Earthquake Terms • Earthquake • Seismometer Chapter 13 • Seismic Wave • Seismograph • Focus • Richter Magnitude • Epicenter • Moment magnitude • Body wave • Subduction Zone Earthquakes – P-wave • Tsunami – S-wave • Surface wave • Silent Earthquake – Rayleigh wave – Love wave Earthquakes Earth’s Interior • An earthquake is a trembling or shaking of the ground caused by a sudden release of energy stored in the rocks beneath the Earth's surface. Earthquake Energy • The stresses that cause earthquakes are Earthquake Fracture applied by tectonic forces ultimately derived from convection in the Earth's interior driven by decay of naturally • The focus is the point radioactive elements, principally U, Th, (underground) of fracture of the and K. earthquake. • The stresses cause elastic strain of the rocks of the lithosphere. • The epicenter is the point on • The elastic strain can be released by the surface directly above the brittle fracture of the rocks. focus. • The released kinetic energy radiates as seismic waves. 1 Earthquake Fracture Earthquake Fracture Clicker Question Clicker Question The underground location of the The underground location of the actual fault movement that causes actual fault movement that causes an earthquake is known as the: an earthquake is known as the: • a) fracture zone • a) fracture zone • b) Benioff zone • b) Benioff zone • c) subduction zone • c) subduction zone • d) focus • d) focus • e) epicenter • e) epicenter Clicker Question Clicker Question The point on the Earth’s surface The point on the Earth’s surface directly above the point of rupture that directly above the point of rupture that causes an earthquake is known as the: causes an earthquake is known as the: • a) fracture zone • a) fracture zone • b) Benioff zone • b) Benioff zone • c) subduction zone • c) subduction zone • d) focus • d) focus • e) epicenter • e) epicenter 2 Seismic waves are classified by their propagation mechanism. • Body waves travel through the Earth. Body Waves: – P-waves are primary and fast. P and S waves – S-waves are secondary and slower. travel through the • Surface waves travel on the Earth’s Earth surface. – Surface waves are slower than either P or S waves, but may have large amplitudes near the epicenter. Surface Waves: Body Waves Rayleigh and Love Waves • P-waves propagate by compression like sound waves. – P-waves travel through solids, liquids. or gasses. • S-waves are secondary or shear waves and propagate by shear of a solid (rock) medium. – S-waves only travel through solids. Surface Waves Seismometers • Surface waves travel more slowly than body waves but do most of the damage • A seismometer is a device for near the epicenter. measuring seismic waves from • Like body waves, there are two types of earthquakes. surface wave. • A seismometer is just a heavy • Rayleigh waves propagate by circular weight suspended on springs with vertical displacement like ocean waves. devices to measure the movement of • Love-waves propagate by horizontal the ground relative to the weight. shear displacement. – Love-waves only travel on the surface of a solid. 3 Seismometers Seismometers • P and S waves travel at different velocities in the Earth. • The time difference between the P and S arrivals can be used to measure the distance to the earthquake. • Three seismic stations can then be used to determine the location of the earthquake. Triangulation Clicker Question Clicker Question The exact location of the epicenter of The exact location of the epicenter an earthquake can generally be of an earthquake can generally be determined from: determined from: • a) The exact time of P-wave arrivals at different stations • a) The exact time of P-wave arrivals at different stations • b) using P- and S-wave arrival times at three stations to • b) using P- and S-wave arrival times at three determine distance and then using triangulation. stations to determine distance and then using • c) the exact time of the surface wave arrivals at three triangulation. different locations • c) the exact time of the surface wave arrivals at three • d) the direction of first movement of the S-wave arrivals at different locations three different stations. • d) the direction of first movement of the S-wave arrivals at • e) the magnitude of the P-waves at three different three different stations. stations. • e) the magnitude of the P-waves at three different stations. 4 Richter Scale Moment Magnitude • Open-ended scale (no Maximum) • Rather than ground motion, • Logarithmic (base 10) measure of moment magnitude is defined on ground motion at epicenter. the displacement vector and • Magnitude 6 quake gives 10 times rupture area on the fault. more ground motion than a • Moment magnitude numbers are Magnitude 5. similar to Richter scale magnitudes • Richter scale is roughly base-30 logarithmic in total energy. Epicenters: Where do Earthquakes Occur? • At Plate Boundaries. • Occasionally they occur elsewhere (Intra-plate earthquakes). Epicenters: Where do Earthquakes Occur? 5 Velocity Structure and Earthquakes at N. Tonga Depths of Earthquakes (Vander Hilst Nature (1995) • Most earthquake foci are shallow (<100km). • Intermediate depth quakes occur at depth of 100 - 250km • Deep focus earthquakes occur at 250- 670km. • Intermediate and deep earthquakes occur only at subduction zones. • No earthquake foci at depth below 670 km. Epicenters: Convection in solid mantle Where do Earthquakes Occur? Shallow and Deep Earthquakes Velocity Structure and Earthquakes at N. Tonga (Vander Hilst Nature (1995) 6 Predicting Earthquakes Tsunamis • Stresses build over years or centuries. • A tsunami is a seismic sea wave. • Fracture initiation occurs in a few • Tsunamis are caused by earthquakes, seconds. underwater landslides, and impacts. • Few (if any) precursors. • Tsunamis can travel at speeds up to • The best we can do is identify active 600 km/h in open ocean. zones. • The wavelength may be several • Build for earthquakes in active zones. hundred km and height up to 1 m. – Building codes • In Dec., 2004 a huge tsunami off – Motion sensors (Stop trains and shut off Sumatra killed nearly 300,000 people gas mains) Deep Sea P Sensor Tsunamis • A tsunami is a seismic sea wave. • Tsunami waves can be detected in the open ocean by ocean bottom pressure sensors. • The Pacific Ocean is instrumented. • The Indian Ocean and the Atlantic Ocean are being instrumented. 40 Suggested global distribution of deep sea pressure sensors (to be Deep-sea pressure sensors defined) (Germany) |41 |42 7 Silent Earthquakes Nuclear Explosions • A silent earthquake is movement on a • It is possible to distinguish an fault that creeps without generating underground nuclear explosion from seismic waves. an earthquake. • Creep is episodic with movement • An underground test is shallow lasting several days to several weeks. (<1km). • These have only been recognized • First motion studies show direction of since the early 1990s with accurate P-wave arrival. Explosions are GPS measurements. outward in all directions. Earthquakes are outward in two quadrants only. Clicker Question Clicker Question A seismic sea wave is called a: A seismic sea wave is called a: • a) P-wave • a) P-wave • b) S-wave • b) S-wave • c) Tsunami • c) Tsunami • d) Rayleigh wave • d) Rayleigh wave • e) Love wave • e) Love wave Clicker Question Clicker Question An earthquake resulting from slow An earthquake resulting from slow movement on a fault and which does movement on a fault and which does not generate discrete seismic waves is not generate discrete seismic waves is called a: called a: • a) Fault • a) Fault • b) Silent earthquake • b) Silent earthquake • c) Tsunami • c) Tsunami • d) Subduction • d) Subduction • e) Epicenter • e) Epicenter 8 Earthquake Terms • Earthquake • Seismometer • Seismic Wave • Richter Scale • Focus • Moment • Epicenter magnitude • Body wave – P-wave • Subduction Zone – S-wave • Tsunami • Surface wave – Rayleigh wave • Silent earthquake – Love wave 9.