The earthquake that cracked a mountain?

Not the fault, but a fracture

The September 8, 2023 Mw 6.8 earthquake in Morocco occurred at 23:11 local time, when most people were asleep. Hours later, the sun rose over a changed landscape.

One dramatic change was immediately noticed by people living near the Tizi n’Test Fault, which is the best candidate for the fault that ruptured. A video posted by the news site @Hepress on X (Twitter) showed a steep mountain face traversed by vertical cracks. The video was flagged by user @kymyk, who translated part of the audio into English for us: “The mountain split in two parts… local people never saw the crack before.” They passed us the coordinates of the peak and asked for any information about what had happened. So, let’s try to find out.

Screenshots from the video posted by @Hespress. Source: https://twitter.com/hespress/status/1701570100569194580. We do not know what the text says!

Disclaimer: Landslide and rockfall hazard is a serious problem in steep mountain areas, and our analysis cannot say whether this hazard has increased or decreased after the earthquake. On-the-ground engineering analysis and monitoring are likely needed in this case.

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Where is the mountain?

We used the provided coordinates to match the video viewpoint in Google Earth. Once we knew where the peak was located, we searched old topographic maps (ca. 1941) for place names, and determined that the mountain is probably named Djebel Ifilillis ( 30.852481°N, 8.517052°W).

Clip of geological map of the area; white arrow highlights peak Djebel Ifilillis. Basemap source: Kharitat al-Maghrib al-jiyulujiyah (Carte Géologique du Maroc), 1985. See text below for a link that hopefully works!

Link to 1985 geological map of Morocco: https://exhibits.library.cornell.edu/barazangi-map-collection/catalog/60-2763

By matching that location to a geological map - a map of rocks and rock fabrics - we can see that the mountain peak is located just near the edge of the Tichka Massif, a body of rock formed by the intrusion of magma into existing, older rock. This intrusion occurred several hundred million years ago, during the collision of continents that built the supercontinent Pangaea.

Geological map and cross-section of the Tichka Massif, showing multiple stages of intrusion and steep foliation fabrics within the rocks. White arrow on the map shows location of the mountain Djebel Ifilillis. Source: Lécuyet et al. (2017).

The massif - and the mountain peak - are located in the hanging wall of the Tizi n’Test Fault (i.e., above the fault). Surrounding the massif is old (Cambrian, ~540-485 million years old) sedimentary rock. Bedding remains visible in satellite imagery - layers of rock deposited horizontally but now tilted steeply, the result of an immensely long history of deformation.

Geological map overlain on Google Earth topography, highlighting the location of the cracked mountain. Pz = Paleozoic; Tr = Triassic; K = Cretaceous (rock ages reflecting different periods of geological time). Note that the Paleozoic is a time period that includes the Cambrian. Black lines are mapped faults that are part of the Tizi n’Test system.

The crack is one of many fractures - and not a fault surface rupture

So, what can we say about the crack? First, we note that while it is located near the fault that slipped, it is not on it; the fault lies further to the southeast. Using imagery from Google Earth, we can identify a number of apparent cracks in these ancient rocks. Some of these cracks also align with drainages. This is natural: cracks weaken the rock, making it easier for these locations to erode.

Many fractures can be mapped in the rocks. One of those fractures lines up with the post-earthquake observation of a cracked mountain.

In fact, a photo of the same mountain shows that prior to the earthquake, there was already apparent cracking cutting through it, near the peak, in a location similar to the crack evident in the recent video.

Photo of the mountain taken prior to the earthquake by abbi fares. Available at: https://lh5.googleusercontent.com/p/AF1QipOQwtVDoNgC5l8b9WOdOIR8T3WKal1vHu5KX4rO=h1440. Black arrow highlights apparent crack.

As highlighted by Simone Atzori (@SimoneAtzori73) at INVG, both seismic data and radar-based satellite imaging indicate that the M6.8 earthquake was probably blind - the rupture did not extend to the surface. This means that we do not expect to see direct observations of fault offset at the surface. However, areas above the tip likely experienced both strong shaking in the earthquake, and deformation and tilting in response to the deeper slip. It is easy to imagine that rocks in the area may have cracked, or that existing cracks may have moved, in response to these stresses.

Comparison of the slip modeled by the USGS using recorded seismic waves vs. slip modeled by INGV using surface deformation measured by radar satellites. Both ,methods show that the slip likely did not reach the surface. Source: https://twitter.com/SimoneAtzori73/status/1701491361344606337

What do post-earthquake satellite images show?

We downloaded satellite images of the mountain before (September 4) and after (September 11) from Planet Labs to see what we could find. The first thing we noticed was that after the earthquake, many parts of the area are brighter than before, especially along drainages (blue arrows below).

These brighter areas likely reflect exposure of fresher rock - probably caused by rock falls. This is a very rugged region with many steep slopes, so earthquake-triggered landslides and rock falls are expected.

When we mapped out the cracks from the video on the satellite imagery, we found that they partially aligned with some of these brighter areas. Cracking of the mountain could also expose fresh rock surfaces, or trigger rock falls. Some bright regions are visible in the video, although without a matching pre-event photo, it isn’t possible to say for sure that those are new.

Without the video and description of change by locals, we would not have highlighted this mountain as a site of significant change. This reflects the limitation of remote sensing, and the need for more detailed studies using field observations. It is likely that we will see a variety of field-based results in the coming months and years, documenting the earthquake and looking for other geological evidence of past ruptures in the range.

References:

Kharitat al-Maghrib al-jiyulujiyah (Carte Géologique du Maroc), 1985. Accessed at: https://exhibits-int.library.cornell.edu/barazangi-map-collection/catalog/60-2763.

Lécuyer, C., Gasquet, D., Allemand, P. et al. Cooling history of nested plutons from the Variscan Tichka plutonic complex (Morocco). Int J Earth Sci (Geol Rundsch) 106, 2855–2872 (2017). https://doi.org/10.1007/s00531-017-1463-z

Vogel, T.A. and Walker, B.M., 1975. The Tichka Massif, Morocco—an example of contemporaneous acidic and basic plutonism. Lithos, 8(1), pp.29-38. https://doi.org/10.1016/0024-4937(75)90028-6