On the construction of Joyner-Boore distance (Rjb) for PESMOS and COSMOS databases

This paper is aimed at constructing Rjb for PESMOS and COSMOS database representing the earthquakes triggered in Indian sub-continent. Rjb, by definition, is the shortest distance from the site to the horizontal projection of rupture plane and that can be constructed using simple geometry provided the required information is available. In practice, the uncertainty associated with this information offers several challenges which are addressed in this paper. First, a vector algebra-based approach is proposed for estimating epicentral distance and azimuth. Second, a set of empirical relationships is proposed to estimate the rupture plane from the moment magnitude using a dataset of 354 earthquakes based on tectonic settings and focal mechanisms. Third, a step-by-step process of computing Rjb is developed considering the uncertainty in the location of hypocenter on the rupture plane. Two approaches are considered for this purpose, namely, (i) areal grid representation and (ii) hypocenter distribution model. While the former assumes equally likely hypocenter over the rupture plane, the latter requires construction of hypocenter distribution model from the prior database. Fourth, the process is extended to account for the uncertainty in available information of strike and/or dip. The proposed framework is assessed against a total of 4247 records from PEER database with Rjb reported based on geometry and location of rupture plane. The framework is finally applied to compute Rjb associated with PESMOS (474 records) and COSMOS (148 records) database, and the results are expected to serve as a valuable resource while constructing GMPEs of shallow focused earthquakes.