⛏️Intro to Geology Unit 10 – Earthquakes and Seismic Hazards

What's Shaking? Earthquake Basics

• Earthquakes are sudden, rapid shaking of the Earth's surface caused by the release of energy in the Earth's lithosphere
• This energy release is typically associated with the sudden movement of tectonic plates along fault lines
• The point underground where an earthquake starts is called the hypocenter (focus), while the location directly above it on the Earth's surface is the epicenter
• Earthquakes can cause ground shaking, landslides, liquefaction, and in some cases, tsunamis
• The majority of earthquakes occur along plate boundaries, but they can also happen within plates (intraplate earthquakes)
• Foreshocks are smaller earthquakes that precede the main shock, while aftershocks are smaller earthquakes that follow the main shock
• The size of an earthquake depends on factors such as the amount of energy released, the depth of the hypocenter, and the type of fault movement

Earth's Layers and Plate Tectonics

• The Earth is composed of four main layers: crust, mantle, outer core, and inner core
• The crust and the uppermost part of the mantle form the lithosphere, which is broken into several tectonic plates
• Tectonic plates are constantly moving due to convection currents in the mantle, driven by heat from the Earth's interior
• There are three main types of plate boundaries: divergent (plates move apart), convergent (plates collide or subduct), and transform (plates slide past each other)
  • Divergent boundaries often form mid-ocean ridges and rift valleys (East African Rift)
  • Convergent boundaries can create subduction zones, deep-sea trenches, and mountain ranges (Andes, Himalayas)
  • Transform boundaries are characterized by shallow earthquakes and lateral motion (San Andreas Fault)
• The theory of plate tectonics explains the movement and interaction of Earth's lithospheric plates
• Plate tectonics is responsible for the formation and distribution of landforms, earthquakes, volcanic activity, and other geological phenomena

Fault Types and Seismic Waves

• Faults are fractures in the Earth's crust along which movement occurs during an earthquake
• There are three main types of faults: normal (extensional stress), reverse (compressional stress), and strike-slip (shear stress)
  • Normal faults occur when the hanging wall moves down relative to the footwall
  • Reverse faults happen when the hanging wall moves up relative to the footwall
  • Strike-slip faults involve the horizontal movement of rock on either side of the fault
• Oblique-slip faults exhibit a combination of strike-slip and dip-slip (normal or reverse) motion
• Seismic waves are energy waves generated by earthquakes that travel through the Earth
• There are two main types of seismic waves: body waves (P-waves and S-waves) and surface waves (Love waves and Rayleigh waves)
  • P-waves (primary waves) are compressional waves that travel fastest and can move through solids, liquids, and gases
  • S-waves (secondary waves) are shear waves that travel slower than P-waves and can only move through solids
  • Surface waves travel along the Earth's surface and are responsible for most of the damage during an earthquake

Measuring the Rumble: Earthquake Scales

• Earthquake magnitude is a measure of the energy released during an earthquake
• The Richter scale, developed by Charles Richter in 1935, is a logarithmic scale that measures the amplitude of the largest seismic wave recorded by a seismograph
  • Each increase of one unit on the Richter scale represents a tenfold increase in wave amplitude and a 32-fold increase in energy released
• The moment magnitude scale (Mw) is a more accurate scale that measures the seismic moment, which is proportional to the energy released by an earthquake
• Earthquake intensity is a measure of the effects of an earthquake on people, structures, and the environment at a specific location
• The Modified Mercalli Intensity (MMI) scale is a 12-level scale that describes the intensity of an earthquake based on observed effects
  • The MMI scale ranges from I (not felt) to XII (total destruction)
• Seismographs are instruments used to detect and record seismic waves, which help determine the location, magnitude, and intensity of earthquakes

Earthquake Hotspots and Prediction

• Earthquake hotspots are regions with a high frequency of seismic activity, often located along plate boundaries or near active faults
• Some notable earthquake hotspots include the "Ring of Fire" (circum-Pacific belt), the Alpide belt (Mediterranean to Southeast Asia), and the Mid-Atlantic Ridge
• Intraplate earthquakes can occur in areas far from plate boundaries due to localized stresses in the Earth's crust (New Madrid Seismic Zone)
• Earthquake prediction involves estimating the time, location, and magnitude of future earthquakes
• Short-term earthquake prediction is currently not possible due to the complex nature of the Earth's crust and the many factors that influence seismic activity
• Long-term forecasting uses historical data, geologic evidence, and probabilistic models to estimate the likelihood of earthquakes in a given region over a specified time period
• Seismic hazard maps show the probability of ground shaking of a certain intensity in a given area over a specific time frame, helping to guide building codes and emergency preparedness efforts

When the Ground Moves: Seismic Hazards

• Ground shaking is the primary seismic hazard caused by the passage of seismic waves through the Earth's surface
• The intensity of ground shaking depends on factors such as the earthquake's magnitude, distance from the epicenter, local geology, and soil conditions
• Liquefaction occurs when water-saturated sediments temporarily lose strength and behave like a liquid due to strong ground shaking
  • Liquefaction can cause buildings to sink, tilt, or collapse, and can lead to the formation of sand boils and lateral spreading
• Landslides can be triggered by earthquakes, particularly in areas with steep slopes, unstable soils, or heavy rainfall
• Tsunamis are large ocean waves generated by the displacement of water during underwater earthquakes, landslides, or volcanic eruptions
  • Tsunamis can travel at speeds up to 500 mph (800 km/h) in the open ocean and can cause significant damage and loss of life in coastal areas
• Fires can break out following earthquakes due to ruptured gas lines, damaged electrical systems, or overturned appliances
• Seismic hazards can cause significant damage to buildings, infrastructure, and critical facilities, emphasizing the importance of seismic-resistant design and construction practices

Staying Safe: Earthquake Preparedness

• Earthquake preparedness involves taking steps to minimize the impact of seismic hazards on individuals, communities, and infrastructure
• Developing an emergency plan and practicing earthquake drills can help people respond quickly and safely during an earthquake
• Creating an emergency kit with essential supplies (water, food, first-aid, flashlight, radio) can help sustain individuals and families in the aftermath of an earthquake
• Securing furniture, appliances, and other potential hazards can reduce the risk of injury and property damage during ground shaking
• Retrofitting older buildings and constructing new ones according to seismic-resistant design standards can minimize structural damage and collapse
• Participating in community preparedness efforts, such as awareness campaigns and training programs, can enhance resilience and recovery capabilities
• Understanding the local seismic hazard, including the location of active faults, historical seismicity, and potential secondary hazards (liquefaction, landslides) is crucial for effective preparedness
• Regularly reviewing and updating emergency plans, supplies, and communication strategies can ensure that preparedness measures remain current and effective

Real-World Quakes: Case Studies

• The 1906 San Francisco earthquake (Mw 7.8) resulted in widespread destruction, fires, and over 3,000 deaths, highlighting the need for improved building codes and emergency response
• The 1960 Chile earthquake (Mw 9.5), the largest recorded earthquake in history, triggered tsunamis that affected various locations along the Chilean coast and caused damage as far away as Hawaii, Japan, and the Philippines
• The 1964 Alaska earthquake (Mw 9.2) caused significant ground shaking, liquefaction, and tsunamis, demonstrating the far-reaching effects of large subduction zone earthquakes
• The 1976 Tangshan earthquake (Mw 7.6) in China resulted in the loss of over 240,000 lives, making it one of the deadliest earthquakes in recorded history
• The 1995 Kobe earthquake (Mw 6.9) in Japan exposed vulnerabilities in the country's infrastructure and emergency response, leading to significant improvements in seismic-resistant design and disaster management
• The 2004 Sumatra-Andaman earthquake (Mw 9.1) and subsequent Indian Ocean tsunami caused widespread destruction and over 227,000 deaths across 14 countries, emphasizing the need for improved tsunami warning systems and coastal preparedness
• The 2010 Haiti earthquake (Mw 7.0) resulted in over 200,000 deaths and significant damage to buildings and infrastructure, highlighting the vulnerability of developing nations to seismic hazards
• The 2011 Tohoku earthquake (Mw 9.0) and tsunami in Japan led to the Fukushima Daiichi nuclear disaster, underscoring the importance of considering multiple hazards in risk assessment and emergency planning