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Learning Objectives Saturday, August 22, 2009 5:05 PM LO2-1 Describe the physical characteristics of the earth's layers. 1. crust 2. lithosphere 3. asthenosphere 4. mantle 5. outer core 6. inner core LO2-2 Describe scientific evidences that support plate tectonics. LO2-3 Describe the different types of plate boundaries and the types of earthquakes they generate. Give an example or location of each of these boundaries. 1. spreading or rift zone (divergent plate boundary) 2. convection zone (convergent plate boundary) 3. Transform fault boundaries LO2-4 Describe the characteristic of fault types in terms of relative movement of the fault plane. 1. dip slip 2. strike slip 3. oblique slip LO2-5 Describe the various earthquake magnitude scales and how to calculate each. 1. Local magnitude 2. Surface wave magnitude 3. Body wave magnitude 4. Moment magnitude Lecture 2 - Science of Earthquakes Page 1 Learning Objectives Saturday, August 22, 2009 5:05 PM LO2-6 Know how to calculate the distance to the earthquake focus from p and s-wave arrival information. LO2-7 Know how to calculate earthquake magnitude, maximum and average fault displacement from geological data. LO2-8 Define the following: focus epicenter epicentral distance hypocentral distance right lateral fault fault directivity fault fling elastic rebound theory seismic moment modified Mercalli intensity scale maximum credible earthquake characteristic earthquake asthenosphere continental shelf mid Atlantic ridge fault dip fault strike return period active fault Lecture 2 - Science of Earthquakes Page 2 Important Concepts Tuesday, August 18, 2009 3:07 PM 1. The earth is a dynamic planet. The earth’s crust and landforms have changed throughout geologic time. The planet materials have segregated by density into distinct layers: (1) core, (3) mantle, (3) lithosphere, (4) surface fluids (i.e., water and atmosphere). 2. The theory of continental drift (plate tectonics) was proposed in the early 1900s and was supported by a variety of geologic evidence. However, without a knowledge of the nature of the oceanic crust, a complete theory of earth dynamics could not be developed. 3. A major breakthrough in the development of plate tectonic theory occurred in the early 1960s when the topography of the ocean floors was mapped and magnetic and seismic characteristics were determined. 4. The earth’s tectonic system involves the movement of material from the earth’s interior which results in seafloor spreading, creation of new crust, continental drift, volcanism, earthquakes and mountain-building. 5. The lithosphere can be divided into a series of plates bounded by the oceanic ridge, trenches, mountain ranges, and transform faults. 6. The plates move apart where the convecting mantle rises and spread laterally beneath the oceanic ridge at rift zones. The plates descend (subduct) into the mantle beneath ocean trenches and are consumed in the mantle by heat in subduction zones. 7. The energy for plate tectonics is internal heat, probably generated by radioactivity in the asthenosphere. 8. There are three types of plate boundaries: (1) rift or spreading zones, (2) subduction zones, (3) transform faults. Subduction zone boundaries can produce the largest earthquakes, followed by transform faults and then rift zones. 9. Faults can have relative movements which consist of: (1) dip slip, (2) strike slip, and (3) oblique. Lecture 2 - Science of Earthquakes Page 3 Important Concepts Sunday, August 23, 2009 7:45 PM 10. The energy-releasing function of earthquakes suggests that a period of time for strain energy accumulation should be expected between large earthquakes at the same location. It also suggests that earthquakes should be most likely to occur along portions of a fault for which little seismic activity has been observed-unless the plate movement has occurred aseismically. 11. Earthquake intensity is a qualitative measure of the effects of an earthquake at a particular location. It is related to the size of the earthquake but is also influenced by other factors. Isoseismal maps can be used to describe the spatial variation of intensity for a given earthquake. Because no instrumental measurements are required, historical accounts can be used to estimate intensity values for pre-instrumental earthquakes. 12. Earthquake magnitude is a quantitative measure of the size of an earthquake. Most magnitude scales are based on measured ground motion characteristics. The local magnitude is based on the trace amplitude of a particular seismometer, the surface wave magnitude on the amplitude of Rayleigh waves, and the body wave magnitude on the amplitude of p-waves. Because these amplitudes tend to reach limiting values, these magnitude scales may not accurately reflect the size of very large earthquakes. The moment magnitude, which is not obtained from ground motion characteristics, is able to describe the size of any earthquake. 13. Earthquake magnitude scales are logarithmic, hence a unit change in magnitude corresponds to a 10-fold change in the magnitude parameter (ground motion characteristic or seismic moment). The energy released by an earthquake is related to magnitude in such a way that a unit change in magnitude corresponds to a 32-fold change in energy. Lecture 2 - Science of Earthquakes Page 4 Internal Structure of Earth Sunday, August 23, 2009 8:26 PM The internal structure of the earth as deduced from variations in velocities of seismic waves at depth, The velocity of both P and S waves increases to a depth of approximately 100 km; then it decreases abruptly to a depth of 200 km. This lowvelocity layer is called the asthenosphere. There is then a general increase in velocities of P and S waves to a depth of about 3000 km, where both change abruptly. The S wave does not travel through the central part of the earth, and the velocity of the P wave decreases drastically. This is the most striking discontinuity in the earth and is considered to be the boundary between the core and the mantle. Another discontinuity in P waves occurs at a depth of 5000 km, indicating an inner core. Lecture 2 - Science of Earthquakes Page 5 Physiography of Atlantic Ocean Sunday, August 23, 2009 8:40 PM Lecture 2 - Science of Earthquakes Page 6 Physiography of Pacific Ocean Sunday, August 23, 2009 8:45 PM 1 2 3 4 6 5 1 = Kuril Trench 2 = Japan Trench 3 = Mariana Trench (Challenger Deep 36198 feet) 4 = Philippine Trench 5 = Tongan Trench 6 = Pacific Rift Zone Lecture 2 - Science of Earthquakes Page 7 Plate Boundaries Sunday, August 23, 2009 9:11 PM (a) Earthquake Epicenters (dots) and Subduction Zones (sawtooth lines) (b) Major Plate Tectonic Features and Direction of Movement of Plates Lecture 2 - Science of Earthquakes Page 8 Plate Boundaries Sunday, August 23, 2009 9:01 PM Lecture 2 - Science of Earthquakes Page 9 Subduction Zone Sunday, August 23, 2009 9:03 PM Subduction zone at converging plate margins Cascadian subduction zone Lecture 2 - Science of Earthquakes Page 10 Fault Geometry Sunday, August 23, 2009 9:17 PM Lecture 2 - Science of Earthquakes Page 11 Fault Movement Sunday, August 23, 2009 9:20 PM Lecture 2 - Science of Earthquakes Page 12 Fault Movement Sunday, August 23, 2009 9:25 PM Lecture 2 - Science of Earthquakes Page 13 Fault Solutions from P-waves Sunday, August 23, 2009 9:31 PM positive polarity = compression negative polarity = extension Lecture 2 - Science of Earthquakes Page 14 Polarity from direction of first wave arrival Elastic Rebound Theory Monday, August 24, 2009 7:41 AM Elastic Rebound Theory (Video) Lecture 2 - Science of Earthquakes Page 15 Elastic Rebound Theory Monday, August 24, 2009 7:44 AM see Multiple Epicentral Locations - 1964 Alaska Earthquake see Fault Directivity see Fault Fling Lecture 2 - Science of Earthquakes Page 16 Multiple Epicentral Locations - 1964 Alaska Earthquake Thursday, January 17, 2013 11:43 AM © Steven F. Bartlett, 2013 Lecture 2 - Science of Earthquakes Page 17 Fault Directivity Thursday, January 17, 2013 11:45 AM Earthquake directivity is the focusing of wave energy along the fault in the direction of rupture. This means that, exclusive of local site conditions such as soft soil, the stronger ground motions (and damage, if the earthquake is large enough) will be distributed in an elongated pattern centered along the axis of the fault. In other words, distance to the fault is not the only consideration for ground motion amplitude: direction is also important. When a fault ruptures unilaterally (with the epicenter at or near one end of the fault break), the radiated waves are stronger in one direction along the fault. Pasted from <http://earthquake.usgs.gov/regional/nca/rupture/directivity/> Peak accelerations and epicenter of the San Juan Bautista earthquake of August 12, 1998. The epicenter is near the southeast end of the fault break. Notice the asymmetrical distribution, with higher values in the northwest, along the strike of the fault. Pasted from <http://earthquake.usgs.gov/regional/nca/rupture/directivity/> © Steven F. Bartlett, 2011 Lecture 2 - Science of Earthquakes Page 18 Fault Fling Thursday, January 17, 2013 11:45 AM see more from: Effects of Fling Step and Forward Directivity on Seismic Response of Buildings Erol Kalkan,a… S.M.EERI, and Sashi K. Kunnath,a… M.EERI © Steven F. Bartlett, 2011 Lecture 2 - Science of Earthquakes Page 19 Elastic Wave Propagation Thursday, September 10, 2009 10:52 AM Elastic Properties and Wave Propagation The elasticity of a homogeneous, isotropic solid can be defined by two elastic constants, K and G. • K = bulk modulus = 27 x 1010 dynes per cm2 (for granite) = 2.0 x 1010 dynes per cm2 (water) (Note that 1 dyne = 1 x 10 -5 N) • G = shear modulus (sometime also called the modulus of rigidity, ) = 1.6 x 1011 dynes per cm2 (for granite) = 0 (for water) (water cannot sustain a shear stress) Speed of wave propagation in an elastic body • Velocity of P wave = ((K + 4/3*G1/2 where = mass density • Velocity of S wave = (G/)1/2 Location of Earthquakes The difference in arrival time between the first P wave and the first S wave can be used to calculate the distance to the earthquake source. t = D(Vp - Vs )/(Vp Vs ) = D [1/Vs - 1/Vp ] where t = difference in arrival time between the P and S wave D = distance to the earthquake focus VP = P wave velocity Vs = S wave velocity D2 D1 D3 Lecture 2 - Science of Earthquakes Page 20 Seismic Moment and Moment Magnitude Monday, August 24, 2009 7:45 AM Lecture 2 - Science of Earthquakes Page 21 Other Measures of Magnitude Monday, August 24, 2009 7:56 AM Lecture 2 - Science of Earthquakes Page 22 Earthquake Intensity Monday, August 24, 2009 8:01 AM Lecture 2 - Science of Earthquakes Page 23 Earthquake Intensity - Masonary Classification Monday, August 24, 2009 8:02 AM Lecture 2 - Science of Earthquakes Page 24 Earthquake Intensity Monday, August 24, 2009 8:00 AM Isoseismal map for the 1811 New Madrid Earthquake Lecture 2 - Science of Earthquakes Page 25 Earthquake Intensity Monday, August 24, 2009 8:00 AM Isoseismal map for the 1886 Charleston, South Carolina Earthquake Lecture 2 - Science of Earthquakes Page 26 Magnitude from Geological Data Monday, August 24, 2009 8:04 AM Lecture 2 - Science of Earthquakes Page 27 Magnitude from Geological Data Monday, August 24, 2009 8:05 AM Should use coefficients for all slip type (All) in this table. Lecture 2 - Science of Earthquakes Page 28 Magnitude from Geological Data Monday, August 24, 2009 8:10 AM Lecture 2 - Science of Earthquakes Page 29 Blank Wednesday, August 17, 2011 12:45 PM © Steven F. Bartlett, 2011 Lecture 2 - Science of Earthquakes Page 30