# 5.3: Earthquake Basics

• 5.3.1: Introduction
The problem is that earthquakes start out many miles beneath the surface, too deep for us to observe them directly. So we study them from afar by (1) observing the geological changes at the ground surface, (2) analyzing the symphony of earthquake vibrations recorded on seismographs, and (3) monitoring the tectonic changes in the Earth’s crust by surveying it repeatedly, using land survey techniques for many years and now using satellites.
• 5.3.2: Elastic Rocks; How They Bend and Break
If you blow up a balloon, the addition of air causes the balloon to expand. If you then squeeze the balloon with your hands, the balloon will change its shape. Removing your hands causes the balloon to return to its former shape. If you take a thin board and bend it with your hands, the board will deform. If you let the board go, it will straighten out again. These are examples of a property of solids called elasticity.
• 5.3.3: A Classification of Faults
Most damaging earthquakes form on faults at a depth of five miles or more in the Earth’s crust, too deep to be observed directly. But most of these faults are also exposed at the surface where they may be studied by geologists. Larger earthquakes may be accompanied by surface movement on these faults, damaging or destroying human-made structures under which they pass.
• 5.3.4: Paleoseismology, the Slip Rates and Earthquake Recurrence Intervals
Major earthquakes are generally followed by aftershocks, some large enough to cause damage and loss of life on their own. The aftershocks are part of the earthquake that just struck, like echoes, but last for months and even years. But if you have just suffered through an earthquake, the aftershocks may cause you to ask: when will the next earthquake strike? I now restate this question: when will the next large earthquake (as opposed to an aftershock) strike the same section of fault?
• 5.3.5: What Happens During an Earthquake?
Crustal earthquakes start at depths of five to twelve miles, typically in that layer of the Earth’s crust that is strongest due to burial pressure, just above the brittle-ductile transition, the depth below which temperature weakening starts to take effect. Slab earthquakes like the one that started in the Juan de Fuca Plate underlying the continent, at greater depths but still in brittle rock. These depths are too great for us to study the source areas of earthquakes directly by deep drilling.
• 5.3.6: Measuring an Earthquake
The chorus of high-frequency and low-frequency seismic waves that radiate out from an earthquake indicates that no single number can characterize an earthquake, just as no single number can be used to describe a Yakima Valley wine or a sunset view of Mt. Rainier or Mt. Hood. The size of an earthquake was once measured largely on the basis of how much damage was done.
• 5.3.7: Measuring Crustal Deformation Directly Using Tectonic Geodesy
As stated at the beginning of this chapter, Harry F. Reid based his elastic rebound theory on the displacement of survey benchmarks relative to one another. These benchmarks recorded the slow elastic deformation of the Earth’s crust prior to the 1906 San Francisco Earthquake. After the earthquake, the benchmarks snapped back, thereby giving an estimate of tectonic deformation near the San Andreas Fault independent of seismographs or of geological observations.
• 5.3.8: Summary
Earthquakes result when elastic strain builds up in the crust until the strength of the crust is exceeded, and the crust ruptures along a fault. Some of the fault ruptures do not reach the surface and are detected only by seismograms, but many larger earthquakes are accompanied by surface rupture, which can be studied by geologists. Reverse faults are less likely to rupture the surface than strike-slip faults or normal faults.