Damaging Magnitude 7.3 earthquake along the Iran-Iraq border was preceded by Magnitude 4.3 foreshock

Ross S. Stein, Ph.D., Temblor

A powerful, shallow earthquake struck along the Iraq-Iran border today. Damage is unfortunately heavy, with about 350 reported deaths, and numerous collapsed buildings. This was tragically demonstrated when the 2003 M=6.3 Bam, Iran, earthquake took 26,000 lives due to the almost complete collapse of ancient adobe dwellings. Also, a M=5.3 aftershock hit 10 minutes after the mainshock, which is large enough to bring down a building damaged by the first event.

 

The quake struck in a region of very low background seismicity

Although both Iraq and Iran are seismically active, and even though the quake lies only 100 km (60 mi) from the compressional boundary between the Arabian and Eurasian plates, there were no M≥4.5 quakes within about 60 km (35 mi) of today’s epicenter during the past 20 years.

 

The completeness magnitude for this region is likely about M=4.5 since 1997, and so we use those to assess the background, or preceding seismicity, and find it to be very sparse.

 

M=4.3 foreshock an hour before the mainshock

Nevertheless, unless the EMSC catalog suffers from timing errors, there was a M=4.3 ‘foreshock’ one hour before the mainshock, located about 60 km (35 mi) to the southwest of the future mainshock. Given how low the background rate is, this occurrence might indicate that the gently-dipping thrust fault on which the mainshock struck was undergoing precursory creep. Although the occurrence of foreshocks is rare, as indicators of future mainshocks or even creep, they are nevertheless unreliable.

 

The foreshock struck rather far from the mainshock, but could indicate deep precursory creep.

 

Is this a very rare event?

According to the ISC-GEM seismic catalog, the closest large events since 1900 were a pair of M=6.7 and M=6.8 events in 1957-1958, some 200 km (120 mi) to the southeast of today’s mainshock.

Broadly, the Arabia tectonic plate is being shoved against Eurasia plate along the Bitlis Suture and Zagros fold belt at a speed of 26 mm/yr (1 in/yr). This is the same slip rate as the San Andreas Fault. But because almost no M≥5.8 quakes struck in this region for the past 40 years, and because the local strain rate is not adequately measured by GPS, the Global Earthquake Activity Rate (GEAR) model shown in Temblor expects only a M=5.5-5.8 in a typical lifetime in this area. But the political and military conflicts in the region have prevented adequate GPS monitoring.

Today’s earthquake sequence struck along two borders: political and tectonic.

So, either this event is indeed quite rare, or the absence of GPS data has created artificial quake ‘hole’ in the GEAR model.

References: USGS ANSS catalog, ESMC catalog, ISC-GEM catalog

Updated: 8:30 AM PDT 13 Nov 2017