M=6.1 earthquake in Iran: A near miss

Temblor

8 years ago

By David Jacobson and Ross Stein, Temblor

Mashhad, Iran, the second most populous city in the country and home to 3.3 million people experienced light shaking in today’s M=6.1 earthquake. (Photo from: letsgopersia.com)

At 9:39 a.m. local time today (5 April 2017), a M=6.1 earthquake shook northeastern Iran, 87 km southeast of the city of Mashhad, the second most populous city in the country, and home to over 3 million people. According to USA Today, 2 people are confirmed to have died, while 11 others are injured. However, there are concerns that more people may have died in smaller towns closer to the epicenter, which fortunately was in a sparsely populated region.

This Google Earth image shows the location of today’s M=6.1 earthquake in the extremely arid northeastern corner or Iran. The red lines in this image represent faults from the USGS World Energy Project. The city of Mashhad, which is home to 3.3 million people and can be seen to the northwest of the epicenter, experienced light shaking.

Based on the USGS focal mechanism, today’s earthquake was primarily thrust in nature, with a small strike-slip component. The compressional regime of the region is a product of the Arabian Peninsula moving at a rate of 31 mm/yr towards the Middle East, which tectonically-speaking, this is quite fast. In northeastern Iran, approximately 6.5 mm/yr of this movement is accommodated across the Kopet-Dag fault system.

A quake that would kill 3 people in California would take 3,000 lives in Iran

While it is fortunate that there are only 2 confirmed fatalities, the same size earthquake underneath a populated center could have dramatic effects. A 2010 analysis of global earthquake fatalities over the past forty years by Kishor Jaiswal and David Wald of the USGS (the brains behind the PAGER damage and fatality alert system) has shocking implications for Iran: The country is a thousand times more vulnerable to earthquakes than California.

This figure modified from Jaiswal and Wald, 2010 shows fatality rates from earthquakes around the world. What this illustrates is that Iran is 1,000 times more vulnerable to earthquakes than California. This difference stems almost exclusively from building quality and construction. (Figure from: Kishor Jaiswal and David Wald (2010), An Empirical Model for Global Earthquake Fatality Estimation, Earthquake Spectra, 26, 1017–1037, DOI: 10.1193/1.3480331)

How could this be?

First, it has nothing at all to do with the character of shaking in Iran. A M=6 in Iran would shake the ground the same as a M=6 in California. Instead, the difference stems almost exclusively from building construction quality, particularly old unreinforced masonry and adobe buildings, and to a lesser extent, modern but inadequately reinforced or poorly constructed buildings. Densely populated cities with narrow streets also limit the speed of emergency response, as was seen in the 2016 Amatrice, Italy earthquake.

This means that had today’s M=6.1 shock been beneath vibrant and rapidly growing Mashhad, with 3.3 million residents, it could have killed about 100,000 people rather than 2. In airline parlance, today’s quake was a “near miss.”

A once-in-a-lifetime quake

Based on the Global Earthquake Activity Rate (GEAR) model, which uses global strain rates, and seismicity since 1977, today’s earthquake falls within the 1% chance per year range. What this means is that an event of this magnitude was likely in your lifetime for this location. Therefore, while not necessarily surprising, this quake could provide a wakeup call to a region susceptible to damaging earthquakes. This global earthquake forecast is available in Temblor, and to view it, click here.

This Temblor map shows the location of today’s earthquake as well as the faults across Iran. This map also has the Global Earthquake Activity Rate (GEAR) model overlain, which shows that today’s quake was not surprising. In fact, an earthquake of this magnitude was likely in your lifetime for this location.

References
USGS
USGS World Energy Project (Iran Faults)
Kishor Jaiswal and David Wald (2010), An empirical model for global earthquake fatality estimation, Earthquake Spectra, 26, 1017–1037, DOI: 10.1193/1.3480331
Ph. Vernant, F. Nilforoushan, D. Hatzfeld, M. R. Abbassi, C. Vigny, F. Masson, H. Nankali, J. Martinod, A. Ashtiani, R. Bayer, F. Tavakoli and J. Chery, Present-day crustal deformation and plate kinematics in the Middle East constrained by GPS measurements in Iran and northern Oman, Geophys. J. Int. (2004) 157, 381–398, doi: 10.1111/j.1365-246X.2004.02222.x