EARTHQUAKES

Earthquakes are caused by the passage of seismic waves through and over the surface of the Earth.
Seismic waves are commonly generated by a "stick-slip" movement of rocks along faults.
This is illustrated by the San Andreas fault, a strike-slip fault that marks the boundary between the North American and Pacific plates.

SEISMIC WAVES

Types of seismic waves:

A. Body waves: travel through Earth's interior

1. P waves (or compressional waves)

compress and dilate rocks in the direction of wave motion during their passage
primary waves: the fastest seismic waves, the first to arrive at a seismograph station

2. S waves (or shear waves

twist and shear rocks in the direction normal to wave motion during their passage
secondary waves: the second waves to arrive at a seismograph station

B. Surface waves:

1. radiate outwards across the Earth's surface from the epicenter (the point on the surface directly above the focus)

2. the slowest moving seismic waves, and the last to arrive at a seismograph station

3. cause roller-coaster and shearing motions of the ground that damage buildings and other surface structures

SEISMOGRAMS

Seismograms are graphic recordings of the arrival times and amplitudes of seismic waves. Such data are used to locates the epicenters of earthquakes and measure their magnitudes.

Refer to Figure 8-3 in your lab manual

ANALYZING SEISMOGRAMS

Locating the epicenter1:

P waves travel faster than s waves. The time lag between their arrivals at a seismograph station is proportional to the distance from the epicenter.

Measuring the magnitude2:

The Richter magnitude scale is one measure of the strength of a quake. Given the maximum amplitude of the p or s waves and the distance to the epicenter, it can be calculated with a nomogram.

EARTHQUAKE INTENSITY

A second measure of the strength of an earthquake is the Mercalli intensity scale, which subjectively describes its effect on humans and surficial structures.

SEISMIC STRATIGRAPHY

Seismic stratigraphy is the application of seismology to the study of Earth's interior.

The analysis of seismic waves reveal the structure and composition of Earth's interior because:

A. they move at different velocities in different rocks:

for example, at the same depth, they travel more rapidly through crystalline limestone than shale
for example, for the same rock, they travel more rapidly through deeper buried (and denser) rocks

B. they are sometimes refracted or reflected as they pass from one rock type into another

Seismic profiles are cross-sectional images of Earth's crust produced by the analysis of seismic waves.
Seismic waves are mechanically created at the surface and then travel down into the crust, where they are sent back to the surface by reflecting horizons.
Reflecting horizons usually mark the contacts between rocks and structural features such as faults.
The returning waves are detected by geophones that feed their arrival times into a computer, which in turn generates a seismic profile from these data.
Seismic profiles show the locations of the major reflecting horizons and outlines the rock layers between them.
The thickness of a rock layer is expressed on the vertical scale of the seismic profile as the two-way travel time: the time required for seismic waves to travel down and back up through them.
Geologists have measured the typical travel times for the different rocks in Earth's crust. If the composition of the rock layer is known, its thickness can be estimated from its travel time.

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