4.4: Earthquakes

What was that? Was that an earthquake?

“What was that?” is one of the questions heard most often when a small earthquake occurs; the other is “Was that an earthquake?” Fortunately, most people live their lives without ever experiencing the ground becoming mobile beneath them, but not in California. California is one of the most seismically active areas on Earth because the entire length of the state is relatively near, if not actually on some type of plate boundary. Plate boundaries are basically just large fault zones that allow two plates to move by one another (see Tectonic Plates, Plate Motions, and Plate Boundaries).

Earthquakes can occur anywhere the Earth is deforming, but most medium to large earthquakes happen at or near plate boundaries. As shown in Figure \(\PageIndex{1}\), the largest earthquakes, with magnitudes greater than 9, occur at subduction zones. An example of a subduction zone in California is the Cascadia Subduction Zone in northern California.

Figure \(\PageIndex{1}\): World map of earthquakes larger than magnitude 6.0 from 1900 to 2017. Earthquakes greater than 9.0 occur in subduction zones. Earthquakes from 8.0 up to 9.0 occur in subduction zones, collision zones, and occasionally on transform plate boundaries. Earthquakes between 6.0 and 8.0 occur at any type of plate boundary. Also note, earthquakes larger than 6.0 rarely occur away from a plate boundary. "Map of Earthquakes" by Phoenix7777 via Wikimedia Commons, is licensed under CC BY-SA 4.0.

And the next smallest earthquakes, magnitude between 8 and 9 can also occur on large and long faults, an example in California would be the San Andreas fault system. Therefore, the potential for big earthquakes exists in California. For a Californian, knowing about earthquakes and being prepared for them should also be part of daily life (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): USGS Mobile Laser Scanning Truck collecting data near the zone of maximum surface displacement for the magnitude 7.1 Searles Valley earthquake of July 6, 2019. This is the top of a rupture that extends 45 km (28 miles) in length and 8 km (5 miles) in depth. "Near Zone of Maximum Surface Rupture" by Ben Brooks, USGS, is in the public domain.

What is an Earthquake?

Earthquakes occur when an area is experiencing a force or stress that builds up over time until a fault finally breaks, and each side of the fault moves past the other side. The energy released then propagates as waves through Earth and is what is called an earthquake. Elastic deformation is the basic mechanism used to describe Earth’s response when stress is applied. Elastic is like a rubber band, if the stress were removed, the rock would return to its original shape. However, just like pulling on a rubber band, if the stress is too great, or continues for too long, a break, or fault, can occur. The rocks on either side of the fault return to their previous unstressed condition, just in a new location. Figure \(\PageIndex{3}\) illustrates rock in a pre-stressed state (left), stressed rock experiencing elastic deformation (center), and rock that is unstressed after a rupture (right).

Figure \(\PageIndex{3}\): Three figures showing the progression of elastic deformation to rupture. (a) Undeformed rock. (b) Rock under stress, deformed elastically—if stress is removed, it returns to (a) with no permanent record. (c) Stress exceeds the fault’s strength, causing rupture and stress release. "Elastic deformation and Rupture" by Steven Earle is licensed under CC BY 4.0.

This is called elastic rebound and was first hypothesized as the mechanism for the 1906 magnitude 7.9 San Francisco earthquake. The 1906 earthquake changed the way Americans perceived earthquakes because up until then, no major U.S. city had ever been destroyed by one.

A commission was formed to look at the observable aspects of what had happened and why. One of the members of the Commission, Harry Fielding Reid, studied geodetic survey records for the 50 years before the earthquake and found evidence that the ground had deformed by more than 3m (10 ft) during those 50 years. He hypothesized that the ground deformation before the earthquake was from the buildup of elastic deformation of the rock. This deformation increased the stress on the fault, and when the build up of stress became larger than the strength of the fault, it broke. The 1906 San Francisco earthquake was the result. Modern seismic studies of earthquake waves and ground motion before and after earthquakes using GPS data support his observations and hypothesis.

It’s also important to recognize that an earthquake is not just a point inside Earth. While the earthquake starts at a single point, it actually ruptures an area on a fault inside Earth within a matter of seconds.The rupture doesn't spread equally in all directions. Rock properties, stress buildup, and the amount of stress influence the direction and size of the rupture. Also, when an earthquake occurs and stress is relieved in one area, it can increase the stress in surrounding areas. This transfer of stress from a fault to surrounding rocks can help trigger another earthquake.

Earthquakes therefore may have foreshocks, aftershocks, or both. A foreshock is a smaller earthquake that occurs in a fault zone or related fault zone before a larger earthquake. An aftershock is a smaller earthquake that occurs in a fault zone or a related fault zone after a larger earthquake. Not all earthquakes have foreshocks and not all earthquakes have aftershocks.

Video \(\PageIndex{1}\): Foreshocks - Mainshock - Aftershocks

A series of earthquakes in the Alaska Range are used to illustrate how foreshocks and aftershocks are related to an earthquake.

A California example of both foreshocks, a mainshock, and aftershocks is the 2019 Ridgecrest Earthquake Sequence (Inset Box \(\PageIndex{1}\).)

Box \(\PageIndex{1}\): The 2019 Ridgecrest Earthquake Sequence

The 2019 Ridgecrest earthquake sequence began in the California desert on July 4, 2019 and was widely felt in central and southern California and Nevada.

Based on statistics derived from prior California earthquakes, there was a 6% estimated probability that this earthquake was a foreshock for another larger earthquake within the next 3 days, and that is what happened. Two days after the initial magnitude 6.4 earthquake, a magnitude 7.1 earthquake occurred. The basic statistics and the weekly predictions for three weeks after the earthquake sequence began are summarized in Box Figure \(\PageIndex{1}\). Note that there are many smaller aftershock earthquakes.

Box Figure \(\PageIndex{1}\): Infographic on the 2019 Ridgecrest earthquake sequence. "2019 Ridgecrest Earthquake Sequence" by USGS Earthquake Hazards Program is in the public domain. Access a detailed description.

Box Video \(\PageIndex{1}\): A Tear in the Mojave

Watch how the data from the earthquakes were compiled and analyzed. Of special note is notice how the foreshocks and aftershocks are part of a sequence that occurred on 2 faults at right angles to each other.

One of the interesting results of the research done on this earthquake sequence is that it may have increased the possibility of a major earthquake on the nearby Garlock fault.

Query \(\PageIndex{1}\)

Not all earthquakes are the same. Sometimes stress can be released within a fault zone by processes other than earthquakes. One of these processes is episodic tremors and slip (ETS) that occur on both subduction zones and parts of the San Andreas fault zone (Inset Box \(\PageIndex{2}\)).

Box \(\PageIndex{2}\): Episodic Tremor and Slip (ETS): When an Earthquake is Not Really an Earthquake

Episodic Tremor and Slip (ETS) is a seismic phenomenon observed in many subduction zones. This is nearly imperceptible seismic rumbling, known as tremor, and slow slip along the plate interface. ETS events are imperceptible to humans, but not to seismometers, and do not cause damage.

So, why do we care?

Scientists are interested in studying ETS because it may provide insight into how stress is transferred within the crust, and how larger more destructive megathrust earthquakes are generated.

In the Cascadia subduction zone, where the Juan de Fuca Plate is subducting beneath the North American Plate, the cold upper part of the fault within the subduction zone is considered “locked” because of friction between the plates (Inset Box Figure \(\PageIndex{2}\)). When slip occurs here, it generates very large earthquakes. In the warm, lower part of the fault, sliding is continuous because of how warm and weak the rock is. Between the two zones is the ETS zone, where rocks are not cold enough to be stuck long term but aren’t warm enough to slide continuously. Rather, this part of the boundary slips episodically on average every 14 months for a duration of about 2 weeks, moving a few centimeters at a time. These episodes are known as slow slip events.

Box Figure \(\PageIndex{2}\): Episodic tremor and slip along the Cascadia subduction zone. The Juan de Fuca plate is locked to the North American plate at the top of the subduction zone, but lower down it is slipping continuously. In the intermediate (ETS) zone, the plate alternately sticks and slips on a regular schedule. "Episodic tremor and slip" by Steven Earle is licensed under CC BY 4.0.

These episodes are detected using geodetic measurements, such as GPS. GPS markers above the locked zone of the Cascadia subduction zone are steadily moving to the east, because the Juan de Fuca Plate is frictionally locked with the North American plate and dragging it eastward. GPS markers above the continuous slip zone are not moving, indicating that there is no friction between the plates here. Above the ETS zone, the North American plate generally moves eastward, then every 14 months or so, it reverses course for about two weeks and then it returns to its previous eastward motion.

Box Video \(\PageIndex{2}\): Subduction Zone - Deformation Zones (educational)

The following video illustrates how geodesy, the science of measuring Earth's shape, position, and movement, is used to detect Episodic Tremor and Slip (ETS).

It might seem like this periodic slip would help release stress buildup, thus reducing the risk of larger, more destructive earthquakes. However, the movement along the ETS zone actually appears to transfer stress to the adjacent locked zone, increasing the chance of a large earthquake.

ETS has been observed in a number of subduction zones including the Cascadia, Hikurangi, Mexican, and Nankai subduction zones. In addition to subduction zones, ETS has also been detected in transform faults like the San Andreas. Tremors have been observed along the Parkfield segment of the San Andreas which is considered the transition between the southern locked segment and the northern creeping section.

The Role of Fluids

Tremor is often associated with the movement of underground magmatic or hydrothermal fluids. In the case of the San Andreas fault zone and the subduction zones previously mentioned, hydrothermal fluids, not magma, are believed to be the cause of the tremor. Geoscientists have suggested that minerals undergoing metamorphic dehydration reactions could be a source of these non-magmatic fluids. As minerals are buried deeper, they experience increased temperatures and become unstable. Some minerals undergo metamorphic reactions that may include the release of water resulting in increased pore fluid pressure.

Interestingly, one mineral that might be the culprit of fluid induced tremor along parts of the San Andreas fault zone may actually be serpentine, the mineral that makes up California’s state rock serpentinite. Serpentine will undergo dehydration to olivine, talc, and water at increased temperatures. The ancient subduction zone that preceded the transform San Andreas fault zone (see A Brief Geologic History) is known to have produced significant quantities of serpentinized rock and those rocks may still be undergoing dehydration and producing water. These increased pore-fluid pressures may be responsible for lowering the frictional stress and generating tremor along parts of the San Andreas fault zone.

Seismic Waves

Waves occur frequently in nature and in many different situations. What makes a wave a wave? Consider a wave in the ocean or a ripple in a pond. The wave moves forward, but the water doesn’t go with the wave. If you watch a cork floating on the water, it moves up and down as the wave moves the water under it, but it ends up where it started. When an earthquake occurs, the fault slips, and seismic waves are created and propagate away from the fault rupture in all directions. Like a water wave, the seismic wave propagates onward, but the particles of rock just oscillate and end up where they started. There are two major types of seismic waves - body waves and surface waves. Body waves move through materials; surface waves move along surfaces.

Body waves are P-waves and S-waves. The difference between them has to do with how the particles of the material move or vibrate as the wave passes through. The following two videos illustrate P-wave motion and S-wave motion. Neither video has audio, but both will be discussed in the text afterwards.

Video \(\PageIndex{2}\): P-Waves

The following animation uses a mass of small cubes to represent an imaginary rock. Watch as the cubes are compressed and extended as the p-waves move through it. This video contains no audio.

Video \(\PageIndex{3}\): S-Waves

The following animation again uses a mass of small cubes to represent an imaginary rock. Watch as the cubes move up and down as the S-waves move through it. This video contains no audio.

The imaginary rock in both Videos \(\PageIndex{2}\) and \(\PageIndex{3}\) has the shape of a brick, which has been divided into small cubes. The wave in both videos propagates from one end of the brick to the other. The movement of the small cubes within the brick shows the particle motion. And the difference between P-waves and S-waves is the difference in the movement of the small cubes.

With the P-wave, the small cubes compress and then expand with motion in the same direction as the wave motion. Notice that they do not move side to side, or up and down. With the S-wave, the small cubes do not compress or expand. Instead they move up and down as the wave moves through the brick. This is like people doing the wave in a sports stadium, where the “wave” moves through the crowd as people raise their arms while standing up and then sitting down. The people in the sports stadium, and the little cubes in the brick start and end in the same place.

P-waves are compressional waves. The particle motion of a P-wave is in the same direction as the wave motion. P-waves can propagate through solids, liquids, and gases. When you hear someone speak, you are listening to a P-wave. P-waves are also called primary waves because they propagate the fastest, and on seismograms, arrive before S-waves. Seismograms are the recordings made of the arrival of earthquake waves. Now imagine that the brick in the P-wave video was rotated vertically and you were standing on its end. When a P-wave arrives, the motion you experience would be up and down because of the compression and expansion of the little cubes beneath your feet.

S-waves are shear waves. The particle motion of an S-wave is perpendicular, or at right angles, to the wave motion. The motion of the little cubes in the video is vertical, but an S-wave could also move horizontally, like the motion of a snake. Unlike P-waves, S-waves can only propagate through solids. They cannot propagate through liquids or gasses. S-waves are also called secondary waves because they are slower and come in second on a seismogram, after the P-waves. Now imagine the brick in the S-wave video rotating to vertical like you imagined the rotation of the brick in the P-wave video. Both the vertical and the horizontal versions of an S-wave in this orientation would cause side-to-side shaking.

Query \(\PageIndex{2}\)

When a P-wave or S-wave arrives at the surface of Earth, most of the wave reflects back into the Earth. However, some of the wave energy becomes trapped near the surface of the Earth making new waves, surface waves.

There are two types of surface waves, Rayleigh waves and Love waves (Figure \(\PageIndex{4}\)). They move slower and have larger amplitudes than P-waves or S-waves; therefore, they cause much of the damage that occurs during earthquakes. Love waves are just trapped S-waves and are experienced as strong side to side shaking. Rayleigh waves are generally experienced as a rolling motion like that of an ocean wave; their motion along the surface is actually elliptical as is indicated in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Block models showing the motion of surface waves: Rayleigh waves roll, while Love waves move side to side. Both are confined to the near-surface and weaken with depth. "Surface Waves" by Steven Earle is licensed under CC BY 4.0.

When an earthquake occurs the P-waves arrive first, then S-waves, and finally surface waves. Figure \(\PageIndex{5}\) is a record of the arrival of seismic waves, a seismogram. Notice also that the seismogram records the time of the arrival of the different seismic waves.

Figure \(\PageIndex{5}\): Simplified seismogram showing the ground motion of P-waves, S-waves, and surface waves. “First Arrivals of Seismic Waves” by Steven Newton is licensed under CC BY-NC 4.0.

Video \(\PageIndex{4}\): Seismic Waves from an Earthquake

The following animation describes the arrival of seismic waves.

Query \(\PageIndex{3}\)

Query \(\PageIndex{4}\)

Epicenter and Hypocenter

When an earthquake occurs another question that is immediately asked is “Where’s the epicenter?” While the epicenter is related to the earthquake, it’s not where the earthquake occurs. The epicenter is the location on Earth’s surface directly above where the earthquake begins; the hypocenter, or focus, is on the fault inside the Earth (Figure \(\PageIndex{6}\)). The hypocenter is where the earthquake begins and the fault propagates away from there. For a medium to large earthquake, magnitude 6 or greater, the fault will typically propagate 10s of kilometers from its starting point at the hypocenter. The epicenter is a useful location to refer to on a map that is commonly used for the earthquake’s location.

Figure \(\PageIndex{6}\): Cross-section within the Earth showing a fault with the hypocenter (or focus) of an earthquake. The epicenter is at the surface directly above the hypocenter. "Epicenter-Hypocenter" by Ansate via Wikimedia Commons is licensed under CC BY-SA 1.0.

Historically, large or notable earthquakes are given names based on where the maximum damage from the earthquake occurred. This is because there was no way to calculate the actual location of the hypocenter of an earthquake until 1880 when modern seismometers, machines to measure seismic waves, were invented. With the invention of the seismometer, it became possible to locate the hypocenter of an earthquake and find its epicenter, but referring to an earthquake by the location of maximum damage was still common. An example would be the 1933 magnitude 6.4 Long Beach earthquake. This earthquake is the second most deadly earthquake in modern times in California, 120 people died and Long Beach suffered massive damage, but the hypocenter and its associated epicenter were actually to the south 20 kilometers (12.5 miles) near Huntington Beach.

As global seismic networks became more common, calculations of hypocenters improved, and epicenters are now reported by latitude and longitude with major or notable earthquakes named by the city, town, or geographic feature closest to the geographic location of the epicenter. For example, the 1989 magnitude 6.9 Loma Prieta earthquake, is named after Loma Prieta Peak in the Santa Cruz mountains, even though extensive damage happened in the cities of Santa Cruz and San Francisco.

Query \(\PageIndex{5}\)

Locating an Earthquake

Finding where an earthquake is located can be complicated and several methods that can be used. Automated systems can calculate an estimate of its hypocenter (and epicenter) very fast, but not necessarily with the high level of accuracy needed to study the earthquake. But people want to know about the earthquake, now! They don’t want to wait for the results of intensive analysis that might take hours, days, or even longer.

Therefore, a compromise occurs, a fast automated estimate of the earthquake’s location is usually reported almost immediately, and then the information is updated and improved as more data are analyzed. This does not mean that a mistake is being corrected because the location changes; it means that the first report was done using methods that are quick, computerized, with an emphasis on speed and good enough to help people responding to the earthquake, but the emphasis is on fast and useful, not on highest level of accuracy possible. This initial automated estimate of an earthquake’s location will be within a few kilometers of the actual location (and the estimated magnitude will be within a few tenths).

Commonly the automated estimate is updated within an hour, after the initial estimate was reported. It may then be updated again within a few days. Eventually when all of the data recorded are analyzed a “final” hypocenter is found and it is listed in databases and reported in scientific papers, but in some cases this can take a year or more. While the epicenter may slightly change, it’s usually very close to the original quick estimation of the automated systems from when it occurred. The 1994 magnitude 6.7 Northridge earthquake is however an example of what can happen.

Originally the epicenter was reported in Northridge, California. More than a year later, once all the data were analyzed, it turned out the epicenter was actually 1.6 km (1 mile) to the south-southwest in Reseda, California. Even though the epicenter is now known to be in Reseda, it’s still however referred to as the Northridge earthquake. As Earth models are improved, and the physics of earthquakes and faulting is better understood, new methods are developed to study seismic waves and sometimes decades later the hypocenter of an earthquake may be recalculated yet again (see Inset Box 4.4.3).

One common way to locate an earthquake, starts with reading seismograms (Figure \(\PageIndex{7}\)). It begins by finding the arrival times for the first P-wave and first S-wave. These waves travel the nearly same path within Earth, and subtracting the arrival time for the P-wave from the arrival time for the S-waves finds the time difference between their arrivals. Because the P-waves travel faster than the S-waves, the further apart they are on a seismogram, the farther the distance to the earthquake (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{7}\): Simplified seismograms for an earthquake recorded by three different seismometers at different locations with the P-wave and S-wave first arrivals indicated. The time difference is given in minutes along with a distance in km. "Measuring Seismograms" by IRIS is licensed under CC BY 4.0.

The time difference can then be used to estimate the distance to the earthquake using a travel-time curve. Travel-time curves are graphs of distance versus time for different seismic waves (Figure \(\PageIndex{8}\)). Their construction is described in Video \(\PageIndex{5}\).

Video \(\PageIndex{5}\): Travel-Time Curves

This video explains how travel-time curves are constructed.

Figure \(\PageIndex{8}\): Seismograms overlaid on a travel-time curve to show how earthquake distance is determined. The seismogram is rotated to align time on the vertical axis, while matching the P-wave and S-wave time time difference with the travel-time graph. "Distance Traveled" by Karla Panchuck is licensed under CC BY-NC-SA 4.0.

Query \(\PageIndex{6}\)

Query \(\PageIndex{7}\)

Each seismogram provides the distance to the earthquake from that seismometer but tells nothing about the direction to the earthquake. To actually find the location of an earthquake, requires data from at least three seismometers using a method called triangulation. On a map, draw a circle around each seismometer with a radius equal to the distance to the earthquake, and where they intersect is the epicenter of the earthquake (Figure \(\PageIndex{9}\)).

Figure \(\PageIndex{9}\): Triangulating data from three seismic stations to locate the earthquake's epicenter. The distance to the earthquake is determined by the method shown in Figure \(\PageIndex{8}\). The epicenter is located where the circles intersect. "Locate" by IRIS is licensed under CC BY 4.0.

Box \(\PageIndex{3}\): When Earthquake Locations Change

An extreme example of how the location of an earthquake can change over time, as either new information becomes available, or as understanding of the physics of earthquakes changes, or new analysis methods are discovered is what used to be called the 1812 San Juan Capistrano earthquake.

It was large, did massive damage throughout the missions of Alta California, but the worst damage was at San Juan Capistrano and the missions to the north. At San Juan Capistrano the “Great Stone Church” was destroyed and 40 people died. The earthquake occurred before seismometers were invented, so there are no seismograms to read. It was originally assumed to be an earthquake on the Newport-Inglewood fault zone (Box Figure \(\PageIndex{3}\)), because that fault was reasonably close and has a historical record of large earthquakes which includes the 1933 magnitude 6.4 Long Beach earthquake.

Later when computer modeling became common and as Earth models for the geology of southern California were improved the damage patterns for older earthquakes were being reassessed. Everything that could be discovered about the damage done at the time, and how the buildings were constructed, and the geology of the area was put into models to determine where and how big the earthquake must have been. A better match to the damage was if the earthquake were located on the San Jacinto fault zone to the east.

But it did not end there, new research on how fault zones can interact with each other has again changed the understanding of where this earthquake may have begun and how large it might have been. Now the earthquake is believed to have started on the San Andreas fault zone.

It seems that the epicenter of the earthquake was on the San Andreas fault near Wrightwood, California and the rupture jumped fault zones. The San Jacinto fault zone is part of the greater San Andreas fault system and near Cajon Pass it jumped from the San Andreas fault zone to the San Jacinto fault zone based on patterns mapped in the faults and subsequent paleoseismic evidence from tree ring analysis. By transitioning between the San Andreas and San Jacinto faults, the rupture surface expanded, leading to a greater release of energy. The earthquake today is estimated to be a magnitude 7.5 starting on the San Andreas fault zone and then transferring to the San Jacinto fault zone, and it is now called the 1812 Wrightwood earthquake.

Box Figure \(\PageIndex{3}\): Simplified fault map of southernmost California “Faults of Southern California” by Debra Brooks, a derivative of "Southern California Faults" by BrucePL is licensed under CC BY-NC 4.0.

References

1. Berkeley Seismological Lab. (2004). Where Can I Learn More About the 1906 San Francisco Earthquake? https://web.archive.org/web/20080327145305/http://seismo.berkeley.edu/faq/1906_0.html
2. Bolt, B. A. (1999). Earthquakes. (4th ed.). W. H. Freeman and Company.
3. Brumbaugh, D. S. (1999). Earthquakes - Science and Society. Prentice Hall.
4. Earle, S. (2019). Physical Geology. (2nd ed.). PressBooks. https://opentextbc.ca/geology/
5. Earthquakes Hazards Program. (n.d.). 1906 Marked the Dawn of the Scientific Revolution. U.S. Geological Survey. https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/revolution.php
6. Hough, S. E., & Graves, R. W. (2020). The 1933 Long Beach Earthquake (California, USA): Ground Motions and Rupture Scenario. Scientific Reports. 10(10017). https://doi.org/10.1038/s41598-020-66299-w
7. Lemelson - MIT. (n.d.). John Milne Seismograph. Massachusetts Institute of Technology. https://lemelson.mit.edu/resources/john-milne
8. Panchuck, K. (2021). Physical Geology (H5P ed.). BCcampus. https://opentextbc.ca/physicalgeologyh5p/
9. Reid, H. F. (1910). Permanent Displacements of the Ground. In A. C. Lawson (Chairman), The California Earthquake of April 18, 1906 - Report of the State Earthquake Investigation Commission. Vol. II The Mechanics of the Earthquake. (pp. 16-28). The Carnegie Institution of Washington.
10. SAGE-IRIS Consortium. (n.d.). Education & Outreach Series No. 6: How are Earthquakes Located? EarthScope Consortium. https://www.iris.edu/hq/inclass/fact-sheet/how_are_earthquakes_located
11. Seismological Society of America. (2023, June 28). What Are the Characteristics of Foreshocks for Large Earthquakes? https://www.seismosoc.org/news/what-are-the-characteristics-of-foreshocks-for-large-earthquakes/
12. Southern California Earthquake Data Center (2013). Earthquake Information [data set]. Caltech. DOI:10.7909/C3WD3xH1
13. U.S. Geological Survey. (n.d.). How do Seismologists Locate an Earthquake? https://www.usgs.gov/faqs/how-do-seismologists-locate-earthquake
14. Wald, L. (n.d.). Earthquakes Hazards Program: The Science of Earthquakes. U.S. Geological Survey. https://www.usgs.gov/programs/earthquake-hazards/science-earthquakes

Inset Box \(\PageIndex{1}\) References

1. Barnhart, W. D., Hayes, G. P., & Gold R. D. (2019, October 15). The July 2019 Ridgecrest, California, Earthquake Sequence: Kinematics of Slip and Stressing in Cross-Fault Ruptures. Geophysical Research Letters, 46(21) 11859-11867. https://doi.org/10.1029/2019GL084741
2. California Institute of Technology Division of Geological and Planetary Sciences (2024). Earthquake Information: Earthquake Probabilities. Southern California Earthquake Data Center. https://scedc.caltech.edu/earthquake/probabilities.html
3. Lozos, J. C., & Harris, R. A. (2020, March 12). Dynamic Rupture Simulations of the M6.4 and M7.1 July 2019 Ridgecrest, California Earthquakes. Geophysical Research Letters, 47(7) https://doi.org/10.1029/2019GL086020

Inset Box \(\PageIndex{2}\) References

1. Beroza, G. C., & Ide, S. (2011). Slow Earthquakes and Nonvolcanic Tremor. Annual Review of Earth and Planetary Sciences, 39(1) 271–296. https://doi.org/10.1146/annurev-earth-040809-152531
2. Kirby, S. H., Wang, K., & Brocher, T. M. (2014). A large mantle water source for the northern San Andreas fault system: A ghost of subduction past. Earth, Planets and Space, 66(1), 67. https://doi.org/10.1186/1880-5981-66-67
3. Rogers, G. & Dragert, H., 2003, Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip, Science, 300(5627) 1942-1943. DOI: 10.1126/science.10847

Index Box \(\PageIndex{3}\) References

1. Rodriguez Padilla, A. M., Oskin, M. E., Rockwell, T. K., Delusina, I., & Singleton, D. M. (2021). Joint earthquake ruptures of the San Andreas and San Jacinto faults, California, USA. Geology, 50(4) 387-391. https://doi.org/10.1130/G49415.1
2. Southern California Earthquake Data Center (2013). Earthquake Information – Chronological Earthquake Index [data set]. Caltech. https://scedc.caltech.edu/earthquake/wrightwood1812.html