Ecuador subduction ” supercyclic ” behavior highlighted after the Pedernales earthquake of April 16th, 2016
The Pedernales earthquake (magnitude 7.8) violently struck Ecuador's central coast on April 16, 2016, causing nearly 700 casualties and more than 16,000 injuries.
Publication date: 09/01/2017
Research
Related teams :
Tectonics and Mechanics of the Lithosphere
Related themes : Natural Hazards
This earthquake is a continuation of the intense seismic activity in this area during the 20th century (earthquake of magnitude ~8.6 in 1906, followed by earthquakes of magnitude 7.7-8.2 in 1942, 1958 and 1979, see Figure 1). Based on this past seismicity, various projects have been set up since 2007 in collaboration with the Institute of Geophysics in Quito, to develop our capacity to observe earth deformations in this area, by installing GPS, seismometers and accelerometers. Initially instigated by the Geoazur laboratory, these projects have since been extended to the wider French community (IPGP, ISTEP) and have benefited from the ongoing involvement of the IRD and the support of the ANR.
These field observations, supplemented by satellite interferometry techniques (InSAR) and global seismic recordings (by the GEOSCOPE network, among others), have enabled us to reconstruct the earthquake rupture process in detail. As shown in Figure 2, the rupture lasted 45s and propagated from north to south over a distance of 100km, successively rupturing two zones along the Ecuadorian coast at a depth of between 15 and 30km. The second zone is characterised by a strong slide of 6 metres over an area of 30×30 km2 at a depth of 20 km below the coast. The associated sharp drop in stress led to very high ground accelerations (1.4 times the value of the gravity field). The preferential direction of the rupture also created a strong amplification of the seismic waves for the zones located to the south of the rupture (directivity effect). This effect explains at least in part the severe damage observed in the towns of Portoviejo and Manta, while the towns to the north of the rupture (Esmeraldas) were little affected.
As well as being useful for analysing the earthquake itself, the GPS data can be used to determine the stresses that accumulate along the Equator subduction line year after year. These measurements reveal that the displacements induced by the earthquakes of 1942, 1958, 1979 and 2016 are greater than the potential accumulated by the movement of tectonic plates since 1906. In other words, the high recurrence of major earthquakes since 1906 corresponds to the release of forces accumulated over several centuries. Recent marine palaeoseismology data acquired during an offshore campaign in 2000 on the Ecuadorian margin (Migeon et al., 2016) confirm a long period of seismic silence before the 20th century and a similar sequence at the end of the Middle Ages. The North-Ecuador and South-Colombia subduction therefore seems to follow a behaviour known as an earthquake ‘supercycle’, already observed for continental faults and the Sumatra subduction. In this behaviour, long periods of energy accumulation are followed by successive large earthquakes, each releasing only part of the accumulated energy. This behaviour makes it very difficult to understand the risk of earthquakes, and means that we should not always follow the intuitive but over-optimistic idea that a major earthquake reduces the risk of earthquakes.
Find out more:
- Main reference: Supercycle at the Ecuadorian subduction zone revealed after the 2016 Pedernales earthquake, J.-M. Nocquet, P. Jarrin, M. Vallée, P. A. Mothes, R. Grandin, F. Rolandone, B. Delouis, H. Yepes, Y. Font, D. Fuentes, M. Régnier, A. Laurendeau, D. Cisneros, S. Hernandez, A. Sladen, J.-C. Singaucho, H. Mora, J. Gomez, L. Montes and P. Charvis, Nature Geoscience, 2016.
- Interesting reworking of the results of the article: Why have there been so many large earthquakes in Ecuador?
- Related reference: Earthquake-triggered deposits in the subduction trench of the north Ecuador/south Colombia margin and their implication for paleoseismology, S. Migeon, C. Garibaldi, G. Ratzov, S. Schmidt, J.-Y. Collot, S. Zaragosi, L. Texier, Marine Geology, 2016.
