Rapid estimation of faulting extent for large earthquakes by locating the end of rupture: application to the 2004, Mw=9.0 South Asia mega-thrust

Anthony Lomax

Anthony Lomax Scientific Software, Mouans-Sartoux, France, anthony@alomax.net, www.alomax.net

Feb 2005

To be presented at: European Geosciences Union, April 2005, Vienna. (EGU Abstract)


The 26 December 2004, Mw=9.0, South Asia earthquake caused a tsunami which devastated coasts around the eastern Indian Ocean within 3 hours.  Tsunami hazard warning and emergency response for future large earthquakes in this and related regions would benefit greatly if knowledge of the extent of earthquake rupture were available within minutes after the event.  Currently the size and extent of rupture of very large earthquakes are first estimated from moment tensor determinations based on long-period seismic recordings and from examination of aftershock locations; these estimates are not available until several hours or more after the event. 

Seismic P waves contain information on the earthquake rupture and are the earliest signal to arrive at distant recording stations.  Within about 15 minutes after the event the arrival times of the initial P wave are routinely used to locate the point of initiation of earthquake rupture, or hypocenter.  The first available information about the termination of rupture is contained in the last P-wave energy radiated from the source.  Here we introduce a method to extract arrival times for this energy through analysis of the shape of the short period, P-wave signal.  We use these arrival times to estimate the location of rupture termination in the same manner as for the hypocenter location.  The required P wave recordings from global seismic stations are available about 20 to 30 minutes after an event.  A few minutes later, this procedure provides an estimate of the rupture-termination location, and consequently the extent of rupture for the earthquake, and the rupture duration.  This information can aid in rapid assessment and modeling of tsunami hazard and of damage distribution. 

Application to the 2004, Mw=9.0, South Asia earthquake gives a rupture termination location near the Andaman Islands, about 1100 km north-northwest of the hypocenter, and a rupture duration of about 8 minutes.  These results imply mainshock rupture throughout the zone delimited by early aftershocks and an average rupture velocity of about 2.3 km/s.

Below: Map showing results for the 26 December 2004, Mw=9.0, South Asia earthquake.  A density cloud (red cluster of points) indicates the pdf for the termination of rupture location; this location is more likely to be in the denser regions of this pdf cloud.  Also shown are the mainshock hypocenter (red star), aftershocks located by NEIC within one week of the mainshock (white dots), and principal plate boundaries (violet lines).

20041226 Sumatra-Andaman - End of rupture

For Google Earth users: to view this figure as an Image Overlay click here (or save the Image Overlay kmz file available
here and drag it into My Places).

See also:

Anthony Lomax, Anthony Lomax Scientific Software, Mouans-Sartoux, France, anthony@alomax.net, www.alomax.net