Gonzalez-Huizar, and two co-authors, reported on November 4, 2021 that on September 7, 2021, a magnitude-7.1 earthquake struck near the coast of Guerrero, Mexico. The earthquake was widely felt — even in Mexico City, more than 220 miles (350 kilometers) away — and caused damage to buildings in the nearby tourist city of Acapulco. The earthquake struck very close to an area that had not seen a large earthquake in the past century, leading scientists to wonder if it could trigger an earthquake that would fill a so-called “gap” along Mexico’s otherwise seismically active Pacific coast.
In this region, the Cocos tectonic plate subducts underneath the North American plate. This tectonic boundary — one of the most seismically active in the world — has a long history of large, damaging earthquakes. In 1787, a magnitude-8.4–8.6 earthquake, the biggest in Mexico’s history, occurred in the region (Suárez and Albini, 2009). In 1985, the Mexico’s deadliest earthquake, a magnitude-8.0, struck along the boundary, causing about 10,000 deaths, most of which occurred in Mexico City, far from the epicenter.
Other past, damaging earthquakes in Acapulco and nearby towns include a magnitude-8.2 in 1907, a magnitude-7.8 in 1909, a magnitude-7.8 in 1957, and two earthquakes of magnitude-7.1 and -7.2 in 1962, which occurred just eight days apart. Given the poor precision in the estimated location of these historic earthquakes, the September 2021 earthquake may have occurred in the same rupture area.
Location Uncertainty
September’s earthquake struck roughly seven miles (11 kilometers) southwest of Acapulco, according to the Servicio Sismólogico Nacional (SSN), Mexico’s earthquake monitoring agency. The U. S. Geological Survey (USGS) and other global agencies estimate it struck between 10 and 15 miles (17 and 23 kilometers) to the northeast of this location. However, agencies that use global seismic networks to locate earthquakes in the Mexican subduction zone, such as the USGS, systematically mislocate such events by roughly 16 miles (26 kilometers) to the northeast (Singh and Lermo, 1985; Hjörleifsdóttir, et al, 2016). The errors are attributed to lateral variations in the mantle structure, which affect the velocity of the seismic waves detected by seismometers. These errors become exaggerated when stations far from the epicenter are used (Hjörleifsdóttir, et al, 2016).
The Guerrero Seismic Gap
The epicenter of the recent Acapulco earthquake lies at the eastern edge of the Guerrero Seismic Gap, a zone of apparent seismic quiescence embedded in this highly active seismic region. The last large earthquake — greater than or equal to magnitude-7.0 — in the Guerrero Gap occurred more than a century ago, in 1911, whereas for the rest of the tectonic plate boundary, large earthquakes occur approximately every 30 to 60 years (Nishenko and Singh, 1987). Moreover, ruptures from previous, proximal events of magnitude-8.0 and larger did not propagate into the gap.
Seismic gaps can indicate that stress, usually released during earthquakes, is building up along a locked plate boundary. In the case of the Guerrero Gap, some researchers estimate enough stress has accumulated to cause a magnitude-8.0 or larger earthquake (Singh and Mortera, 1991) and a major tsunami. Such a large event could cause significant damage to nearby coastal cities such as Acapulco, as well as to Mexico City, where seismic waves are amplified by the underlying soft sediment (an ancient lakebed).
Alternatively, recent studies suggest that within this gap, the plate boundary is sliding freely. In this case, stress imparted by the tectonic plates sliding past one another is nearly continuously released. If true, the probability of a big earthquake occurring is significantly lower (Husker et al., 2018; Plata-Martinez et al., 2021). The latter hypothesis stems from studies that suggest the Guerrero Gap’s deep-seated zone where slow slip events occur — one of the largest in the world — could reach into the shallower depths where large earthquakes are usually generated. (Kostologlov et al., 2003).
Triggering by the Acapulco Earthquake
An earthquake can change the physical conditions of nearby faults, potentially triggering more earthquakes (termed static triggering). A parameter known as “Coulomb stress change” describes where earthquakes are more likely to be triggered. In general, stress changes greater than 0.1 bar (force per unit area) suggest a high probability of triggering (Hill, 2008).
We modeled the Coulomb stress change (Toda et al., 2011) caused by the Acapulco earthquake, and discovered that a large area of the Guerrero Seismic Gap was brought closer to triggering. Stress change values above 0.1 bar could be reached as far as the epicenter of 1911 earthquake. Earthquakes can be triggered in other ways. During an earthquake, seismic waves radiate outward from the ruptured area. Ground deformation related to the passing of seismic waves can also generate Coulomb stress change values large enough to trigger earthquakes — a phenomenon known as “dynamic triggering” (Gonzalez-Huizar et al., 2012). In some cases, the triggering occurs during the passing of the seismic waves, but in other cases, there is a delay between the waves passing and the triggering of the earthquakes (Castro et al., 2015, Gonzalez-Huizar and Toda, 2021). It is not clear, however, what controls this delay time. It is possible that the passing waves initiate a small slow slip event or cause permanent damage on the faults. Such events could gradually trigger earthquakes (Parsons 2005; Shelly et al., 2011).
The authors modeled the Coulomb stress change caused by the seismic waves from the Acapulco earthquake passing through the Guerrero Gap. Values larger than 0.1 bar were generated inside the gap (lower panel in the following figure), which might have implications in the occurrence of future events along the gap.
Fewer aftershocks following the Acapulco earthquake occurred inside the Guerrero Seismic Gap than to the opposite side of the epicenter, outside the gap. This suggests that, like large earthquakes, smaller magnitude aftershocks are not readily produced within the Guerrero Gap, indicating that this area has stored relatively little tectonic stress. This observation might support the hypothesis that the Guerrero seismic gap is undergoing nearly continuous slip. However, the aurthors' modeling shows that the Acapulco earthquake and its seismic waves increased stress within the gap — enough to trigger a future large magnitude earthquake.
The authors indicated that their results are preliminary, and as the scientific community obtains better models for the geometry of the rupture of the Acapulco earthquake, the location of its epicenters, and the physical characteristics of the gap, their results can be refined.
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