lecture 12 seismology - ernet
TRANSCRIPT
Energy release
• Energy release:
• Increase of one level of
magnitude corresponds to:
⇒Amplitude increase: 101 = 10
⇒Energy increase: 101.5 ≈ 30
• Energy release increases very
rapidly with magnitude
!
log10 E = 4.4 +1.5MS
u = A cos(kx – ωt) K.E. = ½ ρω2A2 sin2(kx- ωt) 0 ≤ E ≤ = ½ ρω2A2 log10E = 1.5MS + 4.8 • Energy released increases very rapidly with magnitude • Total energy of all small earthquakes << energy released by one big earthquake
Energy Released by Earthquakes
Seismic Hazard - Ground acceleration - Secondary effects (liquifaction, landslides, fires,
tsunamis)
• Seismic Potential è Probability of an earthquake of a given magnitude to occur
• Seismic Attenuation è Geometrical spreading,
scattering
• Site Response è softness of soil, sediment thickness
Site response
• Ground acceleration decreases
with distance, but can vary by a
factor of 10 for 2 sites at the
same distance to an earthquake
⇒ site response
• Site response depends on
geological factors:
– Softness of soil or rocks near
the surface: ground motion
amplified by soft rocks
– Sediment thickness above
bedrock: ground motion
amplified by thick sediments
Snapshots of simulated wave propagation in the LA area for the
hypothetical SAF earthquake (K. Olsen, UCSB) Source: E. Calais @Purdue
Event Id:37301704 Sun Jan 4 03:18:09 2015 Mag:4.2 7 miles NNW of Castaic Lake dam, CA Latitude:34.6197 Longitude: -118.6300 shakemovie.caltech.edu
Event Id:12245763 Wed May 24 04:20:26 2006 Mag:5.4 31 miles SE of Calexico, CA Latitude:32.3067 Longitude: -115.2278 shakemovie.caltech.edu
Fault Plane Solutions
9
Physics and chemistry of the Earth’s interior – The earthquake source
Fault planes
Rock ruptures and slides releasing stress
Stress builds deforming the rock
Physics and chemistry of the Earth’s interior – The earthquake source
First motionThe first motion direction observed at many stations around an earthquake can tell us the type of fault and orientation of the fault planes.
Consider a strike-slip earthquake on the San Andreas Fault:
Fault plane solutions
Vertical view
Plan view
R. Allen lecture notes
Normal Fault StressNormal Fault Stress
normal fault
• the maximum principal stress is vertical
dips at 60º
ı1
ı3
ı2
The maximum principal stress is vertical
Reverse (Thrust) Fault StressReverse (Thrust) Fault Stress
• the minimum principal stress is vertical
reverse fault
dips at 30º
ı3
ı1
ı2
The minimum principal stress is vertical
StrikeStrike--slip Fault Stressslip Fault Stress
• the intermediate principal stress is vertical
strike-slip fault
ı1
ı3
ı2
ı1
ı1
ı1
ı1
ı3
ı3
ı3
ı3
ı2
ı2
dips at 90º
The intermediate principal stress is vertical
Stereographic projection
• A method of projecting half a sphere onto a circle.
• e.g. planes cutting vertically through the sphere plot as straight lines
http://www.learninggeoscience.net/free/00071/index.html
Forecasting Earthquakes
Difference between Predictions and Forecasts: - Predictions have specific times, locations, and
magnitudes for future earthquakes - Forecasts are more long-term estimates of
earthquake occurrences. Often they include probability information
Credit: Walter Mooney, USGS
‘Now, when will earthquake prediction be possible and an efficient Forewarning service available ? …if we start the project presented here we should be able to answer the question with sufficient certainty within ten years.’ - The Japanese Blueprint (Tsuboi et al, 1962)
‘Based on an assessment of worldwide observations and findings over the past few years, it is the panel’s unanimous opinion that the development of an effective earthquake prediction capability is an achievable goal. …with appropriate commitment and level of effort, the routine announcement of reliable predictions may be possible within ten years…’ - Panel of the US National Research Council (Allen et al., 1976)
Optimism in the 1960’s and 1970’s
Credit: Walter Mooney, USGS
Prediction of the 1975 Haicheng, China Earthquake (M7.3)
Prediction based on foreshocks and animal behavior saved many lives
Credit: Walter Mooney, USGS
For short-term precursors, there are currently more ‘negative’ results than ‘positive’results.
Forecasting Earthquakes
Difference between Predictions and Forecasts: - Predictions have specific times, locations, and
magnitudes for future earthquakes - Forecasts are more long-term estimates of
earthquake occurrences. Often they include probability information
Long-term Probability Estimates
Credit: Walter Mooney, USGS
“… the Earthquake Research Committee announced on May 24, 2003, there was 10-20 percent chance of occurrence of a M8-class Off-shore Tokachi Earthquake over the next 10 years starting from January 1, 2003, and a 60 percent chance over the next 30 years”
Tokachi-oki earthquake September 26, 2004 M8.0
Credit: Walter Mooney, USGS