Earthquake

This chapter will help you understand the nature and origin of earthquakes. We discuss the seismic waves created by earthquakes and how the quakes are measured and located by studying these waves. We also describe some effects of earthquakes, such as ground motion and displacement, damage to buildings, and quake-caused fires, landslides, and tsunamis.

Earthquakes are largely confined to a few narrow belts on Earth. This distribution was once puzzling to geologists, but here we show how the concept of plate tectonics neatly explains it.

As geologists learn more about earthquake behavior, there is the possibility that we will be able to forecast earthquakes. We conclude the chapter with a look at this developing branch of Earth study.

Key points

What is earthquake? What are the causes of earthquake?

Different type of seismic waves?

Size of earthquake: distinguish magnitude and intensity. How to determine size of earthquake?

Relation between tectonics and earthquake

What are the effects of earthquake?

What is liquefaction?

How is the location of earthquake locus is determined?

Technical Terms

aftershock

Small earthquake that follows a main shock.

body wave

Seismic wave that travels through Earth’s interior.

depth of focus

Distance between the focus and the epicenter of an earthquake.

earthquake

A trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the surface.

elastic rebound theory

The sudden release of progressively stored strain in rocks results in movement along a fault.

epicenter

The point on Earth’s surface directly above the focus of an earthquake.

focus

The point within Earth from which seismic waves originate in an earthquake.

intensity

A measure of an earthquake’s size by its effect on people and buildings.

Love wave

A type of surface seismic wave that causes the ground to move side to side in a horizontal plane perpendicular to the direction the wave is traveling.

magnitude

A measure of the energy released during an earthquake.

modified Mercalli scale

Scale expressing intensities of earthquakes (judged on amount of damage done) in Roman numerals ranging from I to XII.

moment magnitude

An earthquake magnitude calculated from the strength of the rock, surface area of the fault rupture, and the amount of rock displacement along the fault.

P wave

A compressional wave (seismic wave) in which rock vibrates parallel to the direction of wave propagation.

Rayleigh wave

A type of surface seismic wave that behaves like a rolling ocean wave and causes the ground to move in an elliptical path.

Richter scale

A numerical scale of earthquake magnitudes.

seismic sea wave

See tsunami.

seismic wave

A wave of energy produced by an earthquake.

seismogram

Paper record of earth vibration.

seismograph

A seismometer with a recording device that produces a permanent record of Earth motion.

surface wave

A seismic wave that travels on Earth’s surface.

S wave

A seismic wave propagated by a shearing motion, which causes rock to vibrate perpendicular to the direction of wave propagation.

travel-time curve

A plot of seismic-wave arrival times against distance.

tsunami (seismic sea wave)

Huge ocean wave produced by displacement of the sea floor; also called seismic sea wave.

Extra readings

1. Earthquakes are the sudden release of strain energy, usually along faults, but also associated with volcanism and mineral transformations. Elastic rebound theory accounts for this stored strain, but a weak-fault model, involving small stress has been suggested for some earthquakes, and not all earthquakes are associated with faults.

2. Earthquakes produce seismic waves. Body waves originate at the focus, the point of initial movement along a fault. Surface waves originate from the epicenter, point on earth's surface directly above the focus. P waves are compressional body waves vibrating parallel to wave propagation that arrive first at a recording station. S waves are transverse body waves that vibrate perpendicular to wave propagation and arrive after P waves. P waves pass through fluids, but S waves do not. Surface waves are slowest, but cause the most damage. They include Love waves (vibrate perpendicular to propagation) and Rayleigh waves (cause ground to move in elliptical path).

3. Seismometers detect seismic waves by measuring ground motion. Seismograms are the records of earth motions produced by seismographs, that are recording seismometers.

4. Difference in arrival times of P and S waves at a seismograph tell its distance from the earthquake focus, using a travel-time curve. Location of the epicenter requires three stations to triangulate their distances from the focus. Depth of focus can be determined by analyses of seismograms: shallow< 70 km, intermediate 70-350 km, deep 350-670 km. Earthquakes do not occur below 670 km, and shallow focus account for 85% of the total earthquake energy released.

5. Earthquake strength is measured by either intensity (damage) or magnitude (energy released). The modified Mercalli scale expresses intensities in Roman numerals (I-XII). The Richter scale determines magnitude by measuring the height of a particular wave on a seismogram. Recorded Richter magnitudes range from a little more than 0 to 8.6. It is logarithmic so that a difference of one on the scale is 10 times the ground vibration and 32 times the energy released. Moment magnitude, determined by rock strength, surface area of rupture, and amount of rock displacement along a fault in the field, is a new method of calculating magnitude, and is more accurate for magnitudes of 7 or greater. Moment magnitudes can exceed 9.0, the theoretical limit for the Richter scale.

6. Earthquake can occur in Vietnam. Most earthquakes in Vietnam occur in North West region because it is relating to some active faults. Historic earthquakes east of the Rockies occur along old diverging plate boundaries and aulacogens. They have had Richter magnitudes of 5.0-6.0 . A seismic risk map is illustrated as Fig. below.

7. Earthquake damage is caused by ground motion that topples buildings, produces fires (broken gas mains), landslides, liquefaction, permanent land displacement, and tsunamis (seismic sea waves). Foreshocks and aftershocks precede and follow the main earthquake and they can cause destruction as well.

8. Vertical motion of the sea floor is most conducive to tsunami formation and most are associated with subduction zones. Tsunami wavelengths can reach 160 kilometers with speeds of 725 kilometers per hour. A breaking tsunami can reach 30 meters. Devastating tsunami have struck Hawaii, California, Alaska, New Guinea, and Japan. An Early Warning System was developed after the 1946, Hilo, Hawaii, tsunami to minimize loss of life around the Pacific coast.

9. Most earthquakes are concentrated in the circum-Pacific belt, which produces 100% of deep focus, 90% of intermediate focus, and 80% of shallow focus earthquakes. The Mediterranean-Himalayan belt is the second major concentration of earthquakes. Benioff earthquake zones begin at ocean trenches and slope and deepen toward island arcs and continents. Benioff zones account for the world's deep and intermediate focus earthquakes.

10. First-motion studies determine whether the fault producing the earthquake was undergoing tension or compression.

11. Plate boundaries are identified and defined by earthquakes. Diverging plate boundaries produce rift valleys from normal faulting indicated by first-motion studies. First motion studies indicate that transform boundaries experience shallow strike-slip motion. Converging boundaries are marked by either continental collision, or subduction. A subducting plate initially undergoes tension at a trench as it is bent, but may experience either tension or compression as it descends into mantle based on first-motion studies. Subduction angles vary from gentle to steep, and plates may even break-up at depth. Deepest earthquakes may be caused by mineral transformations or dehydration.

12. A variety of observations have been used to predict earthquakes: microseisms and changes in rock properties, such as magnetism, change in water levels in wells, radon emission, changes in geyser eruption intervals, surface tilting and elevation change, animal behavior and foreshocks. Most prediction is based currently on seismic history and identification of seismic gaps, areas quiet for long periods of time.

13. Timed release of stored strain along faults could potentially allow the control of earthquakes. Lubricating the fault with water is one method to release small, timed earthquakes rather than a large unpredicted one.

Earthquake harzard

website by USSG that you can find recent or historic earthquakes, lists, information on selected significant earthquakes, earthquake resources by state, or find webservices.