1872 North Cascades earthquake facts for kids

The 1872 North Cascades earthquake happened on December 14, 1872, at 9:40 p.m. local time. It shook central Washington Territory, which is now Washington state. People felt the shaking strongly in several places, with a maximum Mercalli intensity of VIII (Severe). This means it caused serious damage.

Less intense shaking was felt in many other areas, including parts of Washington, Oregon, and British Columbia. Some places reported V (Moderate) to VII (Very strong) shaking. Even far away in Idaho and Montana, people felt light shaking (intensity IV). Because the earthquake happened in a remote area, and there were also many strong aftershocks (smaller earthquakes that follow a main one), not many buildings were damaged. Only a few cabins close to where the shaking was strongest were affected.

Since this earthquake happened before seismometers (machines that measure earthquakes) were used in the region, its exact size and location are not known for sure. Scientists have studied reports from people who felt the earthquake to guess its strength and where it started. One study suggested it was a magnitude 6.5–7.0 earthquake, possibly near Lake Chelan on the east side of the Cascade Range. Another study thought it might have been even bigger, placing it in the North Cascades, near Ross Lake, just south of the Canada–United States border.

What is an Earthquake?

Earthquakes happen when parts of the Earth's outer layer, called the crust, suddenly move. This movement usually occurs along cracks called faults. When the ground shakes, it sends out seismic waves. The strength of the shaking is measured in two main ways:

• Magnitude: This is a number that tells us how much energy an earthquake releases. It's like measuring the total power of the earthquake at its source.
• Intensity: This describes how much the earthquake was felt and how much damage it caused in different places. It's measured using scales like the Mercalli intensity scale, which goes from I (not felt) to XII (total destruction).

The Pacific Northwest region, which includes Washington, Oregon, and British Columbia, has many earthquakes. Some happen deep inside the Earth's crust, while others are closer to the surface. For example, a very large earthquake around 900 C.E. on the Seattle Fault might have caused a giant wave, called a tsunami, in Puget Sound.

Studying the 1872 Earthquake

This map shows how strongly the 1872 earthquake was felt in different areas. The lines connect places where the shaking was of similar intensity.

Because there were no seismometers in Washington state in 1872, scientists had to use old reports from people who felt the earthquake. These reports helped them create a map showing how strong the shaking was in different areas. This is called an Isoseismal map.

Scientists used a special method to figure out the earthquake's magnitude and where it started (its epicenter). They looked at how strong the shaking was in different places for twelve other earthquakes that happened later in the Pacific Northwest and had been measured by instruments. They used this information to understand the 1872 event.

One study suggested the earthquake's epicenter was near the south end of Lake Chelan. They estimated its magnitude to be between 6.5 and 7.0. However, other possible starting points were also considered, including areas north and northeast of the lake.

Another group of scientists used a similar approach. They compared the shaking patterns of the 1872 earthquake with those of other recorded earthquakes in the region. They knew that sometimes, the actual epicenter of an earthquake isn't exactly in the middle of where the strongest shaking was felt. This can happen because of the different types of ground and rocks, which can either make the shaking weaker (attenuate) or stronger (amplify).

These scientists looked at three possible locations for the 1872 earthquake. One was near Lake Chelan, where people reported a lot of ground disturbance. Their favorite spot was near Ross Lake. A third idea was a location north of the Canada–United States border. They chose the Ross Lake site because its shaking pattern matched their findings best. They also thought the earthquake probably happened between 25 and 37 miles deep, which is common for earthquakes in the Northwest.

More recently, scientists used a special technology called LIDAR to find a crack in the ground in Spencer Canyon. They believe this crack was caused by the 1872 earthquake, showing where the ground actually broke.

Damage from the Earthquake

Even though the earthquake was felt over a very large area, from the Pacific Ocean to Montana, and from British Columbia to Oregon, the area where it hit hardest was mostly empty. There were very few homes there.

One log building near the Wenatchee River had its roof logs fall off, and its kitchen separated from the rest of the house. Another log cabin between Entiat and Winesap also had damage to its roof. Even in the western part of Washington, near the more populated Puget Sound region, the shaking was strong (intensity VI).

Aftershocks

Aftershocks are smaller earthquakes that happen after a main earthquake. They occur as the Earth's crust settles down after the big shift. The 1872 earthquake had many aftershocks. For the first 24 hours, these aftershocks were strong enough to be felt over a wide area, from Idaho to southern British Columbia.

Over time, the aftershocks became weaker. However, they continued for at least a year, still being felt in places like Wenatchee, Lake Chelan, and Entiat. Scientists believe that having so many aftershocks is a strong sign that the main earthquake happened relatively close to the Earth's surface, not very deep down.

The Entiat area continued to have small earthquakes well into the 20th century. Some people wondered if these were very long-lasting aftershocks from the big 1872 event.