Frequency Response of the Seismic System and Character of its Records

The response of the standard USGS seismic system can be described as broadband intermediate frequency range. It is flat (to constant peak ground velocity) from about 1 to about 25 Hz. The lower frequency cutoff corresponds to the seismometer free period, and the upper frequency cutoff is accomplished electronically in the discriminators to suppress system noise, including cross modulation from adjacent telemetry channels. The most serious limitation of the system is its relatively low dynamic range, about 40 dB. Overall system performance also depends on the mode of recording: poorest for Helicorders and Develocorders, better for compensated tape playbacks, and best for on-line digitization at the discriminator outputs. Overall responses of the high- and low-gain USGS systems are compared with those of the big Benioff (JAS) and the standard Wood-Anderson in fig. 2.

Between frequencies of 0.2 and 30 Hz the shape of the USGS system response curve is approximately the inverse of the spectral amplitude of quietsite Earth noise (QSN, fig. 3a). This relationship insures that the amplitude of recorded Earth noise is relatively independent of frequency within that range (QSN, fig. 3b) and that the detection of signals that are only slightly larger than background noise is independent of frequency. Earthquake signals are also transformed spectrally in the recording process. Logarithmic

Figure 1
Northern California Seismic Net. Star = 1965 UC Berkeley station.
Triangle = Telemetered USGS station, vertical plus horizontal.
Dot = Telemetered USGS station, vertical only.

Figure 2
Magnification curves for the standard and low-gain USGS
seismic systems, the standard Wood-Anderson seismograph,
and the 100-kg Benioff seismograph.

spectral ground displacement curves for magnitude 2 and 4 earthquakes, according to the Brune source model with an average stress drop of 5 bars, are compared with that of quiet-site Earth noise and with the USGS system magnification curve in figure 3a. Such curves for magnitude 1 through magnitude 7 earthquakes, for a recording distance of 100 km, were combined with the magnification curve to produce the logarithmic spectral record amplitude curves in figure 3b. The peaks in these curves should correspond to the dominant frequencies in the records. For earthquakes between magnitude 1 and just over magnitude 4, these peaks also correspond to the respective corner frequencies in the ground displacement spectral amplitude curves. For quakes of magnitude 5 and larger, the record spectral peak and predominant frequency remain constant at 1 Hz and correspond to the natural frequency of the seismometers.

As a specific example, the predominant frequency in the record of a magnitude 3.0 earthquake should be about 4 Hz, and record amplitudes should decrease at a rate of about 6 dB/octave toward both higher and lower frequencies. Records obtained from tape playbacks of the low-gain vertical and north-south components of a magnitude 3.0 earthquake, recorded at station

Figure 3
a) Comparison of USGS system
response curve with quiet-site ground
noise displacement spectrum (QSN) and
Brune earthquake ground displacement
spectrum curves (at 5-bar stress  drop)
for magnitudes 2 and 4 earthquakes.
b) Comparison of USGS system record
spectral amplitude curves for magnitude
1 through 7 earthquakes (at 100 km
distance and for a 5-bar stress drop)
and for quiet-site noise.

HQR from a source 3 km deep and 22 km away, are shown in figure 4. In accordance with the expectation discussed above, the peak record amplitudes of this magnitude 3.0 earthquake fall in the 2.5–5.0-Hz band, and amplitudes fall off gently within the range 1–20 Hz and more abruptly at higher and lower frequencies.