Lecture 12

Seismology

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

www.seismo.ethz.ch

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

Examples

USGS

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