Earthquakes and Volcanoes

Question

Step-by-step solution

1. Step 1

   Convergent (destructive) boundary: two plates move towards each other. When an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the continental plate in a process called subduction. Both earthquakes (caused by friction as the plates grind against each other) and volcanoes (magma rises through the overlying plate) occur. Example: the Pacific Plate subducting beneath the Eurasian Plate off the coast of Japan — responsible for the 2011 Tohoku earthquake (Mw 9.0).

2. Step 2

   Divergent (constructive) boundary: two plates move apart. Magma rises from the mantle to fill the gap, creating new oceanic crust. Volcanoes are common (typically shield volcanoes); earthquakes also occur but are generally less severe than at convergent boundaries. Example: the Mid-Atlantic Ridge, where the North American and Eurasian plates diverge; Iceland sits on this ridge and experiences both volcanic activity and frequent earthquakes.

3. Step 3

   Conservative (transform) boundary: two plates slide horizontally past each other with no creation or destruction of crust. No volcanic activity occurs, but earthquakes are common and can be very powerful, as stress builds up when the plates lock and is released suddenly. Example: the San Andreas Fault, California, where the Pacific Plate slides north past the North American Plate — responsible for the 1906 San Francisco earthquake.

Question

Step-by-step solution

1. Step 1

   Primary hazard 1 — Ground shaking: The sudden release of energy at the focus (hypocentre) approximately 30 km beneath the ocean floor sent seismic waves — both P-waves (compressional) and S-waves (shear) — radiating outward. When these waves reached the surface, the ground shook violently. In the Tohoku earthquake, shaking reached intensity VII–VII (Modified Mercalli Scale) across a wide area, collapsing unreinforced buildings and bridges.

2. Step 2

   Primary hazard 2 — Tsunami: The Tohoku earthquake occurred on a subduction zone offshore. The sudden vertical movement of the sea floor (the overlying plate snapped upward by several metres) displaced an enormous volume of seawater, generating a series of tsunami waves. Waves reached heights of up to 15 m when they struck the coast, travelling inland up to 10 km. The tsunami caused approximately 19,000 deaths — far more than the direct ground shaking — and destroyed entire coastal communities including Minamisanriku.

3. Step 3

   Secondary hazard 1 — Liquefaction: In areas of poorly consolidated, saturated sediment (such as reclaimed land around Tokyo Bay), violent shaking caused soil particles to lose contact with each other and the ground to behave temporarily like a liquid. Buildings sank and tilted; pipes fractured; roads buckled. Liquefaction was observed across suburban Tokyo, hundreds of kilometres from the epicentre.

4. Step 4

   Secondary hazard 2 — Fires from ruptured gas mains: The earthquake ruptured underground gas pipelines in urban areas. Gas ignited and fires spread — particularly where water mains were also broken, preventing fire-fighting. Multiple fires burned in Kesennuma city, destroying buildings that had survived the ground shaking.

Question

Step-by-step solution

1. Step 1

   Shape and structure: Shield volcanoes have broad, gently sloping sides, like an upturned warrior's shield — typically less than 8° slope angle. They can be enormous in volume (Mauna Loa, Hawaii is the world's largest volcano by volume). Composite (stratovolcano) volcanoes have steep, cone-shaped sides (30–40° slope) built up from alternating layers of lava and solidified ash/pyroclastic material — hence the name 'strato' (layers).

2. Step 2

   Type of lava/magma: Shield volcanoes produce basaltic magma — low silica content (~50%), low viscosity (flows easily), low dissolved gas content. This magma originates at divergent boundaries or hot spots. Composite volcanoes produce andesitic or rhyolitic magma — high silica content (60–75%), high viscosity (thick, flows slowly), high dissolved gas content. This magma originates at convergent/subduction boundaries.

3. Step 3

   Explosiveness: Shield volcanoes are non-explosive (effusive). Low viscosity allows gas to escape gradually; lava flows slowly and can be observed in advance. Composite volcanoes are highly explosive. High viscosity traps gas under high pressure; when pressure exceeds the strength of the magma, violent explosions occur — like uncorking a shaken bottle.

4. Step 4

   Hazards produced: Shield volcanoes: lava flows (slow-moving, allows evacuation time; destroys property and farmland; Mauna Loa lava flows have destroyed communities in Hawaii). Composite volcanoes: pyroclastic flows (superheated gas, ash and rock at 700°C, 200+ km/h; unstoppable — Mt Pinatubo 1991), ash fall (disrupts aviation, smothers crops, causes roof collapse), lahars (volcanic mudflows from mixing of ash with water), ballistic projectiles, volcanic gases (SO₂, CO₂, H₂S). Examples: Mauna Loa, Hawaii (shield); Mt Pinatubo, Philippines / Mt Vesuvius, Italy (composite).

Question

Step-by-step solution

1. Step 1

   Monitoring techniques used at Pinatubo: Scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the USGS deployed multiple instruments:

   (1) Seismographs — recorded increasing numbers of small earthquakes (over 100 per day) beneath the volcano from late March 1991, indicating magma fracturing rock as it rose through the crust. The swarm of earthquakes migrated towards the surface over weeks.

   (2) Ground deformation monitoring (tiltmeters/GPS) — detected inflation of the volcano's flanks as magma accumulated in the magma chamber, causing the volcano to 'swell'. Tiltmeters measured changes of only a few microradians.

   (3) SO₂ gas measurements — a COSPEC (correlation spectrometer) measured sulphur dioxide emissions from the summit. SO₂ flux increased dramatically from April 1991 (from ~500 to >5,000 tonnes/day), indicating that fresh, gas-rich magma was rising close to the surface.

   (4) Observational monitoring — scientists directly observed phreatic (steam-driven) explosions, new fumaroles opening, and the growth of a lava dome in the summit crater from mid-May.

2. Step 2

   Successes of the Pinatubo prediction and management:

   (1) Early evacuation saved tens of thousands of lives. Based on the monitoring data, PHIVOLCS issued escalating alert levels (1–5). At Alert Level 5, a 40 km permanent danger zone was declared and around 60,000 civilians and 18,000 US military personnel at Clark Air Base were evacuated before the climactic eruption on 15 June 1991.

   (2) Use of a volcano hazard map — volcanologists had prepared a detailed map (the 'Pinatubo hazard zonation map') showing areas most at risk from pyroclastic flows and lahars, allowing targeted evacuation.

   (3) International scientific collaboration — USGS volcanologists worked alongside Philippine scientists, bringing expertise in monitoring methods from earlier eruptions (e.g. Mount St Helens 1980).

   (4) Proportionate response to uncertainty — scientists communicated probabilistic risk to government officials honestly, including the possibility of a smaller (non-catastrophic) eruption, allowing risk-based decisions rather than false certainty.

3. Step 3

   Limitations and failures:

   (1) 800 people still died — many deaths occurred in lahar events in the months and years following the eruption, as heavy monsoon rains remobilised millions of tonnes of volcanic ash deposited on the slopes. Lahars buried communities up to 40 km from the volcano. The lahar hazard persisted for years — the total economic cost exceeded $700 million.

   (2) Not all evacuees complied — some residents refused to leave due to reluctance to abandon homes, crops and livestock, or disbelief about the risk. Some returned to the evacuation zone prematurely.

   (3) Timing uncertainty — even with good monitoring, the exact timing of the climactic eruption could not be predicted to the hour. The window between Alert Level 5 and the climactic eruption was narrow (~2 days), requiring rapid mass evacuation under difficult conditions (simultaneously, Typhoon Yunya struck the Philippines on 15 June, compounding the crisis).

   (4) Resource inequality — communities nearest the volcano (indigenous Aeta people) were among the most vulnerable and had least capacity to evacuate quickly or access assistance.

Question

Step-by-step solution

1. Step 1

   Engineering technique 1 — Base isolation: Large rubber and steel pads are installed between a building's foundation and the structure above. During an earthquake, the base absorbs and dampens the seismic energy, allowing the building to move independently from the ground shaking. The upper structure moves far less, preventing collapse. Tokyo Skytree and hundreds of Japanese hospitals use base isolation systems — hospitals must remain functional immediately after an earthquake for rescue operations.

2. Step 2

   Engineering technique 2 — Cross-bracing and moment-resistant frames: Steel diagonal cross-bracing ('X-bracing') is built into the structural frame between floors, distributing lateral (sideways) forces evenly across the building and preventing the 'pancake collapse' (progressive floor-on-floor failure) common in poorly designed structures. Moment-resistant frames use strong, flexible connections between beams and columns that can absorb rotational forces without snapping.

3. Step 3

   Engineering technique 3 — Tuned mass dampers (TMDs): Large masses (typically 100–700 tonnes) suspended at or near the top of tall buildings are connected to the structure via hydraulic actuators or pendulum systems. As the building sways due to an earthquake (or wind), the mass moves in the opposite direction, counteracting the motion and reducing oscillation amplitude by up to 40%. Taipei 101 (Taiwan) and Citicorp Center (New York) both use TMDs.

4. Step 4

   Evaluation — is building design alone sufficient?

   Advantages — why building codes are essential: The 1995 Kobe earthquake (Mw 6.9) killed ~6,400 people in Japan, primarily because many older buildings (pre-1981 building codes) collapsed. By contrast, buildings constructed after Japan's revised 1981 seismic code performed significantly better in the 2011 Tohoku earthquake. Building design directly addresses the primary cause of earthquake death — structural collapse.

   Limitations — why building design is not sufficient alone:

   (1) Cost and inequality: Earthquake-resistant construction is expensive. Japan, as a high-income country, can afford this; many earthquake-prone LICs (Nepal, Haiti, Afghanistan) cannot. The 2010 Haiti earthquake (Mw 7.0) killed ~220,000 people — partly because buildings were constructed of unreinforced concrete with poor materials and no seismic codes. Even in wealthy countries, older building stock may not be retrofitted.

   (2) Secondary hazards cannot be engineered against alone: Building design cannot prevent tsunamis (Tohoku's main killer), liquefaction, landslides, or fires. Japan also built tsunami walls, established the Jishintsu system of earthquake early warnings, and maintains underground cisterns for fire-fighting — building codes are only one layer.

   (3) Land use planning is equally critical: Building earthquake-resistant structures on active fault lines or unstable ground (liquefaction-prone reclaimed land) reduces but does not eliminate risk. Restricting development near fault zones and on poor ground is also necessary.

   (4) Emergency preparedness: Even if buildings survive, communities need evacuation plans, search-and-rescue training, stockpiled emergency supplies, and international aid coordination frameworks (Japan uses J-ALERT for rapid public warnings).

Question

Step-by-step solution

1. Step 1

   Why people live near volcanoes:

   (1) Fertile soils: Volcanic ash weathers rapidly to form nutrient-rich soils (andosols). The slopes of Vesuvius, Java's volcanoes, and Mount Etna support intensive agriculture — producing grapes, citrus, and vegetables of exceptional quality. In densely populated agrarian societies (e.g. Java, Indonesia), these productive soils support far more people per km² than surrounding lowland areas.

   (2) Economic opportunities: Geothermal energy (volcanic heat used to generate electricity and provide heating — Iceland meets 25% of electricity needs from geothermal; Kenya's Olkaria geothermal field provides ~45% of national electricity); tourism (volcano tourism generates significant income — Vesuvius and Etna attract millions of visitors per year); mining of volcanic minerals (sulphur, pumice, basalt building materials).

   (3) Low perceived risk: Volcanic eruptions occur on timescales of decades to centuries — far beyond human experience of immediate threat. Many communities have lived near volcanoes for generations without experiencing a major eruption. Risk perception is low when the last eruption was before living memory.

   (4) Lack of alternatives: In many LICs, fertile volcanic land is the only economically viable option. Relocation may mean moving to less productive or urban informal settlements with worse livelihoods.

2. Step 2

   Arguments FOR relocation:

   (1) Safety from catastrophic eruption: A composite volcano can produce pyroclastic flows, lahars, and ash falls with little warning. The last eruption 40 years ago was within a human lifetime — another eruption is plausible. Permanent relocation removes the risk of mass casualties.

   (2) Lesson from Nevado del Ruiz (Colombia, 1985): A moderate eruption melted glacial ice, triggering lahars that buried the town of Armero 74 km from the volcano, killing approximately 23,000 people. Scientists had predicted the hazard and recommended evacuation, but officials delayed — the town was destroyed in minutes. Relocation before an eruption would have saved all 23,000 lives.

   (3) Long-term economic cost of disaster: Rebuilding after a major eruption is more expensive than managed relocation. Post-disaster aid, infrastructure reconstruction, agricultural recovery, and long-term health costs (respiratory disease from ash) may exceed relocation costs over 20 years.

3. Step 3

   Arguments AGAINST relocation:

   (1) Cost: Relocating 50,000 people requires building new housing, infrastructure, schools, hospitals, and businesses at an alternative site. This is extremely expensive — potentially billions of dollars — and may not be affordable for an LIC government.

   (2) Livelihoods and food security: If the community depends on farming volcanic soils, relocation to less fertile land may cause poverty and food insecurity. Farmers may refuse to move if their livelihoods cannot be replicated elsewhere.

   (3) Cultural and social ties: Long-established communities have cultural, religious and ancestral connections to their land. Forced relocation can cause social breakdown, loss of identity, and mental health impacts — as seen in resettlement programmes for dam-affected communities.

   (4) Uncertain timing of eruption: The volcano has not erupted for 40 years and may remain dormant for decades more. Relocation at this moment may be politically and socially unjustifiable given that monitoring can provide warning time.

   (5) Compromise strategy: Rather than full relocation, the government could invest in: improved monitoring systems, community evacuation drills, lahar-resistant land use zoning (no building in lahar channels), and evacuation route improvements — reducing risk without destroying livelihoods.