CHAPTER 4: Earthquakes

Brittle Behavior = Fault

•   Fault: Planar feature along which a visible displacement is observed from one side relative to the other
•   Creep: Aseismic slip: Fault where movement takes place through a long period of time.  Therefore it does not cause losses of life.

Elastic Rebound?
 
 

     Earthquake damage: proportional to energy accumulated and released

Focus = hypocenter?
 
 

Epicenter = point over the earth's surface directly above the focus

Earthquake locations                 Intraplate (severe)
  boundaries                               deep focus earthquakes =  subduction zones
                                                  shallow focus earthquakes: Midocean ridges

Seismic Waves
Body waves             P = compr, Sol+liq =>   Fastest, limited shadow zone
                                S = shear, only sol   slower, large shadow zone

Shadow zone
 

Surface waves =>  over the surface - (like water waves)
                                can cause serious damage
 

Location of epicenter: seismographs at 3 different locations
                    Measurement of the delay between the arrival of S and P waves to the station.
                    epicenter located at overlap area of 3 circles

Magnitude: related to vertical displacement or shaking
                     waves represent energy release
Magnitude = proportional to the total amount of energy released by EQ at its source

Richter

•  logarithmic scale
•  e.g. Magnitude 4 is 10 times larger than Magnitud 3
•   and also  is 100 times larger than Magnitud 2.
•      energy increases about 30 times with every magnitude unit.

• measures earthquake's effect on human or surface features = Mercalli Scale
•      subjective (varies)

Earthquake related hazards
same magnitude,                                    local geology
different damage result                          proximity to coast
                                                              angle of slopes

Hazards
1) Ground motion:   shaking of the ground (surface waves)
                                displacement alongside faults
 Prevention    => Do not build near fault zones
                      =>"giving" rather than "breaking" pipes and       powerlines
                      =>Earthquake resistant buildings
                              how and where to build

 adequate building codes, solid bedrock better (soft bedrock =>  collapse)

     Aftershocks (postdating main EQ)
     Duration of EQ
     Problems of older buildings = not prepared

 3) Ground failure
   landslides =  in hilly areas, do not build there
   Liquefaction: problem in wet places =>  close to H2O table
   (improve drainage)                                  filled land close to coast

4) Tsunami's - Coastal flooding
               speed 1000 km/h - breakers 15 m high
               subsidence of plates   flooding
               uplift also possible   harbor = turned useless

Earthquakes prediction
Seismic gaps: places of active faults where no displacement has been recently recorded
                     =>stress is building up - Fault locked by friction

Precursors:
 things that happen or changes in rock properties prior to Earthquake.

•    uplift or tilt of terrain
•    change in speed of P waves -
•    change in ratio of speeds between P & S waves
•    electrical resistivity of rocks -

Government-sponsored EQ prediction programs
  Japan
  China                        monitoring active faults
  Soviet Union           experimentation
  U.S.A. predictions on timing and magnitude still not reliable

Earthquake Control
1) Unlocking locked faults =>  need carefully controlled method (small nuclear explosion)
2) Fluid injection (water):  lubrication  => diminish friction =>  unlock fault
 Problem = sudden release of stress
 Useful to avoid stress build up in recently active faults

Earthquake awareness & response: Concerns about predictions

•    Dropping property values
•    Logistics of massive evacuations
•    failure of prediction to happen

Public response China - Japan - U.S.
Authorities =

•  Improvements in building codes, siting and land use
•  Adequate disaster response program
•  Public education

EARTH PROCESSES AND ENVIRONMENTS
EARTHQUAKES: REVIEW QUESTIONS AND TOPICS
1)  Earthquake location. Depth of earthquake focus according to tectonic setting: mid-ocean ridges, subduction zones, intraplate.
2)  Elastic reound theory.
3)  Types of waves: body waves and surface waves. Characteristics: media they travel through, relative velocities, type of movement (with reference to the direction of travel, are they compressive?, shear?). Shadow zones: causes and relative extension (only for P and S waves). MAKE SURE YOU DO NOT MIX UP S WAVES WITH SURFACE WAVES!
4)  Compare and contrast magnitude and intensity.
5)  Hazards related to earthquakes. Which preventive or mitigating measures can be taken. The problem of old buildings. Behavior of different type of materials (sediments and hard rocks) during an earthquake. Man-made earthquakes.
6)  The status of earthquake predictions. Seismic gaps. Earthquake precursors.
7)  Areas of high risk in the U.S.
8)  Proposed measures for earthquake control. Basis of the theory and feasibility.
9)  Reasons for earthquake risk in the East Coast or the Midwest.
12) Definitions for all the Key terms at the end of the chapter.