Earthquakes and Plates
Mankind has long dreamed of a way to predict earthquakes and escape their terrible power. Scientists and amateurs alike have tried to link quakes to phenomena as diverse as animal behavior, tides, weather, the movements of the planets, the rise and fall of water in wells, even to psychic visions. Unfortunately, none of these factors can be linked consistently enough with earthquakes to provide a useful forecasting tool.
For now, the most reliable clues to future quakes are the patterns of past earth movements. These allow geologists to suggest where earthquakes are most likely to strike. Areas most at risk of earthquakes can develop and implement strategies to minimize harm to people and property when the inevitable occurs. Geologists are still seeking a way to predict exactly when a quake will happen, so that people can be evacuated and valuable or dangerous substances can be secured.
The map below shows the type of data that helped geologists understand where and why most earthquakes occur. Study the global distribution and depth of large earthquakes that occurred between 1975 and 1995. Then click on the map to compare the locations of earthquakes with plate boundaries. Compare the location of earthquakes and plate boundaries. Then answer the questions that follow.
- Notice that earthquakes are mainly concentrated in narrow bands. Explain why.
The map shows that most earthquakes happen along plate boundaries. It seems that the majority of earthquakes are caused by tectonic plates breaking in two, or banging and sliding against one another.
- Which parts of the United States have the most frequent earthquakes?
Alaska and the west coast states, near where the North American plate meets the Pacific plate, experience the most frequent earthquake activity.
Look again at the map, and see that earthquakes can begin close to the Earth's surface or deep inside the crust. Geologists classify earthquakes as shallow (those that start 70 kilometers or less underground), deep (earthquakes beginning 300 km or more below the surface), and intermediate (quakes between 70 and 300 km). Earthquakes can only start within the solid rock of the lithospheric plates, not in the soft mantle below.
- Shallow earthquakes occur at all plate boundaries, and even in the middle of plates. Deep earthquakes only occur at some plate boundaries - why?
Deep earthquakes can happen only where there is solid rock hundreds of kilometers below the surface. That only occurs at convergent boundaries, where plates are sometimes pushed deep beneath the surface and into the mantle.
- Why might the depth of an earthquake be related to the amount of damage it causes?
The deeper the earthquake, the farther it is from the people and buildings on the surface. Damage will be less severe in a deep earthquake because the energy of the earthquake will be dissipated by the distance it must travel before it reaches the surface.
Now lets consider two of the most important earthquakes to strike the United States.
In 1906, a shallow earthquake struck San Francisco. The ground surface jumped horizontally as much as 30 feet along a 300-mile long crack. Thousands of people died in collapsed buildings and in fires that swept the city after the quake.
- Locate the site of the San Francisco earthquake on the map above - it began at 38ºN 122º W. Is this close to a plate boundary?
Yes, the boundary between the North American and Pacific plates runs right through San Francisco.
- What type of plate boundary was responsible for the earthquake?
Since the ground moved sideways in the earthquake, it must be a transform boundary, where plates slide past each other.
The largest known earthquake to hit the continental United States began near the town of New Madrid, Missouri in 1812. It affected an area more than 10 times larger than the 1906 San Francisco earthquake - the shaking even rang church bells more than 1,000 miles away. The Mississippi River was thrown out of its channel and ran backwards for a time, and the terrain was so distorted that lifelong residents got lost on their own land.
- Locate the site of the New Madrid earthquake on the map above - it began at 37.5ºN 90º W. Is this close to a plate boundary?
No, there are no plate boundaries near the central and eastern United States. Even though the vast majority of earthquakes strike near the edge of a plate, some do hit the middle of plates.
- Was this a shallow or deep earthquake?
It must have been a shallow earthquake - deep earthquakes are possible only at some plate boundaries. And the extreme amount of damage is also more consistent with a shallow earthquake.
Critical ThinkingThe 1812 New Madrid earthquake was actually only one of a series of powerful quakes to hit the area in the winter of 1811-1812, including 3 that are among the 10 largest earthquakes in U.S. history. Fortunately, very few people were killed in these events. The next time the area experiences such an enormous quake, how do you think the death toll would compare? Justify your answer.
Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers.
The death toll will undoubtedly be very high in the next large earthquake. In the early 1800’s, there weren’t too many people living in that part of the country, but now there are millions, especially in the large cities of St. Louis and Memphis. And this area seems like such a surprising place to have big earthquakes that it is unlikely that the people living there are prepared for the next one. In a place like California, earthquakes happen all the time, so there are strict building codes and most residents are aware of ways to safeguard their property. In the Mississippi River area, earthquakes are uncommon, so building standards are lower and most inhabitants are unaware of what to do to protect themselves before or during an earthquake.