Spatial distribution of geophysical hazard events

Geophysical hazards, such as earthquakes, volcanoes, and landslides, are not randomly distributed across the globe. Their occurrence is closely linked to specific geological and environmental factors, particularly tectonic activity, which influences where these events are most likely to happen.

Distribution patterns of earthquakes

• Plate boundary association - Most earthquakes occur near tectonic plate boundaries. For instance, significant activity is observed along the Mid-Atlantic Ridge in the centre of the Atlantic Ocean and around the edges of the Pacific Ocean.
• Variation by boundary type - Broad belts of earthquakes are linked with subduction zones and collision boundaries, while narrower belts are associated with constructive plate margins. Conservative boundaries typically show even narrower earthquake zones.
• Human-induced earthquakes - Some earthquakes are triggered by human activities like the building of large dams, mining operations, and weapons testing, often occurring away from tectonic plate boundaries.

Distribution patterns of volcanoes

• Plate boundary concentration - The majority of volcanoes are located at tectonic plate boundaries, with around three-quarters of the Earth's historically active volcanoes situated along the Pacific Ring of Fire.
• Hotspot exceptions - Not all volcanoes are tied to plate boundaries. Some, like those in Hawaii, form over hotspots, which are areas where hot material rises from deep within the Earth's mantle, creating volcanic activity in the middle of oceanic plates.

Distribution patterns of landslides

• Global occurrence with specific risks - Landslides can happen worldwide, but fatal landslides are more frequent in regions with specific conditions:
  • Active tectonic processes causing high uplift rates and occasional earthquakes.
  • High rainfall, especially intense short-term downpours.
  • High population density, increasing exposure to risk.
• High-risk locations - Fatal landslides are more common in low-income countries lacking protective infrastructure, with notable areas including:
  • The southern edge of the Himalayas.
  • Central China.
  • South-west India.
  • The western boundary of the Philippine Sea plate.
  • Central Indonesia.
  • The Caribbean and central Mexico.
  • The western edge of South America.

The relevance of hazard magnitude and frequency for risk management

Understanding the magnitude and frequency of geophysical hazards is crucial for effective risk management. These factors help predict the likelihood and potential impact of events, guiding preparation and response strategies.

Understanding recurrence intervals

• Definition of recurrence interval - Also known as the return period, this measures the expected frequency, in years, of an event of a specific size occurring.
• Frequency-magnitude relationship - Smaller events tend to happen frequently with short return periods, while larger, more destructive events occur less often with longer return periods.
• Implications for management - Low-frequency, high-magnitude events, such as major earthquakes or volcanic eruptions, cause the most destruction and require the most intensive management efforts, compared to high-frequency, low-magnitude events which are more common but less damaging.

Earthquake frequency and magnitude measurements

Earthquakes vary widely in their strength and occurrence, which are measured using specific scales to assess their potential impact on communities and infrastructure.

Measuring earthquake magnitude

• Richter scale - Developed in 1935, this logarithmic scale quantifies earthquake magnitude. An increase of 1.0 on the scale represents a tenfold increase in power. For example, a magnitude 6.0 earthquake is 10 times stronger than a 5.0 event.
• Moment Magnitude Scale (M) - Increasingly used by scientists, this scale measures the energy released during an earthquake, providing values similar to the Richter scale. Each 1.0 increase corresponds to over 30 times more energy released, and each 0.2 increase doubles the energy output.

Frequency and impact of earthquakes

Measuring the strength of volcanic eruptions

Volcanic eruptions are assessed based on their explosiveness and the scale of their impact, which helps in understanding their potential threat to life and property.

Volcanic Explosive Index (VEI)

• Measurement criteria - The Volcanic Explosive Index (VEI) evaluates eruptions based on the volume of material ejected, the height of the eruption column, and the extent of damage caused.
• Scale significance - Eruptions above VEI 5 are considered extremely powerful and violent. A VEI 8 eruption, classified as a supervolcano, releases over 1,000 km³ of material, which is ten times greater than a VEI 7 event.
• Historical context - The most recent VEI 8 eruption occurred approximately 76,000 years ago, indicating the rarity of such mega-colossal events.

Magnitude and frequency of volcanic eruptions