Scientifiques files
 

Earthquake of magnitude 7.8 close to the Iran-Pakistan border on 16 April 2013.


An earthquake of magnitude 7.8 occurred on 16 April 2013 at 10h44 UTC in the extreme east of Iran.
This deep seismic event (87 km) was very widely felt over the entire area of the Persian Gulf up to distances of more than 1500 km.

Seismotectonic context:

The Arabian plate is descending under the Makran mountain chain at a rate of almost 3 cm per year. This tectonic context is responsible for the occurrence of strong subduction-related earthquakes at the front of the accretionary prism (zone of accumulation of sediments): for example, the very strong tsunamigenic earthquake of magnitude 8.1 on 28 November 1945 (see map on Figure 1). Nearly 400 km to the north of this subduction front, the Arabian plate is present at depths of more than 50 km, and is bending elastically under the weight of one of the many “geological blocks” (i.e. the Helmand Block) making up the southern margin of the Eurasian plate, which is said to be undergoing flexure. This flexure causes the nucleation of deep earthquakes within the Arabian plate. The strong earthquake of magnitude 7.8 on 16 April 2013 (see map on Figure 1) corresponds to a seismic event on a WSW-ENE-striking normal fault located at a depth close to 80 km. In view of this depth and focal mechanism (Figure 2), it thus most probably represents one of the normal fault-related seismic events, otherwise known as extrados earthquakes, which rupture the upper part of the downflexured Arabian plate. This earthquake occurred 200 km to the WSW of the Dalbandin earthquake of 18 January 2011, which displays a similar mechanism.


 Figure 1: Map showing location of the earthquake of 16/04/2013 and the seismicity of Makran. The seismicity presented used here is taken from the USGS database (historical earthquakes in yellow and instrumental earthquakes in orange or red according to their magnitude).


 Figure 2 Focal mechanism obtained by inversion of the W-phase at teleseismic distances ((Z. Duputel, L. Rivera, H. Kanamori and G. Hayes, 2012. W-phase fast source inversion for moderate to large earthquakes (1990 - 2010), Geophysical Journal International, v. 189, iss. 2, p. 1125-1147).