Vertical seismic noise at very low frequencies

Abstract

We use records from the Project IDA modified LaCoste gravimeters to investigate ground noise at. frequencies from 1 to 10 mHz. At most sites the level between 2 and 10 mHz is nearly flat and close to 2 × 10−18 m2 s−3 Much higher values which are observed at island and coastal stations are due to loading by waves trapped along the shore. Data from the superconducting gravimeter at Piñon Flat Geophysical Observatory show that the noise power increases as ƒ−2.7 for frequencies between 1 and 0.001 mHz.

On ISCPPandpPdata and their use in delay-time tomography of the Caribbean region

“…As can be seen from the figure, the noise level of the IDA network's Lacoste-Romberg gravimeters is only slightly above that of the superconducting instrument used in the Agnew & Berger (1978) study. Thus, by stacking the IDA records we can expect to achieve a noise level comparable to that attained in some of the previously mentioned studies using the superconducting instruments.…”

“…The sloping, heavy line indicates the noise spectrum for a superconducting gravimeter (Agnew & Berger 1978), and the horizontal line indicates the spectral level of a 15ngal oscillatory signal. As can be seen from the figure, the noise level of the IDA network's Lacoste-Romberg gravimeters is only slightly above that of the superconducting instrument used in the Agnew & Berger (1978) study.…”

“…The theoretical monthly and fortnightly tidal amplitudes are indicated by the inverted triangles. The horizontal tine denotes the amplitude of a 15ngal sinusoid, and the heavy, sloping line is a rough approximation to the noise curve of a superconducting gravimeter published inAgnew & Berger (1978).…”

Structure of the inner core inferred from observations of seismic core modes

“…The noise level so estimated is of the order of lO-"m s -~ in a frequency range of the three core modes. Note that in the frequency band between lop3 and lo-' Hz, acceleration amplitude of the ground noise is in general of the order of to 10-'0ms-2 over a 1/3 octave bandwidth (Agnew & Berger 1978;Agnew et al 1986). The above test thus shows that, if the core mode zSz has a high-Q value, it is possible for us to detect it with the Sompi method.…”

Reply

“…The ocean effects can also be significant since SY is located near coast (e.g., Agnew and Berger, 1978). Nagata et al (1993) reported a seasonal variation of the amplitudes of ocean tides near SY.…”