Earth Science
Earthquakes / Seismology
An earthquake is a sudden motion or shaking of the Earth's crust, caused by the abrupt release of slowly accumulated strain stored in the rocks beneath the surface.
The first earthquake detector Movement of the vase dislodged a ball from a dragon's mouth into the mouth of a frog below. For instance, is a ball from the dragons on either the east or west sides of the vase were dislodged, then the earthquake must have come from one of those two directions. |
Seismology, the study of earthquake, emerged as a true science during the 1880s when instruments were developed to effectively record earthquake waves. As early as A.D. 132, the Chinese scholar Chang Heng invented the first earthquake detector, but it only reveal that an earthquake took place in one of two directions from the device. By 1880s, scientists tried using pendulums to record earthquake waves, but with little success until 1875, when Filippo Cecchi in Italy made the first successful seismograph, an instrument that detectd, records and measures the vibrations produced by an earthquake. By 1880 improvements in seismograph sensitivity began yielding meaningful data still use suspended masses to detect earthquake waves, most now employ electronic sensors, and, of course, computer printouts have largely replaced the strip charts of earlier seismographs.
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 A horizontal-motion seismograph. Because of its inertia, the heavy mass that contains the marker remains stationary while the rest of the structure moves along with the ground during an earthquake. As long as the length of the arms is not parallel to the direction of ground movement, the marker will record the earthquake waves on the rotating drum. This seismograph would record waves from north or south, but to record waves from the east or west another seismograph at right angles to this one is needed |
A vertical-motion seismograph. This seismograph operates on the same principle as a horizontal-motion instrument and records vertical ground movement. |
The point within Earth where fracturing begins - that is , the point at which energy is
first released - is an earth focus, or hypocenter.
What on usually hears in news reports, though, is the location of the epicenter, the point at the surface directly above the focus.
 The focus of an earthquake is the location where rupture begins and energy is released. The place on the surface vertically above the focus is the epicenter. Seismic wave fronts move out in all directions from their source, the focus of an earthquake. |
Seismologists recognize three categories of earthquakes based on focal depth. | |
| shallow-focus | focal depth less than 70 km from surface | |
| intermediate focus | foci between 70 and 300 km | |
| deep focus | foci more than 300 km deep | |
| Approximately 90 % of all earthquake foci are at depths of less than 100 km, whereas only 3% of all earthquake are deep. Shallow focus earthquake are, with few exceptions, the most destructive. | ||
An interesting relationship exists between earthquake foci and plate boundaries. Earthquakes generated along divergent or transform plate boundaries are invariably shallow- and nearly all intermediate- and deep-focus earthquakes occur along convergent margins. Furthermore, a pattern emerges when the focal depths of earthquakes near island arcs and their adjacent ocean trenches are plotted.
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The focal depth increases beneath the
Tonga Trench in a narrow, well defined zone that dips approximately 45 degrees. Dipping
seismic zones, called Benioff zone, are common to convergent
plate boundaries where one plate is subducted beneath another. Such dipping seismic zones
indicate the angle of plate descent along a convergent plate boundary. <----Focal depth increases in a well-defined zone that dips approximately 45 degrees beneath the Tonga volcanic arc in the South Pacific. Dipping seismic zones are called Benioff zones. |
SEISMIC WAVE
Body wave
An earthquake generates two type of body waves: P-waves (primary waves) and S-waves (secondary waves).
P-waves are the fastest seismic waves and can travel through solida, liquids, and gases. P-waves are compresional, or push-pull, waves and are similar to sound waves in that they move material forward and backward along a line in the same direction that the waves themselves are moving.
S-waves are somewhat slower than P-waves and can only travel through solids. S-wave are shear waves because they move the material perpendicular to the direction of travel, thereby producing shear stresses in the material they move through. Because liquid are not rigid, they have no shear strength, and S-wave can not be transmitted through them.
The velocities of P- and S-waves are determined by the density and elasticity of the materials through which they travel.
Surface wave
surface waves travel along the surface of the ground, or just below it, and are slower than body waves. Unlike the sharp jolting and shaking that body wave cause, surface waves generally produce a rolling or swaying motion, much like the experience of being in a boat. Several types of surface waves are recognized. The two most important are Rayleigh waves ( R-waves ) and Love waves ( L-wave ), named after the British scientists who discovered them, Lord Rayleigh and A.E.H. Love.
Rayleigh waves (R-waves) move material in a elliptical path in a plane oriented parallel to the direction of wave movement. The arrival of R-waves causes the surface to undulate and shake from side to side.
Love waves (L-waves) moev amterial back and forth in a horizontal plane perpendicular to the direction of wave movement. The arrival of L-waves causes the surface to undulate and shake from side to side.
From Physical Geology Exploring the Earth 4th Edition, James S. Monroe, Reed Wicander, Center Michigan University, brooks/cole
The destructions of earthquake
Earthquake Escarpment An
aerial view of the "Stanford Hill" escarpment, formed by the Landers earthquake
on June 28, 1992. Visible is Cal Tech's Kerry Sieh, who studied the scarp. |
Researching Old Earthquakes Dr.
Kerry Sieh has used carbon dating of sediment layers and evidence of faulting to date
earthquakes on the San Andreas Fault. Layers are marked with dates to indicate when quakes
occurred. |
Earthquake Fault in a Desert Aerial
view of the desert north of Landers, California. Cracks in the surface, indicating the
presence of a fault, are visible at the base of some low hills. |
Old Earthquake Fault Close-up
of an ancient fault exposed when geologists dug a trench. |
Earthquake Prone Town Aerial
view of the small town of Petrolia. It was heavily damaged in an earthquake in 1992, and
is threatened by its proximity to active faults.
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Cracked Street After Earthquake © www.corbis.com/Michael S. Yamashita |
Brick Facade Cracked by Earthquake The
layer of bricks on the outside of a San Francisco house shows damage caused by the Loma
Prieta Earthquake. |
Car on Street Cracked by Earthquake A
car sits upon a large crack in a street, cause when the Loma Prieta earthquake cause the
street to buckle and break. |
Earthquake Fault Between Houses Aerial
view of the desert north of Landers, California. A crack in the surface, indicating the
presence of a fault, runs between two homes.
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Quake-Damaged Road A
large crack runs across a road. The offset yellow line on either side shows how far the
highway was shifted by the large earthquake. |
Fault in a Southern California Desert Cracks
from a fault on the surface of a desert, after a large earthquake. |
Quake-Damaged Apartment Building Close-up
of cracks on the exterior of the Champagne Towers Apartments in Santa Monica. The building
was damaged in the 1994 Northridge earthquake, and is held up by steel braces.
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Most recent Earthquake
Current Earthquake Information - includes a near real-time earthquake bulletin. From the USGS National Earthquake Information Center.
How to read the display of Drum Recorder - offers knowledge of reading the display of drum recorder
Earthquake News - tracks news and crises around the world.
Latest California Seismic Events - offers near real-time earthquake maps for the area.
Latest New Zealand Earthquake Report - date, time, location, and magnitude of the latest earthquake recorded by the Seismology Department of the Institute of Geophysics at Victoria University of Wellington.
Recent Central United States Earthquakes
Up-to-the-minute Southern California Earthquake Map - shows the last approximately 500 earthquakes detected by seismographs in southern California. Requires Java.
World-Wide Earthquake Locator - provides basic information about earthquakes within hours of their occurrence.
Earthquakes - information from the British Geological Survey's Global Seismology & Geomagnetism Group.
Earthquake Prediction
Earthquake Clouds and Short Term Prediction - attempts to predict earthquakes by reading "earthquake clouds," which ancient Chinese and Italians studied, believing they were indicative of impending earthquakes.
Earthquake Prediction - based on geophysical, geomagnetic, and astronomical circumstances.
Earthquake Warning Research - all about detecting signals from the earth which tell animals, and some people, that an earthquake is about to occur.
International Institute of Earthquake Prediction Theory and Mathematical Geophysics (IIEPT) - research focused on the application of mathematics to seismology and adjacent earth sciences.
Syzygy - earthquake prediction newsletter.
Related Website in Canada
British Columbia Earthquake Resources
Canadian National Earthquake Hazards Program (NEHP) - goal is to understand the causes of, and hazards associated with, earthquakes in Canada.
Lithoprobe Seismic Atlas of Canada - view deep crustal seismic reflection data from across Canada.
Current Earthquake Reports
National Earthquake Hazards Program: Western Canada - offers current and other earthquake information for British Columbia and all of western Canada.
Earthquake Risk and Hazard Mitigation
National Earthquake Hazards Program - works to understand the causes of, and hazards associated with, earthquakes in Canada.
Seismic Networks
Southern Ontario Seismic Network (SOSN) - earthquake maps and waveform plots from the Toronto-Hamilton-Niagara area.
Specific Past Earthquakes
February 28, 1925 Charlevoix-Kamouraska Earthquake - retrospective look at the Canadian earthquake which measured 6.7 on the Richter scale.
Access Earthquake Data through Internet
Council of the National Seismic System (CNSS)
CNSS authoritative composite catalog
The Consortium of Organizations for Strong-Motion
Observation Systems (COSMOS)
Customized phase
arrival-time calculator (by: USGS and University of Alaska)
CTBT Prototype International Data Centre
Geotechnical Engineering Web Resources
International Association of Seismology and Physics of
the Earth's Interior (IASPEI)
International Seismological Centre, United Kingdom
IRIS Headquarters
Multidisciplinary Center For Earthquake Engineering
Research (MCEER)
NOAA - National Geophysical
Data Center - Earthquake Data
USGS - National Strong Motion Program
USGS National Earthquake Information Center (NEIC)
US National Seismic Net
Earthquake Hazards Program
Swiss Seismological Service list of world-wide
AutoDRM sites (originator of the AutoDRM)
UCSD IDA/IRIS
SURFING THE INTERNET FOR STRONG
MOTION DATA (provided by Dave Wald)
Reference list to
seismology software available on the Internet (provided by ORFEUS)
Seismic information relating to volcanic activity
Alaska Volcano Observatory (AVO)
Cascades Volcano Observatory (CVO)
Mount Erebus Volcanic Observatory
(New Mexico Tech)
Hawaiian Volcano Observatory (USGS/HVO)
USGS Volcano Hazards Program - Long Valley Caldera
Yellowstone Caldera seismicity - Univ. of Utah
Links to more general Volcanology information
Michigan Technological University
Volcanoes Page
Jonathan Dehn's "The
Volcanic Homepage" (most complete listings found)
Smithsonian Institution - Global Volcanism Program
US Geological Survey general volcano pages
Volcanosurfing pages by Hugues Dufumier
American Geophysical Union
Council of the National Seismic System (US)
Earthquake Engineering Research Institute (EERI)
European Geophysical Society
European Seismological Commission
Seismological Society of America
Cascade Region Earthquake Workgroup (CREW) - public-private coalition
Central United States Earthquake Consortium
Missouri Seismic Safety Commission
Western States Seismic Policy Council (WSSPC)
Seismolinks to a comprehensive
list of general seismological resources
General Tsunami
information and resources - C.E. University of Washington