Engineering Geology and Geohazards Lecture 13: Ground Response

Question 1

What are the two main ways in which geology affects seismic waves

Answer 1

Amplitude
Attenuation

Question 2

Define Amplitude

Answer 2

• Amplification is the increase in surface ground motion in unconsolidated sediments relative to the motion in solid bedrock.

Depends on the elastic properties of the sediments
overlying bedrock. Bedding in the sediments increases amplification as seismic shear waves are reflected between bedding planes. Confirmed by down-hole seismographs and intensity records.

More amplification in loose, unconsolidated sediment, like landfills or unconsolidated beaches

Question 3

Define Attenuation

Answer 3

Attenuation is the decrease in seismic signal amplitude caused by spreading of the wave and absorption and scattering of seismic energy by the materials of the Earth as a wave propagates from a source. Attenuation varies with elastic properties of crustal rocks.

There is less attenuation in bedrock, seismic waves can travel far with low amplitude

Question 4

What is the relationship between Amplification and Attenuation

Answer 4

As amplification increases, so does attenuation

Meaning the more amplification, the less far the seismic wave can travel

Less amplitude = travels further
More amplitude = travels less

Amplitude and attenuation are determined by the geology present, e.g. bedrock or unconsolidated sediment

Question 5

Describe the various factors affecting the amount of ground response/shaking in an earthquake

Answer 5

1. the strength of the surface rocks (elastic moduli)
2. the thickness of surface sediments
3. variations in subsurface rock units, eg. presence of buried channels
4. topographic effects - resonance

Question 6

Define resonance in an earthquake

Answer 6

Earthquake resonance is the phenomenon that occurs when the natural frequency of a building matches the frequency of seismic waves during an earthquake.

This causes the building to oscillate with a higher amplitude, which can lead to severe damage.

Question 7

Describe the Loma Prieta earthquake 1989

Answer 7

The 1989 Loma Prieta earthquake was a 6.9 magnitude earthquake that struck Northern California on October 17, 1989

The Loma Prieta earthquake of 1989 demonstrated a wide range of effects.

The Santa Cruz mountains suffered little damage, even though they were close to the epicentre.

However, the centre of the city of Santa Cruz, about 24 kilometres away from the epicentre, was almost completely destroyed.

More than 80 kilometres away, the cities of San Francisco and Oakland suffered selective but severe damage, including the loss of more than 60 lives.

The greatest destruction occurred in areas where roads and elevated structures were built on ground underlain by weak, unconsolidated sediments.