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Aftershocks of the Morocco earthquake
More than 200 events detected so far; temporary seismic network planned
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How many aftershocks have occurred since the September 8 earthquake in Morocco? Well… it depends on who you ask - and all datasets are all wrong, or at least incomplete.
Of the three international databases that we use, the Euro-Mediterranean Seismological Centre (EMSC) seems to have the most complete record: more than 200 events. In second place, the International Seismological Centre (ISC), with ~60. In last place, the United States Geological Survey (USGS): 2.
These differences highlight the importance of a dense local seismic network. Most of the aftershocks that have occurred since the mainshock are small, below magnitude 4. These events can only be accurately identified and located using seismometers that are relatively close - the seismic waves are not detected at distant stations.
The aftershocks are almost entirely shallower than the mainshock, which occurred at ~25 km depth. An inversion of the slip in the mainshock by researchers at CNR-IREA and INGV, using satellite-based radar imaging, suggests that the earthquake was blind - the slip only occurred on deeper parts of the fault, not reaching the surface.
The aftershocks are mostly located in either a large, shallow cluster southeast of the mainshock, or in a more dispersed shallow region to the northeast of the mainshock. The first cluster is roughly centered around the Tizi n’Test Fault, which we highlighted recently as the mostly likely candidate to have hosted this earthquake. The second zone is located around a set of low-angle décollement faults that extend northward below the basin, as we also discussed in a prior post. As of yet, it remains unclear what faults are slipping to produce these aftershocks. In fact, many of these events may not be occurring on major faults - small earthquakes can occur on many small fractures within the crust that do not connect to form large faults. The pattern of blind slip could be causing stress concentrations along the updip projection of the slip patch, which could be contributing to the density of shallow aftershocks in this region.
It is possible that some of the shallowest aftershocks may actually represent landslides, which can also create seismic signals.
In the day or so after the mainshock, there was a trend of aftershocks apparently propagating northward, towards Marrakesh. However, that seems to have stopped, and most of the more recent events are more closely clustered near the mainshock.
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While most of the earthquakes have individual depths assigned to them, some are still located at default depths - in particular, there is a strong band of events at 10 km depth, presumably poorly located.
The dataset provided by the EMSC is irregular: during some periods, it includes very small events - as small as M1.5, too small to feel. During other periods, however, only events above M3 are reported. The EMSC collates data from a number of different sources; this seems to be a a variation in data reporting by data providers.
The smaller events appear to be coming from the Centre National de la Recherche Scientifique et Technique (CNRST), a governmental research institute in Morocco. We do not know why they have reported data for some periods but not others; however, we assume that they are very busy, and suspect that this might simply reflect how quickly they can process the data that they are recording.
According to their Facebook page, the CNRST is planning to deploy a temporary seismic network around the area of the earthquake, which should help to track small-scale seismicity and better locate events.
So far, the aftershocks have been relatively small, although people in the area likely continue to feel unsettled by shaking. Aftershocks are a normal part of a sequence like this, and tend to follow a specific pattern: the rate of seismicity decreases, on average, as 1/time. In other words, by day 10, we should be seeing about 10% of the seismicity that was observed on day 1.
Nevertheless, it is important to remember that aftershocks are irregular, and larger events are still possible. This is a period of elevated earthquake risk, and homes that are vulnerable to earthquake shaking - including, but not limited to, those that are obviously damaged by the mainshock - could experience additional damage or collapse in a large aftershock.
It is possible, but not common, for one large earthquake to trigger another one of equal size or larger. Such triggering can happen within days, months, or even decades. Unfortunately, it is not possible to predict earthquakes, and there is no way to know if another large earthquake will happen. The best approach to managing earthquake hazard is to design and enforce appropriate seismic building codes.
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