Litho: Seismic Activity

Question 1

Describe the formation of earthquakes (part1)

Answer 1

1) rocks in the outermost layer of the crust are strong yet brittle.
2) when plates move, rocks at boundaries undergo compression, tension, shear.
3) buildup of pressure within crustal rocks
4) concentrated release of energy, forming a fault

Question 2

Describe the formation of earthquakes (part2)

Answer 2

1) lithosphere glides smoothly over the underlying asthenosphere
2) when movement approaches plate boundaries, friction between plates increases
3) continuous drag that MCC exert on underside of plates ensure plates continue to move
4) exert great stresses on surrounding rock

Question 3

Describe the formation of earthquakes (part3)

Answer 3

1) crustal rocks bend or deform
2) energy stored in rocks released when limits of deformation are exceeded
3) rocks in opposite side of the fault rebound
4) releases energy, producing an earthquake

Question 4

Describe ground motion as an effect of earthquakes.

Answer 4

A building will sway back and forth in a given period of time at a frequency that depends on its height and type of construction. (Natural frequency). Resonance is when the building amplifies the vibrations during an earthquake until the buildings shake themselves apart. 10-20 storey buildings most prone to sway in time with the 2-second period shaking of the ground. (Eg. 1985 Mexico City earthquake: 70% of 10-20storey buildings damage)

Question 5

Describe a tsunami as an effect of earthquakes.

Answer 5

Tsunamis are a series of large waves caused by an abrupt displacement of water. Eg. 2004 M9.3 earthquake that occurred below the Indian Ocean offshore of Sumatra)

Question 6

What is the formation of tsunamis?

Answer 6

1) technic up thrust displace large quantities of water, generating massive energy
2) energy causes large waves to move over ocean surface
3) wavelength increase, wave periods long
4) as tsunamis waves approach coastline, they slow down significantly due to friction of collusion with rising sea bed
5) velocity decrease, wavelength decrease, amplitude increase
6) wavelength compresses and heightens as wave collides with shore

Question 7

Describe landslides and liquefaction as an effect of earthquakes.

Answer 7

A soil layer on the side of the hill will liquefy and loosen during seismic shaking and flow as landslide. Liquefaction occurs in water saturated sediment when grains settle during ground shaking, displacing the water upward. Water pressure moves the grains apart. Soil in a state of liquefaction has no strength and cannot bear any load. This causes the ground surface to subside. (Eg. Nigata earthquake in nigata Japan in 1964 destroyed many buildings)

Question 8

Describe fires as an effect of earthquakes.

Answer 8

Blocked streets and damaged water supplies exacerbate fires. Can be caused by down power lines, broken gas lines etc. (Eg. San Fran 1906 earthquake 700deaths. Developed ‘smart pipes’ that will quickly and automatically shut off sections of gas and water pipes where pressures fall suddenly due to a break)

Question 9

What are the 6 prediction methods of earthquakes?

Answer 9

1) Tiltmeters
2) Seismic monitoring
3) Recurrence intervals
4) Foreshocks
5) Satellite monitoring
6) Animal behaviour

Question 10

Describe tiltmeters as a prediction of earthquakes.

Answer 10

Tiltmeters utilise a liquid bubble inside a chamber that responds to changes in tilt to measure ground tilts. It is used to monitor earthquake activity deep underground and data can be transmitted electronically to monitoring centres.

Question 11

Describe seismic monitoring as a prediction of earthquakes.

Answer 11

Involves the use of seismographs to detect faint ground motions from seismic events. International organisations (eg. Global Seismic Network) cooperate to locate and determine the size of earthquakes around the globe and disseminate info to mitigate future earthquakes.

Question 12

Describe recurrence intervals (average time between 2 earthquakes) as a prediction of earthquakes.

Answer 12

The measurement of the average time between large fault ruptures on a single fault can assess the probability of an earthquake occurrence. Stresses build up overtime as plates move at a continual rate, the historical record of fault displacements should provide the return period or frequency of earthquakes in a region. eg. rate of plate movements 10cm/yr, average fault displacement is 5m, then fault should be active every 50 years. Seismic gaps are prime sites for future earthquake activity.
Limitations: method is imprecise because rates of stress accumulation is not truly constant. (eg. prediction that a section of a fault near Parkfield, California will producce M6.0 earthquakes approx ever 22 years. Prediction was inaccurate as the predicted quake struck 11 years later)

Question 13

Describe foreshocks as a prediction method for earthquakes.

Answer 13

Foreshocks precede the mainshock of the earthquake, representing small displacements of the weaker parts of the fault surface as stresses rises. (eg. moderate earthquake hits San Andreas Fault near Parkfield, California. Scientists estimated a main shock will follow)
Limitations: foreshocks do not always occur

Question 14

Describe satellite monitoring as a prediction method for earthquakes.

Answer 14

Involves the use of satellite-based radar interferometry to detect ground deformation. However, these documents the slow build up of strain rather than indicating where or when a rupture is likely to occur.

Question 15

Describe animal behavior as a prediction method for earthquakes.

Answer 15

Involves observation of suspicious animal behaviour but this method is not foolproof.
Success: 1969, zookeepers in China reported that animals were behaving strangely before the M7.4 earthquake struck closeby. Haicheng Earthquake 1 million people evacuated.
Failure: Scientists failed to predict the 1976 Tangshan Earthquake.

Question 16

Describe earthquake alarm and automated systems as a mitigation measure of earthquakes.

Answer 16

Alarm systems can detect a P-wave signal of a nearby major earthquake. Allows an automated system to enact basic precautions such as cutting power to high speed electric trains and setting all traffic signals to red. eg. Japan

Question 17

Describe building design as a mitigation measure of earthquakes.

Answer 17

Isolated base technology: involves using a coil or other flexible support and placing it between the structure and the foundation of the building. This counteracts the motion of seismic waves, enabling the building to stay completely stationary.
Tie the walls to the roof and any intervening floors, ensure that whole building sways in one direction
Cross supports between frame supports, or between the corners of the frame. eg. 1989 M7.1 Loma Prieta earthquake. Top floor of transAmerica pyramid swayed for 1min, no one injured
Dig extra deep foundations so the structure rests on solid bedrock where amplitudes of vibrations is likely too be less.

Question 18

Describe hazard mapping as a mitigation measure of earthquakes.

Answer 18

Highlight locations susceptible to failure after examining the geology of the area, mapping locations of potentially active faults.

Question 19

Describe controlled earthquakes as a mitigation measure of earthquakes.

Answer 19

Involves releasing strain that has been accumulating in rocks by pumping water, under high pressure conditions, into areas w a lack of recent seismic activity. The pressure of water triggers controlled earthquakes and act as a lubricant for crustal blocks to skip past each other gently. Released in smaller, timed earthquakes.