Earthquake Odds and Ends Lind Gee Waves waves

Earthquake Odds and Ends Lind Gee " Waves, waves " All things "seismo" " " Quantifying earthquakes Earthquake Monitoring

Elastic Rebound

Earthquake waves

Static Offset

Waves, waves

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/cm 2) m = modulus of rigidity (dynes/cm 2) r = density (g/cm 3) 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 " <. 92 Vs Love waves � Shear motion in the horizontal plane � Requires � Vs 1 layered velocity structure < Vs 2

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 � World � Dog catfish (Japan) supported by turtle (Mongolia) shaking snow from his coat (Russia) � Thunderbolts from God Namazu

Seismometry (then) 132 China 1844 England 1877 Italy 1880 Japan

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 1700 s-mid 1800 s - 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 "1880 s - 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, 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 Pwave 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 Pwave 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 1930 s 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 M 6 earthquake produces an amplitude 10 times greater than an M 5, 100 times greater than an M 4, and 1000 times greater than an M 3 This concept has been expanded over time: mb = log (A/T) + Q(h, D) - body-wave magnitude Ms = log (A 20) + 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

Seismoscopes

Earth Structure

Earth Structure