lesson plan

EOS 170 – Natural Hazards Lecture Summary

Lecture 4 — faulting and seismic waves

Today we will study earthquake faulting, the earthquake cycle, and types of seismic wave. We’ll also

briefly introduce the two major fields of geophysics used to study earthquakes, geodesy and seismology.

Earthquakes occur when faults slip

Many large continental earthquakes generate surface ruptures which are shown to accommodate relative

displacement, or slip, of the crust on either side. These ruptures are the surface expression of faults,

planar features which extend down through the upper crust, typically to depths of 10–20 km (below

which, rocks are normally too hot to be brittle). Rare exposures of fault planes show striations and

corrugations which indicate the direction of slip during earlier earthquakes. Note that earthquakes do

not involve the ground ‘opening up’ (faults 6= fissures); if they did so, there’d be nothing to cause to ground to shake. Earthquakes do not occur at points in space (though they are often plotted this way)

or time. The hypocenter is merely the point on the fault plane where slip starts, and the epicenter is

the point at the surface directly above the hypocenter. The origin time is just when the slip starts;

the largest earthquakes can last several minutes, with the rupture front migrating at 2–3 km/s.

Faults are often classified by their relative sense of motion, or mechanism. Normal faults accommodate

horizontal extension; one side of the fault slides down the other as the two blocks move apart. The

fault dip angle is usually rather steep (often ∼60◦, measured from the horizontal). Thrust faults take up horizontal compression; one side of the fault is ‘thrust’ over the other as the two blocks

converge. The dip angle is usually gentle (∼30◦); steeper compressional faults are known as reverse faults. Finally, strike-slip faults accommodate simple shear, the two sides moving laterally past one

another. If the opposite block moves right, the fault is right-lateral; if it moves left, it is left-lateral.

Strike-slip faults are usually close to vertical (∼90◦). Focal mechanisms are a graphical representation of the type of faulting involved in earthquakes. Each ‘beach ball’ displays a top-down hemispheric

view of the radiation pattern of seismic P waves (more on this later). Directions that feel a push from

the earthquake waves are shaded; those that feel a pull are left unshaded. Different types of plate

boundary have distinct patterns of focal mechanisms that illustrate their tectonics.

The earthquake cycle describes how stress on a fault builds up slowly over time, driven by plate tecton-

ics, before being released suddently in an earthquake. During the interseismic phase, steady motion

occurs a long way away from the fault but the fault itself is locked due to friction. The earthquake itself

marks the coseismic phase (co means ‘during’), in which the crust ‘rebounds’ elastically to produce

an offset along the fault. Geodesy is the study of the shape of Earth and its change through time, and

provides the means to image different parts of the earthquake cycle (nowadays, satellites are used).

Seismic waves

Elastic rebound in an earthquake releases stored energy in the crust as seismic waves. These are

recorded on seismometers as seismograms (graphs of ground motion versus time) and studied using

seismology. There are two main categories: body waves, which pass through the body of the Earth, and

surface waves, which pass along its surface. Body waves include P waves (primary, pressure), which

being the fastest wave always arrive first at any seismometer, and S waves (secondary, shear), which

are slower and arrive second. Of the two, only P waves can pass through liquids; this is how the liquid

outer and solid inner core are distinguished. Surface waves are slower, travelling at ∼2–3 km/s, but also much more damaging. Again there are two types: Rayleigh waves which involve ‘ground roll’ in

the direction of wave propagation, and Love waves which involve side-to-side motion.

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