Earthquakes (Chapter 13)
Lecture Outline • What is an earthquake? • Seismic waves • Epicenter location • Earthquake magnitude • Tectonic setting • Hazards
Earthquake • An earthquake is caused by the deformation that occurs when one mass of rock slides quickly past another – time scale less than about 1 hour (slower is called creep) • Most common along plate boundaries, but may occur in plate interiors
Seismic (Earthquake) Waves • Travel outward from focus • Focus: site of initial rupture • Epicenter: point on surface above the focus
Elastic Rebound and Fault Rupture • Forces applied, often due to plate tectonics • Rocks accumulate strain (deformation) • Elastic rebound when fault ruptures • Seismic waves radiated • Slip along fault apparent if near the surface • Frequency of earthquakes depends both on earthquake size and rate of strain accumulation
Seismographs • Seismic waves are recorded by seismographs – now mostly digital recordings • Record both horizontal and vertical earth movements
Types of Seismic Waves • Seismographs detect three main wave types: • P Waves (primary waves) the fastest of the two body types (~6-8 km/s); they are the first to arrive • S Waves (secondary or shear waves) body waves travel typically half as fast as P-waves • Surface Waves travel at and near the air-earth interface, are the slowest and last to arrive, travel at speeds lower than shear waves.
P waves (compressional) 6–8 km/s. Parallel to direction of movement (slinky), also called primary waves. Similar to sound waves.
S waves (shear) 4–5 km/s. Perpendicular to direction of movement (rope), also called secondary waves. Result from the shear strength of materials. Do not pass through liquids.
Surface Waves • Travel at and near the surface-air interface • Amplitude of motion decreases exponentially with depth (scaled by wavelength) • Usually largest amplitude, longest period, and most destructive
Locating Earthquakes • If the P- and S- velocities are known, then differences in travel times of P and S waves may be used to estimate distance from the station to the epicenter
Determining Epicenter Location and Focal Depth by Triangulation • With multiple stations, the location of the epicenter can be estimated • Focus (depth) can be inferred from travel times and ‘depth’ phases due to free-surface reflections
Earthquake Magnitude • Earthquake strengths range from imperceptible to catastrophic. Several scales are used: • Richter Magnitude – a logarithmic measure of how much the ground moved at the seismograph as seismic waves pass by • Moment magnitude – a logarithmic measure proportional to total area of fault rupture and seismic energy released • Modified Mercalli scale – Not a magnitude, but a “measure” of the perception of the earthquake – what people felt, and how much damage there was, useful for historical events preceding the invention of the seismograph. This scale is useful for studying historic earthquakes that occurred prior to modern seismographs.
Large earthquakes occur much less frequently than smaller ones - longer recurrence interval
“First motion” studies can determine the type of fault that produced an earthquake
Convergent Plate Boundaries • Deep earthquakes typically are found only at convergent plate boundaries • Shallow earthquakes also occur at these boundaries
Intraplate Earthquakes • Earthquakes also occur distant from plate boundaries, typically with shallow-foci • Some of the most destructive earthquakes (e.g. New Madrid) have been intraplate, but may be associated with ancient plate boundaries. Their cause is a mystery.
Severity of damage is controlled by more than just earthquake magnitude - Ground material properties also affects Shaking Amplitude: bedrock is better than loose soil
Tsunamis • Tsunamis are waves generated by fault motion or slumps on the seafloor • Tsunamis can travel at speeds up to 800 km/hr and form waves over 20m high as they break • Would not be observed if they passed by your boat unless you were in shallow water
Earthquake Prediction -Still Science Fiction • Large earthquakes do tend to follow a cycle of rupture, followed by declining aftershocks and a period of quiescence • During the quiet period strain is building toward another rupture • Recurrence intervals vary from 10’s of years to 100’s • Great interest in seismicity prior to written history • -Helps us predict where, but when is another matter
Seismic Gap Method • Applying this cycle to the plate boundary can yield areas where there has been a long time since the last major earthquake