1838 San Andreas earthquake facts for kids

The 1838 San Andreas earthquake was a powerful earthquake that likely happened in June 1838. It caused a break along the northern part of the San Andreas Fault. This break stretched for about 100 kilometers (62 miles). It went from the San Francisco Peninsula all the way to the Santa Cruz Mountains.

This earthquake was very strong. Scientists estimate its size was between 6.8 and 7.2 on the moment magnitude scale. This makes it one of the biggest earthquakes known in California's history. At that time, not many people lived in the area. However, buildings were damaged in places like San Francisco, Oakland, and Monterey. We don't know if anyone died. Based on studies of the ground, the fault moved about 1.5 meters (5.0 feet) during the quake.

For many years, people thought another big earthquake happened two years earlier, in 1836, on the Hayward fault. But now, experts believe that story was actually about the 1838 San Andreas earthquake. There is no real proof that a large earthquake hit the region in 1836.

Life in California in the 1830s

During the 1830s, California had very few large towns. Most settlements had only a few hundred people. This was true even in the area between the San Francisco Peninsula and the Santa Clara Valley.

Native American tribes did not keep written records. Communication between villages was also poor. There were no local newspapers, and no reporters from the distant United States.

After 1833, the Spanish missions were no longer controlled by the church. These missions used to be the main source of information about past earthquakes. After they changed, there was less information from Mexican sources too.

Later, the California Gold Rush brought many changes. New newspapers started in the Sierra Nevada mountains and the San Francisco Bay Area. Scientists later used these newspaper records to study earthquakes in the area.

Studying Earthquakes in the Past

It was hard for scientists to learn about old earthquakes. This was because there were not many records. The science of seismology (the study of earthquakes) was still very new. There were no special tools to measure or record earthquakes. Even if there were tools, there were no scientists in California to use them.

Special instruments called seismometers were invented later. John Milne designed some by 1896. But it wasn't until 1926 that the Wood-Anderson seismograph [pt] became widely used in California.

Old earthquake records show that the years 1836–1840 were very active for earthquakes.

How Earthquakes Happen in California

The San Andreas Fault (SAF) system is a group of active faults. These faults move sideways past each other. They are part of a larger, complex boundary between the Pacific and North American plates. Think of it like two giant puzzle pieces of the Earth's crust slowly grinding past each other.

This zone where the plates move extends east into the Basin and Range Province of Eastern California and western Nevada. The faults run both on land and offshore along California's coast. Near San Diego, they are about 150 kilometers (93 miles) wide. Near San Francisco Bay, the different fault lines are about 80 kilometers (50 miles) wide.

The San Andreas Fault system ends in the north at the Mendocino Triple Junction. Here, the San Andreas Fault meets other fault zones. In the south, it gradually ends near the Salton Sea. There, the movement changes to a series of other faults and spreading centers.

The northern part of the San Andreas Fault has four main sections. These are the Santa Cruz Mountains (or Loma Prieta), Peninsula, North Coast, and Offshore sections. All these sections broke during the huge 1906 San Francisco earthquake.

History of Earthquakes on the San Andreas Fault

Scientists learn about old earthquakes on the northern San Andreas Fault by studying the ground. This is called paleoseismic investigation. They look for signs of past quakes before 1776, when the first missions were built in California.

Scientists have found signs of several past earthquakes. The most recent one before 1838 happened between 1660 and 1780. This quake likely broke a very long section of the fault, over 250 kilometers (155 miles). It affected the Offshore, North Coast, and Peninsular sections.

Since 1776, possible earthquakes on the fault include events in 1836, 1838, 1839, 1865, 1890, and 1906.

• The 1836 event is now thought to have been a smaller earthquake, and it's not clear which fault it was on.
• The 1839 event is now believed to be a mistaken report of the 1838 earthquake.
• The 1865 event only affected the Santa Cruz Mountains. It was a different type of fault movement, not on the San Andreas Fault itself.
• The 1890 earthquake likely happened on the Santa Cruz Mountains section. Its estimated size was between 6.0 and 6.3.
• There has not been a large earthquake on the northern part of the San Andreas Fault since 1906.
• The 1989 Loma Prieta earthquake was also a different type of fault movement, not directly on the San Andreas Fault.

Damage from the 1838 Earthquake

We don't have many detailed reports about the damage from this earthquake. Most of what we know comes from a few written accounts. Many of these were written long after the earthquake happened.

Damage was reported from the northern end of the San Francisco Peninsula down to Monterey in the south.

• On the peninsula, some buildings at the Presidio had cracked walls. The walls of the Mission San Francisco Dolores were badly damaged.
• Across the bay, large cracks appeared in the ground near the waterfront in Oakland.
• An adobe (mud-brick) house was destroyed in San Leandro. A large landslide was seen in the nearby hills.
• The mission buildings in San José and Santa Clara were badly damaged. A house collapsed in San Jose town.
• In Woodside, adobe buildings cracked badly. People reported that the "ground (moved) in waves like the ocean."
• A long crack in the ground was described, stretching from near Lone Mountain to the Santa Clara mission.
• Many trees were pulled out of the ground and thrown sideways. This was also seen during the 1906 earthquake.
• In Santa Cruz, reports of damaged buildings shortly after the quake might be from this event.
• In Monterey, some adobe walls were damaged, and dishes and glassware broke.

Scientists have also studied large rockfalls in the Sierra Nevada mountains. They use a method called lichenometry to figure out when these rockfalls happened. Lichen grows on rocks, and its size can tell us how old the rockfall is.

They found four main groups of rockfall ages: 1817, 1837, 1857, and 1909. Each age has about a 10-year uncertainty. These rockfalls are thought to be linked to major San Andreas earthquakes in 1812, 1838, 1857, and 1906. The number of rockfalls from 1837 was similar to those from 1857 and 1909. This suggests the 1838 earthquake had a similar effect on the Sierra Nevada mountains as the other two major quakes.

The Earthquake Details

Because there are so few records from that time, we are not completely sure about the exact time, date, size, or how much of the fault broke during the 1838 earthquake.

We know the earthquake happened in late June 1838. It occurred shortly after midday.

Scientists estimate the earthquake's size based on:

• How large an area was affected.
• How strong the shaking was (maximum intensity).
• How long the fault broke and how much it moved.

This gives a magnitude range of 6.8 to 7.5. The strongest shaking was estimated to be at least VII–VIII (very strong to severe) on the Modified Mercalli intensity scale. This is based on reports of damage to mission buildings in Santa Clara, San Francisco, and San Jose. Some experts think the shaking might have even reached IX (violent).

The description of the earthquake in Woodside, along with reports of a possible break on the surface in the same area, suggests that part or all of the Peninsula section of the fault broke. Studies of ancient earthquake activity (paleoseismic investigations) north of Woodside also support this.

The shaking in Monterey was similar to that in the 1906 earthquake. This suggests that part or all of the Santa Cruz Mountains section of the fault also broke. The longest possible break is 140 kilometers (87 miles), if both sections of the fault broke completely.

Scientists have dug trenches across the fault in the Santa Cruz Mountains to look for signs of the 1838 earthquake. They found evidence consistent with the 1838 earthquake at several sites. This suggests that at least 14 kilometers (8.7 miles) of this fault section broke.

The amount the fault moved during the 1838 earthquake has been estimated at several sites. It moved about 1.6 meters (5.2 feet) at Filoli, 1.5 meters (4.9 feet) at Hazel Dell, and 1.0 meter (3.3 feet) at Mill Canyon and Arano Flat. If we assume the total break was about 100 kilometers (62 miles) long, this gives a magnitude range of 6.8–7.2.

Aftershocks

One old account mentions many aftershocks. These aftershocks continued for at least two and a half months. Some larger aftershocks in 1840 and 1841 are also thought to be related to the 1838 earthquake. These quakes were about magnitude 6. They happened near San Juan Bautista. This supports the idea that the 1838 break extended into the Santa Cruz Mountains.