Geotechnical Hazards L1

Input to risk evaluation

Defines geographical position

Given the hazard, decisions about the structure resilience and where to build determine risk

Plot prob dist of resistance against prob dist of loading, make contour plot of likely scenarios and maximise length of centroid to the line where failure can occur

Relate it to design life and exceedance rate. This rate can be described by a Poisson dist:

Then expected exceedance

By solving for P(n>=1)

Evalute P(n=0) with Poisson distribution, and subtract it from 1

By finding rate of exceedance annually for each level of intensity and making a plot. Knowing the exceedance rate that we want for our probability of failure we can find out the intensity measure to keep below.

Determining the average rate of exceedance of an earthquake of magnitude M.

A ground motion hazard curve can assist in creating this. 

This is defined probabilistically as

Occur due to shear dislocations occurring over 1 or more surfaces (faults) in crust

Crust is 35km thick, only 10-20km can support earthquakes (seismogenic portion or schizosphere)

In virgin material, causing new fault.

In old surface of reduced strength (existing fault) - can predict with Mohr Circle and geological mapping.

Mantle has temperature gradient causing convective cycles which induce basal shear on crust surface leading to compression or tension.

Plates separate and mantle material fills void, creating crustal material

Plates converge together. Can lead to collision boundaries and mountain building of continental crust, or subduction zones where oceanic crust falls below continental crust.

Transform boundaries

More earthquakes and deformations observed here due to plate interaction, but not all events occur at these interfaces

Reverse faulting - compression

Normal faulting - extension

Strike-slip faulting - transform/lateral motion

direction of intersection of fault with hz surface rel to north

Angle between fault and hz plane

Movement of hanging wall rel to horizontal

It is the hanging wall which moves relative to the footwall

Crust is loaded in tectonic processes

Eventually shear stress exceeds resistance of the crustal structure

An earthquake results which dissipates strain energy built up in crust - elastic seismic waves

It is not possible for earthquakes to occur again until the critical stress is exceeded

In strike-slip the occurrence of many ruptures, or parts of a fault that are discretised (not the whole fault) can be modelled

It models a spring attached to a rigid body. The body moves when the force of the spring, kx, exceeds the static resistance us *mg.

The situation then becomes dynamic, with ma + kx = ud * mg (dynamic friction coef)

When velocity equals 0, the frictional resistance increases and motion stops.

The crust/spring stores energy

The motion occurs due to plate tectonics

There is resistance on the plane interface (between plates = block with floor)

Seismograph plots velocity of ground motion against time

Accelerometer measures forcing on ground in terms of acceleration

Seismograph sensitive - good at measuring weak motions

Accelerometer better for strong motions - can measure intense movements.

1SD = 68%

2SD= 95%

3SD= 99.7%

Compression as opposed to tension

Equal and opposite stresses rise on auxillary plane

1. Shear stresses on fault.

2. Eq and opposite stresses on auxillary plane balance moment.

3. These can be modelled as normal stresses if rotated 45 deg.

4. The compression forces lead to dilational waves, tension leads to compressive waves

Low angle thrust event - has large shallow rupture width and thus associated with high magnitude earthquake

Tension generated in slab top, compression on bottom

Low tensile strength of ocean crust means cracks form

This is a high angle normal fault.

Usually smaller magnitude than interface event due to attenuation

Local magnitudes are computed from observed amplitudes at the recording station and focus upon a particular frequency band. If the local site conditions were associated with resonant effects in this frequency band then this would have a clear impact upon the amplitudes observed – and hence the magnitude.