10/23/ build up of strain. Elastic Rebound Theory: -- apply stress. What is an Earthquake?

Transcription

1 Earthquakes Earthquakes Where Do Earthquakes Occur? Topics Where do earthquakes occur? Earthquake Fundamentals What is an Earthquake? Seismic Waves Locating an Earthquake Sizes of Earthquakes Earthquakes and Plate Tectonics Earthquake Predicition Most earthquakes concentrated at Mid-ocean ridges; Circum-Pacific belt; Mediterranean-Himalayan belt What is EQ What is an Earthquake? The vibration of the earth produced by rapid release of energy Elastic Rebound Theory: -- build up of strain -- apply stress -- causes by sudden movement along a fault Seismic waves Seismic Waves -- when forces exceed the elastic limit à rupture -- release the stored energy/strain -- sent out waves of energy through the earth -- Waves of energy produced by an earthquakes -- point of initiation: "focus" or hypocenter San Andreas Fault after 1906 earthquake Point Reyes National Seashore -- the point on the surface directly above the focus :"epicenter" 1

2 Types of seismic waves Types of seismic waves P & S waves (2) Surface waves: travel on the earth's surface; rolling motion (1) Body waves: travel through the earth's interior P (primary) waves: direction of wave propagation parallel to the direction of particle motion S (secondary) waves: direction of wave propagation perpendicular to the direction of particle motion Which type of earthquake waves cause the most damage during an earthquake? -- Surface waves Body waves Chang Heng Measurement of Ground Vibrations 1st prototype instrument by Chang Heng in 132 A.D. 1st modern Seismograph: ~ 1890 Basic principle: inertia. Resistance of a large mass to sudden movement Record of earthquakes: Seismogram Order of arrivals: P - S - Surface waves Locating an Earthquake Animation: How Seismographs Work : Use travel time difference between P & S waves interactiveanimations/079_seismograph_hs_gg_stu.html?te10 S-P travel time curve 2

3 e.g., The Sizes of Earthquakes Animation: Creating Travel Time Curve (A) Magnitude: : based on the size of ground motion measured by seismograph M Amp mm of ground vibration Log scale:1 unit increase in magnitude = a 10-fold increase in amplitude of the ground motion 5 23 mm Factor of Amplitude Richter Scale: based on amplitude factor 0.1 x 1 x mm x 2300 mm x = 10 n aotm/12/iristraveltime_nobounce_480.mov (n = difference in magnitude) Richter scale is open-ended (theoretically no maximum value) However, in practice M ~ 9 is maximum. Why? The Sizes of Earthquakes Richter Scale: based on amplitude of ground vibration Moment Magnitude: based on energy release (reflects in both amplitude and duration of ground motion) Rocks have finite strength, tend to break after a certain amount of energy is stored in the rocks (A) Magnitude: : based on the size of ground motion measured by seismograph Difference in Magnitude Difference in Amplitude Difference in Energy ~ 3 times (i.e.,100.5) ~ 5.5 times (i.e., 320.5) 1 10 times (i.e.,101) ~ 32 times (i.e., 321) times (i.e.,102) ~ 1000 times (i.e., 322) 0.5 Maximum Surface Fault Displacement vs. Earthquake Moment Magnitude, Mw The Size of Earthquakes (B) Intensity- Modified Mercalli Scale : A measure of earthquake damage at a given location; range from I to XII The Size of Earthquakes (B) Intensity- Modified Mercalli Scale : A measure of earthquake damage at a given location from I to XII Intensity depends on: -- size of EQ -- strength of structures -- proximity to the epicenter -- local geology Soft mud amplifies ground vibration the most Intensity map, 1971 California earthquake (Mw = 6.7) 3

4 Instrumental Intensity (ShakeMap) 1994 Northridge, CA, Mw = Nisqually, WA, Mw = 6.7 Magnitude & frequency Effects of geologic structures on ground shaking Plate boundaries (A) Divergent Boundaries -- Mid-ocean ridge -- Continental rift Earthquakes and Plate Tectonics Divergent Shallow earthquakes Frequency and focal depth for earthquakes with magnitude 5 and greater recorded since 1900 (B) Transform Boundaries (C) Convergent Boundaries convergent (C) Convergent Boundaries Collision zones Shallow to intermediate earthquakes Subduction zones collision (Wadati-Benioff zone) Transform boundary Shallow to deep earthquakes Deepest: Shallow earthquakes ~ 670 KM deep 4

5 Largest EQ ever recorded: 1960 Chile (M = 9.5) Caused by subduction of Nazca Plate beneath South American Plate. Magnitude Russia 1952 Chile 1960 Alaska 1964 Sumatra 2004 Japan 2011 All at subduction zones! Year Great (M > 8) earthquakes since 1900 (figure courtesy of IRIS) Recalling 1960 Chile Quake Recalling 1960 Chile Quake Deadliest subdcution zone earthquake ever reported Sumatra, 12/26/2004, Mw = 9.1 Subduction of Indian Plate beneath Burma Plate tsunami leveled almost all of the homes in Banda Aceh, Sumatra Asian Tsunami - CBS News Most costly natural disaster Tohoku, Japan, 3/11/2011 M= Subduction of Pacific Plate beneath North American Plate. 5

6 Long-Range Earthquake Warning Long-Range Earthquake Prediction Short Range Earthquake Warning P-Wave Warning System, or Shake-Alert Seismic gap -- seismically active area which has not experienced major EQ for a long time Parkfield Gap Seismicity along San Andreas Fault Measuring p-waves to warn the public of impending quakes video Animation: Liqufaction, 1906 SF Earthquake aotm/20/liquefaction_sf1906.mov Animation: How will 3 buildings, engineered equally, on different bedrock react to an earthquake? 6