Earthquakes or How does the crust break? - Click here for a guide to earthquake processes

- The traditional concept is that stress gradually increases until exceeds the friction pressing the rocks together on either side of the breakage plane (fault). If stress increases by a steady amount per decade then, in theory, one can forecast general timing of an earthquake.

BUT - If that theory is correct the amount of friction that builds up should generate huge amounts of heat that would persist for long times (thousands of years) because rocks are such poor conductors. We don't see that heat, which suggests there is a problem with this model.

- Another theory: stresses need not build up to a set level before movement of rocks. Fluids or other factors can temporarily reduce friction, enabling failure at any time. When slip occurs, it is not all at once. Rather a small gap runs down the fault (breakage plane) breaking rock in front and sealing behind.
- This self healing slip model solves heat problem but bodes ill for earthquake predictions

Frequency of earthquake occurrence since 1900

Some idea of energy released is given by the magnitude assigned to the earthquake

 The amount of shaking felt is often reported as seismic intensity, a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.

Earthquake energy is transmitted through the Earth as waves

Information obtained from earthquake waves

The average observed velocity of the waves increases downward gradually but also sudden changes in velocity occur
- The gradual change in velocity reflects a gradual change in properties of the material traversed by the waves.
- Sudden changes in velocity are due to abrupt discontinuities. When there are abrupt discontinuities the waves are

Discontinuities
- first one was detected by a Serbian - Mohorovicic in 1909. This marks the crust-mantle boundary and is called the Moho. It is a sharp discontinuity:

We can observe the crust directly and so we have some idea of the kind of rocks it is made of and its physical properties.

The mantle which lies below the Moho is not so easily investigated. The nature of the mantle is inferred using seismic wave velocities to determine its density and elastic properties. Also:

The next abrupt discontinuity is the mantle-core boundary at 2900 km. This is marked by a sudden distinct velocity decrease. Also s-waves are no longer transmitted by the material below this boundary so we conclude it is liquid

A final discontinuity lies within the core at 5000 km. This is due to the solid inner core

Earthquake distribution - You can see a map showing earthquakes in the last 15 days. This map shows the location and magnitude of events and includes a record of seismicity for the last 5 years. By looking at this and other maps you can see that earthquakes occur in narrow belts at the surface. These belts define plate margins.

• at depth earthquakes are divided into shallow, intermediate, and deep. To see the distribution of earthquakes with depth for various parts of the world access the maps of world seismicity provided by the US Geological Survey.
• Some belts (= plate boundaries) are marked only by shallow earthquakes. These are the divergent and transform boundaries.
• In other belts earthquakes range from shallow to deep. These define inclined planes called Wadati-Benioff zones. These are the convergent boundaries. No earthquakes occur below about 700 km. Wadati-Benioff zones are one of the main clues to plates and plate motions. They mark the position of downgoing slabs. These are colder than the surrounding material and more brittle, thus they break and release earthquake energy.

Check out the locations of the 15 largest earthquakes in the US.

Back to Geo lecture notes

Back to Geo 101 homepage