🤙🏼Earthquake Engineering Unit 1 – Earthquake Engineering: Intro to Seismology

Key Concepts and Terminology

• Seismology studies the generation, propagation, and recording of seismic waves in the Earth and their effects on structures
• Earthquakes occur when stored elastic strain energy is suddenly released, usually along a fault plane
• Fault refers to a planar fracture or discontinuity in a volume of rock across which there has been significant displacement
  • Types of faults include strike-slip, normal, and reverse (thrust) faults
• Seismic waves are elastic waves generated by an abrupt release of energy, such as an earthquake or explosion
• Epicenter is the point on the Earth's surface vertically above the hypocenter (focus) where an earthquake or underground explosion originates
• Magnitude measures the energy released at the source of the earthquake, commonly expressed using the Moment Magnitude scale ($M_w$)
• Intensity measures the strength of shaking produced by the earthquake at a certain location, often expressed using the Modified Mercalli Intensity (MMI) scale

Earth's Structure and Plate Tectonics

• Earth's interior consists of the crust, mantle, outer core, and inner core, each with distinct properties and compositions
• Lithosphere, composed of the crust and uppermost mantle, is rigid and broken into tectonic plates that move relative to each other
• Asthenosphere, the portion of the upper mantle beneath the lithosphere, is ductile and allows for the movement of tectonic plates
• Plate boundaries are classified as divergent (plates move apart), convergent (plates collide or subduct), and transform (plates slide past each other)
  • Divergent boundaries are associated with seafloor spreading and rift valleys (East African Rift)
  • Convergent boundaries can result in subduction zones, oceanic trenches, and mountain building (Andes Mountains)
  • Transform boundaries are characterized by lateral motion and often produce large, shallow earthquakes (San Andreas Fault)
• Convection currents in the mantle, driven by heat from the Earth's core, are the primary driving force for plate motions
• Earthquakes and volcanic activity are concentrated along plate boundaries, where tectonic forces are most active

Types of Seismic Waves

• Body waves travel through the Earth's interior and include P-waves (primary or compressional waves) and S-waves (secondary or shear waves)
  • P-waves are the fastest seismic waves and can travel through solids, liquids, and gases
  • S-waves are slower than P-waves and can only travel through solids due to their shearing motion
• Surface waves travel along the Earth's surface and are typically the most destructive due to their large amplitudes and low frequencies
  • Rayleigh waves exhibit elliptical particle motion and travel along the free surface of the Earth
  • Love waves cause horizontal shearing of the ground and are confined to the Earth's surface
• Seismic wave velocities depend on the elastic properties and density of the medium they travel through
• Seismic waves can be reflected, refracted, or converted when they encounter boundaries between different materials or at the Earth's surface
• Shadow zones are areas on the Earth's surface where direct P- or S-waves are not detected due to the presence of the liquid outer core or sharp velocity contrasts

Earthquake Measurement and Scales

• Earthquake magnitude scales, such as the Moment Magnitude scale ($M_w$), measure the energy released at the source of an earthquake
  • $M_w$ is based on the seismic moment, which depends on the fault area, average slip, and rock rigidity
  • An increase of one unit in $M_w$ corresponds to a 32-fold increase in seismic moment
• Intensity scales, like the Modified Mercalli Intensity (MMI) scale, describe the effects of an earthquake on people, structures, and the environment at a specific location
  • MMI scale ranges from I (not felt) to XII (total destruction) and is based on qualitative observations
• Peak Ground Acceleration (PGA) measures the maximum ground acceleration at a site during an earthquake and is an important parameter for seismic design
• Seismic energy is proportional to the square of the seismic wave amplitude and is used to estimate the potential for damage
• Earthquake duration, frequency content, and site conditions also influence the potential for damage and are considered in seismic hazard assessments

Seismographs and Seismic Networks

• Seismographs are instruments that record ground motion during an earthquake, typically measuring displacement, velocity, or acceleration
  • Traditional seismographs consist of a mass suspended by a spring or pendulum, a damping device, and a recording system
  • Modern seismometers use electronic sensors (accelerometers or velocity transducers) to convert ground motion into electrical signals
• Seismic networks are arrays of seismographs distributed over a region to monitor and locate earthquakes
  • Global seismic networks, such as the Global Seismographic Network (GSN), provide worldwide coverage for detecting and studying earthquakes
  • Local and regional seismic networks offer higher-resolution data for smaller areas and are used for detailed seismic studies and earthquake monitoring
• Seismic data processing involves filtering, amplifying, and digitizing seismic signals to extract meaningful information about earthquakes and Earth's structure
• Seismic tomography uses seismic wave travel times to create 3D images of Earth's interior, revealing variations in seismic wave velocities and material properties

Earthquake Prediction and Forecasting

• Short-term earthquake prediction remains a challenge due to the complex nature of earthquake processes and the lack of reliable precursors
• Long-term forecasting estimates the probability of future earthquakes based on historical seismicity, geological data, and statistical models
  • Seismic gap theory suggests that segments of faults that have not experienced recent large earthquakes are more likely to rupture in the future
  • Recurrence interval estimates the average time between large earthquakes on a specific fault or region based on paleoseismic data and slip rates
• Precursory phenomena, such as foreshocks, changes in seismic wave velocities, or ground deformation, are studied as potential indicators of imminent earthquakes
• Earthquake early warning systems detect the initial P-waves of an earthquake and issue alerts before the damaging S-waves and surface waves arrive
  • These systems aim to provide seconds to minutes of warning for people to take protective actions and for automated systems to mitigate damage (shutting down gas lines, stopping trains)

Seismic Hazard Analysis

• Seismic hazard analysis assesses the probability of exceeding a certain level of ground motion or other seismic effects at a specific site or region
• Deterministic Seismic Hazard Analysis (DSHA) considers worst-case scenarios based on the maximum credible earthquake for a given fault or seismic source
• Probabilistic Seismic Hazard Analysis (PSHA) incorporates uncertainties in earthquake size, location, and frequency to estimate the probability of exceeding various ground motion levels
  • PSHA combines seismic source characterization, ground motion prediction equations, and probability calculations
• Seismic hazard maps display the expected level of ground motion or probability of exceedance for a given return period (e.g., 475 years for a 10% probability of exceedance in 50 years)
• Site-specific seismic hazard analyses consider local soil conditions, topography, and potential for ground failure (liquefaction, landslides) in addition to regional seismic hazard
• Time-dependent seismic hazard models incorporate the concept of elastic rebound and the time elapsed since the last major earthquake on a fault to refine probability estimates

Practical Applications in Engineering

• Seismic design codes and standards, such as the International Building Code (IBC), provide minimum requirements for the design and construction of structures to resist earthquake effects
• Seismic retrofitting involves strengthening existing structures to improve their resistance to earthquake forces, often focusing on older buildings or critical infrastructure
  • Techniques include adding shear walls, reinforcing columns and beams, and installing seismic isolation or energy dissipation devices
• Seismic isolation decouples a structure from the ground motion using flexible bearings or dampers, reducing the seismic forces transmitted to the building
• Energy dissipation devices, such as viscous dampers or yielding metal elements, absorb seismic energy and limit the forces and deformations in a structure
• Performance-based seismic design aims to achieve specific performance objectives (e.g., life safety, immediate occupancy) under different levels of earthquake intensity
• Seismic risk assessment combines seismic hazard analysis with vulnerability analysis of buildings and infrastructure to estimate potential losses and guide risk reduction strategies
• Seismic microzonation studies provide detailed maps of local seismic hazards, considering factors such as soil amplification, liquefaction susceptibility, and landslide potential, to inform land-use planning and emergency response