Maximum expected magnitude, interface coupling and seismic cycle
in the Mexican Subduction Zone
UNAM Seismology Group
Mexico is located in an active subduction zone with one of the most unusual plate
configurations in the world. Instrumental records in the world date back to the beginning
of the last century. In this period, the maximum magnitude recorded in Mexico is 8.2, on
the coast of Colima-Jalisco in 1932. Recently, historical descriptions of tsunamis and
coastal damage were used to determine ​ that an ​M8.4-8.6 earthquake occurred along
the Oaxaca coast in 1787. Apparently, this event was 350 km - 400 km in length
(Suárez and Albini, 2010). However, the subduction zone in Mexico has a length larger
than 1600 km. How can we know the length of rupture of an extreme event? What
would the recurrence period be?
​
The maximum expected magnitude depends on the length and width of the fault that
can be broken in an event. The length depends on barriers restricting the rupture.
These can be geological features along the interface, aseismic areas or deficiency in
accumulation of energy deformation as a result of the occurrence of a recent
earthquake. The width of the fault that can be broken can be estimated from the
geometry of the subducted plate; focal mechanisms of the interplate seismicity and the
degree of interseismic coupling. The Mexican Subduction Zone (MSZ) has
experimented slow-rupture earthquakes close to the trench​ (18 April 2002, ​Mw​6.7),
suggesting that at least some patches close to the trench might have some degree of
coupling. Is this a possible scenario all along the trench?
Although the average recurrence periods of large subduction earthquakes in Mexico are
between 30 and 60 years in a given segment, there are two segment of the MZS, that
have not experienced a significant earthquake
​
rupture (i.e., ​M>6.5) in more than 100
years. One is between the cities Acapulco and Papanoa in Guerrero state, being less
than 300 km away from Mexico City, the capital of the country with more than 20 million
people. It has been estimated that an earthquake in the gap may produce ground
shaking in Mexico City 2 to 3 times larger than those experienced in 19 September
1985 due to the large ​M8.1 Michoacán earthquake. The other segment is where the
Tehuantepec ridge is subducting. No detailed studies exist for this segment, assuming
that it subducts aseismically, and that it acts as a barrier for rupture areas to propagate
to the south.
The Tehuantepec ridge is also where an important divide occurs in the forearc. The
Chortis block moved from the western end of the subduction zone starting near ~90 Ma
to its present position in Guatemala. This event brought with it much of the forearc and
led to subduction erosion that removed the forearc west of the ridge (Ferrari et al.,
2014). A forearc sliver remains east of the ridge, but it appears to have very low
coupling near the trench with evidence of slow tsunamigenic earthquakes (Ye et al.,
2013). The forearc removal to the west of the ridge has led to the longest coastline
(1200 km) within 100 km of the trench in the world (Figure 1). The closest point is only
about ~40 km from the trench. This means that the large megathrust earthquake zone
lies partially beneath the continent in this entire region.
Figure 1. Tectonic setting, slab contours (modified from Pardo and Suárez, 1995), and
permanent (stars) and temporary (squares) seismic stations in Mexico. The Trans Mexican
Volcanic belt is shown in yellow, active volcanos are denoted by brown triangles.
The geometry of the subduction zone west of the Tehuantepec ridge is also unusual.
The Rivera plate in the NW of the subduction zone subducts at a steep angle as does
the SE of the zone in Oaxaca. However, in the middle of the subduction zone in
Guerrero, Cocos plate remains nearly flat at 40 km depth from 150 km to 300 km from
the trench (Figure 1), making it the longest, flattest shallow subduction zone in the world
(Pérez-Campos et al., 2008). In this region there is no deep Benioff zone, and interplate
seismicity is within the flat portion of the plate near 200 km from the trench at ~ 50 km
depth.
In the last decade, about 200 temporary broadband seismic stations have been installed
in different regions of Mexico (Figure 1). They have been part of collaborative projects
between Mexican institutions, mainly UNAM, and US counterparts. The main objective
of these arrays has been the geometry of the subducted plates, Rivera and Cocos.
Also, in the last decade, the permanent national seismic network has incremented its
number of stations (Figure 1), from 25 to 67. This network is denser at the Pacific coast.
This station density has allowed identification of tectonic tremors (TT) and low
frequency earthquakes (LFE) in Guerrero and Oaxaca states.
Geodetic instrumentation in southern Mexico is mainly coordinated by UNAM as well
(Figure 2). This has allowed the detection of periodic slow slip events (SSE) occurring
also in the Guerrero and Oaxaca regions, a phenomenon that strongly determines the
strain accumulation cycle and thus the seismic potential of the entire region. The SSE’s
invade the seismogenic zone within the Guerrero seismic gap thereby reducing the
coupling as observed on GPS stations.There is an ongoing discussion as to what this
means in terms of seismic hazard.
Figure 2.​ GPS stations as of 2015.
There are not enough seismic or geodetic stations to verify the existence or absence of
such phenomena (SSE, LFE, TT) northwest of Guerrero, in the adjacent state of
Michoacán; or further south of Oaxaca, at the Isthmus of Tehuantepec.
The Mexican subduction zone represents an interesting area to study with many
unusual features. There is a clear need for a reliable estimate of the coupling, mainly in
the region close to the trench, and the location of barriers to reach a robust estimate of
the maximum expected magnitude in the MSZ. Up to now there has been no offshore
instrumentation to analyze the structure or the plate coupling in this region. In addition,
onshore regions such as Michoacán and Tehuantepec will require more
instrumentation. Eventually, the forearc sliver east of the Tehuantepec ridge should be
considered as a whole region with studies that cross borders from Mexico to Costa
Rica.
We acknowledge support from NSF through grants EAR-0335782, EAR-0609707,
EAR-0510887, and EAR-0847688; UC MEXUS project 04105384; Conacyt projects
177676, 225919, and J51566-F; UNAM PAPIIT projects IN110913, IN105816,
IX120004, IN119505; the Gordon and Betty Moore Foundation; the Tectonics
Observatory at Caltech; and the CENS at UCLA. We further thank all the volunteers that
participated in the temporary arrays and the Servicio Sismológico Nacional and the
other Mexican permanent networks.
Referencias
Ferrari, L., M. Bergomi, M. Martini, A. Tunesi, T. Orozco-Esquivel, and M.
López-Martínez (2014), Late Cretaceous-Oligocene magmatic record in southern
Mexico: The case for a temporal slab window along the evolving Caribbean-North
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​Tectonics,
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doi:10.1002/2014TC003525.
Pardo, M. and G. Suárez (1995). Shape of the subducted Rivera and Cocos plates in
southern Mexico: Seismic and tectonic implications, ​J. Geophys. Res., ​100​,
12,357-12,373.
Pérez-Campos, X., Y. Kim, A. Husker, P. M. Davis, R. W. Clayton, A. Iglesias, J. F.
Pacheco, S. K. Singh, V. Constantin Manea, and M. Gurnis (2008). Horizontal
subduction and truncation of the Cocos Plate beneath central Mexico, Geophys. Res.
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Suárez, G. and P. Albini (2010). Evidence for great tsunamigenic earthquakes (​M 8.6)
along the Mexican subduction zone, ​Bull. Seismol. Soc. Am., ​99​, 892-896.
Ye, L., T. Lay, H. Kanamori (2013), ​Large earthquake rupture process variations on the
Middle America megathrust, ​EPSL, ​381​, 147-155.