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Notice Concerning Copyright Restrictions NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Geothermal Resources Council TRANSACTIONS, Vol. 19, October 1995 SEISMICITY RECORDED IN THE CERRO PRIETO GEOTHERMAL FIELD DURING 1994-1995 Hubert Fabriol’, Luis Munguia Orozco’, Hector Lira2 and Gilbert0 Guillen2 ‘CICESE, Divisidn Ciencias de la Tierra, Aptdo. Postal 2732,Ensenada 22830 B.C., MOxico *Cornision Federal de Electricidad, Residencia General de Cerro Prieto, Residencia de Esudios, B.C, MOxico ABSTRACT world. The field extends over a 12 km2 area, with more than Seismicity was monitored from August 1994 to April 1995 210 wells drilled among which about 110 are producing in the Cerro Prieto Geothermal Field (Mexico), with 4 analog (Gutierrez and Rodriguez, 1994). Reinjection of waste brine seismographs. That allow the initiation of a catalog of the started in 1989 to avoid discharge of residual brines into seismicity occurring within the production zone. Although surface water bodies and streams. At present (1995), more seismic activity is scattered in time, up to 118 earthquakes than 42% of the extracted fluid is reinjected. were located during this period of time. Magnitudes range The geothermal field is situated on the sediments which from -1 to 4,O and five of these earthquakes had magnitudes fill the Colorado River delta. The liquid dominated reservoir is Mdr3. Earthquake locations showed a significant change in located between 1500 and 4000m in sandstones, sandy pattern from November to December 1994. It is proposed that shales and shales (Lippmann and MaMn, 1987). The heat the October-November 1994 seismicity, located to the source is attributed to a spreading center where the crust is southeast of the field and with depths up to 12-14 km, is pulled apart between Cerro Prieto fault and Imperial fault and . related to the spreading center located between Cerro Prieto igneous rocks are being emplaced. The Cerro Prieto pull and Imperial faults. Seismicity occurring after and until April apart basin is part of the Salton Trough-Gulf of California 1995 is shallower and situated close to the Droduction area. system of spreading centers and transform faults which link Up to now, it is too early to state whether the latter is “normal” the East Pacific Rise to the San Andreas fault system. and linked to the spreading center and strike-slip faults or Seismological studies were carried out in the area since exploitation-related. A longer monitoring would be necessary 1969, first aimed to investigate the deep structure of Mexicali to discriminate both mechanisms. Valley (Lomnitz et al., 1970), then focused on the geothermal area as an exploration tool (Reyes et at., 1979, Albores et al., INTRODUCTION 1980. Majer and McEvilly, 1981, 1982). The high level of seismic activity allowed to delimit the spreading center and Seismicity and geothermics are tightly linked. First, the extremities of Cerro Prieto and Imperial faults. Moreover, because areas of high heat flow are tectonically active, which Majer and McEvilly (1982) reported an increase of the number means high level of seismic activity, second because several of earthquakes of magnitude M~rlin the geothermal examples exist of seismicity generated by geothermal fluid production area between 1978 and 1981. They interpreted it exploitation (e.g. The Geysers, see Romero et al., 1994). A as a consequence of production increase. continuous monitoring of seismicity was undertaken beginning in August 1994 at Cerro Prieto geothermal field (Baja Continuous, local recordings indicated small seismic California, Mexico) with a small array of analog seismographs activity during 1983-1985, and increases of seismic activity in order to study the temporal and spatial distribution of local during 1987 and 1988, which concentrated in swarms (Reyes earthquakes. This is the first attempt to continuously monitor et at. 1989). The regional seismic network (RESNON) has the Cerro Prieto geothermal field seismicity since 1987. A recorded seismicity since 1977. However recording is not telemetered digital network is planned to be setup in October continuous, and earthquake and magnitudes are available 1995 and will replace the analog recorders. In this paper we only since 1987. An increase in the number of earthquakes present the data analysis of 8 months of recording, from can be observed in Cerro Prieto area in 1978, 1980, 1987 and August 1994 to April 1995. Histograms of occurrence of 1992-1993. More recently, Glowacka and Nava, (1993), earthquakes and maps of epicenters are interpreted assuming studied the correlation between seismicity and fluid extraction, that earthquakes in this particular area are very common and using data from USGS-Caltech catalogs for years 1973-1991. that the recording array does not allow location errors greater According to these authors, probabilistic analysis can not than 2 km. reject the hypothesis that strong earthquakes could be triggered by a production increase. The Cerro Prieto Geothermal Field is located in the Mexicali Valley, at the head of the Gulf of California, between From 1987 until now, no systematic monitoring of the the southeast end of the Imperial Faults and the northern geothermal area was carried out, except for a three month extremity of the Cerro Prieto Fault (Figure 1). Exploitation study with seven digital seismological stations from January started in 1973 and, at present, the nominal production is 620 to April 1993 (Fabriol and Rebollar, 1993), focused on MWe, which corresponds to the second largest field in the monitoring the effect of fluid reinjection. More than 250 387 Fabriol and Mungula microearthquakes were recorded in and around the in the right part of the curve, due to the length of recording production area and the duration magnitudes ranged from period respect to the Occurrence of earthquakes of magnitude Md=-1 to Md=2.6. The period of recording was definitely too Mdr2.5. Although in volcanic or geothermal areas 6 is short to point out clear correlations between fluid injection and supposed to be larger or equal to 1, a 6-value of 0.82 is usual seismic events rate. in areas of important tectonic activity and the regression line agrees with the occurrence of earthquakes of magnitude On August 20, 1994, four analog seismographs were Md=4, at least. setup to start the continuous monitoring of the geothermal area, 9 days after a magnitude M~=4.6earthquake occurred. SPATIAL DISTRIBUTION OF EARTHQUAKES This event was located beneath the south part of the As said before, 120 earthquakes were located using the Evaporation Pond (Laguna de Evaporacion), at about 5 km HYPO71 (Lee and Lahr, 1975) algorithm. Many layered depth. From August 1994 to May 1995, five more earthquakes models are available to simulate P-wave velocity in this part of magnitude MD23 were recorded at distances shorter than of Mexicali Valley (see Gonzales, 1986 for a review). We 5 km from the power plants and depths shallower than 5km. chose the model used by the Strong Motion Network of the The local array is made up of four vertical seismometers Northwest of Mexico (Table 1) and a value of VdVs=l.73 for connected to an analog drum seismograph, distributed around hypocenter determination. As a matter of fact, some of the the geothermal area (Figure 2). The array is complemented by Md23 earthquakes shown here are located using the the permanent digital station CPX of RESNON (three accelerometers arrival times of stations GEO and DEL component), located on the Cerro Prieto Volcano, the analog (Munguia, 1995). seismograph of QKP and, as for the largest earthquakes, the Velocity (km/s) Depth (km) strong motion recorders of the Strong Motion Network of the 1.70 0.00 Northwest of Mexico. Analog recorders are certainly less 2.00 0.10. sensitive and accurate than digital ones : precision in arrival 2.30 0.73 time readings is not better than 0.05 s. Nevertheless, they 2.60 1.75 have the advantage to record everything, even if noise is 3.00 2.94 present as it happens very often in the sedimentary-filled 5.00 5.62 Valley of Mexicali. 6.00 10.00 7.80 20.00 TIME AND MAGNITUDE DISTRIBUTION OF SEISMICITY From August 20, 1994 to May 10, 1995, more than 225 Table 1 P-wave velocity model used for hypocenter earthquakes and micro earthquakes were recorded in the determinations (based on a study by McMechan and Mooney, geothermal area, among which more than 50% were located. 1980) Figure 3 shows the histogram of occurrence of events. The The mean errors in hypocenter determination (Le. the seismicity level is relatively low and the mean rate of events average of the errors of the 118 located earthquakes) are as is less than one percent per day. Rate of events reported follows: either by Majer and McEvilly in 1981, or by Fabriol and -rms residual: 0.24 f 0.16 s, Rebollar in 1993, are much more larger : 7 events of ML~1 -horizontal error: 2.0 f 1.9 km, per day, and 3-4 events of MdrO per day, respectively.
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