Large Mw 7.9 earthquake strikes offshore Sumatra

Large Mw 7.9 earthquake strikes offshore Sumatra

  • Earth In The News
03 Mar 2016

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, in partnership with Institute de Physique du Globe de Paris, Indonesian Institute of Sciences, and Schmidt Ocean Institute, 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. This scientific research was made possible through funding from Schmidt Ocean Institute, and the Singapore Ministry of Education and the National Research Foundation, via their support of the Observatory.

(Cover image credit: USGS)

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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 35 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 is based on the information provided by NEIC (Lon.=-4.905°, ; Lat.=94.236°). The point source focal mechanism is obtained by the Cut-And-Paste Method (Zhu and Helmberger, 1996) with additional segments for S-waves. 1D velocity model is extracted from the CRUST2.0 global tomography model (Bassin et al., 2000). The fault plane with strike of 275 degree and dip of 82 degree (based on our point source focal mechanism inversion) is used for the inversion. Our inversion result shows rupture towards east and maximum slip of ~ 8m.

Map view of the historical earthquakes in the region and the location of recent earthquake.Inset: the source time function showing the time evolution of released moment rate.

Depth profile of slip distribution with arrows indicate slip vectors, and color coding shows amplitude, the isochrons show the rupture time. The rupture times are given relative to the onset of slip at the epicenter.

Comparison of the observed (black) and modeled (red) teleseismic seismograms (in displacement). Station names are indicated to the left of the traces along with the azimuths and epicentral distances in degrees. Peak amplitude in micron of data is indicated above the end of each trace.

Point source focal mechanism inversion result, shown by depth versus misfit plot. The detailed source parameters are shown on top. Number above each beach ball represents the corresponding moment magnitude.

Representative waveform fits for the point focal mechanism inversion (see the following figure for summary of all stations). Red seismograms represent the 1D synthetic generate by PREM model, black waveforms stand for the seismic data recorded,with their names hinted at the leftmost of the seismograms. Epicentral distance and azimuth of the stations are printed along with the station name.Synthetic and recorded waveforms are alighed by a certain value of time shift (underneath the seismographs) for each componer. Number beneath the time shift is the cross correlation coe cient (CCC) between the data and synthetics, which express the degree of resemblance.

Spidergram plots of crosscorrelation coeffcient (cc) and time shifts for 6 components involved in the inversion . Numbers right to the phase name indicate the constant shift due to the complex local structure or origin time correction. Time shifts for each phase give independent constrains on the velocity model for they represent the average divergence between the real structure and 1D model on the path of the phase.

 

 

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.

Zhu, Lupei, and Donald V. Helmberger. "Advancement in source estimation techniques using broadband regional seismograms." Bulletin of the Seismological Society of America 86.5 (1996): 1634-1641.

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 Sumatra earthquake, please refer to the USGS site here or the IRIS special event page here.

Research Team: