Transcript Slide 1
Molly Zurheide The Basin, NH Marc Massie Saas Fee Switzerland Olivia Sher Amazon River EARTHQUAKES EFFECTS OF EARTHQUAKES SEICHES (pronounced sayshes) Seismic waves cause water in an enclosed basin to oscillate. Can occur miles from the focus or epicenter of the earthquake. EARTHQUAKES EFFECTS OF EARTHQUAKES TSUNAMI Translated from Japanese means “harbor wave”. Not a “tidal wave”. TSUNAMI is a very long wavelength, high-speed ocean wave, produced by earthquakes. Wavelength can be up to 160 km. Velocities reach 800 km/hr (500 mph). Generally imperceptible at sea, crest only 1 m high. Water bunches up when the wave “feels bottom”. Wave height can reach 65 m (210’). EARTHQUAKES EFFECTS OF EARTHQUAKES TSUNAMI Produced by large, rapid displacement of sea floor due to submarine faulting. Can also be produced by submarine landslides. Tsunami hitting Hawaii following an earthquake in Alaska in 1946 EARTHQUAKES EFFECTS OF EARTHQUAKES TSUNAMI 1964 Anchorage Earthquake EARTHQUAKES EFFECTS OF EARTHQUAKES FIRE Fire is not directly a result of earthquakes, but fire certainly accompanies earthquakes in industrialized setting. Earthquakes rupture gas mains, oil tanks and power lines. They also break water mains which makes fire fighting difficult. EARTHQUAKES EFFECTS OF EARTHQUAKES FIRE 1906 San Francisco Earthquake EARTHQUAKES EFFECTS OF EARTHQUAKES FIRE Kobe, Japan Anchorage, AK EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Most earthquakes happen at plate boundaries. Divergent Boundaries Have shallow focus and low magnitude. EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Transform Boundaries Have shallow focus and high magnitude. EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Convergent Boundaries Have deep focus and high magnitude. Deeper focus due to subduction (depths up to 700 km). EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Benioff-Wadati Zone EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Intraplate Earthquakes Generally have shallow focus and low magnitude. Lower strain due to location away from plate margins. However…... Intraplate rocks are older and colder and more brittle. Less fractured at mid-continent. Thus they transmit seismic waves more efficiently. EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Intraplate Earthquakes New Madrid, MO Largest recorded earthquake in North America. Occurred in 1811, lasted 53 DAYS! Three main quakes had magnitudes of ~8.5! 1500 aftershocks. Moved position of Mississippi River Rang church bells in BOSTON! EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Intraplate Earthquakes New Madrid, MO EARTHQUAKES PRINCIPAL EARTHQUAKE ZONES Intraplate Earthquakes New Madrid, MO (1811) vs. San Francisco, CA (1906) EARTHQUAKES OTHER EARTHQUAKE FEATURES SEISMIC GAP Parts of faults “stick”. Friction prevents movement in that part of the fault. Force is applies, rocks are stressed. When friction is overcome, fault movement occurs. Areas where major earthquakes could occur. EARTHQUAKES SEISMIC GAP EARTHQUAKES OTHER EARTHQUAKE FEATURES TECTONIC CREEP Fault has continuous movement over time. Allows stress to be released without building up. EARTHQUAKES TECTONIC CREEP Hayward Fault, CA EARTHQUAKES A COMPARISON A joule is a measure of stored energy, similar to calories. GEOPHYSICAL PROPERTIES OF THE EARTH What do we know about the interior of the Earth? Most comes from interpretation of SEISMIC WAVES. Generally waves travel in a straight line and at an unchanging velocity when passing through a homogeneous medium at constant temperature and pressure. Seismic waves speed up and slow down. Indicates variation in conducting medium. Allowed interpretation of the Earth’s interior. GEOPHYSICAL PROPERTIES OF THE EARTH PRIMARY OR P-WAVES P-waves travel through solid, liquid, and gas. Travel fastest through solid. Velocity varies with medium. Shadow zones exist because of this. Due to refraction of p-waves through different media (i.e., the Earth’s three layers, crust, mantle and core) GEOPHYSICAL PROPERTIES OF THE EARTH SECONDARY OR S-WAVES S-waves do not go through liquid. In early investigations it was noted that p-waves always arrived at the seismograph station, but sometimes S-waves didn’t. Produced a larger shadow zone than for p-waves. GEOPHYSICAL PROPERTIES OF THE EARTH Focus of Earthquake GEOPHYSICAL PROPERTIES OF THE EARTH Seismic waves allow us to view the interior of the Earth. Indicates that crust is thinner under ocean basins than under continents. Change in P-wave velocity indicates a change in composition between crust and mantle. Boundary is called the MOHOROVIČIĆ DISCONTINUITY or MOHO. GEOPHYSICAL PROPERTIES OF THE EARTH GEOPHYSICAL PROPERTIES OF THE EARTH P-waves go through the upper mantle faster than those that go through the crust. Velocity also slows down as they go through the aesthenosphere. Produces Low Velocity Zone. P-waves also determined that there was a boundary between the mantle and the core. GEOPHYSICAL PROPERTIES OF THE EARTH S-waves helped to determine that the outer core was liquid. Observed through the shadow zone for S-waves. Focus of Earthquake GEOPHYSICAL PROPERTIES OF THE EARTH ISOSTASY Lithosphere floats on the underlying denser, heatsoftened, partially melted aesthenosphere. Areas with greater mass sink deeper into the aesthenosphere. Equilibrium between lithospheric segments (plates) and aesthenosphere beneath them is called ISOSTASY. GEOPHYSICAL PROPERTIES OF THE EARTH ISOSTASY Water GEOPHYSICAL PROPERTIES OF THE EARTH ISOSTASY GEOPHYSICAL PROPERTIES OF THE EARTH ISOSTASY Lithosphere, because of its lower density “floats” or “sinks” to find its own isostatic equilibrium. Continental lithosphere is less dense so a larger proportion of it “floats” above the aesthenosphere. Changes in mass cause it to rise and fall. Similar to a boat unloading or loading cargo. Removal of mass causes it to rise. Addition of mass causes it to sink. Rise after removal of mass is called ISOSTATIC REBOUND. GEOPHYSICAL PROPERTIES OF THE EARTH ISOSTASY GEOPHYSICAL PROPERTIES OF THE EARTH EARTH’S MAGNETIC FIELD Penetrates and surrounds the Earth. Extends into space approximately 60,000 km. GEOPHYSICAL PROPERTIES OF THE EARTH EARTH’S MAGNETIC FIELD Obvious cause would be the Earth’s iron core. But, the core is too hot to be a permanent magnet. However, magnetic fields such as those that surround the Earth can be produced by an electrical current. So, we think of the Earth now as a giant electromagnet. The electrical field is produced by the movement of electrons through the molten iron of the liquid outer core. GEOPHYSICAL PROPERTIES OF THE EARTH EARTH’S MAGNETIC FIELD The rotation of the Earth sets the liquid in motion. Moving currents generate the magnetic field. This, then, generates more electrical currents. Produces a SELF-EXCITING DYNAMO. GEOPHYSICAL PROPERTIES OF THE EARTH MAGNETIC REVERSALS Magnetic pole moves around all the time, but in general it is in close proximity to the geographic pole. Earth’s magnetic field reverses occasionally. Magnetic north and south poles switch position. Happens every ~ 500,000 years. We think it takes ~ 1000 to 5000 years to switch, but evidences suggest it can be only ~100. How do we know….. GEOPHYSICAL PROPERTIES OF THE EARTH MAGNETIC REVERSALS GEOPHYSICAL PROPERTIES OF THE EARTH MAGNETIC REVERSALS Normal is when magnetic pole is in the northern hemisphere. Reversed is when magnetic pole is in the southern hemisphere.