Seismograms
recorded by a 3-component seismograph at Nana, Peru for an earthquake located
near the coast of central Chile on September 3, 1998. The three
seismograms record motion in horizontal (east-west and north-south) and
vertical (Z) directions. P, S, Rayleigh and Love waves are identified
on the record. The S wave arrives significantly after the P-wave
because S-wave velocity in rocks is lower than P wave velocity.
Additional arrivals between the P and the S wave are P and S waves that have
traveled more complicated paths (such as the pP and PP phases and P-to-S
converted phases) from the earthquake location to the seismograph. The
surface waves arrive after the S waves because surface wave velocities in
rocks are lower than the shear wave velocity. The surface waves extend
over a long time interval because surface wave propagation is dispersive (the
velocity of propagation is dependent on the frequency of the wave).
This dispersive character can easily be seen in the Rayleigh wave on the
vertical (Z) component seismogram in that the earliest Rayleigh wave energy
has a longer period (lower frequency) than the later arriving waves. |