Travel Time Curves & Seismic Phase Identification

Harvard EPS55

Interactive demonstration of how seismologists use travel time curves to identify different seismic phases on seismograms

Epicentral Distance (degrees): 45°
Focal Depth (km): 33 km

Seismic Phase Types

P - Direct P-wave
S - Direct S-wave
PP - P reflected at surface
SS - S reflected at surface
PcP - P reflected at core
ScP - S to P at core
PKP - P through core
SKS - S to K to S
Lg - Surface wave

How to Use Travel Time Curves

  1. Measure the arrival time of a phase on the seismogram
  2. Find the epicentral distance using P-S time difference
  3. Use the travel time curve to identify the phase
  4. Check for consistency with expected amplitude and polarity

💡 Pro Tip

The P-S time difference increases by about 8 seconds per degree of distance. This helps quickly estimate epicentral distance!

Body Waves

Travel through Earth's interior:

P & S: Direct waves, always first arrivals
PP & SS: Single surface reflection
PcP & ScS: Core-mantle boundary reflection
PKP: Through outer core (P-wave only)

Core Phases

Interact with Earth's core:

Shadow Zones:
P-wave: 103° - 143°
S-wave: > 103° (blocked by liquid outer core)
PKP phases: Multiple branches (PKPab, PKPbc, PKPdf)

Surface Waves

Travel along Earth's surface:

Lg: Trapped in continental crust
Love waves: Horizontal motion
Rayleigh waves: Elliptical motion
Large amplitude, arrive after body waves