Natural Hazards and Disasters

Class 4: Geohazards; Earthquakes

Class slides

Geohazards

Geohazards are geological hazards that are associated with processes in the solid Earth's interior or at its surface. These hazards include earthquakes, volcanic activity, landslides, ground motion, and tsunamis. Some experts also include floods, droughts, meteorite impacts and the health hazards of geologic materials in geohazards, since they all involve processes at or under the Earth's surface. Many, but by far not all geohazards are related to plate tectonics. Geohazards are increasingly caused by human activities. For example, groundwater, oil and gas extraction can lead to surface subsidence impacting built infrastructure. It also can change the stress field and induce earthquakes. Fracking changes seismicity as well.

Spatial scales of geohazards can range from local events such as a rock slide or coastal erosion to events that threaten the existence of humankind such as a supervolcano or meteorite impact. On temporal scales, geohazards range from rapid rock falls and short earthquakes over prolonged volcanic eruptions to slow slope motion and subsidence that can last for years and more.

The sciences that are most relevant to study these hazards are geology, geophysics, and geochemistry. Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geophysics applies physics to studying the planet, and originally it included meteorology and physical oceanography. Geochemistry studies the chemistry of the planet. For floods and droughts, meteorology, hydrology, and climatology are important, too. For meteorite impacts, astronomy is an important field.

The geohazards can be grouped in those related to tectonics, ground instabilities and movements, anthropogenic ground instabilities, and other geological hazards:

• Tectonics:
  Stress and strain (tectonic movements)
  • Earthquakes
  • Tsunamis
  • Volcanic activity
  • Salt Tectonics
• Ground instabilities and movements:
  • Landslide
  • Soil Creep 
  • Ground Dissolution
  • Collapsible Ground
  • Running Sand/ Liquefaction
  • Shrink-swell clays
  • Compressible Ground
• Anthropogenic ground instabilities:
  • Induced seismicity (reservoirs)
  • Ground water management
  • Oil and gas extraction
  • Mining
  • Underground construction
  • Engineered ground
  • Fracking
• Other geohazards:
  • health hazards of geologic materials:
    • radioactivity (non-human and human caused)
    • atmospheric aerosols
    • chemical elements (e.g. mercury, heavy metals)
    • water quality
    • anthropogenic pollution
  • floods, droughts
  • sediments
  • meteorite impacts

Plate Tectonics

Alfred Wegener (1 Nov. 1880 - Nov. 1930) noted the similarity of the coast lines on both sides of the Atlantic, and he also found similar rocks on both sides of Atlantic. Based on this evidence, he published the idea of continental drift in 1912. However, the theory was not accepted because he could not provide an explanation of the forcing processes for moving the continents on Earth's surface.

Long after his death in Greenland, new evidence for continental drift came from magnetic patterns on the ocean floor of the Atlantic ocean. These patterns could only be explained by a slow generation of the oceanic crust long the mid-Atlantic ridge and a slow drift of the crust away from the ridge.

Today, continental drift, or plate tectonics, are well established paradigms in Earth sciences. Many of the phenomena of the solid Earth, the fluid envelope, and the biosphere direct result from plate tectonics.

Earth's Internal Structure

Earth's continental and ocean crusts are the thinnest, outermost layers of the planet. Earth's outermost crust, on which we live, is often described by analogy with an egg’s shell. Although high mountains and deep ocean trenches may seem enormous, on the scale of the planet as a whole they are almost unnoticeable wrinkles. The continental and oceanic crustal rocks and their underlying lithosphere, which together comprise the tectonic plates, are on average 100 km thick and for the most part they are rigid and brittle.

Earth's lithosphere is chemically and mineralogically part of the upper mantle. Its rock is predominantly peridotite, which is a coarse-grained, dense, igneous rock that is high in iron and magnesium. Beneath the lithosphere is the asthenosphere, which although solid, is capable of flowing slowly due to its high temperature (1300°C). On the surface, the rock would melt at such high temperatures, but the high pressures at depth keep it in a solid state.

Earthquakes are a main means for map Earth's internal structure. The seismic waves generated by earthquakes are useful for mapping Earth’s internal structure.

Earthquakes and very large explosions release energy in the form of seismic waves that travel in all directions through the Earth, as well as along the crust’s surface. Seismic surface waves cause ground shaking and are responsible for most of the damage caused by earthquakes, but the different physical properties of compressional (P) and shear (S) seismic body waves are useful for mapping Earth’s internal structure.

P-waves have a smaller amplitude, shorter wavelength, and travel faster than S-waves through Earth's crust, which is why they are the first to arrive at seismograph stations after an earthquake. However, unlike P-waves, the S-waves cannot travel through fluids and on the opposite side of Earth to an earthquake there is always a 'shadow zone,' within which no S-waves are received by seismographs. The recognition of this shadow zone allowed scientists to infer the existence of Earth's fluid outer core.

Tectonic Plates

Lithospheric plate boundaries are divergent, convergent, or transform, or a combination. Earthquakes and volcanic eruptions occur mainly because Earth’s lithospheric plates are constantly in motion. Relative to an Earth-fixed reference frame, they move with few exceptions with velocities of less than 10 cm/year and relative velocities between neighboring plates can reach up to 25 cm/year. When summed over thousands and millions of years, the plates can accomplish a great deal of motion.

The three main types of plate boundary are: divergent, where plates move apart along mid-ocean spreading ridges; convergent, where one plate moves over the top of another; and, transform, where plates slide past one another. There are also more complex boundaries with a combination of different motion types. The boundaries and relative movement vectors of the larger plates are well-defined, but there are also dozens of micro-plates whose boundaries and relative motions are still the subject of research activity.

• Divergent Plate Boundaries: Divergent plate boundaries are where new lithosphere is created, along mid-ocean ridges.

  At mid-ocean ridges, hot, buoyant basaltic magma reaches the surface where it solidifies into basalt rock in undersea mountain ridges. As the magma cools, it reaches a temperature at which iron atoms in magnetic minerals such as magnetite lock their magnetic field into the same orientation as that of the Earth’s magnetic field. This temperature, called the Curie Point, is about 570°C for magnetite.

  Earth's magnetic field reverses direction about every 200,000 to 300,000 years. When a ship-towed magnetometer crosses a mid-ocean ridge it receives a signal of alternating normal and reverse polarity in the ocean floor rocks. These alternating patterns are consistent with a symmetrically increasing age of the rocks and their overlying sediments on either side of the midocean ridges, and point to ocean lithosphere divergence along the ridges. Hence the term ‘sea floor spreading’.

• Convergent Plate Boundaries: Cold, dense lithosphere is subducted into the mantle at convergent plate boundaries, while the mantle above partially melts to form volcanic arcs.

  Cold, old, oceanic lithosphere is much more dense than continental crust or than hot, young oceanic lithosphere. As the cold lithosphere thickens with distance from the spreading ridge, gravitational forces eventually cause it to sink back into the mantle along subduction zones. Flexure of the sinking slab creates deep ocean trenches at surface, but neither the bending nor the descent of lithosphere into the mantle is smooth. Some of the deepest and largest earthquakes occur in, and just above, the down-going slab. Above the sinking slab, the lithosphere and asthenosphere are heated and partially melted by an upward counter-flow of mantle. The resulting magma rises, heating and melting part of the lithosphere and crust as it goes. These magmas incorporate more crustal rock as they ascend, and the resulting silica-enriched magma erupts in volcanoes to form a volcanic arc.

  When all of the oceanic lithosphere has been consumed in a subduction zone, the result is a convergence of continental lithosphere. The region of Tibet, north of the Himalayan Mountains, has an average elevation of 4,500 m above sea level. One reason for such a high elevation is that the continental crust there is twice its normal 40 km thickness. At about 70 million years ago, oceanic lithosphere beneath a long-gone ocean called Tethys, on the northern margin of the Indian Plate, began to be subducted beneath the Eurasian Plate. India moved quite rapidly northward as the Tethyian oceanic lithosphere was subducted. By 40 million years ago, the convergence of the Indian and Eurasian plates had consumed all the Tethyian lithosphere that had been between them, along with some micro-plates, and the continental crust of India collided with that of Eurasia. The descending lithosphere continued to pull the Indian plate northward, under Eurasia, resulting in a doubling of the crust beneath Tibet. The many large earthquakes and landslides that occur in the Himalayas, Tibet, and even far into China, are due to the continuing adjustments along this continent-continent plate boundary.

• Transform Plate Boundaries: Transform boundaries are where the lithospheric plates slide, jarringly, past one another.

  Earth is not a perfect sphere, but it is spheroidal and therefore as its tectonic plates spread apart in some places and converge in others, some segments of their boundaries must slide past one another. This slip occurs on transform fault zones, such as along the San Andreas Fault zone in coastal California.

  Transform fault zones have a very steep to vertical orientation. The majority of large earthquakes on these faults occur in the most brittle, upper section of crust, at depths of 3 to 4 km. Exactly how deep the brittle behavior extends beneath the base of a typical transform fault system is still a topic of intensive research. What is known is that the many faults that comprise a transform fault zone are not perfectly planar and any one of them can get ‘stuck’ and be unable to slide – until stresses build to the point that the rocks rupture. When the fault finally slips, it releases its potential energy as an earthquake.

There are different processes forcing tectonic plate motion. Ridge push and slab pull are the two main driving forces for plate tectonics. Oceanic crust originates as basaltic magma in the upper mantle that is erupted at mid-ocean ridges. The basalt cools and thickens as the two sides of the ridge move slowly apart. At the same time, the uppermost asthenosphere under the new crust also cools, gradually creating an oceanic lithosphere that eventually reaches a thickness of 50 to 100 km. The dense oceanic lithosphere sags slightly into the underlying asthenosphere under the influence of gravity. This down-and-outward motion of the lithosphere, called 'ridge push,' is a major driving mechanism for plate motion.

An even more important mechanism for plate motion than ridge push is 'slab pull,' which is where cold, dense slabs of old oceanic lithosphere sink back into the mantle down subduction zones, again under the influence of gravity. As the slab descends into the mantle it becomes heated and less rigid, eventually becoming almost indistinguishable from lower mantle. The combination of ridge push and slab pull, together with other, lesser driving forces, creates a circulating flow of material within the mantle that carries the overlying lithospheric plates along.

Other Causes of Geohazards

Processes at the Earth's surface can lead to geohazards. Rockfalls, landslides, and mudslides can move large masses due to instabilities and gravity downhill and cause devastation along their path. In certain geological conditions, sinkholes can open very suddenly and cause locally large damage and loss of lives. Skrinking and swelling clays can impact buildings and other infrastructure built on these clays.

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.

Seismicity can be induced by loading (reservoirs), deloading (groundwater), as well as injection and extraction of subsurface material. Between the years 1973–2008, there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States. This rate has ballooned to over 600 M3+ earthquakes in 2014 and over 1000 in 2015. Through August 2016, over 500 M3+ earthquakes have occurred in 2016. See here for more detail on injection-induced seismicity

Earthquakes

An earthquake is a sudden, violent shaking of the ground, usually caused by the fracturing of rock or movement of magma within Earth’s crust. Earth’s tectonic plates slowly and constantly readjust their positions and shapes in response to internal tectonic forces, and earthquakes are one of the consequences of this constant plate movement. Earthquakes occur when rock in the Earth’s crust breaks and moves suddenly in a fault zone. They are also caused by magma motion beneath volcanoes and during eruptions, and on the rare occasions when a meteorite hits Earth. Earthquakes occur most often along current and ancient plate boundaries, with the largest ones especially common on subduction and transform boundaries. However, they also occur, less frequently, in plate interiors. Destruction and casualties on a massive scale can result when a large earthquake happens beneath or very close to densely populated regions with infrastructure not built to resist the forces resulting from the ground motion.

Disasters triggered by Earthquakes

Earthquakes cause disasters mainly due to failing buildings and infrastructure. “Earthquakes don't kill people, buildings do!” (Bingham, 2011). The largest disasters caused by earthquakes are in areas where the built environment is not able to withstand the ground shaking. Failing buildings, resulting fires, or other casacading effects in the built environment lead to the largest disasters. For off-shore earthquakes, a resulting tsunami can cause a very large impact.

The location of an earthquake is specified by:

• Epicenter: Latitude, longitude; position on the ground directly above the earthquake;
• Focal Depth: depth below ground surface where earthquake rupture occurs;
• Hypocenter (focus): Actual location in the Earth’s crust where the earthquake occurs (needs lat, long, and depth)

Tectonic Plates are cool, thin and 'rigid,' relative to the hot, ductile mantle beneath. A plate consists of the crust and lithosphere, with the later being softer than the crust. Earthquakes occur within or between tectonic plates and within the upper mantle. Earthquakes cluster in three depth regions:

• Shallow earthquakes: less than 70 km;
• Intermediate earthquakes: between 70 and 300 km;
• Deep earthquakes: more than 300 km.

Most earthquakes occur on 'faults' or fracture zones in the Earth's crust when crustal rocks are stretched or slide past each other. When it breaks, it releases a sudden pulse of energy. Faults 'stick' as they slide past one another. The friction (resistance to sliding) causes stress to build up at sticking point. Most earthquakes occur on 'faults' or fracture zones in the Earth's crust when crustal rocks are stretched or slide past each other or above each other.

Three main types of 'faults’'or fracture zones in the Earth's crust:

1. When crustal rocks are stretched;
2. When they slide past each other;
3. when they slide over each other.

The magnitude of an earthquake is a number that characterizes the relative size of an earthquake. It is ased on measurement of the maximum motion recorded by a seismograph.

There are several magnitude scales; most commonly used are:

1. local magnitude (ML), commonly referred to as "Richter magnitude",
2. surface-wave magnitude (M_s),
3. body-wave magnitude (M_b), and
4. moment magnitude (M_w).

The earthquake moment magnitude, M^w is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. It is based on the concept of:
     seismic moment = area x displacement of fault rupture.
It is measured on a logarithmic scale, and it measures total amount of energy released. The subscript 'w' indicates the quantity ‘work’.

The moment magnitude Mw is a dimensionless value defined by Kanamori (1977) as
     M_w = (2/3) log₁₀(M₀) - 10.7
where M₀ is the seismic moment in dyne⋅cm (10⁻⁷ Nm). The constant values in the equation are chosen to achieve consistency with the magnitude values produced by earlier scales.

Earthquakes with M_w of 2.5 or less are usually not flet but recorded by seismographs. They are frequent with more than 900,000 happening every year. Earthquakes with M_w between 2.5 and 5.4 are often felt and can cause minor local damage. Globally, on the order of 10,000 earthquake happen in this magnitude range. For M_w between 5.5 and 6.0, slight structural damage is likely and on the order of 500 of these earthquakes happen each year. In the M_w range between 6.1 and 6.9, significant structural damage can result often leading to loss of lives. On the order of 100 earthquakes happen annually in the magnitude range. Earthquakes in the M_w range of 7.0 to 7.9 are considered as major earthquakes that cause serious damage. If they happen on ocean floor, they can also cause local to regional tsunamis. On the order of 20 earthquakes happen annually in this range. Earthquakes with M_w of 8.0 and greater are great earthquakes that are extremely destructive.

The impact of an earthquake of a given magnitude depends strongly on the depth of the hypocenter and also the mechanism of the earthquake. This is captured by the concept of earthquake intensity. Intensity is a number describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States are:

• Modified Mercalli scale and the
• Rossi-Forel scale.

The earthquake energy is released as a wave through the rocks. The wave has two components:

• Compressional P waves: They travel as a compression and dilatation through the rock;
• Shear S waves: They deform the rock in a shear motion.

Importantly, P and S waves have different velocities. Seismographs record the arrival times of P and S waves. The P waves travel faster and arrive first. The waves also have different amplitudes. Surface waves have the largest amplitudes and they cause the largest ground shaking and subsequent damage. Since the wave velocity within the Earth's interior depends on the depth, and is in general higher deeper inside the Earth, surface waves arrive after the P and S waves, that travel through the interior, which gives very short times as early warning.

The recurrence interval T^r is the average time between two earthquakes of a given magnitude. It gives an indication how likely an earthquake of this magnitude is in a given location.
     T_r = N/n
where N is the number of years in the record and n is the number of events (earthquakes of the specified magnitude). Of main interest is the recurrence interval of large magnitude earthquakes.

Seismic hazard maps are based on recurrence intervals for large earthquakes. Convergent boundaries can have

• subduction zones (mainly in the 'ring of fire' around the Pacific), where very large earthquakes can happen at all depth down to more than 600 km, and where volcanoes are frequent;
• collision zones (for example, the Himalayas), where very large earthquakes can happen at shallow and intermediate depth but hardly any volcanoes are located.

At transform boundaries (e.g., California, New Zealand), earthquakes are mostly at shallow depth, but can have large magnitude and cause destructive ground shaking.

Class Reading List:

PanGEO, n.d., What are Geohazards? html

Kious, W. J., Tilling, R. I., 1996. “Historical perspective”. This Dynamic Earth: the Story of Plate Tectonics (Online ed.). U.S. Geological Survey. ISBN0-16-048220-8.

Murphy, J. B., van Andel, T. H., 2017. Plate Tectonics. Encyclopaedia Britannica, html.

Wikipedia, n.d., Plate Tectonics. html

National Geographic, n.d., Plate Tectonics. html.

Bolt, B. A., 2017. Earthquake. Encyclopaedia Britannica, html

The Editors of Encyclopædia Britannica, 2017. Notable Earthquakes in History. Encyclopaedia Britannica, html.

Plag, H.-P., Brocklebank, S., Brosnan, D., Campus, P., Cloetingh, S., Jules-Plag, S., Stein, S., 2015. Extreme Geohazards — Reducing the Disaster Risk and Increasing Resilience. European Science Foundation. pdf.

Back to Class Schedule ...