Seismicity What is an Earthquake? Seismicity What is an - PDF document

A Violent Pulse: A Violent Pulse: Earthquakes Earthquakes Chapter 8 Chapter 8 part 2 part 2 Earthquakes and the Earth’s Interior Seismicity What is an Earthquake? Seismicity What is an Earthquake? • ‘Earth shaking caused by • Seismicity (‘quake or shake) cause by… a rapid release of energy.’ – Motion along a newly formed crustal fracture (or, – Energy buildup due tectonic fault). stresses. – Cause rocks to break. – Motion on an existing fault. – Energy moves outward as – A sudden change in mineral structure. an expanding sphere of waves. – Inflation of a – This waveform energy can magma chamber. be measured around the – Volcanic eruption. globe. • Earthquakes destroy – Giant landslides. buildings and kill people. – Meteorite impacts. – 3.5 million deaths in the last – Nuclear detonations. 2000 years. • Earthquakes are common. Earthquake Concepts Earthquake Concepts Faults and Earthquakes Faults and Earthquakes • 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 Faults and Fault Motion Fault Types 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). – Reverse fault – Hanging wall (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 • Blocks slide past one another. heads on the hanging wall. • No vertical block motion. Faults and Fault Motion Faults and Fault Motion Fault Initiation (elastic rebound theory) 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. • Elastic strain transforms into brittle deformation Fault location evident by surface tear. – Fault scarp – A small cliff. (rebounds), releasing earthquake energy. • Blind faults are invisible. Fault Motion Fault Motion 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 Amount of Displacement Seismic Waves 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 through solids; – Offset not liquids. • Changes are measureable. • Slower. – Interferometry Seismic Waves Seismology 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 mass or rotating drum. surface. – Deployed worldwide. • Move like ripples on a pond. – 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?) Locating an Epicenter Locating an Epicenter Seismograph Operation Seismograph Operation • Locating an epicenter depends upon the • Straight line = background. different velocities of p and s waves. • Arrival of 1 st wave causes 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 1 st – S-waves 2 nd – Surface waves last. • A seismogram measures… – Wave arrival times – Magnitude of ground motion. 3

Locating an Epicenter Locating an Epicenter 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 • Data from three compared for (at stations needed. least) 3 stations. • The point where • A travel-time three circles graph plots the intersect is the epicenter. distance of each station to the epicenter. Earthquake Size Earthquake Size Earthquake Size Earthquake Size • Magnitude – The amount of energy released. • Two means of describing earthquake size – Maximum amplitude of ground motion from a – Intensity ( Mercalli scale ) seismogram. – Magnitude ( Richter & Moment ) – 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 • Magnitude scales are logarithmic. assigned to different levels of damage. – Increase of 1 Richter unit = 10 fold increase in ground – Damage occurs in zones. motion however this – Damage diminishes in = a 30 fold increase in energy. intensity with distance. Measuring Earthquake Size Earthquake Occurrence Measuring Earthquake Size Earthquake Occurrence • Earthquakes are closely linked to plate tectonic boundaries. • Earthquake energy release • Shallow earthquakes - Divergent and transform boundaries. can be calculated. • Intermediate & deep earthquakes – Convergent boundaries. – Energy of Hiroshima bomb 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