* By David Jacobson and Ross Stein, Temblor *

Updated: 3 p.m. (PDT) 1 May 2017

Since 22 April 2017, an earthquake swarm has been in progress offshore the port city of Valparaíso, which has been widely felt in Chile’s capital, Santiago. This sequence began with a M=6.0, and was followed two days later on 24 April, by a M=6.9 earthquake in the same swarm cluster. Today there were M=5.8 and M=5.9 earthquakes. What are the implications of the swarm, in light of historical earthquakes in the region, for what may happen next?

** Geological Disneyland **

Large megathrust earthquakes are one of three dramatic consequences of the ‘Nazca’ oceanic plate being shoved under the South American continent at about 80 mm/yr (3 in/yr). The second consequence of this ‘subduction’ is the compression and uplift of the Andes, and the third is the string of Pacific rim volcanoes. The Chilean coastline has suffered repeated great earthquakes over the 500 years of its recorded history. The largest earthquake ever recorded, a M=9.5 in 1960, occurred 570 km (350 mi) south of Santiago in southern Chile.

** How big could the next shock be? **

This week’s seismic swarm struck in the center of a 300-km-long span of the megthrust hemmed in by two recent great quakes. In 2010, the M=8.8 Maule earthquake ruptured the region to the south, and in 2015, the M=8.3 Illapel earthquake ruptured to the north (Melgar et al., 2016). The last large earthquakes to have occurred within this span were in 1971 (M=7.9) and 1985 (M=8.0), and so 50-60 years have since elapsed. Because the long term subduction, or slip rate on the megathrust, is about 80 mm/yr, a deficit of roughly 4-5 m of slip has accumulated. If a 300 km x 100 km section of the megathrust suddenly slipped 4-5 m (12-16 ft), that would yield a M~8.3 shock.

** The megathrust is stuck and so great quakes must eventually strike **

The velocities of GPS stations in Chile reveal that the megathrust surface offshore this 300-km-long span is stuck (Klein et al., 2017): The fault is not slipping freely, and so the accumulating stress must ultimately be released by a great quake or a large series of smaller ones. On the other hand, the swarm we witnessed this week is probably the manifestation of creep on at least one spot on the fault. So, its behavior is mixed, but dominated by great quakes.

** The recent shocks ratcheted up the stress on the Valparaíso section **

One can calculate the stress imparted by the 2010, 2015, and 2017 earthquakes to the unbroken span (We use Coulomb 3.4: Toda et al., 2011). The stress transfer is not an observation, but instead a calculation based on a series of reasonable assumptions, so one must take it with a grain of salt. Nevertheless, we find that roughly 3/4 of this span has experienced an estimated stress increase of at least 0.5 bars (shown as dark red in the figure). In general, stress increases of much less than this (0.1 bars) are associated with seismicity increases. One way to think about this result is that earthquake stress drops average 30 bars, and so a 0.5-bar increase represents a 1.5% jump.

So, if we think of great shocks here having about 50-75 year inter-event times, then the next shock has been advanced by several years. But the effect of stress imparted to rupture zones tends to fade with time, and so the impact of this week’s shock is large despite its small size, whereas the effect of the 2010 Maule shock might be smaller today despite its large size. The Global Earthquake Activity Rate (GEAR) model used by Temblor gives a 1% chance per year of a M~8 shock off Valparaíso, and we estimate that the combined effect of the recent shocks might double or triple this to 2-3% per year. So, the best we can say is that it’s a large increase on a small likelihood.

** Could the M=6.9 prove to be a foreshock of a M~8? **

It’s possible, but not obvious. Worldwide, no more than 5-10% of mainshocks are preceded by anything that a seismologist—even in retrospect—would want to call a foreshock. But the 2014 M=8.2 Iquique earthquake (Hayes et al., 2014) in northern Chile offers a stunning counter-example, and so we cannot exclude it down the road at Valparaíso. One can see from the ‘time series’ below that a M=6.7 quake struck 15 days before the Iquique mainshock, and it was followed a week later by many M=5-6 shocks that migrated northward. Ultimately, about half the future rupture zone lit up before the mainshock. In the case of Valparaíso, so far no more than about one fifth of the span has been filled by recent seismicity. But now, take a look at the time series for the 2010 M=8.8 Maule earthquake: No foreshocks whatsoever, and this is far more common.

** Reading the tea leaves **

So, if the pattern of Iquique were followed, in the next few weeks we could see an expansion of the swarm zone leading to a larger rupture. But if seismicity follows a far more common pattern, then the M~8.3 is out there, somewhere in the future.

** References **

Melgar, D., W. Fan, S. Riquelme, J. Geng, C. Liang, M. Fuentes, G. Vargas, R. M. Allen, P. M. Shearer, and E. J. Fielding (2016), Slip segmentation and slow rupture to the trench during the 2015, Mw8.3 Illapel, Chile earthquake, Geophys. Res. Lett., 43, doi:10.1002/ 2015GL067369.

Klein, E., Vigny, C., Fleitout, L., Grandin, R., Jolivet, R., Rivera, E., Métois, M. (2017), A comprehensive analysis of the Illapel 2015 Mw 8.3 Earthquake from GPS and InSAR data, in press, Earth and Planetary Science Letters.

Gavin P. Hayes, Matthew W. Herman, William D. Barnhart, Kevin P. Furlong, Sebastian Riquelme, Harley M. Benz, Eric Bergman, Sergio Barrientos, Paul S. Earle & Sergey Samsonov (2014), Continuing megathrust earthquake potential in Chile, after the 2014 Iquique earthquake, Nature, 512, doi:10.1038/nature13677.

Shinji Toda, Ross S. Stein, Volkan Sevilgen, and Jian Lin, Coulomb 3.3 Graphic-Rich Deformation and Stress-Change Software for Earthquake, Tectonic, and Volcano Research and Teaching—User Guide, U.S. Geological Survey Open-File Report 2011–1060 [https://pubs.usgs.gov/of/2011/1060/]

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