Regional Earthquake Magnitude Conversion Relations for the Himalayan Seismic Belt

We present regional earthquake magnitude conversion relations among different magnitude scales (Mw, Ms, mb, ML, and MD) for the Himalayan seismic belt developed from data of local, regional, and international seismological agencies (International Seismological Centre [ISC], National Earthquake Information Centre [NEIC], Global Centroid Moment Tensor Solution [CMT], International Data Centre [IDC], China Earthquake Administration [BJI], and National Centre for Seismology [NDI]). The intra- (within the same magnitude scale) and inter- (with different magnitude scales) magnitude regression relations have been established using the general orthogonal regression and orthogonal distance regression techniques. Results show that the intra-magnitude relations for Mw, Ms, and mb reported by the Global CMT, ISC, and NEIC exhibit 1:1 relationships, whereas ML reported by the IDC, BJI, and NDI deviates from this relationship. The IDC underestimates Ms and mb compared with the ISC, NEIC, and Global CMT; this may be due to different measurement procedures adopted by the IDC agency. The inter-magnitude relations are established between Mw,Global CMT and Ms, mb, and ML reported by the ISC, NEIC, IDC, and NDI, and compared with the previously developed regional and global regression relations. The duration (MD) and local (ML) magnitudes reported by NDI exhibit a 1:1 relationship. The derived magnitude regression relations are expected to support the homogenization of the earthquake catalogs and to improve seismic hazard assessment in this region.

A seismic hazard analysis considering uncertainty during earthquake magnitude conversion

The magnitude of earthquakes can be described with different units, such as moment magnitude Mw and local magnitude ML. A few empirical relationships between the two have been suggested, such as the model calibrated with the earthquake data in Taiwan. Understandably, such a conversion relationship through regression analysis is associated with some error because of inevitable data scattering. Therefore, the underlying scope of this study is to conduct a seismic hazard analysis, during which the uncertainty from earthquake magnitude conversion was properly taken into account. With a new analytical framework developed for this task, it was found that there is a 10% probability in 50 yr that PGA could exceed 0.28 g at the study site in North Taiwan.