Earthquakes
Experience of an earthquake: Violent shaking that can damage, even destroy buildings, bridges and other structures. These collapse cause death and destruction. Strangely enough only a real danger in an urban environment. Therefore have only been a real hazard since human settlement established 6000 years ago.
Historical earthquakes
1692, Port Royal, Jamaica
1755, Lisbon, Portugal
1811 New Madrid, Missouri
1866 Charleston, S. Carolina
1906 San Francisco
1960 Chile
1964 Alaska
1972 Managua, Nicaragua
1985 Mexico City
1997 Central Italy
Many small earthquakes - temblors
What are earthquakes?
Passage of a wave in the earth - comparable to water wave
What causes earthquakes?
Motion on faults. Sides of faults do not always slide past each other in a smooth manner - if they do is called fault creep
Elastic rebound theory: Stick-slip occurs on faults and sliding is a jerky, intermittent process. When fault is in "stick" mode, motion of blocks continues and elastic energy accumulates like stretching a rubber band, or bending a ruler.
Sides of fault held together by friction, but as stresses build up, eventually fault slips - and does so very rapidly. This generates the waves that radiate outward from the place where fault slips.
Place where fault slips is called the focus or hypocenter .
Epicenter is the position on the map.
Detection of earthquake waves
Seismometers / seismographs
Produce seismograms
Types of earthquake waves
P waves - 8 kms/sec
S waves - 5 km/sec
Surface waves - 4-3 km/sec
Using arrival of waves to locate the hypocenters/epicenters
Travel time curves - these can be deternined for an particuar region (depends on local geology of seismograph station etc.).
Because of different velocities of P and S waves, arrival times of each of these gets greater and greater the further you are away from focus/hypocenter. Thus difference in arrival times of P and S waves is a measure of distance of earthquake source from seismograph station.
Need records of seismic wave arrival from at least three stations to locate the epicenter of an earthquake.
Go to VirtualEarthquake to see how.
Measuring the size of earthquakes
I - XII intensity of shaking at a location. Subjective but in pre seismgraph days (or for historical reconstruction) v. useful. But clearly need to know what energy is released a source to compare earthquakes.
Richter magnitude scale
Related to energy released at hypocenter. Number is actually derived from certain properties of waves recorded by seismograph (period, amplitude etc) this is the "Richter scale"
Richter scale is a logarithmic scale ranges from 0 to about 9 (upper limit imposed by strength and elasticity of rocks). 3, 4 small e/q; 6.5,7,8 very large e/q. - 8.6 largest ever observed.
[log E (in joules) = 4.8 +1.5 M ]
Difference of 1 on Richter scale points to a difference of 31.6 time energy released
Where do earthquakes occur?
In certain restricted zones - basically what we now regard as plate boundaries.
Distribution in 3-D in island arc ("subduction zones").
Seismic hazards
Primary
Shaking - greater the magnitude, closer to the hypoceneter, worse the effect
Effect of local geology - "Jello" effect in basins of unconsolidated rock
Regional changes in land elevation
Secondary
Fires
Differential subsidence
Liquefaction - sand blows, collapse
Landlslides - mass wasting
Tsunamis (if fault scarps submarine, or trigger submarine landslides)
Mitigation of earthquakes hazard-(Hazard reduction)
Locate "active" faults - can be difficult!!
Earthquake engineering
Social enginnering - locate essential services away from zones of greatest risk.
Prediction of earthquakes
We can predict earthquakes - only not precisely or reliably
Desired precision is elusive, however.
GPS - we can now look directly at rate of motion of blocks adjacent to fault zones
Seismic gap theory
Radon, elecrical resistivity
Animal activity ?? - Dogs, roaches, cattle and other myths