The M=7.1 megathrust quake that struck Peru is small by the standards of the region

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Ross S. Stein, Ph.D., Temblor

 

Templo de Yauca, close to the epicenter
Templo de Yauca, close to the epicenter

 

This morning’s earthquake struck at a depth of 36 km. Given its modest size and moderate depth, it did not produce a large tsunami. Nevertheless, one death and 42 injuries have been reported, and Peru will declare a state of emergency in parts of Arequipa, the state lying to the southeast of the epicenter.

The quake occurred in a highly active region of the Peruvian coastline (below right), in which a M=7.1 is not uncommon. In fact, according to the GEAR earthquake forecast, a local resident would be expected to experience a quake of this about size in their lifetime. Within a 300 km (180 mi)-long stretch of coastline from of today’s epicenter, there have been a dozen M≥7 quakes since 1913.

 

In the left panel, regions identified by Chilieh et al (2011) as having a large slip deficit are shown as the ‘Most overdue’ patches. These are the sites where the megathrust is stuck and is accumulating stress at the highest rate, as inferred from GPS measurements. But today’s shock did not strike in or near one of these stuck patches, perhaps because the rupture area of a M=7.1 quake, about 50 x 50 km, is too small to be resolved by GPS.
In the left panel, regions identified by Chilieh et al (2011) as having a large slip deficit are shown as the ‘Most overdue’ patches. These are the sites where the megathrust is stuck and is accumulating stress at the highest rate, as inferred from GPS measurements. But today’s shock did not strike in or near one of these stuck patches, perhaps because the rupture area of a M=7.1 quake, about 50 x 50 km, is too small to be resolved by GPS.

 

Neither the USGS nor the EMSC catalogs show any foreshocks during the past 30 days, nor aftershocks during the past 18 hours. These catalogs have a detection level of about M≥4.2 here, so this is a surprising episode of seismic quiet, as well as a remarkable absence of aftershocks. The Geophysical Institute of Peru (IGP) reported just two aftershocks, a M=3.7 and M=3.8 events in the first 6 hours. These are beneath USGS and EMSC detection levels.

 

There have been four M≥7.7 shocks since 1900 within 125 km (80 mi) of today’s epicenter, in this highly active subduction zone. Image from Google Earth; earthquakes from the ISC-GEM catalog.
There have been four M≥7.7 shocks since 1900 within 125 km (80 mi) of today’s epicenter, in this highly active subduction zone. Image from Google Earth; earthquakes from the ISC-GEM catalog.

 

Sources

USGS
EMSC
IGP
Xinhua News Agency

Mohamed Chlieh, Hugo Perfettini, Hernando Tavera, Jean-Philippe Avouac, Dominique Remy, Jean-Mathieu Nocquet, Frédérique Rolandone, Francis Bondoux, Germinal Gabalda, Sylvain Bonvalot (2011), Interseismic coupling and seismic potential along the Central Andes subduction zone, Journal of Geophysical Research, 116, doi: 10.1029/2010JB008166

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