1857 Fort Tejon earthquake facts for kids

The 1857 Fort Tejon earthquake was a massive earthquake that shook central and Southern California on January 9, 1857. It happened around 8:20 in the morning, Pacific time. This earthquake was one of the biggest ever recorded in the United States.

Scientists estimate its strength was a moment magnitude of 7.9. The earthquake caused a long crack, or rupture, along the southern part of the San Andreas Fault. This crack stretched for about 225 miles (350 km), from a place called Parkfield to Wrightwood. Even though the earthquake started near Parkfield, it's called the Fort Tejon earthquake. This is because Fort Tejon was the place that suffered the most damage. Fort Tejon is located where the San Andreas and Garlock Faults meet, in an area with several mountain ranges.

This earthquake is the last really big one to happen on that part of the San Andreas Fault. The shaking was felt very strongly, reaching an intensity of IX (Violent) on the Modified Mercalli scale. This was especially true near Fort Tejon and along the fault line in the mountains. People's stories about the earthquake varied, including when the main shock happened and when smaller shakes, called foreshocks, occurred earlier that morning.

What Caused This Huge Earthquake?

The 1857 earthquake happened because of the movement along the San Andreas Fault. This fault is a major crack in the Earth's crust. It helps to handle most of the movement between two giant tectonic plates: the North American Plate and the Pacific Plate.

The Pacific Plate is slowly moving north compared to the North American Plate. It moves about 1.5 inches (38 mm) each year. This slow movement builds up stress along the fault. When the stress becomes too much, the fault suddenly slips, causing an earthquake. Scientists have studied this part of the San Andreas Fault. They have found signs of many earthquakes over the last 3,000 years.

The Earthquake's Big Shake

The 1857 earthquake caused a large part of the southern San Andreas Fault to rupture. However, it didn't break the entire fault line. Experts believe the slip stopped near Cajon Pass. This might be because the stress in that area was released during another earthquake in 1812.

On average, the ground moved about 15 feet (4.5 meters) along the fault. In some places, like the Carrizo Plain, the ground moved as much as 20 feet (6 meters)! This 7.9 magnitude earthquake was the last "Big One" for Southern California. The very end of the San Andreas Fault, near the Salton Sea, last ruptured in 1680.

The ground cracking from the earthquake might have gone even further than first thought. People reported seeing fresh cracks in the ground. These were not just old features of the land. In the north, the cracks extended 50 miles (80 km) north of Cholame. In the south, there were fewer people living near the fault. So, observations were mostly limited to the area between Fort Tejon and Elizabeth Lake. This area was along the main road at the time.

Scientists found tilted Jeffrey Pine trees south of Elizabeth Lake. These trees were growing directly over the fault line. Their lower parts were tilted, but their tops stayed straight. By studying their tree rings, scientists confirmed the trees started growing 10 to 25 years before 1857. After the earthquake, their rings grew thicker on the tilted side. This is how trees try to correct their tilt. This showed that the fault's movement caused the ground to shift and tilt the trees.

Smaller Shakes Before the Main One

This map shows how strongly the main earthquake was felt in different areas.

Before the main earthquake, there were smaller shakes called foreshocks. People felt these foreshocks between one and nine hours before the big one. Based on where these smaller shakes were felt, scientists think the main earthquake started (its epicenter) somewhere between Parkfield and Cholame. This is about 60 miles (97 km) to the northwest.

In 1857, people didn't use standard time zones like we do today. Everyone used their own local time, based on the sun. This made it hard to know exactly when the foreshocks happened. For example, the time in San Francisco was different from the time in San Diego. One person even reported different times for the same foreshock to two different newspapers! Standard time zones didn't become common until the 1880s.

The most common reports were about foreshocks felt 1, 2, and 4 hours before the main earthquake. These were called the predawn, dawn, and sunrise shocks.

• The predawn shock was felt in San Francisco, San Jose, and Santa Cruz.
• The dawn shock was felt in those places, plus Fort Tejon and possibly the Carrizo Plain.
• The sunrise shock was felt in San Francisco, Monterey, and Visalia.

Interestingly, Sacramento and Los Angeles did not report feeling any of these smaller shakes.

Scientists have looked at similar earthquakes that happened later. They believe the dawn and sunrise foreshocks were likely smaller earthquakes (between magnitude 5 and 6) that occurred closer to the coast of central California.

What Damage Did It Cause?

Most of the adobe buildings at Fort Tejon were badly damaged. Several people there were also hurt. More buildings were destroyed along a 20-mile stretch between Fort Tejon and Elizabeth Lake. This lake is a "sag pond" that sits right on the San Andreas Fault.

The earthquake also affected water sources. Streams and springs in San Diego and Santa Barbara Counties were disturbed. The Kern River, Kern Lake, and Los Angeles River all overflowed their banks. Farther north, in Santa Clara County, the flow of well water changed. Cracks appeared in the ground near the Pueblo de Los Angeles and in the Oxnard Plain. These cracks were caused by soil liquefaction, where wet ground acts like a liquid during shaking.

At the time, central and southern California didn't have many people. This probably helped limit the damage. But it also meant fewer people were around to report what happened. In downtown Los Angeles, the shaking was felt with an intensity of VI. Some homes and buildings cracked, but there was no major damage. In Ventura, the roof of Mission San Buenaventura fell in, and its bell tower was damaged. Farther north, the front wall of the old adobe Mission Santa Cruz chapel collapsed.

Sadly, two people died because of the earthquake. One woman was killed when her adobe house collapsed in Gorman. An elderly man in the Los Angeles area might have also died from the earthquake.

Shakes After the Main One

After the main earthquake, there were many smaller earthquakes called aftershocks. These continued for at least 3.75 years. However, the total number of large aftershocks was less than expected for such a big earthquake.

Four of the largest aftershocks were estimated to be stronger than magnitude 6.

• On the night of January 9, there was a large aftershock (about magnitude 6.25). It might have been near the Garlock Fault.
• The biggest aftershock happened on January 16 (about magnitude 6.7). It might have been offshore and was felt strongly in Southern California. Santa Barbara and San Bernardino felt it with intensity V, and Los Angeles felt it with intensity V and VI.
• Two important aftershocks happened in the San Bernardino area on December 15–16, 1858. The second one was about magnitude 6.
• The last major aftershock recorded was on April 16, 1860 (about magnitude 6.3). It was near the Parkfield section of the San Andreas Fault.

What About Future Earthquakes?

Scientists and public officials often wonder about the possibility of another very large earthquake in Southern California. They also think about the damage it could cause. The part of the San Andreas Fault that broke in 1857 has been quiet since then. This suggests that when it does break again, it might be another very large earthquake, similar to the 1857 event. After that, it might go quiet again for a long time.

Towns like Frazier Park, Palmdale, and Wrightwood are very close to the San Andreas Fault. But even areas farther away, like much of Los Angeles, could be badly affected if a similar earthquake happened again. A professor from the California Institute of Technology said that if the fault ruptured from Parkfield to Wrightwood again, the Los Angeles area would be severely impacted. The San Fernando Valley would be hit especially hard.

Two very important water systems, the Los Angeles Aqueduct and the California Aqueduct, bring water to Greater Los Angeles. Both of these systems cross the San Andreas Fault in the area that was most damaged by the Fort Tejon earthquake. This is in the Tehachapi Mountains and Sierra Pelona Mountains.

See also

In Spanish: Terremoto de Fort Tejón de 1857 para niños