Ashley Nichole Grijalva

Years participated in RESESS: 2011


An Overview

Academic Affiliation: University of Texas at El Paso


Ashley grew up in El Paso, western Texas, also known as the Sun City. She currently attends the University of Texas at El Paso, where she enjoys the campus architecture modeled after Buddhist temples. Her interest in geology began during family hiking trips in the mountains of New Mexico. She gives credit to her former professor at UTEP for sparking an academic interest in the geological sciences.


Slip rate of the northern San Jacinto and southern San Andreas Faults from GPS measurements

Slip rate is the key determinant of the repeat time of earthquakes rupturing a fault and a critical input into assessments of seismic hazard. To determine slip rates, both geologic and geodetic methods are used. Geologic methods measure historical slip and include the use of LiDAR data or field mapping to measure offsets, and radiocarbon dating to determine the ages of those offsets. Geodetic methods can give accurate estimates of current slip rates by measuring crustal strain accumulation at various distances from a fault. In southern California, there is a discrepancy in slip rates between the two methods; the geologic slip rate estimate for the San Andreas Fault in Cajon Pass is 25mm/year while the geodetic estimate is 5 mm/year. We constructed time series to obtain velocities for our sites. We then conducted two-dimensional elastic modeling to find the ranges of fault slip rates that are consistent with our GPS velocities. The best-fitting model had slip rates of 8 mm/yr for the San Andreas and 14 mm/yr for the San Jacinto fault. These results are consistent with prior geodetic studies, which suggest a low rate for the San Andreas Fault. However, from the combinations that produce relatively well-fitting lines, the San Andreas Fault slip rate could be any- where between 0-24 mm/yr and the San Jacinto Fault slip rate could also be anywhere between 0-24 mm/yr. Nonetheless our results showed that the SJF-SAF system must have a total slip of 20-24 mm/yr on the two faults combined. The best-fitting rate for all faults in the Eastern California Shear Zone combined was 15 mm/yr. The models we tested were that showed the best-fitting line ranged from 0-10 mm/yr. For the faults west of the SJF system, the combined best-fitting rate was 8 mm/yr.