Searching for the origin of slow earthquakes in Guerrero Gorge
We’re on a 48-day expedition near Acapulco on Mexico’s west coast, where the young Cocos oceanic plate dives beneath the North American plate. Much of this subduction zone, often referred to as the “Mexican portion of the Central American Trench,” has produced major earthquakes over the past 100 years, including the 1985 magnitude 8.0 Michoacan earthquake that killed more than 10,000 people in Mexico City. The Guerrero earthquake clearance is one of the exceptions. This part of the Mexican subduction zone has not ruptured in a major earthquake (M>7) since at least 1911. Instead, large, relatively shallow slow-slip events occur—slowly releasing energy over days to months without generating strong seismic waves—about every 3-5 years.
So far, we have not fully understood why the Guerrero gap has a distinct sliding behavior than its adjacent parts, which often rupture in large earthquakes. Scientists typically cite fluids, such as seawater, transported into subduction zones by incoming oceanic plates to explain slow-slip events in other subduction zones around the world, although evidence for fluids in the Guerrero gap and elsewhere remains very limited. Through this project, we hope to better quantify the volume and distribution of fluids flowing into the oceanic plate, their fate at depth, and changes in the amount of fluid between the Guerrero gap and its neighbors, to explore how fluids contribute to the shallowness of this gap. There is a slow-slip event.
A map of our survey plan in and around the Guerrero gap on Mexico’s Pacific coast near Acapulco.
During our voyages, we probe the ocean floor using sound waves to look for plate boundary faults or “great thrusts” (where two tectonic plates meet on the ocean floor) at a depth of approximately 15 kilometers, and characterize the descending and overlying plates structure and properties. For our investigation, we are using R/V from Columbia University Marcus G.Lanceth Operated by the Ocean Action Office at the Lamont-Doherty Earth Observatory. Our survey, funded by the National Science Foundation, will be the first active source seismic imaging study in the Guerrero gap and its adjacent region.
For the first part of the cruise, we used the seafloor seismometers at the Woods Hole Oceanographic Institution’s Seabed Seismograph Instrument Center.We deploy seismometers on the seafloor along pre-defined profiles to record sound waves generated by the R/V’s high quality and powerful sound source Marcus G.LancethThe reflection and refraction of sound waves through the seafloor will provide important information about the properties of the different layers in the subsurface, such as their composition and the presence of fluids.
For the second part of the cruise we will use the same sound source and will be towing a 15km cable containing 1200 hydrophones spaced 12.5m apart. This long cable, or “streamer,” will record echoes from different layers of the subsurface and produce an image of the structure of the seafloor, including the number and type of faults.
R/V Marcus G. Langseth moored at the port of Manzanillo, Mexico Photo: Brandon Shuck
The project involved close collaboration with Mexican and Japanese collaborators. Mexican collaborators Víctor Manuel Cruz-Atienza and Jorge Real-Pérez from the National Autonomous University of Mexico (UNAM) sailed with us. Since 2017, they have deployed a series of amphibious broadband seismometers and geodetic stations in the Guerrero Gap. The array is capable of recording small background earthquakes and tectonic tremors, as well as silent deformations associated with slow-slip events or stress build-up. Combining our active-source seismic observations with their passive-source seismic observations will be a very powerful tool for understanding how this seismic gap works, with the ultimate goal of better assessing the long-term seismic potential of this anomalous region and the associated harm to local Mexican residents.
Manzanillo’s science team for the MGL2204 experiment, then set sail on the R/V Marcus Langseth. Left to right: Tanner Acquisto, Ph.D. LDEO. Candidates; Brian Boston, LDEO Research Scientist; Joshua Burstein, Master Student at Northern Arizona University; Brandon Shuck, LDEO Postdoctoral Research Scientist; Anne Bécel, LDEO Associate Research Professor; Víctor Cruz-Atienza, UNAM Professor. Not shown: Jorge Real-Pérez, UNAM geophysical engineer; Donna Shillington, professor at Northern Arizona University.
Anne Bécel is a marine geophysicist and research professor at Columbia University’s Lamont-Doherty Earth Observatory.
