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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