A Violent Pulse: Chapter 8 part 2

Earthquakes

and the Earth’s Interior

What is an ? Seismicity • ‘Earth shaking caused by a rapid release of energy.’ • Seismicity (‘quake or shake) cause by… – Energy buildup due tectonic – Motion along a newly formed crustal fracture (or, stresses. ). – Cause rocks to break. – Motion on an existing fault. – Energy moves outward as an expanding sphere of – A sudden change in mineral structure. waves. – Inflation of a – This waveform energy can magma chamber. be measured around the globe. – Volcanic eruption. • Earthquakes destroy – Giant landslides. buildings and kill people. – Meteorite impacts. – 3.5 million deaths in the last 2000 years. – Nuclear detonations. • Earthquakes are common.

Faults and Earthquakes Earthquake Concepts • Focus (or Hypocenter) - The place within Earth where • Most earthquakes occur along faults. earthquake waves originate. – Faults are breaks or fractures in the crust… – Usually occurs on a fault surface. – Across which motion has occurred. – Earthquake waves expand outward from the • Over geologic time, faulting produces much change. hypocenter. • The amount of movement is termed displacement. • – Land surface above the focus pocenter.

• Displacement is also called… – Offset, or – Slip • Markers may reveal

the amount of offset. Fence separated by fault

1 Faults and Fault Motion Fault Types • Faults are like planar breaks in blocks of crust. • Fault type based on relative block motion. • Most faults slope (although some are vertical). – Normal fault • On a sloping fault, crustal blocks are classified as: • Hanging wall moves down. – Footwall (block • Result from extension (stretching). below the fault). – Hanging wall – Reverse fault (block above • Hanging wall moves up. the fault). • Result from compression (squeezing). • Miners on a – Thrust fault fault would… • Special kind of reverse fault. – Stand on the • Fault surface is at a low-angle. footwall; – Strike-slip fault – Bump their heads on the • Blocks slide past one another. hanging wall. • No vertical block motion.

Faults and Fault Motion Fault Initiation (elastic rebound theory) • Faults are commonplace in the crust. • Tectonic forces add stress to unbroken rocks. – Active faults – On-going stresses produce motion. • The rock deforms slightly (elastic strain). – Inactive faults – Motion occurred in the geologic • Continued stress cause more stress & cracks. past. • Displacement can be visible. • Eventually, cracks grow to the point of failure. – Fault trace – A surface tear. Fault location evident by surface tear. • Elastic strain transforms into brittle deformation – Fault scarp – A small cliff. (rebounds), releasing earthquake energy. • Blind faults are invisible.

Fault Motion Fault Motion

• Faults move in jumps (rebounds). • When rocks break, stored elastic strain is released. • Once motion starts, it quickly stops due to friction. • This energy radiates outward from the hypocenter. • Eventually, strain will buildup again causing failure. • The energy, as waves, generates vibrations. • This behavior is termed stick – slip behavior. • The vibrations cause motion, as when a bell is rung. – Stick – Friction prevents motion. • Large earthquakes are often… – Slip – Friction briefly overwhelmed by motion. – preceded by , and… • Smaller quakes. • May signal larger event. – followed by . • Smaller quakes. • Indicate readjustment.

2 Amount of Displacement Seismic Waves • Displacement scale varies. • Body Waves – Pass through Earth’s interior. – Large events may rip large fault segments. – Compressional or Primary (P) waves • 100s of kilometers long • Push-pull (compress and expand) motion. • 10s of kilometers deep • Travel through – Smaller events may result in more localized effects. solids, liquids, • Displacement maxima near focus / epicenter. and gases. • Displacement diminishes with distance. • Fastest. • Faulting changes landscapes. – Shear or Secondary (S) waves – Uplift • “Shaking" motion. – Subsidence • Travel only – Offset through solids; not liquids. • Changes are measureable. • Slower. – Interferometry

Seismic Waves • Surface Waves – Travel along Earth’s surface. • Seismology is the study of earthquake waves. – Love waves – s waves intersecting the surface. • Seismographs - Instruments that record seismicity. • Move back and forth like a writhing snake. – Record Earth motion in – Rayleigh waves – p waves intersecting the relation to a stationary surface. mass or rotating drum. • Move like ripples on a pond. – Deployed worldwide. – Can detect earthquakes • These waves are the slowest and most destructive. from around the entire planet. – Seismology reveals much about earthquakes. • Size (How big?) • Location (Where is it?)

Seismograph Operation Locating an Epicenter

• Straight line = background. • Locating an epicenter depends upon the • Arrival of 1st wave causes different velocities of p and s waves. frame to sink (pen goes up). • Because they travel at different velocities, they • Next vibration causes located by comparing p and arrival times opposite motion. from a minimum • Waves always arrive in of three seismic sequence. stations. –P-waves 1st –S-waves 2nd – Surface waves last. • A seismogram measures… – Wave arrival times – Magnitude of ground motion.

3 Locating an Epicenter Locating an Epicenter • A circle with a radius equal to the distance to the epicenter is drawn around each station. • First arrival of p and s waves compared for (at • Data from three least) 3 stations. stations needed.

• The point where • A travel-time three circles graph plots the intersect is the distance of each epicenter. station to the epicenter.

Earthquake Size Earthquake Size • Two means of describing earthquake size • Magnitude – The amount of energy released. – Intensity (Mercalli scale) – Maximum amplitude of ground motion from a – Magnitude (Richter & Moment) seismogram. – Value is normalized for seismograph distance. • Mercalli Intensity Scale – Intensity – The degree of • Several magnitude scales. shaking based on damage – Richter (most common) (subjective scale). – Moment (most accurate) – Roman numerals assigned to different • Magnitude scales are logarithmic. levels of damage. – Increase of 1 Richter unit – Damage occurs in zones. = 10 fold increase in ground – Damage diminishes in motion however this intensity with distance. = a 30 fold increase in energy.

Measuring Earthquake Size Earthquake Occurrence • Earthquake energy release • Earthquakes are closely linked to plate tectonic boundaries. can be calculated. • Shallow earthquakes - Divergent and transform boundaries. – Energy of Hiroshima bomb • Intermediate & deep earthquakes – Convergent boundaries. is ~ 6.0 magnitude quake – Annual energy released by all quakes is ~ 8.9 magnitude. • Small earthquakes are frequent. ~100,000 earthquakes (of >3 magnitude) per year. • Large earthquakes are rare. There are ~ 32 earthquakes of >7 magnitude per year.

4 Convergent Plate Boundaries Earthquake Focal Depths • Populous nations in convergent tectonic settings have to • Shallow – 0-20 km depth content with frequent earthquakes. – Along the mid-ocean ridge. • 80% of all earthquakes occur in the circum-Pacific belt – Transform boundaries. (around Pacific Ocean). Another 15% occur in Mediter- – Shallow part of trenches. ranean-Asiatic belt (Mediterranean – Continental crust. to Himalayas to Indonesia) • Intermediate and deep earthquakes occur along the path of a subducting plate called the Benioff-Wadati zone. – Intermediate – 20-300 km depth as downgoing plate remains brittle. – Deep - 300-670km depth - Mineral transformations? • Earthquakes rare below 670 km because the mantle is ductile.

Continental Earthquakes San Andreas Fault • Earthquakes in continental crust. • Pacific plate meets the North – Continental transform faults (San Andreas, American plate on the Anatolian). western edge of California. – Very dangerous fault. – Continental rifts (Basin and Range, East African rift). – Hundreds of earthquakes – Collision zones (Himalayas, Alps). each year. – Intraplate settings (Ancient crustal weaknesses). – 12 + major temblors since 1800.

Intraplate Earthquakes Earthquake Damage • 5% of earthquakes are not associated with plate • Earthquakes kill people and destroy cities. boundaries. • The death and damage resulting from a large • These intraplate earthquakes are not well understood. earthquake can be horrific and heart-rending. – Possible causes. • Learning about the characteristics of earthquakes, what • Remnant crustal weakness. they do and how they do it, can improve your chances – Failed rifts. of surviving one of these potentially deadly events. – Shear zones. • Stress transmitted inboard. • Isostatic adjustments. • Clusters – New Madrid, Missouri. – Charleston, South Carolina – Montreal, P.Q. – Adirondacks, New York.

5 Earthquake Damage Earthquake Damage • Ground Shaking and Displacement • Ground Shaking and Displacement – Earthquake waves arrive in a distinct sequence. – Earthquake waves arrive in a distinct sequence. – Different waves cause different motion. – Different waves cause different motion. • R waves • P waves • S waves • L waves nd – Last to arrive. –1st to arrive. –2 to arrive. – Follow S-waves – Like ripples in a pond. – Rapid up – down motion. – Back and forth motion. – Ground writhes like a – Stronger than P-wave motion. snake. – May last longer than others.

Earthquake Damage Earthquake Damage • Severity of shaking and damage depends on… • Shaking Effects on Buildings. – Magnitude (energy) of the earthquake. More energy = – Slabs disconnect. more shaking & damage . – Distance from the hypocenter. – Intensity and duration of the vibrations. – The nature of subsurface material. – Facades delaminate. • Bedrock transmits waves quickly = less damage • Sediments bounce – Bridges topple. waves = amplified damage – Wave frequency and resonance. – Bridges come apart.

Earthquake Damage Earthquake Damage

• Shaking Effects on Buildings.

– Masonry disintegrates.

– Buildings collide.

– Slopes collapse.

6 Earthquake Damage Earthquake Damage • Liquefaction • Landslides and Avalanches – Water saturated – Shaking can destabilize slopes to the point of failure. sediments become liquefied when shaken. – Often hazardous slopes bear evidence of ancient slope failures • High fluid pore pressures force grains apart. – evidence that is not recognized. • This reduces friction and – In mountainous landscapes, they move as a slurry. earthquakes can bring down – Sand becomes “quicksand.” rockslides or snow avalanches. – Clay will become “quickclay.” – An earthquake was the immediate – Liquefied sediments flow. precursor to the landslide that • Injected as sand dikes. unleashed the Mount St. Helens • Erupt as sand volcanoes. th eruption, May 18 , 1980. • Preserve distorted layering.

Liquefaction Earthquake Damage • Water saturated sediments turn into a mobile fluid. • Fire – Shaking topples stoves, candles and power lines. • Land will slump and flow. – Broken gas mains and petroleum storage tanks can ignite a • Buildings may founder and topple over intact. conflagration. – Earthquakes destroy infrastructure such as water, sewer, telephone, and electrical lines as well as roads.

– Firefighters often can’t help. • No road access • No water • Too many hotspots – Good planning is crucial.

Earthquake Damage Earthquake Damage • Disease • Tsunamis or Seismic Sea Waves – Earthquake devastation may fuel large – Often incorrectly called “tidal waves.” disease outbreaks. – Caused when earthquakes change the seafloor. • Food, water and medicines are scarce. – Thrust faulting raises the seabed; normal faulting drops it. • Basic sanitation capabilities disabled. – This displaces all the overlying water (up or down). • Hospitals damaged Resulting in a giant mound (or trough) on the sea surface. or destroyed. – This feature may be enormous (up to a 10,000 mi2 area). • Health professionals – The surface feature quickly collapses, creating waves that overtaxed. race rapidly away from the disturbance. • There may be many decaying corpses.

7 Earthquake Damage Tsunami vs. Wind Waves • Destructive tsunamis occur frequently - about once/yr. • Wind waves • Tsunami waves • There have many tsunami disasters in recorded history. – Influence the upper ~100 m. – Influence the entire water depth (avg. 2½ miles). – 94 destructive tsunamis in the last 100 years. – Have wavelengths of several – Have wavelengths of several – 51,000 victims (not including Sumatra in 12/26/04) 10s to 100s of meters. 10s to 100s of kilometers. • Many tsunami disasters lurk in the future of humanity. – Wave height and wavelength – Wave height and wavelength – Larger human population than at any time. related to windspeed. unaffected by windspeed. – Wave velocity maximum – Concentrations of people in low-lying coastal areas. – Wave velocity maximum several 10s of kph. several 100s of kph. • Education about tsunamis can save many lives. – Waves break in shallow – Waves come ashore as a water and expend all stored raised plateau of water that energy. pours onto the land.

Tsunami Behavior The Aftermath • Tsunamis race at jetliner speed across the deep ocean. • Tsunami destruction limited to low-lying coastal land. • Tsunami waves may be imperceptible in the deep ocean. • The magnitude of the run-up is a result of… – Low wave height (amplitude). • Offshore bathymetry. – Long wavelength (frequency). – Broad shallows • As water shallows… • Shallows sap wave energy. – Waves slow from frictional drag. • Waves become higher, but... – Waves grow in height. • Have less energy and dissipate sooner. • Waves may reach 10-15 m. – Rapid deep to shallow offshore. • Waves have maximal energy. • Wave heights are modest. • Water pours onto land as a sheet . • Deadliest condition. • Topography of shore. – Broad low land – maximum damage. – Steep rise of land – less damage.

The Destructive Effects of Earthquakes Dec. 26, 2004 ™Tsunami: Killer Waves a magnitude 9.0 earthquake offshore Sumatra caused deadliest tsunami in history. Dec. 26, 2004

Banda Ache, Sumatra

BEFORE AFTER Banda Ache, Sumatra Stepped Art Fig. 8.16, p. 207 Fig. 8-16b, p. 207

8 Earthquake tsunami at Banda Earthquake tsunami at Phuket, Ache, Sumatra, Dec. 26, 2004 Thailand, Dec. 26, 2004

The Indian Ocean Tsunami The Indian Ocean Tsunami • On December 26, 2004, a strong • Killed more people than any tsunami in recorded history.

(Mw 9.0) originated in the oceanic trench to the west of – ~283,100 deaths with 14,100 still missing (as of 5/05) northern Sumatra. – 1,126,900 people were displaced. • The earthquake was the largest in 40 years. • The death toll was so horrific for several reasons. • A rupture length of 1100km and a rupture width of 100 km – The earthquake was so large. were estimated from aftershocks. – Low-lying coastal areas were heavily populated • Fault displacement was as much as 15 m. • Resorts on the Malaysian Peninsula were full of • The earthquake generated a devastating tsunami that killed Christmas tourists. people in 10 countries surrounding the Indian Ocean. • The tsunami destroyed low-lying coastal areas around the Ocean. • Northern Sumatra was particularly hard hit. Large portions of Banda Aceh were erased from the map.

Source: USGS National Earthquake Information Center 12/26/04 Earthquake webpage http://neic.usgs.gov/neis/eq_depot/2004/eq_041226/

The Indian Ocean Tsunami The Indian Ocean Tsunami

• Western Sumatra is a typical subduction setting. • Destruction limited to land below the “run-up” elevation. – Deep oceanic trench. • Dense coastal development suffered the greatest devastation. – East dipping subduction zone. • In Banda Aceh, the tsunami erased entire communities. – Oceanic volcanic island arc. • The Indian Plate is subducting at an oblique angle (N23E) beneath the Burma Microplate. • This results in a complicated geometry that includes… – Strike-slip (transform) faults – Thrust faults

9 Surviving a Tsunami U.S. Coastal Risk • Heed natural and official warnings. • Coastal Oregon and Washington – An earthquake in a coastal setting. – Cascadia subduction zone. – Retreat of water from the shoreline is sign of an impending tsunami. – Geological evidence of numerous • Expect many waves. tsunami events. – Bigger waves may be next. – Wave arrival may last for hours. • Hawaii - Kileauea volcano • Abandon belongings. • Get to high ground and stay there. • East Coast of the United States • Expect roads to be impassable. – Cumbre Vieja volcano on La Palma • Climb a sturdy building or a tree. (Canary Islands). • Grab something that floats. • Volcano has a gigantic fracture system. • Expect debris. • At some point in the future, eruption will – Sediment cause this fracture to fail. – Wreckage • 500 km3 of rock will enter the ocean. – Corpses • The ensuing tsunami may devastate the • Expect landscape changes. entire U.S. East Coast.

Brian F. Atwater and others, 1999, Surviving a Tsunami – Lessons from Chile, Hawaii and Japan, USGS Circular 1187 Source: Brian F. Atwater and others, 1999, Surviving a Tsunami – Lessons from Chile, Hawaii and Japan, USGS Circular 1187

Tsunami Prediction • Scientific modeling helps to predict tsunami behavior. • Prediction would help reduce catastrophic losses. • Detection systems exist in the Pacific; are planned • Can seismologists predict earthquakes? Yes and no. for Indian Ocean. – CAN be predicted on a long-term (10-100s of years) basis. – Tsunami detectors are placed on the deep seafloor. – CANNOT be predicted in the short-term (hours-months). – Sense increases in pressure from subtle changes in sea thickness. • Data analysis for prediction is “ assessment.” • Prediction / detection can save 1000s of lives. • Seismic hazards are shown on maps of seismic risk. • This information is useful for… – Developing building codes. – Land-use planning. – Disaster planning.

Earthquake Prediction Earthquake Prediction • Long-Term Predictions Long-Term Predictions – Probability of a certain magnitude earthquake occurring • Based upon recurrence time – average time between events. on a time scale of 30 to 100 years, or more. • Historical records. – Based on the premise that earthquakes are repetitive. • Geologic evidence – Requires radiometric dating of events. – Sand volcanoes. – Require determination of seismic zones, by… – Offset strata. • Mapping historical (after ~ 1950). – Drowned forests. • Evidence of ancient earthquakes (before seismographs). • Seismic gaps, places that – Evidence of seismicity – Fault scarps, sand volcanoes, etc. haven’t slipped recently, – Historical records. are likely candidates.

10 Earthquake Prediction Preparing for Earthquakes • Short-Term Predictions • Can’t stop earthquakes but we can be ready. – Goal: warn of the location & magnitude of large earthquakes – Understand what happens during an earthquake. – Currently, no reliable short-range predictions are possible. – Map active faults & areas likely to liquefy during shaking. – But known precursors to earthquakes, including… – Developing construction codes to reduce building failures. • Clustered foreshocks. • Crustal strain. – Land-use regulation to control development. • Stress triggering. – Community earthquake preparedness training. • And, possibly… – Education on safe earthquake behavior and response. – Water level changes in wells. – Increases gases (Rn, He) in wells. – Keep viable stores of emergency supplies. – Unusual animal behavior. – On May 18th, 2005, the USGS began daily 24-hour earthquake hazard assessments for California. http://pasadena.wr.usgs.gov/step/

W. W. Norton & Company Notable Earthquakes Independent and Employee-Owned Location Magnitude Date Comments Deaths NE Sumatra 9.0 Dec 26, 2004 Tsunami 283,100+ Bam, Iran 6.6 Dec 26, 2003 41,000 Denali Fault, Alaska, USA › Nov 3, 2002 Largest in Alaska, huge landslides 0 Nisqually, Wa, USA 6.8 Feb 28, 2001 Liquefaction 0

Kocaeli, Turkey 7.4 Aug 17, 1999 100,000 buildings destroyed 17,439

Kobe, Japan 6.9 Jan 16, 1995 100,000 buildings, $147 billion 5,400 The End Northridge, Ca, USA 6.9 Jan 17, 1994 No surface faulting, $15 billion 56

Loma Prieta, Ca, USA 7.0 Oct 17, 1989 “World Series” 63 T’ang-shan, China 7.6 Jul 27, 1976 242,000 Peru 7.8 May 31, 1970 Large landslide 66,000 Prince William Sound, Ak, USA 8.6 Mar 28, 1964 “Good Friday” - Tsunami 131 › Messina, Italy 7.5 Dec 28, 1908 120,000 San Francisco, Ca, USA 8.25 Apr 18, 1906 700 Charleston, SC, USA Aug 31, 1886 Felt in Boston, Chicago and St. Louis 60 New Madrid, Mo. USA 7.5 Dec 16, 1811 Felt in Boston, changed Miss. R. Several Lisbon, Portugal 1755 Tsunami and fire 70,000 Shen-shu, China 1556 830,000

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