5.6 - Seismic Hazards
Earthquakes as the primary seismic hazard
Earthquakes represent the main hazard resulting from seismic activity, occurring due to stress accumulation at various plate boundaries. This stress is released when tectonic plates suddenly shift, generating powerful vibrations that can cause widespread destruction.
Causes and characteristics of earthquakes
- Plate margin tension - Stress builds up at destructive, constructive, and conservative plate margins where plates interact.
- Shockwave generation - As plates jerk past each other, they release energy in the form of shockwaves, which are vibrations spreading outwards.
- Focus and epicentre - The focus is the underground origin of the earthquake, which may be a single point or extend along a fault line. The epicentre is the location on the Earth's surface directly above the focus, where the shaking is first felt.
- Wave strength - Shockwaves are most intense near the focus, leading to greater damage in these areas.
- Ground effects - Earthquakes cause the ground to tremble and, in some cases, to split open along fault lines, creating ruptures.
Methods for measuring earthquake strength and impact
To assess the power and effects of earthquakes, scientists use specific scales that measure different aspects of these events. Each scale provides unique insights into the nature and consequences of seismic activity.
Scales used to measure earthquakes
- Richter scale - Measures the magnitude or shaking power of an earthquake. It is logarithmic, meaning a magnitude 6 earthquake has a shaking amplitude ten times greater than a magnitude 5. There is no upper limit, and major earthquakes are classified as above magnitude 7.
- Moment magnitude scale (MMS) - Evaluates the total energy released during an earthquake. Also logarithmic with no upper limit, it is considered more precise than the Richter scale, especially for larger events.
- Mercalli scale - Rates the impact of an earthquake based on observed effects, using a scale from 1 to 12. A rating of 1 indicates an event only detected by instruments, while a rating of 12 signifies total destruction.
Secondary hazards triggered by seismic activity
Beyond the initial ground shaking, earthquakes can trigger a range of secondary hazards that amplify their destructive potential. These additional effects often contribute significantly to the overall impact on communities and landscapes.
Types of secondary seismic hazards
- Tsunamis:
- These are massive waves caused by the displacement of large volumes of water, often due to underwater earthquakes shifting the seabed.
- The waves spread out from the epicentre, with greater seafloor movement resulting in larger waves.
- Tsunamis are particularly devastating when they originate near coastlines, as they lose less energy before striking land.
- They travel rapidly in deep water, often providing little warning before impact, and can lead to significant loss of life.
- Landslides and avalanches:
- Earthquake vibrations can destabilise rock, soil, or snow on slopes, causing rapid downslope movement.
- The shaking weakens ground materials, and infiltrating water can add weight, potentially triggering landslides even after the initial shaking ceases.
- Soil liquefaction:
- During an earthquake, saturated soil can behave like a liquid due to intense vibrations.
- This reduces the soil's strength, making it susceptible to deformation and subsidence, particularly under the weight of heavy structures, leading to further structural damage.
Geographic patterns of seismic hazards
Seismic hazards are not randomly distributed but follow specific geographic patterns related to tectonic activity. The location, nature, and severity of earthquakes are influenced by several key factors associated with plate boundaries.
Distribution and influencing factors of seismic hazards
- Plate margin locations - Seismic activity is most common near destructive and conservative plate margins, with some events occurring at constructive margins.
- Factors affecting earthquake nature and magnitude:
- Margin type - The largest and most powerful earthquakes typically occur at destructive margins, especially in subduction zones, while those at constructive margins are generally of lower magnitude.
- Rate of plate movement - Plates shift at varying speeds, ranging from 1 to 15 cm per year, but there is no consistent link between movement speed and earthquake magnitude.
- Depth of focus - The focus of an earthquake can be shallow or deep underground. Deep-focus earthquakes often have higher magnitudes but cause less surface damage because the waves lose energy travelling a greater distance to the surface.
Frequency and challenges in predicting earthquakes
Understanding the frequency and predictability of earthquakes is crucial for managing their risks. While scientists have made progress in identifying vulnerable areas, precise forecasting remains elusive.
Patterns and limitations in earthquake prediction
- Frequency of events - Globally, hundreds of low-magnitude earthquakes happen every day, while high-magnitude events are far less common. The frequency of earthquakes varies from year to year with no predictable cycle.
- Random occurrence - Seismic events do not follow a clear or consistent pattern, making their occurrence largely random.
- Risk identification - Scientists can pinpoint at-risk regions by tracking tectonic plate movements and historical seismic activity, helping to focus preparedness efforts.
- Prediction challenges - Despite advances in monitoring, it is currently impossible to accurately predict the exact timing or magnitude of an earthquake, limiting the ability to provide specific warnings.