August 1, 2019, magnitude-6.8 Chile earthquake reveals stress is building on the megathrust

Tiegan Hobbs, Ph.D., Postdoctoral Seismic Risk Scientist, Temblor (@THobbsGeo)

Sandwiched in the neighborhood of the 1960 M=9.5, 2010 M=8.8, 2015 M=8.3 and 1985 M=7.8 earthquakes, a M=6.8 earthquake went virtually unreported on Aug. 1, 2019. But it has an important story to tell — that stress on the subduction zone is accumulating again.

CITATION: T.E. Hobbs (2019), August 1, 2019, magnitude-6.8 Chile earthquake reveals stress is building on the megathrust, Temblor, http://doi.org/10.32858/temblor.042

 

An earthquake struck the central coast of Chile on Aug. 1, 2019, shaking the small surfing town of Pichilemu as well as the nearby capital, Santiago. The depth of the hypocenter (where the earthquake starts) is being reported at 9 to 10 kilometers by the Centro Sismológico Nacional (CSN) and the U.S. Geological Survey (USGS). This indicates that the magnitude-6.8 event occurred on or just above the subduction zone interface, where the Nazca Plate subducts beneath the South America Plate at a rate of around 7 centimeters per year [Moreno et al., 2010].

 

A photo from Jan and Pete Lowe at the TravelBlog showing the great surfing waves at Pichilemu.

 

Chileans are used to living with earthquakes

This event produced no tsunami, fatalities, or major damage, which is why it was hardly reported. Chile is no stranger to earth shaking. The country has among the highest seismic hazard (https://www.globalquakemodel.org/gem) in the world [Pagani et al., 2018], with the Santiago region having the greatest seismic risk (https://downloads.openquake.org/countryprofiles/CHL.pdf) in the country [Silva et al., 2018]. According to the USGS, the region of the earthquake has experienced more than 12,000 recorded earthquakes since the year 2000. Events like these are so common that the Santiago Times and El País News didn’t even report on this event.

 

Nearby earthquakes have filled the gaps

The magnitude-6.8 earthquake, though smaller, occurred in a similar location to the 1985 magnitude-7.8 and 2010 magnitude-8.8 Maule earthquakes [Comte et al., 1986; Moreno et al., 2010]. Both of these events produced widespread damage along the coast, but the 1985 quake was preceded by 11 days of intense foreshocks.

 

This map, edited from a 2017 Temblor article, shows the location of the Aug. 1, 2019, M 6.8 Pichilemu earthquake (white star) at the edge of the 2010 M 8.8 Maule earthquake. Approximate rupture extents of that event and the 2015 M 8.3 Iquique earthquake are shown with purple ellipsoids, with their corresponding peripheral region of stress enhancement shown in red. The 2017 Valparaiso M 6.9 earthquake and accompanying seismic swarm are also shown. Rough extent of the 1985 earthquake is shown offshore in gray. Red and green dots indicate seismicity prior to the 2017 and 2019 earthquakes.

 

After that rupture, though, part of the fault remained to the south in which no major earthquake had occurred since 1835. That segment then ruptured in the 2010 magnitude-8.8 Maule earthquake, which affected roughly 2 million people, caused approximately 500 casualties, and generated a tsunami almost 2 meters high. It was also the first well-recorded earthquake to fill what’s called a “seismic gap.” That means it’s an area where some scientists suggest that more stress has been built up than has been released.

 

All stress relieved by the 2010 Maule earthquake

After the Maule earthquake, the gap was certainly filled. We know this because of a strange set of aftershocks a couple of weeks later. Two large, extensional ruptures occurred on shallow crustal faults [Ryder et al., 2012]. While the presence of aftershocks isn’t surprising, the presence of large extensional events in the upper plate means that all compressional stress there had been relieved by the mainshock. This was also then seen for the 2011 magnitude-9.0 Tohoku earthquake [Imanishi et al., 2012].

 

Adapted from Ryder et al., [2012], this figure shows aftershocks of the 2010 earthquake (black dots). Many lie along the subduction interface (orange shading), between the Nazca and South American plates. However, a pair of extensional earthquakes ruptured in the overriding plate, on shallow, high-angle faults.

 

Understanding the relationship between large megathrust earthquakes and shallow extensional aftershocks is also very important for characterizing seismic hazard. These shallow events tend to be much nearer to populated areas. Because they are farther landward and closer to the surface, the damage can be greater locally than for much larger aftershocks on the subduction interface.

 

What’s next?

The magnitude-6.8 earthquake at the northern end of the Maule rupture zone indicates that stress is accumulating again after being depleted. This is supported by Bedford et al., [2016], who found that the subduction zone relocked within about a year of the 2010 mainshock, and by a string of M>6 events since 2012. The average rate of recurrence for a large megathrust event is roughly 100 to 200 years anywhere on the Chilean coast [Nishenko, 1985; Beck et al., 1998]. While it has been only 34 years since the 1985 shock, could it re-rupture now?

 

Figure adapted from Métois et al., [2016], showing major earthquakes of the last 400 years along the coast of Chile (left) and background seismicity recorded by the Centro Sismologico Nacional from 2010 to 2014 (right). The 1985 earthquake (purple rectangle) partially fills the gap between the 2015 Illapel and 2010 Maule earthquakes, but is substantially smaller than the previous 1906 event.

 

The Pichilemu-Valparaiso section of the Chilean coast is somewhat different from its neighbors. Large earthquakes tend to occur more often, approximately every 83 years [Comte et al., 1986]. One would then expect the next earthquake on this section of the megathrust to occur in about 2068. However, the most recent event prior to 1985 released roughly two to three  times more energy [Barrientos, 1988]. Since the maximum slip of the 1985 earthquake was only 2.6 meters [Barrientos, 1988] and the plate slides at roughly 79 millimeters per year near Valparaiso, then the 1985 earthquake released at most about 32 years’ worth of built-up stress. Part of the remaining stress may be relieved by quiet (aseismic) slip between major earthquakes [Métois et al., 2016], or some of the stress may still be released as future moderate-sized quakes in the coast offshore Santiago.

 

References

Barrientos, S.E. (1988). Slip distribution of the 1985 central Chile earthquake. Tectonophysics, 145(3-4), 225-241.

Beck, S., Barrientos, S., Kausel, E., & Reyes, M. (1998). Source characteristics of historic earthquakes along the central Chile subduction Askew et Alzone. Journal of South American Earth Sciences11(2), 115-129.

Bedford, J., Moreno, M., Li, S., Oncken, O., Baez, J. C., Bevis, M., … & Lange, D. (2016). Separating rapid relocking, afterslip, and viscoelastic relaxation: An application of the postseismic straightening method to the Maule 2010 cGPS. Journal of Geophysical Research: Solid Earth121(10), 7618-7638.

Comte, D., Eisenberg, A., Lorca, E., Pardo, M., Ponce, L., Saragoni, R., … & Suárez, G. (1986). The 1985 central Chile earthquake: A repeat of previous great earthquakes in the region?. Science233(4762), 449-453.

Imanishi, K., Ando, R., & Kuwahara, Y. (2012). Unusual shallow normal‐faulting earthquake sequence in compressional northeast Japan activated after the 2011 off the Pacific coast of Tohoku earthquake. Geophysical Research Letters39(9).

Métois, M., Vigny, C., & Socquet, A. (2016). Interseismic coupling, megathrust earthquakes and seismic swarms along the Chilean subduction zone (38–18 S). Pure and Applied Geophysics173(5), 1431-1449.

Moreno, M., Rosenau, M., & Oncken, O. (2010). 2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone. Nature467(7312), 198.

Nishenko, S. P. (1985). Seismic potential for large and great interplate earthquakes along the Chilean and southern Peruvian margins of South America: a quantitative reappraisal. Journal of Geophysical Research: Solid Earth90(B5), 3589-3615.

Pagani, M., Garcia-Pelaez, J., Gee, R., Johnson, K., Poggi, V., Styron, R., Weatherill, G., Simionato, M., Viganò, D., Danciu, L., Monelli , D. (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), doi.org/10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1

Ryder, I., Rietbrock, A., Kelson, K., Bürgmann, R., Floyd, M., Socquet, A., … & Carrizo, D. (2012). Large extensional aftershocks in the continental forearc triggered by the 2010 Maule earthquake, Chile. Geophysical Journal International188(3), 879-890.

V Silva, D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore (2018). Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). DOI: 10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1

  • Perseidos

    Hello! By “the 1985 earthquake released at most about 32 years’ worth of built-up stress” do you actually mean “32 years” of stress building up right after the 1906 earthquake?
    If so, would that suggest there might currently be about 80 years of built-up stress still to be released since the 1906 event?

    I’ve read in scientific papers the Los Vilos-Pichilemu section seems to be highly coupled these days even though it’s been only 34 years after the last major shock in the region, however, considering the 1985 quake was significantly smaller than the 1906 event as well as what you explain now in this article, it all just seems to make sense and a big earthquake might not be quite a long time away as it was previously thought.

  • Tiegan Hobbs

    You’re right that the location of the 2019 earthquake is within the region of increased stress from the 2010 earthquake (second image in this article). Therefore, there may have been additional loading from the 2010 event, but it doesn’t need to be one or the other. Both background tectonic loading from convergence and stress transfer from a nearby earthquake, if in the correct geometry, can promote a future earthquake.

    You’re also right that there are seamounts in this region, although they are most numerous along the Juan Fernandez ridge to the north [ex: Ranero et al., 2006]. In the region of the 2010 Maule earthquake, a supposed subducted seamount is thought to have actually stopped the rupture [Hicks et al., 2012]. When a seamount results in less slip, we refer to it as a ‘barrier’. When it tends to be the nucleation zone of large earthquakes we refer to it as an ‘asperity’, which seems more in line with what you’re talking about.

    Hicks, S. P., Rietbrock, A., Haberland, C. A., Ryder, I. M., Simons, M., & Tassara, A. (2012). The 2010 Mw 8.8 Maule, Chile earthquake: Nucleation and rupture propagation controlled by a subducted topographic high. Geophysical Research Letters, 39(19).

    Ranero, C. R., von Huene, R., Weinrebe, W., & Reichert, C. (2006). Tectonic processes along the Chile convergent margin. In The Andes (pp. 91-121). Springer, Berlin, Heidelberg.