Seismic Waves; Body waves — pass through Earths interior. " P-waves (primary or compressional waves). #Waves travel by compressing and expanding material. #Material moves back and forth parallel to wave direction. #P-waves are the fastest. #They travel through solids, liquids, and gases. Body waves — pass through Earths interior. " S-waves (secondary or shear waves). #Waves travel by moving material back and forth. #Material moves perpendicular to wave travel direction. #S-waves are slower than P-waves. #They travel only through solids, never liquids or gases. Surface waves — travel along Earths exterior. ! Surface waves are the slowest and most destructive. " L-waves (Love waves) #S- waves that intersect the land surface. #Move the ground back and forth like a writhing snake. Surface waves — travel along Earths exterior. " R-waves (Rayleigh waves) #P-waves that intersect the land surface. #Cause the ground to ripple up and down like water. How Do We Measure Earthquakes?; Seismometer — instrument that records ground motion. " A weighted pen on a spring traces movement of the frame. Modern seismometers use a magnet and electric coil. They record data digitally. They are able to detect very small ground motions, those that people cannot sense. Seismogram — the data record by a seismometer. It depicts earthquake wave behavior, particularly the arrival times of the different waves, which are used to determine the distance to the epicenter. Seismic waves arrive at a station in sequence. P- ǁaǀes are first. ;P for… priŵarǇ, push pull, preŵierͿ S -waves are second. (S for… seĐoŶdarǇ, shear, striŶg, seĐoŶdaireͿ SurfaĐe ǁaǀes are last. Finding the Epicenter; P-waves always arrive first; then S-waves. " P-wave and S-wave arrivals are separated in time. " Separation grows with distance from the epicenter. " The time delay is used to establish this distance. P-wave and S-wave arrival times can be graphed. A travel- time curve plots the increasing delay in arrivals. The time gap yields distance to the epicenter. Data from three or more stations pinpoints the epicenter. The distance radius from each station is drawn on a map. Circles around three or more stations will intersect at a point. The point of intersection is the epicenter. Earthquake Size; Earthquake size is described by two measurements. " The severity of damage (intensity) – a local measurement " The total radiated seismic energy measured at distance via ground motion (magnitude) ! Mercalli Intensity Scale -- amount of shaking damage. " Roman numerals assigned to different levels of damage. #I = low #XII = high " Damage occurs in zones. " Damage diminishes in intensity with distance. Magnitude is a measurement of size based on the maximum amplitude of seismograph waves. Magnitude is described using different scales. Richter scale (ML) — related to maximum amplitude at a distance of 100 km using a specific type of seimograph. Best for local measurements (near the epicenter). Moment magnitude scale (MW) — the best measure. Based on characteristics of different seismic waves and the area and displacement of fault slip.
- Fall '09
- Plate Tectonics, Magnetic Field, continental crust, plate boundaries