Mission 1: The Atlantic is about 2.5 cm wider every year. What moves the plates?

Plates and their margins

• Earth's rigid outer layer, the lithosphere, is broken into plates that move over the weaker asthenosphere below.
• Divergent margins: plates move apart at spreading centres.
• Convergent margins: plates move towards each other, and one may sink.
• Transform (conservative) margins: plates slide horizontally past one another.

Internal heating

• Heat inside Earth comes from two main sources: radioactive decay and residual heat left over from Earth's formation.
• The heat slowly migrates towards the surface.
• Hotter, lighter material rises and colder, denser material sinks: mantle convection.

Convection, ridge push and slab pull

• Mantle convection: the older view was that convecting mantle drags the plates along.
• Ridge push: new plate at a spreading ridge is hot and stands high, so gravity makes it slide away downhill from the ridge.
• Slab pull: a cold, dense oceanic plate sinking at a subduction zone drags the rest of the plate behind it.
• In its 1996 booklet, USGS called slab pull "the driving force of plate tectonics". Supporting evidence: plates attached to subducting slabs move fastest.

Subduction and collision

• Subduction: at a convergent margin, a denser oceanic plate sinks into the mantle.
• Central America: the Cocos Plate subducts beneath the Caribbean Plate, building a line of volcanoes that includes Fuego (Mission 7).
• Himalaya: when two continents meet, USGS's booklet says "neither is subducted" and the crust crumples in a collision zone. Yet USGS's 2015 Nepal summary calls the India plate "subducting". Terms vary: name your source.

Rifting and spreading

• Rifting: when continental crust stretches beyond its limits, cracks open. A new spreading centre may be forming along the East African Rift.
• Iceland is splitting along the spreading centre between the North American and Eurasian plates. Rifting at Krafla opened about 7 m between 1975 and 1984.
• Spreading rates: the Mid-Atlantic Ridge averages about 2.5 cm a year; the East Pacific Rise, more than 15 cm a year.

Plumes and hotspots

• A mantle plume is a column of exceptionally hot material. Where it reaches the base of a plate it feeds a hotspot.
• Hawaii formed more than 3,200 km from the nearest plate boundary. As the Pacific Plate moves over the hotspot, older islands are carried away: the Big Island's oldest exposed rocks are under 0.7 million years old, Kauai's about 5.5 million.
• Some hotspots lie near spreading ridges, such as beneath Iceland.
• The plume model is debated: newer studies suggest hotspots may be neither deep nor fixed.

Where earthquakes and volcanoes cluster

• USGS: about 81% of "our planet's largest earthquakes" occur in the circum-Pacific belt, where oceanic plates subduct; about 17% in the Alpide belt (Java to the Himalaya, the Mediterranean and the Atlantic). USGS gives no magnitude cut-off for "largest".
• A third belt follows the Mid-Atlantic Ridge.
• There are about 1,350 potentially active volcanoes, not counting the belts on ocean-floor spreading centres. Many are around the Pacific "Ring of Fire".

Exam move: evaluating the mechanisms

• USGS: "none of the proposed mechanisms can explain all the facets of plate movement".
• The forces act too deep to test directly, so present convection, ridge push and slab pull as explanations with different support.
• Strong answers link each mechanism to a margin and a located example: slab pull at the Cocos subduction zone, rifting in East Africa, a plume under Hawaii.

Mission 2: In 1985 a volcano 45 km away killed about 23,000 people in Armero, two hours after it erupted. How?

Why eruptions differ

• A magma's composition, viscosity and gas content largely decide how explosive an eruption is.
• Viscosity is resistance to flow. Silica has the greatest effect on it; most igneous rocks contain about 45–78% silica by weight.
• Low-viscosity lava flows easily: at Kīlauea, flow fronts move up to 10 km/h.
• High-viscosity, silica-rich magma does not spread out; it traps gas, so eruptions tend to be explosive.

Shield volcanoes

• Built almost entirely of fluid basalt lava flows, spreading widely to form a broad, gently sloping cone "like that of a warrior's shield".
• Mauna Loa (Hawaii) is the largest shield volcano and the world's largest active volcano: 13,677 feet (about 4,170 m) above sea level.
• Eruptions are mainly effusive: lava flows rather than explosions.

Composite volcanoes (stratovolcanoes)

• Steep-sided, symmetrical cones built of alternating layers of lava flows, ash, cinders, blocks and bombs.
• They may rise as much as 8,000 feet (about 2,400 m) above their bases.
• Viscous magma makes many eruptions explosive. Nevado del Ruiz (Colombia) and Fuego (Guatemala) are stratovolcanoes.

Cinder cones

• The simplest type: blobs of gas-charged lava blown from a single vent fall back as cinders around it.
• Parícutin (Mexico) started growing on a farm in 1943 and built a cone about 1,200 feet (about 370 m) high.
• It was active for 9 years. Cinder cones are usually small and short-lived.

Measuring eruption size: the VEI

• The Volcanic Explosivity Index (VEI) starts at 0 (non-explosive); 8 is the top of the usual scale, though USGS calls it open-ended.
• Each step is about 10 times the volume of ash, except VEI 1.
• A VEI 5 eruption, such as Mount St Helens in 1980, may release about 1 km³ of ash.
• Most eruptions in Hawaii rate VEI 0 or 1.

Secondary hazard 1: pyroclastic flows

• Avalanches of hot rock fragments and gas that typically travel at more than 80 km/h.
• Temperatures are generally 200–700 °C.
• They knock down, shatter, bury or carry away nearly everything in their path.
• Their speed means evacuation before they start is the only reliable protection.

Secondary hazard 2: lahars

• Lahars are volcanic mudflows: a slurry of debris and water that looks like wet concrete.
• The water can come from melting snow and ice, heavy rain, or lakes and rivers.
• They often grow to more than 10 times their initial size, and can exceed 200 km/h on steep slopes.
• The hazard outlasts the eruption: after Fuego's 2018 eruption, heavy rains produced many lahars in the valleys.

Secondary hazard 3: volcanic landslides

• Volcano flanks can collapse. Mount St Helens' 1980 eruption triggered "the largest landslide in recorded times".
• Volcanic activity can make a slope fail as a "hot" debris avalanche, which can travel close to 100 m/s.
• Debris avalanches can turn into lahars that travel "many tens of kilometers farther downstream".

Armero, 1985: the event

• Nevado del Ruiz is a glacier-topped stratovolcano, 5,389 m high.
• Late on 13 November 1985 an explosive eruption, hidden by a storm, melted ice and sent lahars racing down the river valleys.
• Armero, built on old lahar deposits 45 km from the volcano, was hit about two hours after the eruption.
• Within minutes about 23,000 people, most of the town, were killed. The eruption killed more than 25,000 in all.

Armero: why so many died (AO3)

• Physical: ice melt and steep valleys funnelled the lahars to the town.
• Monitoring: Colombian scientists lacked the expertise, government support and equipment to monitor the volcano.
• Governance: guerrilla fighting had broken out in Bogotá, and to the government the unstable political situation was "more pressing".
• Communication: earlier that evening the mayor and priest had told residents they were safe for the night. USGS: two hours was "plenty of time" to reach higher ground.
• When Pinatubo (Philippines) awoke in 1991, the whole volcanological community responded, and a disaster like Armero's was averted.

Mission 3: The central and eastern USA used to have about 25 earthquakes of magnitude 3 or more a year. In 2015 it had 1,010. What changed?

Focus, epicentre and depth

• The focus (hypocentre) is where the rupture starts underground. The epicentre is the point on the surface directly above it.
• Depth of focus: shallow 0–70 km, intermediate 70–300 km, deep 300–700 km.
• Earthquakes deeper than 70 km all occur within slabs sinking into the mantle.
• Shallow earthquakes usually do more damage, because the energy is released close to the surface.

Seismic waves

• P waves (primary): compressional, the fastest, so they arrive first; 5–7 km/s in typical crust.
• S waves (secondary): shear waves, about 60% of P-wave speed (3–4 km/s in crust). They cannot travel through liquids.
• Surface waves (Love and Rayleigh): the slowest, but their ground motions are often much larger, and they cause buildings to fail.

How plate margins make earthquakes

• Convergent margins: thrust (reverse) faults, where the upper block moves up and over the lower one. Nepal 2015 was thrust faulting between India and Eurasia.
• Divergent margins: normal faults, where the upper block moves down. Iceland, on the Mid-Atlantic Ridge, has had earthquakes "as large as at least M6.9".
• Transform margins: strike-slip faults, where blocks slide past each other. New Zealand's South Island boundary is mainly transform faulting.

Magnitude and intensity

• Magnitude measures the size of the earthquake and does not depend on distance. Each whole number up means 10 times the measured amplitude and about 32 times the energy.
• USGS uses moment magnitude (Mw); Richter's scale is now used mainly for small, local earthquakes.
• Intensity describes the effects of shaking at a place, and usually falls with distance. The US uses the Modified Mercalli Intensity scale.
• Agencies can differ: Nepal's National Seismological Centre recorded the 2015 earthquake as 7.6; USGS gives Mw 7.8. Name the source.

Human triggers 1: dams and reservoirs

• A large reservoir loads the crust and raises water pressure in rocks below, which can trigger slip on faults that are already stressed.
• The Koyna Dam in western India has been seismically active since its reservoir was filled in 1962.
• In 1967 an M6.3 earthquake there "claimed 200 human lives" and damaged the dam.
• Bursts of M5 earthquakes followed in 1973, 1980 and 1993–95. In the latest, activity tracked water levels in both reservoirs, more in the new Warna reservoir.

Human triggers 2: resource extraction

• Oil production brings up waste fluids, which are injected deep underground. USGS: the increase in the central USA is "primarily caused by disposal of waste fluids", not by fracking itself.
• In Oklahoma, about 2% of earthquakes can be linked to hydraulic fracturing; the rest are induced by wastewater disposal.
• Oklahoma's M3+ earthquakes: 41 in 2010, 585 in 2014, 888 in 2015 (USGS count, March 2016).
• The M5.8 Pawnee earthquake (Oklahoma, 2016) is the largest induced by wastewater disposal (Moschetti et al., 2019, listed by USGS).

Secondary hazard: tsunami

• When a fault under the sea moves the seafloor up or down, the water above is displaced: a tsunami.
• 2004, Indian Ocean (M9.1): USGS's event page gives more than 283,100 killed and 14,100 missing in 10 countries; its FAQ says the tsunami "killed over 230,000 people". Tolls differ by source and date.
• Kaikōura 2016: offshore faults ruptured; the first wave arrived within 10 minutes, with a 6.9 m run-up at Goose Bay.

Secondary hazard: landslides

• Shaking triggers landslides on steep slopes: at least 25,000 in Nepal in 2015; over 10,000 at Kaikōura in 2016.
• At Kaikōura, more than 200 landslides dammed rivers.

Secondary hazard: liquefaction

• Liquefaction: loosely packed, saturated sediments lose strength in strong shaking.
• Buildings may sink or crack, and buried pipes can float up.
• Christchurch: in March 2012 the government rezoned 251 properties in Central City South, Linwood and Richmond South to red, saying "there has been widespread liquefaction" there. Mission 9 shows what happened to the red-zoned land.

Mission 4: About 1,600 magnitude-5 earthquakes strike each year, and about one magnitude-8. Which should planners prepare for?

Magnitude and frequency

• Small events are far more frequent than large ones. Averages calculated from USGS counts for 2000–2021: about 1,613 earthquakes of M5–5.9 a year, 137 of M6–6.9, 14 of M7–7.9 and 1.2 of M8 or more.
• Over the long term (since about 1900), USGS expects about 15 M7 earthquakes and one M8 or larger each year.
• On a log scale the counts fall steadily as magnitude rises: the magnitude–frequency relationship.

Recurrence intervals

• The recurrence (return) interval is the average time between events of a given size in a place.
• A "1-in-100-year" event has a 1% chance of happening in any year. It can happen two years running.
• Volcanoes follow the same pattern: each VEI step means about ten times as much ash, and the largest eruptions are the rarest.

Designing for rare events

• USGS hazard maps show the shaking that has a 10% or a 2% chance of being exceeded in 50 years.
• These correspond to return periods of about 475 years and 2,475 years (calculated: 1 − (1 − 1/475)⁵⁰ ≈ 10%).
• Planners choose a level of risk to accept: for example, a hospital may be built for rarer, stronger shaking than a garage.

Rare events cause most deaths

• In USGS's list for 2000–2019, two years made up about 65% of all earthquake deaths: 2004 (298,101, all earthquakes that year) and 2010 (226,050), out of 808,717.
• The 2010 total rests on Haiti's disputed toll: officially 316,000, but USGS notes other estimates "as low as fewer than 100,000".
• Haiti was an M7.0, a size that happens about 14 times a year: rare in impact, not in magnitude.
• UNDRR: risk models tuned to the largest events often miss "smaller, often recurrent and in sum equally damaging events".

Showing magnitude and frequency (skills)

• A logarithmic scale fits counts from 1 to 1,600 on one axis.
• A risk matrix plots likelihood against impact. Rare, high-impact events sit in one corner; frequent, low-impact events in the other.
• Exam move: label the scale and name the data source and years on any graph you draw.

Mission 5: In one Aceh village, the 2004 tsunami killed four females for every male. Why would a wave "choose"?

Hazard, exposure and risk

• A hazard is the process that may cause harm: shaking, a lahar, a debris flow.
• Exposure: the people and assets in harm's way.
• UNDRR: disaster risk is "the potential loss of life, injury, or destroyed or damaged assets", determined "as a function of hazard, exposure, vulnerability and capacity".

Vulnerability, capacity and resilience

• Vulnerability: conditions "which increase the susceptibility" of people or assets to a hazard.
• Capacity: "the strengths, attributes and resources" available to manage and reduce disaster risk.
• Resilience: the ability to "resist, absorb, accommodate, adapt to, transform and recover".
• Shorthand: risk = hazard × exposure × vulnerability ÷ capacity. It is a way of thinking, not a formula to calculate.

Economic factors

• Development pays for building codes, monitoring and warning systems.
• UNDRR (all disaster types, 2000–2019): low-income countries had 23% of disaster deaths but under 10% of the world's population, and the most deaths per event (284).
• High-income countries had 67% of economic losses, US$1.99 trillion.
• Richer places tend to lose more money; poorer places lose more lives.

Social factors: education

• Education: people who know the warning signs and what to do can act sooner.
• A review of eleven studies found "consistent and robust evidence" that formal education reduces vulnerability.
• Countries with a higher share of women with at least secondary education suffered lower disaster mortality.

Social factors: gender

• Oxfam, 2005: in Kuala Cangkoy (Aceh), four females died for every male. In Cuddalore (India), 391 women died, against 146 men.
• Oxfam's explanations: many women stayed behind to look for their children; men more often than women can swim or climb trees.
• In Aceh the wave struck on a Sunday morning, when women were at home and the men were away from the seafront.

Demographic factors

• Density: more people in a hazard zone means more exposure.
• Age structure: in Japan's 2011 earthquake and tsunami, people over 60 were about 31% of the affected area's population (1 September 2011) but about 65% of the dead (counted by 11 April 2011). (Second panel of D20.)
• Children, older people and disabled people may need help to evacuate.

Political factors: governance

• Governance shapes monitoring, warnings, building rules and land use.
• Armero, 1985: a government focused on political unrest, and an under-resourced monitoring team.
• Freetown, 2017: the World Bank assessment describes "highly fragmented governance" and an unclear legal status for land and building permits.

Geographic factors: place, time and isolation

• Rural or urban: in Nepal (2015), poorer rural areas were hit harder than towns and cities because their houses were weaker.
• Time of day: Nepal's earthquake struck on a Saturday, when schools were closed, at midday. The government's assessment says a night-time quake would "certainly" have killed more.
• Isolation: road damage cut off rural Nepal; landslides cut Kaikōura's road and rail links.

Synthesis: models of vulnerability

• Pressure and Release (PAR) model (Blaikie, Cannon, Davis and Wisner, 1994): vulnerability builds from root causes (limited access to power and resources), through dynamic pressures (lack of training, rapid urbanisation), to unsafe conditions (fragile housing on dangerous land), and meets a hazard.
• Reducing the pressures at each stage "releases" vulnerability.
• Degg's model (1992): a disaster happens only where a hazard overlaps a vulnerable population.
• Strength: both show risk comes from human as well as physical processes. Limitation: the pressures are hard to measure.
• Exam move: the guide asks how risk comes from "spatial interactions between different human and physical processes". Link both sides with a located example.

Mission 6: Nepal 2015 and Kaikōura 2016 were both magnitude 7.8. One killed more than 8,790 people; the other killed 2. Why?

Nepal, 25 April 2015: event profile

• Mw 7.8 (USGS) on Saturday 25 April at 11:56 local time. The epicentre was near Barpak, Gorkha district, about 76 km north-west of Kathmandu (Nepal's assessment; USGS says about 80 km).
• Thrust faulting where India converges with Eurasia at about 45 mm a year; the focus was about 8 km deep.
• The rupture spread east, towards Kathmandu, so strong shaking reached the city.
• Aftershocks included an M7.3 on 12 May.

Nepal: secondary hazards

• At least 25,000 landslides across the steep Himalaya, mapped from satellite images.
• In Langtang, an avalanche mostly of snow, followed by rockfalls, destroyed the village.
• An avalanche killed 20 people at Everest Base Camp.
• Weakened slopes left the region more prone to landslides and floods in the monsoon.

Nepal: impacts on well-being

• Over 8,790 deaths and 22,300 injuries by June 2015. The lives of about 8 million people, almost a third of the population, were affected.
• Over half a million houses destroyed; nearly 7,000 schools completely or significantly damaged.
• Damage and losses: about US$7 billion, equal to about one-third of GDP (2013–14). The World Bank estimated at least 700,000 more people would fall into poverty.
• Data: USGS lists at least 8,669 killed and 384 missing in the earthquake and the 12 May aftershock; UNDRR's EM-DAT figure is 8,969. Counts differ by date and source.

Nepal: why vulnerability varied

• Between communities: poorer rural areas were hit harder than towns and cities; the damage exposed houses that "did not have any seismic-resistant features".
• The six lowest-HDI districts with high losses per person included Sindhupalchowk, Gorkha and Dolakha.
• Within communities: more women and girls died than men and boys, partly because of gendered roles that kept women indoors.
• Isolation: road damage further isolated rural areas in the central and western regions.

Nepal: perception and preparedness

• Nepal had no integrated national search-and-rescue capacity before 2015, but trained army and police teams carried out rescue.
• Fear of being trapped drove many families, even with undamaged homes, to sleep in the open.
• Neighbours, especially young people, dug others out of the rubble.

Kaikōura, 14 November 2016: event profile

• Mw 7.8 at 12:02 a.m. on Monday 14 November; the focus was about 15 km deep.
• Oblique-reverse faulting at the boundary between the Pacific and Australia plates, which move about 40 mm a year relative to each other.
• The rupture "unzipped" about 180 km of coast in nearly 2 minutes. More than 20 faults ruptured, a world record.

Kaikōura: secondary hazards

• Over 10,000 landslides; more than 200 of them dammed rivers.
• Coastal uplift of up to 6.5 m.
• Tsunami: a 6.9 m run-up at Goose Bay.

Kaikōura: impacts and recovery

• Two deaths: one from a heart attack at Mt Lyford, one when a homestead collapsed in Kaikōura.
• Landslides cut road and rail links, stranding many visitors in the town. Rail reopened in September 2017 and State Highway 1 in December 2017, about 10 and 13 months later.
• About 40,000 claims for building, contents or land damage went to the Earthquake Commission (now the Natural Hazards Commission).
• Scale: the quake triggered slow-slip events on the Hikurangi subduction zone up to about 600 km away.

Comparing the two (AO3)

• Buildings: many Nepali houses had no seismic-resistant features. New Zealand's Building Code requires that buildings "withstand likely loads, including wind, earthquake".
• Exposure: the Nepal rupture spread towards Kathmandu and its valley.
• Recovery capacity: New Zealand homeowners had state hazard insurance.

Mission 7: On Fuego, in 2018, a golf resort evacuated safely. A village two kilometres further on was buried. Why?

Two plate boundaries

• Fuego, Guatemala: a stratovolcano in the Central American arc, where the Cocos Plate subducts beneath the Caribbean Plate (convergent).
• Reykjanes Peninsula, Iceland: where the Mid-Atlantic Ridge rises above sea level; a rift zone on oceanic crust (divergent).
• Expect explosive eruptions and pyroclastic flows at Fuego, and effusive fissure eruptions of runny lava at Reykjanes (Mission 2).

Fuego, 3 June 2018: event profile

• The first pyroclastic flows came at 10:00. Explosive activity peaked at about 13:10, with a column 16–19 km high.
• The first big pulse began at 15:05 and reached the Las Lajas bridge at 15:09, overflowing onto the RN-14 road towards San Miguel Los Lotes.
• About 15.1 million m³ of pyroclastic flow material was deposited in Las Lajas (Albino et al., 2020, cited in S41): more than double the average since 1999.
• In the rainy weeks that followed, rain produced many lahars.

Fuego: impacts on well-being

• About two-thirds of the north-western part of San Miguel Los Lotes suffered "total inundation, destruction, and burial" by the flows.
• An estimated 5,000 people lost their homes.
• The death toll is uncertain. Official counts of the missing differ: 332 (Naismith et al., 2020) and 430 (Charbonnier et al., 2023, whose abstract says "at least 430" were killed). Survivors' estimates are far higher.

Fuego: vulnerability between communities

• The private golf resort of La Reunión evacuated successfully; Los Lotes, about two kilometres further south, suffered extensive loss of life.
• The researchers found that La Reunión had "a culture of preventative self-evacuation".
• They call the residents' idea that wealth decided who survived "debatable".
• Separately, communities on Fuego's west and east flanks differed in their past experience of eruptions and in their communication with the monitoring and civil protection agencies.

Fuego: perception and self-evacuation

• Scientists issued information from 06:30, and authorities "persistently tried to remove people from high-risk areas".
• But authorities expected residents to recognise changes in the volcano and evacuate themselves. Locals weighed security fears and livelihoods.
• Perception: residents clearly remember only eruptions that disrupted daily life. Smaller flows had triggered evacuations in 2012 and 2017.

Reykjanes, 2023–24: event profile

• From 27 October 2023, magma accumulated beneath Svartsengi and the ground rose; on 10 November it forced a dike beneath the Sundhnúkur crater row and Grindavík.
• The town of 3,700 people was evacuated the same day (IFRC; the Global Volcanism Program gives about 4,000).
• By December 2023 the IFRC reported that half the town had dropped by more than a metre.
• Fissure eruptions followed from December 2023. On 14 January 2024 a fissure opened at the edge of town and its lava burned three homes.

Reykjanes: responses and impacts

• Barriers of earth and rock, built from November 2023, diverted some lava away from Grindavík and the Svartsengi power plant.
• Iceland's Natural Catastrophe Insurance (NTÍ) covers damage to buildings from volcanic eruptions, earthquakes and landslides.
• Under a 2024 law (Act 16/2024), the state company Þórkatla offered to buy residents' homes; by 27 March 2024 it had 546 applications.
• The cost to well-being: a whole community displaced from its homes, schools and harbour town.

Comparing the two (AO3)

• Hazard: Fuego's flows spilled from their channel; Reykjanes' lava was slow enough to divert.
• Capacity: Iceland had insurance and a state buy-out; at Fuego, the documented capacity gap is in evacuation practice and communication.
• Exam move: the plate setting explains the type of hazard; development and governance explain the impact.

Mission 8: In 2017 a hillside above Freetown collapsed; about 1,141 people were declared dead or missing. A Californian landslide has been moving since 1956 and is still destroying homes. Are they the same hazard?

Classifying mass movement

• Mass movement: rock, soil or debris moving downslope under gravity, by falling, toppling, sliding, spreading or flowing.
• Varnes' classification sorts them by type of movement and by material:

• A complex landslide combines two or more types.

Liquidity and speed of onset

• Liquidity: from dry falls to saturated flows. A debris flow is loose soil, rock, organic matter, air and water moving as a slurry.
• A mudflow is wet enough to flow rapidly and is at least 50% sand, silt and clay.
• Speed: creep is "imperceptibly slow", usually under 1 m per decade. Debris flows are "rapid to extremely rapid": USGS gives 56 km/h as an example, depending on consistency and slope. Debris avalanches can travel close to 100 m/s.

Cause, duration, extent and frequency

• Physical causes: weak or weathered rock, erosion of the slope's toe, saturation by rain or snowmelt, earthquakes.
• Human causes: excavation, loading the slope, deforestation, irrigation, mining, leaking water pipes, lowering reservoirs.
• Duration: from seconds (a rockfall) to decades (creep). Frequency: some slopes fail every wet season; others once in centuries.
• Extent: from one boulder to tens of square kilometres of creep.

Where fatal landslides happen

• From 2004 to 2016, 4,862 fatal non-seismic landslides killed 55,997 people.
• Asia had 75% of these events: along the Himalayan Arc, in India and south-eastern China, Indonesia and the Philippines. Other clusters were in Central and South America and East Africa.
• Most were triggered by rain. The authors report that landslides triggered by human activity (construction, illegal mining, hill cutting) are increasing, but they ascribe part of the rise in their record to "improved event capture".

Freetown, Sierra Leone, 14 August 2017: event profile

• 1,040 mm of rain fell between 1 July and 14 August.
• At about 6:50 a.m. the lower part of a valley-side slope below Sugar Loaf mountain, in Regent, began to slip; about ten minutes later a "more destructive stage" followed.
• Clayey soil and boulders of up to 40 m³ became a saturated, highly mobile debris flow, advancing down the river channel like a "tidal wave" along a 7 km corridor.
• Classification: fast and loose: a wet debris flow.

Freetown: impacts

• In the rapid assessment, about 6,000 people were affected and about 1,141 were declared dead or missing; it says "substantiated numbers remain scarce".
• In Regent, 349 buildings were destroyed.
• Damage and losses: about US$31.65 million.

Freetown: why so vulnerable

• "Nearly all the poor in the Western Area live in informal housing".
• The city's slopes "are steep and subject to ongoing slope instability", and settlement is expanding "into the more elevated, steeper and forested central mountain belt".
• Deforestation: the World Bank assessment says it "cannot be attributed directly as the landslide trigger".

Portuguese Bend, California: event profile

• One of three active landslides inside an ancient landslide complex on the Palos Verdes Peninsula (Los Angeles County), studied since 1946.
• CGS attributes its reactivation in 1956 to road fill placed for the extension of Crenshaw Boulevard: a human cause, loading the slope.
• A peer-reviewed study, citing CGS, classifies the major landslides there as "rock slides", deep-seated and moving on bedding planes in the Altamira Shale.
• After the wet years of 2022–24 it sped up: about 4 feet (1.2 m) a month from June to August 2024 (City of Rancho Palos Verdes, reported by CGS).
• Classification: slow and solid: a coherent mass sliding on a slip surface, lasting decades.

Portuguese Bend: impacts and responses

• Damage to homes, roads and utilities, with indefinite gas and electricity shut-offs for hundreds of residents.
• Dewatering wells at the toe removed over half a billion gallons of groundwater in two years. In late 2024 the City reported movement slowing, after dewatering and drier weather.
• A voluntary buy-out offers the pre-disaster appraised value. FEMA covers 75% of eligible costs; the other 25% is deducted from the seller's proceeds. Homes are demolished and the land kept as open space.
• The programme began at US$42 million in 2024; by October 2026 over US$54 million had been awarded for 27 homes. The City received 85 applications.

Perception and preparedness: the two landslides

• Freetown: the Regent landslides happened in locations already marked on landslide hazard maps, but "flooding is a much more common hazard than large-scale landslides" there.
• Freetown: "people were in the process of evacuating, some unsuccessfully, before the landslide". Two failures ten minutes apart left little time.
• Portuguese Bend: movement is slow and monitored (studied since 1946), so residents and the City have time to act.
• Exam move: speed of onset shapes perception and preparedness. A slow slide is seen and planned for; a fast flow gives almost no second chance.

Comparing the two (AO3)

• Speed decides the threat: a fast flow threatens lives; a slow slide mainly threatens property, services and roads.
• Capacity: Rancho Palos Verdes draws on federal and state funds; Freetown's poorest lived in informal housing.
• Synthesis: in both places, human actions (road fill; building on steep slopes) interacted with heavy rain.

Mission 9: Large earthquakes are not becoming more common. So why do experts expect more people to be affected by geophysical disasters?

Event frequency: no upward trend

• USGS: catalogues list more earthquakes "not because there are more earthquakes, but because there are more seismic instruments".
• Year-to-year swings are normal. USGS's FAQ gives 2010 as the year with most major earthquakes (23) and 1989 with 6. Its statistics table counts 24 of M7 or more in 2010 (23 of M7–7.9 and one M8).

Disaster trends

• Recorded earthquake disasters rose from 445 (1980–1999) to 552 (2000–2019); volcanic disasters from 84 to 102.
• In 2000–2019, earthquakes and tsunamis were the deadliest type of disaster: 58% of all disaster deaths.
• Exam move: more recorded disasters does not mean more earthquakes. Better reporting and more exposed people and assets both raise the counts.

Exposure is rising

• The EU's Joint Research Centre found that people living in seismic areas rose from about 1.41 billion in 1975 to 2.72 billion in 2015. It counts people where a 475-year earthquake would give at least moderate shaking.
• People within 100 km of the 220 most dangerous volcanoes rose from about 227 million to 414 million, but their share of world population stayed about 5.5%: the rise follows population growth.
• Brown and others put the number within 100 km of any active volcano at about 800 million. The two volcano figures use different lists of volcanoes.

Future projections

• World population is projected to grow from 8.2 billion (2024) to about 10.3 billion in the mid-2080s (UN projection).
• The JRC: "With a growing population and urbanizing area, also exposure is expected to increase".
• Landslides triggered by human activity may be increasing, though part of the rise in the 2004–2016 record reflects better reporting.
• So, while event frequency is steady, rising exposure points to more people at risk, unless vulnerability falls.

Government planning: land-use zoning

• California's Alquist-Priolo Act, passed after the 1971 San Fernando earthquake: buildings for people cannot be placed over an active fault, and must generally be set back about 50 feet (about 15 m).
• Limitation: it covers only surface fault rupture, not shaking, liquefaction or landslides, which a separate Act maps.
• Managed retreat: the owners of almost 5,500 Christchurch properties accepted government offers. The 602-hectare red zone, land that once held more than 5,000 homes, is now a river-corridor regeneration area.

Personal resilience: preparedness

• Preparedness: drills teach "Drop, Cover, and Hold On". In 2015, more than 40 million people in 60 countries registered for ShakeOut drills.
• Knowing what to do saves time: the people most at risk are those who must work it out during the shaking.

Personal resilience: insurance

• New Zealand: homeowners with private home insurance also get state natural-hazards cover, generally up to NZ$300,000 plus GST, through the Natural Hazards Commission (the Earthquake Commission at the time of Kaikōura).
• Iceland: NTÍ covers volcanic, earthquake and landslide damage to buildings.
• California: only 15.24% of homeowners' policies included earthquake cover in 2025.

New technology: early warning

• Earthquake early warning detects the fast, weaker P waves and sends alerts "seconds to minutes" before the stronger S and surface waves arrive.
• Japan invested US$600 million after the 1995 Kobe earthquake. In 2011 no trains derailed, and 90% of citizens polled said it was worth it.
• Android phones now act as a global network: alerts in 98 countries. In user feedback, 85% of people alerted said they felt shaking, and 36% received the alert before it began.
• Limit: it gives warning, not prediction.

Perspectives on adaptation (AO3)

• Governments: zoning and buy-outs protect many people, but uproot communities (Christchurch, Grindavík, Portuguese Bend).
• Households: insurance, retrofitting and drills depend on income and knowledge. At Portuguese Bend, sellers themselves pay 25% of the buy-out cost.
• Technology: phone alerts reach places without seismometer networks, but warning times can be short.
• Exam move: weigh who pays, who benefits, and over what time scale.

Mission 10: By 2013, Hong Kong said it had cut the landslide risk from its man-made slopes by about 75%. Is preventing disasters worth more than responding to them?

The management cycle

• Pre-event: mitigation (reduce the hazard's effects or the vulnerability) and preparedness (warning systems, plans, drills).
• Post-event: rescue (hours to days), rehabilitation (weeks to months: shelter, water, services), reconstruction (years).
• Each stage feeds the next: how a place is rebuilt sets its vulnerability for the next event.

Park's disaster-response model

• Park's model (Park, 1991) plots quality of life against time: a sharp fall at impact, then relief, rehabilitation and reconstruction.
• Better preparation makes the fall shallower and the recovery faster.
• Limitation: it is a generalised curve, not a measurement, and recovery is uneven between groups.

Pre-event: slope stabilisation

• Keep groundwater down: cover the slope, drain surface water away, drain groundwater out, reduce irrigation.
• Add support: a retaining structure or a rock and soil berm at the toe; or remove weight from the top.
• Hong Kong set up a slope-safety office in 1977, after the Sau Mau Ping disaster of 1976. In 2013 it said the risk from man-made slopes had fallen by 75%; its engineering department now says "less than 20% of the 1977 level".

Pre-event: building design for earthquakes

• Braced frames and shear walls give buildings rigidity against sideways forces.
• Base isolation: rubber or sliding bearings between a building and its foundations change how it responds and add damping.
• Codes matter. In Mashiki (Kumamoto, Japan, 2016) wooden houses built before 1981 collapsed far more often than newer ones.
• So retrofitting older buildings is a priority.

Pre-event: tsunami defences

• Japan, 2011: the tsunami overtopped coastal defences, mainly 5–10 m seawalls. More than 15,000 people died. Inundation was much wider than hazard maps had predicted.
• Japan now plans for two levels: coastal protection for frequent tsunamis; for the largest possible ones, structures plus measures centred on evacuation, such as hazard maps.
• Walls of about 12 m are planned against tsunamis expected about every 200–300 years. Critics argue that moving shrinking communities may be more efficient.

Pre-event: GPS monitoring of volcanoes

• GPS monitoring: Hawaii's permanent GPS stations find their positions daily, usually to better than a centimetre, revealing magma moving underground.
• At Svartsengi, GPS showed 35 cm of uplift by mid-December 2023.
• Estimates of the magma volume are compared with the volumes that came before earlier eruptions, to judge when the next is likely.

Pre-event: lava diversion

• Earth barriers at Fagradalsfjall (2021) delayed lava by up to 16 days.
• At Heimaey (1973), seawater was sprayed on the lava to cool and slow it, saving the harbour.
• Limit: at Grindavík, on 14 January 2024, a fissure opened at the edge of town outside the barrier, and its lava burned three homes.

Post-event: rescue and rehabilitation

• Nepal, 2015: the government asked for international help within hours. 134 international search-and-rescue teams from 34 countries came.
• Helicopters made 4,236 flights; 7,558 people were rescued by air and 4,689 by land.
• Rehabilitation: transit shelters were set up in Kathmandu, but there were not enough tarpaulins.

Post-event: communications technology

• Mapping: after Nepal's earthquakes over 6,000 volunteers added data to OpenStreetMap, used by the Nepal army, the Red Cross and others. Satellite images mapped at least 25,000 landslides.
• Tracking displaced survivors: anonymised data from 12 million mobile phones showed an estimated 390,000 more people than normal leaving the Kathmandu Valley.
• Limit: "Phone ownership is skewed towards certain population groups", and some areas had little or no network coverage.
• Development: Nepal's housing programme recorded agreements in a digital system, and engineers inspected homes with a tablet app.

Post-event: reconstruction and continuing development

• Nepal's housing programme built over 700,000 earthquake-resilient houses. The World Bank-supported project funded over 300,000.
• Grants of NPR 300,000 came in three parts: NPR 50,000 → 150,000 → 100,000. The second and third were paid only after engineers certified safe construction.
• Masons and engineers were trained. More than 18,000 vulnerable people (such as single women, older and disabled people) who had not finished rebuilding were identified for extra help.
• Kaikōura's road and rail recovery work was scheduled to finish on 15 December 2020.

Evaluating management (AO3)

• Mitigation can pay: a US study found federal mitigation grants "save $6 per $1 spent", and adopting model building codes $11.
• Hong Kong's 75% is the government measuring its own programme: name the source when you use it.
• Pre-event measures can be exceeded by rarer events (Tōhoku's walls); post-event rescue saves lives within hours, and reconstruction decides how vulnerable people will be next time.
• Scale: local (barriers, slope drains), national (building codes, insurance), global (phone alerts, volunteer mapping).

How Option D is examined (Paper 1)

• Paper 1 is 35% of the grade at both levels. SL: two options, 1 hour 30 minutes, 40 marks. HL: three options, 2 hours 15 minutes, 60 marks.
• For each option: a structured question (10 marks), then one extended answer from a choice of two (10 marks). About 45 minutes per option.

What the top band rewards (extended answer, 9–10 marks)

• The answer is "in-depth and question-specific (topic and command term)".
• Examples, statistics and details are "integrated", not listed: give each case study's place, date and figures.
• Evaluation discusses other perspectives and the "strengths and weaknesses of evidence", such as disputed death tolls and differing magnitudes.
• The conclusion is "balanced and aligned with the evidence", and diagrams are annotated and support the argument.

The big ideas

• Internal heat drives convection; slab pull, ridge push and plumes move the plates, so hazards cluster at margins and hotspots.
• Magma viscosity sets eruption style and volcano shape; secondary hazards such as lahars can kill far from the vent (Armero).
• Earthquakes vary by depth, wave type and margin; people can trigger them (Koyna, Oklahoma).
• Small events are frequent and large ones rare; a few high-impact events cause most deaths, so planners use return periods.
• Risk = hazard × exposure × vulnerability ÷ capacity: economic, social, demographic, political and geographic factors (Aceh, Japan, Nepal).
• Contrasting cases: Nepal and Kaikōura; Fuego and Grindavík; Freetown and Portuguese Bend.
• Exposure is rising; adaptation, pre-event measures and post-event recovery decide the future toll.

Final screen. Score, the mission list ticked, and "Next: test yourself in the Verify stage" (links to the Verify quiz).