The Faults on our Earth


CSU geology experts study the active land California inhabits to better understand earthquakes and predict the location and intensity of future temblors.


“More than 70 percent of [California’s] population resides within 30 miles of a fault where high ground shaking could occur in the next 50 years.”
California Geo​logical Survey

We call it “The Big One.” No one knows exactly when and where it will hit. This hypothetical California earthquake—defined as a 7.8 or higher magnitude event along the southern section of the San Andreas Fault—could be 44 times stronger than the 1994 Northridge earthquake​.

In 2008, a group of scientists drew up The ShakeOut Scenario to describe what could happen after “The Big One” and its aftershocks hit Southern California. The consensus: 1,800 deaths, 50,000 injuries, $213 billion in losses, severed sewage and electricity lines that would take months to repair, disconnected and contaminated water sources, collapsed and unstable buildings, fires, landslides and damaged roads and transportation systems preventing aid and emergency services from reaching the area.

Following the 2019 earthquakes in Ridgecrest—a magnitude 6.4 followed by a magnitude 7.1—the likelihood of such an earthquake taking place increased. But as any California resident knows, the state will continue to experience any number of earthquakes before “The Big One” strikes. Just consider that California is second only to Alaska for the U.S. state with the most earthquakes and experiences the most earthquakes that cause damage in the country.

Dr. Sally McGill working with a student in the field.

Dr. Sally McGill works with a student in the field.

What's Shaking?

Earthquakes occur when energy in the form of seismic waves ripple the earth, causing the ground to shake. While this energy can come from volcanic activity or manmade triggers like an explosion, most often they emanate from fault lines where two sections of landmass called tectonic plates meet. About 15 kilometers (nine miles) below the earth’s surface, these massive plates can slowly creep past each other, both horizontally and vertically; but above that depth, they become stuck because of friction. As the lower part of the plates move, the upper part of the plates bend across the fault until energy builds up enough that a plate springs free, sending out seismic waves.

A major part of anticipating and preparing for tectonic earthquakes is understanding a region’s seismic hazard: the likelihood of earthquakes occurring in an area, their frequency and intensity in those areas and related effects like landslides and tsunamis.

“The primary goal is for people to be prepared for earthquakes,” says Sally McGill, Ph.D., a geologist and associate dean of California State University, San Bernardino’s College of Natural Sciences. “We know the San Andreas Fault is an active fault and it's going to continue to produce larger earthquakes that are going to impact large sections of Southern California and Central California. We want people to understand where the faults are and the level of shaking that could be expected from these earthquakes.”

Using that information empowers the state to predict which areas will experience the most damage during an earthquake and make decisions around where to construct commercial and residential buildings, which existing buildings need retrofitting, where to build infrastructure like roads and utility lines and how to ensure residents have access to services in the aftermath.

Research from CSU geologists and seismologists provides the state with critical information to help protect citizens and ensure industries survive “The Big One.”

CSUSB Master of Science students, Seth Clemen Saludez (left) and Andrew Suarez (right), performing a scan with mobile backpack LIDAR technology at Mill Creek Fault in Mill Creek, San Bernardino Mountains.

CSUSB Master of Science students, Seth Clemen Saludez (left) and Andrew Suarez (right), perform a scan with mobile backpack LIDAR technology at Mill Creek Fault in Mill Creek, San Bernardino Mountains.

A View from Above

For earthquake researchers, the first step in seismic hazard analysis is often mapping a region’s geological features, like faults and debris flows following wildfires—a key focus for Kerry Cato, Ph.D., a licensed engineering geologist and Cal State San Bernardino geology professor.

Employing drones and LIDAR (Light Detection and Ranging) technology, which measures distances using a laser light, Dr. Cato captures hundreds of images of an area that he then digitally stitches together into 3D models. When these areas are mapped at different points in time, he can then detect movement and changes in a landscape. “If it's a fault, we can tell which strands actually moved and ruptured the ground surface,” he says. “When we see this from the sky, we send geologists to look at it and provide the ground truth.”

With recent funding from benefactor Caroline Amplatz and a W.M. Keck Foundation grant, CSUSB’s geology department is acquiring more of this technology to continue mapping California’s seismic hazard zones, as well as give students an opportunity to practice digital mapping.

Following the 2019 Ridgecrest earthquakes, Cato began taking students ​to the locations where the ground ruptured to map the area. He is also working to map that region with graduate student Frank Jordan, who works as a geologist for San Bernardino County and is studying local faults, mountain forming and seismically induced landslides. The county can use Jordan’s maps (along with maps from the state) to develop its official seismic safety maps and building plans.

“Where the earthquakes are in California, that's a teaching moment,” Cato says. “It's good to [study their effects], because people forget about hazards. ... Earthquakes are years apart or decades apart. It's hard to keep that public mass consciousness up there, but earthquakes definitely would disrupt our way of life in a huge way.”

Under the direction of Dr. McGill, Bryan Castillo studies sediment layers of a 40-meter section of a trench dug along the San Andreas Fault.

Under the direction of Dr. McGill, Bryan Castillo studies sediment layers of a 40-meter section of a trench dug along the San Andreas Fault.


Responding to Cato’s call, enter geologists like Dr. McGill, who studies the frequency at which faults rupture and the speed at which two plates move past each other along a fault line, called slip rates.

Currently, she’s looking at a series of three alluvial fans (a fan pattern formed when water deposits sediments at the mouth of a canyon) along the San Andreas Fault that have been offset over time by earthquakes. By dating sediment samples from the fans and measuring the distance by which they’re offset from the canyon using digital mapping, she can calculate the slip rate along that section of the fault. Her current graduate student, James Burns, is using the same method to map and date offset landforms along the Garlock Fault in the Mojave Desert.

“Most of the time, the fault is not moving at all; it's locked and only moves during the earthquake,” McGill explains. “But if you add up all those earthquakes over 5,000 years or 20,000 years, we can calculate, on average, how fast that fault [is moving], how many millimeters per year or how many meters per 1,000 years that fault is moving. And that's useful for a seismic hazard analysis because the faults that are moving faster are probably going to have more big earthquakes and generate more seismic hazards.”

Another tactic involves digging trenches across active faults and analyzing the sediment layers to determine which sections ruptured and when. McGill’s former graduate student and a current department lecturer, Bryan Castillo, led an excavation of a section of the San Andreas Fault near Palm Springs, where he documented eight prehistoric earthquakes—while another recent graduate student, Kyle Pena, did the same on a section of the Garlock Fault.

“We're able to tell, roughly, how frequently the fault produces earthquakes,” she says. “And that's also relevant for a seismic hazard to know [if it’s] every 200 years, every 500 years, every 1,000 years.” This shows how soon the fault may rupture again.

Dr. Kim Blisniuk collecting samples from a section of the San Andreas Fault in the Coachella Valley.

Dr. Kim Blisniuk collects samples from a section of the San Andreas Fault in the Coachella Valley.

Kimberly Blisniuk, Ph.D., geologist, geochronologist and San José State University associate professor of geology, is similarly collecting slip rate data from sites on Northern and Southern California sections of the San Andreas Fault, the San Gregorio Fault near Half Moon Bay and Mavericks and the Rogers Creek Fault in Sonoma County. “Not only are we understanding how the landscape is changing as the result of earthquakes, but slip rate data has a direct impact on society and people,” she says.

The data collected is added to the Uniform California Earthquake Rupture Forecast, a model of seismic events in California that public and private entities can use to make decisions around earthquake preparation and mitigating earthquake damage. “This model basically compiles all published data we know about faults and their seismic activity—for example, how fast they move and where they're located—to estimate earthquake probabilities,” she explains. “All this information is then used by insurance companies or [Pacific Gas and Electric Company] or builders or whatnot to make informed decisions on how and where to build.”

CPP student James East swinging a hammer, which acts as a seismic source, for a refraction experiment to determine the width of the Ridgecrest fault zone during a field class

CPP student James East swings a hammer, which acts as a seismic source, for a refraction experiment to determine the width of the Ridgecrest fault zone during a field class.

A Quick Shake

While geologists can help locate where and when the next earthquake is likely to occur, seismologists can help determine how intense and destructive that earthquake’s shaking could be.

“This provides important information on the site effects that govern damage caused by local and regional earthquake activity,” says Jascha Polet, Ph.D., seismologist and professor of geophysics at California State Polytechnic University, Pomona.

Dr. Polet’s research in the San Gabriel, Chino and San Bernardino basins focuses on seismic site response: using ambient noise ground motion data (the persistent vibration of the ground not due to earthquakes) to study how various locations in an area react differently during an earthquake. This helps her determine the ground motion amplification and resonance frequency—that is, the intensity and duration of shaking that will occur during an earthquake—in these regions.

In addition, Polet analyzes ground motion and ground deformation data from earthquakes in near-real time to determine factors like depth and magnitude (called source characterization)—​​as well as finds faults and determines their subsurface geometry by measuring how their presence affects gravity, electrical currents or magnetic forces. By understanding past earthquakes, she can better predict what future earthquakes may look like.

“Better knowledge of where faults are located, how large the earthquakes may be that these faults can produce and how the ground will move when an earthquake happens can all help mitigate earthquake hazards,” she says.


While California is using research like that at the CSU to ensure it can endure a massive earthquake, all residents should also work to protect themselves, their families and their homes. Take these actions to make sure you’re prepared.

  • Participate in the Great ShakeOut to practice how to be safer during earthquakes. International ShakeOut Day happens on the third Thursday of October, but you can get resources and practice safety measures year-round.
  • Follow the Earthquake Country Alliance’s Seven Steps to Earthquake Safety guide to learn what to do before, during and after an earthquake.
  • Sign up for earthquake warnings and download the MyShake App through the California Governor's Office of Emergency Services’ Earthquake Warning California, the first statewide earthquake warning system in the U.S.
  • Check if ​you live in an earthquake hazard zone with the California Geologi​cal Survey’s California Earthquake Hazards Zone Application ("EQ Zapp").