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

Worldwide distribution of EarthquakesWorldwide distribution of Earthquakes

Release of pent-up energyRelease of pent-up energy

Causes of EarthquakesCauses of Earthquakes

Sudden release of accumulated strain energy – at shallow depths, stressed rocks accumulate strain energy

Creation of new faults by rupturing rocksShifting of rocks at preexisting faultsSporadic recurrence of Earthquakes:

Accumulation of Energy ---- Sudden release ------ Rocks lock back in place

Seismic WavesSeismic Waves(Earthquake’s energy is transmitted through the earth as

seismic waves) Two types of seismic waves

Body waves- transmit energy through earth’s interior

Primary (P) wave- rocks vibrate parallel to direction of wave

Compression and expansion (slinky example) Secondary (S) wave- rocks move perpendicular to wave

directionRock shearing (rope-like or ‘wave’ in a stadium)

Surface waves- transmit energy along earth’s surface

Rock moves from side to side like snake Rolling pattern like ocean wave

Primary WavesPrimary Waves

Secondary WavesSecondary Waves

Two most common types of surface wavesTwo most common types of surface waves

Functioning of SeismographFunctioning of Seismograph

Measuring of EarthquakesMeasuring of Earthquakes Seismograph- device that measures the magnitude of

earthquake Seismogram is visual record of arrival time and

magnitude of shaking associated with seismic wave

Mercalli Intensity scaleMeasured by the amount of damage caused in human terms- I

(low) to XII (high); drawback: inefficient in uninhabited area

Richter Scale- (logarithmic scale)Magnitude- based on amplitude of the wavesEarthquake total energy- uses moment magnitude scale

Measuring Earthquake-contd.Measuring Earthquake-contd. Richter Scale

– Amplitude scale is logarithmic (10-fold increase for every whole number increase)

– Scale 1 ---- 0.001 mm; 2---- 0.01 mm; 5---- 10mm; 7---- 1 meter

– Earthquake Energy: Each whole number represents a 33-fold increase in Energy; Energy difference between 3 & 6 means ~1000 times

– Drawbacks: California Rocks Based on Antiquated Wood-Anderson

Seismographs Measurment Past Magnitude 7.0 ineffective –

Requires Estimates (Scale 8 corresponds to 10m)

Measuring Earthquake-contd.Measuring Earthquake-contd.Moment-Magnitude Scale

– 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

– Shallow Earthquakes occur at– Oceanic Divergent zones, continental rift and collision zones,

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)

Animal Behavior??

Earthquakes in MoonEarthquakes in Moon