16 April 2016 Ecuador M=7.8 quake likely a repeat of a 1942 M=7.8 shock; such repeats are rare but important

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Click for the New Ecuador Earthquake Article

17 April 2016  |  Quake Insights

The event severely shook the coastal cities of Muisne, Tosagua, and Pedernales, with 34,000 inhabitants. The quake was broadly felt throughout Ecuador and southern Colombia. On the basis of historic earthquakes, including the 1906 M~8.3 shock, as well as a high strain rate, the region was known to have a high seismic hazard.

The megathrust ruptured along the Ecuador Trench, along which the Nazca plate subjects beneath South America at about 55-61 mm/yr. The rupture area, as determined thus far only by Gavin Hayes at the USGS in Golden, CO, using seismic waveforms, is about 50 x 100 km in extent (30 x 60 mi), and is inclined (‘dipping’) at about 15° under the coastline. The rupture propagation took about a minute, with the greatest pulse in the first 30 seconds. The peak slip is about 5 m (16 ft), directly under the coastline. The shallow depth, rapid rupture, and high slip beneath populated areas probably all contributed to the damage, which thus far has killed 238, and will likely rise considerably.

Rupture areas of historical quakes near the 16 April 2016 M=7.8 event. The 1942 rupture area nearly coincides with the 2016 quake. There was also a M=7.1 shock in 1998 (Chlieh et al., 2014), which lies within, or just south of, today’s aftershocks. The 2016 event could also trigger a a subsequent megathrust quake to the north, or on the right-lateral Jama-Quininde fault (Collot et al. 2004).

The 16 April 2016 quake appears to lie in almost the exact location as the 1942 M=7.8 quake studied by Senneson and Beck (1998). The distribution of observed seismic intensities for the 1942 shock closely match those predicted by the USGS for yesterday’s quake, and their aftershock distributions are similar. In addition, the large aftershocks of the 16 April 2016 shock lie on either side of the 1942 rupture zone. This would be expected if the 2016 rupture were smooth (rather than spiky), with aftershocks in the Coulomb stress ‘butterfly zones’ just outside of the rupture patch.

The ISC-GEM seismic catalog shows that the M=7.8 epicenter is surrounded by large quakes over the past century, whereas in southern Ecuador, there have been no great quakes (top). Nevertheless, the strain rate is high along the entire Ecuador coastline (bottom). This part of the boundary could slip aseismically, or in less frequent but larger earthquakes. These invaluable datasets were produced by the GEM Foundation, which is building a seismic risk model for the world, and is currently working on its Ecuador model together with South American scientists.

There is also evidence that 50 km to the south of the 2016 M=7.8 quake, there was a ‘slow slip event,’ essentially a M=6.3 silent earthquake. This event is shown in the figure below from Chlieh et al. (2014); the event can only be detected by GPS receivers because it did not excite any seismic waves. Many subduction zones produce such slow slip events. It is possible that theses events, if frequent enough, relieve the stress that would otherwise accumulate to produce great quakes.

In this figure from Chlieh et al (2014), ‘ISC’ means inter-seismic coupling. If it is red (1.0), the megathrust is stuck and must produce quakes; if blue (0.0), the plate is freely slipping and so large quakes will not strike. Notice that this model shows the site of the 2016 quake is in the blue (at the site of the 1942 star), which means this 2016 quake just disproved this ICS theory. Science is hard.

Is a repeat of the 1942 event reasonable, given that only 75 years has since elapsed?

The answer is yes. At the 60 mm/yr subduction rate, 75 years is enough to produce another quake with 4-5 m (12-16 ft) of slip. Given the area of the two shocks, a M=7.8 today would be right on target if the site did not suffer much larger quakes at other times. But that’s the problem: The site did suffer a M~8.3 event in 1906, and so repeats of M=7.8 events cannot be the whole story. Near-exact repeats of earthquakes are very rare; in general, we find a wide diversity of rupture behavior. And considering that this site was part of the ten-times larger event in 1906, it is particularly baffling.

What next?

A subsequent M~8 event to the south might be considered unlikely, both because of the M=7.1 event in 1998 and the slow slip event in 2010. But the region to the north of the 16 April 2016 event is different. There is an 80 km (50 mi) region that has not ruptured since 1906 that would seem able to host another M~7.8 shock. Further, there is the right-lateral Jama-Quininde fault, parts of which could have been brought closer to Coulomb failure by this recent 2016 megathrust event. Both should now be closely watched.

Click for the New Ecuador Earthquake Article

Ross Stein and Volkan Sevilgen, Temblor

Sources: USGS, GEM Foundation (globalquakemodel.org), and,

Chlieh, M., P.A. Mothes, J.-M. Nocquet, P. Jarrin, P. Charvis, D. Cisneros, Y. Font, J.-Y. Collot, J.-C. Villegas-Lanza, F. Rolandone, M. Vallée, M. Regnier, M. Segovia, X. Martin, H. Yepes (2014), Distribution of discrete seismic asperities and aseismic slip along the Ecuadorian megathrust, Earth Planet. Sci. Letts., 400, 292-301, http://dx.doi.org/10.1016/j.epsl.2014.05.027

Collot, J.-Y., B. Marcaillou, F. Sage, F. Michaud, W. Agudelo, P. Charvis, D. Graindorge, M.-A. Gutscher, and G. Spence (2004), Are rupture zone limits of great subduction earthquakes controlled by upper plate structures? Evidence from multichannel seismic reflection data acquired across the northern Ecuador–southwest Colombia margin, J. Geophys. Res., 109, B11103, doi:10.1029/2004JB003060.

Sennson, Jennifer L, and Susan L. Beck (1996), Historical 1942 Ecuador and 1942 Peru subduction earthquakes and earthquake cycles along Colombia-Ecuador and Peru subduction segments Pure Appl. Geophys., 146, 67-101, doi: 10.1007/BF00876670