15.4: Geologic Hazards of the Peninsular Ranges

In a place so lovely, what could possibly go wrong?

Geologic hazards occur everywhere. Every area has its potential geologic hazard be it earthquakes, volcanoes, tsunamis, mass wasting, subsidence, floods, swelling clays, quicksand, beach erosion, sea level rising, saltwater intrusion, or groundwater contamination, just to name a few. In the Peninsular Ranges province, the rock types might not be that varied compared to many parts of California, but the landscape and the topography are varied, which leads to a host of potential hazards. Therefore, while there are no active volcanoes here (anymore), and tsunamis generally arrive with plenty of warning and much smaller amplitudes, making them much less of a problem than in northern California, everything else on that list can and does happen here. Many of these problems, while important – especially if they are affecting your home or your business – are fortunately mostly local and not wide-ranging within the province. This chapter will focus on the challenges that can and eventually may affect many people or large areas of the province.

Earthquakes

Earthquakes are generally number one on the list of California hazards. Earthquakes occur when tectonic stress builds up and the rock snaps, breaks, and moves with the best place for this to happen being at or near a plate boundary (see Earthquakes). The local plate boundary, the San Andreas fault zone (SAFZ), is not in the Peninsular Ranges province but is just outside it as is indicated in Figure 15.1.6. Two of the three major fault zones within the Peninsular Ranges are considered to be part of the greater San Andreas fault system – the San Jacinto and the Elsinore. The third major fault zone, the Newport-Inglewood-Rose Canyon fault zone, while not part of the greater San Andreas, parallels it, to the west, just onshore in some places, and just offshore in others. While the SAFZ represents a significant hazard, the local fault zones are closer and therefore present a greater local hazard. Especially since they all have a history of moderate to large earthquakes in recent times.

Query \(\PageIndex{1}\)

The San Jacinto fault zone (SJFZ) is one of the most seismically active fault zones in all of southern California. Estimates are that up to 80% of the slip rate between the North American and Pacific plates in southern California is occurring because of earthquakes in the SJFZ. Estimates of the largest possible earthquake on the SJFZ is M_W7.5. It splays off the SAFZ in Lytle Creek in the San Gabriel Mountains of the Transverse Ranges province and crosses into the Peninsular Ranges province near Loma Linda, California, and then crosses into the Colorado Desert province south of the Santa Rosa Mountains and can be traced on the Interactive Fault Activity Map of California.

There is a long history of moderate to large earthquakes in the SJFZ. The most famous historic earthquake in the Peninsular Ranges, the 1812 San Juan Capistrano earthquake, was originally believed to have been on the Newport-Inglewood fault zone, then on the SJFZ, but now recent studies on interactions between fault zones indicates it occurred as a joint rupture of the SJFZ and the SAFZ. Seismologists now believe that the rupture most likely began on the SAFZ, then jumped between the two fault zones in Cajon Pass to the SJFZ, which increased the amount of fault rupture and magnitude. It has an estimated magnitude of M_W7.5 and is now being referred to as the 1812 Wrightwood earthquake, even though most of the damage occurred at the Missions along the coast, especially San Juan Capistrano (Figure \(\PageIndex{1}\)). In all, at least 6 earthquakes with M6 or higher have occurred on the SJFZ since 1899. The largest being the 1918 M_L6.8 near the town of San Jacinto.

Figure \(\PageIndex{1}\): The remains of the Great Stone Church at Mission San Juan Capistrano, destroyed in a large earthquake on December 8, 1812. Extensive damage occurred throughout Alta California and 40 people died in the collapse of the church. "Great Stone Church 02" by Bernard Gagnon via Wikimedia Commons is licensed under CC BY-SA 3.0.

The next major fault zone to the west, the Elsinore fault zone (EFZ), is much less seismically active. It is one of the quieter fault zones in southern California in historic times but has the potential for earthquakes as large as M_W7.5. The last major earthquake to occur was a M_L6.0 in 1910. There were however two larger earthquakes in the 1890s, estimated M_W6.8 and M_W6.5 that may have occurred on either the EFZ or the SJFZ. This is before California had many seismometers and the epicentral locations calculated from those older more distant seismometers are not as precise as they are today. Since the 1890s, earthquakes as large as M7 have also occurred in Mexico on the southern extension of the EFZ, the Laguna Salada fault zone.

The third major fault zone in the Peninsular Ranges province is the Newport-Inglewood – Rose Canyon fault zone. Frequently it is referred to by either its northern name, the Newport-Inglewood fault zone (NIFZ), or its southern name, the Rose Canyon fault zone (RCFZ). This fault zone hugs the coast and passes from just onshore to just offshore, as seen in the Interactive Fault Activity Map of California. Potential largest magnitude estimates are M_W7.5 for the NIFZ, which is particularly scary because its northern end is in the heavily populated greater Los Angeles area, which includes Orange county. The RCFZ is a southern extension of the NIFZ and is mostly offshore and not as well studied, which is also scary because San Diego is another heavily populated area.

The NIFZ, however, is the location of one of the most important historical earthquakes in California, the 1933, M_W6.4 Long Beach earthquake. It is the second deadliest earthquake in California, 120 people died and it destroyed among other things 70 schools and damaged another 50. Luckily school was out for the day, or it is estimated that thousands of children would have died or been injured. This led to the passage of the Field Act, which requires state approval for all public-school buildings with respect to earthquake safety (see The Hazard of Place). The epicenter for this quake was not in Los Angeles county near the Inglewood end of the fault, but rather south of Huntington Beach. It’s called the Long Beach earthquake because that is where maximum death and destruction occurred, even though the epicenter was not near Long Beach.

Evidence of a new potential increase to earthquake hazards in the Peninsular Ranges province comes from an unlikely location; Oklahoma. In the early 2000s, Oklahoma experienced an alarming increase in earthquake activity with their epicenters closely aligned with producing oil fields. Water was being injected into oil fields where fracking had occurred to help with the recovery of the remaining oil and in the process was lubricating long dormant fault zones. The Los Angeles Basin is also a major oil producing area and seismologists studying the earthquake history of the region noted a similar alignment of some earthquakes prior to 1935 with oil fields. Further investigation of the production practices pre-1940s indicate that several historic earthquakes, including the 1933 Long Beach earthquake may have been related to oil production. There is no indication that this is continuing to occur today, the geology of the Los Angeles basin and the Oklahoma oil fields is different and modern oil production practices are not the same as they were in the 1930s.

Because earthquakes are a major potential hazard in the Peninsular Ranges province, in order to estimate what could happen, the Southern California Earthquake Center (SCEC) in 2014 made a series of videos. These videos estimate fatalities and damage following a moderate earthquake on local faults in highly populated areas of the state. The following videos (Videos - 15.4.1, 15.4.2, 15.4.3, 15.4.4, 15.4.5) for the highly populated areas within the Peninsular Ranges are from that series.

Video \(\PageIndex{1}\): Magnitude 6.8 Earthquake on the Newport-Inglewood Fault

What if? What would happen to Torrance, Long Beach, Huntington Beach, if an earthquake approximately the size of the 1994 Northridge earthquake were to happen on the northern, onshore part of the Newport-Inglewood fault near the Los Angeles-Orange County line? This area which was sparsely populated when the 1933 Long Beach earthquake occurred is now densely populated and the story is no longer the same. This video has no sound. Access a written description.

Video \(\PageIndex{2}\): Magnitude 6.9 Earthquake on the San Joaquin Fault

What if? What would happen to Huntington Beach, Costa Mesa and Santa Ana, California, if an earthquake approximately the size of the 1994 Northridge earthquake were to happen on the San Joaquin fault? The San Joaquin or San Joaquin Hills thrust fault is a blind thrust fault onshore of the Newport-Inglewood fault in central Orange County, California. This video has no sound. Access a written description.

Video \(\PageIndex{3}\): Magnitude 6.5 Earthquake on the Palos Verdes Fault

What if? What would happen to Compton, Long Beach, and Huntington Beach, if an earthquake approximately the size of the 1994 Northridge earthquake were to happen on the Palos Verde fault? The Palo Verde fault is offshore between Santa Catalina Island and the Orange county coastline. This video has no sound. Access a written description.

Video \(\PageIndex{4}\): Magnitude 6.5 Earthquake on the Rose Canyon Fault

What if? What would happen to San Diego and Chula Vista, California, and Tijuana, Mexico, if an earthquake approximately the size of the 1994 Northridge earthquake were to happen on the Rose Canyon fault? This video has no sound. Access a written description.

Video \(\PageIndex{5}\): Magnitude 6.5 Earthquake on the Elsinore Fault

What if? What would happen to Corona, Yorba Linda, and Santa Ana, if an earthquake approximately the size of the 1994 Northridge earthquake was to happen on the northern Elsinore fault? This video has no sound. Access a written description.

The situation has changed since 1933, notice that if any of these earthquakes were to happen, it would immediately move to at least second place on the death and destruction list of California earthquakes. All of these models indicate that such an earthquake happening today will potentially kill or injure more people than did the 1933 Long Beach earthquake.

Query \(\PageIndex{1}\)

Floods

Usually when people think of flooding in California they think of the Great Valley or the Delta, not southern California. With a Mediterranean climate along the coast and relatively dry climates within the mountains, flooding would not seem to be much of an issue in southern California, and usually, it isn’t. Flash floods occur in the mountains after rainstorms, but usually they are local events and not wide ranging. However, some of the largest and most damaging floods in the history of California have occurred, at least partly, in southern California.

There is a long history of mostly coastal flooding in the Peninsular Ranges province. Since California became a state, significant flood events have occurred in one part or all of the Peninsular Ranges in 1862, 1891, 1916, 1938, 1969, 1980, and 2005. Of particular note are the Great Flood of 1862 (Box 15.4.1) and the flood of 1938.

Box \(\PageIndex{1}\): California’s Great Flood of 1862 – in southern California

In 1862, the western front of the Civil War came to a standstill during what is one of the largest flood events ever experienced on the west coast of the United States. Now known as the Great Flood of 1862, continuous rain fell for anywhere from 28 to 42 days on the west coast and major flooding occurred from British Columbia to Mexico and was especially bad in the states of California, Oregon, and Nevada. In California, much of the Central Valley flooded and Sacramento was under 4 feet of water. On January 10, 1862, the newly elected Governor of California, Leland Stanford, arrived at his inauguration at the State Capital in a rowboat.

Within two more weeks the flooding in Sacramento was so bad that the State Legislature temporarily moved to San Francisco. The rain and flash flooding that occurred in the mountainous areas of the state, including the Peninsular Ranges, destroyed many of the mining camps, most of which were never rebuilt. Statewide, an estimated 4000 people are believed to have died, over 200,000 cattle, most of the bridges in the state were destroyed, as was approximately one-third of all property, and the state went bankrupt.

In southern California, approximately 35 inches of rain fell over 28 consecutive days starting on December 24, 1861. The Santa Ana River, the major river system in the northern Peninsular Ranges, flowed wall-to-wall in Santa Ana Canyon and when the canyon opened onto the coastal plain near Anaheim, it flooded Anaheim and destroyed it along with its vineyards and farms.

By February the flood waters of the Santa Ana River had merged on the coastal plain with those of the San Gabriel and Los Angeles Rivers to create a temporary inland sea from Signal Hill to Huntington Beach that was over 29 km (18 miles) long and with standing water over 1.2m (4 feet) deep. The water did not recede for over 3 months. In San Diego, high tides and the storm surge from an offshore storm backed up the San Diego River and it cut a new channel to the harbor. The hills and marine terraces were heavily eroded, and many were cut by gullies and channels. A 15.25 m (50 foot) arroyo was eroded at San Luis Rey and like Sacramento to the north, Mission Valley was underwater.

While the Great Flood of 1862 did not start the California citrus industry, it helped set the stage for the citrus industry to flourish in the 1870s. Prior to the Great Flood, much of southern California was cattle ranches and over 10,000 cattle are estimated to have died in the flood in southern California. Many of the ranchers became farmers because they could not afford to replace the cattle they had lost. And the climate in many of the destroyed areas of southern California, such as Riverside, Anaheim, and San Diego was perfect for citrus.

In southern California this flood still stands as the worst flood to have occurred in modern times. Sedimentological studies indicate that large flood events may happen as frequently as every 100-200 years. Also, while it took the floodwaters until summer to recede in many areas, once the flood was over, California went into a period of drought that lasted until 1864.

The flood of 1938 (Figure 15.4.2) is what finally mobilized the state into taking action to prevent massive flooding and started an era of dam building for both flood prevention and water storage with Prado Dam on the lower Santa Ana River completed in 1941.

During the 1938 flood, it rained for 5 days, 87 people died and damage was $78 million ($1.69 billion in today’s dollars). The Santa Ana River broke its levee near Santa Ana Canyon and the cities of Anaheim, Buena Park, and Santa Ana flooded. Flood waters in Anaheim and Santa Ana were almost 2m (6 feet) deep. Almost every bridge and railroad crossing over the river was destroyed.

Figure \(\PageIndex{2}\): The mouth of the Santa Ana River near Huntington Beach, California, during the flood of 1938 with the Pacific Ocean and Santa Catalina Island in the background. Major breaks in the levee system are visible. US Hwy 101 along the coast and major street locations are indicated within the flood waters. "Santa Ana River during the flood of 1938” is copied courtesy of the Orange County Archives and is licensed under CC BY 2.0. Access a detailed description of this figure.

Why this history of massive flooding? It’s partly because of ARkStorms, for Atmospheric River 1000 Storms, storms that occur every 500-1000 years when atmospheric rivers bring days of extreme weather that overwhelm existing water control systems (Video 15.4.6). While not every flood event is an ARkStorm, the 1862 flood was. With global warming, it’s possible that storms of this magnitude will occur even more frequently than every 500 to 1000 years in the future.

Video \(\PageIndex{6}\): ARkStorm

An overview by the U.S. Geological Survey of the danger an ARkStorm could bring.

Saltwater Intrusion

Saltwater or seawater intrusion is a coastal problem. It occurs when the local groundwater aquifer is open to seawater and there is a possibility that saltwater will advance inland (see Threats to California’s Water). Problems occur when the balance between saltwater and freshwater is disturbed, which generally happens when fresh water is pumped out faster than it can be replenished, or as sea level rises. As sea level rises, there is more water exerting pressure on the seafloor sediments and this can help drive more saltwater into the aquifers near shore.

In Orange and San Diego counties the most common method used to try and prevent this from happening is the use of injection wells close to the coast (Figure \(\PageIndex{3}\)). Rather than try to build a solid barrier, freshwater, frequently recycled water (yellow in the figure), is pumped or injected into the ground closer to the coast to form a fluid barrier between the saltwater (red) and the freshwater (blue). Freshwater wells are then sited inland further from the coast.

Figure \(\PageIndex{3}\): Cross-section of a coastal aquifer with injection wells. Fresh water (yellow) is being pumped or injected into the ground between saltwater (red) and freshwater (blue). "Freshwater Injection" by Los Angeles County Public Works is in the public domain.

In coastal Orange county, there are two groundwater barriers created by injection wells, the northernmost is the Alamitos Barrier Project, a joint project near the Los Angeles and Orange county line and the Talbert Barrier near Huntington Beach and Fountain Valley, California. The Alamitos Barrier is a joint project between Los Angeles and Orange counties and maintains a system of 59 injection wells near Seal Beach and Long Beach, California. The Talbert Barrier by the Orange County Water District is a series of 102 injection wells over three miles. The injection wells vary from 100 to 685 feet deep and are located to form a fluid barrier.

The area between these two barriers is known as the Sunset Gap and is currently being monitored because in 2015 a water well 3 miles inland in Huntington Beach had to be taken offline because of saltwater intrusion. Currently the monitoring of salt levels by both geophysical and chemical methods shows that saltwater is slowly moving onshore and approaching the production wells. New monitoring wells were drilled and data from them is being used to design a new injection well barrier.

There are other techniques that can be used to monitor the movement of saltwater onshore. The most common ones are geophysical methods based on changes in the electrical resistivity of the water. Geophysical techniques such as this are generally much cheaper than drilling monitoring wells and can cover larger areas of interest, but do not provide the detailed water quality information gathered by a monitoring well.

Two factors will lead to this becoming an even more important problem to mitigate in the future – population growth and rising sea level. Both will contribute to an increased risk and preventing saltwater intrusion will become even more important.

Mass Wasting

Mass wasting is the technical term used to describe what are commonly called landslides. Mass wasting can occur anywhere but is most likely to occur in areas with high relief and steep slopes, or steep slopes and weak rock. In fact, coastal California, including the Peninsular Ranges province has some of the best conditions in the world for mass wasting. Most of the mass wasting that occurs in the Peninsular Ranges province occurs along the coast in the marine terraces. While the slopes in the mountains are steep and have high relief, most of the rock in the mountains is very strong, competent plutonic igneous rock that seldom hosts landslides. Where they do occur in the mountains is in shear zones or other areas where the rock has been highly fractured and while they do occasionally occur, they are relatively uncommon.

The marine terraces are another story. They are frequently oversteepened and with slopes well above the angle of repose (Figure \(\PageIndex{4}\)). The rock is almost always sedimentary, relatively young, and extremely well-watered. All that is needed is a trigger for mass wasting to occur and in most of southern California dozens, if not hundreds, of small earthquakes occur daily. Most of these earthquakes are so small that humans cannot feel them, but they are still large enough to trigger mass wasting. Most of the time, mass wasting will be some type of slide, with slumps being the most common. Slumps occur when material moves downslope as a unit on a curved plane.

Figure \(\PageIndex{4}\): Oversteepened cliffs of Torrey Sandstone at Torrey Pines State Beach, San Diego County, California. Notice the piles of sediment at the base of the cliff. As sediment weathers from the cliff, wave action at high tide removes the finer-grained sediment, leaving only the larger pebbles and cobbles behind. "Torrey Pines 2024-06" by Allison Jones is licensed under CC BY-NC 2.0

An example of what can happen is illustrated in this video (Video 15.4.7) of Black’s Beach in San Diego county. In January 2023, a slump occurred that brought much of the bluff down onto the beach.

Video \(\PageIndex{7}\): Black’s Beach Bluff Collapse

TV report of a slump occurring at Black’s Beach, San Diego County, California.

Much of the time slumps bringing down beach cliffs is only a hazard to the people on the beach below, and unfortunately people are sometimes injured or even die. Sometimes however there is development at the top of the cliff and houses or businesses may be lost. In southern Orange county the railroad right-of-way is at the base of the cliffs and during stormy, wet winter weather, mass wasting takes out the train tracks on a fairly regular basis. The opposite situation occurs in San Diego county, where the railroad right-of-way is on the marine terrace at the top of the cliffs and so far, there has not been a problem, but the potential for one exists.

Query \(\PageIndex{3}\)

Climate Change

As the world’s climate warms, the effects on coastal communities will be huge. Most of what is likely to occur however is that the hazards already existing, floods caused by storms, saltwater intrusion, mass wasting, will become more frequent and exacerbated over time. Added to this will be flooding from rising sea level and the possibility for beaches to disappear. Most coastal communities are dependent upon their beaches and tourism for much of their economy, which means that even if ways are found to help the residents stay in their homes, if the beaches are gone, the communities may die anyway.

References

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9. Edwards, B. D., & Evans, K. R. (2005, May 17) Saltwater Intrusion in Los Angeles Area Coastal Aquifers – the Marine Connection. USGS Fact Sheet 030-02 https://pubs.usgs.gov/fs/old.2002/fs030-02/
10. Ellsworth, W. L. (1990). Earthquake history, 1769-1989. In R. E. Wallace (Ed.), USGS Professional Paper 1515: The San Andreas Fault System, California. (pp. 153-181). U.S. Geological Survey. https://pubs.usgs.gov/pp/1990/1515/
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13. Hart, M. W. (1991). Landslides in the Peninsular Ranges, Southern California. In M. J. Walawender & B. B. Hanan (Eds.), Geological Excursions in Southern California and Mexico: Guidebook for the 1991 Annual Meeting Geological Society of America San Diego, California. (pp. 349-371). Department of Geological Sciences, San Diego State University.
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15. Hough, S. E., & Page, M. (2016). Potentially Induced Earthquakes during the Early Twentieth Century in the Los Angeles Basin. Bulletin of the Seismological Society of America, 106(6) 2419-2435. https://doi.org/10.1785/0120160157
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References for Box \(\PageIndex{1}\): California’s Great Flood of 1862 – in southern California

1. Dettinger, M. D., & Ingram, B. L. (2013). The Coming Megafloods. Scientific American, 308(1), 64–71. http://www.jstor.org/stable/26017895
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3. Moore, A. (2024, March 1). The Great Flood of 1862. California Historical Society. https://californiahistoricalsociety.org/blog/the-great-flood-of-1862/
4. Philpott, T. (2020, August 26). The Biblical Flood That Will Drown California. Mother Jones. https://www.motherjones.com/environment/2020/08/california-flood-arkstorm-farmland-climate-change/
5. Smith, J. (2003, June 26). San Diego’s five worst floods. San Diego Reader. https://www.sandiegoreader.com/news/2003/jun/26/unforgettable-san-diegos-five-worst-floods/
6. Taylor W. L., & Taylor, R. W. (2007). The Great California Flood of 1862. The Fortnightly Club of Redlands, California. https://www.redlandsfortnightly.org/papers/Taylor06.htm