The Himalayan Frontal Thrust: Attributes for seismic hazard
The 2500 km long Himalaya Frontal Thrust (HFT) fault system is one of the great fault systems of the world with many attributes similar to oceanic subduction zones. Despite a series of large historical earthquakes and recent paleoseismic studies, surface rupture traces of the fault remain poorly mapped with little attendant fault characterisation parameters. Therefore, there are insufficient data to embed the fault in to the GEM Faulted Earth database structure. Instead, in this project we have used remote sensing data, calibrated where possible with detailed published studies, to create a GIS shapefile of the fault traces at a scale of 1:200,000, and have attributed traces or fault segments from the published data. Several possible rupture segments have been identified and parameters are provided so hazard modellers can create earthquake event sets. However, we have little confidence that the defined segments fully represent all future earthquake scenarios because the segments are based on only minimal data – either the past c. 500 years of historical earthquakes or sparse geological evidence for prior ruptures 500-1000 years ago. We therefore propose an alternate method for creating an earthquake event set to that used on the GEM Faulted Earth database. This involves defining a maximum magnitude earthquake that could occur anywhere along the HFT, and allowing these earthquakes to ‘float’ along the length of the fault and be supplemented with smaller events whose fault area is scaled with magnitude, and activity rate determined by the Gutenberg- Richter ‘b value’, the convergence rate across the fault, and the proportion of the convergence that could be released without seismic energy release (the aseismic slip factor). These steps define a Gutenberg-Richter distribution of seismicity which we suggest should truncated at a lower magnitude of Mw 7.0. The parameters are all defined in this report.