2. Seismic Phases and Arrival Times

Seismograms carry a great deal of information about remote earthquakes. There are four basic types of seismic vibrations. Each type travels at a different speed, some via different paths, so that they arrive at the seismograph station at different times. The difference in arrival times grows larger the further away the earthquake is so that it can be used to calculate how far away the earthquake is.

Two of the basic vibrations are body phases, that is they travel through the body of the earth. The P-phase travels the fastest and is a compressional, longitudinal vibration, the vibration being in the same direction that the energy is propagating. The S-phase is also a body phase but is a shearing, transverse vibration, that is the vibration is perpendicular to the direction of propagation.

The Love and Rayleigh phases are surface phases. They result from the interaction of body waves with big impedance change at the surface of the earth. It is the surface phases that cause the most of the damage in an earthquake and they are usually the biggest part of the seismic signal on a Lehman type of seismograph. The surface displacement due to the Great Sumatra-Andaman Island earthquake of December 2004 was a minimum of 1 centimeter everywhere on earth.

The velocity of the body phases changes with depth so the seismic “ray” is refracted or bent as it travels. There are also discontinuities in the velocity profile, most notably at the outer and inner core, but also elsewhere. These discontinuities give rise to reflected phases.

I have plotted several seismograms on a single graph window. Each is a 5000 second segment of data with the zero time corresponding to the time of the earthquake at the epicenter and offset vertically to give a zero amplitude value centered on the epicentral distance. Their amplitudes are scaled to show the weak phases so they do not show relative amplitudes between earthquakes. This display shows the runout of the various seismic phases and some interesting observations from solid earth geophysics.

Travel Time Graph
The seismograms used are:

Magnitude and Location  Distance       Date           Depth
M3.5  Ohio               001.39     Jun 20, 2006      z=5 km
M5.6  WesternMontana     022.83     Jul 26, 2005      z=5 km
M6.7  VancouverIsland    033.85     Nov 02, 2004      z=10 km
M7.2  Columbia           035.94     Nov 15, 2004      z=10 km
M7.5  Peru               046.29     Sep 26, 2005      z=127.4 km
M6.9  Argentina          067.72     Mar 21, 2005      z=557.8 km
M6.6  Japan              100.00     Mar 20, 2005      z=10 km
M7.6  Pakistan           102.2      Oct 08, 2005      z=10 km
M7.3  NicobarIsland      131.2      Jul 24, 2005      z=16 km
M8.7  Sumatra            137.55     Mar 28, 2005      z=30 km

 

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