Application of Point Measurements from InSAR to Study Slow Earthquakes on the Central San Andreas Fault

Ingrid A. Johanson and Roland Bürgmann

Introduction

In 1992, 1996 and 1998, creepmeters and strainmeters on the San Andreas fault (SAF) near San Juan Bautista recorded slip transients identified as slow earthquakes (SEQs) (Linde et al., 1996; Gwyther et al., 2000). These authors determined that SEQs in this area had moment magnitudes equivalent to the largest seismic quakes ($M_{w}$4.8-5.1) in this region. Their size and frequency suggest that SEQs are an important contributor to strain release. Additionally, SEQs primarily occur in areas of transition between locked and stably sliding faults, and may offer clues as to why and how these transitions take place. However, the low density of creep- and strain-meters makes it difficult to determine basic rupture parameters, such as the area or depth of the slip patch, with much certainty. In fact, the instrument density is so low that the network may fail to record many slow earthquakes. We are using high resolution geodetic measurements adjacent to the fault in an attempt to significantly improve our observations of SEQs and to possibly identify unrecorded SEQs. In particular, we would like to know: how common slow earthquakes are, how much of the fault's total slip they accommodate, whether they consistently rupture the same fault patch and how their occurrence depends on depth. Ultimately, our results should give us a better understanding of how episodic, aseismic slip events fit into the overall mechanics of fault systems.

InSAR

The extensive spatial coverage, good resolution and sub-centimeter precision of Interferometric Synthetic Aperture Radar (InSAR) makes it an ideal tool for measuring surface deformation due to slow earthquakes. However, the ground cover for this section of the SAF is largely vegetation and many interferograms are reduced to random scatter as a result of decorrelation noise (Figure 23.2). We must remove any decorrelation noise before we can identify a geophysically based signal. Our observations of InSAR amplitudes and air photos reveal that small buildings and rock outcrops exist within the vegetated areas that could provide stable and accurate measurements (Figure 23.1). These isolated stable patches (`stable scatterers'), ranging in size from a small town to a single pixel, can be extracted from the sea of decorrelation noise and pieced together to increase the useful area of an interferogram.

Figure 23.1: InSAR amplitude image of the San Juan Valley showing the locations of `stable scatterers' (black circles). The dotted line delimits the swath box from which the profile in Figure 3 is extracted. Also shown is the San Andreas Fault (solid line).

Figure 23.2: Wrapped interferogram corresponding to the area covered in Figure 1 and spanning from May 19, 1996 to August 8, 1998; with a perpendicular baseline of 7 meters. Although the image has been spatially filtered to enhance any signal, random noise still dominates.

Stable Scatterers

Ferretti et al. (2001) proposed, as a part of their Permanent Scatterer Method, that the stability of individual amplitude and phase measurements across many interferograms be used to identify stable pixels. Buildings (which are often picked as `Permanent Scatterers' because of their good stability) have corner reflecting walls that reflect much more brightly than any surrounding vegetation. `Stable scatterers' can therefore also be identified as points with consistently high amplitudes (Johanson and Bürgmann, 2001). We are implementing this adaptation of the Permanent Scatterer Method to accurately measure surface deformation due to slow earthquakes on the central SAF. InSAR amplitude suggests that there is a sufficient number of potential `stable scatterers' to make this approach viable (Figure 23.1). Across-fault profiles of stable scatterers from our preliminary results are more coherent than randomly chosen points and exhibit geophysically reasonable motions (Figure 23.3). However patches of high scatter in the profile show that additional refinement of our `stable scatterer' definition may be necessary.

Figure 23.3: Unwrapped profile across the San Andreas Fault of `stable scatterers' contained within the swath box as shown in Figure 1, extracted from an unfiltered version of Figure 2. Wrapped range change values can vary between 1.4 $\&$ -1.4 cm. This profile represents a 26$\%$ reduction in angular deviation as compared with randomly selected points.

Future Work

With nearly 100 available interferograms of the central SAF, we plan to construct a time series that can be analyzed together with a campaign GPS data set spanning 1989-2002. Yearly observations of a GPS network covering the Santa Cruz, Hollister, and Salinas areas, have yielded precise measurements of the post-seismic deformation from the 1989 Loma Prieta earthquake and of the regional interseismic deformation. The pattern of ground movement from these processes overprints the deformation pattern from SEQs and should be removed from the InSAR analysis. Another benefit of GPS data is that it measures ground movement in three components and can provide a basis for differentiating between horizontal and vertical ground movement in the interferograms.

References

Ferretti, A., C. Prati, and F. Rocca, Permanent scatterers in SAR interferometry, IEEE Transactions on Geoscience and Remote Sensing, 39 (1), 8-20, 2001. Gwyther, R.L., C.H. Thurber, M.T. Gladwin, and M. Mee, Seismic and aseismic observations of the 12th august 1998 San Juan Bautista, California m5.3 earthquake, in 3rd San Andreas Fault Conference, pp. 209-213, Stanford University, 2000. Johanson, I.A., and R. Bürgmann, Using point measurements from InSAR to detect transient deformation (abstract), EOS Transactions, AGU, 82 (47), 266, 2001. Linde, A.T., M.T. Gladwin, M.J.S. Johnston, R.L. Gwyther, and R.G. Bilham, A slow earthquake sequence on the San Andreas Fault, Nature, 383 (6595), 65-8, 1996.

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