Anomalous seismic activity associated with 1988 Udayapur earthquake

Abstract

The Udayapur earthquake of August 20, 1988 (Mb 6.4, Ms 6.6, Origin time: 23:09:10, epicenter at 26.8 °N 86.6 °E and Focal depth 57 km) occurred in the Himalayan front arc which caused severe damage in the region will loss of 1100 lives. In the same epicentral region, a strong event of mb 5.4 has occurred on 20 April 1988. Anomalous seismic activity associated with these earthquake in Eastern Nepal and its adjoining region in an area bounded by 26.0°-29.0° N and 84.6°-88.4° E have been studied using seismicity data from 1975-1988. The preparatory zone is delineated using the temporal and the spatial distribution of earthquakes, considering the events with cutoff magnitude mb ≥ 4.3. Based on daily number of events as well as cumulative number of earthquake with time within the preparatory zone anomalous seismicity is recognized. Accordingly four anomalous episodes: Normal/ background (N); Anomalous/ swarm (A); Precursory gap (G) and Mainshock sequence (M) are identified. It is observed that the first mainshock (mb 5.4) was related to the second mainshock (mb 6.4) which was preceded by well defined patterns of anomalous seismicity/ precursory swarm which commenced about ten years and six months prior to mainshock. Keywords: Anomalous seismicity; Central Himalaya; South Eastern Nepal; Precursory swarm DOI: 10.3126/bibechana.v7i0.4050BIBECHANA 7 (2011) 76-82