VII: Earthquakes

Sudden movements between plates create earthquakes

1. Where do earthquakes happen?

During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges.

[ 24 hour Earthquake monitor http://www.iris.edu/seismon/ ]

[ Broadband Seismic Data Collection Center http://eqinfo.ucsd.edu/

2. Earth’s Seismic Zones

This section describes the relationship between seismic zones and plate tectonics

Earthquakes happen at plate boundaries, whether the plates are converging, diverging, or transforming.

This website can be used to locate the four types of siesmic zones.


[ Maps of Earthquakes Worldwide & their Depth ]

Four types of seismic zones –

a. Mid-Ocean ridge - Activity is low, and it occurs at very shallow depths. The lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Choose North Atlantic Ocean, point out: Iceland, Azores, Tristan da Cunha.

b. Shallow focus event, no volcano - In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault. Choose Western United States, point out: San Andreas fault. Choose Europe, point out: Anatolian fault in Northern Turkey.

c. Oceanic/Continental plate collision - One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. This type of earthquake can be shallow, intermediate, or deep, according to its location on the downgoing lithospheric slab. Such inclined planes of earthquakes are know as Benioff zones.Choose Pacific Rim, point out: 1) Philippines(lat. 20N, long. 120E) ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. 2) Peru - Chile trench(lat. 30S, long.70W) the Pacific plate is being subducted under the South American plate which responds by crumpling to form the Andes.

d. Continental/Continental plate collision - Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Choose Central Asia, then the Middle East, then Africa to show areas of great seismicity: Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to Gilbraltar and the Caucasus.

3. Epicenter and Focus

Focus is the location within the earth where underground rock moves and sends out earthquake waves. The epicenter is the location on the surface of the earth directly above the focus of an earthquake.


[ difference between epicenter and focus ]

4. Detection at different distances

P, S & L Waves - Every earthquake creates three kinds of waves. P and S waves travel downward into the earth, and L waves travel along the top of the crust.

Primary (P) waves: Pressure waves caused when rock is pushed or pulled forward or backward. Primary waves, the fastest waves sent out by an earthquake, travel down into the earth rather than along the surface. Their movement is like that of a slinky: push-and-pull.

Secondary (S) waves: Shear waves caused when rock is shaken or whipped from side-to-side, like the wavy motion of a slithery snake. Secondary waves, the second-fastest wave sent out by an earthquake, travel down into the earth rather than along the surface.

Surface (L) waves: Up-and-down (rolling) or side-to-side motion of the earth surface. Surface waves, the slowest earthquake waves, travel along the surface of the earth rather than down into the earth. Although they are the slowest of all earthquake waves, L waves usually cause more damage to society than P or S waves.

[ Flash graphic differentiating P and S waves ]

Seismology- Seismologists record the waves that are generated by the earthquake. By combining the data collected from many different locations, they are able to determine the location of the epicenter.

Earth-wide Wave Detection- Earthquake detection can be used to determine what the interior of the earth looks like.

[ P and S waves traveling through the Earth's interior ]

5. Effects of earthquakes on the land

Soil liquifaction - The 1989 Loma Prieta earthquake made this apartment building in San Francisco's Marina District sink, putting the third story at ground level. The earthquake caused the soil foundation under the building to shift and flow like water - a process called soil liquefaction.

[ Earthquake damage, Loma Prieta]

Shifting soil levels - When the land elevation falls in a coastal region, some fresh-water lowlands drop below sea-level. Vegetation in these areas is exposed to seawater and dies. There are many areas that show this kind of devegetation along Washington’s southern coast, from Grays Harbor to the lower Columbia River. The most obvious of these buried lowland areas are the standing groves of dead Western red cedar common along the coast between the Copalis and Columbia rivers.

[ Dead vegetation ]

6. Earthquake data from SIO, including rotatable depth of earthquake data


[ Download IView3D Software and Data Sets ]