🌋Seismology Unit 7 – Seismogram Analysis: Phases & Interpretation

Key Concepts & Terminology

• Seismology studies the propagation of seismic waves through the Earth to understand its internal structure and properties
• Seismograms are visual representations of ground motion recorded by seismometers, which measure displacement, velocity, or acceleration
• Seismic waves are elastic energy that propagates through the Earth's interior and along its surface, generated by earthquakes, explosions, or other sources
• 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, causing compression and rarefaction of the material they pass through (push-pull motion)
  • S-waves are transverse, causing shearing deformation perpendicular to the direction of wave propagation
• Surface waves travel along the Earth's surface and include Rayleigh waves and Love waves
• Seismic phases refer to specific arrivals of seismic waves on a seismogram, characterized by their travel paths and velocities (e.g., P, S, PP, SS, PcP)
• Travel time is the duration taken by a seismic wave to propagate from its source to a receiver, dependent on the wave type and the Earth's velocity structure

Seismogram Components

• Seismograms display the ground motion as a function of time, with the horizontal axis representing time and the vertical axis representing amplitude
• The main components of a seismogram include the P-wave arrival, S-wave arrival, and surface wave arrivals (Rayleigh and Love waves)
• Seismograms can be recorded in three orthogonal directions: vertical (Z), north-south (N), and east-west (E) to capture the full 3D ground motion
• The amplitude of seismic waves on a seismogram depends on factors such as the magnitude of the event, distance from the source, and local site conditions
• Seismograms also contain information about the frequency content of the seismic waves, with higher frequencies attenuating more rapidly with distance
• The signal-to-noise ratio (SNR) is an important consideration in seismogram analysis, as high noise levels can obscure seismic phases and make interpretation challenging
• Seismograms may need to be processed (e.g., filtered, deconvolved) to enhance the visibility of seismic phases and remove unwanted noise or instrument effects

Types of Seismic Waves

• Seismic waves are classified into body waves and surface waves based on their propagation paths and particle motion
• Body waves travel through the Earth's interior and include P-waves and S-waves
  • P-waves are the fastest seismic waves, with typical velocities ranging from 5-8 km/s in the Earth's crust and mantle
  • S-waves are slower than P-waves, with velocities approximately 60% of P-wave velocities, and cannot propagate through fluids (e.g., outer core)
• Surface waves travel along the Earth's surface and are generated by the interaction of body waves with the free surface
  • Rayleigh waves have retrograde elliptical particle motion in the vertical plane containing the direction of propagation
  • Love waves have transverse horizontal particle motion perpendicular to the direction of propagation
• Seismic wave velocities vary with depth in the Earth due to changes in composition, temperature, and pressure
• Seismic waves can undergo reflection, refraction, and conversion at interfaces between materials with different elastic properties (e.g., Moho, core-mantle boundary)
• The dispersive nature of surface waves (velocity dependent on frequency) can be used to study the Earth's shallow velocity structure

Reading a Seismogram

• Identify the time scale and amplitude scale on the seismogram to understand the duration and relative size of the recorded ground motion
• Recognize the characteristic shapes and relative arrival times of different seismic phases (e.g., P, S, surface waves) to infer the distance and depth of the seismic event
• Use the polarities (up or down) of the first motion of the P-wave to determine the sense of motion at the source (compression or dilatation)
• Measure the time difference between the P and S arrivals to estimate the distance to the event epicenter using a travel time curve or lookup table
• Identify later arriving phases (e.g., PP, SS, PcP) to constrain the depth of the event and study the Earth's deep interior structure
• Look for patterns in the waveforms across multiple stations to locate the event epicenter and determine its focal mechanism (e.g., fault orientation, slip direction)
• Analyze the frequency content and amplitude decay of the seismic waves to infer the attenuation properties of the Earth and the source characteristics (e.g., rupture size, duration)

Phase Identification Techniques

• Use the expected arrival order of seismic phases based on their velocities and travel paths (P before S, direct phases before reflected/refracted phases)
• Compare the observed arrival times with predicted travel times from a reference Earth model (e.g., IASP91, AK135) to identify phases
• Look for characteristic phase amplitude ratios and polarities (e.g., P/S amplitude ratio, P-wave first motion polarity) to distinguish between different phases
• Analyze the particle motion of the seismic waves using three-component seismograms to identify phase types (e.g., linear for P, elliptical for Rayleigh)
• Use phase picking algorithms (e.g., STA/LTA, cross-correlation) to automatically detect and identify seismic phases, especially for large datasets
• Apply polarization filters (e.g., rectilinearity, planarity) to enhance the visibility of specific phase types and improve signal-to-noise ratios
• Compare waveforms from multiple stations and events to identify common phases and build a consistent phase interpretation

Travel Time Curves & Distance Calculation

• Travel time curves show the relationship between the travel time of seismic phases and the distance from the event epicenter
• Different seismic phases have distinct travel time curves due to their unique propagation paths and velocities in the Earth
• The Jeffreys-Bullen (JB) and Preliminary Reference Earth Model (PREM) are commonly used 1D velocity models for calculating theoretical travel times
• Measure the time difference between the P and S arrivals on a seismogram to estimate the epicentral distance using a travel time curve or table
  • The S-P time increases with distance, allowing for an approximate distance calculation
• Use the arrival times of multiple phases (e.g., P, PP, S, SS) to improve the accuracy of the distance estimation and constrain the event depth
• Apply time corrections for station elevation, sedimentary layers, and other local site effects to refine the distance calculation
• Develop regional travel time curves using well-located events and dense seismic networks to account for lateral variations in Earth structure

Earthquake Location & Magnitude Estimation

• Earthquake location involves determining the epicenter (latitude, longitude) and depth (focal depth) of the seismic event
• The most common method for earthquake location is the time-distance method, which uses the arrival times of P and S waves at multiple stations
  • Measure the P and S arrival times at each station and calculate the S-P time differences
  • Use the S-P times to estimate the epicentral distances to each station using a travel time curve or lookup table
  • Plot the distance circles around each station and find the intersection point, which represents the epicenter location
• Earthquake depth can be estimated using the arrival times of depth-sensitive phases (e.g., pP, sP, PcP) and comparing them with theoretical travel time curves
• Magnitude is a measure of the size and energy release of an earthquake, with several scales used depending on the seismic wave types and frequency bands analyzed
  • Local magnitude (ML) is based on the maximum amplitude of the horizontal component of the seismogram and the epicentral distance
  • Body wave magnitude (mb) uses the amplitude of the P-wave and a distance correction factor
  • Surface wave magnitude (Ms) is calculated from the maximum amplitude of the surface waves (Rayleigh waves) with a period of around 20 seconds
  • Moment magnitude (Mw) is derived from the seismic moment, which is proportional to the product of the rupture area, average slip, and rock rigidity

Advanced Interpretation Methods

• Focal mechanism solutions (beach balls) represent the orientation and sense of motion of the fault plane using P-wave first motion polarities and amplitude ratios
  • Compress