Factors affecting severity of earthquakes: – focus and epicentre

9.1 Earthquake and Volcanic Hazards – Factors Affecting the Severity of Earthquakes

9.1.1 Learning Objective

Students will be able to:

• Explain how the focus (hypocentre) and the epicentre of an earthquake, together with depth, distance and local conditions, control the intensity of ground shaking and the resulting impacts.
• Describe the global distribution of earthquakes and volcanoes and the tectonic processes that generate them.
• Identify primary and secondary hazards of both earthquakes and volcanoes and evaluate how physical and human factors influence vulnerability.
• Assess risk‑management strategies, using quantitative and qualitative criteria, and evaluate their effectiveness with real‑world examples.

9.1.2 Global Distribution of Earthquakes and Volcanoes

• More than 90 % of the world’s earthquakes occur along plate boundaries; subduction zones alone account for ~70 % of the total seismic moment release.
• Divergent boundaries – mid‑ocean ridges and continental rifts (e.g., East African Rift) produce shallow (<20 km) normal‑fault earthquakes and basaltic fissure eruptions.
• Convergent (subduction) boundaries – oceanic plate sinks beneath another plate, generating the deepest (0–700 km) and most powerful earthquakes and the majority of stratovolcanoes (e.g., Japan, Andes). The depth is controlled by slab‑pull forces and the mantle‑wedge dynamics that concentrate stress at the plate interface.
• Transform boundaries – lateral slip along strike‑slip faults (e.g., San Andreas) produces shallow (<25 km) earthquakes; volcanic activity is rare.
• Intraplate settings – less common but can be significant (e.g., New Madrid, USA; intraplate volcanism in the East African Rift).

9.1.3 Tectonic Processes that Generate Earthquakes

9.1.4 Earthquake Hazards & Impacts

• Primary hazards – ground shaking, surface rupture, fault displacement.
• Ground‑motion parameters
  • Peak Ground Acceleration (PGA) – maximum acceleration of the shaking; closely linked to structural damage.
  • Peak Ground Velocity (PGV) – controls landslide and liquefaction potential.
  • Duration of strong shaking – longer durations increase cumulative damage.
• Secondary hazards
  • Liquefaction – loss of strength in saturated, loose sediments.
  • Landslides & rockfalls – triggered on steep slopes, especially where shaking is strong and the focus is shallow.
  • Tsunamis – generated by vertical displacement of the seafloor on sub‑marine thrust faults.
  • Aftershocks – can cause further damage to already weakened structures.
• Socio‑economic impacts
  • Immediate loss of life and injuries.
  • Disruption to essential services (water, electricity, transport).
  • Long‑term displacement of populations and loss of housing.
  • Economic loss measured in billions of dollars (e.g., US $200 bn for the 2011 Tōhoku quake).

9.1.5 Volcanic Hazards (linking processes to hazards)

• Eruption styles
  • Effusive – low‑viscosity basaltic magma flows out as lava; limited explosive energy, primary hazard is lava flow inundation.
  • Explosive – high‑viscosity, gas‑rich magma fragments violently, producing an eruption column. The rapid expansion of gas creates:
    • Pyroclastic flows – dense, hot mixtures that travel down slopes under gravity.
    • Volcanic ash clouds – fine particles that can travel thousands of kilometres, causing roof collapse, respiratory problems and damage to machinery.
    • Volcanic gases (SO₂, CO₂) – released from magma degassing; SO₂ forms acid rain, CO₂ can accumulate in low‑lying areas and suffocate.
• Secondary volcanic hazards
  • Lahars – volcanic mudflows generated when ash mixes with rain or snow, travelling along river valleys.
  • Sector collapses – flank failure can displace large volumes of water, generating tsunamis (e.g., 1883 Krakatoa).
  • Volcano‑tectonic earthquakes – stress changes from magma movement can trigger local seismicity.

9.1.6 Physical & Human Factors of Vulnerability

• Physical factors
  • Local geology – soft sediments amplify shaking (e.g., Mexico City sits on lake‑bed clays, leading to extreme damage in 1985).
  • Topography – steep slopes increase landslide risk (e.g., Nepal’s Himalayan foothills).
  • Coastal setting – exposure to tsunami waves.
• Human factors
  • Population density and settlement patterns – densely built‑up areas suffer higher casualties.
  • Construction quality – unreinforced masonry is vulnerable; seismic‑resistant design reduces damage.
  • Socio‑economic status – wealthier societies can afford retrofitting, early‑warning systems and rapid recovery.
  • Land‑use planning – zoning away from flood‑plains, liquefaction‑prone soils and steep slopes reduces exposure.

9.1.7 Risk Assessment & Management Strategies (with evaluation)

• Prediction & monitoring
  • Seismograph networks, GPS crustal‑deformation monitoring, and tsunami buoys provide real‑time data.
  • Volcanic monitoring – seismicity, gas emissions, satellite thermal imagery, and deformation measurements.
• Mitigation measures
  • Seismic building codes (e.g., Japan’s 1995 code, Chile’s post‑2010 code). Evaluation: Reduction in casualty rates from ~6 % in 1960 to <1 % in recent events indicates high effectiveness.
  • Land‑use zoning – exclusion of critical infrastructure from high‑risk zones; success measured by the limited damage to new developments in low‑risk areas.
  • Early‑warning systems – EEW (Earthquake Early Warning) in Japan and Mexico; performance judged by the seconds of warning gained and the corresponding reduction in injuries.
  • Public education & regular drills – community preparedness scores improve after each drill; post‑event surveys show higher compliance with safety procedures.
• Effectiveness criteria
  • Casualty reduction (deaths/injuries per unit magnitude).
  • Economic loss per unit GDP.
  • Speed of recovery (time to restore essential services).
  • Compliance rates with building codes and evacuation orders.

9.1.8 Key Geometrical Concepts

• Focus (hypocentre) – point within the Earth where rupture initiates.
• Epicentre – point on the surface directly above the focus.
• Depth of focus – vertical distance from the surface to the focus; controls attenuation of seismic energy.
• Horizontal distance from epicentre – determines the intensity of shaking at a specific location.
• Magnitude vs. intensity – magnitude (e.g., $M_w$) quantifies total energy released; intensity (e.g., Modified Mercalli Scale, MMI) describes observed effects at a site.

9.1.9 How Focus Depth Affects Severity

Seismic waves lose energy as they travel through rock; the deeper the focus, the more material they must traverse, reducing surface amplitudes.

• Shallow focus (< 70 km) – highest surface amplitudes; most destructive.
• Intermediate focus (70–300 km) – moderate surface effects; can be felt over a wider area.
• Deep focus (> 300 km) – low surface intensity but felt over very large distances.

9.1.10 How Distance from the Epicentre Affects Severity

Ground‑motion intensity declines with distance, approximately following an exponential attenuation relationship:

\( I = I_{0}\,e^{-k d} \)

• \(I\) – intensity at distance \(d\)
• \(I_{0}\) – intensity at the epicentre
• \(k\) – attenuation coefficient (depends on geology, wave frequency and focus depth)

Interpretation: Roughly, intensity drops by about 10 % for every 10 km away from the epicentre on hard rock, but the reduction is slower on soft sediments.

9.1.11 Combined Effect of Depth and Distance

9.1.12 Secondary Earthquake Hazards

• Liquefaction – occurs in saturated, loose sediments; ground behaves like a fluid, causing settlement and loss of bearing capacity.
• Landslides & rockfalls – triggered on steep slopes when shaking exceeds shear strength.
• Tsunamis – generated by vertical seafloor displacement on sub‑marine thrust faults (e.g., 2004 Sumatra‑Andaman).
• Aftershocks – smaller quakes that can cause additional damage to already weakened structures.

9.1.13 Detailed Case Studies (one earthquake, one volcanic event)

1. 2015 Gorkha (Nepal) – Shallow‑focus earthquake (M_w 7.8, depth ≈ 15 km)
   • Prediction & monitoring: Limited pre‑event warning; regional seismograph network detected foreshocks but no clear imminent‑event signal.
   • Response: Immediate national emergency declared; international aid arrived within 48 h; evacuation of landslide‑prone villages.
   • Evaluation (using effectiveness criteria):
     • Casualties ≈ 9 000 – high, reflecting vulnerable unreinforced masonry.
     • Economic loss ≈ US $5 bn (≈ 10 % of GDP) – substantial.
     • Recovery of essential services (electricity, water) took ~3 months; reconstruction of schools took >2 years.
     • Post‑event reforms: stricter building codes and a new national seismic‑hazard map; early‑warning pilot tests began in 2017.
2. 1991 Mount Pinatubo (Philippines) – Explosive Plinian eruption
   • Process–hazard link: High‑viscosity, gas‑rich magma fragmented violently, producing a 25 km high eruption column that generated pyroclastic flows and widespread ash fall. The collapse of the eruption column caused a sector collapse, displacing ~10 km³ of rock and generating a local tsunami.
   • Monitoring & prediction: Increased seismicity, rapid ground deformation (InSAR) and rising SO₂ emissions were detected months before the eruption.
   • Response: A phased evacuation of ~200 000 people was carried out based on the volcano‑alert level system; evacuation routes and shelters were pre‑identified.
   • Evaluation:
     • Deaths reduced from an estimated 5 000–10 000 (without evacuation) to < 100 – 200 actual fatalities.
     • Economic loss from ash‑fall damage to agriculture estimated at US $500 M; however, the successful evacuation saved lives and reduced medical costs.
     • Long‑term benefit: the eruption injected ~20 Mt of SO₂ into the stratosphere, causing a measurable global cooling of ~0.5 °C for 2 years – a reminder of the far‑reaching impacts of volcanic activity.

9.1.14 Implications for Hazard Assessment

• Analyse historic depth‑distribution of regional faults to estimate likely shaking intensities for future events.
• Map population density, critical infrastructure and vulnerable building stock relative to probable epicentral zones.
• Identify geological amplifiers (soft sediments, alluvial plains) and incorporate them into intensity‑attenuation models.
• Assess secondary‑hazard potential (landslides, liquefaction, tsunamis, lahars) using depth‑distance relationships and terrain analysis.
• Integrate seismic and volcanic monitoring data into a unified risk‑management framework that feeds directly into early‑warning and land‑use planning.

9.1.15 Summary

• Focus depth – shallow foci generate the strongest surface shaking; deeper foci produce lower intensities but can be felt over larger areas.
• Distance from the epicentre – intensity declines exponentially with distance; local geology can modify the rate of attenuation.
• Primary and secondary earthquake hazards (ground shaking, liquefaction, landslides, tsunamis) and volcanic hazards (pyroclastic flows, ash fall, lahars, tsunamis) are all governed by the same geometric relationships.
• Physical (geology, topography) and human (construction, population, socio‑economic status) factors determine vulnerability.
• Effective risk assessment combines knowledge of depth‑distance effects with robust monitoring, stringent building codes, strategic land‑use planning and community education; success is measured by reduced casualties, lower economic loss and faster recovery.