14.5: Geologic Hazards of the Transverse Ranges Province

Earthquakes in the Transverse Ranges

Like much of California, the Transverse Ranges Province is a zone of considerable seismic hazard. The region is crisscrossed by numerous active faults with considerable potential for damage. Two important examples are the 1971 San Fernando Earthquake and the 1994 Northridge Earthquake. These events occurred on reverse faults typical of this region and they exemplify the impact of such structures on the region’s communities.

The 1971 M 6.7 San Fernando earthquake ruptured the Sierra Madre fault. This fault is part of a reverse fault system at the foot of the San Gabriel Mountains. Studies of this fault system indicate it has ruptured at least two times in the past 15,000 years, with likely magnitudes as high as 7.5 or 7.6 Mw. In addition to the uplift caused by the San Fernando quake, severe damage occurred in the surrounding areas in response to shaking (reaching XI, or extreme) on the Modified Mercalli Intensity Scale; Figure \(\PageIndex{1}\)). Damage to a large dam in the area caused the evacuation of approximately 80,000 downstream residents for four days and major transportation corridors were obstructed. Of particular concern was the partial collapse of two medical facilities (Olive View Medical Center and Veterans Administration Hospital). Both facilities were situated near the fault: some of the Medical Center buildings were within 300 m of the fault, and the VA Hospital buildings were within 5 km (3.1 mi) of the rupture. Of the 65 fatalities associated with this event, more than half occurred due to the failure of these medical facilities.

Figure \(\PageIndex{1}\): The 1971 San Fernando earthquake, also known as the Sylmar earthquake — Los Angeles County, Southern California. Shake map of the magnitude 6.7 earthquake occurring in the San Fernando Valley on February 9, 1971. Epicenter is marked at the star. Shaking maximum is Intensity IX northeast of the epicenter. "San Fernando earthquake" by USGS is in the public domain. Access a detailed description of this image.

The 1994 Northridge earthquake occurred on a previously unknown, buried reverse fault that did not break the surface (now known as a “blind thrust” after the fact that it did not break the surface). This 6.7 Mw earthquake killed 57 people and caused massive devastation across much of the LA basin due to seismic amplification within basin sediments; shaking was felt as far as 137 km (85 mi) away (Figure \(\PageIndex{2}\)). Damage was particularly pronounced in basin areas of Santa Monica, Simi Valley, and Santa Clarita and as far away as the city of Anaheim located 80 km (50 mi) away. The shaking caused highway collapse and rail line ruptures, as well as pancaking of apartment buildings with weak first floor levels or garages, and ruptured gas and water lines among other challenges (see Chapter on earthquake hazards). As if it weren’t bad enough, shaking stirred up clouds of fine dust that contained spores of a fungus (coccidioidomycosis) that led to an outbreak of Valley Fever. This fungal infection of the lungs is very difficult to eradicate and can cause severe long-term health impacts.

Figure \(\PageIndex{2}\): ShakeMap computed for the 1994 Northridge earthquake (see trinet for this and other archived events); the square indicates the estimated location and surface projection of the fault in this case. Maximum shaking was north of the epicenter. Intensity levels were as high as X+. "Shake Map for Northridge" by USGS is in the public domain. Access a detailed description of this image.

Video \(\PageIndex{1}\): What the Northridge earthquake felt like

This video will give you an account of what people who lived near the epicenter of the Northridge earthquake experienced.

Video \(\PageIndex{2}\): A geologist talks about the Northridge earthquake

This video is an interview with a professional U.S.G.S. geologist concerning the Northridge earthquake.

Following the Northridge event, scientists revisited geophysical data from across the region and identified additional “blind thrusts” like the one that ruptured in the Northridge event (Figure \(\PageIndex{3}\)). In the case of the Northridge earthquake, scientists realized that the structure that ruptured in 1994 might be related to an earlier rupture in 1987 (the M6 Whittier Narrows Earthquake). Continued work revealed other potentially hazardous structures buried beneath the Los Angeles Basin that could be associated with earthquakes of M6.2-6.7 and as high as Mw7.1. This potential, along with the possibility of seismic amplification in the many basin areas of the Transverse Ranges Province presents considerable risk for the residents.

Figure \(\PageIndex{3}\): This cross-section beneath the San Fernando Valley shows models designed to trace rupture areas using seismic data from the Northridge Earthquake and the San Fernando Earthquake. This cross section is oriented with north to the right. Models of the fault planes of the 1994 Northridge dip to the south and do not reach the surface. Model results from the 1971 San Fernando earthquakes dip to the north and suggest that movement on the buried thrust fault responsible for the Northridge earthquake terminated about 5 kilometers beneath the surface. This movement may have terminated against one of the faults that moved in 1971. The hypocenters of the two shocks are at the lower limits of each of the fault models, and the arrays of dots indicate locations of aftershocks. "San Fernando cross section of fault models" by USGS is in the public domain.

Inset Box \(\PageIndex{1}\) State Responses to Major Earthquakes

Each time a major seismic event impacts California, we learn more about our risk and exposure to the hazard. In the case of earthquakes, enhanced instrumentation tells us more about the type of shaking we can expect. This information guides building design and code development that keeps us safe. The San Fernando and Northridge earthquakes led to the development and expansion of several important programs that keep Californians safer.

Following the San Fernando event, the Alquist-Priolo Special Studies Act was enacted. This act restricts construction of buildings designed for human occupancy across potentially active faults. It relies on maps of Alquist-Priolo Earthquake Fault Zones, which are created based on mapping information provided by the California Department of Conservation (CDC). In addition to the enhanced focus on residential buildings, the state created the The Hospital Safety Act of 1972 to ensure that new or retrofitted hospital buildings are sited, designed, and constructed so that they are able to remain operable following earthquakes. This event also highlighted the need for better communication among scientists, engineers, and emergency responders, leading to the creation of the first Earthquake Clearinghouse established by the CDC to promote better communication between.

The state also sought to better describe and understand the potential shaking damage to buildings via implementation of the California Strong Motion Instrumentation Program. This program’s goal is to maximize the volume of data by furnishing and maintaining instruments at selected lifeline structures, buildings, and ground response stations. Information learned from this network informs development of building design to withstand potential shaking. To date, The program has installed more than 900 stations, including 650 ground-response stations, 170 buildings, 20 dams and 60 bridges.

Following the Northridge event, the California Legislature created the California Earthquake Authority (CEA), which is a publicly managed but privately funded organization that offers minimal earthquake damage coverage. In addition, a substantial effort was also made to reinforce freeway bridges against seismic shaking, and a law requiring water heaters to be properly strapped was passed in 1995. In addition, the Federal Emergency Management Agency (now under the U.S. Department of Homeland Security) awarded the California Geological Survey nearly $20 million to accelerate the zoning of earthquake hazards under the Seismic Hazards Mapping Act of 1990. These Seismic Hazard Zones, or Zones of Required Investigation, delineate where there is a high likelihood of these hazards occurring in future earthquake events. Local city and county permitting agencies are required to request “special geological and engineering studies” be performed within these zones prior to land development and construction.

Flooding in the Transverse Ranges

We typically think of California as the land of earthquakes. However, residents know that flooding is a periodic hazard for many of our communities. You can learn more about this hazard in our chapter on California Water. In the case of the Transverse Ranges Province, flooding is a major concern in low lying regions. This is because our steep ranges collect a lot of moisture from storms arriving from the Pacific Ocean to the west. When rainfall is very high, the water cannot infiltrate the ground and instead runs off into our heavily inhabited basin regions. Such dramatic episodes lead to the debris flows that we’ve seen in the Santa Barbara area, and coastal flooding observed throughout the state.

Historical flooding in the Los Angeles basin led to dramatic decisions to ensure the safety of area residents: the paving and channelization of major rivers and the construction of range front debris basins. (Figure \(\PageIndex{4}\)) This action, taken by the US Army Corps of Engineers, was largely in response to serious flooding in 1938. This 50-year flood event was caused by the confluence of 2 major storms that deposited more than 1 year of precipitation in a few days. Flooding killed more than 100 people in the Transverse Range region (Figure \(\PageIndex{5}\)). The event was so severe that the Los Angeles river’s course was actually changed!

Figure \(\PageIndex{4}\): The concrete channel of a portion of the Los Angeles River. Water is in the central part of the channel. Sloped banks prevent flooding and erosion. "MTA Los Angeles River" by anarchosyn, is licensed under CC BY-SA 2.0.

Figure \(\PageIndex{5}\): Los Angeles River. View upstream from above Victory Blvd. showing breaches in paved levees in and below a sharp curve in channel alignment. River mile 32.0. This image is from the Report on Engineering Aspects, Flood of March 1938 by the U.S. Engineer Office in Los Angeles and compiled in August 1938. The Los Angeles River and the river's tributaries flooded after a very wet rainy season and one particularly bad storm on March 1-3, 1938. After this flood, the Army Corps of Engineers set about ensuring extensive flooding would not cause such extreme damage in the future. "Los Angeles River Flood" by the United States Army Corps of Engineers is in the public domain.

Inset Box \(\PageIndex{2}\): The Saint Francis Dam Disaster

The epic 1938 flooding of Los Angeles was not the first major event impacting this region. In 1928, a man-made flood killed more than 350 residents of the western Transverse Ranges when the Saint Francis Dam broke. This dam was built by the Bureau of Water Works and Supply (now Los Angeles Department of Water and Power) under the leadership of William Mulholland in response to growing water needs in Los Angeles and was in San Francisquito Canyon of the Western Transverse Ranges (Inset Figure \(\PageIndex{1}\)).

Inset Figure \(\PageIndex{1}\): View of the St. Francis dam looking to the north, with water in its reservoir, in February 1927, before it failed.
"St. Francis Dam" by H.T. Stearns/USGS is in the public domain.

The dam was placed above a reddish conglomerate/sandstone unit that contained gypsum and rested in fault-contact with a foliated mica schist interspersed with talc. The fault separating these units was the San Francisquito Fault, which was determined to be inactive. Throughout the period of filling and use, areas of seepage beneath and adjacent to the dam were observed, along with cracks within the dam (which were sealed upon inspection). None of these issues were deemed to be of significant concern. However, the dam catastrophically failed just before midnight on March 12, 1928. While there are no surviving witnesses to what followed, reconstructions of the event indicate that a 37 m (120 ft) high flood wave had traveled 1.6 - 2.4 km (1-1.5 mi) at an average speed of 29 km/hr (18 mi/hr). A large piece of the dam itself, weighing approximately 9,000 metric tons was found about three-quarters of 1.6 km (1 mi) below the dam site. The devastation was complete (Inset Figure \(\PageIndex{2}\)).

Inset Figure \(\PageIndex{2}\): Damage from the St. Francis Dam Flood March 12-13, 1928, Los Angeles County, California. Taken from the same location as Inset Figure 5.1 showing the remains of the dam and reservoir floor. The dam failed at 11:58:30 p.m. Monday March 12, 1928. The left (west) abutment of the dam was entirely swept away and the inactive San Francisquito Fault is clearly visible, being located along the contact zone of schist and conglomerate. Adventurous hikers should note that this dam remnant no longer stands, as it was deemed an “attractive nuisance,” and unfortunately removed. "St. Francis Dam after the 1928 failure" by H.T. Stearns/USGS is in the public domain.

Flooding damaged the towns of Fillmore and Santa Paula and ultimately washed out to the Pacific Ocean south of the town of Ventura 87 km (54 mi) downstream. Subsequent investigations suggested that dam failure was in part due the fact that its unstable foundations were built on unsuitable bedrock. Superintendent and chief engineer of the Los Angeles Water Department William Mulholland was in the habit of personally intervening in dam designs, and he in fact inspected the St. Francis Dam hours before its deadly destruction. It should be noted that Mulholland had no formal degrees or expertise in engineering and was, for want of a better term, a self-taught engineer. An inquest considered, but did not advocate for, criminal charges against Mulholland.

As a result of this failure, enhanced dam safety legislation was enacted, and civil engineers were required to be licensed by the state.

Inset Box Video \(\PageIndex{1}\): The Story of the St. Francis Dam

This short video reviews the story of this dam and the ultimate disaster that unfolded.

Landslides

Due to the steep relief of the ranges within the Transverse Ranges Province, and the relatively unconsolidated or foliated nature of the rocks in these ranges, landslides are an additional hazard of this region. Some of these landslides are dramatic, forceful events and others are long-term creeping events. The two examples discussed here demonstrate that these processes are ongoing in this region. There are many other examples, however; one only needs to monitor the news!

The Blackhawk Landslide is a prehistoric landslide that can be observed to the north of the San Gabriel Mountains. The debris and trail of this mass of material can be readily viewed using aerial imagery (Figure \(\PageIndex{6}\)). It appears as a long “river” of sediment that meets a fan deposit at its foot. The structure is approximately 8 km (5 mi) long and 9-30 m (30-100 ft) thick. Fossils preserved in ponds on its surface indicate the slide is no younger than 17,000 years old.

Figure \(\PageIndex{6}\): The Blackhawk Slide was a mammoth landslide that came off of Blackhawk Mountain, on the north side of the San Bernardino Mountains, an estimated 17,000 years ago, which is recent in geologic time. It is spread across the Lucerne Valley in Southern California. "Blackhawk Slide" by Doc Searls is licensed under CC BY 2.0.

The story of this slide is incredible: it’s thought that roughly 17,000 years ago, Pennsylvanian limestone in the headwall of the slide (now situated to the south, in Blackhawk Canyon) collapsed as weathered metamorphic rock beneath it gave way. Once loose, this rock avalanche moved down-canyon at high speeds that reached 270 km/hr (170 mi/hr). Once the slide reached the flat areas to the north, it hit a small hill that acted as a ramp that launched the mass of debris as much as 120 m (400 ft) above the ground. Once airborne, the debris then traveled along a cushion of air at speeds as high as 385 km/hr (270 mi/hr) before collapsing. All told, the mass of rock fell more than 1200 m and traveled approximately 8 km (5 mi) in approximately 80 seconds!

Another example of an important mass wasting event in the Transverse Ranges Province is the Portuguese Bend Landslide Complex on the south side of the Palos Verdes Peninsula (Figure \(\PageIndex{7}\)) This large area of slope failure includes multiple slide areas that have been more or less continually moving since 1956, but initiated as early as 600,000 years ago. The site is situated on ancient landslides that appear to have been active as early as 400,000 to 600,000 years ago. The cause of the slide is related to the orientation of the main geologic unit here (the Monterey Formation, a series of shales, diatomites and tuffs that contain a type of clay called bentonite, which swells when wet). These mass movements are a combination of earth flow, and translational and rotational slides that are driven by gravity sliding along bedding planes. The sliding is amplified by the addition of moisture via irrigation and natural rainfall.

Figure \(\PageIndex{7}\): A photo of the Portuguese Bend Reserve on March 12, 2016. In the foreground is the Portuguese Bend landslide area. In the background are Inspiration Point, Portuguese Point, Long Point, and the Terranea Resort. "Portuguese Bend Reserve" by Pricetveyron is licensed under CC BY-SA 4.0.

Modern sliding in the Portuguese Bend region was first noticed when the area was under development in 1956. Despite this movement, large numbers of homes were built in the area. These homes were subsequently destroyed by sliding that occurred throughout the last part of the 20th century and into the 21st century at rates of approximately 0.2-11 m/yr (1 in/yr up to 35 ft/yr). As a result, the area of active sliding has been converted to a natural area with trails. However, the effects of ongoing sliding are apparent in the continued buckling of roads that cross the area and the hummocky nature of the region (Figure \(\PageIndex{8}\)). In addition, active slides in similar lithologies along the northern limb of the Palos Verdes anticline formed in 2023 in response to heavy winter rains. These slides have destroyed houses and roads in this heavily populated and desirable area.

Figure \(\PageIndex{8}\): Upset earth with small cracks and offsets less than 1 meter in the Portuguese Bend Landslide, March 12, 2016. "Portuguese Bend Landslide" by Pricetveyron is licensed under CC BY-SA 4.0.

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