Deadly Quake in Nepal; Aftershocks, Weather Hamper Rescue: AIR Analysis

April 27, 2015

As scenes of devastation flashed around the world, the destruction and loss of life from the most severe earthquake to hit Nepal in 80 years has become apparent. Catastrophe modeling firm AIR Worldwide said the quake “struck Nepal on Saturday, April 25, at 6:11 UTC (11:56 local time).”

It was initially estimated at magnitude 7.5, but the USGS (U.S. Geological Survey) later revised their estimate to magnitude 7.9 and finally to magnitude 7.8. The earthquake occurred at an estimated depth of just 10 kms [6.2 miles],” making this a shallow and therefore extremely destructive event,” AIR said.

AIR’s report and widely broadcast images detailed the destruction in the country’s capital Kathmandu, which has an estimated population of 1 million people. “Unreinforced masonry buildings— some of them centuries-old temples and towers—have collapsed,” AIR said. “It has been reported that around 250 people may have been in the Dharara tower when it collapsed. From around the city, there are widespread reports of collapsed or cracked walls, broken windows, roads made impassable by large cracks and fallen telephone poles.”

The death toll, originally estimated at 1000,has risen dramatically with the most recent reports stating that more than 3200 people are confirmed dead and more than 6500 have been injured, many of them seriously. An avalanche, triggered by the earthquake, has also reportedly killed at least 17 people at the base of Mount Everest.

AIR noted that while “most of the fatalities have occurred in Nepal, deaths have also been reported in India (where shaking was felt as far away as Delhi), Tibet, and Bangladesh. The earthquake has been followed by several significant aftershocks, one of which was M6.6. In Kathmandu, residents are remaining outdoors for fear that more buildings will collapse.”

There have been at least 15 aftershocks so far that have been measured at magnitude 4.5 and above. Severe weather in the Himalayas with rain, wind and thunderstorms are also hampering rescue efforts. It has been impossible to reach outlying areas, many of which are located in mountainous terrain.

Kathmandu’s airport was shut down after the quake, and communications from outlying towns are generally poor. “One report, however, from nearby Lalitpur indicates that while damage was considerable, most buildings remain standing,” AIR said.

According to AIR, the “earthquake occurred on the Main Himalaya thrust fault zone (MHT) along the plate boundary zone between India and Eurasia plate. The Main Himalaya thrust fault zone is a 2500 kms [1553.5 miles] long active thrust fault zone that absorbs about half of the 4.5 cm [app. 1 3/4 inches] per year convergence between the India – Eurasia collision zone.

“The seismic activity along the main Himalaya fault zone is very active in the known history with at least 4 earthquakes larger than 7.5 occurred since 1900, including the 1934 M8.1, the 1952 M8.6 Assam earthquake in the eastern segments, and the most recent – the M7.6 Kashmir earthquake in the western segment of MHT that resulted more than 46,000 deaths in 2005.

This most recent earthquake “occurred along the central segment of MHT in central Nepal, the seismically most active fault segment of MHT,” AIR’s report continued. “The epicenter is located just west of the 1934 earthquake, which was one of the largest historic earthquakes in central MHT. The segment west of the 1934 rupture in western Nepal was probably ruptured in the large earthquakes (M8.1) in 1505 and 1833. Today’s earthquake could have ruptured in the segment between the 1905 and 1934. Another larger earthquake in 1833 probably also occurred in this 800 km long central segment of MHT, however locations of both 1505 and 1833 are quite uncertain. Therefore today’s rupture could have occurred on the fault that ruptured in 1505 or 1833.

“The repeat occurrence of large earthquakes along MHT indicates that the MHT is nearly fully locked. Since the location of the 1505 and 1833 earthquake are both poorly known, today’s earthquake could represent a repeat of either earthquake. The segment ruptured in 1505 could have accumulated more than 10 meters of slip, large enough to generate an earthquake as large as today’s earthquake. Even the segment ruptured in 1833 could have accumulated enough energy for a large magnitude earthquake.”

According to AIR, the west segment of MHT from west of Nepal to the western Himalaya syntax has been historically very quiet with only one earthquake in 1905. The accumulated energy in the western segment of MHT is sufficient to general earthquakes larger than 8. The 2005 Kashmir earthquake released only a very small portion of the accumulated energy.”

Sources: AIR Worldwide and news reports

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