Los Angeles’ fractured and filled landscape: a field trip to the sites

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Deborah Painter (US)

The Los Angeles Times reported on 5 April 2021 that a magnitude 3.3 earthquake struck around 4:15a.m., followed by a magnitude 4.4 quake 29 minutes later. Several aftershocks followed. Seismologist Lucy Jones of the Lucy Jones Center reported that two quakes were 19.31km deep, with an epicentre around Inglewood, in the Los Angeles Basin. She reported that the movement was thrust, probably not on any mapped fault.

Californians scarcely even notice an earthquake of magnitude 3.3 at that depth below the surface. That magnitude on the Richter scale is in the order of a large truck driving rather close by, but probably not intense enough to awaken them from sleep at that hour of the morning while being intense enough to warrant a news item.

If one believes television programmes and movies, “The Big One” is going to happen sometime in the near future and part of California is going to slide into the Pacific Ocean and vanish beneath the waves, much like an overloaded barge. The trope of the sinking Golden State gained popularity sometime in the 1960s and should have been thoroughly discredited by now. The film industry helped get this into the general public’s mind and the general public keeps it alive. However, it would be impossible for two reasons:

  • Firstly, tectonics is not going to cause the land to subside as though it were a huge chunk of the crust precariously teetering over the edge of the continent. California is firmly attached to the rest of the crust.
  • Secondly, tectonic motion is slow… between 30mm and 50mm a year. The future fate of Los Angeles is to be adjacent to San Francisco, many millions of years hence.

The San Andreas, the major (and most famous) fault in California, is approximately 64km east of Los Angeles. It is a strike-slip fault – the west flank moves northwest and the east moves southeast. In fact, the Fault is actually being drawn closer together in several areas. There are numerous smaller faults, both mapped and unmapped, occurring in the Los Angeles area. And as we shall see in this discussion, the Los Angeles Basin is actually growing in height.

In 1930, the Los Angeles Basin was producing close to 25% of the world’s petroleum supply. It has decreased substantially from this output, but 16 oil companies still extract petroleum at a localised area within the Inglewood Oil Field, of which the Baldwin Hills field is part.

But why are there fossil fuels beneath Los Angeles? The answer lies in the Basin’s evolution.

The silt, clay and sand that comprise the Los Angeles Basin extend 9.1km until the Catalina Schist bedrock is reached. In essence, the Basin is a gigantic sand filled depression. The walls of this depression are the Santa Monica Mountains and San Gabriels to the north, the Santa Anas to the south and east, and the Palos Verdes peninsula (once an island off the coast) to the west (Fig. 1). The faults under this bowl of sediment make for interesting earthquakes, since the sediment moves unevenly.

Fig. 1. This aerial view from a commercial jet shows the Palos Verdes Hills west of Long Beach extending into the Pacific. The Palos Verdes Hills were once an island but which were eventually surrounded on three sides by sediments. (Credits: Deborah Painter.)

During the Oligocene epoch, approximately 24 million years ago, the Santa Monica Mountains uplifted, rotating clockwise due to subduction of the Farallon Plate beneath the North American Plate, and stretched due to tectonic stresses. (Fig. 2). Some volcanism resulted. A depression was created and silt, clay and other sediments from the Pacific, as well as from now vanished rivers, poured into the resultant depression. Rapid filling of the Basin during the Miocene preserved the organic content of the sediment, that is, both macro- and microorganisms, in stagnant, cool water bodies such as lakes. The organic content formed petroleum and thus the oil rich sedimentary strata came into being.

Fig. 2. Los Angeles’s unusual geography and petroleum reserves are caused by subduction and strike-slip faulting over thirty million years, with the subduction of the Farallon Plate beneath the North American Plate and, later, strike-slip faulting resulting from the Mendocino and Rivera Triple Junctions that altered the Farallon Plate. (Credits: US Geological Survey, Department of the Interior/USGS.)

Continued subduction split the Farallon Plate into two, the Juan de Fuca Plate to the north and the Cocos Plate to the south. The Farallon Plate no longer existed as a single tectonic plate. By ten million years BP, subduction in the area of Los Angeles had ceased and the Southern California coastline was subjected to strike-slip fault movement.

About five million years ago (the Pliocene epoch), this stretching ceased and the crust here began shrinking. The basin continued to fill due to ongoing sedimentation from sea and land sources, while tectonic activity pushed the Basin upward. The former Palos Verdes Island was now no longer an island, since the Basin sediments spread out to meet it. Thus, the Los Angeles Basin is actually expanding horizontally and vertically due to deposition and tectonic activity. An early Pleistocene uplift, due to ongoing tectonic activity, is responsible for the Baldwin Hills, which is a fault scarp. The Hills are composed of sandstone and gravel. (See Fig. 3 for place names and locations.)

Fig. 3. The Los Angeles Basin and surrounding geography with field trip sites labelled. (Credits: US Geological Survey, Department of the Interior/USGS.)

The Basin’s geography, surrounded by three mountain ranges with the sea providing the only opening, created the smog situation prior to today’s emissions regulations. Temperature inversions held it in place. The Basin feature has historically trapped partly burned hydrocarbons and motor vehicle, and oil refinery generated emissions and created what became known as “smog”, which caused the emissions to hang over Los Angeles for days

. Because of this, cars were redesigned to run on unleaded petrol. This made possible the catalytic converter. Now, the state of California has the most stringent air emissions standards in the United States and automobiles must now have “smog checks” completed by qualified mechanics. The state’s Air Resources Board estimates that new automobiles sold in California in 2019 are ten times cleaner than those sold in 1999. Considering that, during that time, there was a 50% increase in motor vehicles licensed to operate, this demonstrates some success.

The geography is also the reason for the pleasant, semi arid climate. Seldom does the temperature go below 10oC in winter. In the late summer and early autumn, when temperatures are highest, the highs generally do not exceed 30oC. Most rainfall occurs in winter and early spring.

If one spends one’s time in the flat Basin and does not explore the hills of the Baldwin Hills near Culver City (a neighbourhood in the Basin) or the taller Santa Monica Mountains, one does not see the fractured terrain that in quiet times reminds one that this is a tectonically active area.

The field trip

The field trip in this article is recommended to be no fewer than five days in length, but can be as long as one wishes. It starts on Aviation Drive near the Los Angeles International Airport automobile rental companies and ends at Malibu Lagoon State Beach through the Santa Monica Mountains. This should take us first to a landmark in the Inglewood area of Los Angeles near Aviation Drive – “Randy’s Donuts”.

This iconic doughnut shop is located several blocks to the northeast (Fig. 4). Take Arbor Vitae west, then take a right to La Cienega Boulevard. Randy’s is at the corner of La Cienega and West Manchester at 805 West Manchester Boulevard. Proceeding east on West Manchester should take us to South La Brea Avenue, which will serve us well for our purposes. Go north. South La Brea Avenue will take us over the Baldwin Hills, one of a few Pleistocene fault scarps in an otherwise flat basin.

Fig. 4. Randy’s Donuts is a landmark that shows the way to South La Brea Avenue. (Credits: Pixabay/WikiImages.)

Stop 1: Nodding Donkeys near the Kenneth Hahn State Recreation Area

Many oil rigs swing rhythmically to and fro and are visible from the roadway (Fig. 5). The Kenneth Hahn State Recreation Area, with picnic tables, a fishing lake, a playground for small children, a sand volleyball court, a soccer field and hiking trails, is near these oil fields at 4100 South La Cienega Boulevard. There is a small parking fee. A shuttle will take us to a scenic overlook. Returning to South La Brea, continue north. Soon, after passing through Vietnamese, Mexican and Orthodox Jewish neighbourhoods, and their shops and wonderful food aromas, we will reach Wilshire Boulevard. Several long city blocks east of this intersection is the famous La Brea “tar pits”, but that is a visit reserved for another article.

Fig. 5. An oil rig “nodding donkey”. (Credits: John R Perry.)

Stop 2: Bronson Caves

Continue north on South La Brea and our roadway becomes North La Brea Avenue. Turn right at Hollywood Boulevard. Continue past the tourist areas of Grauman’s Chinese Theatre, past the Grauman’s Egyptian Theatre, the towering “Babylonian set” shopping courtyard modelled after the Gates of Babylon set used in the 1916 film Intolerance, and the ultramodern Dolby Theatre. Travel east several kilometres on Hollywood, and, when we reach the UNESCO World Heritage Site of Frank Lloyd Wright’s Hollyhock House at the Barnsdall Art Park on the right, we will be close to Sunset Drive.

Cross Hollywood Boulevard at its intersection with Vermont Avenue. Bear to the right to Sunset Drive. At 4365 Sunset Drive, covering half a city block, is the old Monogram Studios (Fig. 6), a former studio for still-popular low budget but entertaining mystery and Western films and television programmes from the 1930s until the 1950s when it became Allied Artists Pictures Corporation and eventually ceased operation.

Fig. 6. Monogram Studios is located within the upper Los Angeles Basin. (Credits: Deborah Painter.)

We are now about to leave the Basin and go to the Santa Monica Mountains. Going west on Sunset Drive, turn in the direction of Hollywood Boulevard, turn right onto Hillhurst, then left onto Franklin Avenue, right onto Canyon Drive, and proceed north until reaching 3200 Canyon Drive, where there is a car park which is part of Griffith Park. The municipal park is the eastern extent of the Santa Monicas.

A short hike will take us to Bronson Caves. There are two, and they are in a former quarry used in many television programmes and films (Fig. 7). Note the broken appearance of the strata in this quarry and the deformed sandstones caused by multiple earthquakes (Fig. 8). The “caves” themselves are tunnels excavated for ease of use by quarry workers and now are open to the public (Fig. 9). The Hollywood Sign is prominent from the vantage point of the quarry, where it is visible for miles from atop Mount Lee.

Fig. 7. Bronson Canyon is mostly an old quarry, now part of Griffith Park. (Credits: Michael Ramsey.)

Fig. 8. Note the deformation of the sandstone strata in the centre of the photo taken at Bronson Canyon. (Credits: Michael Ramsey.)

Fig. 9. The “caves” are used for motion pictures and television programmes. (Credits: Michael Ramsey.)

Stop 3: The Warner Brothers Studio Tour (a non-geological digression)

Leaving this area of Griffith Park, we proceed to Canyon Drive, then to Carolus, then right to North Bronson, and then right onto Franklin. Merge onto US 101 northbound. Take Exit 11 A to Barham Road, merge onto Cahuenga, then right onto Barham, thence Olive Avenue, right onto Riverside, then right onto South Avon, to park at the car park for the Warner Brothers Studio Tour in Burbank, California. There is a charge to take the tour, which includes a museum and both interior and exterior sets. The typical American small town exterior has the Santa Monica Mountains as a backdrop (Fig. 10). Usually, clever camera angles on the part of the cinematographer will keep audiences from seeing the mountains.

Fig. 10. Warner Brothers owns many acres used for exteriors. The snow in this small town set is the variety created for use in ski resorts. The Santa Monica Mountains form a backdrop. (Credits: Deborah Painter.)

Stop 4: Topanga State Park

After completing this tour, head to Topanga State Park using South Avon Street, turning left onto West Riverside Drive, then right at the second cross street onto North Hollywood Way. Turn left onto West Alameda Avenue, and merge onto California 134 West. Take Exit 1A on our left for U. S. 101/Ventura Freeway. After several kilometres, take Exit 27 B for California 27/South Topanga Canyon Boulevard. Take South Topanga Canyon Boulevard and turn left onto Entrada Road. Pay a small parking fee and take advantage of the many hiking trails.

A short climb brings hikers from the woodland near the car park to a fragrant scrub community known as “chapparal”, consisting of elderberry, coffeeberry, poison oak, manzanitas, purple sage, and black sage, where no plant grows taller than about 4.5m (Fig. 11). The most commanding presence in the scenery in this part of the Park is a “spur ridge”, offset from the body of the rest of the mountain by a considerable amount. It, like similar sandstone ridges in the Park, is the result of the endless series of earthquakes that continue to create the mountains (Fig. 12).

Fig. 11. The Santa Monicas are approximately 950m above mean sea level at Topanga State Park. (Credits: Deborah Painter.)

Fig. 12. Fault scarps like this sandstone spur ridge at Topanga State Park result from tectonic activity. (Credits: Deborah Painter.)

Miocene age marine fossils of small shells are visible in the sandstone cobbles along the Santa Ynez Trail. The Topanga Formation displays areas of basaltic rock at the surface associated with 17 million year old (early Miocene) lava flows due to rifting. Near Topanga State Park is the entrance to Nature Conservancy lands. Pulling off at a northward facing car park area here treats us to a marvellous view of the San Fernando Valley with the city of Tarzana in the near distance. It was named for “Tarzan”, the literary creation of Edgar Rice Burroughs, who prospered so much from the Lord of the Jungle that he purchased a large ranch that he later sold off as lots. In the far distance are the Santa Susana Mountains (Fig. 13).

Fig. 13. From the Nature Conservancy land holdings just north of the state park, we can see the San Fernando Valley with the city of Tarzana spread out below and the Sant Fernando Valley, with the city of Tarzana spread out below and the Santa Susanas in the far distance. (Credits: Deborah Painter.)

Stop 5: Malibu Lagoon State Beach

Our last stop on this field trip is the Malibu Lagoon State Beach. Leaving Topanga’s car park off Entrada Road, head northwest on Entrada Road back toward California 27 North. Turn left onto California 27 South/South Topanga Canyon Boulevard. Drive several kilometres and then turn right onto California 1 North. Proceed for several more kilometres, make a U-turn at Webb Way and park at the car park at Malibu Lagoon State Beach, 23200 Pacific Coast Highway, Malibu (Fig. 14). There is a small day parking fee.

Fig. 14. Malibu Lagoon State Beach is easy to reach from the Pacific Coast Highway. (Credits: Deborah Painter.)

The beach, though not especially wide at this point along this erosional coastline, boasts a lagoon with white pelicans and many species of gulls, sparkling sand, tidal wetlands and wildflower gardens (Fig. 15). Malibu Creek drains directly into the sea. The blue Pacific Ocean entices us and we can swim in confidence if a lifeguard is present (Fig. 16). It is a peaceful sojourn for relaxation after navigating the often congested streets of the enormous city and vigorous hiking in the mountains.

Fig. 15. Malibu Lagoon provides habitat for fish, shellfish and various species of birds. (Credits: Deborah Painter.)
Fig. 16. The Pacific Ocean shoreline here at Malibu is an erosional coastline typical of most of the West Coast. (Credits: Deborah Painter.)

About the author

Deborah Painter is an ecologist and general environmental scientist specialising in transportation, energy and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.


Bowden, John. Magnitude 4.0 and 3.3 earthquakes rattle Los Angeles The Hill April 5, 2021: https://thehill.com/homenews/state-watch/546459-magnitude-40-and-33-earthquakes-rattle-los-angeles

California Department of Parks and Recreation Kenneth Hahn State Recreation Area page: https://www.parks.ca.gov/?page_id=612

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