Collaborative Research: Deciphering upper plate deformation and faulting processes in Central America with integrated geodetic and seismic analyses

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In subduction zones, where one tectonic plate descends into the Earth beneath another, a portion of the upper plate called the fore-arc sometimes moves horizontally in a direction that is nearly orthogonal to the direction of convergence between the two plates. For such fore-arc migration to occur, a zone of deformation must exist in the upper plate where movement on faults accomodates the motion of the fore-arc. However, the locations and orientations of faults in the zone of deformation, and their relationship to volcanoes in the upper plate, differ from one subduction zone to the next. This study will improve a global understanding of the processes that enable fore-arc migration by studying them in Nicaragua where the Cocos plate subducts to the northeast beneath the Caribbean plate, and the Caribbean plate fore-arc is moving to the northwest. In particular, this work will test whether the volcanoes in Nicaragua create weak zones in the upper plate that concentrate faulting and earthquakes in the stronger parts of the plate that lie between volcanoes. GPS data will be used to measure where shearing related to fore-arc motion is localized. The locations of large earthquakes and their aftershocks will be improved and used to determine the orientations of active faults. Seismic waves will be used to probe the structure of the upper plate and determine where partial melt rising up to the volcanoes has weakened the upper plate. In addition to improving understanding of fore-arc transport processes, this work will help in assessing where earthquake faults pose hazards to the population of Nicaragua. The project will contribute to the education and career development of graduate students at Penn State and Brown. At least one undergraduate will work on this project through the Leadership Alliance (Brown), and the PA Space Grant (Penn State), programs that recruit students from groups underrepresented in STEM fields. The project will also reach a broader group of students and postdocs at Penn State and Brown through research group meetings and courses, and will be featured in outreach with elementary schools in Providence, RI.Strain partitioning and the formation of migrating fore-arc terranes due to oblique convergence have been documented in numerous subduction zones around the globe, yet major questions remain regarding how the motion of these terranes is accommodated in the upper plate. Studies in different subduction zones have reached varying conclusions about the impact of arc magmatism on upper plate faulting associated with fore-arc transport, and whether and why the apparent geometry of deformation and faulting varies between subduction zones, and in some cases along the strike of a single fore-arc sliver. This project will address these global questions by testing the hypothesis that fore-arc transport in Nicaragua represents an end-member case where crustal weakening due to magma beneath volcanic centers strongly mediates the geometry of faulting and deformation. Along-arc changes in upper plate deformation are revealed by a dramatic rotation in the direction of interseismic velocities from margin-normal in central Costa Rica to margin-parallel in Nicaragua where fore-arc sliver transport is most pronounced. In Nicaragua, evidence for bookshelf faulting and the rotation of fore-arc blocks about vertical axes is provided by margin-normal faults with large earthquakes. However, the distribution of bookshelf faulting versus arc-parallel strike-slip faults, their roles in accommodating fore-arc transport, and the relationship of faulting to a rheologically weak arc, remain uncertain. This project will use geodetic and seismic data to test the hypothesis that zones of rheological weakness associated with magmatic centers significantly alter the mode of upper plate deformation and faulting. Four predictions of this hypothesis will be tested: 1) deformation associated with fore-arc transport is localized in the arc; 2) faulting is localized in zones of strong crust between volcanic centers; 3) deformation is accommodated by bookshelf faulting and margin-parallel strike-slip faulting, where fault orientation is partially controlled by the spacing of volcanic centers; and 4) fore-arc transport is partially accommodated by magmatism. Integrated geodetic and seismic analyses will test the four model predictions. The analysis of new and existing GPS data will indicate the location of the maximum strain gradient (1) and anomalous regions of shear and dilatational strain rates (2-4). GPS-derived coseismic displacements combined with earthquake relocations and analyses of source directivity will indicate fault geometry (2-3). Joint analyses and inversions of surface wave and converted body wave data will improve models of upper plate structure and crust and mantle rheology, including constraints on the distribution of partial melt in the crust (i.e., weak zones associated with the arc). The investigators will work with colleagues at INETER (the agency that monitors earthquakes and volcanoes in Nicaragua) to improve hazard assessment from upper plate earthquakes in relation to population. New GPS network installations will expand the utility of the COCONet GPS network. Another outcome will be software and a workflow to enable the seismologists at INETER to independently carry out double-difference relocation of earthquake sequences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.