By John Elliott (University of Oxford)
Recently published work shows that there is significant remaining seismic hazard at the city of Van resulting from the relatively deep rupture of the previous earthquake from 2011.
In October 2011 a magnitude 7.1 earthquake struck the cities of Van & Ecriş in Eastern Turkey killing over 600 people. Using satellite radar measurements, we were able to precisely map the fault segments that ruptured in this event.. These observations were critical in showing that the slip in the earthquake remained buried between 8 and 20 km underground. However, this leaves a large shallow portion of the fault nearer the surface that has not broken, and field observations show that this fault is capable of rupturing in earthquakes, posing a continuing hazard to this city of a million people.
Earthquakes continue to surprise in their location and size, such as the 2010-2011 Christchurch earthquakes in New Zealand, which occurred on unknown faults. The Van earthquake also occurred on previously unmapped faults, highlighting the need to undertake basic mapping of active fault structures, as well as measure regions building up strain that might rupture in the future.
The InSAR data presented here (Figure 1) permit us to determine an accurate location and the depth extent of faulting for the 23 October Mw 7.1 earthquake as 8–20 km, along a pair of fault planes, each 14 km in length. This relatively deep earthquake leaves an unruptured upper portion of the fault in the shallow crust, which is now under greater stress from this deeper slip.
The projection of these fault segments to the surface is just 10 km north of Van (Figure 2). For such a large earthquake, very little of the slip occurred at the surface (Figure 3a) – the maximum slip of 9 m occurred at 14 km depth. However, given that the surface trace of the fault is clearly visible in the geomorphology of the mountain range to the north of Van and that fault gouge was found in Quaternary sediments at the surface (Figure 3b), it is very likely that the upper portion of the crust is seismogenic, and could still break in the near future.
A hiatus of months to years in rupture between two similarly sized earthquakes is not unprecedented, as found for a pair of equal sized earthquakes in China which occurred on the same fault – the first deeper in 2008, and the second much shallower ten months later in 2009.