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The San Simeon Earthquake was a magnitude 6.5 earthquake on the Central Coast of California, about 7 miles northeast of San Simeon. It occurred at 11:15 AM on December 22, 2003. The earthquake probably occurred on the Oceanic fault zone in the Santa Lucia Mountains. It was caused by reverse faulting and propagated southeast from the hypocenter for 12 miles (19 km).

The area around the epicenter is sparsely populated and the most severe damage occurred in Paso Robles, 24 miles (39 km) east-northeast, where the earthquake's only fatalities occurred. The Acorn Building, an unreinforced masonry building built in 1892, completely collapsed, killing two people. Other unreinforced masonry buildings, some more than a century old, in the city's historic downtown area also had extensive damage. However, none of the buildings that had even partial retrofitting collapsed. There is currently a wrongful death lawsuit against the owners of the Acorn Building.

Two sulfur hot springs in Paso Robles erupted after the earthquake. One was underneath the parking lot of City Hall. Hot water and sediment were released at a rate of about 1,300 gallons per minute (4,900 liters per minute), forming a large sinkhole. There was formerly a bath house at the location and the spring was capped after it closed down. Another hot spring flowed out of the embankment at the Paso Robles Road exit on U.S. Highway 101.

Outside of Paso Robles the damage was less severe, with unreinforced masonry buildings taking minor to moderate damage. Brick veneers were also disproportionately affected. In addition, water tanks in Paso Robles, Templeton and Los Osos were damaged. Residential buildings, predominantly one to two story wood frame structures, weathered the quake with little or no damage. The damage that did occur was mostly limited to chimneys, although a house in Atascadero suffered severe damage when it moved off of its foundation. The damage was probably caused by poor construction. There were fewer nails connecting the plywood siding to the sill than is required and many of them did not actually hit the sill. Some wineries, especially those near the epicenter along California Highway 46, reported damage such as barrels toppling and bursting.

• California Integrated Seismic Network
• Earthquake Engineering Research Institute (PDF)
• United States Geological Survey

• Cal Poly
• University of California, Berkeley