I was asked to provide some written comment to a Chilean newspaper, so I thought I'd take this opportunity to write a blog post about the current seismic events offshore of Valparaiso, Chile.
Seismic swarm
There was a clear sequence of seismic events that led up to last night's M6.9 earthquake.
The first event of this seismic sequence occurred on 22 April with a magnitude 4.9 quake. The following day, there were many more small and moderate sized events, including a M5.9 quake. We call this sequence a seismic swarm because it contained many similar sized events occurring in the same location and within a short time of each other. There is some indication a number of these events may have occurred on splay faults that branch from the main subduction fault (see cross-section below).
The first event of this seismic sequence occurred on 22 April with a magnitude 4.9 quake. The following day, there were many more small and moderate sized events, including a M5.9 quake. We call this sequence a seismic swarm because it contained many similar sized events occurring in the same location and within a short time of each other. There is some indication a number of these events may have occurred on splay faults that branch from the main subduction fault (see cross-section below).
Cross-section through the central Chile subduction zone. Red circles give the location of small past earthquakes. The yellow stars give the location of the 2017 foreshock and mainshock earthquakes. Original figure from Armijo et al. (2010).
Eventually, this swarm seems to have culminated (for now, at least) with the M6.9 earthquake that occurred yesterday evening; this is by far the strongest earthquake of the sequence so far. Preliminary data shows that this earthquake may have ruptured the megathrust plate boundary between the Nazca and South American plates. This type of faulting mechanism is responsible for some of the world's largest earthquakes.
Knowing what we do now, we can classify the earlier sequence as a foreshock swarm of the M6.9 earthquake. Naturally, this earthquake will have its own aftershock sequence. The largest aftershock so far has been a magnitude 5.4 earthquake that occurred just ~30 minutes after the mainshock. As part of ‘normal’ aftershock sequences, we can expect events as large as around 1 unit of magnitude less than the mainshock (i.e. M5.9-6). The most likely scenario for the near future is that the aftershock rate will gradually decrease with time. However, we cannot rule out the possibility of more, similarly strong, or even stronger events.
Knowing what we do now, we can classify the earlier sequence as a foreshock swarm of the M6.9 earthquake. Naturally, this earthquake will have its own aftershock sequence. The largest aftershock so far has been a magnitude 5.4 earthquake that occurred just ~30 minutes after the mainshock. As part of ‘normal’ aftershock sequences, we can expect events as large as around 1 unit of magnitude less than the mainshock (i.e. M5.9-6). The most likely scenario for the near future is that the aftershock rate will gradually decrease with time. However, we cannot rule out the possibility of more, similarly strong, or even stronger events.
Historical precedence
| The characteristics of this sequence are fairly similar to what occurred during the seismic sequence that led to the M8.2 Pisagua / Iquique earthquake in northern Chile in 2014. In the 2014 case, the foreshock sequence lasted weeks to months, whereas so far, it seems the Valparaiso sequence lasted a matter of just days. It is possible that this type of behaviour is somewhat characteristic of large earthquakes along the Chile subduction zone. Even the 1960 M9.5 Valdivia earthquake (the largest earthquake ever recorded) had large foreshocks that preceded it in the same area in the days before. The last strong earthquake in the Valparaiso region occurred in 1985 (magnitude 7.8). Scientific studies have shown that this 1985 earthquake had its own intense foreshock sequence. Going back even further in time, earthquakes in this region typically occur every 75-90 years (see the graph to the right). |  Past large earthquakes along the central Chile subduction zone. Original figure from Comte et al. (1985) and updated to include large ruptures since 1985. This figure shows that possible 'seismic gaps' still lie to the north and south of the April 2017 Valparaiso earthquake. |
The April 2017 earthquake doesn’t seem to fit this apparently regular pattern. This discrepancy may be due to several factors: the low sample size of historical events, the little detail on magnitudes of ancient earthquakes, and a possible lack of data on moderate to large magnitude events that were of a similar size to yesterday’s quake.
Outlook
| Overall, as yesterday’s event and the past history continues to tell us that the central Chile subduction plate boundary is very active and the region has a very high seismic hazard. We also know that the subduction megathrust fault offshore of Valparaiso is highly locked and may have the potential to host a M>7.5 earthquake in the future. There are still possible 'seismic gaps' to the north and south of the April 2017 Valparaiso earthquake. The map to the right shows the high degree of fault coupling in the area of the 2017 earthquake. With the nearby population centres of Valparaiso and Santiago, earthquakes in this part of Chile have the potential to produce significant damage. We can be sure though that our seismologist colleagues at the CSN (Centro Sismológico Nacional, Universidad de Chile) will be working very hard to inform the local public of any further developments in this current seismic sequence. |  Distribution of fault coupling along the central Chile subduction zone. Darker shading indicates parts of the fault that are more strongly coupled. Map from Metois et al. (2012). |
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