Years participated in RESESS:
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Major: Geosciences with Geophysics emphasis
Academic Affiliation: University of Arizona
Research Mentors: Anne Sheehan and Jefferson Yarce
Communications Mentor: Sarah Evans
Born and raised in southeast Arizona, Enrique’s familiarity with the Basin and Range Province became useful as he turned to study geology. He is ultimately interested in using physics to describe and understand Earth’s natural processes. These interests include large-scale structural geology, tectonics, and other hazard-related phenomena. This summer he worked on developing seismic focal mechanisms for subduction zone earthquakes recorded offshore of the North Island, New Zealand, during the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. Understanding the spectrum of faulting behaviors at the Hikurangi subduction zone is important for understanding slow slip events and for determining the likelihood for damaging earthquakes and tsunamis.
Earthquake Focal Mechanisms from the HOBITSS Ocean Bottom Seismology Experiment, Hikurangi Subduction Zone, New Zealand
Subduction thrust systems are important to understand because they generate the world’s largest, most devastating tsunamis. The Hikurangi subduction zone, with the trench just offshore of Gisborne, North Island, New Zealand, has been the site of episodic slow slip events that have major implications for seismic hazard assessment models in New Zealand (Wallace et al., 2016). In an effort to better define the faulting parameters at the plate interface and in the overriding plate, we used first arrival polarity data to determine seismic focal mechanisms for subduction zone earthquakes recorded during the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. A better understanding of the relationship between slow slip events and earthquake source parameters is needed to determine the likelihood of damaging earthquakes and tsunamis along the Hikurangi margin.
The HOBITSS project included the deployment of 35 ocean bottom seismometers and seafloor pressure gauges offshore of Gisborne. The addition of ocean bottom seismometers to New Zealand’s preexisting land seismic network, GeoNet, provides a new opportunity to study offshore earthquakes near their source. The GeoNet seismic catalog was used to identify events larger than M_w2.9 that occurred near the coast of Gisborne between May of 2014 and June of 2015. Specific earthquakes were chosen to correspond with the dates and locations of the HOBITSS instrumentation deployment so that adequate azimuthal station coverage was guaranteed for each solution. Data were used from both the GeoNet land network and the HOBITSS ocean bottom seismic network to relocate the events and to determine their focal mechanisms.
Resolvable patterns exist in our lower-hemisphere plots that help to define the geometry of faulting during each of our selected earthquakes. However, when we compare the results of our event relocations to New Zealand’s national seismic catalogue, we see notable differences in the depth determinations and epicentral locations. This is likely an artifact of several things. First, the locations of offshore events made by GeoNet may have increased uncertainty since the events are outside of their network. Second, our locations may lack the desired precision because of the velocity model that we used. GeoNet uses velocity models that have been developed to represent New Zealand at the regional scale, whereas the iasp91 model that we used describes the global velocity structure of the Earth. Although many of our relocations might be improvements to GeoNet’s offshore location values, further analysis will be done before any faulting parameters are drawn from our results. To better constrain the event-to-station travel time information needed to develop accurate focal mechanisms a more detailed regional velocity model will be implemented.