Earthquake Science
Lisbon, 1755
Why the Earth Shakes and How we Measure it
What is an Earthquake?
The release of accumulated energy caused by the sudden movement of a block of the Earth’s crust along a fault.
4th Ave, Anchorage, 1964
Faults
Accumulated energy comes from application of a force (stress)
Types of Stress
Stress results in Deformation
Types of Deformation Elastic
not permanent
Plastic
Folds
Origin of Earthquakes
Brittle
Faults
Factors Effecting Deformation
• • • • • Strength of Material Temperature Pressure Magnitude of Stress Rate Stress is Applied
Quakes tend to occur in: Shallow, cold, rock where energy accumulates past the breaking point
Types of Faults
Dip-Slip Faults
Strike-Slip Faults
Defining Strike and Dip
Strike-Slip Fault
Transform Boundary
Dip-Slip Faults
(Cross Section)
Normal Fault
Divergent Boundary
(Reverse fault)
Thrust Fault
Convergent Boundary
Intermission - Fault Type?
Intermission - Fault Type?
Fault Activity
Active
-moved in last 10,000 years (Holocene)
Potentially Active
-moved between 10,000 to 2,000,000 years (Pleistocene)
Inactive
-movement older than 2,000,000 years
Accumulation of Energy: Elastic Rebound Theory
May be foreshocks and aftershocks
13 weeks after 2004 Indonesian Quake
What happens when the fault ruptures?
Energy is Released as Seismic Waves
Seismic Waves - Body Waves
P Waves
S Waves
Seismic Waves - Surface Waves
Love Waves
Rayleigh Waves
Wave Summary
Wave Type P (Primary) Waves Motion Body: Pressure (“pushes” & “pulls”) Body: Shear (up & down) Surface: Up & down, rolling Velocity (crust) 5-7 km/sec Notes First to arrive, travels fastest through solids Can’t travel through fluids Similar to water waves Usually a little faster than Rayleigh Waves
S (Secondary) Waves
3-4 km/sec
Rayleigh (R) Waves
2-4 km/sec
Love (L) Waves
Surface: Side to 2-4 km/sec Side
Wave animations: http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm
Measuring Seismic Waves: Seismograph
Seismograph
Seismogram
Seismogram
Measuring Earthquakes
Richter Scale
Increase of 1 unit on Richter Scale equals: Measure of the amplitude of the shaking measured on a seismograph 10x increase in wave amplitude 30x increase in energy released 4 5
6
Degree of Shaking related to:
What we want to quantify • • • • Amount of energy released Distance (including depth) from quake Type of material the waves travel through Limits of the Seismograph
Determining Richter Magnitude
Degree of Shaking related to:
What we want to quantify • • • • Amount of energy released Distance (including depth) from quake Type of material the waves travel through Limits of the Seismograph
Main problem that result in the use of a different scale today
Measuring Earthquakes
Moment Magnitude Scale
Seismic moment = (Total length of fault rupture) x (Depth of fault rupture) x (Total amount of slip along rupture) x (Strength of rock)
Equivalent Energy Output
Frequency of Earthquakes
All earthquake shaking is not equal
All earthquake shaking is not equal
1989 Loma Prieta Quake
Shaking on sediments vs. bedrock
Factors that impact shaking:
Energy released by quake (magnitude) Distance from epicenter Duration of shaking Foundation (sediment vs. bedrock) Building style
1971 San Fernando Valley Quake (M = 6.6)
Modified Mercalli Intensity Scale
Intensity Map
1906 San Francisco Quake
Intensity Map
1811/12 New Madrid Quake
Intensity Map Exercise
Intensity Map
2001 Nisqually Quake