Editor’s Note: This article orginally ran March 16, 2023 as “12 earthquakes rattle Tres Pinos area.” Following another string of earthquakes in Tres Pinos and Paicines in late February, 2025, BenitoLink is republishing this earthquake guide.
Lea este artículo en español aquí.
For people living in San Benito County, the rocking and rolling of the earth is a somewhat frequent event. On March 14 and 15 the Tres Pinos area experienced 12 earthquakes ranging in magnitude from 2.4 to 3.4 on the Richter scale in a 30 hour-span.
With the help of the UC-Berkeley Seismology Lab’s website and its Project Scientist—Earthquake Early Warning specialist and seismologist Angie Lux—BenitoLink here answers common questions and dispels a few myths.
San Benito County sits on several earthquake faults, including the San Andreas and Calaveras, both of which have several offshoots. The San Andreas Fault is very active in the county though it is unclear why.
What is an earthquake?
The term earthquake describes both the sudden slip of a fault and the radiated seismic energy and ground-shaking caused by the slip.
What is a fault?
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement can occur rapidly, in the form of an earthquake, or slowly, in the form of creep. Faults range in length from inches to thousands of miles. Two major faults run through San Benito County. From south county going west through San Juan Bautista is the almost 750-mile San Andreas fault; running east through Hollister and toward Morgan Hill and East San Jose is the 76-mile Calaveras fault.
Why do plates move?
The inside of the Earth is hot because of radioactive rocks and heat left over from the immense pressures of the Earth’s formation. The Earth is still cooling off. As the Earth cools, hot rock within the mantle (the layer of rock between the earth’s crust and outer core) is very slowly coming up to the surface, while cold rock is very slowly sinking down toward the core. This is called convection.
How are earthquakes measured?
Seismologists have several different methods for determining the size of an earthquake—some based on body waves (which travel deep within the structure of the earth), some based on surface waves (which primarily travel along the uppermost layers of the earth) and some based on completely different methodologies.
Moment Magnitude (Mw) is the latest concept in magnitude determination. Unlike the older methods which are all based on the maximum amplitude of ground movement at the measuring station, Mw is based on the seismic moment at the source, or hypocenter, of the earthquake.
Other methods are described here.
Can small earthquakes relieve stress thus preventing large ones?
The simple answer is no. The more complex answer according to the UC Berkeley Lab is:
Earthquake statistics in most regions of the world, including California, show that for every magnitude 5 earthquake on the Richter scale, there are about 10 that have a magnitude of 4, and for each magnitude 4, there are 10 with magnitude 3. Unfortunately, this means there are not enough small earthquakes to relieve enough stress to prevent large events. In fact, it would take 32 magnitude 5s, 1000 magnitude 4s, or 32,000 magnitude 3s to equal the energy produced in one magnitude 6 event.
During an earthquake, should you head for the doorway?
Only if you live in an old, unreinforced adobe house. In modern homes, doorways are not stronger than any other parts of the house, and the doors in them could swing and cause injury. It is safer to practice “drop, cover and hold on,” under a sturdy piece of furniture.
Will California eventually fall off into the ocean?
No. The San Andreas Fault System, which crosses California from the Salton Sea in the southeast to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving northwest with respect to the North American Plate at approximately 46 millimeters, or about 2 inches per year. The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, in the very distant future, Los Angeles and San Francisco may one day be adjacent to one another.
Can the weather influence the occurrence of earthquakes?
This is another no. According to UC Berkeley, however, very large low-pressure systems, such as hurricanes, have been known to cause episodes of fault slip (slow earthquakes), which are not very damaging.
Lux suggested that when we experience a larger quake, we might associate other factors such as the weather at the time with the experience and project it as part of a pattern.
Do animals sense earthquakes in advance?
According to Lux there is no scientific evidence that animals sense an earthquake before it happens. She did suggest that they notice the smaller ones at a higher rate than their human counterparts because they are not as busy as humans.
Does the ground open up during an earthquake?
Shallow crevasses can form during earthquake-induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not open up during an earthquake. Movement occurs along the plane of a fault, not perpendicular to it. If faults opened up, no earthquake would occur because there would be no friction to lock them together.
According to the California Office of Emergency Services there are tools and resources available through Earthquake Warning California. The California MyShake App, which is available free for both Android and iPhone, could provide a few seconds of advance warning. The app uses technology to rapidly detect seismic waves as an earthquake begins, calculates the maximum expected shaking, and sends alerts to MyShake users in the surrounding area to take cover before damaging shaking arrives.
For official information about the California Earthquake Early Warning System, visit: earthquake.ca.gov.
Lux said that if outside when shaking begins, move away from power lines and wait out the shaking. She also advised people to be prepared for earthquakes by having a basic earthquake emergency kit on hand. See more here.
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