Earthquakes • Earthquakes are vibrations in the earth. • Earthquakes are recurring phenomena, affecting areas repeatedly. • Earthquakes are waves resulting from • Slip-lock motion: the release of stored elastic energy from motion along faults; • Implosions: sudden volume changes in subducting oceanic crust due to changes in PT conditions and resulting changes in mineral phase • Other causes like explosions and asteroid impacts. v 0030 of 'Earthquakes' by Greg Pouch at 2011-03-23 11:59:08 LastSavedBeforeThis 2011-03-23 11:54:51
Earthquakes Terminology 3 Vocabulary 4 Vocabulary Processes 5 Motion 6 Measuring Earthquakes > Instrumentation 7 Measuring Earthquakes> Location Distribution 8 Distribution and Causes > Proximate 9 Distribution and Causes>Plate tectonics 10 Distribution and Causes>Plate tectonics>Details 11 Magnitude 12 Magnitude Formulas 13 Magnitude Table with Energies Products 14 Destruction 15 Tsunamis 16 What to Do During an Earthquake 17 Prediction and Control
Vocabulary • Earthquakes are measured using devices called seismometers (the sensor) or seismographs (sensor + recorder) on records called seismograms (paper or computer file). (Same endings as with telegrams) • Seismic refers to vibrations of the ground. • Seismology is a branch of geophysics dealing with earthquakes, their causes, and the planetary distribution of seismic velocity (whole-earth seismology) or using artificially-induced vibrations to explore the subsurface (exploration seismology). • Foreshocks occur before a major earthquake, aftershocks occur after it. When it happens, you don't know whether an earthquake is a foreshock to something bigger or just a regular earthquake.
Vocabulary • The focus is the point where an earthquake initiates, but since an earthquake is generated by an area slipping or volume imploding, distance from the focus is not the same as distance from the source, especially for large earthquakes. (Large earthquakes are large because a large segment of fault slips, so distance from the focus is not as important as it seems at first.) • The epicenter is the point on the surface above the focus. The depth is the vertical distance from the focus to the epicenter, and is harder to determine: often, an initial guess is used, and depths of 5 or 10 or 100 km should be taken cum grano salis.
Motion • An earthquake is a pulse of motion emanating from some source, be it fault tear, implosion, explosion, asteroid impact, footsteps… At least for atomic bombs where we know the source's energy and the waves' energy, relatively little energy is radiated away as elastic waves (like 1 to 10%). • Types of motion • Body waves travel through the body of the material (1/R2 fall-off: energy distributed on sphere) • P-waves are compressional waves, like sound in air and are the fastest. • S-waves are vibrations at right angles to the direction of propagation, like light, and are second fastest. • Surface waves travel along an interface, as between air and ground, or loose materials and bedrock and cause most of the damage in earthquakes. (1/R fall-off: energy distributed on circle) • Rayleigh waves travel along the rock-air interface, and cause the most damage and are like water waves • Love waves are transverse and travel along solid-solid boundaries, like bedrock.
Measuring Earthquakes > Instrumentation • Seismometers have a large mass loosely coupled to the ground (loose springs) and other parts tightly coupled to the ground, so that when the ground moves (and the loose mass doesn't), part of the instrument moves relative to the other; then there is a bunch of mechanical or electrical engineering wizardry to magnify this and make it easily detectable. What is recorded directly is usually amplitude of ground displacement (linear), and can often read to micro-meter or less: what is interesting from the whole-earth geophysics point is energy, from the civil engineering point, acceleration. Both can be derived from the displacement-time relationships.
Measuring Earthquakes> Location • P-waves always travel faster than S-waves, and the delay depends on how far away they originated. The velocity depends on the material, and this is used to find the structure of the earth and for exploration. • By knowing the distance from several fixed points to the earthquake (based on travel time), you can determine the location. (The Global Positioning System works just the opposite: by knowing the travel-time from several satellites, you can determine your location.) • Knowing the direction of first motion also helps determine the type of fault motion.
Distribution and Causes > Proximate • Elastic Rebound Theory • Lock-Slip motion accounts for most near-surface earthquakes • Plates move relative to each other. • Under certain circumstances, this movement is accommodated by no motion for a while, during which energy is stored elastically. Eventually, the rock cannot store any more energy and slips. This slipping changes the stress field nearby and causes vibration, which can cause other nearby rock near its breaking point to slip. (Examples: string instruments, grating machinery, shovel and tree root.) • Sometimes, the rock does not store elastic energy, and simply creeps along. This is known as fault creep. • Phase changes • More than one mineral can have the same chemical composition. • These minerals (polymorphs) often have different PT stability, and physical properties. • Of particular relevance, high-density phases are usually favored at high pressure (LeChatlier's principle). • Sometimes this phase transition is gradual, occurring in small steps. Sometimes, it happens as dramatic implosions. • Olivine (common in oceanic crust) becomes unstable and changes to a higher density form at the depths of deep-focus earthquakes, and this is the probable mechanism for many deep earthquakes. (olivine->spinel at 470km, spinel->perovskite at 600km) • Volcanoes and magma movement • Explosions • Asteroids
Distribution and Causes>Plate tectonics • Most earthquakes are due to plate motion and concentrate at and define plate boundaries. • At mid-ocean ridges, there are small, shallow quakes on normal faults. • During rifting, there can be large, shallow quakes on normal faults. • Subduction zones have • Shallow quakes due to fracturing from bending (Think of a subduction zone as a really big thrust fault.) • Deep quakes due to phase-change implosions • Collision zones have shallow quakes on thrust faults and strike-slip faults, and perhaps secondary normal and reverse faults. • Transform boundaries have strike-slip faults with shallow- to intermediate- focus quakes, often very severe. • Volcanic eruptions and intrusions often cause small earthquakes. • Plates do not behave entirely rigidly, and there are also intra-plate seismic zones, such as New Madrid, MO, which might be driven by loading-unloading or differences in thermal expansion or…
Magnitude • Magnitude refers to description of the size of an earthquake. (how big?) A single number for the magnitude does not describe an earthquake very fully, but it is standard. • Mercalli Intensity: This scale assigns magnitude based on the effect on structures, not the energy. • Richter Scale magnitude is based on the displacement amplitude on a Wood-Anderson torsion seismometer which Richer owned. Richter magnitude can be converted to energy (see next slide) and measures seismic energy (the radiated waves) • The Richter scale is logarithmic, meaning that for each increase of 1 on the scale, the property increases by a multiplicative factor. For the Richter scale, the step size for energy is SQRT(1000) ~ 32X, so a magnitude 6 earthquake has ~32X as much energy as a magnitude 5, a magnitude 7 has 1000X as much as a magnitude 5. An increase of Richter Magnitude by 0.2 means a doubling of energy (Ignore the book: the authors are confused about logarithms, and seem kind of weak on physics as a whole.) • A Richter magnitude 6 earthquake has the energy equivalent to the Bikini atoll hydrogen bomb, a Richter 9 earthquake has radiated 475 megatons_TNT of energy as "elastic" waves. • Both the Mercalli and Richter scale have modified versions available, and newscasters tend to not tell what exactly they're reporting.
Magnitude Formulas (PowerPoint doesn't deal well with superscripts and subscripts, so I'm using X_Y where I'd like to use XY) See http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=33 • To compare seismic energy E in two earthquakes A and B of Magnitude M_A and M_B E_A/E_B=10( 1.5*(M_A-M_B ) )=10^( 1.5*(M_A-M_B ) ) • To convert an earthquake magnitude M_S (Ricter, or Moment Magnitude, which is the USGS standard way of reporting) to energy E_S in Joules E_S=10(4.8+1.5*M_S)=10^(4.8+1.5*M_S) • To convert energy in Joules to "Tons of TNT", divide by the USGS's conversion factor of "One ton of TNT has an energy of 4.2x109=4.2E+09 Joules." Divide that by 1,000 for kilotons, or 1,000,000 for megatons, or 1e9 for gigatons. • A magnitude 9 earthquake has an energy of 475 megatons_TNT as waves.
Destruction Note that the authors are all at California State University at Sacramento. • Vibration Most building are built on the assumption it won't oscillate much. The vibrations associated with quakes impart forces that are not well-designed for, especially horizontal accelerations. • Vibrations are most severe on low-density materials, such as landfill and recent sediments, compared to bedrock. Resonance effects can also play a major role in worsening the effects of a quake. • Displacement Most building are built on the assumption one part will not move relative to another. Displacements can cause destruction of structures built across faults. • Tsunami is a water wavetrain caused by an earthquake. Also known as, very incorrectly, a tidal wave. Water waves cause much of the damage • Fire With other things breaking, natural gas lines, fuel tanks, and water pipes often break. • Liquefaction Some soils become liquid upon jarring. Building can sink or rise, and rarely due so perfectly uniformly. • Landslides can be triggered by earthquakes • Avoidance is the best policy. Avoid earthquake zones, and particularly areas that are built on loose sediments rather than bedrock. • "The Big One" is a very dangerous misconception. The implication is that there will be a single large earthquake, then you don't have to worry. In reality, earthquakes are a recurring phenomenon like floods, and are as predictable (Caveat: there is no reason why two large floods can't occur back to back, but two large earthquakes in the same place would be unusual. On the other hand, you don't really know whether you had the large earthquake, or just a foreshock, or what…)
Tsunamis • A tsunami, also known as a tidal wave (no relation to tides), is a wavetrain (series of waves) generated by the sudden displacement of the sea-bottom due to an earthquake, or, more rarely, a submarine landslide, an asteroid impact, or a volcanic explosion. (Pretty much anytime the water column gets smacked: it's a lot like the waves from moving in a bathtub.) • The Samoan tsunami of 2009 had four or five surges. • Tsunami are very long wave-length waves, so they move fast (747 fast) and feel bottom at great depths. • In deep water, tsunamis have fairly small amplitude (like a half-meter or meter). When they feel bottom and start to crest, a huge amount of water stacks up and can result in devastating coastal flooding. • Sometimes, a tsunami falls then rises, others it rises than falls. This depends on where the site is relative to the fault, and which way the fault moved. It's a lot like the first motion of a P-wave (see Box on beachball diagrams). • If there is any possibility of tsunami--you’re in a coastal area and 1) have felt an earthquake, 2) you have heard about a large earthquake offshore, 3) you have been warned by authorities, 4) the water at a beach suddenly rolls out--, get to higher ground immediately: your life depends on it. Abandon any possessions if need be. • If you are in deep water on a boat, stay in deep water. If you are in a boat in shallow water, either get ashore and get to high ground, or get into deeper water. • Red Cross http://www.redcross.org/portal/site/en/menuitem.86f46a12f382290517a8f210b80f78a0/?vgnextoid=740d5d795323b110VgnVCM10000089f0870aRCRD&vgnextfmt=default
What to Do During an Earthquake • From the American Red Cross’s website, page http://www.redcross.org/disaster/safety/guide/earth.htmlSee http://www.fema.gov/hazard/earthquake/eq_during.shtm • Drop, cover, and hold on! Move only a few steps to a nearby safe place. Most injured persons in earthquakes move more than five feet during the shaking. It is very dangerous to try to leave a building during an earthquake because objects can fall on you. Many fatalities occur when people run outside of buildings, only to be killed by falling debris from collapsing walls. In U.S. buildings, you are safer to stay where you are. • If you are in bed, hold on and stay there, protecting your head with a pillow. You are less likely to be injured staying where you are. Broken glass on the floor has caused injury to those who have rolled to the floor or tried to get to doorways. • If you are outdoors, find a clear spot away from buildings, trees, streetlights, and power lines. Drop to the ground and stay there until the shaking stops. Injuries can occur from falling trees, street-lights and power lines, or building debris. • If you are in a vehicle, pull over to a clear location, stop and stay there with your seatbelt fastened until the shaking has stopped. Trees, power lines, poles, street signs, and other overhead items may fall during earthquakes. Stopping will help reduce your risk, and a hard-topped vehicle will help protect you from flying or falling objects. Once the shaking has stopped, proceed with caution. Avoid bridges or ramps that might have been damaged by the quake. • Stay indoors until the shaking stops and you're sure it's safe to exit. More injuries happen when people move during the shaking of an earthquake. After the shaking has stopped, if you go outside, move quickly away from the building to prevent injury from falling debris. • Stay away from windows. Windows can shatter with such force that you can be injured several feet away. • In a high-rise building, expect the fire alarms and sprinklers to go off during a quake. Earthquakes frequently cause fire alarm and fire sprinkler systems to go off even if there is no fire. Check for and extinguish small fires, and, if exiting, use the stairs. • If you are in a coastal area, move to higher ground. Tsunamis are often created by earthquakes. (See the "Tsunami"section for more information). • If you are in a mountainous area or near unstable slopes or cliffs, be alert for falling rocks and other debris that could be loosened by the earthquake. Landslides commonly happen after earthquakes. (See the "Landslide" section for more information.)
Prediction and Control • Prediction • Short term: not yet, maybe never. • An amazing variety of techniques have been attempted for predicting earthquakes. None seem reliable. Many are based on the incorrect assumption that earthquakes are associated with rupturing of pristine, un-fractured rock: even in the seventies, this was known not to be true. • Earthquake prediction is one of the most fertile grounds for quacks and bad science. • Long term: easily. They occur in the same places over and over and are as predictable as floods and lightning • Recurrence studies, where sediments are examined for evidence of earthquakes and dated, provide a rough estimate of how often earthquakes happen. (I’m involved with trying to find sand dikes from airphotos for this.) They probably under-estimate the number of earthquakes (due to missing sediments) and thus over-estimate the recurrence interval. • Stress-field studies look for displacements and stresses likely to cause rupture. • Earthquakes do not happen on a an exact schedule like taxes; earthquakes are a recurring process with known causes but an irregular schedule, like armed robberies of convenience stores, or deaths. • Control • It is possible, sometimes, to trigger an earthquake, such as by introducing water (thus altering the effective stress) or by detonating explosives (In the 1950s, there was serious talk of using .atomic bombs along a fault to relieve stress.) • Even if this were completely feasible and there were no additional side-effects, you need a LOT of small earthquakes to release the stress that would cause one big earthquake. To avoid one magnitude 8.0 quake over the course of 100.0 years, you would need (numbers rounded) • 32 M7 earthquakes (one every three years or so) • 1,000 M6 events (10 per year, one every 37 days) • 31,663 M5 events (317 per year, or one every 28 hours) • 1,000,000 M4 events (10,000 per year, or one every 68 minutes) which is a terrible idea, because prolonged low-amplitude vibration is not all that good on buildings either.
Earthquakes • Earthquakes are vibrations of the ground, often due to sudden release of stress along a fault or implosion of subducted material. • Seismometers are used to measure ground motion. The distance from a station to the earthquake can be estimated by the difference in S and P arrivals. • The Richter scale is a widely used earthquake magnitude system. It mainly measures energy, and is a logarithmic scale with a step factor of about 32x. An increase of magnitude of 0.2 ~doubles the energy. • Destruction from earthquakes comes from vibration and acceleration of small structures, these plus displacement on some large structures, and fire and tsunami. • Short-term earthquake prediction is unlikely to ever work. Long-term hazard studies help design how earthquake resistant structures should be.