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

elastic rebound 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

Fault Structures - Normal Faults

Fault-Block Structures
  • Horst (uplifted block)
  • Graben (or rift valley - downdropped block)
  • Tilted Fault Block

Fault Structures - Reverse Faults

Nappes or Overthrusts are shallowly-dipping thrust faults found in almost all mountain ranges. Because they are nearly horizontal, they often have very complex outcrop patterns.

 Overthrust Fault A Window (W) is an opening where erosion cuts through a shallowly-dipping thrust fault to expose the rocks below. A klippe (K) is an isolated remnant of a thrust fault block.

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.

How Seismographs work 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.

Seismic Waves

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

Isoseismals, 1906 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

San Francisco, 1906 San Francisco, 1906

At left is an isoseismal map for San Francisco itself . Everything was shaken hard, and of course intensities were extremely high close to the fault. But note how in the city intensity can vary by two levels within a couple of hundred meters. At right is a geologic map. Note that low intensity correlates closely with bedrock at or near the surface (Franciscan metamorphic rocks and serpentine).

San Francisco, 1906 San Francisco, 1906

When we examine intensity compared to depth to bedrock (right) the pattern becomes even clearer. 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

isoseismals, New Madrid 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

Seismic Magnitude Scale Magnitude and energy for large earthquakes. Near-surface earthquakes are measured in terms of their surface waves, but deep earthquakes don't produce much surface waves. 

Deep earthquakes are measured in terms of their P- and S- waves. The two scales are defined to coincide as well as possible for normal deep earthquakes. 

Seismic Magnitude Scale There are not too many familiar analogies for very large earthquakes, but very small events overlap the energies of many familiar phenomena.

Strategies of Earthquake Prediction

Eastern North America Earthquakes 1534-1994

Source: USGS Data

eastern earthquakes

U.S. Earthquakes, 1973-2002

Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot is below 100.

U.S. Earthquakes

Seismic Risk Level Maps for the U.S.

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

U.S. seismic hazard

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

  U.S. seismic hazard

Seismic Gaps

seismic gaps seismic gaps
seismic gaps

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.

Earthquake frequency

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.

The graph above shows earthquake fatalities since 1800 from the U.S. Geological Survey list of significant earthquakes. The totals are not exact for any year but give an idea of trends. For example, the database for 1892 lists only two fatalities. Does anyone really believe there were only two earthquake fatalities worldwide in 1892, let alone the gaps where there are no reported fatalities?

Note that the scale is logarithmic. The dozen or so events with more than 100,000 fatalities account for a large fraction of the total. Even in recent decades there have been quiet years with only a few hundred fatalities. There have been about 2.9 million earthquake fatalities since 1900, 3.8 million since 1800, and 7.7 million since 1500. The total for all recorded earthquakes is around 12.5 million. It's hard to know how to interpret those results, since ancient casualty totals were often exaggerated, but also many fatalities would have gone unrecorded.

There is an overall increasing trend, partly due to better reporting, partly due to larger populations in at-risk areas, and population pressures forcing people into ever more dangerous ground. However, some seismologists believe we have not seen the worst. World population has tripled since 1950 and that is too short a time for us to conclude we have seen the worst case scenarios. A repeat of the 1923 Tokyo earthquake at the worst possible time, or a tsunami like 2004 but directed north toward Bangladesh, could conceivably produce disasters with million-plus fatalities.


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Created 15 Jan 1997; Last Update 14 December 2009
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