Faults and Earthquakes

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Some Important Earthquakes

Greatest Earthquakes and Volcanic Eruptions


What Causes Earthquakes?

Most Quakes Occur along Faults (Fractures in Earth's Crust)

Elastic Rebound Theory

Here we have a landscape with a road, a fence, and a line of trees crossing a fault. As the crust moves, the rocks adjacent to the fault are deformed out of shape (in reality the deformation is spread across many kilometers - if it were this obvious, earthquake prediction would be easy).

Eventually the rocks are so stretched out of shape that they cannot bear the stress any longer. The fault slips, and the stage is set for the next cycle of strain buildup and release.

Epicenter and Focus

Focus 
Location within the earth where fault rupture actually occurs 
Epicenter 
Location on the surface above the focus

Types of Faults

Faults Are Classified on the Basis of the Kind of Motion That Occurs on Them


Major Hazards of Earthquakes

Safest & Most Dangerous Buildings


Tsunamis


Seismology

Ideally, we'd like to be able to hover above the earth during and earthquake and watch the earth move beneath us. Since my anti-gravity belt is in the shop for repairs, the closest we can come is with a pendulum.

Contrary to intuition, an earthquake does not make the pendulum swing. Instead, the pendulum remains fixed as the ground moves beneath it.

A pendulum with a short period (left) moves along with the support and registers no motion. A pendulum with a long period (right) tends to remain in place while the support moves.

The boundary between the two types of behavior is the natural period of the pendulum. Only motions faster than the natural period will be detected; any motion slower will not. 

Since earthquake vibrations can have periods of many seconds, we need a pendulum with a very long period. We can construct a pendulum with a very long arm, or we can build a compact instrument by building a horizontal pendulum. If the pendulum is built like a swinging gate, the restoring force (force pulling it back toward the center of its swing) can be made very weak, and the pendulum can have a period as long as we like.

Seismic Waves

Seismic waves come in several types as shown below:

P-Waves
Primary (they arrive first), Pressure, or Push-Pull. Material expands and contracts in volume and particles move back and forth in the path of the wave. P-waves are essentially sound waves and travel through solids, liquids or gases. Ships at sea off the California coast in 1906 felt the earthquake when the P-wave traveled through the water and struck the ship (generally the crews thought they had struck a sandbar).
S-Waves
Secondary (arrive later), Shear, or Side-to-side. Material does not change volume but shears out of shape and snaps back. Particle motion is at right angles to the path of the wave. Since the material has to be able to "remember" its shape, S-waves travel only through solids.
Surface Waves
Several types, travel along the earth's surface or on layer boundaries in the earth. The slowest waves but the ones that do the damage in large earthquakes.

Magnitude and Intensity

Intensity

How Strong Earthquake Feels to Observer

We can plot earthquake intensity by gathering reports from observers. Although the reports will be subjective, and vary somewhat, most observers will agree on the intensity criteria, for example, feeling the quake while driving. For very strong quakes, damage provides fairly objective measures of intensity.

Isoseismals from the 1906 San Francisco Earthquake

Overall, the pattern is pretty simple: high intensity close to the San Andreas Fault, dropping off with distance. But why is there a disconnected island of high intensity in central California?

The band of low (IV) intensity parallel to the coast coincides with the Coast Ranges. Soils here are very shallow - usually less than a meter to bedrock. Observers here felt mostly a sharp jolt.

In contrast, the high intensity in central California coincides with the Central Valley, where young and unconsolidated sediments are kilometers deep. Unconsolidated material shakes like jelly in an earthquake.

Note how intensity VI follows the shoreline of San Francisco Bay, where there are also thick unconsolidated sediments.

Intensity and Geology in San Francisco

At left above is a map of seismic intensity for the 1906 San Francisco earthquake. At right is a map of depth to bedrock. The pattern is clear: the greater the depth to bedrock, the stronger the shaking. Candlestick Park, where game 3 of the 1989 World Series was about to begin, owes its reputation for being a windy ball park to being near a steep hill. Its location on bedrock meant that fans felt a sharp jolt, there were a few cracks in the concrete, and little else. (The First Amendment gives San Francisco the right to call it 3-Com Park if they like - it also gives me the right to ignore them.) The Marina District was shaken badly because it's on artificial fill, in fact, much of it is rubble from the 1906 earthquake. The deep filled valley in northeastern San Francisco is occupied by the commercial center of the city but the modern construction is steel-frame and was undamaged in the 1989 earthquake.

San Francisco and New Madrid Compared

The map at left compares the isoseismals from the 1906 San Francisco earthquake and the 1811-1812 New Madrid quakes.

There is a lot less intensity data for the New Madrid events so local details are missing. Intensity estimates are based on reports from places shown as blue dots.

Although the New Madrid events were big, they owe their vast felt areas to the layer-cake geology of the Midwest. The flat strata and relative lack of geologic complexity (especially compared to California) mean that seismic waves travel very efficiently for long distances with little loss of energy.

Magnitude - Determined from Seismic Records

Richter Scale:

A Seismograph Measures Ground Motion at One Instant
But --

Seismic - Moment Magnitude

Magnitude and Energy


Strategies of Earthquake Prediction

Eastern North America Earthquakes 1534-1994

Source: USGS Data

U.S. Earthquakes, 1973-2002

Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot (extreme upper left) is below 100.

Seismic Risk Level Maps for the U.S.

Probable ground acceleration in 50 years. Blue = small, red = large

Probability of damage in 100 years. Blue = negligible, green = low, red = high.

 

Seismic Gaps

Areas that haven't had earthquakes in a long time are prime candidates for the next one.

Are Earthquakes Getting More Frequent?

It was only in 1885 that a seismograph in Europe detected an earthquake in Japan, and we have global coverage, even for very large events, only since 1900 or so. Below is a graph, based on USGS data, for the annual number of M=7.5 and M=8 earthquakes from 1900 to 2001.

The high levels between 1900 and 1918 were real. The instruments might have overrated some events, but also it is still possible that some events were missed in those years.

There was a steady decline between 1968 and 1984. Curiously, not a single person during those years asked me whether earthquakes were becoming less frequent.


Seismology and Earth's Interior

Successive Approximation in Action

Assume the Earth is uniform. We know it isn't, but it's a useful place to start. It's a simple matter to predict when a seismic signal will travel any given distance.

Actual seismic signals don't match the predictions

We conclude:

Inner Structure of the Earth

The overall structure of the Earth.

Seismic Tomography

Seismic tomography is a method of using seismic signals to map the earth's interior in three dimensions.


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Created 15 Jan 1997; Last Update 02 March 2010
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