Studying earthquakes and a continental triple junction in Guatemala

Andi Ellis and Enrique Molina in Guatemala

Riddled with active tectonic faults and volcanos, the country of Guatemala is both a geologist’s dream and a hazard planner’s nightmare. A M=7.6 earthquake in Guatemala in 1976, which left 20% of the country’s population homeless, stimulated the first studies of Guatemala’s primary faults. Earth science research in Guatemala however languished during its long-lasting civil war and only resumed in earnest in the late 1990s. The primary earthquake hazard in Guatemala is the Motagua-Polochic fault zone (defined by the red lines in the associated figure), which bisects the country and has ruptured in a series of damaging historic earthquakes (most recently in 1976). For decades, this major fault zone was believed to continue westward to a poorly defined, underwater intersection with the Middle America subduction zone. More recent work instead suggests a different story, whereby the Motagua/Polochic fault zone terminates at a series of poorly understood on-land faults that accommodate movement of a coastal sliver wedged between the volcanic arc and Middle America subduction zone. This unusual configuration of the regional plate boundaries suggests a higher and more complicated degree of earthquake hazard involving the Motagua/Polochic fault zones, numerous faults in the volcanic arc, and the Middle America subduction zone.

    In 2012, the National Science Foundation awarded faculty members Chuck DeMets and Basil Tikoff a five-year grant to use high-precision GPS instruments and a variety of other geophysical and structural techniques to better understand the unique tectonics of this region and the earthquake cycles of Guatemala’s major faults. The project is part of a longer-term regional study by DeMets and Tikoff of the tectonics of northern Central America, including Honduras, El Salvador, Guatemala, southern Mexico, and Nicaragua. An important aspect of the project is engage faculty and students from these countries in our field and scientific efforts, as well as to hold a series of training workshops and mini-conferences in each of the host countries in the next few years.

   Between 2000 and 2012, we installed ~50 GPS measurement sites in El Salvador and Honduras (indicated by the red-filled circles in the figure) and another five continuous GPS stations in those countries (red stars in the figure). The GPS velocity field for these two countries revealed for the first time the pattern of active geologic deformation due to a combination of moving tectonic plates and actively locked tectonic faults (by shada at dhead). Modeling of this velocity field by former UW student and native Honduran Manuel Rodriguez showed that the present deformation is controlled largely by the curved geometry of the Motagua/Polochic fault zone and tectonic plate motions.

  In late 2012, UW graduate student Andria Ellis and long-time department engineer Neal Lord spent 4 weeks working with Professors Enrique Molina and Omar Flores of Guatemala’s national university to both install a network of GPS marks in the southern half of Guatemala (open red circles in figure) and occupy ~10 existing GPS pins installed by our French collaborators in the late 1990s. Ellis et al. logged many miles and hours on rough back roads to find suitably stable and secure locations for GPS measurements (see the photo of Andria in action!). DeMets and Ellis returned to Guatemala in January 2013 to continue the field work and initiate a collaboration with Guatemala’s national geodetic agency IGN, which has operated a 17-station national continuous GPS network since 2009. Our first year was as successful as we’d hoped – we installed 5 continuous GPS instruments and 18 annual sites; we located and occupied 10 existing French GPS monuments for long-term use; and we now receive daily GPS observations from IGN’s network. During 2014, we will hold training workshops and mini-conferences in Guatemala and El Salvador, and also occupy our numerous existing GPS stations in Honduras and El Salvador.

  Though early in the project, we already have our first result thanks to a M=7.4 earthquake that occurred on Nov. 7, 2012 off Guatemala’s Pacific coast (epicenters shown by the black stars) The blue arrows in the associated figure shows the sudden movements that we measured at ~20 continuous GPS sites that were operating during the earthquake. Rapid station motions of up to 40 millimeters (1.6 inches) toward the earthquake rupture zone occurred during the earthquake as the crust quickly released elastically-stored strain that had accumulated gradually during the decades preceding the earthquake. Andria Ellis is presently modeling the distribution of offshore fault slip that is required to fit the measured GPS offsets. From IGN and French measurements taken during the years before the earthquake, she will also model the distribution of frictional locking offshore that preceded the earthquake. Together, these will be our first steps toward an improved understanding of the earthquake cycle in this region.