Subsections

Creep Measurements on the Concord Fault from PS-InSAR

Ingrid Johanson, Roland Bürgmann, Alessandro Ferretti (TRE, Milan) and Fabrizio Novali (TRE, Milan)

Introduction

We use PS-InSAR (permanent scatterer interferometric synthetic aperture radar) measurements to study surface creep on the Concord fault in the Eastern Bay Area. Creep on the Concord fault has previously been measured in two locations using alinement arrays (McFarland et al., 2009). The alinement arrays found that creep occurs at rates of 2.5-3.5 mm/yr and mostly in episodes every 3-5 years. InSAR provides dense measurements of ground motion that allow us to measure fault creep along several cross-fault profiles and gain a sense of the distribution of creep along-strike. The PS-InSAR method identifies and integrates individual points with stable phase measurements (outcrops, buildings, utility poles, etc.) in all SAR acquisitions of an area of interest. PS-InSAR provides time-series of range-change measurements that we can use to resolve deformation rates at high precision ($< $1 mm/yr). This may allow us to also resolve some of the variability in the Concord fault's creep rate.

Figure 2.8: Overview map showing mean velocity of each PS-InSAR points from 46 InSAR scenes spanning 1992-2001. Dashed boxes are the areas included in the swath averages, which are then projected onto the solid black centerlines. The two alinement arrays, CSAL and CASH, are shown as white triangles and fall mostly within Profile 3 (P3).
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Measuring creep from PS-InSAR

We use 46 acquisitions from the European Space Agency's ERS1 & 2 satellites, on descending track 70, frame 2853, and spanning from 1992 through 2001. The data were processed using the PS-InSAR method of Ferretti et al. (2001) to produce time series of range change (change in distance between the ground and satellite) for each point shown in Figure 2.8. To measure creep on the Concord fault, we look at motion along several profiles through the Permanent Scatterers (PSs), crossing the Concord fault. We use swath averaging to construct each profile. Within each swath, shown as dashed lines in Figure 2.8, points within 0.25 km bins perpendicular to the fault are averaged together and projected onto the centerline. For example, all points between 0.5 and 0.75 km west of the fault are averaged together to provide one point on the profile and their standard deviation provides an estimate of the profile point's uncertainty.

Figure 2.9: An example of a swath average profile; this is for profile 3 on 12/9/2001, referenced to 5/6/1992. Grey circles are the actual PS-InSAR points within the dashed box in Figure 2.8 and projected onto the centerline. Black circles are the derived swath averages; averages of all points inside a 0.25 km bin. Error bars are one standard deviation of the PS-InSAR points comprising each swath average. The dashed black line is fit to the data using a linear inversion and the fault creep is measured from the offset of the profile at the fault trace.
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A swath average profile is constructed for all 46 acquisition dates on each of the five profiles, for a total of 230 profiles (an example is shown in Figure 2.9). A linear inversion is performed on each profile to obtain the offset at the fault trace, produced by the shallowly creeping fault. Each offset value represents the amount of creep on the Concord fault since the beginning of the time series in 1992.

Figure 2.10: Surface creep through time for the five profiles shown in Figure 2.8. Individual time series are offset from each other for clarity. The creep rate as determined from a weighted linear inversion is printed next to each series. Alinement array measurements from sites CSAL and CASH are plotted with Profile 3 (P3) for comparision.
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Long-term creep rates

Time series of profile offsets for all five profiles are shown in Figure 2.10. By fitting a line to the offsets, we can obtain a measurement of the creep rate at each profile. The two northern-most profiles (P1a & P1b) have creep rate of 2.3-2.4 mm/yr; this is very low, but measurable within the calculated uncertainty of 1.1-1.6 mm/yr. The creep rate continues to be low southward through profile 2 (P2), where the measured rate could be zero within uncertainty. Creep picks up fairly suddenly at profile 3 (P3), which is near the portion of the fault known to creep from alinement arrays. Time series of creep measurements from the alinement arrays are plotted with the time series from P3 in Figure 2.10. Both arrays measure slightly lower creep rates than is indicated by P3, but support the idea that the creep rate is increasing toward the south. CSAL is the more northern alinement array and is on the north edge of P3 (see Figure 2.8); it has a lower creep rate (2.8 mm/ur) than CASH (3.7 mm/yr), which is located toward the center of P3. P3 incorporates more data to the south of CASH, which may explain its higher creep rate. Higher rates of creep continue in profile 4 (P4), our southern-most profile.

Time-variable creep

Creep on the Concord fault has also been shown by alinement array measurements to be episodic, with a 3-5 year period. This variability is within the uncertainty in the profile offsets and could not be independently detected using the PS-InSAR data. However, offsets for P3 match some of the variability in alinement array CASH, particularly post-1998, suggesting that there is information available in the PS-InSAR data on the time variability of Concord fault creep.

Acknowledgements

This work was supported by USGS-NEHRP external grant #08-HQGR-0095. SAR data are copyrighted by the European Space Agency and were obtained via the WInSAR consortium.

References

Ferretti, A., C. Prati, and F. Rocca, Permanent scatterers in SAR interferometry, IEEE Trans. Geosci. Remote Sens., 39, 8-20, 2001.

McFarland, F., J. Lienkaemper, and S. J. Caskey, Data from theodolite measurements of creep rates on San Francisco Bay region faults, California; 1979-2009, USGS Open file report 09-1119, 2009.

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