Earthquakes and volcanoes

• Earthquake is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves
• Earthquake


  Also called a quake, tremor, or temblor
• Earthquakes can range in intensity, from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities
• Seismic activity


  The frequency, type, and size of earthquakes experienced over a particular time
• Seismicity


  The average rate of seismic energy release per unit volume at a particular location in the Earth
• Earthquakes can occur naturally or be induced by human activities, such as mining, fracking, and nuclear tests
• Hypocenter


  The initial point of rupture
• Epicenter


  The ground level directly above the hypocenter
• Earthquakes are primarily caused by geological faults, but also by volcanic activity, landslides, and other seismic events
• Significant historical earthquakes


  • 1556 Shaanxi earthquake in China (over 830,000 fatalities)
  • 1960 Valdivia earthquake in Chile (largest ever recorded at 9.5 magnitude)
• Effects of earthquakes


  • Ground shaking
  • Soil liquefaction
  • Damage and loss of life
  • Tsunamis
  • Landslides
• Tectonic movements


  Normal faults, reverse (thrust) faults, and strike-slip faults
• Elastic-rebound theory


  Energy release and rupture dynamics governed by the gradual build-up of strain and stress punctuated by sudden earthquake failure
• Efforts to manage earthquake risks

  • Prediction
  • Forecasting
  • Preparedness
  • Seismic retrofitting
  • Earthquake engineering
• Similar seismic phenomena, known as marsquakes and moonquakes, have been observed on other celestial bodies, indicating the universality of such events beyond Earth
• Fault ruptures


  • Rocks hotter than about 300 °C (572 °F) flow in response to stress; they do not rupture in earthquakes
• The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1,000 km (620 mi)
• The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault, the North Anatolian Fault, and the Denali Fault, are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter
• Normal faults


  • Occur mainly in areas where the crust is being extended such as a divergent boundary
  • Earthquakes associated with normal faults are generally less than magnitude 7
  • Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where the thickness of the brittle layer is only about six kilometres (3.7 mi)
• Reverse faults


  • Occur in areas where the crust is being shortened such as at a convergent boundary
  • Reverse faults, particularly those along convergent boundaries, are associated with the most powerful earthquakes (called megathrust earthquakes) including almost all of those of magnitude 8 or more
  • Megathrust earthquakes are responsible for about 90% of the total seismic moment released worldwide
• Strike-slip faults


  • Are steep structures where the two sides of the fault slip horizontally past each other
  • Transform boundaries are a particular type of strike-slip fault
  • Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8
  • Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km (6.2 mi) within the brittle crust
  • Earthquakes with magnitudes much larger than 8 are not possible
• There exists a hierarchy of stress levels in the three fault types: Thrust faults are generated by the highest, strike-slip by intermediate, and normal faults by the lowest stress levels
• Greatest principal stress


  The direction of the force that "pushes" the rock mass during the faulting
• In the case of normal faults


  The rock mass is pushed down in a vertical direction, thus the pushing force (greatest principal stress) equals the weight of the rock mass itself
• In the case of thrusting

  The rock mass "escapes" in the direction of the least principal stress, namely upward, lifting the rock mass, and thus, the overburden equals the least principal stress
• Strike-slip faulting


  Is intermediate between normal and thrust faulting in terms of stress regime
• Differences in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions
• Energy released in an earthquake

  Proportional to the area of the fault that ruptures and the stress drop
• For every unit increase in magnitude, there is a roughly thirty-fold increase in the energy released
• An 8.6-magnitude earthquake releases the same amount of energy as 10,000 atomic bombs of the size used in World War II
• Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees, thus the width of the plane within the top brittle crust of the Earth can reach 50–100 km (31–62 mi), making the most powerful earthquakes possible
• Shallow-focus earthquakes


  Earthquakes occurring at a depth of less than 70 km (43 mi)
• Mid-focus or intermediate-depth earthquakes


  Earthquakes with a focal depth between 70 and 300 km (43 and 186 mi)
• Deep-focus earthquakes


  Earthquakes that occur at much greater depths (ranging from 300 to 700 km (190 to 430 mi)) in subduction zones, where older and colder oceanic crust descends beneath another tectonic plate
• Deep-focus earthquakes occur at a depth where the subducted lithosphere should no longer be brittle, due to the high temperature and pressure
• Mechanism for deep-focus earthquakes

  Faulting caused by olivine undergoing a phase transition into a spinel structure
• Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes
• Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes and can be recorded by seismometers and tiltmeters and used as sensors to predict imminent or upcoming eruptions
• Rupture dynamics

  A tectonic earthquake begins as an area of initial slip on the fault surface that forms the focus, and the rupture then propagates away from the focus, spreading out along the fault surface
• In most cases, the rupture speed approaches, but does not exceed, the shear wave (S-wave) velocity of the surrounding rock