4.7 GEOLOGY AND SOILS This section describes the geology of the Planning Area and analyzes issues such as potential exposure of people and property to geologic and soil hazards. In addition, potential seismic hazards such as ground shaking and ground failure from earthquakes, as well as differential settlement on bay fill, are discussed. 4.7.1. EXISTING SETTING TOPOGRAPHY AND LOCAL GEOLOGY The City of San Mateo encompasses a variety of upland, hillside, valley, and alluvial fan land forms. The city is situated along the northeasterly flank of the central Santa Cruz Mountains but is separated from the range both geologically and topographically by the San Andreas fault and its associated rift valley. The bedrock types that underlie the City are different from most of those found to the southwest across the San Andreas fault. GEOTECHNICAL CONDITIONS Bedrock types in San Mateo comprise the sedimentary-volcanic-metamorphic rocks of the Fransican Formation and the conglomerate, sandstone and minor mudstone of the Santa Clara formation. The rocks exist in masses having the form of blocks, slices, and wedges, which are, for the most part, separated by long inactive faults. The broad, rolling upland area which characterizes much of the western hills of San Mateo is part of an extensive uplifted surface which is scarred by the drainage canyons of Laurel and San Mateo Creeks. To the northeast, soil conditions are that of bay mud and imported fill material. These soil conditions roughly extend from San Francisco Bay to areas near the Bayshore Freeway (US 101), representing the approximate location of the shoreline in the San Mateo area before the Bay was filled. GEOLOGIC HAZARDS, FAULTS, AND SEISMICITY The severity of an earthquake can be expressed in terms of both intensity and magnitude. For detailed descriptions of these terms, please see Table 4.7-1. TABLE 4.7-1 MAGNITUDE AND INTENSITY Magnitude Intensity Description 1.0 – 3.0 I Not felt except by a very few under especially favorable conditions. Felt only by a few persons at rest, especially on upper floors of buildings. 3.0 – 3.9 II – III Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking 4.0 – 4.9 IV – V building. Standing motor cars rocked noticeably. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. 5.0 – 5.9 VI – VII Damage negligible in buildings of good design and construction; slight to moderate in well- built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. City of San Mateo General Plan Update July 2009 Draft Environmental Impact Report 4.7-1 4.7 GEOLOGY AND SOILS Magnitude Intensity Description Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. 6.0 – 6.9 VIII – IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Some well-built wooden structures destroyed; most masonry and frame structures destroyed 7.0 and VIII or with foundations. Rails bent. higher higher Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. Damage total. Lines of sight and level are distorted. Objects thrown into the air. The San Francisco Bay Area is recognized by geologists and seismologists as one of the most seismically active regions in the United States. The significant earthquakes that occur in the Bay Area are generally associated with crustal movement along well-defined, active, fault zones of the San Andreas fault system, which regionally trend in a northwesterly direction. The San Andreas fault, which generated the great San Francisco earthquake of 1906, is located approximately 2 miles to the west of the City of San Mateo. The Hayward fault is located approximately 14 miles northeast of the project area. There are no known active faults or Alquist-Priolo earthquake hazard zones in the City of San Mateo. The inactive faults which are present are older features which do not exhibit indications of recent motion. There is no reason to expect a recurrence of movement along these fault traces. Many of the geologic processes active in San Mateo are related to earthquake-induced ground shaking, common to the area. The entire San Francisco Bay Area is occasionally subjected to strong earthquakes that originate on the San Andreas, Hayward, and Calaveras faults. Ground Shaking In populated areas, the greatest potential for loss of life and property damage is a result of ground shaking from a nearby earthquake. The degree of damage depends on many interrelated factors. Among these factors are the Richter magnitude, focal depth, distance from the causative fault, source mechanism, duration of shaking, high rock accelerations, type of surficial deposits or bedrock, degree of consolidation of surficial deposits, presence of high groundwater, topography, and design, type, and quality of building construction. The City of San Mateo could be affected by strong ground shaking caused by a major earthquake during the next 30 years. This hazard is common to all development in the San Francisco Bay Area. Ground shaking may affect areas hundreds of miles distant from the earthquake’s epicenter. Historic earthquakes have caused strong ground shaking and damage in the San Francisco Bay Area, the most recent being the 6.9 (moment magnitude) Loma Prieta earthquake in October 1989. The epicenter for this event was approximately 40 miles southeast of the project area; the earthquake caused strong ground shaking for about 20 seconds and resulted in varying degrees of structural damage throughout the Bay Area. The areas that experienced the worst structural damage due to the Loma Prieta earthquake were not those closest to the fault, but rather those with soils that magnified the effects of ground shaking. Areas that are underlain by bedrock tend to experience less ground shaking than those underlain by unconsolidated sediments such as artificial fill. Alluvial-type soils have a moderate to high potential of amplifying ground shaking during an earthquake. Damage to buildings and utilities General Plan Update City of San Mateo Draft Environmental Impact Report July 2009 4.7-2 4.7 GEOLOGY AND SOILS is likely to be greatest on those sites underlain by deep, loose, compressible deposits such as bay mud. These areas include the residential neighborhoods east of US 101 and the Mariner’s Island Area. No active or potentially active faults underlie the City of San Mateo based on published geologic maps. The Planning Area is not located within an Alquist-Priolo Fault Study Zone and surface evidence of faulting has not been observed. However, due to the proximity to the San Andreas Fault Zone, Hayward Fault Zone, and other active faults, the Planning Area may experience severe ground shaking during a seismic event. Six dams affect the City of San Mateo: Crystal Springs, San Andreas, Laurel Creek and East Laurel Creek (2), and Tobin Creek in Hillsborough. Lower Crystal Springs Dam retains water supply for San Francisco and most cities within San Mateo County. In 1977, the seismic safety of the dam was studied, finding that the risk of structural damage to the dam with a maximum magnitude of 8.3 on the Richter scale earthquake is low and that landslides which might be triggered by such an earthquake would not generate waves capable of overtopping the dam. Although the probability of Lower Crystal Spring Dam’s failure is remote, should such an event occur, the San Mateo Area Office of Emergency Services (OES) estimates that a population of 70,000 would be affected, with inundation occurring from the downtown area north to the Burlingame Recreation Lagoon and south to the Ralston Avenue/US 101 interchange. San Andreas Dam is located on San Andreas Creek in Burlingame and is also used to impound water for San Francisco and much of San Mateo County. Seismic safety studies in 1979 and 1983 indicated that the dam would probably remain stable during strong seismic shaking. Laurel Creek Dam is located at the end of Laurelwood Drive and reduces the peak stormwater runoff of 600 cubic feet per second (cfs) in half. The most recent reports by the California Division of Safety of Dams (DSOD) indicate that the dam is structurally safe and will perform without failure during a major seismic event. East Laurel Creek Dam is located at the end of East Laurel Creek Drive and is also used to control peak storm runoff. The dam is too small to be regulated by DSOD, and its seismic stability is unknown. Two other small dams are located in Belmont (East Laurel Creek) and in Hillsborough (Tobin Creek). Ground Failure Ground failure is a secondary effect of ground shaking and can include landslides, liquefaction, lurching, and differential settlement. Buildings can tilt or sink, utility lines can rise to the surface, and levees can fail. If soils are poorly consolidated, the ground can subside. Landslides Problems of slope instability are most prevalent in the hillside areas where landsliding has occurred previously and where landslide deposits can be found. Landslide failures have also occurred in areas where slopes were modified by grading.