Seismic Measurements Help
When a fault cracks, it causes deformation of surface features. For example, when someone builds a road, railroad, pole line, or fence line across a fault, they may be surprised when they come back after an earthquake, to find everything has changed. The previously straight line is shifted into a shape with higher displacement near the fault, a process known as elastic rebound . Figure 12-5 shows how this stress is exerted and then rebounds.
The first known intensity scales used in Europe were used to compare the vibrational effects of one earthquake to another. An intensity scale helped to estimate the overall severity of any one earthquake, compared to others that had taken place previously.
By the early 20th century, the two most commonly used standards of intensity were the Rossi - Forel and the Mercalli scales, which used Roman numerals. Intensities were rated from I to X on the Rossi-Forel scale, and from I to XII on the Mercalli scale. Higher numbers were equal to stronger earthquakes.
In 1931, the Modified Mercalli Intensity Scale was described by Harry Wood and Frank Neumann. It measured the destructiveness of an earthquake and used the same I to XII scale as the original Mercalli scale. The 1906 San Francisco quake was given a XI on the Modified Mercalli Intensity Scale.
Seismographs And Seismograms
Seismology is based on the measurement and observation of ground motion. Originally, people judged an earthquake’s strength by the amount of damage it caused.
The first pendulum seismograph used to measure the shaking of the ground during an earthquake was developed in 1751, but it wasn’t until 1855 that geologists realized faults were the source of earthquakes. The first seismographs using time and motion indicators were built in the late 1800s. Long before electronics, scientists built huge spring - pendulum seismographs to measure long-period quake motion. A seismograph, three stories high, in Mexico City is reportedly still in use.
A seismograph is an instrument that records seismic waves (vibrations) onto a tracing called a seismogram.
Richter Magnitude Scale
Seismographs record a zigzag trace that shows the changing amplitude of ground movement beneath the instrument. Sensitive seismographs greatly magnify these vibrations and can detect earthquakes all over the world. The time, location, and magnitude of an earthquake can be found from the information gathered by seismograph stations. Figure 12-6 shows a typical trace from a seismograph.
Fig. 12-6. Seismographs record earthquake primary waves, secondary waves, and wave amplitude.
The Richter Magnitude Scale was developed in 1935, by Charles F. Richter of the California Institute of Technology, as a math tool to compare earthquake sizes. He got the idea from his early studies of astronomy. He knew astronomers gave stars a magnitude level based on their brightness. By using high-frequency data from nearby seismograph stations, Richter came up with a way to measure the magnitude of an earthquake. This became known as the Richter local magnitude ( M L ).
The equation for Richter Magnitude is shown below:
M L = log 10 A (mm) + (Distance correction factor)
A is the amplitude, in millimeters, measured directly from the paper record of a seismograph. The distance correction factor comes from a table in Richter’s Elementary Seismology book.
Earthquake magnitude is found from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the distance differences between the several seismographs and the earthquake epicenter. On the Richter scale, magnitude is written in whole numbers and decimals. It is an open-ended scale (10 is not the highest number).
Because the scale is logarithmic, each whole number increment in magnitude actually represents a tenfold increase in amplitude or energy.
Earthquakes with a magnitude of 2.0 or less are often called microearthquakes . Most people don’t feel them and only nearby seismographs record their movement. Earthquakes of 4.5 or greater magnitudes (thousands each year) are strong enough to be recorded by sensitive seismographs worldwide.
Large earthquakes, such as the 1964 Alaskan earthquake, have magnitudes of 8.0 or higher. On average, one earthquake of 8.0 occurs somewhere in the world every year.
Practice problems of this concept can be found at: Earthquake Practice Test
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