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 Earthquakes

Release of pent-up energy

Causes of Earthquakes Sudden release of accumulated strain energy – at shallow depths, stressed rocks accumulate strain energy Creation of new faults by rupturing rocks Shifting of rocks at preexisting faults Sporadic recurrence of Earthquakes: Accumulation of Energy ---- Sudden release Rocks lock back in place

Seismic 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 direction  Rock 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 Waves

Secondary Waves

Two most common types of surface waves

Functioning of Seismograph

Measuring 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 scale  Measured 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 waves  Earthquake total energy- uses moment magnitude scale

Measuring Earthquake-contd. Richter Scale – Amplitude scale is logarithmic (10-fold increase for every whole number increase) – Scale mm; mm; mm; 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. 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 Classfication) Use Arrival time at a recording station (time lag between P & S waves) to locate the epicenter of an earth quake  Need three stations to determine the epicenter Depth of Focus  Shallow focus EQ < 70 km (45 mi) most earthquakes  Intermediate focus EQ km ( mi)  Deep focus EQ- > 300 km (> 180 mi)

Seismograph Waves

Epicenter

Liquefaction

Frequency 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 km from surface – 1995 Kobe, Japan km from surface Magnitude vs Depth: a) Shallow: Up to 9.5 RM (Moment-Magnitude Scale) b) Intermediate: Up to 7.5 RM c) Deep: Up to 6.9 RM

Earthquake Depth

Map of Tennessee Earthquake

Graphs & Maps of Denver Earthquake

Graphs & Maps of Denver Earthquake-contd.

Effects 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 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; 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 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 zone

Hidden faults

Maps of seismic gaps around the Pacific Ocean

A Close-up Map of S. California

Dilatancy of stressed rocks

Dilatancy of stressed rocks-contd.

Coping with Earthquakes Earthquake zone-identification Plate boundaries Assessing local seismic history and future risks Land use planning-situating critical facilities Quake reinforcement of building/structures: wood, steel, reinforced concrete are preferable (heavy masonry, unreinforced concrete, etc are not good) Short term and Long term forecast Contingency plan

USGS-Survey Plan

USGS-Survey Plan-contd.

Earthquake Prediction Successful Prediction in 1975 in China, but no prediction in 1976 Reduction 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 Moon