Download Ch 2 Sec 3: Monitoring Earthquakes

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Ch 2 Sec 3: Monitoring Earthquakes
Guide for Reading
■ How do seismographs work?
■ How do geologists monitor faults?
■ How are seismographic data used?
Many societies have used technology to try to determine when and where
earthquakes have occurred. During the early 1900s, scientists developed
seismographs that were much more sensitive and accurate than any earlier
devices.
A simple seismograph can consist of a heavy weight attached to a frame by
a spring or wire. A pen connected to the weight rests its point on a drum
that can rotate. As the drum rotates slowly, the pen draws a straight line on
paper that is wrapped tightly around the drum.
Seismic waves cause the seismograph’s drum to vibrate (But the
suspended weight with the pen attached moves very little) Therefore, the
pen stays in place and records the drum’s vibrations by tracing a line onto
the paper.
The pattern of lines, called a seismogram, is the record of an earthquake’s
seismic waves produced by a seismograph.
1st zig zag= P wave
2nd zig zag= S wave
3rd zig zag = surface wave- produces the largest disturbance
To determine the arrival time- draw line down from first peak of first
zigzag.
To monitor faults, geologists have developed instruments to:
1. measure changes in elevation- A network of Earth-orbiting
satellites called GPS satellite helps scientists monitor changes
in elevation (vertical) as well as horizontal movement along faults.
Measure movement in markers set upon opposite sides of faults.
2. tilting of the land surface- A tiltmeter measures tilting or
raising of the ground- vertical movement. Like a carpenter’s level.
3. ground movements along faults- A creep meter uses a wire
stretched across a fault to measure horizontal movement of the ground.
4. A laser-ranging device uses a laser beam & reflector to detect
Horizontal fault movements.
(Figure 14 p. 63)
Seismographs and fault-monitoring devices provide data used to
1. map faults- Geologists use the data from seismic waves to
map faults, which are often hidden by a thick layer of rock
or soil. When wave encounters a fault it reflects off of it so
scientists detect reflected waves to determine where faults
are located. This practice helps geologists determine the
earthquake risk for an area.
2. detect changes along faults- Geologists use fault-monitoring
devices to study the types of movement that occur along faults.
Friction is the force that opposes the motion of one surface as it
moves across another surface.
Where friction along a fault is low, the rocks on both sides of the fault
slide by each other without much sticking. Stress does not build up,
and large earthquakes are unlikely.
Where friction is high, the rocks lock together. Stress builds up until an
earthquake occurs.
3. develop a method of predicting earthquakesEven with data from many sources, geologists can’t predict
when and where a quake will strike only the probability that one
may occur sometime.