Earthquakes

Modified Mercalli Intensity Scale

- 12 categories - based on the amount of people and property effected - qualitative scale

Seismic Waves

- Body waves = P and S waves - Primary (P) waves = moves the fastest in a push and pull motion - travels through liquids and solids - Secondary (S) waves = moves more slowly in an up and down motion - travels only through solids - surface waves = seismic waves when the P and S waves reach earth's surface move along it - moves slower than body waves and causes more destruction

Haiti Earthquakes

- M7.0 earthquake - Jan. 12 2010 - death toll = 220 000 - epicentre = 25km from capital city (where most building were destroyed- poorly constructed buildings) - strike slip earthquake - landslides effected the slums that were located hillside

Tectonic Creep

- aka fault creep - slow movement of rock or sediment along a fracture caused by stress - sudden displacements = minor earthquakes - can damage roads and buildings

Intraplate Earthquakes

- an earthquake on fault in the interior of a continent, far from a plate boundary - typically smaller than plate boundary earthquakes - still may cause heavy destruction as people are no prepared - due to dense, continental bedrock, the earthquake is felt over a greater area

Earthquakes

- caused by rupture of rocks along a fault - energy = seismic waves - measured by a seismograph and compared by its magnitude - epicentre = point on Earth's surface - focus = point directly below Earth's surface where earthquake originated

Earthquake Process

- earthquakes are most common at or near plate boundaries - due to motion at plate boundaries not being smooth or constant - friction along rocks = stress= strain or deformation - when stress overcomes the rock = sudden movement = focus =seismic waves in all directions

Plate boundary earthquakes

- earthquakes that occur on faults separating lithospheric plates - three types: 1) Strike Slip earthquakes= occur along transform faults (San Andreas) 2) Thrust (subduction) earthquakes = occur on faults that separate converging plates (subduction associated) - strongest earthquakes and potentially lead to tsunamis 3) Normal fault earthquakes = occur on faults associated with divergent plate boundaries - common along mid ocean ridge (earthquakes occur under ocean and with magnitude of <M6)

Earthquake magnitude

- expressed to 1 decimal place - Richter Scale (no longer in use) = the strength of a wave 100km away from the epicentre

Ground Rupture

- fault scarps = face of vertical cliff = a linear escarpment of Earth's surface formed by movement along a fault during an earthquake - strong earthquakes can produce fault scarps that extend for hundred of kms - ability to uproot trees, destroy pipelines, tunnels and bridges

Landslides

- ground motion caused by earthquake can cause rock and sediment to move downslope

Fires

- ground shaking and rupture can cause severe power and gas line damage = fires - appliance can topple over = gas leaks

Earthquake Cycle

- hypothesis that explain successive earthquakes on a fault - based on the idea that strain drops abruptly after an earthquake and then slowly accumulates until the next earthquake - as stress increases, the material will rupture - involved are two plates - one experiences a strain the other exerts the stress Several stages: 1) inactive period - the strain = minor earthquakes - period of foreshocks prior to the major release of stress (does not always occur) 2) mainshock - period where the accumulated stress is released 3) aftershock period - always occurs as the rocks need to settle back down after a major release - occurs in the same general area as the mainshock

Amplification

- increase in ground motion during an earthquake - as body waves slow, the energy is transferred into surface waves

Dip slip faults

- involves two walls on an incline: footwall (where miners put their feet) and hanging wall (where miners hang their lantern) - three types: 1) reverse fault = hanging wall moves up at angle >45 2) thrust fault = hanging wall moves up at angle <45 3) normal fault = hanging wall moves down at angle >45

Moment Magnitude Scale (M)

- logarithmic scale - measures the amount of rock that actually moved determined by: - the area ruptured along the fault - the amount of movement along the fault - the elasticity of the crust at the focus

Factors of Earthquake shaking

- magnitude - distance from epicentre - focal depth (the greater the focal depth, the less intense shaking) - direction of rupture/ fault (shaking occurs along fault) - local soil and rock types (energy is transmitted quickly in homogeneous, dense crust while energy is slowed in heterogenous, folded, faulted crust) - local engineering and construction practices

Effects of earthquakes

- primary (physical) effects =ground shaking and surface rupture - secondary effects =liquefaction, land-level change, landslides, fire, tsunamis

Earthquake Hazard Reduction Programs

- programs have 5 goals : 1) improve national seismograph networks 2) develop awareness of earthquake sources 3) determine earthquake potential 4) predict effects of earthquakes on buildings 5) communicate research to educate the public

Fault Types

- strike slip faults = horizontal displacements (ie San Andreas fault) - dip slip faults = vertical displacements

Liquefaction

- transformation of water saturated sediment from solid to liquid - during strong earthquakes, water pressure is high and the sediment within the soil becomes suspended - when pressure decreases, the sediment compacts and regains strength

Locating an Earthquake

- use a seismograph to graph the P and S waves - the time difference between the waves are recorded and used to determine how far the epicentre is from the seismograph - circles are drawn around the station with the distance found as the radius - need at least 3 circles to determine location of epicentre - where the 3 circles intersect = epicentre = process is triangulation

Precursors to earthquakes

1) pattern and frequency of earthquakes - based on foreshocks and microearthquakes 2) land- level change - uplift or subsidence may precede earthquakes 3) seismic gaps along faults - areas along faults that have not yet experienced an earthquake more likely to experience one 4) physical and chemical changes - changes in groundwater levels and chemistry -> rocks expanding prior to earthquake

Fault Activity

Active = movement during 11600 years Potentially active = movement during 2.6 million years Inactive = no movement during 2.6 million years