Figure 17-New



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.