Modes of Elastic Wave Propagation P-wave aves/WaveDemo.htm

Modes of Elastic Wave Propagation S-wave aves/WaveDemo.htm

Modes of Elastic Wave Propagation Rayleigh wave aves/WaveDemo.htm

Modes of Elastic Wave Propagation Love wave aves/WaveDemo.htm

What does a seismic signal look like? Broadband (short time) pulsed signal – Superposition of many sine waves with different amplitude and frequency From Dobrin and Savit, 1988

Rayleigh Waves ace_Theory/Rayleigh/rayleigh_index.html

Modes of wave propagation

Location of Earthquakes Hypocenter or earthquake focus Epicenter Earthquakes do not occur at points but occur due to stress release within small volumes or along fault planes

Complications Earth is not homogeneous or flat P & S waves velocities are not constant Earthquakes don’t normally occur at the surface Errors in traveltime measurement – Precise solution requires many seismometers

Earthquake location Distance between epicenter and seismometer is called the epicentral distance – Shortest distance between two points on a sphere is along the great circle connecting those two points – Epicentral distance is the length of the great circle arc connecting the epicenter and seismometer – With measurement of t s-p can estimate epicentral distance with compilation of global traveltimes

Classical Seismology Global Seismology – Use energy released by earthquakes (or nuclear tests) to image Earth’s deep interior Source:

Earthquake magnitude and moment Concept of magnitude was first introduce by C.F. Richter in 1935 – Based on local earthquakes in Southern California – Based on amplitude of first arriving P-wave as measured on a particular seismograph (12Hz Wood-Anderson seismometer)

Earthquake magnitude and moment All magnitude scales are logarithmic and have the form

Richter MagnitudesDescriptionEarthquake EffectsFrequency of Occurrence Less than 2.0MicroMicroearthquakes, not felt.About 8,000 per day Minor Generally not felt, but recorded. About 1,000 per day Minor Often felt, but rarely causes damage. 49,000 per year (est.) Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.) Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well- designed buildings. 800 per year Strong Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas. 120 per year Major Can cause serious damage over larger areas. 18 per year Great Can cause serious damage in areas several hundred miles across. 1 per year Great Devastating in areas several thousand miles across. 1 per 20 years 10.0+Epic Never recorded; see below for equivalent seismic energy yield. Extremely rare (Unknown)

Richter Approximate Magnitude Approximate TNT for Seismic Energy Yield Joule equivalentExample 0.01 kg (2.2 lb)kglb4.2 MJ kg (12.4 lb)23.5 MJLarge Hand grenadeHand grenade kg (70 lb)134.4 MJConstruction site blast kg (392 lb)747.6 MJWWIIWWII conventional bombs 2.01 metric tonmetric ton4.2 GJLate WWII conventional bombs metric tons23.5 GJWWII blockbuster bombblockbuster bomb metric tons134.4 GJMassive Ordnance Air Blast bomb metric tons747.6 GJChernobyl nuclear disasterChernobyl nuclear disaster, kilotonkiloton4.2 TJSmall atomic bombatomic bomb kilotons23.5 TJ kilotons134.4 TJ Nagasaki atomic bombNagasaki atomic bomb (actual seismic yield was negligible since it detonated in the atmosphere. The Hiroshima atomic bomb was 15 kilotons ) Lincolnshire earthquake (UK), 2008Hiroshima atomic bomb Lincolnshire earthquake (UK), kilotons625 TJ Chino Hills (Los Angeles) earthquake (CA, USA), kilotons747.6 TJ Little Skull Mtn. earthquake (NV, USA), 1992 Alum Rock earthquake (CA, USA), megatonmegaton4.2 PJ Double Spring Flat earthquake (NV, USA), megatons23.5 PJRhodes (Greece), megatons67.9 PJNorthridge earthquake (CA, USA), megatons112.2 PJ San Francisco Bay Area earthquake (CA, USA), megatons134.4 PJ megatons210 PJ Energy released was equivalent to that of Tsar Bomba, the largest thermonuclear weapon ever tested.Tsar Bomba megatons747.6 PJ Kashmir earthquake (Pakistan), 2005 Antofagasta earthquake (Chile), megatons2.4 EJTangshan earthquake (China), gigatongigaton4.2 EJ Toba eruptionToba eruption 75,000 years ago; which, according to the Toba catastrophe theory, affected modern human evolution San Francisco earthquake (CA, USA), 1906 Queen Charlotte earthquake (BC, Canada), 1949 México City earthquake (Mexico), 1985 Gujarat earthquake (India), 2001 Chincha Alta earthquake (Peru), 2007 Sumatra earthquake (Indonesia), 2007 Sichuan earthquake (China), 2008 (initial estimate: 7.8)Toba catastrophe theoryhuman evolution San Francisco earthquake (CA, USA), 1906 Queen Charlotte earthquake (BC, Canada), 1949 México City earthquake (Mexico), 1985 Gujarat earthquake (India), 2001 Chincha Alta earthquake (Peru), 2007 Sumatra earthquake (Indonesia), 2007 Sichuan earthquake (China), gigatons23.5 EJ gigatons134.4 EJ gigatons379.7 EJAnchorage earthquake (AK, USA), gigatons477 EJ Indian Ocean earthquake, 2004Indian Ocean earthquake, 2004 (40 ZJ in this case) gigatons747.6 EJ Valdivia earthquake (Chile), 1960Valdivia earthquake (Chile), 1960 (251 ZJ in this case) teratonteraton4.2 ZJNever recorded.

Earthquake magnitude and moment Shallow focus (<50km) teleseismic earthquakes (20°<Δ<160°)

Earthquake magnitude and moment Deep-focus earthquakes us P and S body waves – Gutenberg-Richter Scale (1956) q~ 6: Δ=10°, ~6.5: Δ=80°, ~8: Δ=110° Wordwide average of m b -M S

Earthquake magnitude and moment Seismic moment Moment magnitude

Identifying Seismic phases P: P-wave in the mantle S: S-wave in the mantle K: P-wave in outer core I: P-wave in inner core J: S-wave in inner core c: mantle outer core reflection i: inner core outer core reflection p: reflected P-wave close to focus from surface s: reflected S-wave close to focus from surface LR: Rayleigh Wave LQ: Love Wave

Identifying Seismic phases PKP PKIKP PKJKP PKiKP sSP

Identifying Seismic phases