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UNITED STATES DEPARTMENT OF THE INTERIOR ...channel Develocorder (galvanometric, 16-mrn automatic...
Transcript of UNITED STATES DEPARTMENT OF THE INTERIOR ...channel Develocorder (galvanometric, 16-mrn automatic...
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UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY
Suggestions on the Analysis of 16-mm
Seismic Data from Local Networks
Open-File Report 80-990
1980
By
Richard L. Dart
This report is preliminary and has not beenedited or reviewed for conformity with
U.S. Geological Survey editorial standards.Any use of trade names is for descriptive
purposes only and does not implyendorsement by the USGS.
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Contents
Page
Introduction........................................................... 1
Data evaluation......................................................... 2
Qualitative evaluation of P- and S-phase arrivals.................. 2
Suggestions on scaling earthquakes................................. 4
Phase scaling procedure........................................... 5
Trace character remarks............................................ 6
References.............................................................. 21
A: Alternate weighting method.......................................... 22
B: International time-zones map........................................ 23
C: Timing systems: WWV, WWVH, WWVB and IRIG............................ 24
D: Suggested reading................................................... 26
ILLUSTRATIONS
Figure 1. Puerto Rico Seismic Network.................................. 9
2. Examples of first motion weighting........................... 10
3. Timing examples.............................................. 11
4. Local earthquake............................................. 12
5. Regional earthquake.......................................... 13
6. Teleseism.................................................... 14
7. Teleseismic surface waves.................................... 15
8. Questionable earthquake...................................... 16
9. Quarry blast................................................. 17
10. Large earthquake............................................. 18
11. Film timing.................................................. 19
12. Seismic data record......................•••••••••••••••••••• 20
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Contents
B-l. International time-zones map................................. 23
C-la. mv and WWVH time-code format................................ 25
C-lb WWVB time-code format........................................ 25
TABLES
Table 1. Phase weighting method........................................ 3
A-l. Alternate weighting method.................................... 22
11
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SUGGESTIONS ON THE ANALYSIS OF 16-MM SEISMIC DATA FROM LOCAL NETWORKS
By
Richard L. Dart
INTRODUCTION
This report is intended as a training aid in the analysis of data
acquired by local seismographic networks. It is focused towards the project
analyst who is responsible for the location of earthquake hypocenters using P-
and S-phase arrival times, the directions of first motions, the determination
of magnitudes from coda durations, and station location quality from the
station trace clarity. To this end, the analyst must distinguish the seismic
activity from cultural, transient electrical, and microseismic background
noise.
All examples are from data acquired by the Puerto Rico Seismic Program
(fig. 1). The Puerto Rico seismic network consists of 19 seismometers at 15
permanent station sites (15 vertical and 4 horizontal Geotech S-13
seismometers). At each station there are one or more seismometers, an
ainplifier-vco/transmitter package, and a transmitting antenna. All station
data are transmitted via radio telemetry and to the central recording site
located at the San Juan Geophysical Observatory at Cayey, Puerto Rico. A 20-
channel Develocorder (galvanometric, 16-mrn automatic film-recording and
developing system) at the observatory records the 19 data channels plus a WWVB
radio time code (Tarr and King, 1976). Forty-eight hours of data are recorded
on each film from which the analyst must locate and evaluate seismic and
nonseismic activity.
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DATA EVALUATION
This section deals with timing, determining the P- and S-phase arrival
times, and coda duration of local, regional, and teleseismic earthquakes
(figs. 3, 4, 5, and 6). First, a word on definitions: phases, P and S, are
seismic waves (Bolt, 1978; Hodgson, 1964) that radiate from the earthquake
focus, or hypocenter, which is the earthquake's point of origin within the
earth. P- and S-waves travel at different speeds and thus are received by the
station seismometer at different times. This time difference is a reflection
of the distance from the earthquake hypocenter to the seismographic station.
Coda duration, or the duration of the recorded signals, refers to length of
time, in seconds, that is required for the amplitude of the recorded
earthquake signal to fall below a certain level, measured in millimeters.
Timing an earthquake consists of determining the time of the P- and S-phase
arrivals: year, month, day, hour, minute, second, to the nearest half-tenth
second. A detailed scheme for evaluating P- and S-phase arrivals and P-phase
first motions, the direction up or down, of the signal is included (tables 1
and 2), and a list of trace character remarks used in film scanning is
provided.
QUALITATIVE EVALUATION OF P- AND S-PHASE ARRIVALS
A phase arrival is indicated by a distinct change in signal character and
usually a sharp increase in its amplitude (see figs. 3 and 4). The phase
arrival is also referred to as signal response, a unique change in trace
character. Phase arrivals are either impulsive (IP or IS) or emergent (EP or
ES) depending on the amount of initial increase in the amplitude of the signal
when compared to the normal trace or background noise level (table 1). The
weighting or scaling of the signal response is an arbitrary method of
determining the quality of the phase reading. In table 1 the weighting
portion of the first motion weight, the evaluation of the signal response, and
2
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TABLE 1
P- AND S-PHASE WEIGHTING METHOD USED IN DATA EVALUATION
Phase
Impulsive P
Emergent P
Impulsive S
Emergent S
First Motion Weight
C* IP 0**
D
IPC ID
EP+2
EP+3
EP 4
IS 0
IS 1
ES 2
ES 3
Amplitude Contrast
A>5a
AX3a
A>_2a
A<2a
A<2a
A>5a
A>3a
A>_2a
A<a
Signal Response
very strong
strong
moderate
weak
very weak
very strong
strong
moderate
weak
Degree of Certainty
100%
100%
100-75%
75-25%
25%-guess
100%
100%
100-25%
25%-guess
A=initial P or S phase amplitude, a=normal trace amplitude level
*First motion direction: impulsive P-phase, Up = C, Down = Demergent P-phase, Up = +, Down = -
**Weighting of signal response: 0 = impulsive1 = impulsive2 = emergent3 = weak4 = perceptible or guess
The first motion weight scheme, column 2, is the actual notation used when recording your evaluations of P and S phases.
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the estimation of the degree of certainty are all arbitrary and are based
totally on the analyst's judgment. Degree of certainty is meant to aid the
analyst in a self-evaluation of his judgment.
SUGGESTIONS ON SCALING EARTHQUAKES
1. All possible P and S-phase arrivals should be picked and coda durations
for local and regional earthquakes determined, (figs. 3, 4, and 5).
(See the section on Trace Character Remarks.)
2. All earthquakes are timed if at least three P-phase arrival times can be
determined (figs. 2 and 3).
3. The ability to locate epicenters for regional earthquakes becomes
questionable if the signal amplitude on at least three stations never
exceeds 3 cm and if the S-minus-P time, that is the S-phase arrival time
minus the P-phase arrival time, for the first station reporting is
greater than 25 s (200 km is ~ 25 s S-minus-P time) (figs. 3 and 5). In
cases like these the magnitude of the earthquake is usually small, <2.0,
and the distance between the hypocenter and the nearest station is great
enough that P-phases will be quite emergent, thus, limiting the
analyst's ability to accurately determine their arrival times.
4. All possible P-phase arrivals should be picked and coda durations
estimated whenever possible for all well-recorded teleseisms with
distinct, clearly discernible first motions (figs. 3, 5, and 6).
5. Teleseisms are timed if the first motion response for three station traces
is clearly distinguished from the normal trace noise (figs. 3 and 4).
(See the section on Trace Character Remarks.)
6. It is possible to measure coda duration accurately if the trace amplitude
for an earthquake exceeds 1 cm (fig. 3).
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7. Duration is measured from the P arrival to a point where the trace
amplitude falls below 1 cm (Lee and others, 1972) or where the
earthquake trace amplitude is equal to twice the amplitude of the normal
trace or background noise level for at least 3 seconds. This is known
as the "3-second rule" (fig. 3). A station trace for a given earthquake
is considered WEAK if the P-phase first-motion amplitude is not at least
double the amplitude of the trace background noise and if. the trace
amplitude for the earthquake never exceeds 1 cm (figs. 2 and 3).
PHASE SCALING PROCEDURE
The actual timing, determining the phase arrival times, of earthquakes
and (or) blasts is accomplished by first projecting onto a reading surface
that portion of the data film containing the event. Projection of 16-mm
negative film on a Geotech Film Viewer, model 6585 or a 3M-brand, "500"
reader-printer or some other visual display system allows the analyst to make
fast, accurate phase-time readings. With the aid of a 30-cm scale, scaling
measurements to the nearest .05 second can be easily made. Data as recorded
on the 16-mm film is continuous for 48 consecutive hours. The beginning of
each minute is indicated by a four digit number which appears above the
uppermost trace (fig. 11). The first two digits are the hour and the last two
the minute, in Coordinated Universal Time. Thin vertical fiducial lines cross
all the traces from top to bottom at 10-second intervals. By placing a
centimeter scale against the display screen and alining two of the film's
10-second lines with the scale's 0- and 10-cm tick marks, accurate time
readings, one second per cm and one-tenth second per mm (fig. 11), can be made
and recorded. After the P- and S-phase arrival times have been determined and
recorded (fig. 12), the various station traces should be reexamined and
evaluated for first-motion weights.
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TRACE CHARACTER REMARKS
Listed here are commonly used remarks pertaining to trace character.
Trace character remarks are recorded along with other permanent data when
timing earthquakes. The use of trace character remarks will aid in reviewing
the data.
1. NOISE: Any of a number of nonseismic signal responses (cultural,
environmental, or electrical problems). Noise is either continuous,
which is a steady frequency with usually an amplitude >2 mm; or
intermittent, which is regularly, or irregularly spaced signal
bursts. Noise can completely distort or interfer with usable data
(fig. 10). Cultural noise includes noise created by highway traffic,
road construction, and so forth; environmental noises might be
livestock grazing near the seismometer, or severe storms or high wind;
and electrical noise can be attributed to any malfunction in or
adjustment to the telemetry equipment.
2. SM: Stretch marks—gaps or spaces in the station traces occurring on all
traces simultaneously (fig. 9). (An electrical problem due to
Develocorder malfunction.)
3. DT: Drifting trace—the station trace has shifted from its normal
position or has exchanged positions with or is overlapping another
trace (fig. 8). (An electrical problem.)
4. CF: Cross feeding—the character of one trace is interfering with that
of another trace. Usually such traces are adjacent. (A telemetry
problem.)
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5. SP: Spiking—an extremely sharp burst of electrical noise of short
duration, usually less than one second, often at regular intervals
(figs. 4 and 6).
6. ?ID: Questionable identification of station code—this remark is used
where crossfeeding or drifting traces are suspected.
7. ?P: The P-phase time is questionable, and an alternate time is given;
example of alternate, ?P = 0736.50.
8. 7SIGN: Not certain of the first motion given.
9. VERY-E: Very emergent response—the amplitude of the P-phase is very
weak or impossible to distinguish from the normal background activity
(fig. 3).
10. LOCAL: Local earthquake—the S-minus-P time for the first reporting
station is <^ 10 seconds (0 to 80 km), (figs. 3 and 4).
11. N-REG: Near regional earthquake—the S-minus-P time for the first
reporting station is _MO and _<20 seconds (180 to 160 km).
12. REG: Regional Earthquake—The S-minus-P time for the first reporting
station is _>10 and _<30 seconds (160 to 240 km), (figs. 3 and 5).
13. D-REG: Distant regional earthquake—the S-minus-P time for the first
reporting station is >30 sec (^240 km).
14. TELE: Teleseismic earthquake—a very distant earthquake, the S-minus-P
time is >75 sec (>1,000 km). (See Richter, 1958 and figs. 3, 6, and
7.)
15. SWARM: One of a series of similar earthquakes. (The sequence in which
the stations report is the same for all earthquakes in the swarm.)
16. FSHOCK: Foreshock—a smaller earthquake that precedes a main shock.
Both earthquakes have the same or similar hypocenters.
17. MSHOCK: Main shock—a large earthquake followed by aftershocks (fig. 9),
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18. ASHOCK: Aftershock—an earthquake that follows the main shock.
Aftershocks typically have the same or similar hypocenters.
19. 7QUAKE: Questionable earthquake—a response on two or more traces that
has an appearance of an earthquake with emergent or impulsive P-phases
and indistinct S-phases (figs. 3 and 8). A degree of uncertainty
exists in the mind of the analyst. The event could just be noise!
20. BLAST: A quarry blast usually has impulsive P-phases and indistinct S-
phases (fig. 9). In Puerto Rico blasts are usually recorded by no more
than three stations, depending on station proximity to the quarry or
road construction. The closest station shows a very impulsive signal
and the rest are contrastingly emergent.
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PIK
RT
O
RIC
O
SKIS
MO
GK
AP
IIIC
N
F/n
VO
RK
Atla
ntic
Ocea
n
Des
ech
ea
(ID
E)
A
Mo
no
(I
MO
)
A
(CS
B-Z
, C
SB
-E)
A
La P
ereg
rma
(LP
R-Z
L
PR
-NW
)
Las
Mes
as
(LS
P)
'S
anta
An
a
Mic
row
ave T
ow
er
Mag
uayo
(M
GP
) A
^ • L
as P
inas
Re
pe
ate
r J*
(CD
P-Z
C
DP
-N.
CD
P-E
) +
*^
X^
Cay
ey
(SJG
C) ^
\C
err
a L
a P
and
ura
(C
PD
)P
enu
elas
(P
NP
) C
oam
o
(CC
A)
Caribbean
10
0 10
20
30
40
50
Se
a
CENT
RAL
RECO
RDIN
G SI
TE
if
SEIS
MOG
RAPH
STA
TION
A
REPE
ATER
STA
TIO
N •
MIC
ROW
AVE
LINK
—
——
-
RADI
O L
INK
——
——
——
FIG.
1.—Puerto Ri
co Se
ismi
c Ne
twor
k.
Vert
ical
-com
pone
nt se
ismo
mete
rs are
installed
at each si
te;
stations
with horizont
al components co
ntai
n su
ffix
es in their
code
na
me to
indicate the
orie
ntat
ion
(Z=sve
rtic
al,
N=north,
E=east,
and
NW=northwest).
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IPDO
EP+2 EP-2
•f. EP-3
FIG. 2.—Examples of first-motion weighting: -^ indicates phase arrival
10
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LOCAL EARTHQUAKE-INDISTINCT DURATION
10 sec
LOCAL EARTHQUAKE—WEAK,DISTINCT DURATION
10 sec
LOCAL EARTHQUAKE-IMPULSIVE,DISTINCT DURATION
10 secREGIONAL EARTHQUAKE-VERY EMERGENT.NOT TIMED
I \ j ^ jj
\f4^\^^or i /-> I *10 sec
REGIONAL EARTHQUAKE WEAK
^^tifatyfy*^^
'lO sec
REGIONAL EARTHQUAKE IMPULSIVE I
TELESEISM
V V10 sec ~ \j
QUESTIONABLE EARTHQUAKE-INDISTINCT S PHASE
10 sec
FIG. 3.—Timing examples: -vindicate phase changes
11
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jL_rjL_n_nj
LTuULTUUI
FIG. 4.—Local ea
rthq
uake
: 1,
WWVB
10
-s ti
me line (4
0th
s);
2, min
change (1
4th
h, 55th min);
3, WWV
B timing code;
4,
firs
t station
(closest station
to th
e hypocenter);
5, spiking (NOISE);
6, P-phase;
7, S-phasej
8, las
t station
(far
th
est
station
from the
hypo
cent
er);
9,
distant station
(thi
s station
is too
far
from
th
e hypocenter to
record th
e ea
rthq
uake
); 10
,ver
tica
l channel;
11, ho
rizo
ntal
ch
anne
l;
12, most distant
stat
ion
trac
e.
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FIG.
5.—Regional ea
rthq
uake
: 1,
WWV
B ti
ming
code wa
s not
recorded;
2,P-
phas
e; 3,S-phase; 4,dead tr
ace
(int
erru
ptio
n in signal transmission due
to me
chan
ical
or el
ectr
ical
failure).
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FIG
. 6.—
Tel
esei
sm:
l,P
-phas
e«,
2,p
has
e ch
ange
(S
-phas
e)
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FIG.
7.—Teleseismic su
rfac
e waves: 1,
phas
e ch
ange
; 2,spiking,
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FIG.
8.—Questionable earthquake:
1,qu
esti
onab
le P-
phas
e; 2,overlapping traces;
3,noisy
trac
e.
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FIG. 9.—Quarry blast: 1,
phas
e ar
riva
l; 2,
stre
tch
mark
. St
retc
h marks
are
foun
d in al
l stations across th
enetwork.
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00
FIG.
10.—Large ea
rthq
uake
, magnitude
(ni) 4.
5- 1,
nois
y trace; 2,temporary
loss of ra
dio
tele
metr
y transmissions
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^\^v^^ ovv
\AV*
vvvy
v^
EYP
CPD
CS
BC
DP
CD
P-N
CD
P-E
APR
FIG.
11.—Film ti
ming
. The
P ph
ase
on station
trac
e APR
is read in th
e 6th
minu
te of
th
e 19
th ho
ur at
49.0 s.
The
firs
t mo
tion
is IP
DO.
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U.T
.C.:
hr. Ob
min.
ST
AT
ION
GR
OU
P:
Pue
rto
Ric
on N
etw
ork
ro
o
STA
.
Ey
iP-w
v
CP
O
CS
i
CD
P
CD
PN
CD
P-I
AP
R
PN
P
MG
P
ISP
MC
P
In<?
IMO
-C
CA
MO
V
its IDE
SJG
C
SJC
C E
SJG
C-N
P1
(C 0
| 1
t* H
J-'
Cl
X'C
o
r*C
c
f=r-
Z
p p
X^
'D o
f~ ^
+ 2
.t>
-Z
t>-Z
l^"p
.j-3
JP
Co
p p p p
f 3 p p
YR.
7G 7b
MO
.
tf& ot
DY
.
13 i^
HR
.
l°1 /c;
MIN
.
07
o(>
REP
oo. e
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56"
7o
S'2
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:
y-7,
ocS
'i-J
tT
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^G
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^b
"o
45
X
V
b"7
7c
AD
S
Si
i
s S S S S S S S S S S S S S S S S S S
AM
P.
(mm
.)PE
R.
MIN
.RE
RP
EAD
SR
EM
AR
KS
/Vw
sHot
ta
OU
R.
3(>l
- o
^u?-
<-
3do
^
FIG.
12.—Example of
the
Seismic
Data
Record for
the
larg
e ea
rthq
uake
in Fi
gure
s 10 and
11.
This
ea
rthq
uake
wa
s th
e main shoc
k for
several
hund
red
aftershocks.
P-phase
time
s we
re re
cord
ed fo
r each station
(ver
tica
l component
seis
mome
ters
), bu
t no S
phas
e da
ta co
uld
be determined.
CDP-
N, CD
P-E,
SJ
GC-E
, an
d SJGC-N are
hori
zont
al co
mpon
ent
seis
mome
ters
. Stations CCA, MO
V, LR
S, an
d ID
E we
re installed
afte
r 19
76 and
EYP, IM
R, SJGC-E,
and
SJGC
-N have
sinc
e be
en relocated
and
thei
r na
mes
chan
ged.
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REFERENCES CITED
Astrodata, A handbook of time code formats; Astrodata Inc., P.O. Box 3003, 240
E. Palais Rd., Anaheim, Calif. 92803, 51 p.
Bolt, B. A., 1978, Earthquakes, a primer: San Francisco, Calif., W. H.
Freeman and Company, p. 10, 27-30, 108-111.
Hodgson, J. H., 1964, Earthquakes and earth structure, Englewood Cliffs,
N.J., Prentice-Hall, Inc., p. 69, 70.
Howe, S. L., ed., 1976, NBS time and frequency dissemination services:
National Bureau of Standards, Special Publication 432, 16 p.
Lee, W. H. K., Bennett, R. E., and Meagher, K. L., 1972, A method of
estimating magnitude of local earthquakes from signal duration: U.S.
Geological Survey, Open-File Report, 28 p.
Lee, W. H. K., and Lahr, J. C., 1975, HYP071 (rev.): A computer program for
determining hypocenter, magnitude, and first motion pattern of local
earthquakes: U.S. Geological Survey, Open-File Report 75-311, 113 p.
Richter, C. F., 1958, Elementary seismology: W.H. Freeman and Company, San
Francisco, Calif., 768 p.
Tarr, A. C., and King, K. W., 1976, Puerto Rico seismic program: U.S.
Geological Survey Open-File Report 76-49, 23 p.
21
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APPENDIX A
Alternate weighting method
The data-weighting scheme illustrated in this report is not absolute.
Problems include its complexity and analytic subjectivity in timing the
data. Furthermore, such a weighting system may complicate the two weighting
methods built into the earthquake location program HYP071 (Lee and Lahr, 1975)
that generates the actual hypocenter solutions and magnitudes.
Some type of first-motion evaluation should be included with the data
when timed. This information will help in further analysis or rereading of
the data. The following weighting method is offered as an alternative.
Impulsiveness and emergence is implied in the phase numbering.
Table A-1.—Alternate weighting method
Numeric weighting First motion
0 C or D
1 C or D
2 + or -
3 + or -
Degree of Phase certainty
P or S 100%
P or S 75%-100%
P or S 50%- 75%
P or S Guess - 50%
Either of the two weighting methods offered in this report are
sufficient. The differences between them are minimal; essentially the former
offers completeness and the latter, because it is briefer and possibly more
easily understood, offers ease of comprehension.
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T) O nj(D fj H^ a* o
p wO rt IP O M
rt H 0 P y rt 3 0 H- ^ P P
O H-c on P^ pj
rt rt
CO COo
03
PJ H ^ rt CO
33 O
O rt
o no
a o • ^C/3 Oi• H-C/3 P• pi
co d P O H-
P <!
pi co V- pi
OQ HP' H-rt S
UTC REFERENCE ZONE
P n OQ '^
rt H'
r
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APPENDIX C
Timing systems
WWV, WWVH, and WWVB: The National Bureau of Standards (NBS) in Boulder,
Colorado, broadcasts Coordinated Universal Time (UTC) by radio signal (Howe,
1976). UTC is equivalent to "Greenwich Mean Time" (GMT) which differs from
local times by an integral number of hours. In order to compensate for
ionospheric conditions, the NBS broadcasts continuously on stations WWV and
WWVB, from Ft. Collins, Colo, and WWVH from Kauai, Hawaii, at a variety of
frequencies. WWV and WWVB use frequencies 2.5, 5, 10, 15, 20, and 25 MHz, and
WWVB uses 60 MHz. UTC is used by seismologists to calibrate seismic
instruments and mark recorded data. Figure Cl shows how UTC would appear on a
seismic record (Howe, 1976).
1R1G standard timing: 1RIG time codes are very similar to WWV, WWVH, and WWVB
in that they use binary number systems that can be recorded on the seismic
record, magnetic tapes, oscillographs, strip charts, printers or film
(Astrodata). The difference between NBS time broadcasts and IR1G is that IR1G
is an internal timing system that is unique to the data recording
instrument. The 1RIG time-code formats used vary according to the
manufacturer.
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FORMAT H, SIGNAL NOOI. is COMPOSED OF THE FOLLOWING:1 ppm FRAME REFERENCE MARKER R = (P 0 AND 1.03 SECOND "HOLE")BINARY CODED DECIMAL TIME-OF-YEAR CODE WORD (23 DIGITS) CONTROL FUNCTIONS (9 DIGITS) USED FOR UTi CORRECTIONS, ETC. 6 ppm POSITION IDENTIFIERS (P THROUGH P ) 1 pps INDEX MARKERS ° '
SECONDHOLE IN CODE FOR 0.8 SECOND PULSE
P,-P, POSITION IDENTIFIERS (0.770 SECOND DURATION)
W WEIGHTED CODE DIGIT (0,470 SECOND DURATION)
C WEIGHTED CONTROL ELEMENT (0.470 SECOND DURATION) CONTROL FUNCTION 16
DURATION OF INDEX MARKERS, UNWEIGHTED CODE. AND UNWEIGHTED CONTROL ELEMENTS
NOTE: BEGINNING OF PULSE IS REPRESENTED BY POSITIVE-GOING EDGE.
UTC AT POINT A • 173 DAYS 21 HOURS 10 MINUTES
o do
' UTiCORRECTION
UT1 AT POINT A • 173 DAYS Zl HOURS 10 MINUTES 0.3 SECONDS
'DAYLIGHT' TIME ZERO DURING STANDARD' TIME
0.170 SECONDS
FIG. C-la.—UTC code format used by WWV and WWVH, (from Howe, 1976)
L- UTr RELATIONSHIP
BINARY CODED DECIMAL TIME-OF-YEAR CODE WORD (23 DIGITS) CONTROL FUNCTIONS (15 DIGITS) USED FOR UTi CORRECTIONS 6 PPM POSITION IDENTIFIER MARKERS AND PULSES (P 0 THRU P 5 )
(REDUCED CARRIER D.8 SECOND DURATION PLUS 0.2 SECOND DURATION PULSE) H * WEIQHTED CODE DIGIT (CARRIER RESTORED IN 0.5 SECOND - BINARY ONE) U . UNWEIGHTED CODE DIGIT (CARRIER RESTORED IN 0.2 SECOND - BINARY ZERO)
NOT!: BE6INNINE DF PULS.E IS REPRESENTED BY NEGATIVE - GOING EDGE.
UTC AT POINT A 258 DAYS 18 HOURS 42 MINUTES
UTI AT POINT A 258 DAYS 18 HOURS 41 MINUTES 59.3 SECONDS
FIQ. Olb.—UTC code format used by WWVB, (from Howe, 1976).
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APPENDIX D
Suggested reading:
Bolt, B,A«, 1978, Earthquakes, a primer; W. H. Freeman and Company, San
Francisco, Calif,, 241 p.
Neumann, F., 1966, Principles underlying the interpretation of seisraograms:
U.S. Dept. of Commerce, Special Publication No, 254, 50 p.
Richter, C. F., 1958, Elementary seismology: W. H. Freeman and Company, San
Francisco, Calif,, 768 p.
Simon, R, B,, 1968, Earthquake interpretations; Colorado School of Mines,
99 p.
GPO 857-C79
26