Section 5: Earthquake Hazards in the City of Downey

to more than 300. The physical cause of such irregular recurrence remains unknown.” 2Damage from a great quake on the San Andreas would be widespread throughoutSouthern California.Earthquake Related HazardsGround shaking, landslides, liquefaction, and amplification are the specific hazardsassociated with earthquakes. The severity of these hazards depends on several factors,including soil and slope conditions, proximity to the fault, earthquake magnitude, and thetype of earthquake.Ground ShakingGround shaking is the motion felt on the earth's surface caused by seismic wavesgenerated by the earthquake. It is the primary cause of earthquake damage. The strengthof ground shaking depends on the magnitude of the earthquake, the type of fault, anddistance from the epicenter (where the earthquake originates). Buildings on poorlyconsolidated and thick soils will typically see more damage than buildings onconsolidated soils and bedrock.Earthquake-Induced LandslidesEarthquake-induced landslides are secondary earthquake hazards that occur from groundshaking. They can destroy the roads, buildings, utilities, and other critical facilitiesnecessary to respond and recover from an earthquake. Many communities in SouthernCalifornia have a high likelihood of encountering such risks, especially in areas withsteep slopes.LiquefactionLiquefaction occurs when ground shaking causes wet granular soils to change from asolid state to a liquid state. This results in the loss of soil strength and the soil's ability tosupport weight. Buildings and their occupants are at risk when the ground can no longersupport these buildings and structures. Many communities in Southern California arebuilt on ancient river bottoms and have sandy soil. In some cases this ground may besubject to liquefaction, depending on the depth of the water table.AmplificationSoils and soft sedimentary rocks near the earth's surface can modify ground shakingcaused by earthquakes. One of these modifications is amplification. Amplificationincreases the magnitude of the seismic waves generated by the earthquake. The amountof amplification is influenced by the thickness of geologic materials and their physicalproperties. Buildings and structures built on soft and unconsolidated soils can facegreater risk. 3 Amplification can also occur in areas with deep sediment filled basins andon ridge tops.Map 5-1: Seismic Zones in CaliforniaEarthquake - 6