Seismology - A Primer

Earth ScienceGeology 007

Focus - location of movement along the fault planeEpicenter - point on surface directly above the focus

Earthquake waves

Earthquake Waves

• Body waves•Travel through the Earth•Speed increases with density•Reflect at layer boundaries = depth•Refract at layer boundaries = density difference•P waves•S waves

• Surface waves•Travel across the Earth’s surface•Love waves•Rayleigh waves

Seismic Waves

(6 km/sec)

Similar to sound waves - can travel through liquidsFastest seismic waves - first to arrive at any point on the surface of the Earth.

Seismic Waves

(3.5 km/sec)s wave

Cannot travel through liquids.Second fastest - second to arrive at any point.

Surface Waves- Arrive last at any point.

A. E. H. Love

Surface WavesArrive last at any point.

Seismometer / Seismograph

P waves arrive firstS waves secondSurface waves last

Time

P S

P - S time lag

950 miles = 3 minute P-S lag3000 miles = 7 minute P-S lag6150 miles = 11 minute P-S lagP-S time lag = distance

Travel time curves for Northridge Earthquake, California

SF = 300 kmLV = 390 kmLA = 189 km

Epicenter

Triangulation to locate the epicenter Seismic Station

Measuring Earthquake Energy / Intensity

• Richter Magnitude• Estimates energy released• Based on wave amplitude and

distance• Single number, -2 to 12

• Modified Mercalli Intensity• Estimates severity of shaking• Based on descriptions of damage• Rated from I to XII• Map of intensity in region of

quake

Charles Richter

Giuseppe Mercalli

Richter Magnitude

Distance (km) P-S lag (sec)

Magnitude

Amplitude (mm)

Amplitude = 18 mm

P-S lag = 24 sec

Magnitude = 4.8

Richter Magnitude

RM Energy (TNT) Equivalent

1.0 30 lbs Large construction blast

4.0 1000 tons Small nuclear weapon

5.0 32,000 tons L5 tornado

7.0 32 million tons Largest nuclear blast

8.0 1 billion tons 1906 San Francisco earthquake

9.0 32 billion tons Largest earthquakes recorded

+1 RM = 32X more energy+2 RM = 1000X more energy

Modified Mercalli Intensity

RM Intensity Description1–3 I Generally not noticed

3–3.9 II-III Felt by few to noticeable rocking motion

4-4.9 IV-V Felt by most, some interior breakage

5-5.9 VI-VII Felt by all, some damage to buildings

6-6.9 VII-IX Considerable damage to buildings

> 7.0 VIII - XII Major to total damage to most structures

Anomolously high intensity due to unstable ground (recent infilling of marshland)

Amplification of intensity due to unstable ground1989, Loma Prieta earthquake

Modified Mercalli intensity map for San Fernando earthquake, 1971

Other Effects of Earthquake Motion• Liquifaction

• Shaking causes uncompacted sand and mud to flow like a liquid

• Buildings sink• Sand and mud erupts to the surface

Turnagain Heights, Alaska, 1964

Sand volcanoes in farm field produced by liquifaction

Sinking of second story to ground level due to liquifaction

Collapse of section of CA Hwy 1 due to liquifaction

Other Effects of Earthquake Motion• Tsunami

• Motion of seafloor causes “tidal wave”• Low amplitude, long wavelength• In open ocean, wave passes unnoticed• In shallow water, wave attains great height• Can travel thousands of miles across oceans

Uplift of shoreline following 1964 Alaska earthquake

Prince William Sound following 1964 earthquake and tsunami

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Travel time to Hawaii for Pacific rim tsunami

Travel time to Los Angeles for Pacific rim tsunami

Tsunami striking Hilo, Hawaii, 1960

Aftermath of Hilo, Hawaii tsunami, 1954