Soft Story Retrofit: Earthquakes in Los Angeles – Part One

Earthquake hazard is high in Southern California (specifically in the Los Angeles region) due to proximity to the great San Andreas Fault and local geology of the Los Angeles basin. In order to improve buildings safety in case of a large earthquake, the City of Los Angeles has mandated a Seismic Soft-Story Program for buildings with tuck-under-parking. Adding structural lateral-force resisting elements to soft-story buildings will greatly increase their performance in terms of earthquake safety. To understand the extent of seismic risk, we present factors that maximize the earthquake hazard and have made Los Angeles one of the most damage-prone regions in the United States.

Plate Tectonics and Earthquakes

Earthquakes mostly happen at the vicinity of plate boundaries, where the largest earthquakes are expected in the location of subduction zones. A subduction zone is a region of the Earth’s crust where tectonic plates meet. Tectonic plates are massive pieces of the Earth’s crust that interact with each other. The places where these plates meet are called plate boundaries.

The San Andreas Fault extends roughly 800 miles through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate. The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the southern segment, which passes within about 35 miles of Los Angeles (Figure 1).

Magnitude of Earthquake and Potential for Damage

Soft soil deposits can amplify the ground motion. Los Angeles is a basin filled with soft deposits of soil sitting on the hard bedrock. Such effect resembles when a home is sitting on top of jelly and rocks more intensely when bottom of jelly is shaken. Earthquake magnitude is a number that describes the level of energy released when an earthquake triggers. The energy released on the fault plane travels in all directions and goes through changes as they travel through the earth’s crust. These changes can decrease or even increase the intensity of ground motion.

In the United States, United States Geological Survey is in the charge of performing research for building codes for earthquake safety and earthquake hazard reduction. Every year, highly qualified scientists are handpicked by USGS to perform state-of-art research to understand and anticipate the effects of earthquake by studying underlying faults and earth structure, and provide recommendations to other agencies to use most recent USGS findings and update building codes.

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