Earthquakes IIGEOL 108Lg 1st Edition Lecture 4Outline of Last LectureI. FaultsII. Plate BoundariesIII. Rupture ProcessIV. Earthquake TerminologyV. Foreshocks and AftershocksVI. Seismic Cycle: Reid’s Elastic Rebound TheoryVII. Stick-Slip CycleOutline Current Lecture I. Surface Waves MotionII. Locating an EpicenterIII. Earthquake Magnitudes & IntensityIV. Empirical laws for aftershocksa. Omori’s Lawb. Bath’s LawCurrent LectureEarthquakes IIFun FactBest place to record seismic waves is the moon! Because no distracting noise or waves.Surface Wave MotionRayleigh waves Rayleigh waves act like the water’s surface, but the particle movement is backwards, thus they are “unsurfable”.They are named after Lord Rayleigh, a scientist who mathematically demonstrated their existence. Rayleigh waves move along the surface of an elastic solid - the earth is a good example. The movement of Rayleigh waves comes as a result of longitudinal compression and dilation. This causes an elliptical motion of points on the surface. Love wavesLove waves travel faster than Rayleigh Waves. They are propagated when the solid medium nearthe surface has varying vertical elastic properties. This causes ONLY back and forth motion, as depicted by the image below. The motion of the wave is perpendicular to the direction of propagation.P waves - the motion is in the direction of the propagation. That motion is mostly vertical and just a little bit horizontal.S waves - the particles are moving back and forth at right angles - most of it is horizontal, with very little vertical motion.Locating an EpicenterP and S Waves move at different speeds. The key to finding an earthquake epicenter lies in thearrival time differences. After identifying P and S time arrivals, you subtract the arrival times toget the S-P delay. Then, you use seismic wave travel time vs. distance chart to determine thedistance to epicenter (Find where S wave line minus P wave line equals the delay time that youoriginally determined - this will correspond to a distance). You repeat these steps for two other seismic stations, then use a compass to plot all possibleepicenter locations surrounding the station - the epicenter is where they all intersect. RECAPP and S waves move at different speedsThe difference between the arrival times of the P and S waves at a seismographic station depends on the distance of the station from the epicenter. Recordings from at least three stations are needed to determine the location of an epicenter.Earthquake MagnitudesRichter magnitude scale : A logarithmic scale based on the maximum amplitude of ground motion, recorded on a standard seismograph, correcting for the distance to the source.ML = log10 (amplitude of seismograph) + distance correctionTotal energy and rupture area are better estimates of earthquake size than the maximum amplitude of seismic waves recorded on a seismograph.Magnitude scales are logarithmic - an increase of 1 magnitude unit = 30 times more energy. Earthquakes less than M=2 are generally not felt by people. Earthquake intensity is measured often by what people felt; surveying the publicOverview of Typical ParametersEarthquakes vs. ExplosionsEmpirical laws for aftershocksOmori's (1894) LawOmari’s Law is an empirical relation for the temporal decay of aftershock rates. It states that the rate of aftershocks is inversely proportional to time from main shock: thus, the rate of aftershocks decreases quickly with time. In simplest case, probability at day two is half that of day one, at day ten a tenth of day one, etc.Bath's LawAftershocks are approximately 1.2 magnitudes smaller than the main shock, regardless of main shock magnitude. Many people do not use this