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HARBOR BLVD. MIXED USE TRANSIT CORRIDOR PLAN FINAL FIR <br />CITY OF SANTA ANA <br />5. Environmental Analysis <br />GEOLOGY AND SOILS <br />feet below sea level near the southern end (OCWD 2011). Therefore, depth to groundwater under the project <br />area is estimated at approximately 90 feet near the northern end and approximately 70 feet near the southern <br />end. <br />Geologic Hazards <br />Seismic Hazards: Faults and Ground Shaking <br />A fault is a break or fracture between moving blocks of rock. Faults showing evidence of surface <br />displacement within about the last 11,000 years are considered active. <br />MeasuEng Earthquakes <br />Energy Released. The energy released by an earthquake is measured as moment magnitude (Mw). The <br />moment magnitude scale is logarithmic; therefore, each one -point increase in magnitude represents a tenfold <br />increase in amplitude of the waves and a 32 -fold increase in energy. That is, a magnitude 7 earthquake <br />produces 100 times (10 x 10) the ground motion amplitude of a magnitude 5 earthquake. <br />Ground Motion. Motion at the ground surface during an earthquake is measured as horizontal ground <br />acceleration in g, where g is the acceleration of gravity. <br />Effects on Buildings and Peoples' Sensations. The Modified Mercalli Intensity (MMI) Scale is a <br />qualitative scale of how earthquakes are felt by people and how they affect buildings. The MMI is a 12 -point <br />scale ranging from Intensity I, which is rarely felt by people, to Intensity XII, in which damage to structures is <br />total and objects are thrown into the air (USGS 2012). In California, the estimated relationship between peak <br />ground acceleration and MMI intensity is shown in Table 5.41. <br />Table 5.4 -1 Estimated Relationship between Peak Ground Acceleration and Intensitv <br />Peak Ground Acceleration, g <br />MMI <br />Effects <br />0.039 -0.092 <br />V <br />Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects <br />overturned. Pendulum clacks may stop. <br />0.092 -0.18 <br />VI <br />Felt by all, many frightened. Some heavy furniture moved, a few instances of fallen plaster. <br />Damage slight. <br />Damage negligible in buildings of good design and construction, slight to moderate in well - <br />0.18-0.34 <br />VII <br />built ordinary structures; considerable damage in poorly built or badly designed structures, <br />some chimneys broken. <br />Damage slight in specially designed structures; considerable damage in ordinary <br />0.34 -0.65 <br />VIII <br />substantial buildings with partial collapse. Damage great in poorly built structures. Fall of <br />chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. <br />Damage considerable in specially designed structures; well -designed frame structures <br />0.65 -1.24 <br />IX <br />thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings <br />shifted off foundations. <br />MMI X: Some well -built wooden structures destroyed, most masonry and frame structures <br />>1.24 <br />X+ <br />destroyed with foundations. Rails bent. <br />MMI XII: Damage total. Lines of sight and level are distorted. Objects thrown into the air. <br />Source: Wald 1999. Notes: a = acceleration of aravity. <br />October 2014 Page 5.4 -3 <br />