EAS-A193 Class Notes

An Introduction to Earthquakes & Earthquake Hazards


What is an Earthquake?

An earthquake is the sudden movement of the ground that releases elastic energy stored in rocks and generates seismic waves.

These elastic waves radiate outward from the "source" and vibrate the ground.

Earthquakes are not the exclusive cause of seismic wave; explosions, planes, wind, storms, and people also vibrate the ground.

Faults & Earthquakes

In an earthquake, the initial movement that causes seismic vibrations occurs when two sides of a fault suddenly slide past each other. A fault is a large fracture in rocks, across which the rocks have moved.


Faults can be microscopic or hundreds-to-thousands of kilometers long and tens of kilometers deep. The width of the fault is usually much smaller, on the order of a few millimeters to meters.

Fault Cartoon

Earthquake Size

The size of an earthquake depends on

  • The area of the fault that ruptured.
  • The distance that the rocks on the two sides of the fault slide past one another.

Small earthquakes rupture small faults or small sections of large faults. Fault movement during such events is quick, small quakes last only a fraction of a second and the rocks on either side of the fault don't move very far.

Large earthquakes rupture faults that are tens to thousands of kilometers long. Such ruptures can take minutes to complete, so strong shaking near the earthquakes can last several minutes and rocks across the fault can be offset tens of meters during very large earthquakes.

Earthquake Magnitude and Frequency

The most commonly used quantification of earthquake size is the magnitude.

Magnitude is an instrumental measure of the amplitude of ground shaking; that is, you must have an instrument called a seismograph to measure the magnitude of an earthquake.

Fortunately, large earthquakes are less frequent that small earthquakes. The temporal distribution of earthquakes by size follows a logarithmic rule.

eq stats table
As a rule of thumb, for each magnitude unit increase, there are 10 times less earthquakes during a specified time interval. For example there are about 20 major earthquakes each year but only one or two great earthquakes.

Earthquake Sequences

Earthquakes are not isolated events, they occur in sequences. Most often, each sequence is dominated by an event with a larger magnitude than all others in the sequence (usually about one magnitude unit larger).

We call the large event the mainshock, and the events that follow are called aftershocks.

Occasionally, the mainshock is preceded by an event or events that we call a foreshock(s).

Sometimes, earthquakes occur in interesting sequences which we call doublets, triplets, multiplets, or swarms depending on how many similar-size events are in the sequence.

What Causes Earthquakes?

Earthquakes are the result of slow-moving processes that operate within Earth.

Earth was hot when it formed, and has been cooling ever since (near the surface, for each km into Earth, the temperature rises by about 30deg. Celsius).

Earth's cooling causes the portions of Earth to move, and that movement is what we call an earthquake.

We will discuss the details later.


Earth Cross Section

Earthquake Catastrophes

Earthquakes in populated regions have killed many people and have destroyed entire communities.

China has been hit especially hard

  • Shensi, China, January 23, 1531 more than 800,000 dead
  • Tangshan, China, July 28, 1976 almost 250,000 dead

In recent years (the 1990s)

  • Iran 40,000
  • Philippines 1,700
  • India 10,000

In the US, we have been fortunate to avoid such disasters, the largest earthquakes in U.S. history occurred decades ago, before the dramatic increase in population in earthquake-prone regions.

A Century of Earthquake-Related Deaths

During the last hundred years there has been an average of about 17 large earthquakes each year.

Millions have lost there lives in significant earthquakes. Although these deaths occur in discrete events, if we average that number killed over the century, we lose about 15,000 people per year.

The Cost of Earthquakes

Cleaning up and repairing damage after earthquakes is expensive.

  • October 1989 - San Francisco Bay Area, California - $5.6 billion
  • January 1994 - Northridge, California - $15 billion
  • January 1995 - Hyogoken-Nanbu (Kobe), Japan - $150 billion

Seismic Hazard in the United States

Alaska is the most seismically active state, the host of seven largest earthquakes in U.S. history. Hawaii is also earthquake-prone, and although dangerous, these events are not often the of the largest size.

California is the most active of the lower 48 states, and its large population increases the potential for a disaster.

Other states that have experienced damaging earthquakes include Washington, Oregon, Nevada, Idaho, Utah, Wyoming, Missouri, Arkansas, and South Carolina.

The 1811-1812 earthquake sequence that struck the New Madrid region of southeast Missouri were large, but they probably do not rank in size with large earthquakes in California and Alaska. Often formally listed as greater than 8, more recent estimates place their magnitudes in the 7.0-7.5 range.

The 10 largest earthquakes in US History

Largest US Quakes List

Seismicity in the Lower 48 States

Seismicity of the Lower 48 states

Seismic Risk

A number of factors must be considered when assessing the likely damage due to an earthquake.

  • The likelihood of an earthquake
  • The distance to a major, active fault
  • The quality of building construction
  • The surface geology (type of ground)
  • The preparedness of the community
  • Others, we will discuss later

Estimated Risks in the Lower 48

Shaking Risk

Preparing for Earthquakes

Preparation is the key to living with earthquakes. Preparations can be long-term or short-term, community or personal.

Here is a list of some of the steps that will help you deal with an earthquake:

  • Construct and maintain an Earthquake Kit.
  • Learn to shut off water and gas supplies to your home.
  • Devise a family communication plan.
  • Discuss a community response plan.
  • Take a first-aid course.
  • Learn what to do during an earthquake.

What You Should Do During An Earthquake

Should rush out of a building during an earthquake?

No, during the 1933 Long Beach, California earthquake, two-thirds of the people killed were crushed by falling masonry as they rushed out of a building during the shaking.

LongBeach Compton School Damage

(Photo from the U.S. Geological Survey)

What you should do:

What to do

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Prepared by: Charles J. Ammon