🌋Seismology Unit 1 – Introduction to Seismology and Earth Structure

Key Concepts and Terminology

• Seismology studies the propagation of seismic waves through the Earth to understand its internal structure and properties
• Earthquakes are the primary source of seismic waves, which are generated by the sudden release of stored elastic energy in the Earth's crust
• Body waves, including P-waves (primary or compressional) and S-waves (secondary or shear), travel through the Earth's interior
  • P-waves are longitudinal and can propagate through solids, liquids, and gases
  • S-waves are transverse and can only propagate through solids
• Surface waves, such as Rayleigh and Love waves, travel along the Earth's surface and are responsible for most of the damage during earthquakes
• Seismic velocity is the speed at which seismic waves propagate through a medium and depends on its elastic properties and density
• Seismic attenuation is the loss of energy as seismic waves travel through the Earth, caused by factors such as absorption, scattering, and geometrical spreading
• Seismic anisotropy refers to the directional dependence of seismic wave velocity in materials with oriented structures (crystal lattices or aligned cracks)

Earth's Internal Structure

• The Earth is divided into three main layers: crust, mantle, and core, each with distinct properties and compositions
• The crust is the outermost layer, ranging from 5-70 km in thickness, and is composed of lighter, silica-rich rocks (granitic continental crust and basaltic oceanic crust)
• The mantle extends from the base of the crust to a depth of about 2,900 km and is composed of denser, iron- and magnesium-rich rocks (peridotite)
  • The upper mantle includes the lithosphere (rigid plates) and the asthenosphere (ductile layer that allows plate motion)
  • The lower mantle is solid but can deform plastically over long time scales
• The core is the innermost layer, composed primarily of iron and nickel, and is divided into the liquid outer core and the solid inner core
  • The outer core is responsible for generating the Earth's magnetic field through convection and dynamo action
  • The inner core is solid due to the immense pressure at the center of the Earth
• Seismic discontinuities, such as the Moho (crust-mantle boundary) and the core-mantle boundary, mark abrupt changes in seismic wave velocities and are used to delineate the Earth's internal structure

Seismic Waves and Their Properties

• Seismic waves are elastic waves that propagate through the Earth, carrying information about its internal structure and properties
• Body waves travel through the Earth's interior and include P-waves (primary or compressional) and S-waves (secondary or shear)
  • P-waves are the fastest seismic waves and can propagate through solids, liquids, and gases
  • S-waves are slower than P-waves and can only propagate through solids, as fluids do not support shear stress
• Surface waves travel along the Earth's surface and are further classified as Rayleigh waves and Love waves
  • Rayleigh waves have a retrograde elliptical particle motion and can exist in both solid and liquid media
  • Love waves have a transverse horizontal particle motion and can only exist in a layered medium with a low-velocity layer overlying a high-velocity layer
• Seismic wave velocity depends on the elastic properties and density of the medium through which they propagate, as described by the equations: $V_P = \sqrt{(K + 4/3\mu) / \rho}$ and $V_S = \sqrt{\mu / \rho}$, where $V_P$ is P-wave velocity, $V_S$ is S-wave velocity, $K$ is bulk modulus, $\mu$ is shear modulus, and $\rho$ is density
• Seismic attenuation is the loss of energy as seismic waves propagate, caused by factors such as absorption (conversion of elastic energy to heat), scattering (redistribution of energy), and geometrical spreading (decrease in amplitude with distance from the source)
• Seismic anisotropy is the directional dependence of seismic wave velocity, which can be caused by intrinsic anisotropy (crystal lattice preferred orientation) or extrinsic anisotropy (aligned cracks or layering)

Seismometers and Data Collection

• Seismometers are instruments designed to measure and record ground motion caused by seismic waves
• Modern seismometers typically use a mass-spring system or a force-feedback system to convert ground motion into electrical signals
  • In a mass-spring system, a suspended mass moves relative to the ground, and its motion is converted into an electrical signal by a transducer (coil-magnet or capacitive)
  • In a force-feedback system, a servo motor keeps the mass stationary relative to the ground, and the current required to maintain this position is proportional to the ground acceleration
• Seismometers are characterized by their frequency response, sensitivity, dynamic range, and noise level
• Seismic networks consist of arrays of seismometers deployed at various locations to monitor seismic activity and collect data for analysis
  • Global seismic networks (Global Seismographic Network) provide worldwide coverage and are used to study large-scale Earth structure and global seismicity
  • Regional and local seismic networks provide higher-resolution data for specific areas of interest and are used to study smaller-scale structures and local seismicity
• Seismic data is typically recorded in a standard format (SEED or miniSEED) and includes information such as the seismometer location, instrument response, and time series of ground motion
• Seismic data quality control involves assessing and ensuring the accuracy, completeness, and consistency of the recorded data, as well as identifying and removing any artifacts or noise

Earthquake Mechanics and Faulting

• Earthquakes occur when the accumulated stress in the Earth's crust exceeds the strength of the rock, causing sudden slip along a fault plane
• Faults are fractures in the Earth's crust along which displacement occurs, and they are classified based on their geometry and sense of motion (normal, reverse, or strike-slip)
• The earthquake source can be represented by a point source (for small earthquakes) or a finite fault model (for larger earthquakes) that describes the spatial and temporal distribution of slip on the fault plane
• The seismic moment ($M_0$) is a measure of the size of an earthquake, defined as the product of the shear modulus ($\mu$), the fault area ($A$), and the average slip ($\bar{D}$): $M_0 = \mu A \bar{D}$
• The moment magnitude ($M_w$) is a logarithmic scale that quantifies the size of an earthquake based on its seismic moment: $M_w = (2/3) \log_{10}(M_0) - 6.07$
• Earthquake focal mechanisms describe the orientation and sense of motion of the fault plane and can be determined from the polarities and amplitudes of seismic waves recorded at different stations
• Coulomb stress transfer is the process by which stress changes caused by an earthquake can influence the likelihood of future earthquakes on nearby faults
• Earthquake triggering refers to the phenomenon where an earthquake can cause a significant increase in seismicity rates in the surrounding region, either due to static stress changes or dynamic stresses from passing seismic waves

Seismic Data Analysis and Interpretation

• Seismic data analysis involves processing and interpreting recorded seismic waveforms to extract information about the Earth's structure and properties
• Seismic data processing steps include:
  • Filtering to remove noise and enhance signal quality
  • Deconvolution to remove the effect of the instrument response and source time function
  • Stacking to improve signal-to-noise ratio by averaging multiple recordings
  • Migration to correctly position reflectors and diffractors in the subsurface
• Seismic tomography is a technique used to create 3D images of the Earth's interior by inverting seismic travel time or amplitude data
  • Travel time tomography uses the arrival times of seismic waves to constrain the velocity structure
  • Amplitude tomography uses the amplitudes and waveforms of seismic waves to constrain the attenuation and velocity structure
• Receiver function analysis is a method used to study the structure of the crust and upper mantle beneath a seismic station by isolating and analyzing converted waves (P-to-S or S-to-P) at seismic discontinuities
• Shear wave splitting analysis is used to study seismic anisotropy in the Earth's interior by measuring the polarization and delay time between fast and slow shear waves
• Seismic attribute analysis involves calculating and interpreting various attributes (instantaneous amplitude, frequency, phase, etc.) from seismic data to highlight specific features or properties of the subsurface

Applications in Geology and Geophysics

• Seismology plays a crucial role in understanding the Earth's internal structure, composition, and dynamics
• Seismic data is used to create detailed models of the Earth's interior, including the crust, mantle, and core, which provide insights into the planet's formation and evolution
• Seismic hazard assessment involves estimating the probability and potential consequences of future earthquakes in a given region, which is essential for risk mitigation and urban planning
  • Seismic hazard maps display the expected ground motion levels for a given probability of exceedance and are used to develop building codes and infrastructure design
  • Probabilistic seismic hazard analysis (PSHA) combines information about seismic sources, ground motion prediction equations, and site effects to quantify the uncertainty in seismic hazard estimates
• Seismology is applied in the exploration and monitoring of natural resources, such as oil, gas, and geothermal energy
  • Reflection seismology is used to image subsurface structures and stratigraphic features, helping to identify potential hydrocarbon reservoirs
  • Microseismic monitoring is used to track the growth and extent of hydraulic fractures during reservoir stimulation and to monitor the stability of underground storage facilities
• Seismology contributes to the study of plate tectonics and the dynamics of the Earth's interior
  • Seismic data is used to map the boundaries and motion of tectonic plates, as well as to study the processes driving plate tectonics (mantle convection, slab pull, ridge push)
  • Seismic anisotropy measurements provide insights into the flow and deformation patterns in the Earth's mantle, which are key to understanding the driving forces behind plate tectonics
• Seismology is used to monitor and study various geological phenomena, such as volcanic activity, landslides, and glacial movements
  • Volcanic seismology involves analyzing seismic signals associated with magma movement, rock fracturing, and fluid migration to assess volcanic hazards and forecast eruptions
  • Seismic monitoring of landslides and glaciers helps to understand their dynamics, stability, and potential hazards

Current Research and Future Directions

• Advances in seismic instrumentation, such as broadband and high-sensitivity seismometers, allow for more detailed and accurate measurements of ground motion
• Dense seismic arrays and large-scale seismic experiments (USArray, AlpArray) provide high-resolution data for imaging the Earth's interior and studying regional-scale structures and processes
• Machine learning and artificial intelligence techniques are being increasingly applied to seismic data analysis, enabling automated event detection, phase picking, and waveform classification
• Seismic interferometry is a technique that uses ambient noise or coda waves to extract Green's functions between seismic stations, allowing for the imaging of the Earth's structure without the need for active sources or earthquakes
• Joint inversion of seismic data with other geophysical data types (gravity, electromagnetic, geodetic) provides a more comprehensive understanding of the Earth's interior and the processes that shape it
• Advances in computational power and numerical modeling techniques enable more realistic and detailed simulations of seismic wave propagation, earthquake rupture, and other geophysical phenomena
• The development of seafloor and borehole seismometers expands the coverage of seismic networks and allows for the study of previously inaccessible regions, such as the deep ocean and the Earth's interior
• Interdisciplinary research combining seismology with other Earth science disciplines (geodynamics, mineral physics, geochemistry) is crucial for understanding the complex processes and interactions within the Earth system
• Future research directions in seismology include:
  • Improving the resolution and accuracy of Earth models through the integration of multiple data types and the development of advanced imaging techniques
  • Investigating the fine-scale structure and properties of the Earth's interior, such as the core-mantle boundary region, the lithosphere-asthenosphere boundary, and the transition zone
  • Studying the interplay between seismic activity, fluid migration, and geochemical processes in the Earth's crust and mantle
  • Developing more accurate and reliable methods for earthquake forecasting and seismic hazard assessment, considering the complex nature of earthquake processes and the uncertainties involved
  • Exploring the seismic signatures of climate change-related phenomena, such as the melting of glaciers and the thawing of permafrost, and their potential impact on Earth's surface and subsurface processes