M=6.0 earthquake punctuates a sustained seismic swarm off Valparaíso, Chile

By Ross Stein and Volkan Sevilgen, Temblor

** Read about the M=6.9 earthquake that followed this swarm **

First Reported: 22 April 2017
Updated: 24 April 2017
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valparaiso-chile
Chile’s port city of Valparaiso has be shaken by several earthquakes over the last few days. (Photo from: Chile Travel)

 

Eight M~5 earthquakes have occurred on or near the megathrust surface, 30 km (18 mi) offshore of the major port city of Valparaiso, Chile. Santiago, Chile’s capital, lies another 100 km (60 mi) inland, where the largest quake was widely felt.

About 4 hours into the sequence, a M=6.0 earthquake struck, which was followed by five more M~5 shocks, as registered by the EMSC of Paris. Sustained sequences of moderate shocks are rare; most instead begin with the largest shock, termed the ‘mainshock,’ followed by ‘aftershocks’ that become less frequent with time, and are only rarely larger than the mainshock. So, in both respects—magnitude and frequency—this sequence looks different. Staccato seismic swarms like this one probably accompany slow slip on either the megathrust, or a thrust fault that splays off the megathrust.

22-Apr-2017-M5.9-Valparaiso-revised
Temblor map of the Chile Trench and the megathrust surface (pink) adapted from SLAB 1.0 (Hayes et al., 2012), along with the rupture zones of the 2010 Maule and 2015 Illapel events (blue), as well as earthquakes during the past month (green) and 24 hours (red). The rupture zones are from Robin Lacassin (IPGP).

 

Here’s what Gavin Hayes from the USGS said to us about the sequence: “The W-phase mechanism for the main shock looks related to the megathrust to me. These events aren’t far beyond the south end of the 2015 M=8.3 Illapel earthquake, and there’s a gap between 2010 M=8.8 Maule and the 2015 rupture zones. The Maule and Illapel earthquakes didn’t have appreciable foreshock sequences, but the 2014 Iquique event in northern Chile did. All food for thought.”

 

Here is the time sequence of the earthquakes in a 30 km x 60 km box centered on today’s sequence. The sequence began with a M~5, which was followed by some smaller aftershocks. But 3 hours later, about 7 M≥5 shocks struck in rapid succession.
Here is the time sequence of the earthquakes in a 30 km x 60 km box centered on today’s sequence. The sequence began with a M~5, which was followed by some smaller aftershocks. But 3 hours later, about 7 M≥5 shocks struck in rapid succession.

 

The seismic swarm is occurring in the site along the Chile Trench with the largest expected earthquake magnitude in a typical lifetime: M=8.0. This is the quake magnitude that has a 1% chance of occurring per year, whereas in Santiago (blue pin), it is M=7.0, which is about 30 times smaller.
The seismic swarm is occurring in the site along the Chile Trench with the largest expected earthquake magnitude in a typical lifetime: M=8.0. This is the quake magnitude that has a 1% chance of occurring per year, whereas in Santiago (blue pin), it is M=7.0, which is about 30 times smaller.

 

The depth and location of the sequence, as well as the ‘focal mechanism’ of the largest shock, suggest that the sequence is occurring on, or more likely above, the megathrust surface, where the Nazca Plate is descending beneath the South American Plate. The world’s great quakes have all struck on megathrusts, and the largest quake ever recorded—a M=9.5 event in 1960—struck along the Chile Trench in 1960 about 800 km (500 mi) to the south of today’s events. Closer to home was the 2010 M=8.8 Maule, Chile, earthquake, which we calculate brought the Valparaiso portion of the megathrust closer to failure.

Subduction of the Nazca plate beneath the deformed and faulted South American plate, from Armijo et al. (2010). Santiago lies at the foot of the Andes. The red dots show the best-located M≥4.0 earthquakes during the period, 2000–2005, by Servicio Sismológico Nacional of Universidad de Chile.
Subduction of the Nazca plate beneath the deformed and faulted South American plate, from Armijo et al. (2010). Santiago lies at the foot of the Andes. The red dots show the best-located M≥4.0 earthquakes during the period, 2000–2005, by Servicio Sismológico Nacional of Universidad de Chile.

 

That earthquakes should occur in this location is consistent with the Coulomb stress transfer from the 2010 M=8.8 Maule shock. One can see on the image below that the megathrust surface offshore Valparaiso was brought closer to failure (turns orange), whereas the stress dropped on the part of the megathrust that slipped in the 2010 shock (turns blue).

Coulomb stress imparted by the 2010 M=8.8 Maule, Chile, earthquake to the surrounding faults. We calculate the stress was increased at the site of today’s swarm.
Coulomb stress imparted by the 2010 M=8.8 Maule, Chile, earthquake to the surrounding faults. We calculate the stress was increased at the site of today’s swarm.

 

References

Armijo, R., R. Rauld, R. Thiele, G. Vargas, J. Campos, R. Lacassin, and E. Kausel (2010). The west Andean thrust (WAT), the san Ramón fault and the seismic hazard for Santiago (Chile), Tectonics, 29, TC2007.

Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.

USGS

EMSC

  • Cliff Thurber

    You called that, a magnitude 7.1 event happened a short while ago.

  • Ross Stein

    We didn’t call it, we only commented on it. But thank you, Cliff (who is a distinguished University of Wisconsin seismologist)