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Explore the world of seismic waves, earthquake monitoring, and elasticity theory. Discover how seismologists quantify earthquakes and the history of seismometry. Learn about intensity scales, location techniques, and magnitude measurements.
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Earthquake Odds and EndsLind Gee • Waves, waves, waves • All things "seismo" • Quantifying earthquakes • Earthquake Monitoring
Body waves • P waves: Compressional or longitudinal wave • Compresses and stretches material in the direction of motion (volume change) • Sound is an example of a compressional wave • People "hear" earthquakes - and the Space Shuttle generates seismic waves • Typical crustal values: 2 - 6.5 km/sec • S waves: Transverse or shear wave • Shears or changes shape of material perpendicular to the direction of motion (shape change) • Does not propagate in fluids • Typical crustal values: 0.5 - 4.5 km/sec
Body waves • P waves: Compressional or longitudinal wave • Vp = (K + 4/3 m)/r • S waves: Transverse or shear wave • Vs = (m/r) K = modulus of incompressibility (dynes/cm2) m = modulus of rigidity (dynes/cm2) r = density (g/cm3) Typically observed that Vp ~ Vs 3 Elasticity theory for an "unbounded" solid well developed by 1822.
Surface waves Initially called "long" or "L" waves Arise from the interaction of P and S waves with the free surface • Rayleigh waves • Elliptical motion in the horizontal and vertical planes • V < .92Vs • Love waves • Shear motion in the horizontal plane • Requires layered velocity structure • Vs1 < V < Vs2
Wave Generation • Earthquakes • Explosions • CTBT verification • Wind, waves, falling trees • Humans (cars, axes, ....)
Seismo Stuff "Seism" - from the Greek seismos meaning earthquake seismic - relating to an earthquake seismicity - earthquake activity seismogram - a record of an earthquake at a particular place seismograph - a seismometer combined with a timing system and a recording device to measure true ground motion seismologist - a scientist who studies earthquakes seismometer - a sensor or instrument for measuring true ground motion seismoscope - an instrument which responds to Earth motion but does not make a record
Seismo Stuff Seismology is a relatively young science .... ... with a rich mythology • Flailing catfish (Japan) • World supported by turtle (Mongolia) • Dog shaking snow from his coat (Russia) • Thunderbolts from God Namazu
Seismometry (then) 1880 Japan 132 China 1844 England 1877 Italy
Seismometry (now) Vault contruction Seismometers Digital data recorder
A selective history • 1755 - Lisbon earthquake; Mitchell associates earthquakes and seismic waves • 1783 - Calabrian earthquakes; first appointed earthquake commission • mid 1700s-mid 1800s - experiments with machines to measure earthquakes - bowls of mercury, pendulums, and some efforts with clocks • 1857 - Naples earthquake; Mallet laid the foundation of modern observational seismology • 1875 - Ceechi constructed the first machine to measure the relative motion of the Earth and a pendulum as a function of time • 1880s - Milne, Ewing, and Gray work in Japan building a network of seismometers • 1900 - Oldham reports the identification of P, S, and surface waves • 1906 - Discovery of the core by Oldham • 1909 - Observation of the discontinuity between the crust and mantle • 1936 - Discovery of the inner core by Lehman
Quantifying earthquakes:Intensity 1783: First intensity scale devised by D. Pignataro with 5 levels (slight, moderate, strong, v. strong, violent) 1823: Expanded to 6 levels with more detail by P. Egen 1846: First use of "isoseismals" to define areas of equal intensity and effectively to "locate" an earthquake 1883: Rossi-Forel scale with 10 divisions. Widely used, although much was "lumped" at level X. 1902: Initial Mercalli Scale. 10 grades 1931: Modified Mercalli Scale with 12 grades. Still in use today 1999: Community Internet Intensity Maps
Quantifying earthquakes:Intensity Subjective measure of damage Depends on population! Use of Roman, rather than Arabic, numbers Comparison of past events with present
Quantifying earthquakes:Location Development of seismometers allowed more precise location - Classic triangulation using S-P travel times
Quantifying earthquakes:Location Today, earthquakes are more commonly located using P-wave travel times - Absolute travel times
Quantifying earthquakes:Location New methods are being developed that yield highly precise locations Earthquakes along the Calaveras fault located by standard methods using P-wave travel times Relocated earthquakes obtained by cross-correlating waveforms.
Quantifying earthquakes:Magnitude Efforts to characterize earthquake size using amplitudes Concept developed independently by Richter and Wadati in the 1930s nomogram
Quantifying earthquakes:Magnitude Ml = log A + log Ao(D) - assuming a "standard" seismograph - referenced to an amplitude of .001 mm at 100 km - A is the maximum amplitude on a record, typically the S-wave - A M6 earthquake produces an amplitude 10 times greater than an M5, 100 times greater than an M4, and 1000 times greater than an M3 This concept has been expanded over time: mb = log (A/T) + Q(h, D) - body-wave magnitude Ms = log (A20) + 1.66 log (D) + 3.3 - surface-wave magnitude Magnitude is a useful, but empirical, measure of earthquake size based on maximum recorded amplitudes. However, these magnitude scales "saturate" for large earthquakes
Quantifying earthquakes:Moment Over the last 20 years, seismologists have developed a measure of earthquake size based on a model of the source as a pair of force couples. Seismic moment can be expressed very simply Mo = m A d where m is the modulus of rigidity, A is the area of the fault rupture, and d is the average displacement along the fault. It has the units of force x distance and is typically measured in dyne-cm. Moment has used to define a new magnitude scale, known as moment magnitude MW Mw = 2/3 log (Mo) - 10.7
