7.2 Identification of seismic phases

Seismic waves tell us a lot about Earth's structure. By studying how they travel and arrive at different places, we can peek inside our planet. P-waves, S-waves, and surface waves all behave differently, giving us clues about what's beneath our feet.

Identifying seismic phases is like solving a puzzle. We look at when waves arrive, how they move, and where they've been. This helps us map out Earth's layers, from the crust to the core, and understand what's happening during earthquakes.

Body Wave Phases

Primary and Secondary Wave Arrivals

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• P-wave arrival marks the first seismic energy detected on a seismogram
• P-waves propagate through both solid and liquid media as compressional waves
• S-wave arrival follows P-waves, indicating the second major seismic phase
• S-waves travel slower than P-waves and only propagate through solid materials
• Time difference between P and S arrivals helps determine earthquake distance
• P-wave particle motion occurs parallel to the direction of wave propagation
• S-wave particle motion happens perpendicular to the direction of wave propagation

Reflected and Refracted Phases

• PP and SS phases represent seismic waves reflected once at the Earth's surface
• PP waves arrive after direct P-waves but before S-waves on seismograms
• SS waves follow direct S-waves and exhibit larger amplitudes than PP waves
• PKP phase denotes P-waves that travel through the Earth's outer core
• PKP waves refract at the core-mantle boundary due to velocity contrasts
• SKS phase indicates S-waves converting to P-waves in the liquid outer core
• SKS waves convert back to S-waves upon exiting the core, providing insights into core structure

Surface Waves

Love Wave Characteristics

• Love waves manifest as horizontal shear waves trapped in surface layers
• Propagate along the Earth's surface with particle motion parallel to the surface
• Require a velocity structure that increases with depth to exist
• Dispersion causes different frequencies of Love waves to travel at varying speeds
• Higher frequency Love waves sample shallower depths in the Earth's structure
• Love waves typically arrive after body waves but before Rayleigh waves
• Cause significant horizontal ground motion, contributing to earthquake damage

Rayleigh Wave Properties

• Rayleigh waves result from the interaction of P and SV waves at the free surface
• Exhibit elliptical particle motion in the vertical plane containing the direction of propagation
• Penetrate deeper into the Earth than Love waves, providing information about deeper structures
• Display retrograde elliptical motion at the surface, becoming prograde at depth
• Dispersion in Rayleigh waves allows for the study of crustal and upper mantle structure
• Rayleigh waves generally have the largest amplitudes on seismograms for shallow earthquakes
• Multiple modes of Rayleigh waves exist, with higher modes sampling deeper Earth structure

Seismic Wave Analysis

Travel Time Curve Interpretation

• Travel time curves graphically represent seismic wave arrival times versus distance from the source
• Plot wave arrival times on the vertical axis and epicentral distance on the horizontal axis
• Different seismic phases appear as distinct curves on travel time plots
• Slope of travel time curves indicates the apparent velocity of seismic waves
• Triplications in travel time curves reveal velocity discontinuities in Earth's interior
• Shadow zones on travel time curves indicate regions where certain phases cannot penetrate
• Travel time curves help identify different Earth layers (crust, mantle, core) based on velocity changes

Moveout Analysis and Applications

• Moveout refers to the difference in arrival times of seismic waves at different receiver locations
• Normal moveout describes the hyperbolic increase in travel time with offset for horizontal reflectors
• Velocity analysis utilizes moveout to determine seismic wave velocities in different layers
• Moveout correction aligns reflections from different offsets to enhance seismic data quality
• Dip moveout accounts for the effect of dipping reflectors on seismic wave travel times
• Residual moveout analysis helps refine velocity models in seismic processing
• Moveout patterns assist in identifying and characterizing different types of seismic events (reflections, refractions, multiples)