Chapter 10: A Violent Pulse: Earthquakes
Earthquakes
Most occur along faults -Crust fractures move rock masses -Movement is called DISPLACEMENT (offset, or slip) -markers can reveal amount of offset
Short Term
No reliable short range predictions -Water levels in wells -Gases in wells -unusual animal behavior -Crustal strain -crustal foreshadows -stress triggers
Rayleigh Waves
P-waves interesecting the surface -like rippling on a pond
San Andreas Fault
Pacific meets North American plate Very active strike-slip fault -hundreds per year -1906- San Fran destroyed -1959 world series
Body Waves
Pass through earths interior Compressional or Primary (P) waves -push & pull (compress & expand) motion -go through solids, liquids, and gas -fastest
Shear Waves
Pass through earths interior Secondary Waves (S) waves "Shaking" motion -goes through only solids, not liquids (not core) Slower
Boundaries
Earthquakes are linked to tectonic boundaries -Shallow- divergent and transform boundaries Intermediate & Deep- Convergent boundaries
Measuring Size
Energy released can be calculated M6.0- energy of hiroshima M8.9- Annual energy released by all other quakes Small quakes are frequent -100,000 magnitude 3 per year Large quakes are rare -32 magnitude 7 per year
Displacement Can Be Visible
Fault Trace - surface tear Fault Scarp- small cliff Blind Faults- invisible
Faults
Faults are like planar breaks in blocks of crust -most slope, some vertical On sloping faults, blocks classified as: Footwall (block below the fault) Hanging Wall (block above the fault)
Fault Motion
Faults move in jumps once starts, stop quickly due to friction Eventually strain builds up again, casuing failure This is called STICK-SLIP BEHAVIOR When rocks break, elastic strain released Engery is sent as a wave, vibrating causes motion Foreshocks and aftershocks common, radiates at hypocenter
Disease Outbreak
Food, meds, water scarce Sanitation disabled Hospitals and healthcare gone
Travel time
Graph plots distance of station to epicenter Requires 3 stations to pinpoint an epicenter -draw a circle around each epiceneter and find the intersection to get the epicenter
Damage
Ground shaking damage -arrive in discrete sequence P-waves- 1st -rapid up and down S-Waves- back and forth motion, stronger than P-waves, extensive (sometimes more) damage Surface Waves- After s-wave, love waves first to follow (writhing like shake) R-waves Last - ripples in pond, may be larger than original. SLower & more damage
Normal Fault
Hanging wall goes down relative to footwall -due to crustal stretching
Reverse Fault
Hanging wall goes up relative to footwall -due to crustal shortening -slope (dip) of fault is steep
Oblique Slip Fault
Hanging wall slips diagonally -combo dip-slip and strike-slip -most display oblique character -PURE dip-strike or strike-slip are rare
Seismograph
Instrument to record seismicity -detect them worldwide Reveals the size and location of earthquakes
Mercalli Scale (intensity)
Intensity- designed for scale based on damage -Roman numerals measure the level of degree -Occur in zones -Damage diminishes with distance
Epicenter
Land surface point above the hypocenter
What is an Earthquake
Rapid release of energy -tectonic stresses cause rocks to break -outward or expanding sphere of waves Destroy buildings and kill people -3.5 million in 2000 yrs
Several Magnitude Scales
Richter Moment (dont know for test) Magnitude scales are logarithmic -Increaes 1-unit means goes up ten fold in ground motion
Love Waves
S-waves intersect the surface -back and forth like writhing snake
Bathymetry
Shape of the area hit determines destruction broad, shallow -supressing deep- quick, maximum condition Low Lands, broad- more damage Steep rise- less damage
Landslides and Avalances
Slopes fail Rockslide and wno avalances follow quakes Quake causes landslide that released Mt St Helen
Strike Slip Fault
no vertical motion -one (laterally, slides past the other) -near vertical fault -caused by near vertical shear
Seismic Gaps
places where an active fault as not slipped for a long time. It may be more dangerous.
Tsunamis
seismic waves, no tidal waves -change in sea floor Normal faulting drops sea bed, -Displace overlying water when it snaps back -Start water moving on sea surface -Jet-liner speed, imperceptible in deep water -Education helps, sensors -Growth of humans in coastal areas and islands causes causulties
Hypocenter (focus)
spot within Earth where earthquake waves originate -usually on fault surface -waves outward from here
Annual Probability
1/recurrence interval
Seismology
Earthquake Study
Buildings
Slabs disconnect, bridges topple and come apart, buildings collide Anchorage Alaska- Tsunami killed Hawiians
Severity of the Shake and Damage
-Magnitude (energy) of quake -Distance from hypocenter -Intensity duration of vibrations -Natures Materials -Bedrock transmit quickly- less dense -Sediments bounce waves- amplified damage
Seismicity
-Motion along formed crustal fracture (fault) , motion on exisiting fault -Sudden change in mineral structure, inflation of magma chamber, volcanic eruptions, landslides, meteorite impacts, nuclear detonations
Intermediate & Deep Earthquakes
-Subduction zones -Wadati-Berioff zone -Intermediate- 20-300 km (plates brittle, move downward) -Deel-300-670 km, mineral transform -Below 670 are rare, mantle is ductile
Shallow Earthquakes
0-20 Km Mid-Ocean Ridge, transform, shallow trench parts, continental crust
Interplate Earthquakes
5% quakes are not near plate boundaries Not well understood -old crust weakness? -Stress transmitted inward -Isostatic Adjustments
Common in Crust
Active - ongoing stresses make motion Inactive- Motion occured in geologic past
Surface Waves
Along Earths surface -Slowest waves and most destructive
Magnitude
Amount of energy released -Max amplitude of motion for the graph -Greater the value the more pen deflection
Recurrence Interval
Average time between sucessive events. Combine this with the idenfitication of seismic zones
Fault Types
Based on relative block motion
Convergent Boundaries
Cites near subduction zones have frequent earthquakes
Fire
Common hazard -Topples stoves, candles -breaks gas and fuel tanks to ignite in CONFLAGRATION -Infrastructure to put out fires is destroyed -Fire fighters powerless
Predicting Earthquakes
Could estimate -Can predict probability (10-10000s of years) -Cant predict (hours to months) Long Term Probability- 30-100 yrs, repetative premise, can determine seismic zones
Prediction
Deformities on the sea floor Change in thickness Hard to predict
Thrust Fault
Special kidn of reverse fault -hanging wall goes up relative to footwall -crustal shortening -slope (dip) of fault is not steep
Stick-slip Behavior
Stick- friction prevents motion Slip- friction briefly overwhelmed by motion
Fault Initiation
Tectonics add force to unbroken rocks -rocks deform slightly (elastic strain) -continental stress causes crack growth -eventually crakcs growto point of failure -When rocks break, elastic strain transforms into BRITTLE DEFORMATIOSN releasing earthquake energy
Continental Crust
Transform Faults (San Andreas, Antatolian, Turkey Continental Rifts (Basin & Range, East Af Rift) Collision Zones (himalayas, Alps) Inter plate Settings- ancient crust weakness
Size
Two means of determining -Magnitude and Intesnsity
Displacement
Varies small to large -large events tear huge faults 100s of kms long -Maximum near hypocenter/epicenter -diminishes with distance -may cause up lift or subsidence -measurable using interferometry
Liquefaction
Water filled sediments are liquefied -High pressure forces grains apart releasing frictoin -flows as SLURRY (quicksand) -Sands bcome quicksand, clay becomes "quick-clay" Ex: Sand dikes, sand volcanos, contorted layers Becomes a mobile fluid -land slopes and flow -buildings and foundations topple over
Seismograph Operation
Wave arrival times and magnitude P-waves - first S-waves- second Surface - last P and S waves travel different velocities, depends on (distance?)
Can Be Ready
codes for builldings train community Map active faults Understand what happens in earthquakes Regulate land use
