The 21 May 2014 M w 5 . 9 Bay of Bengal Earthquake : Macroseismic Data Suggest a High - Stress - Drop Event

A modest but noteworthy Mw 5.9 earthquake occurred in the Bay of Bengal beneath the central Bengal fan at 21:51 Indian Standard Time (16:21 UTC) on 21 May 2014. Centered over 300 km from the eastern coastline of India (Fig. 1), it caused modest damage by virtue of its location and magnitude. However, shaking was very widely felt in parts of eastern India where earthquakes are uncommon. Media outlets reported as many as four fatalities. Although most deaths were blamed on heart attacks, the death of one woman was attributed by different sources to either a roof collapse or a stampede (see E Table S1, available in the electronic supplement to this article). Across the state of Odisha, as many as 250 people were injured (see E Table S1), most after jumping from balconies or terraces. Light damage was reported from a number of towns on coastal deltaic sediments, including collapsed walls and damage to pukka and thatched dwellings. Shaking was felt well inland into east-central India and was perceptible in multistoried buildings as far as Chennai, Delhi, and Jaipur at distances of ≈1600 km (Table 1). In the days following the earthquake, we collected accounts from conventional news outlets as well as social media. Using these accounts, we assigned intensities in keeping with practices described by Martin and Szeliga (2010). The earthquake was reported as “felt” at 310 locations in the eastern and central Indian subcontinent (Fig. 2a;E Table S1). In contrast to available data from the U.S. Geological Survey Community Internet Intensity Map (“Did You Feel It?” [DYFI]) site (Fig. 2b), our intensity map for the 21 May 2014 earthquake (Fig. 2a) confirms initial impressions that the event was remarkably widely felt for an Mw 5.9 earthquake. The purpose of this report is to make available the newly collected intensity dataset and to present preliminary analysis of this noteworthy recent earthquake. We further show that the intensity distribution provides evidence for a high-stressdrop source. These results bear out the observation made two decades ago by Hanks and Johnston (1992, p. 20): “[Our] results suggest that it should be a fairly simple matter to infer a high-stress-drop event from intensity data alone, provided that an instrumental M0 or Mw value is known separately.” Our study illustrates the potential value of carefully determined intensity data for investigations of earthquake source properties, especially when instrumental recordings are sparse. We suggest it may in fact be a more robust way to estimate stress drop than conventional approaches, which require correction of attenuation to estimate pulse width or corner frequency (e.g., Anderson, 1986); the estimate is then cubed to estimate stress drop (Madariaga, 1976). Lastly, we discuss potentially important implications of our results for efforts to characterize probabilistic seismic hazard in the Himalayan region.