We have already seen (Eq. 25) that an external force has the same effect on the seismic mass as a seismic acceleration of the ground. The largest such force is gravity. It is normally cancelled by the suspension, but when the seismometer is tilted, the projection of the vector of gravity onto the axis of sensitivity changes, producing a force that is in most cases undistinguishable from a seismic signal (Fig. 11). Undesired tilt at seismic frequencies may be caused by moving or variable surface loads such as cars, people, and atmospheric pressure. The resulting disturbances are a secondorder effect in welladjusted vertical seismometers but otherwise a firstorder effect [Rodgers 1968,Rodgers 1969]. This explains why horizontal longperiod seismic traces are always noisier than vertical ones. A short, impulsive tilt excursion is equivalent to a steplike change of ground velocity, and will therefore cause a longperiod transient in a horizontal broadband seismometer. For periodic signals, the apparent horizontal displacement associated with a given tilt increases with the square of the period.
Fig. 12
illustrates the effect of barometrically induced ground tilt. Let us
assume that the ground is vertically deformed by as little as m
over a distance of 3 km, and that this deformation oscillates with a
period of 10 minutes. A simple calculation then shows that seismometers
A and C see a vertical acceleration of
m/s^{2} while B sees a horizontal acceleration of
m/s^{2}.
The horizontal noise is thus 100 times larger than the vertical one. In
absolute terms, even the vertical acceleration in our example is by a
factor of four above the minimum ground noise in one octave as
specified by the USGS Low Noise Model (section 5.1).
