Lecture 9: Paleoseismology Flashcards
Difference between body and surface waves
Body waves can travel through the earth’s interior, but surface waves can only move along the surface of the planet like ripples on water
Surface waves are associated with damage and destruction of EQ
P waves
Compressional waves
Push and pull
Particles move in the same direction that the waves are moving in
Can move through any medium
S waves
Move particles up and down, perpendicular to the direction the wave is travelling
Can only move through solid rock
Love wave
Produce horizontal motion in the crust
Rayleigh waves
Rolls along the ground, like a wave in the ocean
Ground moves up and down and side to side
This is most of the shaking felt from an EQ
Common EQ magnitude scales (4)
1) Local magnitude (ML or ML), “Richter magnitude”
2) Surface-wave magnitude (Ms)
3) Body-wave magnitude (Mb)
4) Moment magnitude (Mw)
Seismic moment
Measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting.
(Calculated precisely from seismograph data)
Formula for seismic moment
Moment: shear modulus x area of rupture x average displacement during rupture
Focal mechanisms
Direction of slope in an earthquake and the orientation of the fault on which it occurs
Relationship between fault length and magnitude of surface waves
Larger the fault length, higher the magnitude
Cascadia subduction zone
Juan de Fuca plate subducts underneath North American plate
Between EQ, the two plates stick together, causing strain to build up
Sometime in the future, plates will unlock and generate a huge subduction EQ
Last one occurred in 1700, and they occur between 300-800years
Can predict the movement of plates, depts of subducting plate, and magnitude when plates slip - cannot predict when
What about EQ can we predict? (4)
1) Location (particularly locked areas on fault plane with high stress)
2) Moment magnitude (based on time and displacement if not lockd)
3) Depth (based on type of fault system)
4) Intensity of damage using Mercali scale
Can you predict when an EQ will occur?
Very difficult
Use statistical, geodynamics and chaos models, and precursor events
What tools do we use to try to predict when an EQ will occur? (6)
1) Changes in patterns and frequencies of EQ
2) Seismic gaps (locked zones)
3) Increase in radon gas concentration and other gases, emitted as precursors
4) Animal behaviour
5) Shoreline subsidence precursor (foraminifera)
6) Numerical models
NONE ARE RELIABLE
Kelis Borok model
Current best model for prediction of EQ
Not completely reliable, but predicts when
Tools to mitigate damage from EQ (7)
1) More paleoseismology
2) Good maps and communication
3) High resolution and continuous monitoring
4) Warning systems
5) Reduction of energy “build up” along segments of individual faults - lubrication
6) Better building codes, upgrades
7) Make insurance available to cover EQs
Paleoseismology
The study of the location, timing, and magnitude of prehistoric (pre-instrumental) earthquakes
Paleoseismology focuses on extremely rapid deformation that triggers an earthquake (seconds to hundreds of seconds), but seeks to find the evidence over the past century to million years ago.
Logarithm of return period
Larger magnitude events have longer intervals of time between them
Periodicity
Pattern or predictability
Methods for paleoseismology (2)
1) Find prehistoric faults, and use their length and displacements to estimate the prehistoric magnitude
2) Where there is no surface rupture, use relationships between EQ magnitude and other processes, such as sandblows or landslides
Using marine records for paleoseismology
Potential for well preserved record over a long temporal span
Radiometric dating often possible
Submarine landslides
Can be caused by EQ - slope instability
Include mass transport deposits and turbidites
If you can figure out when deposits occurred, you can figure out when EQ occurred
Evidence for a earthquake triggered landslides (2)
1) Regional correlation and synchronous triggering of landslide deposits
2) Comparison with historical records
How do we find pre-historic landslides under the sea-floor? (3)
1) Multibeam Bathymetry
2) Seismic Data
3) Sediment cores
Multibean bathymetry
Using transductor and beams to survey seabed
Seismic data
Using source and receiver to determine acoustic wave paths and composition of seabed
Sediment cores
Take a long, core cut of ground
Can analyze layers in core
Radiocarbon dating
Radiocarbon dating is a method to date organic material.
In a marine environment this could include: shells, foraminifera, plant fragments.
Observe radio of Carbon 12 and Carbon 14
Pond Inlet, NU
Baffin Bay is seismically active
Largest earthquake North of the Arctic Circle (Ms 7.3 in 1933)
Earthquakes are associated with strike slip and rift system faults
Do not know reoccurrence as there is not enough data
Use Pond Inlet (fjord fed by glaciers and streams) to see record to EQ triggered submarine landslide deposits
Hemipelagic sediments
Normal deposits
Composed of mud
Typical sediment found in deep sea environments
Results from slow settling of mud in low energy marine environments
Mass transport deposits
Inclined stratified mud
Folded mug
Mudclast conglomerate
Turbidites
Sorted sand and silt deposits
Erosional base
Fining upwards
Above MTDs
