Mexico landslide kills at least two people.

Two people, including a three-year-old girld, have now been confirmed dead and another two are still missing following a landslide that hit the city of Tlalnepantla in Mexico State, Mexico, on Friday 10 September 2021. The landslide was caused by the collapse of part of the Cerro del Chiquihuite hillside following heavy rain in the area, which dislodges a series of large boulders, estimated to weigh up to 200 tons, onto a residential district.Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. However, in this case the rains are thought to have been only part of the problem, with the area having been shaken by a Magnitude 7.0 Earthquake which occurred near Acapulco on the coast of Guerrero State, 400 km to the south of Tlalnepantla, on Tuesday 7 September.

  

Boulders lying on top of residential housing in Tlalnepantla, Mexico, following a landslide on 10 September 2021. Eduardo Verdugo/AP.

The 7 September 2021 Acapulco Earthquake was recorded by the United States Geological Survey as occurring at a depth of 20 km, roughly 17.7 km to the north of the city, slightly after 8.45 pm local time (slightly after 1.45 am on Wednesday 8 September GMT). The event was felt over much of southern area, and caused minor damage to many buildings, but no major casualties or destruction.

 

The approximate location of the 7 September 2021 Acapulco Earthquake. USGS.

Mexico is located on the southernmost part of the North American Plate. To the south, along the Middle American Trench, which lies off the southern coast off Mexico, the Cocos Plate is being subducted under the North American Plate, passing under southern Mexico as it sinks into the Earth. Guatemala is located on the southern part of the Caribbean Plate, close to its boundary with the Cocos Plate, which underlies part of the east Pacific. The Cocos Plate is being pushed northwards by expansion of the crust along the East Pacific Rise, and is subducted beneath the Caribbean Plate along the Middle American Trench. This is not a smooth process, and the plates frequently stick together then break apart as the pressure builds up, causing Earthquakes on the process.

 

The position of the Cocos, Nazca and Rivera Plates. MCEER/University at Buffalo.

 

The Cocos Plate is thought to have formed about 23 million years ago, when the Farallon Plate, an ancient tectonic plate underlying the East Pacific, split in two, forming the Cocos Plate to the north and the Nazca Plate to the south. Then, roughly 10 million years ago, the northwesternmost part of the Cocos Plate split of to form the Rivera Plate, south of Beja California.

 

In a paper published in the Journal of Geophysical Research, in 2012, a team led by Igor Stubailo of the Department of Earth and Space Sciences at the University of California Los Angeles, published a model of the subduction zone beneath Mexico using data from seismic monitoring stations belonging to the Mesoamerican Seismic Experiment, the Network of Autonomously Recording Seismographs, the USArray, Mapping the Rivera Subduction Zone and the Mexican Servicio Sismologico Nacional.

 

The seismic monitoring stations were able to monitor not just Earthquakes in Mexico, but also Earthquakes in other parts of the world, monitoring the rate at which compression waves from these quakes moved through the rocks beneath Mexico, and how the structure of the rocks altered the movement of these waves.

 

Based upon the results from these monitoring stations, Stubailo et al. came to the conclusion that the Cocos Plate was split into two beneath Mexico, and that the two plates are subducting at different angles, one steep and one shallow. Since the rate at which a plate melts reflects its depth within the Earth, the steeper angled plate melts much closer to the subduction zone than the shallower angled plate, splitting the Trans-Mexican Volcanic Belt into sections above the different segments of the Cocos Plate, and causing it to apparently curve away from the subduction zone.

 

Top the new model of the Cocos Plate beneath Mexico, split into two sections (A & B) subducting at differing angles. (C) Represents the Rivera Plate, subducting at a steeper angle than either section of the Cocos Plate. The Split between the two has been named the Orozco Fracture Zone (OFZ) which is shown extended across the Cocos Plate; in theory this might in future split the Cocos Plate into two segments (though not on any human timescale). Bottom Left, the position of the segments on a map of Mexico. Darker area is the Trans-Mexican Volcanic Belt, orange circles are volcanoes, brown triangles are seismic monitoring stations, yellow stars are major cities. Bottom Right, an alternative model showing the subducting plate twisted but not split. This did not fit the data. Stubailo et al. (2012).

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