Most Quakes Occur along Faults (Fractures in Earth's Crust)
|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.
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
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 come in several types as shown below:
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.
|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.
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.
|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.
Source: USGS Data
Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot (extreme upper left) is below 100.
Probable ground acceleration in 50 years. Blue = small, red = large
Probability of damage in 100 years. Blue = negligible, green = low, red = high.
Areas that haven't had earthquakes in a long time are prime candidates for the next one.
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.
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
|The overall structure of the Earth.|
Seismic tomography is a method of using seismic signals to map the earth's interior in three dimensions.