The April 25, 2015 Nepal Earthquake

The April 25, 2015 Nepal Earthquake

Earth In The News
26 Apr 2015

UPDATE: 27 April 2015

Slip models derived by Assistant Professor Wei Shengji

The source model is obtained by inversion of GSN broadband data downloaded from the IRIS DMC. We analyzed 40 teleseismic P and 37 SH waveforms selected based upon data quality and azimuthal distribution. Waveforms are first converted to displacement by removing the instrument response and then used to constrain the slip history based on a finite fault inverse algorithm (Ji et al, 2002). The epicenter location and point source mechanism (Global Moment Tensor Solution) are based on the information provided by NEIC (Lon.=84.708°, ; Lat.=28.147°). 1D velocity model is extracted from the CRUST2.0 global tomography model (Bassin et al., 2000). The fault plane with strike of 293 degree and dip of 7 degree (based on GCMT solution) is used for the inversion. Our inversion result shows a unilateral rupture towards east and maximum slip is larger than 5m.

Slip model

Comparison of the observed (black) and modeled (red) teleseismic seismograms (in displacement).

This finite rupture model can be downloaded here.

Refererences:

Ji, C., D.J. Wald, and D.V. Helmberger, Source description of the 1999 Hector Mine, California earthquake; Part I: Wavelet domain inversion theory and resolution analysis, Bull. Seism. Soc. Am., Vol 92, No. 4. pp. 1192-1207, 2002.

 

Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000.

 

USGS National Earthquake Information Center: http://neic.usgs.gov

 

Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF).

For more information on the Nepal earthquake, please refer to the USGS site here, or the IRIS special event page here.

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The magnitude (Mw) 7.9 April 25th, 2015 earthquake in central Nepal occurred where the Indian and Eurasian plates collide. The earthquake ruptured the large, gently dipping Main Himalayan Thrust fault, which represents the boundary between the plates, at shallow depth (between 10-15 km based on preliminary moment tensor solutions).

 

India and southern Tibet are converging at a rate of ~17-21 mm/yr (Ader et al., 2012), and the resulting strain is periodically released by earthquakes along the faults in the frontal Himalaya. The last large earthquake in Nepal occurred in 1934 and killed 16,000 people. That earthquake has recently been shown through trenching to have produced a surface rupture with ~5-7 m of displacement, lifting up ground on one side of the fault by as much as 4-5 m along a minimum width of ~150 km along the range front (Sapkota et al., 2012; Bollinger et al., 2014). Several other large and damaging earthquake have happened in the last century along faults in the frontal Himalaya: (from west to east) 2005 Mw7.6 Kashmir, 1905 Ms7.8 Kangra, 1934 Mw8.2 Bihar-Nepal, and 1950 M8.6 Assam.

Location of the April 25, 2015 earthquake (red star) and previous major earthquakes on the Himalayan frontal system (white stars).

 The April 25, 2015 earthquake occurred about 200 km west of the 1934 earthquake. It ruptured a segment of the fault that likely slipped in 1344 AD (Bollinger et al., 2015 submitted to Nature Communications). This is similar to the time interval between the last two earthquakes on the fault system to the east, which ruptured in both 1934 and 1255. We expect that this 2015 earthquake produced surface ruptures in the frontal Himalaya, as the 1934 earthquake did.

The distribution of aftershocks, which extend up to 130 km to the east of the epicenter, suggests that the rupture may have propagated from west to east, potentially leading to more severe destruction in Kathmandu.

Location of main shock (red star) and aftershocks (orange circles) for the April 25, 2015 earthquake. 

As figure 3, with USGS ShakeMap contours overlain.  Roman numerals indicate Mercalli intensity: see http://earthquake.usgs.gov/earthquakes/eventpage/us20002926#impact_shakemap

Schematic section (a-a’) showing the location of the mainshock (Mw~7.8) and aftershock (Mw~6.7) at depth and the possible rupture extension in red.

1934 surface rupture. Inferred co-seismic rupture (thick red line), superimposed on SRTM topography. Red and white stars are instrumental and macroseismic epicenters of 1934 and 1833 events, respectively. Thin continuous and dashed coloured lines with roman numbers are 1934 macroseismic isoseismals (MSK64), (extracted from Sapkota et al.2013, Nature Gesocience).

Although it is not possible to predict when an earthquake will occur, scientists have been warning of the possibility of an earthquake like this for years. The extent of loss of life and property in an earthquake depends greatly on whether buildings are constructed to withstand earthquakes.

The most recent comparable event occurred in 2008 on the eastern boundary of the Tibetan plateau (2008 Mw7.9 Wenchuan). That earthquake caused over 80,000 fatalities, and massive landsliding within the steep gorges blocked roads and made access to villages within the region slow and difficult. In addition, these landslides created temporary dams in the rivers and led to a risk of catastrophic dam break and flooding downstream. We also expect that this earthquake will have produced significant landsliding, hampering rescue efforts. Damage will likely be widespread, throughout central Nepal and northern India. Strong shaking has been felt as far as Calcutta, 750 km away.