principles and applications of microearthquake networks

Glossary of AbbrciTintions 229
said to have the correct polarity if the up (or down) direction of the first P-motion on the
seismogram corresponds to the up (or down) direction of the ground motion. Otherwise, it
is said to have reversed polarity.
P-phase: The part of the seismic signal recorded on a seismogram, which is interpreted to
represent the ground motion associated with P-waves.
P/SV amplitude ratio: Ratio of the maximum amplitude of the P-phase to the maximum
amplitude of the S-phase of an earthquake as measured from the vertical component
seismogram recorded at a given station. It is assumed that the travel path from the
earthquake source to the receiving station is the same for the P-phase and the S-phase in
question. In general, the SV-wave motion (see Aki and Richards, 1980, p. 531) is not
entirely in the vertical direction. Thus, in practice, it is usually the ratio of the maximum
amplitude of the vertical component of the P-phase to the maximum amplitude of the
vertical component of the SV-phase that is measured.
P-waves: Compressional elastic waves are called P-waves in seismology, where P stands
for primary.
Q: Quality factor of an imperfect elastic medium. It is a dimensionless measure of the
internal friction in a volume of material. [See Aki and Richards (1980, p. 168-169)l.
Seismometer: Instrument that detects ground motion and converts it to a signal that can be
amplified and recorded. This definition is consistent with the current usage, and differs
from an earlier definition which defines seismometer as a calibrated seismograph.
Singular matrix: A square matrix is said to be singular if its determinant is zero.
S-P time interval: Time interval between the S-wave arrival time and the P-wave arrival
time.
S-phase:
The part of the seismic signal recorded on a seismogram, which is interpreted to
represent the ground motion associated with S-waves.
SVD: Singular-value decomposition. See Section 5.1.4.
Swarm: See Earthquake swarm.
S-waves: Elastic shear waves are called S-waves in seismology, where S stands for secondary.
Time-code generator: A crystal-controlled pulse generator that produces a train of pulses
with various predetermined widths and spacings. It is designed such that the time of day
(and sometimes the Julian day of the year) can be decoded from the pulse train. It is used
to provide a continuous time base for physical measurements.
P-wave arrival time or travel time, depending on the context. It is most commonly used
tp:
for the first P-wave that arrives at a given station.
Tripartite array: Array of seismographs deployed at the comers of a triangle. The sides of
the triangle are usually 1 km or so in length, and the triangle is usually equilateral in shape.
A fourth station often occupies the center of the triangle, and serves as a central recording
site. Although this type of array was popular at one time, it is no longer commonly
deployed. The reason is that the benefits from such a geometry often are not balanced by
the logistical difficulties.
t,: S-wave amval time or travel time, depending on the context.
tq/tD: Ratio of S-wave travel time to P-wave travel time. In determining this ratio, it is
commonly assumed that the S-wave and the P-wave travel the same path from an earthquake
source to a given station.
uhf: Ultrahigh frequency. The band of frequencies from 300 to 3000 MHz in the radio
spectrum is referred to as the uhf band.
USGS: United States Geological Survey.
UTC: Universal Time Coordinated: A time scale defined by the International Time
Bureau and agreed upon by international convention.