Factors influencing the severity of tectonic hazard impacts

Tectonic hazards, such as earthquakes, tsunamis, and volcanic eruptions, arise from movements in the Earth's tectonic plates. These events can cause significant social, economic, and environmental effects, but their severity varies based on several interconnected factors.

Key factors affecting hazard impacts

• Level of development - Developed countries often experience fewer fatalities due to better preparation and protection. In contrast, developing countries may see higher death tolls and more homelessness.
• Economic considerations - Rebuilding costs tend to be higher in developed countries, leading to greater overall economic losses.
• Political influences - Access to information, like hazard warnings and preparation advice, and the speed and quality of emergency responses are influenced by political factors.
• Type of hazard - Earthquakes and tsunamis generally cause more widespread damage than volcanic eruptions because they often occur without warning.
• Population displacement - In developing countries, hazards frequently leave large numbers homeless without state-provided temporary accommodation.
• Timing of the event - The time of day or season plays a role in influencing the impacts it creates.

These factors interact in unique ways for each hazard.

Contrasting earthquake impacts by level of development

Earthquakes occur when tectonic plates release built-up energy along fault lines, measured by magnitude (e.g., on the moment magnitude scale, $M_W$) and focus depth (distance from the surface). Impacts vary greatly depending on a country's development level, which influences building standards, infrastructure resilience, and response capabilities.

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Earthquake in a developed country: Christchurch, New Zealand (2011)

This earthquake struck at 12:51 pm on 22nd February 2011, with a magnitude of 6.3 $M_W$ and a shallow focus depth of 5 km. It occurred along a conservative plate boundary between the Pacific and Australian plates, and was likely an aftershock of a previous event in 2010.

Key contributing factors:

• Structural damage from the earlier quake may have weakened buildings.
• Shockwaves amplified by rebounding off nearby hills.
• The event happened midday, increasing fatalities as people were active.
• Sandy soils with high water tables led to instability.

Impacts by category:

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Earthquake in an emerging economy: Sichuan, China (2008)

This event occurred at 2:28 pm on 12th May 2008, with a magnitude of 7.9 $M_W$ and a focus depth of 19 km, caused by collision between the Indo-Australian and Eurasian plates.

Key contributing factors:

• High economic losses due to recent growth in businesses and housing.
• Variable damage between buildings indicated poor adherence to regulations.

Impacts by category:

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Earthquake in a developing country: Gorkha, Nepal (2015)

This quake hit at 11:56 am on 25th April 2015, with a magnitude of 7.8 $M_W$ and a focus depth of 15 km, resulting from collision between the Indo-Australian and Eurasian plates.

Key contributing factors:

• Mountainous terrain increased landslide risks, worsened by deforestation.
• Most homes were owner-built without proper checks or earthquake-safe training.
• Rural epicentre and difficult access delayed rescues.

Impacts by category:

Contrasting tsunami impacts by level of development

Tsunamis are large ocean waves triggered by underwater earthquakes or other disturbances, often at subduction zones. Their impacts depend on coastal geography, warning systems, and development levels, with waves capable of travelling at high speeds and causing inland flooding.

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Tsunami in a developed country: Tohoku, Japan (2011)

This tsunami followed a 9.0 $M_W$ earthquake at 2:46 pm on 11th March 2011, with a focus depth of 29 km, from subduction of the Pacific plate under the Okhotsk plate.

Key contributing factors:

• Wave refraction and diffraction from islands reduced impacts elsewhere.
• Low-lying east coast allowed waves to penetrate inland, reaching up to 10 m high and 800 km/h in open water.

Impacts by category:

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Tsunami in an emerging economy: Chile (2010)

Triggered by an 8.8 $M_W$ earthquake at 3:34 am on 27th February 2010, with a focus depth of 35 km, from subduction of the Nazca plate beneath the South American plate.

Key contributing factors:

• Tsunami warning system existed but responses were slow.
• Tourists in low coastal areas were disproportionately affected, while locals evacuated to higher ground.

Impacts by category:

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Tsunami in a developing region: Indian Ocean (2004)

Caused by a 9.1 $M_W$ earthquake at 7:58 am on 26th December 2004, with a focus depth of 30 km, from fault movement between the Indo-Australian and Burma plates.

Key contributing factors:

• Affected 14 countries, with Indonesia, Sri Lanka, and Thailand worst hit.
• Low-lying coastlines increased flooding; no warning systems existed.
• North-south fault orientation directed strongest waves east-west, sparing some areas like Bangladesh.

Impacts by category:

Contrasting volcanic eruption impacts by level of development

Volcanic eruptions release tephra (fragments of rock such as ash and volcanic bombs), lava, and gases and are rated by the Volcanic Explosivity Index (VEI). Impacts vary with eruption style, preparation, and development, often including secondary hazards like lahars (mudflows).

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Volcanic eruption in a developed country: Mount St Helens, USA (1980)

Erupted on 18th May 1980 (a Sunday), with VEI 5, involving pyroclastic flows, ash, gas, lava, and bombs, from subduction of the Juan de Fuca plate beneath the North American plate.

Key contributing factors:

• Triggered by an earthquake after minor tremors; a landslide followed.
• Exclusion zone was set up but limited by logging activities.
• Media coverage raised awareness; Sunday timing reduced casualties among loggers.

Impacts by category:

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Volcanic eruption in an emerging economy: Mount Merapi, Indonesia (2010)

Erupted 25-26th October 2010, with VEI 4, producing ash, lava, and pyroclastic flows, from subduction of the Indo-Australian plate beneath the Eurasian plate.

Key contributing factors:

• Precursors like avalanches and dome growth observed a month prior.
• Over 300,000 evacuated from a 20 km zone, though some resisted.
• Ash ruined crops, affecting farmers financially.

Impacts by category:

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Volcanic eruption in a developing country: Mount Nyiragongo, Democratic Republic of Congo (2002)

Erupted on 17th January 2002, with VEI 1, releasing lava and gas, from divergent movement in the East African Rift Valley.

Key contributing factors:

• Civil war drove people to cities like Goma, increasing vulnerability.
• Warnings allowed 350,000 to flee lava flows, but unrest hindered relief.

Impacts by category: