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Large M7.6 earthquake strikes offshore Sumatra

03 Mar 2016

Update: March 3 2016 

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.

Cumulative slip (arrows show slip vectors, and color coding shows amplitude) and isochrons of the seismic rupture. The rupture times are given relative to the onset of slip at the epicenter.

 

 

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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On the evening of March 2, 2016, a magnitude-7.9 earthquake struck off the west coast of Sumatra, Indonesia. According to the United States Geological Survey (USGS), the epicenter of the quake was approximately 800km from Padang, West Sumatra. 

A tsunami warning was issued, but cancelled less than two hours after the temblor. Given the nature of the fault rupture and its great distance from the subduction zone that dives beneath Sumatra and produces great tsunamis (as on Boxing Day in 2004), the chance of a tsunami occurrence was very remote.

Last evening’s earthquake was very much like of a flurry of bigger earthquakes under the Indian Ocean in April 2012, in that the April 2012 and yesterday’s fault ruptures are related to the breakup of two great tectonic plates beneath the Indian Ocean.  Australia rides upon one of these great plates and India rides upon the other.  

Yesterday’s earthquake was caused by a strike-slip rupture, during which the oceanic blocks moved horizontally with respect to one another.  Since horizontal motions don’t cause large uplifts of the seafloor, no large tsunami was generated, according to Earth Observatory seismologist Assistant Professor Wei Shengji.

In terms of how this temblor may affect Singapore, Earth Observatory Director and Professor Kerry Sieh says, “There is no physical threat to Singapore, in that we have no scientific reason to suspect that this could provoke a large, damaging earthquake closer to Singapore.”

However, there is a high possibility of a magnitude 8.8 earthquake occurring within the next two decades, as EOS scientists forecast several years ago. This and its resultant tsunami will be caused by the rupture of a large section of the Sunda megathrust, beneath the Mentawai islands, offshore western Sumatra.

Last June, EOS scientists conducted a scientific expedition to understand the active faults near the region of yesterday’s earthquake, and will embark on another similar expedition this July. These scientific researches are made possible through funding from the Singapore Ministry of Education and the National Research Foundation, via their support of the Observatory.

(Cover image credit: USGS)