Chapter 10 Earthquakes Chapter 10 Earthquakes Definitions Earthquake- the vibration of the ground due to the sudden release of energy accumulated in a deformed rock Focus (Hypocenter)- spot underground where the rock begins to break Epicenter- the point on the land surface directly above the focus Aftershock- tremors that occur as rocks adjust to their new position Seismology- the study of earthquake
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Chapter 10 Earthquakes Definitions Earthquake- the vibration of the ground due to the sudden release of energy accumulated in a deformed rock Focus.
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– Seismic Moment Factors Length of Fault Rupture Depth of Fault Rupture Amount of Slip along Rupture
– Moment = (Total Length of Fault Rupture) X (Depth of Fault Rupture x Total amount of Slip along Rupture x Strength of Rock)
– Measurement Analysis requires Time
Locating Epicenter and Focus Depth (EQ Locating Epicenter and Focus Depth (EQ Classfication)Classfication)
Use Arrival time at a recording station (time lag between P & S waves) to locate the epicenter of an earth quakeNeed three stations to determine the epicenter
Depth of FocusShallow focus EQ < 70 km (45 mi) most earthquakesIntermediate focus EQ- 70-300 km (45- 180 mi)Deep focus EQ- > 300 km (> 180 mi)
Seismograph WavesSeismograph Waves
EpicenterEpicenter
LiquefactionLiquefaction
Frequency vs DepthsFrequency vs Depths
90% of Earthquakes occur within depths less than 100 km
Majority of Catastrophic Earthquakes occur within Depths less than 60-km Deep– 1964 Alaska EQ ---- 33 km from surface– 1995 Kobe, Japan--- 20 km from surfaceMagnitude vs Depth:
a) Shallow: Up to 9.5 RM (Moment-Magnitude Scale)b) Intermediate: Up to 7.5 RMc) Deep: Up to 6.9 RM
Earthquake DepthEarthquake Depth
Map of Tennessee EarthquakeMap of Tennessee Earthquake
Graphs & Maps of Denver EarthquakeGraphs & Maps of Denver Earthquake
Graphs & Maps of Denver Earthquake-contd.Graphs & Maps of Denver Earthquake-contd.
Effects of EarthquakesEffects of Earthquakes Ground Displacement
Lateral and vertical (In 30-Myr, Rocks & Landforms on the West Side have shifted ~560-km toward Northwest); Vertical Displacement occur during movement along Dip-slip Faults
-Landslides Liquefaction
Conversion of formally stable fine grain materials to a fluid mass
Seiches The back and forth movement in a semi-closed/closed body of water- could
cause flooding- Alaskan EQ in 1964 – 6000 km away felt in TX Swimming pools
Tsunamis- More from submarine landslide (Large fast-moving sea waves); Sea-floor
displacement during faulting and submarine slides
Fire (Tokyo 1923; San Francisco, 1906)
Principal Earthquake zonesPrincipal Earthquake zones Earthquake zones at Plate Boundaries
and transform boundaries– Subduction-zone earthquake regions are called Benioff-
Wadati zones– In subduction zones: Depth of earthquakes correspond to
depth of portions of descending slab; up to 300-km deep: strong earthquakes; 300-700 km: weaker earthquakes; >700 km: earthquakes are rare
– Magnitude of EQ ~80% of EQ Energy released in Pacific Rim Region; ~20% of world’s EQ energy released in collision Zone from Turkey to Burma
Specific AreasSpecific Areas
Japan: Subduction of Pacific Plate beneath Eurasian Plate; Quakes occur in Tokyo every 69 yrs; 15% of world seismic energy released
Alaska: Pacific Plate subducts beneath North American and Eurasian Plates
Mid-plate Eqs are shallow, weak
Stratigraphy of a fault zoneStratigraphy of a fault zone
Hidden faultsHidden faults
Maps of seismic gaps around the Pacific OceanMaps of seismic gaps around the Pacific Ocean
A Close-up Map of S. CaliforniaA Close-up Map of S. California
Dilatancy of stressed rocksDilatancy of stressed rocks
Dilatancy of stressed rocks-contd.Dilatancy of stressed rocks-contd.
Coping with EarthquakesCoping with Earthquakes
Earthquake zone-identificationPlate boundariesAssessing local seismic history and future risksLand use planning-situating critical facilitiesQuake reinforcement of building/structures: wood,
steel, reinforced concrete are preferable (heavy masonry, unreinforced concrete, etc are not good)
Short term and Long term forecastContingency plan
USGS-Survey PlanUSGS-Survey Plan
USGS-Survey Plan-contd.USGS-Survey Plan-contd.
Earthquake PredictionEarthquake PredictionSuccessful Prediction in 1975 in China, but no
prediction in 1976Reduction of Energy build up along segments of
individual faults – Plans to lubricate San Andreas Fault
Increase in Radon concentration as a precursor (and other noble gases, solubilites of these gases)