APLICATION OF INSAR TO THE STUDY OF GROUND DEFORMATION IN THE MEXICALI VALLEY, B. C., MEXICO.

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1 APLICATION OF INSAR TO THE STUDY OF GROUND DEFORMATION IN THE MEXICALI VALLEY, B. C., MEXICO. O. Sarychikhina (1), R. Mellors (2), E. Glowacka (1). (1) Centro de Investigacion Cientifica y Educaccion Superior de Ensenada (CICESE), km. 107 Carretera Tijuana- Ensenada, Ensenada, B.C., 22860, Mexico. osarytch@cicese.mx, glowacka@cicese.mx (2) San Diego State University (SDSU), 5500 Campanile Drive, San Diego, CA, 92182, USA. rmellors@geology.sdsu.edu ABSTRACT This paper reports the preliminary results of the ongoing ESA CAT-1 project (ID - C1P3508). Radar Interferometry (InSAR) has been used together with geodetic (leveling survey) and geotechnical (tiltmeters and extensometers) techniques to monitor ground deformation in the Mexicali Valley and to distinguish between possible causes (tectonics and fluid extraction) and mechanisms (aseismic and seismic slip). A clear signal of a magnitude 5.4 earthquake that occurred on May 24, 2006 is observed. 1. INTRODUCTION Mexicali Valley is situated in northeastern Baja California, Mexico and is located within an active tectonic region of the southern part of San Andreas Fault system, in the boundary between North American and Pacific tectonic plates. There are two main faults in the Mexicali Valley, The Imperial and Cerro Prieto, seismically very active and with concentrated deformation associated with them. A zone of dispersed seismicity and deformation also occurs in the pull-apart centre of Cerro Prieto (Fig. 1). In 1973, fluid extraction began for electric power production in the Cerro Prieto Geothermal Field (CPGF), and in 1989 the brine injection was initiated. These operations influenced the state of stress in the zone and caused variations in deformation and seismicity rates. Since the deformations and the seismicity in the Mexicali Valley are influenced by active tectonic, hydrothermal processes and human activity, the Mexicali Valley is a natural laboratory for the study of ongoing ground deformations. Ground deformation in the studied area has been monitored by repeated ground surveys using both precise leveling and GPS. It is also currently monitored by a network of geotechnical instruments (tiltmeters and extensometers). The measurements from these ground surveys have revealed subsidence, likely due to fluid extraction coupled with the specific tectonic context of the area [1, 2]. The geometry of the deforming area is controlled by faults. The geotechnical instruments measurements report the presence of vertical (6cm/yr) and horizontal (2cm/yr) mainly aseismic creep in the Saltillo branch of Imperial fault, and vertical (3cm/yr) mainly aseismic creep in the northern end of the Cerro Prieto fault. Using information about current tectonics [3] and data from precise leveling surveys, modeling of tectonic and anthropogenic parts of subsidence has been done by Sarychikhina [4] and published in Glowacka et al. [5]. The obtained results show that tectonic subsidence constitutes only ~4% of the measured subsidence. Anthropogenic subsidence was evaluated using a model of rectangular tensional cracks, based on the hydrological model of CPGF proposed by Lippmann et al. [6], together with the Coulomb 2.0 software [7, 8]. The final model consists of 5 tensional rectangular cracks which represent 3 geothermal reservoirs, located in the CPGF production zone (p), superficial recharge aquifer that cover wide zone between two major faults and local recharge aquifer located between CPGF production zone and Saltillo fault (recharge zone - r) (Fig. 2). In order to supplement these results and improve the spatial resolution of the monitoring, the InSAR has been used. Previous studies realized by Carnec and Fabriol [9] and Hanssen [10] have shown the feasibility of INSAR in this area. In [9, 10] the images acquired by the ERS 1 and ERS 2 satellites in and periods were used respectively. In this project we use ENVISAT satellite data together with ERS1/ data in order to improve the spatial and temporal resolution of the ground deformation monitoring in the Mexicali Valley. 2. PROCESSING SOFTWARE AND DATA SET SAR images (single-look complex) were acquired from ESA and inteferogram processing was conducted using the DORIS InSAR [11], Delft orbits [12], and SRTM elevation data. Interferogram unwrapping was performed using the SNAPHU software developed by Chen [13]. Our dataset includes 24 images acquired by the ERS-1, ERS-2 and ENVISAT satellites in both descending (track 84, frame 2961) and ascending (track 306, frame 639) passes between September 1993 (ERS1) and September 2006 (ENVISAT). The spatial coverage of these images is presented in Fig. 1. Fifty interferograms were created. The interferograms were evaluated and Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 several were discarded due to poor quality. The interferograms have the perpendicular baseline (B ) less than 400m and a time span (B temp ) from 1 month to 2 year. In this paper we present analysis of 4 Envisat interferograms which were selected based on correlation and lack of obvious atmospheric artifacts. Details are listed in Tab PRELIMINARY RESULTS ANALYSIS The InSAR results, geocoded coherence images (a1-a3), wrapped phase interferograms (b1-b3) and maps of surface displacement away from the satellite along the radar line of sight (LOS) (c1 & c2), derived from the 3 interferometric pairs are shown in Fig. 2. Coherence decreases dramatically and interferograms spanning more than three months were generally of very poor quality. (Fig. 2 a3, b3). Only in the mainly desert area of CPGF the coherence is sufficient to produce reliable interferometric data over periods of time longer than three months. For comparison, displacement measurements obtained from precise leveling data were converted into radar line-of-sight and normalized for the corresponding time span period (Fig. 2 d1 & d2). This allows the possibility to compare the ground deformation data obtained by two techniques: precise leveling survey and InSAR. The dominant feature that shows the highest subsidence rate is a NE-SW elliptical area. The maximum subsidence rate ~1.5 cm/month, for the analyzed period, is located in recharge zone (r) (Fig. 2 c1 & c2, d1 & d2). A second centre of subsidence is located below the CPGF production zone (p). If we compare the location and subsidence rate in this centre for two different periods b1, c1 and b2, c2 in Fig. 2 we can observe that these are not the same. This information suggests that the subsidence is a dynamic process which varies in space over time. This conclusion we couldn t obtain from the leveling data where the subsidence is considered a constant process between two leveling surveys which were 9 years apart. The interferograms (03/12/16-04/02/24 and 04/12/19-05/01/23) show that the ground deformation area is bounded in the east by the Saltillo fault and in the west by the more diffuse (~ 2km) Cerro Prieto Fault Zone (Fig.2 b1 & b2, c1 & c2). This is a clear improvement on the result of the leveling survey which possessed low spatial resolution in these areas. (Fig. 2 d1 & d2). The rate of subsidence appeared to differ in the two interferograms. A vertical creep event of 1.2 cm observed by vertical extensometer installed on Saltillo fault falls in the time spanned by the first (03/12/16-04/02/24) interferogram and we suspect that this creep event contributes to the signal. In May 2006 an M w 5.4 (05/24/06) and four M L 4 earthquakes occurred in this area and are clearly observed on the inteferogram. In the Fig. 4 the coherence image (a), wrapped phase interferogram (b) and map of surface displacement away from satellite along the radar LOS (c) for the 4 th interferometric pair are presented. The M w 5.4 earthquake had a normal mechanism. A field survey revealed up to 30 cm of vertical displacement and surface cracks over a length of more than five kilometers [14]. The ground displacement map Fig.4 (c) shows the effects of this earthquake combined with the general subsidence of the area due to geothermal fluid extraction. The area affected by the earthquake is ~ km, with the maximum surface displacement up to 20 cm and a rupture length of roughly 10 km. The deformation is larger than expected for an M w 5.4 earthquake (e.g. Wells and Coppersmith [15]) and indicates that the event was very shallow or triggered an additional (not tectonic) subsidence. We are currently modeling the InSAR data to place further constraints on the rupture parameters. 4. CONCLUDING REMARKS The paper presented preliminary results from InSAR monitoring in the Mexicali Valley. The ground deformation due to fluid extraction, earthquakes and creep events occurrence has been detected. The combination of the InSAR with the available ground truth data (leveling and geotechnical instruments) is powerful and has already yielded useful results. The modeling of the deformation phenomenon observed by InSAR, geodesic and geotechnical techniques is currently in process. A difficulty is the relatively high rate of temporal decorrelation in the areas covered by agriculture. In our next step, we hope to construct time series of deformation perhaps combined with advanced methods to use point scatterers. 5. ACKNOWLEDGEMENT The European Space Agency's (ESA) ERS1/2 and ENVISAT satellites have been used to collect the interferometric data. The data were obtained as a part of ESA Cat-1 Project (ID - C1P3508). This research was sponsored in part by CONACYT, project number F and CICESE internal funds. The authors are grateful to Alejandro Hinojosa Corona for his valuable help with GIS tools during the work on this project. 6. REFERENCES 1. Glowacka, E., Fabriol, H., Munguía, L. & Gonzalez, J.J. (1997). Seismicity and surface deformation around the Cerro Prieto Geothermal Field. In:

3 Rockbursts and Seismicity in mine (Ed. G. Lasocki), Balkema, Rotterdam, pp Glowacka, E., Gonzalez, J.J. & Fabriol, H. (1999). Recent Vertical deformation in Mexicali Valley and its Relationship with Tectonics, Seismicity and Fluid Operation in the Cerro Prieto Geothermal Field. Pure and Applied Geophysics 156, Bennett, R.A., Rodi, W. & Reilinger, R. E. (1996). Global Positioning System Constrains on Fault Slip Rates in Southern California and Northern Baja, Mexico. Journal of Geophysical Research 10, Sarychikhina, O. (2003). Modelación de subsidencia en el campo geotérmico Cerro Prieto. Tesis de Maestría, CICESE, México. 5. Glowacka, E., Sarychikhina, O. & Nava, F.A. (2005). Subsidence and stress change in the Cerro Prieto geothermal field, B.C., México. Pure and Applied Geophysics 162, Lippmann, M. J., Truesdell, A. H., Mañón, A. M. & Halfman, S. E. (1991). A review of the hydrogeologic-geochemical model for Cerro Prieto. Geothermics 20, King, G. C. P., Stein, R. S. and Lin, J. (1994). Static Stress Changes and the Triggering of Earthquakes. Bulletin of the Seismological Society of America 84(3), Toda, S., Stein, R. S., Reasenberg, P. A. & Dieterich, J. H. (1998). Stress transferred by the Mw=6.9 Kobe, Japan, shock: Effect on aftershocks and future earthquake probabilities. Journal of Geophysical Research 103, Carnec, C. & Fabriol, H. (1999). Monitoring and Modeling Land Subsidence at the Cerro Prieto Geothermal Field, Baja California, Mexico, Using SAR Interferometry. Geophysical Research Letters 26(9), Hanssen, R.F. (2001). Radar Interferometry; Data Interpretation and Error Analysis. Kluwer Academic Publisher, Dordrecht, Netherlands. 11. Kampes, B. (1999). Delft Object-oriented Radar Interferometric Software User s manual, Delft University of Technology, Nederlanden. 12. Scharroo, R., Visser, P. N. A. M. & Mets, G. J. (1998). Precise orbit determination and gravity field improvement for the ERS satellites. Journal of Geophysical Research 103(C4), Chen, C. W. & Zebker, H. A. (2001). Twodimensional phase unwrapping with use of statistical models for cost functions in nonlinear optimization. Journal of the Optical Society of America A. 18, Suárez-Vidal, F., Munguia-Orozco, L., González- Escobar, M., González-García, J. & Glowacka, E. (2007). Surface rupture of the Morelia fault near the Cerro Prieto Geothermal Field, Mexicali, Baja California, Mexico, during the M w 5.4 earthquake of 24 May Seismological Research Letters 78 (3). 15. Wells, D. L. & Coppersmith, K.J. (1994). New Empirical Relationship among Magnitude, Rupture Width, Rupture Area, and Surface Displacement. Bulletin of the Seismological Society of America 84 (4),

4 Table 1. The interferometric pairs used in this paper for demonstrating the terrain displacement in Mexicali Valley. A and D indicates respectively ascending and descending track. Pairs Satellite Track_Frame Master Image Slave Image B (m) B temp (days) (yy/mm/dd) (yy/mm/dd) 1 ENVISAT 306_639 (A) 2003/12/ /02/ ENVISAT 84_2961 (D) 2004/12/ /01/ ENVISAT 84_2961 (D) 2005/05/ /10/ ENVISAT 306_639 (A) 2006/05/ /09/ Figure 1. Regional map of the study area. Landsat image is used as the background. Large white rectangles indicate the spatial coverage of the ERS1/2 and ENVISAT tracks along which SAR data were recollected. A and D indicates respectively ascending and descending track. Blue rectangle represents the area of study. CPGF is Cerro Prieto Geothermal Field which area is represented by the yellow rectangle. SF is Saltillo Fault. The tectonic situation modified from [14].

5 (a1) 2003/12/ /02/24 (a2) 2004/12/ /01/23 (a3) 2005/05/ /10/30 306_639 (A) 84_2961 (D) 84_2961 (D) IF IF IF p. r. SF p. r. p. r. SF SF CPFZ CPFZ CPFZ Coherence (b1) (b2) (b3) Wrapped Phase cm (c1) (c2) Kilometers (d1) (d2) (cm) LOS Displacement Figure 2. The geocoded coherence image (a1-a3), wrapped phase interferogram (b1-b3) and surface displacement away from the satellite along the radar line of sight (LOS) (c1 & c2) of the interferometric pairs 1-3 listed in Tab. 1 are shown respectively. d1 & d2 are the LOS displacement obtained from precise leveling data normalized for the corresponding time span period. The area of low coherence (<0.1) is masked in b & c. Brown squares (d1 & d2) are the leveling points. CPFZ is Cerro Prieto Fault Zone. IF is Imperial Fault. SF is Saltillo Fault. Yellow rectangle is Cerro Prieto Geothermal Field. p and r is respectively CPGF production zone and recharge zone.

6 Figure 3. The geocoded coherence image (a), wrapped phase interferogram (b) and surface displacement away from satellite along the radar line of sight (LOS) of the interferometric pair 4 listed in the Tab. 1. The area of low coherence (<0.1) is masked in b & c. CPFZ is Cerro Prieto Fault Zone. IF is Imperial Fault. SF is Saltillo Fault. Earthquake mechanism is from CMT Harvard.