Understanding hazard profiles

To create a hazard profile, the magnitude of the hazard must be measurable, as this forms a core component. Magnitude refers to the size and intensity of a natural hazard event, typically quantified using a numerical score on a scale based on scientific measurements from instruments like seismographs.

Challenges in measuring magnitude

• Scales can oversimplify the complexity and full extent of a hazard.
• Assessments based on impact may be unfair, as a country's level of economic development influences the observed effects, with less developed areas potentially suffering more due to poorer infrastructure.

Scales for measuring tectonic hazard magnitude

Different scales are used to quantify the magnitude of tectonic hazards, each focusing on specific aspects like energy released or observed impacts. These scales allow for consistent comparison across events.

The Moment Magnitude Scale (MMS)

The Moment Magnitude Scale (MMS), also denoted as $M_W$, measures the size of an earthquake based on the total energy released at the moment of occurrence, known as the seismic moment. This is calculated from data at the epicentre by assessing the shift along the fault line and the resistance of the affected lithosphere.

The MMS is a logarithmic scale and has no upper limit. It is more accurate for large earthquakes than older scales like the Richter scale, which it has largely replaced.

The Volcanic Explosivity Index (VEI)

The Volcanic Explosivity Index (VEI) grades volcanic eruptions on a logarithmic scale from 0 to 8, where each increase of one unit represents a tenfold increase in explosivity. This composite index considers multiple factors to determine magnitude.

Key factors in the VEI:

• Volume of material ejected
• Height of the eruption plume
• Duration of the eruption
• Qualitative observations

Examples of VEI levels:

The Mercalli Intensity Scale

The Mercalli Intensity Scale measures the impacts of an earthquake through observations, such as reports from witnesses and photographs, rather than instrumental data. It ranges from 1 to 12, with higher numbers indicating greater observed effects.

This scale is subjective, as it relies on human perceptions, which can vary. It is difficult to assign a single value to an entire earthquake, as impacts differ by location, and witnesses may have differing opinions on the event.

Examples of Mercalli Intensity levels:

Key characteristics in hazard profiles

Hazard profiles evaluate tectonic hazards based on six physical characteristics. These help in assessing the overall risk and potential impacts, linking directly to how hazards affect people and environments.

The six characteristics explained

• Magnitude - The strength of the hazard; higher magnitude increases the likelihood of fatalities, injuries, and damage to buildings and infrastructure.
• Speed of onset - The rate at which the hazard develops; rapid onset leaves less time for preparation and reaction, reducing people's ability to respond effectively.
• Duration - The length of time the hazard event lasts; longer durations delay emergency responses and prolong recovery efforts.
• Areal extent - The size of the area affected; larger areas put more people and resources at risk.
• Spatial predictability - The ability to forecast where and when a hazard will occur; high predictability, such as on well-mapped plate boundaries, allows better planning and preparation.
• Frequency - How often the hazard occurs; frequent events may lead to better management systems, but repeated occurrences can hinder full recovery before the next one strikes.

Using hazard profiles to compare tectonic hazards

Hazard profiles enable comparisons between similar hazards in different locations or times, as well as between different types of hazards, such as earthquakes and volcanic eruptions. They act as prediction tools by linking characteristics to direct impacts, though each hazard is unique and unpredictable.

Comparison of global tectonic events

Relationship between magnitude and frequency

The interplay between magnitude and frequency is crucial for assessing hazard risks and aids in preparation and planning. Generally, high-magnitude events occur less frequently, while low-magnitude ones are more common.

Patterns in earthquakes

Hundreds of low-magnitude earthquakes occur worldwide daily, but very high-magnitude ones are rare. Their occurrence is largely random, with global numbers varying yearly, making patterns hard to discern.

Patterns in volcanic eruptions

Volcanic events range from small, slow lava flows to massive eruptions of lava, ash, and gas. Frequency varies: some active volcanoes erupt every 100,000 years, others every few months. Less-frequent eruptions are typically larger and more damaging. Some volcanoes erupt at regular intervals, while others remain dormant for centuries before multiple eruptions in quick succession.

Implications for hazard management

The random nature of hazards means profiles provide indications rather than precise predictions, as each event is unique and does not follow previous patterns exactly.