Lecture 7 earthquakes 1

Question 1

Give a few examples of famous/large earthquakes

Answer 1

Turkey February 2023 - occurred at night when people were inside asleep. Few large aftershocks occurred.
Alaska 1964 - less people live here and so less people were effected.
Sumatra - boxing day 2004 - subduction zone earthquake slip on plate boundary. Boxing day - tourists coming to this area for winter sun - tsunami devastated these areas. The tsunami caused saltwater damage to agricultural land in these poor areas.

Question 2

There are other sources of seismic activities - not just earthquakes name the 4 natural causes

Answer 2

Tectonic earthquakes
Volcanic earthquakes
collapse (e.g. landslide) earthquakes
Oceanic microseisms

Question 3

name the 4 anthropogenic events that cause seismic sources

Answer 3

• Explosions and seismic surveys
• seismic noise (heavy traffic, people jumping up and down at a football match)
• reservoir and fluid injection induces earthquakes e.g. fraking.
• mining-induced earthquakes.

Question 4

What is an earthquake?

Answer 4

A release of accumulated elastic strain causing a rupture of a rock along a fault. a sudden transient motion or series of motions of the
ground originating in a limited region and spreading out in all directions.

Question 5

What’s the epicentre and hypocentre (focus)

Answer 5

EPICENTRE: The location on the earths surface immediately above the point of the first energy release.
HYPOCENTRE: location within the earth where the first energy is released.
depth of the earthquake below the surface = the focal depth

Question 6

What is the elastic rebound theory

Answer 6

elastic rebound of previously stored elastic stress - this causes an earthquake.

Question 7

What is a strain

Answer 7

A measure of the amount of deformation that has been experienced
by a volume of rock (translation, rotation, compression, shear).

Question 8

What is a stress

Answer 8

Stress: The force acting per unit area on any defined plane within the volume of rock. (Shear stress, normal stress). The relationship between
strain and stress in earth materials depends on their elastic properties.

Question 9

Where do most earthquakes occur?

Answer 9

At boundaries between tectonic plates, where deformation is concentrated.

Question 10

What is the epicentral distance

Answer 10

The distance from the epicentre to a seismic recording station (Km or degrees)

Question 11

Earthquake Focal Depths - how many km’s is considered shallow, intermediate and deep.

Answer 11

Shallow 0-70 km
Intermediate 70-300 km
Deep 300-700 km

Question 12

How can depth error be improved

Answer 12

Can be improved if a network of local
seismometers are available close to the event.

Question 13

foreshock, main shock, aftershock

Answer 13

self explanatory, foreshock happens before, main during and after afterwards.

Question 14

What is a seismic hazard?

Answer 14

Population density, the location and strength of
buildings and infrastructure are all linked to seismic hazard. Statistical estimates of the likelihood, frequency and size of damaging earthquakes in a given area constitute the other main factor. Aspects of these issues rely on interpretation of incomplete data, so can be
controversial. Local effects can also be important (e.g., soft sediment vs bedrock).

Question 15

Earth structure definition.

Answer 15

Earth structure Earthquakes produce seismic waves that give us
primary information about deep Earth structure.

Question 16

Tectonics definition

Answer 16

Tectonics Location, size and character of earthquakes can tell us about tectonics. For example, we can map faults that are invisible at
the surface, determine the thickness of the brittle zone, and work out the type of faulting from remote observations.

Question 17

Earthquake fatalities

Answer 17

Long term average of 10,000 earthquake related deaths globally per year - 1.3 million car accident related deaths however an earthquake happens in one place so can completely wipe somewhere out at once.
- even though somewhere may have a higher magnitude the level of preparedness effects deaths for example Haiti was 6.0 and Japan much higher but Japan had an early warning system for the tsunami, Haiti is poor, with low food supplies, not prepared.

Question 18

Secondary natural hazards list (8)

Answer 18

• ground shaking
• Ground failure/movement
• landslides
• Avalanches
• mudflows
• liquefaction
• subsidence
  -Tsunamis
  in Japan there was subsidence and tsunami which emphasised the wave power.

Question 19

What are the anthropogenic hazards

Answer 19

• damage/collapse of buildings, bridges, viaducts, other structures.
• floods from dam failures and water pipeline breaks.
• failing objects (masonry, cables,)
• disruption of transportation, communications, power, water supply, sewage systems - knock on effect as people cannot access the help they need.
  Radioactive leaks from nuclear reactors, power stations.

Question 20

Explain tsunamis

Answer 20

• generated by seafloor movement (e.g. submarine earthquakes, landslides, volcanic eruptions)
• significant - effects are wide reaching
• Complex source characteristics and runup height relative to local shelf bathymetry and coastal morphology.
• technology is advancing and modelling techniques can predict wave heights.

Question 21

Explain submarine landslides

Answer 21

• Slope failure occurs in terrestrial and submarine environments.
• they can cause tsunamis as a secondary hazard.
• can impact locally or distant populations, infrastructure and land use.
• to fully understand the hazard - need high resolution data of past events and deposits and laboratory/ numerical modelling of the flow process.
• concerns this could happen in the canary islands which would cause a tsunami to the US.

Question 22

How would you determine an earthquakes potential?

Answer 22

Instrumented records (e.g., seismology)
* Historic records
* “Paleoseismology” - geological earthquake
records
- Sedimentary records, e.g., of landslides, tsunamis, earthquake secondary effects
* Predictive probability based on past patterns of frequency, magnitude, location
* Other important factors: source characteristics, parameters at destination/region being assessed.