62G473 Environmental GeologyIntroduction to Earthquakes and SeismologyI. Importance of EarthquakesA. Death and Destruction1. Examplesa. Turkey, August, 1999(1) Mag. 7.4 quake(2) 15,000 dead, 600,000 Homeless(a) 15,000 more missingb. 16th Century China(1) 850,000 deadc. 1923 Tokyo(1) 143,000 deadd. 1995 Kobe Japan(1) 5000 dead(2) 100 billion $ in damagee. San Francisco 1906(1) 700 dead(2) 25 million $ damagef. 1989 Loma Prieta, CA (1) 62 dead(2) 5 billion$ in damageg. Northridge, CA 1994(1) 61 dead(2) 30 billion$ in damageII. BASIC DEFINITIONSA. Earthquake - vibration of earth produced by rapid release of energyB. Focus of Earthquake- point source of earthquake on or within earth's interiorC. Epicenter of Earthquake- map position vertically above focus, at earth's surfaceD. Seismic waves- energy in wave form which radiates in all directions from the focus,vibrational waves in the earth. Wave energy dissipates with distance away from focusE. Source of Earthquakes1. Volcanic eruptions2. Fault movement3. ExplosionsF. Faults-fractures within the earth along which movement occurs on either side, i.e. afault is a plane of discontinuous movement. Earthquakes are often associated withsudden movement and release of stress along a fault.631. Movement along fault may be horizontal, vertical, or oblique (combo of two). Vertical component of movement along a fault may be expressed at earth'ssurface via a fault scarp.2. Fault Typesa. Normalb. Reversec. Strike-Slip(1) left-lateral (sinistral)(2) right lateral (dextral)3. Surface Fault Traces vs. Buried or "Blind" Faultsa. Surface Traces = fault intersects earth surfaceb. Blind Fault = fault at depth, no surface expression4. Other Fault Termsa. Fault Strand = individual faultb. Fault Zone = zones of multiple faultsc. Fault Segment(1) Individual lengths of fault zones that act as coherent unitsd. Inactive vs. Active Faults(1) active - displacement in last 10,000 yrsG. Nature of Seismic Events or Earthquakes1. Rock Deformation Styles in Response to Stress(Stress = Force / unit area)a. Ductile(1) plastic, permanent deformationb. Elastic(1) deformation recovers once stress is removedc. Brittle(1) permanent rupture / deformation of rocksd. Elastic Rebound(1) Follows brittle rupture, rocks elastically re-adjust shape2. Process: crustal rocks are placed under force or stress, the rocks undergoelastic deformation until frictional/atomic forces within the rock are exceeded,and slippage or rupture along a fault occurs. a. Slippage and release of stress allows rocks to "snap back" or reboundafter the release of stress ("elastic rebound"), the elastic rebound is thecause of earth quake or vibrations.b. Aftershocks- periodic mechanical adjustments that occur in rocksfollowing an initial rupture along a fault. Aftershocks are generally weakerthan primary seismic event, but may cause damage to already weakenedstructures.(1) Aftershocks result from elastic rebound64c. Foreshocks- seismic activity that precedes a major earthquake on order ofdays to years.3. Stick-slip motion along a fault- process of discontinuous movement along a fault:build up of stress-elastic deformation (stick)-sudden release along fault (slip)-earthquake-build up of stress-elastic deformation (stick)4. Creep- continuous movement along a fault, very little build-up of strain, lowerearthquake potentialIII. Earthquakes Sources vs. Plate TectonicsA. Intraplate Earthquakes1. Epicenter/focus located within interior of tectonic platesa. e.g. New Madrid, MO seismic zone2. Focus within the center, vertically, of tectonic plates3. Crustal Earthquakesa. focus within earth's crustB. Plate Boundary Earthquakes1. Interface quakes - focus at plate boundaries / shear zonesIV. EARTHQUAKE WAVES (SEISMOLOGY)A. Seismology-study of earthquake waves, dates back 2000 years to ancient Chinese andprimitive seismographs used to detect earth vibration.1. Seismograph-instrument to detect seismic waves or earthquake activity.a. mechanism- weight is freely suspended from a support imbedded inbedrock, weight is attached to a pen and when an earth quake hits, theweight will remain stationary while the frame/support shakes with theearth, the support is connected to a rotating drum with paper on it, andthe relative intensity of quake waves is recorded by the pen.b. seismograms-records from the seismographsB. Seismic waves- elastic energy traveling through earth's crust that is propagated ortransmitted outward radially from the focus of the earthquake. Wave types....(directionof movement)1. Surface Waves - seismic vibrations that move along the outer layer of the earth'ssurface. Have both a vertical and horizontal components of motion, horizontalcomponents cause the most damage.a. Love Waves(1) horizontal vibration, perpendicular to travelb. Rayleigh Waves(1) rolling / orbital ground vibration(a) like ocean waves2. Body waves- seismic vibrations that move through earth's interior65a. Primary waves or P waves- push-pull waves, waves move back and forthin the direction in which the wave is travelling. Change both volume andshape of material in which they pass. Affect and can pass through solids,liquids, and gasses (as they all exhibit resistance to change in volume)b. Secondary waves or S waves-vibrations occurring at right angles todirection of wave propagation, shaking motion. Result in changing onlyshape of material they travel through. Affect and can pass through onlysolid materials (only solids offer resistance to change in shape).3. Seismograms : plot of time vs. intensity of motion, First waves to arrive atseismograph are P waves, 2nd are S waves, and 3 rd are Surface waves. Arrival of waves relates to velocity with which they are transmitted through theearth's crust. Vel. P wave = 1.7(vel S wave), Vel Surface = .90 (vel S waves). Surface waves ride immediately behind S waves which are moving in layerdirectly beneath them.V. LOCATION OF EARTHQUAKESA. Earthquakes can be located by use of several seismic stations at diff. locations on theearth's surface, and by use of arrival times of seismic waves at each stationB. Epicenter of Earthquake- location on earth's surface directly above the focus of theearthquake.C. Focus: actual point of origination of seismic event, may be at depth below earth'ssurfaceD. Location method: difference in velocity of P and S waves provides method ofdetermining epicenter. Based on Vel P > Vel S waves, the greater the distance oftravel, the greater the time lag between first