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Geology - Earthquakes

Geology 101 - Gale Martin - Class Notes

Elastic Rebound Theory

Rocks along faults rarely glide along in a smooth, frictionless fashion. Movement along a fault is sporadic and unpredictable. Behavior along the fault is modeled using the Elastic Rebound Theory. The faults tend to 'lock-up' and resist motion. Stress being applied stretches, twists and distorts the rock resulting in a build up of energy between the grinding rocks. When the fault finally slips, energy is released when rock bounces back to its original shape. As the Earth is jerked and jarred by the shifting fault, vibrations radiate from the break producing an earthquake.

The point along the fault where the movement occurs is the focus of the earthquake. Directly above this is the area of most intense damage along the surface. This area is known as the epicenter. The destruction that occurs on the surface is dependent on several factors. This includes the type of rock (bedrock versus valley fill), the depth of the movement and the type of building structures that are present on the land surface.

When a movement occurs, only a small portion of the fault shifts. This does not 'relieve' the built up energy along the entire fault plane. Regions where the rock remains 'locked' may develop even more stress. The movement often propagates beyond the initial break as energy is released in other areas along the fault plane. These smaller shifts are referred to as aftershocks. (A similar even might be an accident on the expressway -- traffic gets tied up behind it and more accidents occur as drivers 'jockey' for the best way out.)

Seismic Waves

The vibrations produced by the energy release are known as seismic waves. The waves radiate from the focus in all directions, passing through the Earth's interior and along the crust. Several different waves are produced. The behavior of each wave is based on the velocity and the way the wave propagates through the rock.

Body Waves

Body waves can travel through the interior of the Earth; bouncing, bending and reflecting around for long periods of time (hours, even!). They consist of two general types: P- and S-waves. P-waves, or primary waves, are the fastest seismic waves. They can travel at several kilometers per second but the speed will vary depending on the rock type and depth (pressure). As a P-wave moves through rock, it compresses and expands material parallel to the wave motion. This is known as a compressional, or longitudinal, style of wave motion (ex.: a 'slinky'). S-waves, known as shear waves or secondary waves, travel at about half the speed of a P-wave. Rock affected by an S-wave will shift back-and-forth, or in a transverse style, as the wave travels through (ex.: a 'cracked' rope or sidewinder snake). S-waves can only travel through solid material. When they enter a region that is 'liquid', the S-wave will cancel out.

Surface Waves

Several types of waves travel only along the outer crust of the Earth. These waves are referred to as surface waves. The motion of surface waves are complex. For example, one of the surface waves travels similar to ocean waves, rolling in an up and down motion. (Hence people often report a 'rolling' feeling during an earthquake.) Surface waves are much slower in speed and larger in amplitude (size). The energy released by a surface wave quickly dissipates and the motion is felt in a limited region around the epicenter.

Measuring Earthquakes

There are commonly two ways to report the size of an earthquake. The Mercalli Scale is used to determine the intensity of an earthquake (i.e., how it was felt and the amount of damage). The Mercalli Scale is a subjective scale set up to record the effects of the earthquake in the region around the epicenter. A survey is performed and a regional map created with areas labeled using 12 divisions (I - XII). The levels are descriptive in nature; ranging from effects not being felt, to objects falling from cupboards, to the highest being total devastation. (See your text for the division descriptions.)

To compare the size of an earthquake with other global readings, the Richter or Magnitude Scale is used. This type of scale reports the magnitude, or amount of energy released, during an earthquake. A network of instruments, known as seismographs, are set up to record the presence of seismic activity. (The instrument is firmly attached to 'bedrock'. When the ground moves, a spring (or small arm) remains quietly suspended. The movement is recorded as a graph known as a seismogram.) The magnitude scales report the size on a logarithmic scale from 1 to 10. (Each number on the scale is 10x the magnitude of the number before it. The amount of energy can be shown, through mathematical calculations, to be over 30x greater between numbers.) Earthquakes of magnitudes less that 4.0 are common events in regions prone to movement; magnitudes of 6.0 or greater usually cause serious destruction to a region.

Epicenter Location

Based on arrival times between P- and S-waves, seismic readings are used to locate an earthquake's epicenter. Think of it as a race between a sports car (P-waves), a slow sedan (S-waves) and an old junker (surface waves). When the race begins, all the vehicles (seismic waves) start at the same point. A few miles from the starting line, the difference in speed results in gaps between the cars. As the race continues, the old junker's engine dies out (energy loss in surface waves) and the faster sports car continues to gain distance on the slower sedan. The further the seismograph is from the epicenter (race start), the larger the gap in time between the P- and S-wave arrival (finishing times between the remaining cars).

Though a single seismogram can give the distance to an earthquake's epicenter, it cannot provide the direction in which it occurred. Several seismograms for an individual event are plotted on a single map. Each seismograph is plotted at the center of a circle (i.e., multiple unknown directions). The radius represents the distance from the seismograph to the earthquake. The area where the circles intersect is the location of the earthquake. A minimum of three widely spaced seismographs are needed to 'triangulate' the location.

The Earth's Interior

Seismic activity can be used to interpret the configuration of the Earth's interior. The velocity of the body waves varies with different rock types and densities. As the waves penetrate the Earth, changes in rock layers are recorded by changes in body wave velocities and directions. (See your textbook for wave property explanations.) Several 'layers' have been identified in the Earth's interior, including the crust, the upper and lower mantle and the outer and inner cores. This class will cover a very simple model of the outer most layers of the Earth.

The crust consists of two distinctive types: oceanic and continental. The two crusts are firmly attached to one another but do not overlap. Oceanic crust is thin (10 kilometers or less) and consists mostly of fine grained basalt. The high density of basalt means the oceanic crust tends to 'sink' and form 'low lying' regions on the Earth's surface. Water often floods the low lying regions creating oceans. Continental crust is thicker (20 to 40 kilometers, or more) and is composed of silica-rich rocks, such as granite and shale. The 'granitic' continental crust tends to 'float' on the surface with deep 'roots' below large mountain ranges (similar to ice cubes in a glass of water).

The upper mantle lies beneath the crust and can be divided into several layers. Each layer is determined by a change in rock type, density or other characteristics (ex.: plastic nature) that result in shifts in seismic wave velocity. Directly below the crust is a rigid portion of the upper mantle approximately 70 km in thickness. The thickness varies, especially near mid-ocean ridges, but it occurs even under extensive mountain roots. The rigid layer is very brittle and is firmly attached to the overlying crust. This unit (crust and rigid upper mantle) is known as the lithosphere. Below the lithosphere is a region known as the asthenosphere. This portion of the upper mantle is partially molten and acts as the source of magma for igneous activity. The region is plastic in nature; 'giving' and shifting, accommodating mountain roots and filling in regions near thinner lithospheric areas. Deep seated earthquake foci occur to a depth of approximately 700 km, usually considered to be the lower limit of the asthenosphere.

This configuration of the Earth's interior is the basis for the theory of plate tectonics. The movement of broken fragments of lithosphere along the flexible asthenosphere is used to explain the complex geology that occurs in long linear features along the Earth's crust.

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