Mexico was shaken by a pair of earthquakes

The earthquake, which struck along the Pacific coast of Mexico, redistributed the pressure in the region.

Written by Hector Gonzalez-Huezar, Ph.D., Center for Scientific Research and Higher Education in Ensenada, Baja California (CICESE), Chioli Pérez Campos, Ph.D., Institute of Geophysics, National Autonomous University of Mexico (UNAM), Caltech (xyolipc), and Diego Melgar, Ph.D. University of Oregon (geosmx)

Quote: Gonzalez-Huezar, Perez-Campos, X., Melgar, DE, 2022, Mexico shaken by a pair of earthquakes, Temblor, http://doi.org/10.32858/temblor.271

This article is also available in Spanish.

On Monday, a 7.7-magnitude earthquake struck near the town of Colcumen in Michoacan, Mexico. This event was widely felt in various parts of the country, including Mexico City where the seismic alert from the Mexico Seismic Early Warning System (SASMEX, Suárez, 2022) was activated and heard by many people a few seconds before the strong shaking began . The earthquake was also felt in northern Mexico.

In the 24 hours following the earthquake, the Mexican Seismological Service (SSN) reported more than 800 aftershocks. On Wednesday, about 60 hours after Monday’s earthquake, a powerful 6.9-magnitude aftershock occurred, activating the Mexico City seismic warning device. It is very likely that strong aftershocks will continue to occur over the following days to weeks.

The earthquake struck the town of Takatzcuaro, Michoacan, about 180 kilometers (112 miles) from the epicenter. Credit: Madrigal Tehuan.

September 19 has become an iconic date for Mexicans in terms of earthquakes. On September 19, 1985, the most dangerous earthquake in the history of Mexico occurred – a magnitude of 8.0. Despite the official casualty figure of about 10,000, other organizations estimate that about 40,000 people died from this earthquake, most of them in Mexico City. 32 years later, on September 19, 2017, a 7.1-magnitude earthquake caused extensive damage in Mexico City and nearby cities and killed about 370 people.

On September 19 every year, earthquake drills are held in various cities in Mexico to commemorate the 1985 earthquake. In Mexico City, a seismic alert is broadcast using public loudspeakers. In 2017, two hours before a 7.1-magnitude earthquake, the drills and alerts went out.

The history repeated itself on Monday, when people felt the coal quake in Mexico City after about 45 minutes of rehearsals. The occurrence of these events made many people in Mexico believe that September is the month of earthquakes. However, multiple scientific studies – Aguilar (2021) and Ávila-Barrientos (2021) – have shown that large earthquakes are not governed by any date, month, or season. When we analyze large catalogs of earthquakes, including those with very ancient historical events, we conclude that earthquakes occurred in Mexico (and throughout the world) in similar numbers during the twelve months of the year. Therefore, the probability of an earthquake is completely independent of the date.

History of major earthquakes

In Mexico, four earthquakes of magnitude greater than or equal to 8 have occurred since 1900. The epicenter of Monday’s Coalcoman earthquake was very close to and between the foci of three of these large earthquakes. These earthquakes occurred along the Central American Trench, where the tectonic plates Rivera and Cocos slope down the North American plate, near a triple tectonic junction, where these three plates intersect.

Tectonic plates whose interaction causes most earthquakes in Mexico. The red start marks the epicenter of Monday’s earthquake. The dark circles represent the foci of earthquakes greater than 8.0 that have occurred in Mexico since 1900.

Geologically speaking, this is a very active region where volcanoes, silent earthquakes, tectonic earthquakes and large-scale earthquakes occur (Husker et al., 2019). The western coast of Mexico has experienced more than 40 earthquakes greater than or equal to 7.0 in the last century. This week’s 7.7-magnitude earthquake appears to have ruptured in an area similar to the 7.5-magnitude earthquake of January 30, 1973 (Reyes et al., 1979), and the 7.7-magnitude earthquake on April 15, 1941 ( Keeler). et al., 1973).

When an earthquake deforms the crust, the transfer of pressure can loosen a nearby fault and/or increase the pressure acting on it. These stress changes can enhance or impede future earthquakes at these faults. We can calculate how this pressure changed after the coal earthquake. This helps seismologists estimate where earthquakes are likely to occur in the future (Toda et al., 2011). The red areas in the map below indicate where stress has increased, and earthquakes are most likely to occur in the future. The coal earthquake may have increased the likelihood of future earthquakes at or very close to the rupture zones in the 1985 and 1995 earthquakes (both of magnitude 8.0).

The designated area corresponds to the small square in the first image. The map shows the calculated pressure change from the 7.7-magnitude coal earthquake (red star), and aftershocks from this event (open circles). The dark circles represent the foci of earthquakes greater than 8.0 that have occurred in Mexico since 1900. The red square is the epicenter of the magnitude 6.9 earthquake.

modest tsunami event

Of great importance to the region are the effects of tsunamis caused by earthquakes such as these. The Coalcoman earthquake produced a modest amplitude tsunami, which lasted for more than 16 hours, as recorded by tide gauges operating in the area by Servicio Mareográfico Nacional.

Although the peak wave amplitude was close to one meter in the city of Manzanillo, there are no water level gauges near the epicenter. Larger amplitudes could be, perhaps two to three meters (seven to ten feet), but must be confirmed by a post-event tsunami survey. For comparison, the 1995 earthquake of magnitude 8.0 produced tsunamis of up to five meters (16 ft) in size in the area (Borero et al., 1997), causing significant damage.

The amplitude of the tsunami was recorded at four regional tide gauges operated by the National Tide Survey.

In addition to determining the danger posed by large waves, tsunami observations are key to understanding whether slip during Mexican earthquakes extends all the way to the surface along the trench.

When an earthquake slides under the ocean all the way to the surface of the sea floor, sudden deformation of the sea floor can displace large amounts of water, causing a tsunami.

For most earthquakes in the Mexican subduction zone, it is widely accepted that fault slip does not extend to the surface, but stops at a depth of 10 km (six miles), reducing their ability to generate tsunamis. This may indicate that the tsunami risk is generally modest in the country. However, earlier events on instruments such as the 1787-magnitude San Sixto earthquake and more recent events such as the 1932-magnitude 8.2 earthquake have produced large tsunamis that indicate the possibility of a larger slip. Understanding the precise nature of the shallow slide of the coal earthquake will be fundamental to assessing future risks in the country.

references

Aguilar, M. (September 14, 2021) Is September really earthquake month?, https://planeteando.org/2021/09/14/es-septiembre-remente-el-mes-de-los-sismos/amp /

Ávila-Barrientos, L. (2021), General Review on the Characteristics and Consequences of Great Earthquakes in Mexico, JIOS, Vol.41, No. 2.

Borrero, c. , Ortiz, M, Titov, F, and Sinolakis, C (1997). The Mexican Tsunami Field Survey is producing new data and unusual images. Eos, Transactions American Geophysical Union, 78 (8), 85–88. https://doi.org/10.1029/97EO00054

Husker, A, Frank, WB, Gonzalez, G, Avila, L, Kostoglodov, F, and Kazachkina, E (2019). Characteristic tectonic tremor activity is observed during multiple slow-slide cycles in the Mexican subduction zone. Journal of Geophysical Research: Solid Earth, 24 1, 599-608. https://doi.org/10.1029/2018JB016517.

Kelleher, J., Sykes, L., & Oliver, J. (1973), Possible criteria for predicting earthquake locations and their application to the main plate boundaries of the Pacific Ocean and the Caribbean Sea, J. Geophys. Res, 78 (14), 2547-2585, doi: 10.1029/JB078i014p02547.

Reyes, A., Brune, JN & Lomnitz, C. (1979). Source Mechanism and Study of Aftershocks of the Colima, Mexico Earthquake of January 30, 1973. Bulletin of the American Seismological Society, 69, 1819-1840.

Suárez G (2022) The Earthquake Early Warning System of Mexico (SASMEX): A Retrospective View and Future Challenges. In front of me. geology. 10: 827236. doi:10.3389/ feart.2022.827236

Toda, S., Stein, R.S., Sevilgen, V., & Lin, J. (2011), Coulomb 3.3 Graphic-rich deformation and stress shift software for earthquake, tectonics, and volcano research and teaching – User’s guide: US Geological Open File Survey Report 2011–1060, 63 pages, available at https://pubs.usgs.gov/of/2011/1060/.

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