A Violent Pulse: Earthquakes Chapter 8 part 2 Earthquakes and the Earth’s Interior What is an Earthquake? Seismicity • ‘Earth shaking caused by a rapid release of energy.’ • Seismicity (‘quake or shake) cause by… – Energy buildup due tectonic – Motion along a newly formed crustal fracture (or, stresses. fault). – Cause rocks to break. – Motion on an existing fault. – Energy moves outward as an expanding sphere of – A sudden change in mineral structure. waves. – Inflation of a – This waveform energy can magma chamber. be measured around the globe. – Volcanic eruption. • Earthquakes destroy – Giant landslides. buildings and kill people. – Meteorite impacts. – 3.5 million deaths in the last 2000 years. – Nuclear detonations. • Earthquakes are common. Faults and Earthquakes Earthquake Concepts • Focus (or Hypocenter) - The place within Earth where • Most earthquakes occur along faults. earthquake waves originate. – Faults are breaks or fractures in the crust… – Usually occurs on a fault surface. – Across which motion has occurred. – Earthquake waves expand outward from the • Over geologic time, faulting produces much change. hypocenter. • The amount of movement is termed displacement. • Epicenter – Land surface above the focus pocenter. • Displacement is also called… – Offset, or – Slip • Markers may reveal the amount of offset. Fence separated by fault 1 Faults and Fault Motion Fault Types • Faults are like planar breaks in blocks of crust. • Fault type based on relative block motion. • Most faults slope (although some are vertical). – Normal fault • On a sloping fault, crustal blocks are classified as: • Hanging wall moves down. – Footwall (block • Result from extension (stretching). below the fault). – Hanging wall – Reverse fault (block above • Hanging wall moves up. the fault). • Result from compression (squeezing). • Miners on a – Thrust fault fault would… • Special kind of reverse fault. – Stand on the • Fault surface is at a low-angle. footwall; – Strike-slip fault – Bump their heads on the • Blocks slide past one another. hanging wall. • No vertical block motion. Faults and Fault Motion Fault Initiation (elastic rebound theory) • Faults are commonplace in the crust. • Tectonic forces add stress to unbroken rocks. – Active faults – On-going stresses produce motion. • The rock deforms slightly (elastic strain). – Inactive faults – Motion occurred in the geologic • Continued stress cause more stress & cracks. past. • Displacement can be visible. • Eventually, cracks grow to the point of failure. – Fault trace – A surface tear. Fault location evident by surface tear. • Elastic strain transforms into brittle deformation – Fault scarp – A small cliff. (rebounds), releasing earthquake energy. • Blind faults are invisible. Fault Motion Fault Motion • Faults move in jumps (rebounds). • When rocks break, stored elastic strain is released. • Once motion starts, it quickly stops due to friction. • This energy radiates outward from the hypocenter. • Eventually, strain will buildup again causing failure. • The energy, as waves, generates vibrations. • This behavior is termed stick – slip behavior. • The vibrations cause motion, as when a bell is rung. – Stick – Friction prevents motion. • Large earthquakes are often… – Slip – Friction briefly overwhelmed by motion. – preceded by foreshocks, and… • Smaller quakes. • May signal larger event. – followed by aftershocks. • Smaller quakes. • Indicate readjustment. 2 Amount of Displacement Seismic Waves • Displacement scale varies. • Body Waves – Pass through Earth’s interior. – Large events may rip large fault segments. – Compressional or Primary (P) waves • 100s of kilometers long • Push-pull (compress and expand) motion. • 10s of kilometers deep • Travel through – Smaller events may result in more localized effects. solids, liquids, • Displacement maxima near focus / epicenter. and gases. • Displacement diminishes with distance. • Fastest. • Faulting changes landscapes. – Shear or Secondary (S) waves – Uplift • “Shaking" motion. – Subsidence • Travel only – Offset through solids; not liquids. • Changes are measureable. • Slower. – Interferometry Seismic Waves Seismology • Surface Waves – Travel along Earth’s surface. • Seismology is the study of earthquake waves. – Love waves – s waves intersecting the surface. • Seismographs - Instruments that record seismicity. • Move back and forth like a writhing snake. – Record Earth motion in – Rayleigh waves – p waves intersecting the relation to a stationary surface. mass or rotating drum. • Move like ripples on a pond. – Deployed worldwide. – Can detect earthquakes • These waves are the slowest and most destructive. from around the entire planet. – Seismology reveals much about earthquakes. • Size (How big?) • Location (Where is it?) Seismograph Operation Locating an Epicenter • Straight line = background. • Locating an epicenter depends upon the • Arrival of 1st wave causes different velocities of p and s waves. frame to sink (pen goes up). • Because they travel at different velocities, they • Next vibration causes located by comparing p and s wave arrival times opposite motion. from a minimum • Waves always arrive in of three seismic sequence. stations. –P-waves 1st –S-waves 2nd – Surface waves last. • A seismogram measures… – Wave arrival times – Magnitude of ground motion. 3 Locating an Epicenter Locating an Epicenter • A circle with a radius equal to the distance to the epicenter is drawn around each station. • First arrival of p and s waves compared for (at • Data from three least) 3 stations. stations needed. • The point where • A travel-time three circles graph plots the intersect is the distance of each epicenter. station to the epicenter. Earthquake Size Earthquake Size • Two means of describing earthquake size • Magnitude – The amount of energy released. – Intensity (Mercalli scale) – Maximum amplitude of ground motion from a – Magnitude (Richter & Moment) seismogram. – Value is normalized for seismograph distance. • Mercalli Intensity Scale – Intensity – The degree of • Several magnitude scales. shaking based on damage – Richter (most common) (subjective scale). – Moment (most accurate) – Roman numerals assigned to different • Magnitude scales are logarithmic. levels of damage. – Increase of 1 Richter unit – Damage occurs in zones. = 10 fold increase in ground – Damage diminishes in motion however this intensity with distance. = a 30 fold increase in energy. Measuring Earthquake Size Earthquake Occurrence • Earthquake energy release • Earthquakes are closely linked to plate tectonic boundaries. can be calculated. • Shallow earthquakes - Divergent and transform boundaries. – Energy of Hiroshima bomb • Intermediate & deep earthquakes – Convergent boundaries. is ~ 6.0 magnitude quake – Annual energy released by all quakes is ~ 8.9 magnitude. • Small earthquakes are frequent. ~100,000 earthquakes (of >3 magnitude) per year. • Large earthquakes are rare. There are ~ 32 earthquakes of >7 magnitude per year. 4 Convergent Plate Boundaries Earthquake Focal Depths • Populous nations in convergent tectonic settings have to • Shallow – 0-20 km depth content with frequent earthquakes. – Along the mid-ocean ridge. • 80% of all earthquakes occur in the circum-Pacific belt – Transform boundaries. (around Pacific Ocean). Another 15% occur in Mediter- – Shallow part of trenches. ranean-Asiatic belt (Mediterranean – Continental crust. to Himalayas to Indonesia) • Intermediate and deep earthquakes occur along the path of a subducting plate called the Benioff-Wadati zone. – Intermediate – 20-300 km depth as downgoing plate remains brittle. – Deep - 300-670km depth - Mineral transformations? • Earthquakes rare below 670 km because the mantle is ductile. Continental Earthquakes San Andreas Fault • Earthquakes in continental crust. • Pacific plate meets the North – Continental transform faults (San Andreas, American plate on the Anatolian). western edge of California. – Very dangerous fault. – Continental rifts (Basin and Range, East African rift). – Hundreds of earthquakes – Collision zones (Himalayas, Alps). each year. – Intraplate settings (Ancient crustal weaknesses). – 12 + major temblors since 1800. Intraplate Earthquakes Earthquake Damage • 5% of earthquakes are not associated with plate • Earthquakes kill people and destroy cities. boundaries. • The death and damage resulting from a large • These intraplate earthquakes are not well understood. earthquake can be horrific and heart-rending. – Possible causes. • Learning about the characteristics of earthquakes, what • Remnant crustal weakness. they do and how they do it, can improve your chances – Failed rifts. of surviving one of these potentially deadly events. – Shear zones. • Stress transmitted inboard. • Isostatic adjustments. • Clusters – New Madrid, Missouri. – Charleston, South Carolina – Montreal, P.Q. – Adirondacks, New York. 5 Earthquake Damage Earthquake Damage • Ground Shaking and Displacement • Ground Shaking and Displacement – Earthquake waves arrive in a distinct sequence. – Earthquake waves arrive in a distinct sequence. – Different waves cause different motion. – Different waves cause different motion. • R waves • P waves • S waves • L waves nd – Last to arrive. –1st to arrive. –2 to arrive. – Follow S-waves – Like ripples in a pond. – Rapid up – down motion. – Back and forth motion. – Ground writhes like a – Stronger than P-wave motion. snake. – May last longer than others. Earthquake Damage Earthquake Damage • Severity of shaking and damage depends on… • Shaking Effects on