Nick Horspool

Tsunami inundation from heterogeneous earthquake slip distributions: Evaluation of synthetic source models

This study investigates whether eight different synthetic finite fault models (SFFM) can simulate stochastic earthquake-tsunami with similar statistical properties to “real” earthquake-tsunami events, where the latter are represented using heterogeneous slip distributions from 66 Finite Fault Inversions (FFI) for oceanic subduction interface earthquakes. A new method is derived to estimate SFFM parameters from FFI, and predictive relations between the earthquake moment magnitude and the SFFM corner wave numbers are developed to support model applications. SFFM with more capacity to spatially localize slip are better able to simulate higher slip regions on the FFI, and this strongly influences their associated tsunami inundation, which is computed in two dimensions over idealized topography. The best performing SFFM generates tsunami inundation which envelopes the FFI inundation in 81% of cases using 10 synthetic events (close to the ideal value of 82%), while the other SFFM show greater tendencies to underestimate inundation. These differences are related to the capacity of each SFFM to produce spatially localized slip distributions. None of the SFFM showed a tendency to overpredict inundation. The results highlight that SFFM cannot be assumed to reliably quantify uncertainties in the tsunami inundation of real earthquakes, and the use of untested SFFM could create nonconservative bias in tsunami hazard assessments. However, the most successful model used here performs quite well, although it may still underestimate inundation more often than an optimal model.

An evaluation of onshore digital elevation models for modeling tsunami inundation zones

A sensitivity study is undertaken to assess the utility of different onshore digital elevation models (DEM) for simulating the extent of tsunami inundation using case studies from two locations in Indonesia. We compare airborne IFSAR, ASTER and SRTM against high resolution LiDAR and stereo-camera data in locations with different coastal morphologies. Tsunami inundation extents modelled with airborne IFSAR DEMs are comparable with those modelled with the higher resolution datasets and are also consistent with historical run-up data, where available. Large vertical errors and poor resolution of the coastline in the ASTER and SRTM elevation datasets cause the modelled inundation extent to be much less compared with the other datasets and observations. Therefore ASTER and SRTM should not be used to underpin tsunami inundation models. a model mesh resolution of 25 m was sufficient for estimating the inundated area when using elevation data with high vertical accuracy in the case studies presented here. Differences in modelled inundation between digital terrain models (DTM) and digital surface models (DSM) for LiDAR and IFSAR are greater than differences between the two data types. Models using DTM may overestimate inundation while those using DSM may underestimate inundation when a constant Manning’s roughness value is used. We recommend using DTM for modelling tsunami inundation extent with further work needed to resolve the scale at which surface roughness should be parameterised.

Assessing tsunami hazard using heterogeneous slip models in the Mentawai Islands, Indonesia

Abstract Tsunami hazard maps are generated for the coastline of the Mentawai Islands, West Sumatra, Indonesia, to support evacuation and disaster response planning. A random heterogeneous slip generator is used to forward model a suite of earthquake rupture scenarios on the Mentawai Segment of the Sunda Subduction Zone. Up to 1000 rupture models that fit constraints provided by coral and geodetic records of coseismic vertical deformation from major earthquakes in 1797, 1833 and 2007 are used to model inundation and to define a maximum inundation zone that envelopes all of these scenarios. Comparison with single-scenario hazard assessments developed by experts and agreed through scientific consensus shows that there is value in modelling a suite of scenarios in order to obtain a more robust and conservative estimate of potential inundated areas. Although both the model presented here and the single-scenario models are based on assumptions about the characteristics of future events using knowledge of past events, by sampling a range of plausible outcomes we gain a more robust estimate of which areas may be inundated during a tsunami within the bounds of the assumptions applied.

A probabilistic seismic hazard assessment for Sulawesi, Indonesia

Abstract A probabilistic seismic hazard assessment that includes the effect of site amplification is undertaken for the island of Sulawesi, Indonesia. High seismic activity rates, both along fast-slipping crustal faults including the major Palu-Koro–Matano Fault System and in regions of distributed deformation, contribute to moderate–high earthquake hazard over all but the SW part of the island. Of particular concern in terms of seismic risk are the numerous cities sited on soft sedimentary basins that have formed due to movement along presently active structures and that can be expected to amplify earthquake ground motions, including the provincial capitals of Palu and Gorontalo.