GEOS 110
NATURAL DISASTERS
 
Earthquakes-I
 
1. EARTHQUAKE BASICS
  - Global distribution of earthquakes.
 - Earthquakes are an energy release in the form of seismic waves caused by the   sudden rupture of strained rocks. Strain is deformation of rocks resulting   from stress (e.g., tectonic forces).
 - Earthquakes occur along faults (fracture where rocks have been displaced).    Focus =
  Epicenter =
 
  - Three basic types of faults
   1) Strike-slip
 
   2) Normal
 
   3) Reverse
 
  - Fault activity can be described by slip-rate and recurrence interval.
 
2. ELASTIC REBOUND THEORY OF EQ's
 - Rocks on either side of a fault undergo elastic strain as they are stressed by    tectonic forces. When they finally rupture, the built-up stress is released   and the rocks "rebound" to their original undeformed shape.
 - The recurrence interval is the time it takes to accumulate sufficient elastic    strain to cause the next EQ.
 
 - Stages of Elastic Rebound ModelEQ Cycle
  Stage 1 - long period of seismic inactivity during build up of elastic strain.
  Stage 2- increased seismicity as elastic strain approaches rock strength.
  Stage 3- foreshock activity (small to moderate EQ's before the main event)
  Stage 4- the main earthquake event.
 
3. THE DILATANCY-DIFFUSION MODEL OF EQ's
 -More recent model, involving fluid pressure.
 Stages of Dilatancy-Diffusion Model
  Stage 1 - build-up of elastic strain
  Stage 2 - elastic strain eventually causes rocks to dilate (increase in     volume) when stress on rocks = 50% of the rock strength. Open    fractures develop with minor seismicity.
  Stage 3 - influx of water into open fractures increases fluid pressure. This    lowers the rock strength and facilitates rupture.
  Stage 4 - Rupture occurs and fluid pressure and stress on rocks is released.
 
 
4. TYPES OF SEISMIC WAVES
 2 Basic Types:
  1) Body waves: travel within the Earth.
   - Two Types:
    a) P-waves:  (compressional)  alternating compression and dilation in the direction of wave propagation (~5.5 km/s).  Can travel through solids, liquids, and gases.
 
    b) S-waves: (shear or secondary)  up-down (vertical) motion that is perpendicular to the direction of wave propagation (~3.5 km/s). Can travel through solids only.
 
 
  2) Surface waves: travel on or near the Earth's surface (~2 km/s); very destructive
   - Two Types:
    a) Love waves: complex horizontal (side-to-side) motion.
 
    b) Rayleigh waves: rolling or elliptical motion in the vertical plane, like a waves on the ocean surface, a little slower than Love waves; 
 
Prof. Gorring         Jan. 29, 1998
GEOS 110
NATURAL DISASTERS
 
Earthquakes-II
 
EARTHQUAKE MAGNITUDE AND INTENSITY
 
 1. Modified Mercalli Intensity Scale: (ranges from I to XII)
  - Subjective measure of the kind of damage and human reaction.
  - EQ affected areas can be mapped where intensities values are similar.   - Provides approximate location and size of EQ, as well as the effects of local geology and building construction.
 
 2. Richter Magnitude Scale: (from 0 to infinity; values of >9 are unlikely)
  - Measure of the energy released; more quantitative the Mercalli scale.
  - Based on the largest amplitude seismic wave measured on a      seismograph ~100 km from the EQ focus.
  - Base-10 logarithmic scale; thus M=7 EQ has wave amplitude on a     seismograph 10x larger than M=6 EQ. Energy increase is ~30x per    number on Richter scale.
  - Generally used for local, moderate-sized EQ's (ie. M?7).
  - Symbols:
   M_L = Local magnitude regardless of wave used (P, S, or surface).
   M_b = magnitude based on body waves (P or S).
   M_s = magnitude based on surface waves (Love or Rayleigh).
 
 3. Seismic Moment: (M_w) more quantitative and accurate than Richter Scale.   - Based on energy released determined by three factors:
   1) amount of slip on fault
   2) rupture surface area
   3) rock strength.
  - Seismic Moment (M_o) = (slip) x (surface area) x (shear strength)
            M_w = 2/3logM_o -10.7
  - Much better than Richter for estimate of energy release for large EQ's (ie.    M>7). Richter Scale underestimates the magnitude of large EQ's.
  - Largest EQ ever recorded: South-Central Chile, 1960 (M_w = 9.5; M_s = 8.5)
 
Prof. Gorring         Feb. 5, 1998
GEOS 110
NATURAL DISASTERS
 
Earthquakes-III
 
DESTRUCTIVE EFFECTS OF EARTHQUAKES
 
 1. Ground Motion (shaking):
  - Passage of seismic waves causes ground to oscillate.
  - The intensity of ground motion depends on:
    1) EQ magnitude
    2) distance from EQ
    3) local geology
    4) the period of seismic waves (time to successive wave crests)
 
  - The duration of ground motion only depends on EQ magnitude.
 
 
  - Wave attenuation-
   P and S waves have shorter periods (0.1 second to 1 second) than surface waves (1-3 seconds) and are attenuated (damped or diminished) faster than surface waves.  Therefore, the longer period waves (Love, Rayleigh) are relatively stronger at greater distances from the epicenter of EQ's.
 
 
  - Natural frequency (or period) of buildings sometimes match that of seismic waves (0.1 second period = 1 story; 0.5s = 4-5 story; 1-2s = 10-20 story).  "Swaying" or resonance of buildings is reinforced or amplified so much that building may collapse.
 
 
 
  - Material Amplification-  intensity of ground motion is amplified in soft, unconsolidated or water-saturated materials (ie. sand, mud, artificial fill)
 
 
 
 2. Fires
  - Caused by broken gas and electrical lines
  - Broken water lines and impassable roads and highways hinder fire     fighting capability.
  - Fires can account for a large percentage of the death and destruction.
   1906 San Fransico- 80-90% of buildings
   1923 Tokyo- 70-100% of buildings, 40% of deaths
 
 
 3. Ground Failure
  1) Rupture - surface cracking usually accompanied by horizontal and/or    vertical displacements. Large displacements during large EQ's (e.g. 1-10 meters) can cause direct destruction of man-made structures.
 
  2) Landslides - EQ's triggered; occur in hilly/mountainous areas.
   - Potentially cause catastrophic damage and death.
    1970 Peru- ~30% of deaths caused by giant landslide in the      Andes Mtns. (we will talk about this later!).
 
  3) Liquefaction -  the loss of shear strength of water-saturated, unconsolidated materials.  Material is liquified and man-made structures collapse or sink.
 
 
 
 
 
 4. Tsunami (seismic sea waves; "tidal" waves)
  - Caused by sudden displacement of sea floor or submarine landslide     triggered by EQ's. Dangerous for shorelines populations.
    - Mostly restricted to the Pacific Ocean: are ~1 m in heigth and travel at speeds of ~600 km/hr in the open ocean; can cross the entire ocean.
    - Slow to <100 km/hr and can grow up to ~65 m in height (commonly only ~20 m for a large tsunami) when they encounter shallow water.
    -Geometry of shoreline and seafloor topography can amplify tsunami waves, especially in shallow harbors and inlets.
    - Tidal gauges and seismic networks are now used to give warning and minimize tsunami hazards.
 
Prof. Gorring         Feb. 11, 1998
GEOS 110
NATURAL DISASTERS
 
Earthquakes-IV
 
 
Regions that Produce Large Destructive EQ's
 
 1. Convergent Plate Boundaries
 
  - Collision Zones between 2 continental plates
   - along southern margin of Asia where the African, Arabian, and     Indian plates are colliding with Eurasian plate
 
    1998 Afghanistan (Ms = 6.1)
 
    1990 Western Iran (M = 7.7)
 
    1988 Armenia (M = 7.0)
 
 
 
 
  - Subduction Zones*
   - generate the largest EQ's (M >8.5); trigger other natural disasters.
 
 
    1960 southern Chile (Mw = 9.5)* largest EQ ever recorded
 
    1964 Alaska (Mw = 9.2)** 2nd largest EQ recorded
 
    1985 Mexico City (Ms = 8.1) good example of material       amplification and resonance.
 
 
   - Cascadia Subduction Zone (in the Pacific NW) has potential!
 
 2. Transform Plate Boundaries
 
  San Andreas Fault (~1200 km strike-slip fault; has three segments)
  1) Northern ("locked")
   1906 San Francisco (M ~ 8.2) 430 km fault rupture, 6 m       displacement; 5,000 died; SF almost totally destroyed by fire.
   1989 Loma Prieta (Ms = 7.1) 42 km rupture, 2.3 displacement, ~50 died
 
 
  2) Central ("creeping" segment; steady motion)
   Parkfield experiments; no EQ's bigger than M~6; although M = 5-   6's have occurred every 20-30 yrs.
 
 
  3) Southern ("locked")
   1857 (M ~ 8.3) near the "Big Bend"
   1994 Northridge (Mw = 6.7) 3.5 m displacement, 67 died;
    $30 billion in damage
Prof. Gorring         Feb. 16, 1998
GEOS 110
NATURAL DISASTERS
 
Earthquakes-V
 
Intraplate EQ's
 - 1811-1812 New Madrid sequence (four EQ's Mw ~7.8 and 8.3)
 - 1886 Charleston, SC (Ms = 7.7)
 
 - New England and St. Lawerence River Valley EQ's
 
 - New Jersey EQ's
 
 
 Cause(s)
  1) Regional stress from unknown source "activate" ancient faults.
 
  2) fracture zone hypothesis
 
 
Earthquake Risk and "Prediction"
 - Probabilistic methods of evaluating long-term "prediction" or risk.
 
  1) seismic hazard maps- ususally show:
   - past EQ history
   - probability of max horizontal ground motion
 
  2) conditional probability analysis
   - estimate of the probability that an EQ of a given size will occur on     a specific fault segment in a given time period.
 
   - based on:
    1) historical EQ records
    2) geologic EQ records
    3) slip-rate on active faults
    4) frequency and magnitude of recent EQ's (seismicity, "seismic gaps" -  lack of seismicity along a "locked" portion of a fault)
 
 
 - Short-term Prediction (<1 yr. down to days; based on precursor phenomena:
   1.  patterns and intensity of foreshocks:    usually increase in magnitude and may cluster or migrate down a fault to the place where the main shock will eventually occur.
 
 
 
   2.  ground deformation:  rapid increase in ground deformation in the focal area.
 
 
   3.  fluctuations in water well levels:  rapid fluctuations in water levels in groundwater wells.
 
 
   4. Changes in local radio wave characteristics (Loma Prieta EQ):  dramatic increase in long-wave electromagnetic radiation (e.g. radio waves) just prior to Loma Prieta EQ
 
 
   5.  anomalous animal behavior:   response to #4??