Lecture 9: Paleoseismology

Question 1

Difference between body and surface waves

Answer 1

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

Question 2

P waves

Answer 2

Compressional waves
Push and pull
Particles move in the same direction that the waves are moving in
Can move through any medium

Question 3

S waves

Answer 3

Move particles up and down, perpendicular to the direction the wave is travelling
Can only move through solid rock

Question 4

Love wave

Answer 4

Produce horizontal motion in the crust

Question 5

Rayleigh waves

Answer 5

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

Question 6

Common EQ magnitude scales (4)

Answer 6

1) Local magnitude (ML or ML), “Richter magnitude”
2) Surface-wave magnitude (Ms)
3) Body-wave magnitude (Mb)
4) Moment magnitude (Mw)

Question 7

Seismic moment

Answer 7

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)

Question 8

Formula for seismic moment

Answer 8

Moment: shear modulus x area of rupture x average displacement during rupture

Question 9

Focal mechanisms

Answer 9

Direction of slope in an earthquake and the orientation of the fault on which it occurs

Question 10

Relationship between fault length and magnitude of surface waves

Answer 10

Larger the fault length, higher the magnitude

Question 11

Cascadia subduction zone

Answer 11

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

Question 12

What about EQ can we predict? (4)

Answer 12

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

Question 13

Can you predict when an EQ will occur?

Answer 13

Very difficult

Use statistical, geodynamics and chaos models, and precursor events

Question 14

What tools do we use to try to predict when an EQ will occur? (6)

Answer 14

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

Question 15

Kelis Borok model

Answer 15

Current best model for prediction of EQ

Not completely reliable, but predicts when

Question 16

Tools to mitigate damage from EQ (7)

Answer 16

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

Question 17

Paleoseismology

Answer 17

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.

Question 18

Logarithm of return period

Answer 18

Larger magnitude events have longer intervals of time between them

Question 19

Periodicity

Answer 19

Pattern or predictability

Question 20

Methods for paleoseismology (2)

Answer 20

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

Question 21

Using marine records for paleoseismology

Answer 21

Potential for well preserved record over a long temporal span
Radiometric dating often possible

Question 22

Submarine landslides

Answer 22

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

Question 23

Evidence for a earthquake triggered landslides (2)

Answer 23

1) Regional correlation and synchronous triggering of landslide deposits
2) Comparison with historical records

Question 24

How do we find pre-historic landslides under the sea-floor? (3)

Answer 24

1) Multibeam Bathymetry
2) Seismic Data
3) Sediment cores

Question 25

Multibean bathymetry

Answer 25

Using transductor and beams to survey seabed

Question 26

Seismic data

Answer 26

Using source and receiver to determine acoustic wave paths and composition of seabed

Question 27

Sediment cores

Answer 27

Take a long, core cut of ground

Can analyze layers in core

Question 28

Radiocarbon dating

Answer 28

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

Question 29

Pond Inlet, NU

Answer 29

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

Question 30

Hemipelagic sediments

Answer 30

Normal deposits
Composed of mud
Typical sediment found in deep sea environments
Results from slow settling of mud in low energy marine environments

Question 31

Mass transport deposits

Answer 31

Inclined stratified mud
Folded mug
Mudclast conglomerate

Question 32

Turbidites

Answer 32

Sorted sand and silt deposits
Erosional base
Fining upwards
Above MTDs