🌋Physical Geology Unit 10 – Earthquakes and Seismic Hazards

What's Shaking? Intro to Earthquakes

• Earthquakes are sudden, rapid shaking of the ground caused by the shifting of rocks deep underneath the earth's surface
• Occur when stress builds up in rocks until they break or slip past each other
• Release energy in the form of seismic waves that travel through the earth and cause the shaking we feel
• Can cause significant damage to buildings, roads, and other structures (bridges, dams, pipelines)
• Often associated with other geological hazards like landslides, liquefaction, and tsunamis
• Most earthquakes occur along the boundaries of Earth's tectonic plates where there is a lot of stress and movement
• Earthquakes can also occur within plates, along faults or zones of weakness in the crust
• Vary in size from small tremors that are barely noticeable to massive events that can level entire cities (San Francisco 1906, Haiti 2010)

Earth's Layers and Plate Tectonics

• Earth is composed of several layers: crust, mantle, outer core, and inner core
• Crust is the thin, rocky outer layer that we live on
  • Oceanic crust is thinner (~5-10 km) and denser, made of basalt
  • Continental crust is thicker (~30-50 km) and less dense, made of granite
• Mantle is the thick, hot, dense layer beneath the crust
  • Upper mantle is solid rock
  • Lower mantle is slowly flowing, viscous rock
• Outer core is liquid iron and nickel
• Inner core is solid iron and nickel
• Lithosphere includes the crust and uppermost mantle and is broken into several large tectonic plates
• Asthenosphere is the portion of the upper mantle that flows and deforms over long periods of time
• Tectonic plates move and interact at their boundaries, driven by convection currents in the mantle
• Three main types of plate boundaries: divergent (plates move apart), convergent (plates collide or one subducts under the other), and transform (plates slide past each other)

Fault Types and Earthquake Mechanics

• Faults are fractures or zones of fractures between two blocks of rock
• Faults allow blocks to move relative to each other, either vertically or horizontally
• Three main types of faults: normal (extensional stress), reverse (compressional stress), and strike-slip (shearing stress)
  • Normal faults occur where crust is being pulled apart, hanging wall moves down relative to footwall
  • Reverse faults occur where crust is being compressed, hanging wall moves up relative to footwall
  • Strike-slip faults occur where crust is sliding past each other horizontally, classified as left-lateral or right-lateral
• Earthquakes occur on faults when the stress exceeds the strength of the rocks, causing sudden slip
• The point on the fault where slip begins is called the focus or hypocenter
• The point on Earth's surface directly above the focus is called the epicenter
• Faults can be small and only generate minor earthquakes, or can be large systems that generate massive earthquakes (San Andreas Fault)

Measuring the Rumble: Earthquake Scales

• Earthquakes are measured in terms of magnitude and intensity
• Magnitude is the amount of energy released by the earthquake, measured on a logarithmic scale
  • Each one-point increase in magnitude corresponds to a 10-fold increase in ground motion and ~32 times more energy
• Richter scale was the first magnitude scale, developed in the 1930s, measures earthquakes up to M7
• Moment magnitude scale is used for larger earthquakes, takes into account the area of the fault that ruptured and the amount of slip
  • Largest recorded earthquake was M9.5 (Chile, 1960)
• Intensity is a measure of the strength of shaking at a particular location, depends on distance from epicenter and local geology
• Modified Mercalli Intensity Scale is a qualitative measure that ranges from I (not felt) to XII (total destruction)
  • Based on observations of damage and human perception of shaking
• Seismometers are instruments that measure ground motion and are used to locate and measure earthquakes
  • Seismograms are the recordings of earthquake waves from seismometers

Seismic Waves and Their Effects

• Seismic waves are the energy from an earthquake that travels through the Earth
• Two main types of seismic waves: body waves (travel through the interior of the Earth) and surface waves (travel along the Earth's surface)
  • Body waves include P-waves (primary, compressional) and S-waves (secondary, shear)
  • Surface waves include Love waves (horizontal motion) and Rayleigh waves (rolling motion)
• P-waves are the fastest seismic waves and the first to arrive at a seismic station
  • Travel through solids, liquids, and gases
  • Cause rocks to compress and expand in the direction the wave is traveling
• S-waves are slower than P-waves and arrive second at a seismic station
  • Can only travel through solids
  • Cause rocks to move perpendicular to the direction of wave travel
• Surface waves are the slowest seismic waves but cause the most damage
  • Confined to the outer layers of the Earth
  • Cause the ground to move in elliptical or side-to-side patterns
• Seismic waves can be reflected, refracted, or attenuated as they travel through the Earth
  • Provide valuable information about the Earth's interior structure and composition

Earthquake Hotspots and Risk Zones

• Most earthquakes occur in specific zones around the world, primarily at plate boundaries
• The "Ring of Fire" is a zone of frequent earthquakes and volcanic eruptions that encircles the Pacific Ocean
  • Includes subduction zones (Cascadia, Aleutian, Japan) and transform boundaries (San Andreas)
• Other major earthquake zones include the Alpide Belt (Mediterranean, Middle East, South Asia) and the Mid-Atlantic Ridge
• Intraplate earthquakes can also occur far from plate boundaries (New Madrid Seismic Zone, USA)
• Seismic hazard maps show the probability of damaging earthquake shaking in a given area over a certain time period
  • Based on past earthquake locations, geological data, and fault slip rates
• Seismic risk takes into account the hazard level as well as the vulnerability of the built environment and population
• Urban areas in high hazard zones are at the greatest risk for catastrophic damage and loss of life (Tokyo, Los Angeles, Istanbul)
• Developing countries are particularly vulnerable due to poor building construction and lack of resources for mitigation and response (Haiti, Nepal)

Predicting and Preparing for Quakes

• Short-term earthquake prediction (days to weeks in advance) is not currently possible due to the complex nature of earthquake processes
• Long-term forecasting is possible in some areas based on past earthquake patterns and geologic evidence
  • Estimates the probability of an earthquake of a certain magnitude occurring in a specific area over a certain time period (usually decades)
• Earthquake early warning systems can provide a few seconds to minutes of warning before strong shaking arrives
  • Based on the detection of P-waves from a quake that has already begun
  • Can be used to automatically shut down critical systems, stop trains, and alert the public to take cover
• Preparing for earthquakes involves a combination of personal preparedness, building codes and retrofitting, and emergency response planning
• Individuals should have emergency kits, communication plans, and know how to "drop, cover, and hold on" during shaking
• Buildings can be designed or retrofitted to withstand strong shaking (base isolation, reinforced concrete, steel moment frames)
• Cities and regions should have emergency response plans, including search and rescue, medical care, and temporary shelter
• Earthquake insurance can help individuals and businesses recover financially from damage

Seismic Hazards Beyond the Shake

• Earthquakes can trigger a variety of secondary hazards that can be just as damaging as the shaking itself
• Landslides and rockfalls can be triggered by the shaking, particularly in steep, mountainous areas
  • Landslide dams can form in narrow valleys, potentially causing catastrophic flooding if they fail
• Liquefaction occurs when water-saturated sediments temporarily lose strength and behave like a liquid
  • Can cause buildings to sink or tilt, roads to buckle, and underground pipes to float to the surface
• Tsunamis are large ocean waves generated by the displacement of water from an undersea earthquake or landslide
  • Can travel across entire ocean basins and cause devastating damage and loss of life along coastlines (Indian Ocean 2004, Japan 2011)
• Ground rupture can occur along the fault, damaging buildings and infrastructure that cross the fault
  • Offset streams, roads, and fences are common indicators of past fault rupture
• Fires can be triggered by broken gas lines, downed electrical wires, or overturned stoves and heaters
  • Urban conflagrations after major quakes have destroyed large parts of cities (San Francisco 1906, Tokyo 1923)
• Flooding can result from dam or levee failures, tsunamis, or changes in ground elevation affecting drainage patterns