Earthquake geology, and why it matters

Earthquakes, geology, and why it mattersStress causes students rocks to crack with disastrous consequences

Dr. Brendan DuffyThings to think aboutWhy does rock break to form faults?How does faulting rock cause earthquakes?What do faults tell us about the processes that build our land?How do earthquakes shape our world?What are the effects of these processes for societyHow do earthquakes shake our world?Why do rocks break to form faults?Stress!!!

Rocks break under compression, extension and shear

Types of FaultsFaulting accommodates movement of one part of the earth relative to anotherFaults Are Classified According to the Kind of Motion That Occurs on ThemJoints - No Movement Strike-Slip - Horizontal Motion Dip-Slip - Vertical Motion Typically accommodate either extension or shorteningDip-Slip faultsNormal increase the surface area of crustVertical stess maximum the weight of the earth on your shouldersThrust decrease the surface area of the crustHorizontal stress maximum caught between a rock and a hard place

11

Strike-Slip FaultsLeft lateralRight lateral

Why and how do earthquakes shape our worldWhere does the stress come from in the first place?How is the stress manifest in earthquakesWhat are the geomorphological effects (ie the effects on the shape of the earths surface)Different scalesHow do we measure them?What can we understand from them?

Ridge pushHeat transfer introduces the energy, gravity does the work via: Slab pull, ridge push, slab suctionFundamental processes like plate motionThe source of our stress: NZs tectonic settingRelative motion of the Pacific and Australian plate

Crust getting deformed differently in different parts of the country

Different earthquakes and faults in different parts of the country

extensionStrike slipshorteningGPS GeodesyNew Zealand is deforming continuously

Arrows show movement of parts of NZ relative to Australian plate

Small movements are on the Australian plate

Large movements are on the Pacific Plate

Alpine faultOpening of the Whakatane riftComplicating factorsInherited faults from 60 million years ago

Continuously deforming then rebounding abruptlyReid, 1910

FaultStable zone during stretchingIncreasing amount of stretching (strain) prior to snappingEdgecumbe

This rift

Was built like this

A few major South Island faultsThe right lateral plate boundaryRight side moves toward usLeft side gets pushed away and up over the top

SeddonClarence faultAwatere faultPacific PlateAustralianplate

You cant break all the rock at once!Creates localized zones of extension and shortening

Zones of enhanced stress

Followed by more earthquakes somewhere else

extensionextensionshorteningshortening

Elliot et al 2012Recent seismic sourcesConcentrated areas of slip on as many as 8 individual fault segments; surface rupture only on 3 with Mw 6.6 16Mountains like these growOne earthquake at a timeBig and small increments

Continuously deforming then rebounding abruptly

FaultStable zone during stretchingIncreasing amount of stretching (strain) prior to snappingEarthquakes occur due to frictional stick-slip instabilityFis the resistive force of friction

is the coefficent of friction

sis the coefficient of static friction for the two surfaces k is the coefficient of sliding friction, when the fault blocks are moving

Nis the normal or perpendicular force pushing the two objects together

When elastic forces exceed the static friction, an earthquake occurs. The slip travels along the fault at rupture velocity vr, so there is also a finite rupture time TR

At one point on the fault slip takes a finite time (called rise time TD):

Time taken to ruptureTimeSlipTDFault= ruptureMap viewUSGS20In a simple model of the processes at a fault, the rupture propagates along the fault plane a velocity vr, but in addition to this, at each point of the fault the slip takes a finite amount of time to occur. The finite slip at each point can be modeled as a ramp function (the derivative of this is a boxcar). The propagation of the slip along the fault is also most simply modeled as a boxcar (the duration of which is related to the length of the fault, vr and the direction between the fault and the recording station.Southern California Earthquake Centre simulation

450 kmWe mostly feel earthquakes that arent rupturing the groundMost earthquake hazards are associated with shaking, not ground rupture

Only 3 segments ruptured the surface, and all in one earthquake!The most important characteristics of strong ground motion are1) Seismic wave intensity (amplitude)

2) Seismic wave frequency content (how amplitude is distributed amongst different frequencies)

3) Duration of shaking

23Geologic influences on shakingFault type (ss, rev, norm)Fault size (Magnitude of earthquake)Rupture depth and locationRupture directivityFault strength / stress drop / radiated energy

Bedrock lithology fracture density?Basin effects

Topography (elevation, geometric effects)Soil type, fluid content, and depth-spatial distributionOthers?

SiteeffectsSeismic source effectsPatheffects

24Wave characteristicsA given frequency wave passing into a lower velocity (softer) medium will reduce its wavelengthTo conserve its energy it is amplifiedSedimentary basins have lower velocities than bedrock and can therefore produce intense amplification

25Hz

Stiffness

Amplification controls ground accelarationCan be estimated using 1D site response softwareInput data includes:Soil physical propertiesSoil dynamic propertiesSoil thickness

Input rock motion at the base of the soil column (a function of source and path effects)

These factors can be used to estimate site amplification

26Spectral acceleration Period is inverse of frequencyFor a given wavelength, long period waves are associated with low velocity material

Amplification27Buildings damaged by horizontal waves close to their own natural freq.

Some useful linksGNS Science Active Faults Databasehttp://maps.gns.cri.nz/website/af/viewer.htmSlinky seismometerhttp://cgiss.boisestate.edu/~kasper/geoph297wiki/index.php/How_To_Build_a_Slinky_Seismometer

29So what if you come and do geology here?

4 Sept 2010 - DarfieldTimor 2009, 10 and 13

Kaikoura 2011

Castle Hill 2012

San Francisco 2011