Chapter 9

 

EARTHQUAKES

 

Introduction

Vibration of Earth caused by sudden release of energy stored in rocks in the Earth.

-Energy release is usually a result of sudden movement along a fault.

-Fault- fracture separating blocks of rock that have moved relative to one another; commonly do not reach the surface.

-Aftershock- smaller earthquake(s) following larger earthquake due to adjustments along the fault.

 

Elastic rebound theory

 

Rocks behave elastically under large stresses; over time (decades), tectonic stresses elastically deform rock; elastic strain builds up

until strength of the rock exceeded; rock suddenly breaks (movement along fault) releasing stored energy; gives off seismic waves (fig. 9.3).

 

Seismology

 

The study of earthquakes and seismic waves.

 

-Seismograph- instrument that detects, records, and measures the vibrations produced by an earthquake (fig. 9.5).

 

-Seismogram- records of the seismic waves detected by a seismograph.

 

-Focus (hypocenter)- point within Earth where earthquake originates (fig. 9.6).

Shallow focus-  < 70 kilometers

Intermediate focus- 70 – 300 kilometers

Deep focus- 300 – 700 kilometers

 

-Epicenter- point on surface directly above focus (fig. 9.6).

 

-Relationship between earthquake foci and plate boundaries (fig 9.7 & 9.8).

 

Earthquakes- where and how often

 

About 95% of earthquakes take place along specific belts called “seismic belts”.  These belts correspond to plate boundaries.

Circum-Pacific belt

Mediterranean-Asiatic belt

 

The remaining 5% are intra-plate (within the plate) earthquakes (e.g. The New Madrid Seismic Zone).

 

 

Seismic waves

Waves that are generated by an earthquake (or explosion).

 

                        Body waves- travel through Earth's body.

 

P-waves (primary, compressional)(like sound waves)-particles move back and forth in direction of wave propagation; fastest (fig. 912).

                     S-waves (secondary, shear)- particles move at right angles to direction of wave propagation;        slower than P- wave, can only

travel through solids (not liquid or gas)(fig. 9.12).

 

Surface waves-travel only on surface of Earth (like stone dropped in calm lake); slowest moving and often cause most damage (fig. 9.13).

 

Love(L-)waves- material moves side-to-side in a horizontal plane perpendicular to the direction of propagation (fig. 9.13).

Rayleigh(R-)waves-material moves in an elliptical path (like water waves); slower than L (fig.9.13).

 

Locating an earthquake

 

Based on difference in arrival time between P&S waves; P&S waves generated at the same time, but gradually separate because they travel at different speed;

P-S interval increases with increasing distance from earthquake; if know P-S interval, can tell the distance to an earthquake from a time-distance graph (fig.9.15);

determine epicenter from intersection point of 3 circles with radiusfrom 3 seismograph stations equal to the determined distance (fig. 9.16).

 

Earthquake Strength

 

Intensity-qualitative assessment of damage done by an earthquake; Modified Mercalli Intensity Scale used to determine intensity

(table 9.2; figs. 9.17, 9.18). Range of values are from I to XII.

 

Magnitude-quantitative measurement of the amount of energy released by an earthquake; Richter Magnitude Scale measures energy released at source

(tables 9.1, 9.3); determined by measuring maximum amplitude on seismograph (fig. 9.19); a logarithmic scale therefore an increase by a single number is

a 10 times increase in amplitude (and ~30 times increase in energy released). Magnitude scale is an open-ended scale.

 

Earthquake Effects

 

Earthquakes are the most destructive natural phenomena. Earthquake effects dependent upon many factors including: magnitude, duration, proximity, population

of area, time of day, local geology, type of structure, etc.

 

Ground Shaking- Depends on magnitude, distance, underlying material.

Fire-Is a major hazard, particularly in urban areas (fig. 9.23).

Tsunami- Seismic sea waves. Destructive waves generated by submarine earthquakes (submarine volcanoes or landslide may also cause tsunami)(fig. 9.25).

Ground Failure-Surface rupture by fault displacement; liquefaction (fig. 9.21); landslides (fig. 9.27)

 

Earthquake Prediction

Predicting time frame, location, and strength of an earthquake. A goal of geologists to reduce deaths & injuries; rarely successfully done.

 

Earthquake Precursors. Long- and short-term changes within the Earth preceded earthquakes. These changes include:

 

1.      Seismic Gaps (fig. 9.29)

2.      Changes in elevation & titling of land surface

3.      Fluctuation in water levels of wells

4.      Change in seismic wave velocity

5.      Change in radon emission

6.      Change in electrical resistivity

7.      Change in number of seismic events

 

Precursors(most of the above) believed to be related to dilatency(small cracks form) just before failure of rock (fig.9.30).

 

Earthquake Control

Induce numerous small earthquakes instead of occasional major ones by injecting fluids into ground (fig. 9.31); (unlikely; too many problems).

 

Earthquake Engineering- effect of earthquake on a building dependent upon:

1. characteristics of seismic waves reaching site (strength, type [L, P, etc.])

2. material beneath building (unconsolidated sediments amplifies ground shaking; bedrock doesn't)

3. construction and design of building:

-best-small buildings- wood frame (if anchored securely)

        -multistory- steel frame or reinforced concrete; rectangular / box shape

-worst-non reinforced masonry, brick, adobe, etc.(heavy and brittle); irregular shape.

 

Earthquake Preparedness-Table 9-5.