Resolving Vertical Tectonics in the San Francisco Bay Area from Permanent Scatterer Insar and GPS Analysis

Resolving vertical tectonics in the San Francisco Bay Area from permanent scatterer InSAR and GPS analysis

Roland BuÈrgmann Department of Earth and Planetary Science and Berkeley Seismological Laboratory, University of California, Berkeley, California 94720, USA George Hilley Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Alessandro Ferretti   Tele-Rilevamento Europa, Via Vittoria Colonna 7, 20149 Milan, Italy Fabrizio Novali 

ABSTRACT and by loss of coherence in vegetated or high- Using a combination of GPS-measured horizontal velocities of 200 sites and 115,487 relief terrain. A new InSAR processing tech- range-change rates determined with the permanent scatterer interferometric synthetic ap- nique, the permanent scatterer-interferometric erture radar (InSAR) method in the San Francisco Bay Area, we resolve vertical motions synthetic aperture radar (PS-InSAR) method in the region at sub-mm/yr precision. The highest displacement rates are due to nontec- (Colesanti et al., 2003; Ferretti et al., 2000, tonic processes, such as active landslides, subsidence and rebound over aquifers, and rapid 2001), allows for the identi®cation and inte- settling of unconsolidated sediments along the bay margins. Residual displacement rates gration of individual radar-bright and radar- are determined by removing the contribution of the GPS-derived horizontal velocity ®eld phase-stable points (outcrops, buildings, util- from the InSAR range-change rates. To isolate vertical tectonic rates, we use only those ity poles, etc.) in more than 15 SAR images. InSAR measurements made on material that was not Quaternary substrate, which is sus- The phase data from these permanent scatter- ceptible to nontectonic and seasonally varying ground motions. The InSAR residuals in- ers are then used to separate time-dependent dicate signi®cant uplift over the southern foothills of the active Mount Diablo anticlinor- surface motions, atmospheric delays, and ium, the Mission Hills stepover region of the Hayward and Calaveras faults, and the elevation-error components of the range- central and southern Santa Cruz Mountains located along a restraining bend of the San change measurement (Colesanti et al., 2003). Andreas fault. We incorporate 49 European Remote Sensing satellite (ERS-1 and ERS-2) acquisitions col- Keywords: geodesy, San Andreas fault, uplifts, San Francisco Bay region, GPS. lected from 1992 to 2000 of our Bay Area target scene (track 70, frame 2853) (Fig. 1). INTRODUCTION mation rates in the San Francisco Bay Area Points whose displacements vary in a highly The San Andreas fault system in central appear to have been constant (BuÈrgmann et nonlinear fashion, such as a large area of rapid California is a transform plate boundary ac- al., 1997; Lisowski et al., 1991; Savage et al., seasonal uplift and subsidence in the Santa commodating ϳ38 mm/yr of right-lateral mo- 1994). A record of GPS acquisitions, now Clara Valley (Colesanti et al., 2003; Schmidt tion between the Paci®c plate and the Sierra more than ten years' worth, provides precise and BuÈrgmann, 2003), are excluded from the Nevada±Great Valley block (Argus and Gor- horizontal site velocities and reveals the na- ®nal data set. The PS-InSAR data provide don, 2001; d'Alessio et al., 2005). Regional ture of elastic strain accumulation about the range-change rates at 115,487 points, includ- contraction across the San Andreas fault sys- major strands of the San Andreas fault system ing targets in vegetated or mountainous areas tem has occurred at rates of ϳ1±5 mm/yr (d'Alessio et al., 2005). However, GPS is not that are unsuitable for standard InSAR. In ad- since 3±5 Ma (Argus and Gordon, 2001). In able to resolve the low interseismic uplift rates dition, this data set contains a time series of addition to deformation associated with re- that may be associated with the contractional displacement, which allows us to identify regional plate-normal convergence, high rates of structures in the Bay Area (BuÈrgmann et al., gions in which there is a signi®cant time- shortening accommodated by thrust faulting 1994). Historic leveling measurements pro- dependent, often seasonal component to the occur in areas of restraining strike-slip fault vide limited information about possible tec- deformation ®eld. geometries. In the San Francisco Bay Area, an tonic uplift in the eastern Bay Area (Gilmore, ϳ10Њ bend of the San Andreas fault through 1993). InSAR has revealed centimeter-level RESOLVING VERTICAL TECTONICS the Santa Cruz Mountains, the Mission Hills uplift and subsidence patterns related to the Range-change rates recorded by the PS- stepover region between the Calaveras and seasonal drawdown and recharge of the Santa InSAR measurements include the contribution Hayward faults, and a left stepover between Clara Valley aquifer (Schmidt and BuÈrgmann, of horizontal tectonic motions that project into the Greenville and Concord faults (encom- 2003), rapid motions of deep-seated landslides the radar line-of-sight vector. To isolate the passing the Mount Diablo anticlinorium) are (Hilley et al., 2004), and a possible sub-mm/ vertical component of the deformation, we use areas of localized contraction and high topog- yr contribution of an east-side-up, dip-slip the GPS-derived horizontal velocity ®eld to raphy (Fig. 1). Here, we use a new approach component to the range-change offset across eliminate its contribution to each PS-InSAR that allows us to determine a detailed image the creeping northern Hayward fault (Hilley et range-change measurement. To this end, we of vertical motions from nontectonic and tec- al., 2004; Schmidt et al., 2005). use a 200-station subset of a regional 1993± tonic processes by combining precise but InSAR only provides a one-dimensional 2004 GPS velocity ®eld (d'Alessio et al., sparsely distributed GPS-site motions with a measurement of change in distance along the 2005) to constrain a mechanical model of the large data set of satellite-to-ground range look direction of the radar spacecraft, but is horizontal deformation ®eld. In the model, changes measured with interferometric syn- highly sensitive to vertical motion due to its uniform-slip dislocations in an elastic half- thetic aperture radar (InSAR). steep (here 20Њ±26Њ off-vertical) look angle space (Okada, 1985) reproduce the surface SURFACE DEFORMATION (BuÈrgmann et al., 2000). InSAR measure- strain ®eld about the Bay Area faults. Inter- MEASUREMENTS ments that rely on one or a stack of several seismic shear about a locked strike-slip fault Except for the coseismic and postseismic interferograms are often hampered by signi®- is approximated by slip on a buried vertical effects of large earthquakes, the active defor- cant noise introduced by atmospheric delays screw dislocation extending deep below the

᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; March 2006; v. 34; no. 3; p. 221±224; doi: 10.1130/G22064.1; 3 ®gures; Data Repository item 2006041. 221 dient (of 0.094 mm/km) across the scene determined in a joint inversion of both data sets. Scatter in the data at the level of about Ϯ0.5 mm/yr is probably in large part due to the in- herent instability of the objects that re¯ect radar, which mostly consist of built structures. Residual range-change rates represent the combined effects of horizontal and vertical motions that are unaccounted for by the tectonic dislocation model. Where nontectonic horizontal displacement rates are small and the simpli®ed velocity ®eld produced by the dislocation model adequately represents the actual tectonic velocity ®eld, the residual range-change rates may be interpreted in terms of vertical motions. In this case, the inferred vertical displacement rates are 1.08± 1.10 times the residual range-change rate, a factor that slightly varies with the look angle across the area.

RESULTS AND INTERPRETATIONS When the range-change contribution due to horizontal tectonic motions (Fig. DR1; see footnote 1) is removed from the data, we ®nd that the largest motions revealed in the PS- InSAR data are due to various nontectonic hy- drological and surface processes (Fig. DR2). Many of the nontectonic features are highly localized, including active landslides (Hilley et al., 2004) and localized subsidence along the bay due to consolidation of man-made ®ll and bay mud (Ferretti et al., 2004). Regions of uplift overlie young sedimentary basins that Figure 1. Fault map of San Francisco Bay Area in oblique Mercator projection about pole are expanding due to increasing groundwater of Paci®c plate to Sierra Nevada±Great Valley (SNGV) block rotation (d'Alessio et al., 2005). levels during the observation period (Schmidt We include 199 1993±2003 GPS velocities (black arrows tipped with 95% con®dence ellipses, and BuÈrgmann, 2003). not all in map frame; Table DR2 [see footnote 1]) relative to central bay station LUTZ. Yellow To resolve tectonic deformation, we iden- arrows are predicted velocities calculated from dislocation model (model parameters in tify points in the data set in which these non- Table DR1). Color-scaled dots show 1992±2000 PS-InSAR range-change rates of 115,487 permanent scatterers relative to point labeled FIXED. LPÐLoma Prieta; MDÐMount Diablo; tectonic processes severely in¯uence the de- MHÐMission Hills; SCMÐSanta Cruz Mountains; SCVÐSanta Clara Valley. formation measurement. The PS-InSAR data suggest that points located on Quaternary substrate are generally more likely to record de- seismogenic zone. Creep on the Hayward, area that exhibits a systematic mis®t pattern formation related to nontectonic processes, southern Calaveras, and central San Andreas in the residual GPS velocities lies along the and so we use a GIS-produced map of the dis- faults is modeled by shallow dislocation ele- Loma Prieta section of the San Andreas fault, tribution of Quaternary units in the region ments. A simple 17-element dislocation model where a region of apparent residual contrac- (Knudsen et al., 2000) to separate 92,102 (Table DR11) can explain much of the ob- tion and right-lateral shear of Յ5 mm/yr is points located on Quaternary units from those served horizontal displacement ®eld. The observed. The remaining high residuals either on bedrock (i.e., non-Quaternary) (Fig. 2). In summed mis®t to the GPS velocities, the are apparent outliers or are located along addition, we analyze the time series of all PS- (unitless) weighted residual sum of squares creeping fault segments where more detailed, InSAR points to identify points that show (WRSS), of this model is 1173, and the re- slip-distributed models are appropriate strong seasonal deformation (Fig. 3). We ®nd duced mis®t statistic͙WRSS/(n Ϫ p) is 3.1, (Schmidt et al., 2005). that points within bedrock units display long- where n and p are the number of data (199 We use the modeled horizontal velocity term vertical displacement rates (Յ2 mm/yr, north and east velocities relative to reference ®eld to calculate predicted range-change rates Fig. 2) that are far smaller than points on Qua- station) and model parameters (strike slip on along the individually computed line-of-sight ternary substrate with rates up to ϳ10 mm/yr 17 dislocations), respectively. The only large vectors due to horizontal motions at each (Fig. DR2). InSAR data point (Fig. DR1). The modeled Interestingly, even PS-InSAR points pre- 1GSA Data Repository item 2006041, model pa- rates are then subtracted from the observed sumably located on bedrock show substantial rameters (Table DR1), GPS data (Table DR2), and PS-InSAR rates. As the PS-InSAR range- seasonal ¯uctuations that often appear coher- Figs DR1 and DR2, is available online at change ®eld may include a small contribution ent over large areas (Fig. 3). Pre-Quaternary www.geosociety.org/pubs/ft2006.htm, or on request from [email protected] or Documents Secre- from remaining satellite orbit-baseline errors, units may experience some hydrologic expan- tary, GSA, P.O. Box 9140, Boulder, CO 80301- we also estimate and remove the contribution sion or contraction and can be mantled with 9140, USA. of a small regional linear range-change gra- unstable soils. However, the bedrock points'

222 GEOLOGY, March 2006 we are not able to fully characterize the shape of the uplift zone or develop a well- constrained mechanical model of the deformation. The uplift is terminated to the south- east by the Loma Prieta zone of subsidence and residual contraction. Overall, the data suggest that uplift occurs over a broad zone, con- sistent with regional contraction along the restraining bend in the San Andreas fault that is ultimately accommodated by discrete thrust faulting and folding. Points located between the Hayward and Calaveras faults appear to be rising at rates of 0±1.5 mm/yr. The highest uplift rates are localized in a small zone coinciding with the highest elevations of the Mission Hills (Fig. 2). Relocated seismicity and focal mecha- nisms in the stepover region indicate oblique right-lateral slip on a steeply northeast- dipping fault (Manaker et al., 2005). Abun- dant microseismicity and observation of re- peating small earthquakes along this fault at 5±10 km depth (Schmidt et al., 2005) suggest that aseismic fault creep occurs at least over that depth range and may explain the localized nature of the high uplift rates.

DISCUSSION AND CONCLUSIONS Our study shows how contributions from nontectonic processes and time-dependent earthquake cycle deformation make the identi®cation and interpretation of active, vertical tectonics from geodetic measurements a chal- lenging endeavor. The largest vertical motions Figure 2. Residual PS-InSAR rates after removing contribution of tectonic horizontal mo- are due to nontectonic processes such as land- tions and all points located on Quaternary substrate. Inferred uplift rates are 1.08±1.10 times sliding, seasonal and long-term groundwater- the range-change rates depicted, assuming that all remaining deformation is due to vertical level changes, and sediment settling. If we ex- motions (positive range-rate residuals correspond to uplift). Yellow arrows are GPS residual clude all data points located on Quaternary (observed minus modeled) horizontal velocities, and pink lines and diamonds are vertical model dislocation planes and their ends, respectively. Abbreviations are as in Figure 1. substrate to limit contributions to the deformation signal from nontectonic motions, the InSAR range-change rates corrected for hori- magnitude and sign of the seasonal vertical horizontal velocities can explain only ϳ0.5 zontal tectonic motions re¯ect vertical defor- displacement often change over several sea- mm/yr of the residual range-change rate, and mation rates in the region that are Ͻ2 mm/yr. sons, while points lying on Quaternary sub- subsidence at ϳ1.5 mm/yr is indicated. Such The range-change time series of bedrock strate consistently subsided during the dry subsidence is expected due to continued points appear to also contain seasonal varia- season and rebounded during the wet season. postseismic relaxation at depth following the tions due to hydrologic or atmospheric pro- The amplitude of seasonal ¯uctuations tends Loma Prieta earthquake (Pollitz et al., 1998). cesses, which are superimposed over the long- to be ϳ2±3 times larger for the Quaternary A third zone of slow (ϳ0.5 mm/yr) subsi- term uplift patterns we infer over eight years PS-InSAR points than for bedrock points (Fig. dence along the northern San Francisco pen- of PS-InSAR observations. Thus, it is impor- 3). insula may be related to an extensional bend tant to establish a long-term (i.e., decadal) rec- When interpreted in terms of vertical mo- in the San Andreas fault and/or interaction ord of vertical motions to obtain reliable mea- tions, the residual range-change rate ®eld of with the offshore San Gregorio fault zone. sures of tectonic uplift and subsidence. The bedrock target points includes three larger ar- Uplift rates on the order of 1 mm/yr are joint use of continuous GPS and multiple PS- eas of subsidence and three regions of uplift indicated along the southern Santa Cruz InSAR data sets obtained from different ac- at 0.5±1.5 mm/yr rates (Fig. 2). One area of Mountains that ¯ank Santa Clara Valley, along quisition geometries and radar satellites will apparent subsidence is located east of the the peak elevations of the Mission Hills, and allow for future improvements in the accuracy Greenville fault, and we have no obvious ex- within the southern foothills of Mount Diablo. of space-geodetic uplift measurements. planation for this feature. The second area of Inferred uplift is observed on both sides of the The highest uplift rates appear to coincide subsidence at rates of up to ϳ2 mm/yr appears San Andreas fault between 37Њ10ЈN and with known areas of folding and thrusting re- localized about the epicentral region of the 37Њ30ЈN, but appears to be highest to the lated to restraining fault bends and stepovers ϭ 1989, Mw 6.9, Loma Prieta earthquake and northeast overlying a number of Quaternary along the Bay Area strike-slip faults. These coincides with the region of horizontal con- thrust faults (BuÈrgmann et al., 1994). As the include the southern foothills of the Mount traction evident in the residual GPS velocities. Santa Cruz Mountains uplift zone is adjoined Diablo fold and thrust belt, the stepover region The direction and magnitude of these residual by the rapidly expanding Santa Clara aquifer, between the Hayward and Calaveras faults,

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Pollitz, F., BuÈrgmann, R., and Segall, P., 1998, Joint and the Santa Cruz Mountains northwest of southern Santa Cruz Mountains, California, estimation of afterslip rate and postseismic re- the Loma Prieta epicentral region. Active up- deduced from ®ssion track dating, geomorphic laxation following the 1989 Loma Prieta ϳ analysis, and geodetic data: Journal of Geo- earthquake: Journal of Geophysical Research, lift rates of 1 mm/yr in these regions suggest physical Research, v. 99, p. 20,181±20,202, v. 103, p. 26,975±26,992, doi: 10.1029/98JB rapid convergence rates and potentially sig- doi: 10.1029/94JB00131. 01554. ni®cant earthquake hazard associated with the BuÈrgmann, R., Segall, P., Lisowski, M., and Svarc, Savage, J.C., Lisowski, M., and Svarc, J.L., 1994, underlying active thrust faults. J., 1997, Postseismic strain following the 1989 Postseismic deformation following the 1989 Loma Prieta earthquake from GPS and level- (M ϭ 7.1) Loma Prieta, California, earth- ing measurements: Journal of Geophysical Re- quake: Journal of Geophysical Research, ACKNOWLEDGMENTS search, v. 102, p. 4933±4955, doi: 10.1029/ v. 99, p. 13,757±13,765, doi: 10.1029/94JB Eric Fielding, Rob Mellors, and an anonymous 96JB03171. 00507. reviewer provided valuable reviews. Tim Wright BuÈrgmann, R., Rosen, P.A., and Fielding, E.J., 2000, Schmidt, D.A., and BuÈrgmann, R., 2003, Time- provided a program for calculating the line-of-sight Synthetic aperture radar interferometry to dependent land uplift and subsidence in the vectors. SAR data were provided to the WInSAR measure Earth's surface topography and its de- Santa Clara Valley, California, from a large Consortium by the European Space Agency formation: Annual Review of Earth and Plan- InSAR data set: Journal of Geophysical Re- (ESA) through Eurimage. Supported by U.S. Geo- etary Sciences, v. 28, p. 169±209, doi: search, v. 108, 2416, doi: 10.1029/2002JB logical Survey NEHRP External Program grant 10.1146/annurev.earth.28.1.169. 002267. 05HQGR0038. Berkeley Seismolab Contribution Colesanti, C., Ferretti, A., Novali, F., Prati, C., and Schmidt, D.A., BuÈrgmann, R., Nadeau, R.M., and #05-012. 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