Seismicity Flashcards
What percentage of earthquakes that occur each day aren’t associated with plate boundaries?
5%
How many earthquakes each year are considered to be major hazards?
3 or 4
What depth are shallow focus earthquakes?
0-70km
What depth are intermediate focus earthquakes?
70-300km deep
What depth were deep focus earthquake?
300-700km deep
Where do seismic shock waves have their highest energy?
At the focus
What is the place on the earths surface immediately above the focus?
Epicentre
Which waves are recorded on a seismograph?
P and S waves
Which plate margins are the strongest earthquakes related to?
Destructive plate margins
Describe earthquake at constructive plate margins
They are often submarine and usually distant from people, so there’s relatively little hazard
Describe conservative plate margins
The boundary is marked by a fault or series of faults
Name an example of a conservative plate margin
San Andreas Fault in California
Why is there earthquakes in central China? e.g. Sichuan
Due to weaknesses in the crust connected to the collision of India with the Eurasian plate over 50 million years ago
What is the magnitude of an earthquake?
The amount of energy released by a seismic event
What is the Richter scale?
This is a logarithmic scale with each representing a ten fold increase in strength and a 30 fold increase in energy released.
What magnitude do earthquakes have to be to be destructive?
In excess of 6
What is the Modified Mercalli Scale?
It is an alternative measurement of earthquakes. It measures the intensity of the event and its impact.
How many points does the modified mercalli scale have?
A 12 point scale
What are aftershocks and how long do they last after the main seismic event?
They are smaller earthquakes following the main event, they can last for months after the event.
What are the factors that influence the consequences of the earthquake?
- The magnitude and depth of the earthquake
- Geological conditions
- The distance from the epicentre
- Population density, preparation and education
- The design and strength of the buildings
- The time of day
- The impact of indirect hazards, such as fires, landslides and tsunamis
What is soil liquefaction?
When soils with a high water content lose their mechanical strength and start to behave like a fluid.
How much damage did soil liquefaction cause in the 1989 Loma Prieta earthquake?
US$100,000 million
Landslides and avalanches
Slope failure as a result of ground shaking. This can result in further deaths and injuries as well as flooding from blocked rivers creating ‘quake lakes’
What do the effects of earthquakes on people and their built environment depend upon?
Depends upon population density and distance from the epicentre.
What are the long term effects of earthquakes?
- Higher unemployment
- Repair and reconstruction of buildings and infrastructure may take months or years
- Longer term illness as well as reduced life expectancy caused by immediate suffering
- Trauma and grief may take months or years from which to recover
- Emergency pre-fabricated buildings may become permanent fixtures
- Permanent disruption to natural drainage patterns
- Problem in restoring trust in neighbours and civil authorities if there has been civil disorder e.g. As in Haiti
What are tsunamis generated by?
They are generated by seismic activity caused by ocean floor earthquakes or submarine volcanoes
What speeds can tsunamis travel up to?
700km/hour
What factors will a tsunamis effects depend on?
- The height of the waves and the distance they have travelled
- The length of the event that caused the tsunami
- The extent to which warnings were given
- Coastal physical geography, both offshore and the coastal area
- Coastal land use and population density
How many people were killed by the tsunami caused by the 1883 Krakatoa eruption?
35,000 were killed by waves which were up to 40m high
What percentage of tsunamis are generated in the Pacific basin?
90% of all tsunamis
What percentage of all tsunamis are generated in the
Japan-Taiwan island arc?
25%
What are the two devastating tsunamis which have occurred after the December 2004 tsunamis?
- July 2006, South Java coast
2. April 2007, Solomon Islands
What magnitude earthquake caused the south Java coast tsunami and how many people were killed?
generated by a magnitude 7.7 earthquake. More than 600 people were killed
How many people were killed in the Solomon Islands tsunami?
At least 15 people
The Pacific Tsunami Warning System based in Hawaii warns people of tsunamis, what are its levels of warning?
- Important
- Major
- Serious
How do seismic records help predict earthquakes?
They can be studied to look for patterns. Seismic shockwaves are recorded on a seismometer.
How do radon gas emissions help predict earthquakes?
Radon is an inert gas that’s released from rocks such as granite at a faster rate when they’re fractured by deformation
How does measuring groundwater levels help predict earthquakes?
Deformation of the ground can cause water levels to rise (compression) or fall (tension) independently of atmospheric conditions
How does remote sensing help predict earthquakes?
There’s some evidence to suggest that electromagnetic disturbances in the atmosphere directly above areas about to have an earthquake can be detected
What’re the three main types of protection when protecting from earthquakes?
- Making buildings earthquake resistant
- Raising public awareness
- Improving earthquake prediction
What is special about the bottom storey of some Japanese buildings?
They are a ‘soft storey’ which is designed to collapse in the event of an earthquake
In Tokyo what are citizens advised to keep in case of the event of an earthquake?
Supplies such as food, water, blankets and a first aid kit.
What can be installed to reduce the fire risk after an earthquake in some countries?
A ‘smart meter’ which can be fed seismic data and can shut down gas supplies automatically in the event of an earthquake
What percentage of the population of Kobe had earthquake insurance before the 1995 quake?
7%
