Section 4.12 Geology and Soils This section assesses the potential impacts on and from geology and soils that could arise from disturbances and impacts resulting from future development consistent with the General Plan as updated by GPA No. 960, the proposed project. 4.12.1 Existing Environmental Setting – Geology and Soils While the County of Riverside is at risk from many natural and man-made hazards, the event with the greatest potential for loss of life or property and economic damage is an earthquake. This is true for most of Southern California, since damaging earthquakes are frequent, affect widespread areas, trigger many secondary effects and can overwhelm the ability of local jurisdictions to respond. In Riverside County, earthquake-triggered geologic effects that may occur include groundshaking, fault rupture, landslides, liquefaction, subsidence and seiche, all of which are discussed below. Earthquakes can also cause human-made hazards, such as urban fires, dam failure and toxic chemical releases. Earthquakes are caused by movement of rock along a break called a fault. The movement releases pent up strain energy in the form of waves which travel outward in all directions. These seismic waves cause the earth to vibrate and this shaking is what we feel in an earthquake. Most earthquakes occur along plate boundaries. The outer portion of the Earth consists of enormous chunks of rock called plates, which slowly collide, separate and grind past each other. Frictional forces resist plate movement and the plate edges lock together. Much strain energy builds up as the plates keep trying to move. Eventually, frictional forces are exceeded, the locked edges move and all the stored strain energy is released in seismic waves. Earthquake risk is very high in the heavily populated western portion of Riverside County due to the presence of three of California’s most active faults: the San Andreas, the San Jacinto and the Elsinore. Risk is moderate in the eastern portion of the county which includes the Coachella Valley and Blythe. In California, recent earthquakes in or near urban environments have caused relatively few casualties. This is due more to luck than design. For example, when a portion of the Nimitz Freeway in Oakland collapsed at rush hour during the 1989 moment magnitude (Mw) 7.1 Loma Prieta earthquake, it was unusually empty because many were watching the World Series. Nonetheless, California’s urban earthquakes have resulted in significant economic losses. Riverside County is at risk from larger, more damaging earthquakes than the moderate sized, Mw 6.7 Northridge earthquake, which in 1994 caused 54 deaths and $20 to $30 billion in damage. County of Riverside Environmental Impact Report No. 521 Public Review Draft § February 2015 4.12-1 A. Baseline Data Sources The existing setting discussion herein is summarized from Section 5.2 of the 1999 Existing Setting Report prepared for the 2003 RCIP Riverside County General Plan and its Appendix H, “Natural Hazard Mapping, Analysis and Mitigation: A Technical Background Report in Support of the Safety Element of the New Riverside County 2000 General Plan” (“Appendix H” herein). Pursuant to CEQA, the description of the physical environmental conditions provided in this EIR is as they exist at the time the issuance of the Notice of Preparation (NOP), that is, April 13, 2009. This environmental setting constitutes the baseline physical conditions by which the County, as Lead Agency under CEQA, determines whether an impact is significant. However, for geology, soils and seismicity, the 1999 Existing Setting Report and Appendix H remain relevant to existing conditions within the county because geologic conditions change very slowly with time and no major earthquakes have occurred within the study area since the reports were prepared. For this reason, these documents were found to adequately represent the county baseline existing geological and seismic conditions. The various seismic, soils and geology information presented graphically in this section are from the Riverside County GIS Department, generally the Riverside County Land Information System (RCLIS) database, as updated by various means, including through information provided by the State of California (see discussion under Section 4.12.4) and by direction of the Riverside County Geologist in relation to geologic and seismic studies prepared for proposed development sites within the county and submitted to the Riverside County Geologist. Because they are county specific, these data sources were determined to be the best-supported substantial evidence available and were used herein. The sources for the various land use and environmental data sets used in this section are described in their respective sections. B. Fault Hazard Zones Primary ground damage due to earthquake fault rupture typically results in a relatively small percentage of the total damage in an earthquake, but being too close to a rupturing fault can cause profound damage. It is difficult to reduce this hazard through structural design. The primary mitigating technique is to set back from and avoid active faults. The challenge comes in identifying all active faults that could potentially rupture. Faults throughout Southern California have formed over millions of years. Some of these faults are generally considered inactive in terms of present geologic conditions. Other faults are known to be active, meaning either they have generated earthquakes in historical times (the last 200 years) or show geologic and geomorphic indications of relatively recent movement. Faults that have moved in the relatively recent geological past are generally presumed to be the most likely candidates to generate damaging earthquakes in the lifetimes of residents, buildings and communities. Earthquakes in Southern California occur as a result of movement between the Pacific and North American plates. Faults of the San Andreas system are used to mark the boundary between these plates, but the deformation, faulting and associated earthquakes occur in a broadly distributed zone that stretches from offshore to Nevada. Thus, the San Andreas is one of a system of plate-bounding faults. Most of the movement between the plates occurs along the San Andreas Fault, which bisects Riverside County. The rest of the motion is distributed among northwest-trending, strike-slip faults of the San Andreas system (principally the San Jacinto, Elsinore, Newport-Inglewood and Palos Verdes faults), several east-trending thrust faults that bound the Transverse Ranges and the Eastern Mojave Shear Zone (a series of faults east of the San Andreas, responsible for the 1992 Landers and the 1999 Hector Mine earthquakes). Pursuant to state law (see Section 4.12.2), Alquist-Priolo (A-P) Earthquake Fault Zones have been designated by the California Geologic Survey for the Elsinore, San Jacinto and San Andreas fault zones in Riverside County (see Figure 4.12.1 (Alquist-Priolo Fault Zones)). Additionally, the County of Riverside has developed and applied County of Riverside Environmental Impact Report No. 521 4.12-2 Public Review Draft § February 2015 special studies zone criteria for the Agua Caliente fault zone between the Elsinore and the San Jacinto faults in southwestern Riverside County. All of these faults have high rates of displacement and are rapidly accumulating strain energy which will be released in earthquakes. Inevitably, the A-P Zone will expand with time. As faults are studied, more splays are discovered. C. Groundshaking For design and environmental analysis purposes, a worst-case scenario earthquake (the maximum credible earthquake [MCE]) for Riverside County is a magnitude 7.9, based on the rupture of the entire southern segment of the San Andreas Fault from the Cajon Pass to the Salton Sea. While other scenarios would expose portions of Riverside County to intense groundshaking that is locally as severe as the MCE, the MCE exposes most of the county to very high-intensity groundshaking. Groundshaking is simply the movement of the earth resulting from an earthquake. Shaking can cause lateral movement and is the primary reason for collapse of buildings. The strength of seismic groundshaking at any given site is a function of many factors. Factors of primary importance in groundshaking severity include the size of the earthquake, its distance, the paths the seismic waves take as they travel through the earth, the type of rock or soils underlying the site and topography (particularly whether a site sits in a valley or atop a hill). The amount of resulting damage also depends on the size, shape, age and engineering characteristics of affected structures. Interactions between ground motion and man-made structures are complex. Governing factors include a structure’s height, construction and stiffness; a soil’s strength and resonant period; and the period of high-amplitude seismic waves. Waves come in different lengths and thus repeat their motions with varying frequency. Long waves are called long-period or low-frequency. Short waves are short-period or high-frequency. In general, long-period seismic waves, which are characteristic of large earthquakes, are most likely to damage long-period structures such as high-rise buildings and bridges. Shorter period seismic waves, which tend to die out quickly, will most often cause damage near the epicenter of the earthquake, damaging structures such as one-story and two-story buildings. Very short period waves are most likely to cause nonstructural damage, such as to equipment. In different situations, ground displacement, velocity and acceleration can all cause damage. Estimates of several key groundshaking parameters near the fault rupture zone for the Riverside MCE, expressed as a percentage of gravity, are presented in Table 4.12-A (Probable Earthquake Scenarios for Riverside County). Peak ground acceleration, which is the maximum acceleration achieved at a site, often turns out to be the earthquake effect that predicates the most damage to buildings. Wave periods of 0.3 second and 1.0 second are the lengths of seismic waves that commonly damage structures.