GEOS2111 Chap.7 Earthquakes and how they are measured
Earthquakes and how they are measured
Earthquakes are defined in the unit in parts, and each part carries an exception worth learning. They are caused by sudden movement, or rupture, along a fault, but a meteorite strike, a landslide, an underground nuclear test, a collapsing mine or injected fluid will each do it too. They are concentrated along plate boundaries, but they also occur in intraplate settings, which is where Australia sits.
Their signature is seismic waves, which shake the ground, they can result in significant damage if near a population centre, and they cause secondary hazards that are frequently more destructive than the shaking itself. The vocabulary must be exact: the focus or hypocentre is the point at depth where rupture begins, and the epicentre is the point on the surface above it.
Three plate boundary types pair with three fault types, and two wave families matter: body waves, the fast P and S waves radiating through the interior, and surface waves, slower but of higher amplitude. That ordering is the physical basis of earthquake early warning. The central determination of the week is between magnitude and intensity.
Magnitude is quantitative, a measure of energy released, one value per earthquake, reported on the Richter scale historically and on moment magnitude since the 1970s, with a change of one unit corresponding to about a factor of thirty-two in energy.
Intensity is qualitative, the effect at a place, reported on a twelve-level Modified Mercalli scale whose lower levels describe how people responded and whose higher levels describe observed structural damage. Five hazards follow a rupture: ground shaking, surface rupture, liquefaction, fire and landslides, with tsunami added.
Shaking severity depends on magnitude, waves, proximity and near-surface geology, and damage depends on felt intensity and on engineering, which is the basis of the unit's claim that earthquakes do not kill people, buildings do. The week closes on monitoring, on early warning and its crowdsourced substitute, and on a paired comparison in which the smaller event was far deadlier.
What this chapter covers
- 01
The six-part definition, and the exceptions attached to each part
- 02
Focus, epicentre and fault: the terms every later answer depends on
- 03
Three plate boundary types paired with three fault types
- 04
Body waves and surface waves, and why their speed difference makes warning possible
- 05
Global distribution in three belts, the intraplate setting, and induced seismicity
- 06
Magnitude against intensity: energy released against effect at a place
- 07
Richter, moment magnitude and the factor of about thirty-two per unit
- 08
The twelve-level intensity scale, and what its lower and upper halves are built from
- 09
Five hazards from one rupture, site amplification and the three preconditions for liquefaction
- 10
Detection, early warning and the seconds it buys, and its crowdsourced substitute
Why the smaller earthquake killed far more people
- +1Refuse the magnitude comparison first. Magnitude describes energy released at the source and says nothing about how much of it arrives at a populated place. A magnitude 9 released offshore at depth can deliver less shaking to a city than a magnitude 7 released at 12 kilometres directly beneath one.
- +1Assemble the encounter. Depth: 12 kilometres is shallow, so surface intensity is high. Distance: an epicentre close to a dense urban area puts a large population inside the high-intensity contour. Both are properties of the meeting rather than of the earthquake.
- +1Add the ground. Soft near-surface material amplifies shaking, and where liquefaction and landsliding occur the ground stops supporting structures altogether. Site amplification runs from solid bedrock through well-consolidated and poorly consolidated sediment to water-saturated sand and mud.
- +1Put the weight of the answer on the building stock. At a given shaking intensity the proportion of buildings that collapse is set almost entirely by construction type, with adobe and weak masonry failing at intensities that seismically designed reinforced concrete survives. Widespread poverty concentrates a building stock at the fragile end and reduces the capacity to enforce a code.
- +1Extend past the day of the event, and handle the numbers honestly. Deaths continued through a large cholera epidemic produced by destroyed water and sanitation infrastructure meeting a displaced population, which is an indirect impact. Note also that the reported death toll for that event is disputed within the unit's own materials, so name the figure you use and cite it rather than averaging two published numbers.
Key terms
- Focus
- The point at depth at which rupture begins, also called the hypocentre. Depth is one of the two variables that decide how strongly the surface shakes.
- Epicentre
- The point on the Earth's surface directly above the focus. Confusing it with the focus is the most common terminology slip in this topic.
- Normal fault
- A fault produced by tension, in which the hanging wall moves down relative to the foot wall. It is characteristic of divergent settings.
- Thrust fault
- A fault produced by compression, in which the hanging wall moves up relative to the foot wall. Subduction interfaces are thrust faults, which is why they displace the seafloor and generate tsunami.
- Strike-slip fault
- A fault produced by shear, in which the two sides move horizontally past each other. Because vertical displacement is small, a submarine strike-slip rupture generates little or no tsunami.
- Body waves
- The P and S waves that radiate through the Earth's interior from the focus. They are high frequency and the fastest, and the P wave's early arrival is what an early warning system detects.
- Surface waves
- Love and Rayleigh waves, which travel at the Earth's surface with more confined motion. They are low frequency, high amplitude and the slowest, and they do much of the damage.
- Moment magnitude
- A magnitude measure introduced from the 1970s, working energy out from the seismic moment, itself the rigidity of the rock multiplied by the average slip on the fault and the area that slipped.
- Modified Mercalli scale
- A twelve-level intensity scale in roman numerals, developed in 1931, whose lower levels are based on how people responded and whose higher levels are based on observed structural damage.
- Site amplification
- The increase in shaking severity as near-surface material softens, running from solid bedrock through well-consolidated and poorly consolidated sediment to water-saturated sand and mud.
- Liquefaction
- The process in which water surrounds every grain of a saturated granular sediment and eliminates grain-to-grain contact, so the ground flows like a fluid and stops supporting structures. It requires loose granular sediment, a water table within about ten metres, and strong shaking.
- Induced seismicity
- Tremors in the crust caused by human activity, including underground nuclear explosions, mine collapses, withdrawal of fluids and gas, and injection of fluids into underground formations.
Earthquakes and how they are measured FAQ
What is the difference between magnitude and intensity?
Magnitude is quantitative and describes the earthquake; intensity is a qualitative reading of one place. One earthquake has one magnitude and as many intensities as there are locations to observe it.
Magnitude is reported on the Richter scale historically, defined as the logarithm of the maximum wave amplitude on a standard instrument at a reference distance, and on moment magnitude since the 1970s, which calculates energy from the seismic moment.
Intensity is reported on the Modified Mercalli scale, twelve levels in roman numerals, with the lower levels built from how people responded and the upper levels from observed structural damage. It is explicitly a subjective measure, and that is a feature rather than a defect: it records what was experienced where people actually were. A complete description of an event gives magnitude, depth and the intensity distribution.
Why can a magnitude 7 be deadlier than a magnitude 9?
Because magnitude measures energy released at the source, and deaths are produced by shaking at places where people are. Four things intervene. Depth: a shallow focus delivers far more energy to the surface than a deep one. Distance: an epicentre under a city puts a large population inside the high-intensity contour, while an offshore rupture may deliver its energy to open water.
Ground: soft, saturated near-surface material amplifies shaking and can liquefy. And building stock: at a given intensity, the proportion of structures that collapse depends overwhelmingly on construction type, with weak masonry failing where seismically designed reinforced concrete survives. Magnitude opens an answer about an earthquake; it never finishes one about a disaster.
What does it mean that one magnitude unit is a factor of about thirty-two?
It means the scale is logarithmic and cannot be used arithmetically. A magnitude 8 releases roughly thirty-two times the energy of a magnitude 7 and roughly a thousand times that of a magnitude 6, so magnitudes cannot be added, averaged or subtracted in the way ordinary numbers can, and the difference between two nearby magnitudes is far larger than it looks.
Set against frequency, the same scaling produces the pattern seen for tornadoes: events around magnitude 7 occur on the order of ten times a year worldwide, magnitude 8 far less often, and magnitude 9 rarely, yet the rare events dominate the historical death toll. Any hazard programme calibrated on the annual event is calibrated on the wrong thing.
What are the preconditions for liquefaction and why do they matter for planning?
Three, and they must occur together: loose granular sediment such as silt or sand, a water table within about ten metres of the surface, and strong shaking. When they coincide, water surrounds every grain and eliminates grain-to-grain contact, so sediment flows like a fluid and the ground loses its ability to support structures.
The reason it matters for planning is that two of the three are properties of the site rather than of the earthquake, and they are mappable in advance. Susceptibility maps grading terrain from very high to very low can therefore be produced before any event, and they are exactly the kind of product that supports zoning and foundation requirements.
It is one of the clearest cases in the unit where hazard science translates directly into a planning instrument.
If earthquakes cannot be predicted, what is early warning?
Warning after rupture has begun rather than before it. The physics comes from the wave families: a rupturing fault emits fast P waves and slower, more damaging S and surface waves. Sensors detect the P wave and transmit immediately to an alert centre, which determines and updates location and size; a message then reaches a device that works out how hard it will shake there and how many seconds are left.
Because a mobile signal travels faster than a seismic wave, the message can overtake the shaking, buying seconds to minutes. What those seconds are worth is a short list: drop, cover and hold; stop lifts at the nearest floor; brake trains; lift surgical instruments; shut gas valves and industrial processes. Nothing on that list saves a building and everything on it saves people inside one.
Where a dense instrument network is unaffordable, handset motion sensors can substitute, with alerts issued above about magnitude 4.5.
Exam move
Fix the vocabulary first, because everything else in this chapter is unusable without it. Focus against epicentre, the three boundary types paired with the three fault types, and body waves against surface waves with their speed and amplitude properties.
Then build one page on measurement with two columns, magnitude and intensity, and put under each what it measures, how many values there are per event, which scales belong to it, and what it cannot tell you. Add the factor of thirty-two and one worked comparison of two magnitudes so that the logarithmic point is concrete.
Next, learn the five hazards as a list you can produce instantly, with the three preconditions for liquefaction attached, because those three are the kind of precise detail that distinguishes a strong answer. Draw the site amplification sequence as four columns from bedrock to saturated mud, since it explains why intensity maps are patchy rather than concentric.
Rehearse the paired comparison until you can run it cold on any two events: depth, distance, ground, building stock, then indirect impacts.
Finally, carry the two Australian cases, an intraplate event that killed thirteen people and a recent shallow event with thousands of felt reports and an epicentre above a mine, because they let you answer both an Australian-context question and an induced-seismicity question from the same note.
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