Earthquakes

Earthquakes Part 1

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What are Earthquakes?

Imagine bending a stick until it breaks. When the stick snaps, it vibrates, releasing energy. Earthquakes release energy in a similar way. Earthquakes are the vibrations in the ground that result from movement along breaks in Earth’s lithosphere. These breaks are called faults. 

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Why do rocks move along a fault? The forces that move tectonic plates also push and pull rocks along a fault. If these forces become large enough, the blocks of rock on either side of the fault can move past each other. The rocks might move vertically—up or down—or horizontally—sideways.

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The size of an earthquake depends on the amount of force applied to the fault. The greater the force applied to a fault, the greater the chance of a large and destructive earthquake occurring.

Earthquakes can cause billions of dollars in damage. Injuries and fatalities often occur during earthquakes. Earthquakes are common in the state of California. In 1994, the Northridge earthquake along the San Andreas Fault in California caused $20 billion in damage.

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Why do rocks move along a fault? The forces that move tectonic plates also push and pull rocks along a fault. If these forces become large enough, the blocks of rock on either side of the fault can move past each other. The rocks might move vertically—up or down—or horizontally—sideways.

Where do Earthquakes occur?

Few earthquakes occur in the middle of a continent. Most earthquakes occur along active plate boundaries. These areas are generally in the middle of ocenas or along the edges of continents.

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Where do Earthquakes occur?

Stress builds up along plate boundaries. Earthquakes result from the buildup and release of this stress along the active plate boundaries. The deepest and strongest earthquakes occur along convergent plate boundaries. At a convergent plate boundary, plates collide. The denser oceanic plate subducts, or drops down, into the mantle. These earthquakes release great amounts of energy.

Where do Earthquakes occur?

Shallow earthquakes are common along divergent boundaries, where plates separate. They can also occur along a transform plate boundary. Earthquakes occur at varying depths where continents collide. Continental collisions form large, deformed mountain ranges.

Rock Deformation

Force, or pressure, applied along plate boundaries can cause a body of rock to bend and change shape in a process called rock deformation. Over time, this can eventually break the rock.

Faults

A fault is a break in Earth’s lithosphere where one block of rock moves toward, away from, or past another. When rocks move in any direction along a fault, an earthquake occurs. The table on the next slide describes three types of faults. The forces applied to a fault determines the direction the rocks move. 

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Earthquake Focus and Epicenter

When rocks move along a fault, they release energy in seismic waves. Seismic waves originate along the fault where rocks first begin to move. This location inside Earth is the focus of the earthquake. Earthquakes can occur anywhere between Earth’s surface and depths greater than 600 km. In a news report, you might hear a reporter identify the earthquake’s epicenter. The epicenter of an earthquake is the location ON earths surface directly above the focus.

Seismic Waves

During an earthquake, there is a rapid release of energy along a fault. This release of energy produces seismic waves. The waves travel outward in all directions through rock, much like ripples in water. As the waves travel, they transfer energy through the ground and produce the motion associated with an earthquake. The energy released is strongest near the epicenter

Three Kinds of seismic waves

During an earthquake, particles in the ground can move back and forth or up and down. Particles can also move in an elliptical motion parallel to the direction the seismic wave travels. Scientists use wave motion, wave speed, and the type of material the wave travels through to classify seismic waves. The three types of seismic waves are primary waves, secondary waves, and surface waves.

Primary Waves

Primary waves, also called P-waves, cause particles in the ground to move in a push-pull motion, similar to a coiled spring. P-waves move faster than any other seismic waves. They are the first waves detected and recorded after an earthquake.

Secondary Waves

Secondary waves, also called S-waves, cause particles to move up and down at right angles relative to the direction that the wave travels. These waves move like a coiled spring when it is shaken side-to-side and up and down at the same time

Surface Waves

Surface waves cause particles in the ground to move up and down in a rolling motion, similar to ocean waves. Surface waves travel only on Earth’s surface. P-waves and S-waves can travel through Earth’s interior. However, scientists have discovered that S-waves cannot travel through liquid. 

Multiple Choice

Which waves reach a station first?

• S waves

• P waves

• Surface waves

• ocean waves

Information Gathered

Scientists who study earthquakes are called seismologists.

A seismometer measures and records how much the ground moves and can be used to determine the distance seismic waves travel

A seismogram is an illustration of the seismic waves

Determining Earthquakes Magnitudes

The Richter magnitude scale uses the amount of ground motion at a given distance from an earthquake to determine magnitude. Each increase of one unit on the Richter scale represents 10 times the amount of ground motion. For example, a magnitude 8 earthquake produces 10 times greater shaking than a magnitude 7 earthquake does and 100 times greater shaking than a magnitude 6 earthquake does (10 × 10).

Mercalli Scale

 The Modified Mercalli scale measures the intensity of an earthquake based on descriptions of its effects on people and structures. The scale, shown below, ranges from I, an earthquake that people do not feel, to XII, an earthquake that destroys everything. The higher the number is, the greater the effects. An area’s geology also influences earthquake damage. The shaking produces more damage in areas covered by loose sediment than it does in places built on solid bedrock.

Earthquake risks

In the United States, most earthquakes occur near transform faults and covergent plate coundaries. The transform plate boundary in California and the convergent plate boundaries in Oregon, Washington, and Alaska have the highest earthquake risks. However, not all earthquakes occur near plate boundaries. Some of the largest earthquakes in the United States have occurred far from plate boundaries.

Seismologists asses earthquake risk based on past earthquake activity, the geology around a fault, population density, and building design.

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