Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 10: Earthquakes & The Earth’s Interior PowerPoint Presentation Stan Hatfield . SW Illinois College Ken Pinzke . SW Illinois College Charles Henderson . University of Calgary Tark Hamilton . Camosun College Copyright (c) 2005 Pearson Education Canada, Inc. 1 Aug 17, 1999 7.4 Earthquake Izmit Turkey, >10,000 people died 1988 5.9 Saguenay Earthquake What is an Earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy released radiates in all directions from its source, the focus (hypocentre) Energy radiates in the form of seismic waves Sensitive instruments around the world record the event The point on the earth’s surface above the focus is called the epicentre What is an Earthquake? Earthquakes Movements and faults that produce earthquakes are usually associated with large faults in Earth’s crust Most of the motion along faults can be explained by plate tectonics theory Which came first, the big quake or the big fault? What other cause of quakes might there be? Damage from 1906 San Francisco Quake Damage from 1906 San Francisco Quake & Fire What is an Earthquake? Elastic rebound Mechanism for earthquakes was first explained by H.F. Reid (early 1900s) Rocks on both sides of an existing fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome During a quake the stored potential energy is released as elastic wave vibration The strain is released as the fault moves and the cycle repeats. Reid’s Elastic Strain & Rebound Cycle for a Strike-Slip Fault like the San Andreas or the Queen Charlotte What is an Earthquake? Foreshocks and aftershocks Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or even by as much as several years Due to Strain on smaller or adjacent faults A Seismograph is a Mechanical Amplifier This one indicates the direction of first motions. Horizontal Motion Seismograph P, L waves Or Components of Motion. Vertical Motion Seismograph P, S & Rayleigh waves Seismograms & Seismology 2 Types of seismic waves Body waves and Surface Waves Body waves (depend more on rigidity than density) Short period, small amplitude Travel through Earth’s interior refracting with depth Two types based on mode of travel Primary (P) waves Push-pull (compress and expand) motion, changing the volume of material Travel through solids, liquids, and gases Arrive first due to fastest mode of travel Secondary (SV) waves Shear motion at right angles to their direction of travel Slightly greater amplitude than P waves Travel only through solids Generally, in any solid material, P waves travel about 1.7 times faster than S waves Seismograms & Seismology 2 Types of seismic waves Body waves and Surface Waves Surface Waves: Long Period, Large Amplitude, Most damaging 2 types based on mode of particle motion Travel along interfaces, especially the Earth’s surface Formed by P & S conversion at Surface Dispersive, speed depends on frequency or period Two types based on mode of travel Rayleigh waves (R) waves (slowest ~90% of Shear wave 1 to 5 km/s) Retrograde elliptical motion, dampened with depth Travel over solids at ~10X sound in air, ~3 km/s Love waves (SH) waves (2 to 6 km per second) shear motion, dampened with depth Travel through solids Elastic deformation, distortion & particle motion for Body (P, S) and Surface (Love & Rayleigh) waves Seismogram (Vertical motion vs Time) Seismic Travel Time Curves: Arrival time versus Distance VP & VS depend on rock properties VL depends on period Seismic Wave Velocities P-waves: The velocity of P-waves can be calculated from elastic constants of material through which the wave is traveling - one formula is: Vp = (Κ + 4/3 μ) / ρ where K is the bulk modulus (rigidity, change in volume), μ is the shear modulus (change in one linear direction) & ρ is the density Rocks get stiffer with depth more than they get denser so Vp increases & the fastest ray path is curved Seismic Wave Velocities S-waves: The velocity of S-waves can be calculated from elastic constants of material through which the wave is traveling - one formula is: Vs = μ / ρ where μ is the shear modulus & ρ is the density Rocks’ shear strength increases more with depth than density so Vs increases & the fastest ray path is curved The shear modulus is Zero in liquids so Vs = 0 Seismic Wave Velocities Why are P-waves always faster than S-waves? Vp / Vs = {(Κ + 4/3 μ) / ρ} / {μ / ρ} , so: Vp / Vs = Κ/ μ + 4/3 For most rocks the ratio of bulk modulus to shear modulus is about 0.35 to 0.4 so: Vp / Vs ≈ 1.7 An Earthquake Epicentre is Located from 3 or more Seismic Stations: (P-S) gives Distance to Source Locating the Source of an Earthquake A circle with a radius equal to the distance to the epicentre is drawn around each station on a globe of Earth The point where all three circles intersect is the earthquake epicentre Since most earthquakes are shallow the circles usually intersect at a point More seismic stations permit better solutions Locating the Source of an Earthquake Earthquake belts About 95 percent of the energy released by earthquakes originates in a few relatively narrow zones that wind around the globe Major earthquake zones include the Circum-Pacific belt, Mediterranean Sea region to the Himalayan complex, and the oceanic ridge system Subduction zones pierce the entire thickness of the lithosphere and generate quakes up to about 9.5 Transform faults cut the lithosphere vertically and generate quakes up to about 8.5 Ridge systems are warm so the brittle depth is less and they generate quakes up to about 7 Major Earthquake Belts Cold Continental Crust can generate large quakes too! 3 Zones of Earthquake Depths (km): Shallow <70 80< Inter<300 300<Deep<660 Which pose the greatest risk? Why none deeper? Measuring the Size of an Earthquake Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale Based on the amplitude of the largest seismic wave recorded on a seismogram Accounts for the decrease in wave amplitude with increased distance Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard The drawback of intensity scales is that destruction may not be a true measure of the earthquakes’ actual severity The Modified Mercalli Intensity Scale: Turning anecdotal observations into an intensity map. Standardizing Richter Magnitude (S-P) versus Amplitude Equals Richter Magnitude Measuring the Size of an Earthquake Moment Magnitude Scale Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimates the size of very largest earthquakes Derived from the amount of displacement that occurs along a fault zone (displacement X surface area) Can be determined from offsets and the length of the aftershock zone Mr = 9, 1964 Alaska Quake Relocated Valdez & Douched Port Alberni US Death Toll ~131 Damage from an Earthquake 1985 Mexico Earthquake, 8.1 10,000 people killed, $4 billion USD damage 412 buildings collapsed, 312 seriously damaged Severe damage in Mexico City ~400 km from epicentre 1 strike 3 spares Why did some buildings collapse here? Why did this building buckle here? Earthquake Destruction Earthquakes don’t kill people, buildings do! Destruction from seismic vibrations – depends on: Intensity and duration of the vibrations Nature of the material upon which the structure rests Design of the structure All of these factors can amplify surface wave motion (like a kid on a swing)! Sand or Mud Volcanoes above a fracture are formed by liquefaction of saturated beds during a quake. 1989 Loma Prieta California Ground motion depends on substrates! Can Earthquakes be Predicted? Short-range predictions Research has concentrated on monitoring possible precursors – phenomena that precede an earthquake such as measuring uplift, subsidence, strain in the rocks, well level or pressure changes, deformation arrays (Laser ranging, GPS) Long-range forecasts Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more based on strain build up and possible recurrence intervals Can Earthquakes be Predicted? Long-range forecasts Based on the premise that earthquakes are repetitive or cyclic Using historic records or paleoseismology to show quiet zones or seismic gaps Gaps could indicate strain building or mapping errors Are important because they provide information used to Develop construction standards for buildings, dams, bridges, pipelines, mines etc. Assist in land-use planning National Building code is revised every 5 years Buildings over 4 stories require earthquake engineering Can Earthquakes be Predicted? Can Earthquakes be Predicted? No one knew about this fault prior to 1994! Can Earthquakes be Predicted? Activity along the Queen Charlotte Fault & Aleutian Trench 1930-1979: Seismic Gaps Quake Probabilities From California Recurrences: Where would you want to live if you were a renovation contractor? Faults of the Turkish Microplate The squeeze play between Africa & Eurasia Can Earthquakes be Predicted? Activity along the North Anatolian Fault 1939-1999: Westerly Moving Seismicity Hey Meester, you want to buy time share in Istanbul? 20 years worth of earthquakes in Canada 1980 to 2000 See NRCan Website Western Canada’s Plate Boundaries & Significant Historic Earthquakes 2009 2001 Can Earthquakes be Predicted? Epicentres and seismic gap near the Queen Charlotte Islands. The seismic gap is the most likely location for future earthquakes. Tues Nov 17, 2009 M6.5, 7:30 AM Felt to Burns Lake! Damage to a school in Courtenay, B.C. June 23, 1946 Vancouver Island 7.3 Quake 1 death, roads, docks & chimneys collapsed Tsunamis Tsunami (“Harbour Wave” in Japanese) In the open ocean, height is usually <1 metre In shallower coastal waters the water piles up to heights that can exceed 30 metres Displaces the entire water column & can cause major damage to low lying coastal areas Results from vertical displacement along a fault which moves the ocean floor or a Large submarine landslide or a large Submarine volcanic eruption People were unprepared as a tsunami wave hit a beach on Thailand (A) Dec 26, 2004 resulting in extensive damage (B) from an earthquake off the coast of Sumatra (C) Banda Aceh before 12/26/04 Tsunami Banda Aceh during Tsunami withdrawal Banda Aceh before 12/26/04 Tsunami Banda Aceh after 12/26/04 Tsunami Tsunamis: gravity displacement waves Burin Peninsula NF after 1929 Grand Banks Earthquake & Tsunami Port Alberni after 1964 Good Friday Tsunami Tsunami warning systems are now being built for Indian & Atlantic Probing Earth’s Interior: The MOHO Discovery of the Moho using P wave travel to three seismic stations. After Andrija Mohorovicic 1909 P-waves arrive earlier at more distant stations due to velocity increase with depth & refraction. Probing Earth’s Interior Most of our knowledge of Earth’s interior comes from the study of earthquake waves Most of this happened post WWII due to seismic monitoring during the Cold War Nuclear blasts are pure compressional and have no S waves! Travel times of P and S waves through the Earth vary depending on the properties of the materials Variations in the travel times correspond to changes in the materials encountered Analyzing seismograms created tomography of Earth Seismic Tomography was co-opted for Ultrasound imaging, CAT Scans and MRI’s (Serendipity: Fund all research and good ideas spread like wildfire!) Wave fronts are spheres in a uniform solid. Rays are wave front normals. Probing Earth’s Interior The nature of seismic waves P waves can travel through solids and liquids S waves cannot travel through liquids In all materials, P waves travel faster than S waves Body waves increase in velocity with depth and pressure as rocks get stiffer faster than they get denser When seismic waves pass from one material to another, the path of the wave is refracted (bent) Mineral/rock boundaries, different layers are discontinuities Where velocity increases versus depth, ray paths curve A few possible ray paths in a spherically layered Earth. (Seismic arrival times map velocity variations.) Probing Earth’s Interior The core-mantle boundary Discovered in 1914 by Beno Gutenberg Based on the observation that P waves die out at 105 degrees (angular distance) from the earthquake and reappear at about 140 degrees - this 35 degree wide belt is called the P-wave shadow zone Probing Earth’s Interior The P-wave shadow zone. Discovering Earth’s Major Boundaries The P-wave shadow zone. Probing Earth’s Interior Discovery of the inner core Predicted by Inge Lehmann in 1936 P waves passing through the inner core from an atomic blast in the 1960’s show increased velocity suggesting that the inner core is solid C’est Tout!