Department of Civil Engineering,Indian Institute of Science, Bangalore

Rupture Based Seismic Hazard Analysis

Most of the hazard analyses/zonations are being carried out considering the past earthquake location, size, and rate of occurrence of past earthquakes on the fault or in the region for future design of structures. Moderate to major earthquakes need sufficient energy to rupture the faults. Time required to buildup the required energy to create moderate to major earthquakes is a region-specific. So, interval between two consecutive earthquakes in the same location is considerable, but it is accounted poorly in the hazard analysis and future seismic zonation. Earthquakes relive the strain energy that builds up on faults, next earthquake in the region is more likely to occur in areas where little or no seismic activity has been observed for some time (Kramer 1996). Based on the average return period of earthquakes in the region, one can assess the potential of past earthquake location for generating the future similar earthquakes. Let the place/source having earthquake magnitude of M with an average return period of T has ruptured by an amount of R. Amount of rupture depends on the seismotectonic of the region and seismic sources. Maximum magnitude reported in the region is Mmax and M is the average damaging earthquake in the region. If M and Mmax are relatively comparable, the possibility of occurrence of the same M or Mmax in the same (reported past) location is rare up to period T. Hence, for the future seismic zonation for period less than T, these locations can be eliminated or considered as areas with no potential for occurrence of near-future earthquake. But in the conventional hazard analysis for future zonation of time period less than T, these locations are considered and probable magnitude is arrived by adding 0.3-1 more to Mmax. Also, possibility of occurrence of damaging earthquake in other locations/sources is not accounted. In order to account the possibility of occurrence of earthquake in the locations other than past damaging earthquake locations, a new seismic hazard analysis has been attempted in this paper that is named as Rupture Based Seismic Hazard Analysis (RBSHA) for future zonation. Steps for rupture-based seismic hazard analysis are given below:

1. Prepare seismotectonic map of the study region and identify the maximum reported earthquake (Mmax) in the region.
2. Delineate the damaging earthquakes (Mw of 5 for study area) sources/area and minor earthquake source/area.
3. Select appropriate subsurface rupture equation and assess subsurface rupture character of the region. Validate the same if data are available for the region.
4. Mark zone of influence circles for damaging earthquakes based on subsurface rupture length of the event.
5. Identify probable future earthquake location considering minor earthquakes recorded, potential seismic sources, and eliminating damaging earthquake locations (identified in step 4)these locations can be called as Probable Future Earthquake Zones (PFEZ).
6. Estimate maximum characteristic earthquake for study area by considering increased regional rupture characters estimated in step 3.
7. Measure the distance between PFEZ to required site and estimate PGA using regional attenuation model.
8. Identify the maximum PGA at each site/grid and prepare zonation map.

This zonation map is more representative for future design of structures for duration less than T. Site effects and liquefaction vulnerability can be assessed for microzonation based on maximum representative PGA distribution. This microzonation map will be more representative for future seismic disaster management and planning. Seismic hazard of Coimbatore city has been estimated using rupture-based seismic hazard analysis presented above and compared with conventional deterministic seismic hazard analysis in below paper. Anbazhagan P., Prabhu G., and Aditya P. (2011). Seismic Hazard Map of Coimbatore using Subsurface Fault Rupture, Natural Hazard, Published on line. 60:1325:1345

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