principles and applications of microearthquake networks

Glossar? of' A bbreiqiations 227
data and in the same tape track as the data. They are used during the playback of data in
order to correct for the effects of tape-speed variations.
CPU: Central processing unit of a computer. It is the unit that performs the interpretation
and execution of computer instructions.
CPU time for a computer job: Actual time taken by the central processing unit of a computer
to complete a given job. For a computationally intensive job, the CPU time is the
dominant factor in determining the computing cost charged to a user.
dB: Decibel. Unit for describingthe ratio of two powers or intensities. If PI and Pz are two
measurements ofpower, the first is said to be n decibels greater, where n = 10. loglo( P,IP2).
Since the power of a sinusoidal wave is proportional to the square of the amplitude, and if
A, and A2 are the corresponding amplitudes for PI and f2, we have 17 = 20 . log,,(Al/A2).
dB/octave: Decibels per octave. Used as a unit to express the change of amplitude over a
frequency interval. See dB; Octave.
Develocorder: Multichannel (up to 20) galvanometric instrument that records analog signals
continuously on 16-mm microfilm. Processing of the film is performed continuously within
the instrument so that the analog film record can be viewed at lox magnification approximately
10 min. later. It is manufactured by Teledyne Geotech, and is commonly used to
record seismic and timing signals from a microearthquake network.
Direct-mode tape recording : Technique to record analog signals by amplitude modulation
on magnetic tapes.
Discriminator: Electronic co,mponent that converts variations in signal frequency (relative
to a reference or carrier frequency) to variations in signal amplitude. Its function is exactly
opposite to that of a voltage-controlled oscillator.
Duration magnitude: Type of earthquake magnitude. It is based on the duration of seismic
waves (i.e., signal duration). In practice, signal duration is often measured as the interval
from the first P-arrival time to the time where the signal amplitude no longer exceeds some
prescribed value. See Coda magnitude.
Earthquake swarm: Type of earthquake sequence characterized by a long series of large
and small shocks with no one outstanding, principal event.
Euclidean length of a vector: If x is an n-vector with components xi, i = 1, 2, . . . , n, then
its Euclidean length is lbll = (X':=&)1'2. It is the 2-norm of a vector and is the most
commonly used vector norm. See Norm of a vector.
EW: East-west direction.
FDM : Frequency division multiplexing. Technique for combining and transmitting two or
more signals over a common path by using a different frequency band for each signal.
FFT: Fast Fourier transform. It is an algorithm that efficiently computes the discrete
Fourier transform.
First motion of P-waves: Refers to the direction of the initial motion of P-waves recorded on
a seismogram at a given station from a given earthquake. For a vertical component seismogram,
the common convention is that the up and down directions on the seismogram
correspond to the up and down directions of ground motion, respectively. See Section 6.2;
see also Polarity of a station.
FM : Frequency modulation. Technique of converting signals for the purpose of transmission
or recording. In frequency modulation, variations in signal amplitude are converted to
variations in frequency relative to a reference frequency or carrier frequency.
Geophone: Term that is commonly used for an electromagnetic seismometer that is light
weight and responds to high-frequency ground motion (e.g., greater than 1 Hz).
Helicorder: A multichannel (up to three) galvanometric instrument that records analog
signals continuously on a sheet of thermally sensitive paper by means of a hot stylus. The
sheet of paper is wrapped around a cylindrical drum so that a standard-size seismogram is