Week 6 - Earthquakes

Earthquakes

Sudden release of energy bc slippage along crust fault

Seismic waves

Energy released in Earthquake

Focus

Source of seismic waves, radiates outward from here

Epicentre

Points directly above focus

Main categories of waves

• Body waves

• Surface waves

Body waves

Travel through Earth’s interior

Types of Body Waves

Primary

Secondary

Primary waves

Push-pull motion

Change volume of intervening material

Through all states of matter

Secondary waves

Shear motion at 90 degrees and angles to direction of travel

Longer travel time to p waves

ONLY THRU SOLIDS

Surface waves

At Earth’s surface

Rayleigh and Love

Rayleigh waves

Up and down

Love waves

Side to side

Elastic Rebound Theory

Storage + release of energy

Storage - rocks on both sides of fault are deformed, bending and storing ELASTIC ENERGY

Release - a slip occurs at the focus, the weakest point, and an EQ occurs when the rock undergoes ELASTIC REBOUND to spring back to its OG shape

Earthquake Intensity

Semi-quantitative, based on human eyewitness accounts M

MMI Scale

Modified Mercalli Intensity, ranked in I - XII based on severity

High intensity means damage, low intensity means felt observations, animals detext first

Magnitude in EQ

Is constant, intensity decreases with greater distant from epicenter

Local geoglogic factors

Intensity can vary according to characteristic of local geological factors, where unlithified and loose sediment particles move during shaking

Human factors

Devastation relates to poor integrity of human built structures, more relevant in developing countries

Locating Earthquakes in three steps

1. Recording seismic waves

2. Using a P-S Time Travel graph to find epicentral distance

3. Locating epicentre

Recording Seismic waves

P → S → Surface waves, looking at difference in arrival time at different stations to find distance between station and epicentre

Using P-S travel time graph

Longer time difference means greater distance between station and earthquake epicentre

Locating epicentre

Use triangulation, plot epicentral distance on map for each recording station, and find true epicentre by intersection of at least 3 station circles

Impacts - Tsunamis

Often called tidal waves inappropriately, a result of vertical displacement along an ocean floor fault, a large undersea landslide, and waves generated from site of disturbance

Crust Mantle Boundary - Moho Discontinuity

Studies of P-waves indicated lithosphere wasnt uniform, some P-waves arrived sooner than expected at distant stations, if they were more than 200km from the epicentre, shallow path through crust, if less than 200km, a deeper and longer mantle path