You can also go on the Loma Prieta EQ Virtual Field Trip
How to Recognize Active Faults in the Field [use overhead from Envro lab book] Geological Units off set. Offset streams Scarps Linear Valleys Linear Ridges Sag ponds springs Faceted spurs Origin of EQs Stresses build up from plate movement EQs occur at plate boundaries, and help define plate boundaries. Elastic Rebound -- release of energy in elastically deformed bodies of rock. Aftershock an EQ after the main shock that allows surrounding rock to adjust to new positions or stresses. EQs produce Seismic Waves that carry energy away from the EQ site. Types of Seismic Waves (seismos = Greek for EQs) Surface Waves -- travel along the surface -- slowest waves (see below). Body waves --P-Waves -- primary -- travel fast (6 km/s) -- compressional -- can travel through liquid S-Waves -- secondary -- travel slower (3.5 km/s) -- shear -- donŐt travel through liquid --the differences in velocity enable us to determine EQ location. -just like lightning and thunder. Focus -- The center of Energy release during an EQ. Epicenter -- Point on the surface directly above the epicenter. Seismology -- study of EQ waves Seismometers produce seismographs, which record the passing of seismic waves [use brick to demonstrate seismometer] Seismograph is a graph of ground motion vs time [emphasize!] [draw arrival of P-wave before S-wave, then surface waves] Tp = time of day P-wave arrives at seismograph Ts = time of day S-wave arrives at seismograph D = radial distance from epicenter to recording seismograph This is an unknown The S-P time interval (Ts-Tp) is the difference in time between the arrival of the two waves. If tP,S is the time it takes for seismic wave to travel from focus to the seismometer, then velocity (vP,S) = D/tP,S: tP = D/vP and tS = D/vS Two equations, three unknowns: not good But: tS - tP= Ts-Tp now three equations, three unkowns; combining: D/vS - D/vP = Ts-Tp : one equation, one unknown, D!! Show how three seismograph stations are needed to accurately locate an epicenter. Can also figure out tEQ, the time the EQ occured vP = D/tP, tP = D/vP, tEQ = TP - tP Magnitude and Intensity Magnitude -- The amount of Energy released by the EQ -- measured using the height of the S and P waves. -- Each EQ has one and only one magnitude -- The Richter magnitude scale is logarithmic -- factors of 10 between numbers [demonstrate by having 1 vs 10 vs 100 people shout or jump] Intensity -- The amount of shaking felt at any one place, -- This depends on ... 1) Distance from the epicenter 2) Magnitude of the EQ 3) Local geology at the sight, i.e., the type of rocks the ground is made of The stonger the rocks, the smaller the amount of amplitude amplification water bed example Strong rocks are ig rx, or well consolidated sed rx Weak rocks are unconsolidated sediments Liquifaction -- one EQ can have several intensities. -- Table 15.2 lists maximum intensities -- designated using roman numerals Earthquake Prediction (more detail here than in book) --Based on determining recurrance intervals in various segments of faults. Recurrance Interval The period of time needed to build up enough strain so that elastic rebound and an EQ will occur. Fault Segments -- sections of faults that behave differently in terms of recurrance intervals, --often defined by distribution of aftershocks. Characteristic EQ The typical EQ that occurs along a specific fault segment --Inverse relationship between size (magnitude) of characteristic EQ and recurrance interval Seismic Gap -- Section along a fault that has not had recent EQ, therefore higher chance of larger EQ there in the future. [draw section along San Andreas Fault showing segments and seismic gaps. --where would next EQ be??] Creep -- constant movement along a fault due to very frequent, imperceptibly small EQs. -- commmon in Hollister and San Juan Batista. EQ predictions are based on time since last EQ and size of characteristic EQ. [show diagrams for California and Bay Area.] EQs and the Interior of the Earth -- Seismic waves refract (or bend) as they travel deeper in the Earth, or cross physical property boundaries (core-mantle), because of varying density and physical properties (strength) -- Refract the same way light does when passing into water or waves in the ocean. -- Just like ocean waves, seismic waves refract towards the slower medium. -- P-waves travel more slowly in the liquid outer core than in the overlying mantle. Thus, they are refracted downward These effects cause a P-wave shadow, produced by the liquid outer core -- S-Waves cannot travel through liquid outer core -- causes S-wave shadow. [draw picture] Layers of Earth (like asthenospherre) and shape of boundaries are figured out in this manner. Isostasy -- The property of flotational balance among segments of the lithosphere. Operates on density. -- studied using gravitational measurements. Gravity proportional to density. Mantle is denser than crust. Lithosphere can bend, asthenosphere can flow. --Like hardwood and softwood floating in a bathtub. --Glacial ice forming, bending lithosphere, melting, and finally allowing rebound. --Mountain ranges are isostatically balanced with thicker crust. --Isosotasy is impt because it is the main cause of veritcal movement on Earth. (Tectonics is responsible for horizontal movement) --Together they form ŇEarth ForcesÓ.