Stephen Hernández

Years participated in RESESS: 2008, 2006, 2005

An Overview

Major: Mathematics

Academic Affiliation: University of Texas at El Paso


UNAVCO’s RESESS program first launched in the summer of 2005. Stephen Hernández came to UNAVCO from the University of Texas, El Paso, after completing his sophomore year in math and physics. As RESESS’s first intern, Stephen worked with research mentor, Dr. Kristine Larson at the University of Colorado in Boulder, and with UNAVCO staff. His research project focused on the study of errors inherent to GPS measurements of continental plate motions. Specifically, he worked to identify and characterize multipath sources at the Parkfield GPS array. Stephen presented his results at the 2005 SACNAS (Society for Advancement of Chicanos and Native Americans in Science) Conference in Denver, Colorado in September.

RESESS operates in partnership with the well-established SOARS (Significant Opportunities in Atmospheric Research and Science) internship program at UCAR (University Corporation for Atmospheric Research). Stephen and his fellow protégés participated in SOARS-sponsored leadership orientation, team-building exercises, seminars, and writing workshops. These students form a core learning community from which they draw upon for peer support and shared experiences. RESESS aims to strengthen the presence of underrepresented groups within the solid earth sciences and increase the completion of master’s and PhD degrees by these groups in the field.

After he completed his junior year at the University of Texas at El Paso, Stephen returned to Boulder for his second year as a RESESS protégé in 2006. Stephen had an unusual project this year. He worked with David Phillips of UNAVCO to learn processing of high precision GPS data during the regular summer RESESS program and then accompanied Dr. Eric Calais of Purdue University on a field season in Tanzania to obtain GPS data in a small portion of the East African Rift. A crew of scientists, a UNAVCO field engineer, and students from the U.S. joined Tanzanian scientists and support staff for three weeks of installing short term GPS instrumentation. Stephen’s abstract explains that the exact direction and distribution of strain across the East African Rift plate boundary zone remains a mystery due to inadequate data and analysis of the rift. After returning from Africa, Stephen continues to work on taking the raw data from the summer field work and refining it to obtain extremely precise GPS data points which will be used to determine the amount and direction of crustal movement in the field area.


2005- Characterizing Multipath Sources at Seismic Frequencies: A Case Study for the Parkfield GPS Array

Stephen Hernández, University of Texas, El Paso, Kristine M. Larson, University of Colorado, Boulder

Specular multipath remains one of the most limiting error sources compromising position estimates collected from 1-Hertz Global Positioning System (GPS) data. In 2004, after 38 years, the long-awaited Parkfield, California, earthquake was finally captured. Data from Parkfield promises to provide deeper insight into earthquake physics and post-seismic crustal deformation, of which 1-Hz data is particularly well-suited. For GPS to provide sub-centimeter accuracy to geo-scientists, though, confusing multipath error from a broad range of frequencies must first be mitigated. Characterizing multipath at these sites entailed identifying trends from problem sites, satellites, and any other factors contributing to relavant range (position) error. 1-Hz GPS data that included signal-to-noise ratios (SNR) were used to make characterization possible. This included the analysis of power spectra and spectrograms to single out receivers recording uncharacteristically large amplitudes and/or long period interference indicative of specular multipath. In addition, via the use of a forward model, varying frequencies were correlated with a respective estimated distance of antenna from reflector. Preliminary results have identified specific satellites and receivers with significant multipath. Distances of reflectors from antenna, coupled with actual photos of different site monuments, may then aid in predicting when and where significant multipath will reappear. Knowledge gained from multipath characterization can be applied toward the eventual facilitation of multipath at other sites with the Parkfield array as a representative model.

2006Continental breakup on the East African Rift

Although the East African Rift (EAR) is often cited as the premier example of incipient rifting, the shear size and oft times inaccessibility of the rift have limited the analysis and interpretation for accurate determination of the physical processes controlling the deformation of the Earth’s crust. Existing geodetic data are wholly inadequate to address the direction and distribution of extensional strain along and across this incipient plate boundary zone. A new five-year project with three Global Positioning System (GPS) campaigns (in years 1, 3, and 5) distributed across Tanzania will help us obtain interpretable results with a total extension rate across the EAR on the order of 5 mm yr-1. GPS measurements in the EAR are critical to finally establish the kinematic framework of rifting. New GPS measurements spanning the Western and Eastern rifts in Tanzania, combined with the distant data on the surrounding plates, will provide the kinematics of deformation across and along the length of the EAR. In particular, they will allow us to test and further refine the counter clockwise rotation model of the Tanzanian craton suggested by the very scarce geodetic data currently available. GPS measurements will also provide strain distribution across and along the Western and Eastern rifts. In addition to horizontal motions, GPS measurements will provide vertical displacements, critical to test for present-day uplift of the African plateau predicted from the African Superplume upwelling. Using the GAMIT/GLOBK suite of GPS processing software, we present a new, preliminary determination of the horizontal velocities in the EAR at Tanzanian latitudes.

2008- Lineament Analysis for the McMurdo Dry Valleys region, Antarctica

Light Detection and Ranging (LiDAR) data were collected for regions spanning approximately 4,000 km2 in the McMurdo Dry Valleys region of Antarctica. We assess the efficacy of using digital elevation models (DEMs) derived from these data for geomorphic mapping applications in the Dry Valleys. Using the ArcGIS Geographic Information Systems (GIS) software suite, we determine optimal image processing techniques to enhance visibility of geologically and geomorphically significant lineaments including faults, dikes, paleoshorelines, and other rift and glacial features. Optimized filters, shadowing parameters, and the determination of positive and negative relief are applied to regions of interest. Results highlight features that can be used to constrain long-term glacial isostatic adjustment and neotectonic processes related to the West Antarctic Rift System (WARS).

2009- Shear Wave Splitting Analysis from Newly-Installed Seismic Stations in Antarctica

The tectonic fabric of the upper mantle is inferred using SKS and SKKS splitting analysis of data from 15 newly-installed broadband seismic stations in West and East Antarctica. Data collected between December 2007 and December 2008 from 5 stations from the Polar Earth Observing Network (POLENET experiment) and 10 stations from the Gamburtsev Antarctic Mountains Seismic Experiment (GAMSEIS experiment) are used to perform an SKS and SKKS anisotropy analysis of the regions. In West Antarctica, POLENET stations straddle the West Antarctic Rift System (WARS) while in East Antarctica GAMSEIS stations are centered on and around the Gamburtsev Subglacial Mountains (GSM). POLENET stations show splitting times ranging from .85s to 1.25s and fast axes in cursory agreement with the inferred direction of WARS extension. For the larger focus of our study, GAMSEIS stations show a wider range of delay times (.65s to 1.55s) and fast splitting directions that may indicate the existence of 2 distinct tectonic regimes.