1.0 Introduction 1.1 Introduction The M7.2 mainshock of the Sierra El Mayor ­ Cucapah earthquake occurred at 3:40 pm local time on April 4, 2010, which was Easter Sunday. The mainshock was followed by a sequence of aftershocks that were felt during the reconnaissance effort and continue to the present. The mainshock had an estimated focal depth of 10 km (USGS, 2010) and struck in the El Mayor and Cucapah mountains to the southwest of Mexicali, Baja California, Mexico. The epicenter was located at 32.237°N, 115.083°W (USGS, 2010). The earthquake was felt throughout northern Baja California and southern California, and caused damage in a region from the Sea of Cortez in the south to the Salton Sea in the north. This earthquake is the largest to strike this area since 1892. The USGS seismic intensity was IX near the fault rupture, and VIII in Mexicali, which is the nearest densely­ populated area. The largest peak ground acceleration of 0.58g was recorded at McCabe School about 5 km southwest of El Centro, CA. The earthquake ruptured the surface for as much as 140 km from the northern tip of the Sea of Cortez northwestward to nearly the international border, with strike­slip, normal, and oblique displacements observed. The earthquake resulted in two fatalities and hundreds of injuries. Damage occurred to irrigation systems, water treatment facilities, buildings, bridges, earth dams, and roadways. Liquefaction and ground subsidence was widespread in the Mexicali Valley, and left wheat and hay fields submerged under water and destroyed many canals that irrigate fields in the predominantly agricultural region. The largest lasting effect of the earthquake is its impact on agricultural infrastructure (including levees and canals) in the Mexicali Valley and Imperial Valley. This damage is anticipated to cost hundreds of millions of dollars to repair (Personal Communication with Robert Anderson, California Seismic Safety Commission), and the earthquake has provided important lessons regarding water infrastructure in seismically active regions. The GEER reconnaissance effort occurred in multiple phases with many participants, and covered the region shown in Fig. 1­1. Effort among the GEER participants in summarized in Table 1­1. The GEER team coordinated their activities with those of the EERI team, which focused principally on structural damage and lifelines performance (Meneses et al. 2010). The GEER reconnaissance teams' main goal was to quantify the spatial extent and amplitude of surface fault rupture, ground failure, liquefaction, landslides, and geotechnical­related damage to bridges, piers, buildings, lifeline systems and critical structures. Each of the teams was equipped with digital cameras, maps, personal computers, and GPS units for recording track logs and site locations. Track logs and images were uploaded to the GEER ftp repository and were integrated into a single Google Earth KMZ file (Fig. 1­ 1). The epicenter, shaking intensity map, ground motion recording stations, and aftershocks were also superposed on the KMZ file for reference. The KMZ file was useful for identifying which areas had been well­covered and which areas had not, and was critical for guiding our reconnaissance efforts. Table 1­1. Time frame and focus of GEER reconnaissance teams. Figure 1­1. Screen shot of Google Earth kmz file showing area covered by the GEER reconnaissance effort. This reconnaissance report has been organized into chapters that describe the geological and seismological setting (Chapter 2), surface fault rupture (Chapter 3), recorded earthquake ground motions (Chapter 4), liquefaction (Chapter 5), and performance of canals, levees and earthen dams (Chapter 6). This earthquake reconnaissance effort was made possible by funding from the U.S. National Science Foundation, cost sharing by the U.S. Geological Survey, and California Geological Survey and volunteer work by the team participants. This material is based upon work supported by the National Science Foundation under Grant No. CMS­032914. Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsoring organizations. References USGS (2010). "Magnitude 7.2 ­ Baja California, Mexico, 2010 April 04 22:40:42 UTC." http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/ci14607652/. Meneses et al. (2010). "EERI Preliminary Reconnaissance Report on the 2010, M 7.2 El Mayor­Cucapah Earthquake." Earthquake Engineering Research Institute. 2.0 Geological Setting and Seismological Aspects 2.1 Geologic and Plate Tectonic Setting At the latitude of the El Mayor ­ Cucapah earthquake, the Pacific plate moves northwest with respect to the North America plate at about 4.8 cm per year. The principal plate boundary in northern Baja California consists of a series of northwest­trending strike­slip (transform) faults that are separated by pull­apart basins that accommodate northwest­southeast oriented extension. These faults, a subset of which broke in the El Mayor ­ Cucapah earthquake, are distinct from, but parallel to, the main strands of the San Andreas fault system such as the well­ known Cerro Prieto fault. Although some of the faults that broke in this earthquake had been previously mapped, such as the Pescadores and Borrego faults, several others had not (Fig. 2­1). Most notably, the major plate boundary transform fault section that lies southeast of this earthquake’s epicenter is buried by sediments of the Colorado River Delta and had not previously been recognized. This fault has been named, upon its discovery in the field by Javier Gonzalez of CICESE, the Indiviso fault; its existence is clearly a significant new discovery. Figure 2­1. Geological map of the earthquake source area and surroundings (Fletcher & Spelz, 2009) with El Mayor – Cucapah mainshock and aftershocks re­locations overlain by Egill Hauksson, Caltech. 2.1.1 Geologic Setting of the Earthquake The Salton Trough, a geologically young north­south trending sedimentary basin, is located east of the epicenter. The Salton Trough is a structural and topographic depression consisting of the San Andreas transform fault system along the eastern portion of the basin. The basin includes a prime agricultural area north of the U.S. border known as the Imperial Valley, and south of the border in Mexico known as the Mexicali Valley. The Imperial Valley includes the northwest trending San Andreas Fault Zone on the eastern side of the Salton Sea, and the San Jacinto Fault Zone on the western side of the Salton Sea. The northwest trending Imperial Fault is located in the central area of the basin and extends from south of the Salton Sea, crossing the border into the northern portion of the Mexicali Valley. Historically, natural and man­made water ways have traversed the agricultural lands in both of these valleys creating high groundwater levels. The Colorado River enters the basin at the southeastern area of the Imperial Valley and flows south along the east side by the Mexicali Valley to the Gulf of California. At the turn of the century water had been diverted from the Colorado River to irrigate the agricultural soils in the Imperial Valley. However, in 1905 the Colorado River broke through flowing westward across the Mexicali Valley eroding two watercourses, the New River in the west, and the Alamo River in the east. Based on mapping by Godfrey Sykes in 1915, the Salton Sea was filled in the Imperial Valley, and Volcano Lake was mapped in the north central Mexicali Valley (Fig. 2­2). Figure 2­2. Colorado Desert Map by Godfrey Sykes The earthquake epicenter was located in the farming communities on the western side of the Mexicali Valley, a broad flat extensional sedimentary basin. Geologic field observations of the earthquake damage included: surface fault rupture in the mountain range; several rockfalls in the mountain range; liquefaction features consisting of lateral spreading, sand boils/volcanoes and flooding in the valley. Surface fault rupture occurred in the uninhabited area west of the epicenter in the rugged Sierras Cucapah and El Mayor mountain ranges. The mountain ranges are mapped as Cretaceous granitic rocks including metamorphic rocks, part of what is known as the California Batholith. The surface rupture extended northwest from the epicenter area through the Sierra Cucapah mountain range towards the U.S.­Mexico border. The surface rupture broke the Pescadores and Borrego faults, as well as some previously un­named faults, and triggered minor slip on the southeastern part of the Laguna Salada fault system. Surface faulting is treated in more detail in Chapter 3. The Mexicali Valley includes the mapped northwest trending Cerro Prieto Fault in the western area, and the Imperial Fault in the central eastern area of the basin. The Cerro Prieto Spreading Center is mapped between the two faults. Both faults are right­lateral strike­slip faults with a northwest­southeast orientation. Faulting in this area has created a series of northeast­southwest striking secondary faults which are known to act as collectors and distributors of hydrothermal fluids generated by heat transfer along a granitic basement from a probable magmatic ascent zone. Geological and geophysical studies have documented a lithologic sequence in the basin with Cretaceous granitic basement rock at a depth of 4,000m, overlain by sandstones, shales, mudstones, then more sandstones and sands. The sands are then overlain by Quaternary poorly consolidated sediments made up of clays, silts, sands and gravels. The Quaternary sediments are known to range from 600m in the area of Cerro Prieto fault to 2,400m in depth near the Imperial Fault to the east. The Mexicali Valley has been developed with residential communities and farm lands across the wide extensional basin. The area is near sea level and groundwater levels appear to be typically very shallow. Lined and unlined farming irrigation canals traverse the area to provide water to the farms across the basin.