SCEC Award Number 16154 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Effect of Loading-Induced Horizontal Deformation on the Community Geodetic Model
Investigator(s)
Name Organization
Corné Kreemer University of Nevada, Reno Adrian Borsa University of California, San Diego
Other Participants Meredith Kraner (Graduate Student)
SCEC Priorities 1d, 5d, 5b SCEC Groups Geodesy, EFP
Report Due Date 03/15/2017 Date Report Submitted 06/22/2017
Project Abstract
 We investigated the effect of several loading sources on the GPS time-series and on the implied horizontal strain rate field. One of them was the effect of atmospheric pressure loading (APL) on vertical GPS time-series. We tested this in the Basin and Range where other "noise" is believed to be minimal. Yet, we found that APL predictions correlated very poorly with the data. We also looked at how horizontal seasonal GPS motions in central California result in temporal strain variation along the San Andreas fault. We found a strong seasonal strain signal along the SAF creeping segment (with fault-normal contraction in the earlier wetter part of the year and fault-normal extension in the later drier part of the year) but such variation was not found along the locked part.
Intellectual Merit SCEC's community geodetic model (CGM) contains velocities derived from geodetic time-series. Superimposed on the steady-state tectonic motion, the time-series include a range of other signals, varying from earthquake effects, seasonal variation, and transients. The better we understand those sources, the better we can infer the tectonic motion that is driving stress accumulation on faults. Of the various things we looked at, the most important finding is that fault-normal strain rates on the central San Andreas Fault are seasonally modulated, that this signal varies along strike, and that the correlation between those seasonally varying strains and that of seismicity warrant future construction of seasonally varying strain rate fields (regardless of the source of the seasonal motions).
Broader Impacts This project supported one undergraduate student (who has now entered Graduate School) and one female graduate student. The grasuate student's preliminary work through this project helped her secure a NASA Graduate Fellowship.

The paper by Amos et al. [2014] which argued for seasonally varying stresses on the San Andreas fault due to hydrologic (un)loading in the Great Valley received quite a bit of coverage in news outlets. Our follow-up work gives more support to the idea that fault-normal strain are indeed varying throughout the season.
Exemplary Figure Figure 3: Contours of dilatational strain (red is positive/extensional, blue negative/compressional) for the central San Andreas Fault shown in oblique Mercator projection. Also shown are principal strain axes, as well as GPS locations (triangles). Left panel is monthly displacement derived from seasonal model for just the month of February, and right panel is for September. Note the large seasonal variation along the creeping segment and the absence thereof along the locked section.
Credits: M. Kraner, MS. Thesis, University of Nevada - Reno, 2017