13.4 Earthquakes and their impact on landscapes

Earthquakes reshape landscapes in seconds, triggering landslides, altering rivers, and shifting coastlines. These sudden changes kickstart a domino effect of geomorphic responses, influencing long-term landscape evolution and sediment dynamics.

Tectonic stress builds up along fault lines until it's suddenly released, causing earthquakes. This process occurs at plate boundaries and within plates, with each type of boundary producing characteristic earthquake patterns and landscape impacts.

Earthquake Generation Mechanisms

Tectonic Stress and Elastic Rebound

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• Earthquakes result from sudden release of accumulated stress along fault lines in Earth's crust generating seismic waves
• Elastic rebound theory explains energy storage in rocks during elastic deformation under tectonic stress
  • Energy releases when rock strength exceeds causing sudden movement
• Focal mechanism describes fault plane orientation and slip direction crucial for understanding tectonic forces
• Earthquake magnitude scales quantify energy released (Richter, moment magnitude)
• Intensity scales measure observed surface effects (Modified Mercalli)

Plate Boundary Earthquakes

• Plate boundaries serve as primary locations for earthquake generation
• Convergent boundaries produce characteristic earthquake patterns
  • Deep-focus earthquakes occur in subduction zones
  • Shallow earthquakes happen in collision zones
• Divergent boundaries create shallow, low to moderate magnitude earthquakes
  • Associated with rifting and seafloor spreading processes
• Transform boundaries generate shallow, high-magnitude strike-slip earthquakes
  • San Andreas Fault exemplifies this type of boundary

Intraplate Seismicity

• Intraplate earthquakes occur within tectonic plates
• Often result from reactivation of ancient faults
• Can be caused by stress transfer from plate boundaries
• Generally less frequent but can be highly destructive (New Madrid Seismic Zone)

Earthquake Geomorphic Effects

Mass Movements and Ground Deformation

• Seismic shaking triggers various types of mass movements
  • Rock falls, debris flows, and large-scale landslides occur especially in steep or unstable areas
• Surface rupture along fault lines creates scarps and offsets streams
  • Alters local topography forming new cliffs or modifying existing landforms
• Earthquakes induce regional uplift or subsidence
  • Changes coastlines, drainage patterns, and local base levels for erosion
• Seismic waves cause soil compaction leading to localized subsidence
  • Results in formation of ground cracks or fissures

Liquefaction and Hydrological Impacts

• Liquefaction occurs in water-saturated, unconsolidated sediments
  • Ground behaves like liquid potentially causing building collapse and ground failure
• Tsunamis generated by undersea earthquakes cause extensive coastal changes
  • Alters shoreline morphology through erosion, sediment transport, and deposition
• Secondary effects include dam failures and groundwater system changes
  • Can lead to flooding or creation of new water bodies further modifying landscape

Earthquakes and Landscape Change

Immediate Geomorphic Responses

• Earthquakes catalyze rapid geomorphic change accomplishing in seconds what normally takes years
• Coseismic landslides instantly reshape hillslopes
  • Fill valleys with debris and alter local relief and drainage patterns
• Sudden creation of landslide dams impounds rivers
  • Forms lakes and alters longitudinal profiles of stream systems
• Earthquake-induced coastal uplift or subsidence creates new marine terraces or submerges coastal plains
  • Initiates new cycles of erosion or deposition

Long-term Landscape Evolution

• Seismic events trigger cascades of geomorphic responses
  • Initial changes set in motion series of adjustments throughout landscape system
• Frequency and magnitude of earthquakes influence long-term topography evolution
  • Affects balance between tectonic uplift and erosional processes
• Post-seismic relaxation and isostatic adjustment continue modifying landscapes
  • Effects can persist for years to decades after initial event

Earthquake-Induced Landslides and Landscape Evolution

Sediment Dynamics and River System Impacts

• Earthquake-triggered landslides serve as major sediment sources for river systems
  • Often overwhelm normal sediment transport capacities
• Sudden sediment influx leads to aggradation in river channels
  • Alters stream gradients and flow patterns
• Landslide dams significantly impact river longitudinal profiles
  • Create local base levels influencing upstream and downstream geomorphology
• Breaching of landslide dams results in catastrophic flooding and rapid incision
  • Reshapes valley morphology and floodplains

Long-term Geomorphic Consequences

• Increased sediment loads from earthquake-induced landslides persist for decades
  • Influences long-term rates of landscape denudation and sediment flux to basins
• Spatial distribution of earthquake-induced landslides affects pattern of landscape response
  • Some catchments experience more significant changes than others (Wenchuan earthquake, China)
• Interaction between earthquake-induced sediment supply and climate-driven hydrological processes
  • Leads to complex, non-linear responses in landscape evolution over time (Taiwan orogen)