Oscillations, Waves, and Interactions - GWDG

306 Schreiber
Demodulator Signal [V]
1.790
1.780
1.770
mag. 3.23
0 50 100 150 200 250
Time [s]
Figure 27. Example of a regional seismic event recorded on the GEOsensor in Pinon Flats.
on all scales with a particular emphasis on local and regional scales with source
distances of up to 100 km. From the calculation of theoretical seismograms for all six
degrees of freedom of motion, it became apparent that rotational motion information
may contribute the most for the investigation of local and regional earthquakes. One
example out of many of the recorded seismic events in Pinon Flat is shown in Fig. 27.
The basic sensor concept is well suited for the desired application.
7.4 Observations of rotations
Currently there are primarily two types of measurements that are routinely used
to monitor global and regional seismic wave fields. Standard inertial seismometers
measure three components of translational ground displacement and provide the
basis for monitoring seismic activity and ground motion. The second type aims
at measuring the deformation of the Earth (strains). It is well known that there
is a third type of measurement that should be observed in seismology and geodesy
in order to fully describe the motion at a given point, the measurement of ground
rotation.
In fact, Aki and Richards [24, 25] have demanded to use rotations for more than
20 years, but due to the lack of suitable sensors this has not been done in the past.
The recording of the (complete) earthquake-induced rotational motion is expected
to be useful particularly for (1) further constraining earthquake source processes
when observed close to the active faults [30]; (2) estimating permanent displacement
from seismic recordings [32]; (3) estimating local (horizontal) phase velocities from
collocated observations of translations and rotations. As will be shown below, the
consistency of broadband ring laser observations of the vertical component of rotation
rate observed for distant large earthquakes is readily obtained. Furthermore one can
model the observations with numerical simulations of the complete rotational wave
field in a three-dimensional heterogeneous global Earth model.
In order to compare translations (measured by a standard seismometer) with the
vertical component of the vector of rotation – which is what the G-ring is measur-