🪨Intro to Geophysics Unit 3 – Seismology: Studying Earth's Vibrations

Key Concepts and Terminology

• Seismology studies the propagation of seismic waves through the Earth to understand its internal structure and properties
• Seismic waves are elastic waves generated by earthquakes, explosions, or other sources that travel through the Earth
• Body waves travel through the Earth's interior and include P-waves (primary or compressional) and S-waves (secondary or shear)
  • P-waves are longitudinal waves that compress and expand material parallel to the direction of wave propagation
  • S-waves are transverse waves that cause particles to oscillate perpendicular to the direction of wave propagation
• Surface waves travel along the Earth's surface and include Rayleigh waves and Love waves
• Seismographs are instruments that record ground motion caused by seismic waves
• Seismograms are the recorded output of seismographs, displaying the amplitude and arrival times of seismic waves
• Earthquake magnitude is a measure of the energy released by an earthquake, commonly expressed using the Richter scale or moment magnitude scale
• Earthquake intensity is a measure of the effects of an earthquake on people, structures, and the environment, often described using the Modified Mercalli Intensity Scale

Earth's Structure and Seismic Waves

• The Earth is composed of several layers: the crust, mantle, outer core, and inner core
• Seismic waves travel at different velocities through these layers due to variations in density, temperature, and composition
• Discontinuities between layers (Mohorovičić, Gutenberg, and Lehmann) cause seismic waves to refract or reflect
  • The Mohorovičić discontinuity (Moho) separates the crust from the mantle
  • The Gutenberg discontinuity marks the boundary between the mantle and the outer core
  • The Lehmann discontinuity is a subtle boundary within the inner core
• Seismic wave velocities increase with depth in the Earth due to increasing pressure and temperature
• The behavior of seismic waves at layer boundaries provides information about the Earth's internal structure
• Seismic tomography uses the travel times of seismic waves to create 3D images of the Earth's interior
• The Earth's core was discovered by analyzing the shadow zone for P-waves and the absence of S-waves in certain regions

Types of Seismic Waves and Their Behavior

• P-waves are the fastest seismic waves and can travel through solids, liquids, and gases
  • P-wave velocity depends on the bulk modulus and density of the material
• S-waves are slower than P-waves and can only travel through solids
  • S-wave velocity depends on the shear modulus and density of the material
• Surface waves are the slowest seismic waves and are confined to the Earth's surface
  • Rayleigh waves cause particles to move in an elliptical path in the vertical plane
  • Love waves cause particles to move side-to-side in the horizontal plane
• Seismic wave attenuation occurs as waves lose energy due to geometric spreading, absorption, and scattering
• Seismic anisotropy refers to the directional dependence of seismic wave velocity in some materials
• Seismic waves can be converted from one type to another (mode conversion) at layer boundaries or due to changes in material properties

Seismographs and Data Collection

• Seismographs consist of a seismometer, which detects ground motion, and a recording device
  • Traditional seismometers use a mass-spring system to measure ground displacement
  • Modern seismometers use electronic sensors (geophones or broadband seismometers) to measure ground velocity or acceleration
• Seismographs are installed in seismic stations worldwide to monitor seismic activity
• Seismic networks, such as the Global Seismographic Network (GSN), provide real-time data for earthquake monitoring and research
• Ocean-bottom seismometers (OBS) are used to collect seismic data in marine environments
• Seismic data is digitized, timestamped, and stored for analysis
  • Sampling rate and dynamic range are important factors in seismic data acquisition
• Seismic noise, such as cultural noise or microseisms, can interfere with seismic signal detection and interpretation
• Seismic data quality control involves removing artifacts, correcting for instrument response, and filtering unwanted noise

Earthquake Measurement and Scales

• Earthquake magnitude scales, such as the Richter scale and moment magnitude scale, quantify the energy released by an earthquake
  • The Richter scale is based on the maximum amplitude of seismic waves recorded by a seismograph
  • The moment magnitude scale is based on the seismic moment, which considers the fault area, average slip, and rock rigidity
• Earthquake intensity scales, such as the Modified Mercalli Intensity Scale, describe the effects of an earthquake on people, structures, and the environment
• Seismic moment ($M_0$) is a measure of the size of an earthquake, calculated as the product of the fault area ($A$), average slip ($D$), and rock rigidity ($\mu$): $M_0 = \mu AD$
• Earthquake energy ($E$) is related to the seismic moment ($M_0$) by the equation: $E = (M_0 \times 10^{-7})/2$
• Earthquake magnitude and intensity do not always correlate, as intensity depends on factors such as distance from the epicenter and local geology
• Earthquake catalogs compile information about the location, time, magnitude, and other characteristics of earthquakes

Seismic Data Analysis and Interpretation

• Seismic data processing involves filtering, amplification, and other techniques to enhance signal quality and extract useful information
• Seismic phase picking identifies the arrival times of different seismic waves (P, S, and surface waves) on seismograms
• Earthquake location is determined using the arrival times of seismic waves at multiple stations and a velocity model of the Earth
  • The epicenter is the point on the Earth's surface directly above the hypocenter (focus) of an earthquake
• Focal mechanism solutions (beach ball diagrams) represent the orientation and sense of motion of the fault plane during an earthquake
• Seismic waveform modeling compares observed seismograms with synthetic seismograms generated from Earth models to refine our understanding of Earth structure
• Seismic attenuation studies provide information about the physical properties and temperature of the Earth's interior
• Seismic anisotropy analysis reveals the orientation and strength of fabric in the Earth's crust and mantle, which can be related to past and present deformation

Applications in Geology and Engineering

• Seismic hazard assessment estimates the probability of ground shaking, liquefaction, and other earthquake-related hazards at a given location
• Seismic risk analysis combines seismic hazard assessment with vulnerability and exposure data to evaluate potential losses and inform risk mitigation strategies
• Seismic building codes and design standards aim to construct earthquake-resistant structures based on expected ground motion and site conditions
• Seismic microzonation maps provide detailed information about local seismic hazards for land-use planning and emergency response
• Seismic monitoring of volcanoes helps detect magma movement and predict volcanic eruptions
• Seismic exploration techniques, such as reflection and refraction seismology, are used in the oil and gas industry to image subsurface geology and identify hydrocarbon reservoirs
• Seismic site characterization evaluates soil and rock properties for foundation design and ground improvement

Current Research and Future Directions

• Advances in seismic instrumentation, such as fiber-optic seismometers and large-N arrays, enable higher-resolution imaging of the Earth's interior
• Machine learning and artificial intelligence techniques are being applied to seismic data analysis for automated phase picking, event detection, and waveform classification
• Seismic interferometry uses ambient noise or controlled sources to image the Earth's subsurface without relying on earthquakes
• Seismic monitoring of glaciers and ice sheets provides insights into their stability and response to climate change
• Planetary seismology uses seismic data from other celestial bodies (Moon, Mars, etc.) to study their internal structure and evolution
• Integration of seismic data with other geophysical and geological data (gravity, magnetic, GPS, InSAR) enhances our understanding of Earth processes and hazards
• Advancements in computational power and numerical modeling enable more realistic simulations of seismic wave propagation and earthquake rupture dynamics
• Induced seismicity related to human activities (wastewater injection, hydraulic fracturing, geothermal energy production) is an active area of research for hazard mitigation and regulation