Earthquake Science Overview

Earthquake Energy Release

• Energy released by a magnitude 3 earthquake is 32 times greater than that of a magnitude 2 earthquake.

Earthquake Intensity

• Measured by the Modified Mercalli Intensity Scale, which is qualitative, assessing structural damage and human perception.
• Intensity maps illustrate regions of severe damage and shaking.
• Internet data collection enhances the creation of shake maps using seismographic data.

Fault Movements

• Earthquakes occur along faults, weaknesses in the Earth's crust where rocks break and displace.
• Types of faults include:
  • Strike-slip faults: Horizontal displacements (e.g., San Andreas Fault) evidenced by surface cracks.
  • Dip-slip faults:
    • Thrust or reverse faults: Hanging wall moves up relative to footwall.
    • Normal faults: Hanging wall moves down relative to footwall, occurring due to crustal stretching.
• Blind faults: Do not reach the surface, complicating earthquake risk assessment, often formed by mountain-building processes.

Fault Activity

• Active faults: Movement in the last 11,600 years.
• Potentially active faults: Movement in the past 2.6 million years.
• Inactive faults: No movement in the last 2.6 million years.
• Tectonic creep: Gradual fault movement that may not produce felt earthquakes, potentially damaging infrastructure.

Seismic Waves

• Seismic waves: Produced by sudden rock displacement; travel through or along Earth's surface.
• Types of body waves:
  • P-waves (Primary):
    • First to appear on seismographs, faster than S-waves.
    • Can traverse solids, liquids, and gases.
  • S-waves (Secondary):
    • Appear second on seismographs, slower with an up-and-down motion.
    • Can only travel through solids.
• Surface waves:
  • Travel along the Earth's surface, causing horizontal and vertical motion.
  • Responsible for significant damage near the epicenter, includes:
    • Love waves: Cause horizontal shaking.
    • Rayleigh waves: Produce rolling, elliptical motion.

Epicenter Location

• At least three seismic stations needed to identify the epicenter using triangulation.
• Focal depth affects shaking intensity due to wave attenuation.

Earthquake Cycle

• Consists of an inactive period with stress accumulation, followed by small earthquakes and potential foreshocks before a mainshock.
• Major stages include periods of inactivity, buildup of elastic strain, occurrence of foreshocks, and the main earthquake event with subsequent aftershocks.

Geographic Regions

• Approximately 80% of earthquakes occur within the circum-Pacific belt.

Effects of Earthquakes

• Primary effects: Shaking and surface rupture; ground rupture induces surface cracks, while shaking damages structures based on factors like depth and direction of the epicenter.
• Secondary effects: Include liquefaction, land level changes, landslides, fires, tsunamis, and diseases.

Earthquake Forecasting

• Earthquakes cannot be predicted with certainty; only forecasted.
• Indicators for forecasting include patterns of past earthquakes, surface deformations, and seismic gaps.

Animal Behavior

• Historical beliefs exist that animals predict earthquakes through changes in behavior, though evidence remains inconclusive.

Minimizing Earthquake Hazards

• Strategies include:
  • Use of seismic risk maps to assess future earthquake likelihood.
  • Community preparation through education, retrofitting buildings, and locating critical facilities in safe areas.
  • Nationwide economic impact, with $250 billion noted as the costliest natural disaster.

Tsunami Effects

• Primary effects include flooding and erosion, impacting coastal areas and causing deaths.
• Secondary effects involve fires, water contamination, and disease spread.

Tsunami Hazard Minimization

• Detection systems monitor earthquake zones with technologies like seismographs, tidal gauges, and buoy sensors.
• Alerts are communicated through satellite for timely public warnings.

Personal Safety During Tsunamis

• Immediate actions include evacuating low-lying areas upon strong earthquakes or ocean recession.
• Awareness of wave timing is essential, as intervals between waves can be significant.
• Preparedness includes education and community strategies to mitigate tsunami risks.