County of Monterey
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Earthquake


Overview

image of a blue volks wagon hatchback with a tire stuck on a road that cracked due to an earthquake

An earthquake is a sudden motion or trembling caused by an abrupt shift of rock along a fracture in the earth or a contact zone between tectonic plates. Most destructive quakes are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break, and snap to a new position. In the process of breaking, vibrations called “seismic waves” are generated. These waves travel outward from the source of the earthquake at varying speeds. Earthquakes usually occur without warning and, after just a few seconds, can cause massive structural damage, injury, and loss of life, as well as damage to infrastructure networks, such as water, power, gas, communication, and transportation. The effects of an earthquake can be felt far beyond the site of its occurrence. The degree of damage depends on many interrelated factors. Among these are: the magnitude, focal depth, distance from the causative fault, source mechanism, duration of shaking, high rock accelerations, type of surface deposits or bedrock, degree of consolidation of surface deposits, presence of high groundwater, topography, and the design, type, and quality of building construction.


Measuring Earthquakes

The most common method for measuring earthquakes is magnitude, which measures the strength of earthquakes. An earthquake’s magnitude is a measure of the energy released at the source of the earthquake and is related to the total area of the fault that ruptured, the amount of displacement across the fault, and the actual location of the energy release inside the earth. The Richter scale measures magnitude of earthquakes based on the amplitude of the largest energy wave released by the earthquake. Richter scale readings are suitable for smaller earthquakes; however, because it is a logarithmic scale, the scale does not clearly distinguish the magnitude of large earthquakes above a certain level. Therefore, while the Richter scale is the most well-known measurement for earthquake magnitude, the majority of scientists currently use the Moment Magnitude (Mw) Scale to measure magnitude.

There are seven earthquake magnitude classes, ranging from great to micro, which are summarized below:

  • Great: Magnitude > 8; Tremendous Damage
  • Major: 7 ≤ Magnitude < 7.9; Widespread Heavy Damage
  • Strong: 6 ≤ Magnitude < 6.9; Severe Damage
  • Moderate: 5 ≤ Magnitude < 5.9; Considerable Damage
  • Light: 4 ≤ Magnitude < 4.9; Moderate Damage
  • Minor: 3 ≤ Magnitude < 3.9; Rarely Causes Damage
  • Micro: Magnitude < 3; Minor Damage

Although an earthquake has only one magnitude, it can have many intensities. The effects of an earthquake in a particular location are measured by intensity. Intensity at a given site is a function of earthquake magnitude, increasing as magnitude increases; distance from the causative fault, decreasing as distance increases; and underlying site geology, generally increasing in areas with weak, unconsolidated materials. Earthquake intensity decreases with increasing distance from the epicenter of the earthquake.

Earthquake hazard assessment is also based on expected ground motion. The ground experiences acceleration as it shakes during an earthquake. The peak ground acceleration (PGA) is the largest acceleration recorded by a monitoring station during an earthquake. PGA is a measure of how hard the earth shakes in a given geographic area and is expressed as a percentage of the acceleration due to gravity.

History

Earthquakes tend to reoccur where they have struck before. The central California coast has a history of damaging earthquakes, primarily associated with the San Andreas fault, which runs through the southeastern portion of the County for about 30 miles. There have also been a number of 5.0 to 6.5 earthquakes on other faults which have affected large portions of the region. The County has experienced 429 earthquakes since 1900 and 67 earthquakes since 2000 of 4.0 magnitude or higher.


Earthquake Risk

Monterey County is located in a region of high seismicity with numerous local faults, summarized below. The primary seismic hazard for the County is potential ground shaking from these faults.

San Andreas Fault: The San Andreas Fault runs through the southeastern portion of the County for approximately 30 miles and poses the single greatest seismic hazard to the 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. The slip rate of the San Andreas fault is Greater than 5.0 mm/year.

Palo Colorado - San Gregorio Fault: 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 Ano 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 slip rate of the San Gregorio – Palo Colorado fault is between 1.0 and 5.0 mm/year.

Monterey Bay - Tularcitos Fault: 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. The slip rate of the Monterey Bay - Tularcitos fault is between 0.2 and 1.0 mm/year.

California experiences hundreds of earthquakes each year, most with minimal damage and magnitudes below 3.0 on the Richter Scale. Earthquakes that cause moderate damage to structures occur several times a year. According to the California State Hazard Mitigation Plan, earthquakes large enough to cause moderate damage to structures—those of Magnitude 5.5 or larger—occur three to four times a year statewide. Strong earthquakes of Magnitude 6 to 6.9 strike on an average of once every two to three years. Major earthquakes of Magnitude 7 to 7.9 occur in California about once every 10 years. According to the USGS, in the next 30 years, California has a 99.7% chance of a magnitude 6.7 or larger earthquake.

The impact of an earthquake is largely a function of the following components:

  • Ground shaking (ground motion accelerations)
  • Liquefaction (soil instability)
  • Distance from the source (both horizontally and vertically)
Secondary Hazards Caused by Earthquakes

Liquefaction occurs when ground shaking causes the mechanical properties of some fine grained, saturated soils to liquefy and act as a fluid (liquefaction).Liquefaction involves loose sandy soil with a high-water content that undermines the ground’s ability to solidly support building structures during an earthquake. It is the result of a sudden loss of soil strength due to a rapid increase in soil pore water pressures caused by ground shaking.

Landslides occur because 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 extremely high speeds, taking vegetation and/or structures with it. Slide risks increase after an earthquake during a wet winter.

Tsunamis, depending on the location of an earthquake,can be triggered by large earthquakes (generally, a 7.5 magnitude and above). Tsunamis significantly damage many locations beyond what the earthquake struck; however, coastal communities near the earthquake epicenter that are also vulnerable to tsunamis could experience devastating impacts. 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.


Prepare for Earthquakes

  • Practice Drop, Cover, and Hold On with family and coworkers.
  • Make a Plan: Create a family emergency communications plan that has an out-of-state contact. Plan where to meet if you get separated. Make a supply kit that includes enough food and water for several days, a flashlight, a fire extinguisher and a whistle.
    • Being prepared allows you to avoid unnecessary excursions and to address minor medical issues at home, alleviating the burden on urgent care centers and hospitals.
    • Remember that not everyone can afford to respond by stocking up on necessities. For those who can afford it, make essential purchases and slowly build up supplies.
  • Protect Your Home: Secure heavy items in your home like bookcases, refrigerators, water heaters, televisions and objects that hang on walls. Store heavy and breakable objects on low shelves.
    • Consider making improvements to your building to fix structural issues that could cause your building to collapse during an earthquake.
    • Consider obtaining an earthquake insurance policy. A standard homeowner’s insurance policy does not cover earthquake damage.
    • Download the homeowner's guide to earthquake safety for more information
  • Sign up for the State's Earthquake Early Warning system alerts.

Stay Safe During an Earthquake

The safest place to be in an earthquake is where nothing can fall on or under you! Most earthquake-related injuries and deaths are caused by falling or flying objects (e.g., TVs, lamps, glass, bookcases), or by being knocked to the ground. When shaking starts, if possible, quickly move away from glass, hanging objects, large furniture that could fall and cabinets with doors that could swing open.


If an earthquake happens, protect yourself right away

  • If you are in a car, pull over and stop. Set your parking brake.
  • If you are in bed, turn face down and cover your head and neck with a pillow.
  • If you are outdoors, stay outdoors away from buildings.
  • If you are inside, stay and do not run outside and avoid doorways.
Remember to Drop, Cover, and Hold:
  1. Drop (or Lock): Wherever you are, drop down to your hands and knees and hold onto something sturdy. If you’re using a wheelchair or walker with a seat, make sure your wheels are locked and remain seated until the shaking stops.
  2. Cover: Cover your head and neck with your arms. If a sturdy table or desk is nearby, crawl underneath it for shelter. If no shelter is nearby, crawl next to an interior wall (away from windows). Crawl only if you can reach better cover without going through an area with more debris. Stay on your knees or bent over to protect vital organs.
  3. Hold On: If you are under a table or desk, hold on with one hand and be ready to move with it if it moves. If seated and unable to drop to the floor, bend forward, cover your head with your arms and hold on to your neck with both hands.

Stay Safe After an Earthquake

There can be serious hazards after an earthquake, such as damage to the building, leaking gas and water lines, or downed power lines.

  • Expect aftershocks to follow the main shock of an earthquake. Be ready to Drop, Cover, and Hold On if you feel an aftershock.
  • If you are in a damaged building, go outside and quickly move away from the building. Do not enter damaged buildings.
  • If you are trapped, send a text or bang on a pipe or wall. Cover your mouth with your shirt for protection and instead of shouting, use a whistle.
  • If you are in an area that may experience tsunamis, go inland or to higher ground immediately after the shaking stops. Avoid contact with floodwaters as they can contain chemicals, sewage, and debris.
  • Check yourself to see if you are hurt and help others if you have training. Learn how to be the help until help arrives.

Once you are safe, pay attention to local news reports for emergency information and instructions via battery-operated radio, TV, social media or from cell phone text alerts.

  • Never enter a damaged building until it has been inspected by a professional and deemed safe for entry
  • Check the property for gas leaks, chemical spills, damaged electrical wiring and broken water pipes.
  • Take pictures of any damage to your property and home.
  • Contact your insurance agent or company right away to begin your claims process.
  • Keep records of any repair or cleaning costs.
  • Check on your neighbors, especially those who are seniors or disabled.
  • Monitor local radio or television reports about where to get emergency housing, food, first aid, clothing and financial assistance.

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