Anomalous seismic activity associated with 1988 Udayapur earthquake

Abstract

The Udayapur earthquake of August 20, 1988 (M_b 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 m_b 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 m_b ≥ 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 (m_b 5.4) was related to the second mainshock (m_b 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.v7i0.4050

BIBECHANA 7 (2011) 76-82