# Geotechnical Hazards L2

?

## How do earthquake simulators work?

In fault networks, several faults can accommodate deformation. In earthquakes, stresses induced can increase in nearby faults bringing them close to failure or they can reduce, making the faults move away from failure risk.

This can be modelled with the kinematic source model

1 of 30

## What is the seismic moment and its formula?

The seismic moment is considered the energy release of an earthquake in Nm

or

the mu term is known as the shear rigidity and is often known as 3.3x10^10N/m2

2 of 30

## What is the moment magnitude ?

It is a way of quantifying earthquake size

3 of 30

## Why does the seismograph magnitude fall for high M

Cut off happens when the frequency the seismograph measures to relate it to the moment magnitude falls below the corner frequency - it is beyond this frequency that seismographs can't observe the changes in source strength

4 of 30

## What is a fault source?

Fault - 3D surface representing zone of weakness in rocks.

5 of 30

## How is segmentation done in faults?

Faults can be split into segment areas, with areas split and made up of segments according to barriers in rupture propogation or changes in geometry. Combinations of these segments can define ruptures, with the magnitude proportional to the panel area.

6 of 30

## What is an area source? How are they discretised?

An area associated with different level of seismic activity, magnitudes of earthquakes etc., but with a uniform probability of earthquake occurrence.

Area source can be split into cells with possible ruptures mapped out according to the known characteristics of the site.

7 of 30

8 of 30

## What are point sources? Why used?

Represented at a single location

Could be used to model small area faults, or where the closest distance of a rupture to a site is needed and this will not change. Can be computationally more efficient to use points.

9 of 30

## What are background sources?

Areas associated with lower levels of seismic activity which can't be discounted.

10 of 30

11 of 30

## What is source scaling used for?

Source scaling can relate area to magnitude.

i.e.

or vice versa. Even rupture length can relate to magnitude

12 of 30

## Why is there a linear break in source scaling?

Rupture width down the fault dip is limited due to the depth of the seismogenic layer.

This leads to a break in scaling the area to the magnitude (the magnitude levels out)

13 of 30

## Is it easy to find the maximum rupture displacemen

It is often difficult to know if the maximum displacement is observed - many observations are needed for this. This is why approximations are used often relating u and x.

14 of 30

15 of 30

## What is epicentral and hypocentral distance?

Epicentral - 2D dist (x,y)

Hypocentral - 3D dist (x,y,z) between site and earthquake

16 of 30

## What info is needed for GMM in finite faults?

magnitude

strike, dip, depth

fault mechanism

In 1D only magnitude and epicentre/hypocentre needed

17 of 30

## How is rupture characterised with kinematic slip d

If a kinematic distribution is used, coupled with waveform modelling, we need:

magnitude

strike, dip, depth

fault mechanism

spatio-temporal slip distribution over rupture surface

18 of 30

19 of 30

## How are slip rates estimated?

Following a series of earthquakes, average rate can be found:

with

And total uncertainty

20 of 30

## In the offset stream

You can find the history of slips from stream channels.

i.e. for this one channel 2 has been subject to two ruptures, 1 and 3 have been subject to one and the other streams none at all

21 of 30

## What can we learn from offsets?

Knowing the size of the slip, (A), and u, the moment magnitude can be found.

22 of 30

## What can be learned from paleoseismic trenching?

A trench can be excavated across an active fault, assessing offsets at the fault.

Where carbon rich material has been deposited on either side of the soil, the timing of an earthquake can be assessed and bound to assess rupture history

23 of 30

## How do reverse faults work? What can you learn?

can learn rupture history by looking at differential settlements across sediment history

24 of 30

## What can be learned from historical data?

Recurrence intervals

Magnitudes from field investigations

These are likely to be very uncertain

25 of 30

26 of 30

## Reverse Mohr Circle

Max stress is hz component

27 of 30

## Normal Mohr circle

Max component in vertical

28 of 30

## When is seismicity app better?

When the faults are smaller as the geometry cannot be easily found for these faults

29 of 30

## When is geo approach better?

For large sources when geometry can be easily defined

30 of 30