We use two predominant approaches to compute magnitude-frequency distributions (MFDs) and maximum magnitudes of the interface segments. Both use recorded instrumental (and sometimes historical) seismicity that can be attributed to the respective interface segment (classified using the methodolgy described here, fitting a Gutenberg-Richter (a negative exponential) distribution to the seismicity. One approach also includes a characteristic component, computed from the area of the interface surface, the local convergence rate, and the degree of seismic locking (a seismic coupling coefficient). MFD construction is explained in detail here.
Hazard models built by the GEM Secretariat account for intraslab seismicity using non-parametric ruptures (sources with predefined geometry) that fit within a slab volume of uniform thickness. The ruptures correspond to virtual faults within a meshed approximation of the slab volume, and forces ruptures to fit within the slab. Like the interface, the slab volume can be segmented, however here, boundaries only seldom indicate barrier to rupture (such as at a slab tear) and are more commonly used to reflect change in seismicity rate. For each slab segment, we compute a single Gutenberg-Richter MFD from the slab segment subcatalogues produced during tectonic classification, assuming constant rates throughout each segment. Currently, moment rates are distributed uniformly among the computed ruptures, but future development will include a smoothing component.
The Subduction Toolkit: building the geometry of the interface surface and slab
Alongside the PSHA models that incorporate subduction zones, GEM has developed the Subduction Toolkit, which uses an interactive workflow to build the subdction interface and slab top geometry, an integral step in producing the subduction source model.
The subduction geometries are based on trench axes from the GEM Active Faults Database along with several geophysical datasets and models. The toolkit projects swaths of geophysical data onto cross sections along a trench axis, which are used to guide depth picking for the interface and slab upper surface. These depth profiles are then stitched together to form OpenQuake complex fault surfaces, which are used as reference frames for catalogue tectonic classification, and for defining subduction source geometry (described above).
The data plotted on the cross sections is meant to illuminate the subsurface subduction structures and tectonic processes that contribute to seismic hazard. The most commonly used data include:
- hypocenters from ISC-GEM catalogue (Weatherill et al., 2016)
- centroid moment tensors (CMTs) from the Global CMT project (Dziewonski et al., 1981; Ekstrom et al., 2012)
- Moho depth estimates from Lithos1.0 (Pasyanos et al., 2014) and Crust1.0 (Laske et al., 2013)
- Slab depth estimates from Slab1.0 (Hayes et al., 2011) and Slab2.0 (Hayes et al., 2018)
- Shuttle Radar Topography Mission (SRTM) topography (Farr, 2007)
- General Bathymetric Charts of the Ocean (GEBCO) bathymetry (Weatherall et al., 2015)
- Volcano locations
Figure 1: Example cross section of a subduction zone from the Philippines
Initially, the cross sections are automatically generated at a specified increment along the trench axis that balances data density with resolution, with azimuths perpendicular the trench. The cross section origins and azimuths can then be adjusted manually, and additional cross sections added where necessary.
The final depth profiles (or a subset) are stitched together to form an OpenQuake complex fault surface. The Toolkit allows for the full extent of the profiles to be considered in subsequent steps, or a depth range can be defined. We use these capability to separate the subduction interface from the deeper slab, and to segment the surfaces along strike (see above).
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