The spectrogram above (color) represents the frequency power of vertical earth velocity near Leonard Oklahona (TUL) for 12 hours beginning at 1200 UTC on 1999 SEP20. The most noticable events, at least at higher frequencies (ie 0.5 to 2.8 Hertz) are three Oklahoma limestone quarry blasts (two in "QB1,2", the third in "QB3"). The energy in the L shaped area "7.5MW", is from the 1747UTC Taiwan, ROC mainshock. The high frequencies extend upward to only 1.3 Hz, but intense low frequency (below 0.1 Hz, red and brown) is present, particularly during the surface wave arrivals and decay. One Taiwan aftershock, the 2146UTC 6.5MS, has visible surface wave energy in the "6.5MS" box. The "US" box shows the oceanic microseisms which are always present. Beginning with the Taiwan mainshock, microseisms are not visible because of the much larger low frequency energy from the Taiwan earthquake and it's aftershocks. The vertical earth velocity seismogram (black/white), ahows only the Taiwan mainshock and the "6.5MS" aftershock. The frame of this seismogram is about 400,000 arbitrary counts from top to bottom. The quarry blasts are clearly visible if the height is reduced to about 8,000 counts.
The above seismograms were recorded at four samples per second, then lowpass filtered to attenuate frequencies higher than 0.05 Hertz (periods less than 20 seconds). The traces were rotated so that the horizontal traces are radial (mhr), in the direction of the epicenter, and transverse (mht), perpendicular to the direction of the epicenter. Leonard, Oklahoma, USA, is 111.5 great circle degrees (12,402 kilometers) from the epicenter. This puts Leonard in the P-wave shadow zone in which the shadow of the earth's core prevents direct P-waves from travelling to Leonard. Leonard is also in shadowed for direct S. If a sodium lamp or laser is used to make a shadow of a coin or other circular object, The shadow has a spot of light in its center, with concentric rings of light surrounding it. The light waves have been "diffracted" into the shadow. In a like manner, P and S waves are diffracted into the shadow of the earth's core. Pdiff is clear on the vertical and radial seismograms, and Sdiff is prominent on the transverse seismogram. Phases reflected once at the earth's surface about midway between Taiwan and Leonard are PP, SP, and SS. PP is a P-wave reflected as a P-wave, SP is S reflected as P, and SS is S reflected as S. Two other arrival times are shown. PKKPdf and P'P'df both involve travel through the earth's core. It is not certain that these phases are seen on these seismograms. P'P'df may be present on the radial, and PKKPdf may show on vertical and radial. The figures were made with the aid of Geotool and ImageMagick