An earthquake is a sudden motion or trembling caused by a release of strain accumulated within or along the edge of the earth’s tectonic plates. The effects of an earthquake can be felt far beyond the site of its occurrence. Earthquakes usually occur without warning and, after just a few seconds, can cause massive damage and extensive casualties. The most common effect of earthquakes is ground motion, or the vibration or shaking of the ground during an earthquake.


Earthquake Magnitude An earthquakes magnitude is measured using the Moment Magnitude scale. The moment magnitude scale (abbreviated as MMS; denoted as MW or M) is used by seismologists to measure the size of earthquakes in terms of the energy released. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale.
Tectonic Plates and Fault Lines

The San Andreas Fault system is the most active fault system in California. In its entirety, it runs 800 miles down the California coastline, including 30 miles in the southeastern portion of Monterey County. To the north and south of the County, the fault appears to be currently locked with no detectable movement. Between these locked sections, within the County, the San Andreas Fault creeps (slips aseismically). From San Juan Bautista to Parkfield, the creeping section produces numerous small to moderate (mostly magnitude 6.0 and smaller) earthquakes but no large ones. The stretch of the fault between Parkfield and Gold Hill defines a transition zone between the creeping and locked behavior of the fault. Two other active faults are located in Monterey County: the Palo Colorado–San Gregorio Fault zone and the Monterey Bay–Tularcitos Fault zone.
The Palo Colorado–San Gregorio Fault zone connects the Palo Colorado Fault near Point Sur south of Monterey with the San Gregorio Fault near Point Año Nuevo in Santa Cruz County. It is a right-lateral strike-slip fault zone oriented generally north-south consisting of two or more parallel and fairly continuous fault segments that extend at least 60 miles.
The Monterey Bay–Tularcitos Fault zone lies seaward of the city of Seaside, extending northwesterly to the Pacific Ocean. It is composed of short, discontinuous parallel fault segments ranging from 3 to 9 miles in length. The Monterey Bay Fault–Tularcitos zone is either truncated or merges with the San Gregorio fault segment of the Palo Colorado–San Gregorio Fault zone.
Secondary Hazards
In, addition to ground motion, several secondary natural hazards can occur from earthquakes, such as the following:
Surface Faulting is the differential movement of two sides of a fault at the earth’s surface. Displacement along faults, both in terms of length and width, varies but can be significant (e.g., up to 20 feet), as can the length of the surface rupture (e.g., up to 200 miles). Surface faulting can cause severe damage to linear structures, including railways, highways, pipelines, and tunnels.
Liquefaction occurs when seismic waves pass through saturated granular soil, distorting its granular structure, and causing some of the empty spaces between granules to collapse. Pore water pressure may also increase sufficiently to cause the soil to behave like a fluid for a brief period and cause deformations. Liquefaction causes lateral spreads (horizontal movements of commonly 10 to 15 feet, but up to 100 feet), flow failures (massive flows of soil, typically hundreds of feet, but up to 12 miles), and loss of bearing strength (soil deformations causing structures to settle or tip). Liquefaction can cause severe damage to property.
Landslides/Debris Flows occur as a result of horizontal seismic inertia forces induced in the slopes by the ground shaking. The most common earthquake-induced landslides include shallow, disrupted landslides such as rock falls, rockslides, and soil slides. Debris flows are created when surface soil on steep slopes becomes totally saturated with water. Once the soil liquefies, it loses the ability to hold together and can flow downhill at very high speeds, taking vegetation and/or structures with it. Slide risks increase after an earthquake during a wet winter.
Tsunamis: As an Oceanic Plate is subducted beneath a Continental Plate, it sometimes brings down the lip of the Continental Plate with it. Eventually, too much stress is put on the lip and it snaps back, sending shockwaves through the earth’s crust, causing a tremor under the sea, known as an Undersea Earthquake. Factors that affect tsunami generation from an earthquake event include magnitude (generally, a 7.5 magnitude and above), depth of event (a shallow marine event that displaces seafloor), and type of earthquake (thrust as opposed to strike-slip).


Historically, most of the earthquakes that have occurred in Monterey County have originated from movement along the San Andreas Fault system, which runs through the southeastern portion of the county for approximately 30 miles.
  1. The San Andreas Fault is the source of the area’s earliest recorded great earthquake event, which occurred in June 1838. It is believed that this earthquake was a magnitude 7.0 to 7.4.
  2. Monterey County’s next large earthquake occurred almost 20 years later on January 9, 1857. This estimated 8.3 earthquake, dubbed the Fort Tejon earthquake, occurred on the southern segment of the San Andreas Fault, northwest of the unincorporated community of Parkfield.
  3. The next large earthquake, known as the Great San Francisco earthquake, occurred on April 18, 1906. This event lasted 45 to 60 seconds and was in the range of magnitude 7.7–7.9. In Monterey, Hotel Del Monte was nearly destroyed, and four or five people were killed.
  4. Available data suggest that between five to ten small earthquakes have been felt each year in Monterey County and one moderate earthquake has been felt along the San Andreas Fault near Parkfield every 22 years (1857, 1881, 1901, 1922, 1934, 1966, and 2004) over the past 150 years. However, the next large earthquake did not occur for over 80 years, from 1906 until 1989.
  5. On October 17, 1989, the Loma Prieta earthquake occurred near Mt. Loma Prieta in neighboring Santa Cruz County. The earthquake lasted only 10 to 15 seconds, but had a magnitude 6.9 to 7.1. In Moss Landing, liquefaction destroyed the marine laboratory and seriously damaged a power plant.