Observed travel times for Multiply-reflected ScS waves
from a deep-focus earthquake
A . P . ESPESOSA ( * )
Ricevuto il 26 Agosto 1966
SUMMARY. — Tlie deep-focus Argentinean earthquake of December 8
1962, generated multiply reflected ScS phases which were recorded very
clearly at stations of the IGY and the TJSC&GS standardized worldwide
networks and at Canadian stations. The data gathered from this earthquake for the multiply-reflected ScS and sScS were used to construct the
travel times and to extend them to shorter epicentral distances. These
new data brought to light an error in published travel times for the 2(ScS)
phase.
RIASSUNTO. — 11 terremoto profondo avvenuto in Argentina l'8 Dicembre 1962, ha originato onde ScS riflesse più volte, che sono state registrate
molto chiaramente dalle stazioni sismiche della rete mondiale standard dell ' I G Y e deh'USC&GS e dalle stazioni Canadesi.
1 dati raccolti mediante lo studio di questo terremoto per le onde ScS
e sScS, sono stati usati per costruire la curva dei tempi di tragitto ed estesi
a distanze epicentrali più brevi.
Questi nuovi dati, inoltre, hanno messo in luce un errore relativo ai
tempi di tragitto già pubblicati per le onde 2(ScS).
INTRODUCTION
Multiply-reflected ScS and their surface images f r o m the December
8, 1962, deep-focus earthquake in Argentina were unusually well recorde d b y long-period seismographs at stations of the IGY
GS standardized network, and at Canadian stations.
describes a study made on the identification of the
and the
TJSG&
This investigation
multiply-reflected
( * ) Environmental Science Services Administration, U.S. Coast and
Geodetic Survey. Rockville, Maryland.
Fig. 1. - Map showing location of epicenter, stations, and a great circle path through Palisades.
OBS KR V KD T l i A V K L
T I M E S j,\>K M U L T I P I . Y - R F.FLF.CTED.
ETC.
417
phases, and the construction of their travel-times as recorded on longperiod instruments.
T h e observations of the travel-times of the
2(ScS)
at shorter cpicentral distances have permitted the extension of the travelt i m e curve for this phase down to an cpicentral distance of 7°.
Pre-
viously in the literature, the travel-time curve for
been
given for an cpicentral distance greater than 90°.
has
2(Sc8)
T h e n(8cS)
family
of phases has been identified without any doubt b y the presence of the
phases on the recordings.
n(s8cS)
Identification
in the m a j o r circle path is shown.
served travel times of the 2(ScS)
time
curve
for this phase
of
phases
traveling
I n the process of plotting the ob-
phase, an error in the published travel-
has been
found, for a depth of focus of
600 k m .
INTERPRETATION OF THE DATA
T h e epicenter, origin time, and depth of focus for the December 8,
1962, earthquake were located b y
the 7090 IBM
phases, .14 pP
computer. One
and 10 PKP
using
hundred
Bolt's
fifty
(1960)
program
for
readings of impulsive P
readings, were used in the relocation. T h e
pertinent data are listed in Table I .
Table I
Origin Time
(GMT)
Epicenter
h
25.8"
S
25.78° S
63.4°
m
s
Depth
Magnitude
km
W
21 27 22.2
620
62.15° W
21 27 18.0
580
7
PAS
USC&GS
6 3/4 PAL
T h e data used in this study are f r o m the stations shown in Figure 1.
T h e solid line in this figure is a great circle path through Palisades,
N . Y . , and the station code identifying each of the stations corresponds
to the three letter code assigned b y the U8G&G8.
Readings of the phases
which constitute the data were done f r o m long-period recordings having
a
natural period
galvanometer,
of 30 sec
and
100 sec for
the
seismometer
and
respectively.
Figure 2 shows a graphical description of the phases which have
undergone four reflections at the
core-mantle
boundary,
denoted
as
418
A. F.
4(ScS),
ESPINOSA
through the minor circle path and recorded at SGH.
A l s o in
the same figure is shown a phase which travels through the m a j o r circle
path and has been reflected three times at the core-mantle
denoted as 3(ScS)~,
boundary,
and also recorded at the same station.
O
6370k
Pig. 2 - Graphical example of ray paths and notations used for the multiplyreflected phases which have traveled along the minor and major circle paths.
F i g u r e 3 shows a tracing of the E-W
GAB,
component seismogram at
w i t h an epicentral distance of 36.24° and an azimuth f r o m the
station to the epicenter of 174.61°.
T h e original recording was done
on a recorder with a 30 mm/min. revolution rate.
tracing has been reduced b y a factor
of
four.
I n this figure, the
The
portion
of
this
recording is between the arrival t i m e of the S phase and approximately
a minute after the arrival t i m e of the 4(8c8).
On the original seismo-
gram, the peak-to-peak amplitude of the ScS phase is 357 m m .
n(ScS)
and n(sScS)
cation of the n(ScS)
are indicated
on the
seismogram.
The
series has been made certain b y the
its surface images on the recordings.
The
identifi-
arrival
of
(SH)
S
ScS
2(ScS)
4
3(ScS)
2(sScS)
3(sScS)
4(sScS)
MIN
CAR
E-W(30-I00)
F i g . 3 - Seismogram f r o m G A R , December
8, 1962, E - W long-period
component.
(SH)
s
ScS
sScS
2(ScS)
2(sScS)
3(ScS)
3(sScS)
4(ScS)
4(sScS)
•-i
1
BEC
MIN
E-W(30-I00)
Tig. 4 - Seismogram from BEG,
December
8, 1962, E-W
long-period
(ScS)
component.
420
A.
F.
ESIM N o f j A
A n o t h e r typical sei.sinogr.un tracing is shown in Figure 4. This figure
is the E-W
component, at, BEG
with an epioentral distance of
57.86°
and an azimuth of 17*.6" and shows an especially clear recording of the
n(ScS)
and
the nlsScN)
indicated from n -
series.
On
this
record,
the
two
series
are
1 to an order I; however, at, the end portion of this
seismograni, the 5(KcS)
can be .s<-<-n very clearly.
This phase was also
well recorded at st.it.ions close to the epicenter, but there was difficulty
in picking its arrival time since the background noise is high in that
portion of the seismograni.
On inspection of the N-iS' component seis-
mograni, the, horizontally
amplitude,
and
in
the
polarized
vertical
shear
phases
component
are
these
very
phases
small
were
in
not
discernible.
Figure 5 shows a composite of seismograms for nine stations. E i g h t
are due, N o r t h of the epicenter, and the ninth is due East. These recordings were made; on long-period instruments; the E - W component is the
one shown in this figure,
liiiforfiinatcly, t he variation of speed in the
revolution of the drum is different I'or sonic of these stations; some have
a 15 irim/min. and others 30 inm/niin. revolution rate.
W h e n reduction
was done, on each of the seismograms in this composite figure, a one-to-one
correspondence was lost. T h e picture length is made up of four parts
put together, which can be seen in the breaking of the time axis.
This
f a c t also has introduced distortion, and because of these distortions, no
attempt
is made
to
draw
a
travel-time
phase with another for a given
series.
curve
The
ANT,
which
connect
one
and
ABE
BAA,
seismograms were omitted in this figure since their locations are too
close, to the time, axis.
ginals, and the PAL,
T h e RDJ,
HAL,
SGtt,
GAB,
VIC,
BEG
BES,
are tracings of the oriand ALE
pictures of the original seismograms at these stations.
are reduced
T h e time axis
is in minutes, with an interval of 2 minutes, and the, distance axis is
in degrees.
TRAVEL-TIME CURVES.
T h e seismograms obtained f r o m 38 of the IGY,
USC&GS,
and the
Canadian stations w e r e read (sec Figure 1) and a total of 23 station records were used in the filial gathering of the data which is used to construct the travel-time curves f o r the multiply-reflected phases. T o interpret the phases on the recordings, the travel times of impulsive phases were
picked, tabulated, and plotted.
These readings were done on all three
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DISTANCE
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(degrees)
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120
OBSERVED
TRAVEL
TIMES
FOR
M ULTIPI.Y - REFLECTED,
ETC
421
4 500
4000
O O0
4(sScS)
4(ScS)
3500
3(sScS)
3000
,oO
ü
-h
. O-O-O""
3(ScS)
o-no—-O
2500
2(sScS)
0°
<8
I ..-CM
O
-<T<fP"
2000
—O-OO—CD"
--OCT"
2(ScS)
1500
TRAVEL TIMES (sec)
DECEMBER
8,1962
h= 5 8 0 k m
1 0 0 0
5
15 25 35 4 5
55 65 75 85 95
105 115 125 135 145 155
DISTANCE (degrees) — •
Fig. 6 - Observed travel times for the n(ScS) and the n(sScS)
Time in seconds; distance in degrees.
series.
422
A. F.
components.
JSSPINOSA
A f t e r identification of the phases was certain, ellipticity
corrections were applied to each of the readings.
travel times for the n(ScS)
and n(sSaS)
Figure 6 shows the
series obtained f r o m the deep-
< — D I S T A N C E (degrees)
345
325
305
285
265
245
225
205
D I S T A N C E (degrees) — •
Fig. 7 - Observed travel times for the 3(ScS)~ and 3(sScS)~ phases.
in seconds; distance in degrees.
Time
(85
OBSERVED
TRAVEL
T I M E S FOR
MULTIPLY-REFLECTED,
ETC.
423
Fig. 8 - Travel-time curves (after Richter). The dash-line travel-time curve
corresponds to the correct curve for the 2(SoS) phase.
424
A. P.
KSPIXOSA
focus earthquake of December 8, 1962.
T i m e is given in seconds and
distance in degrees. T h e criterion used in the identification of the
n(ScS)
series is t h a t of the presence of its surface images on the recordings (see
Figures 3, 4, and 5).
T h e dashed
for
2{SeS),
curves in this figure
and
3(Sc8),
4(Sc8)
for
correspond to the traArel times
a
depth
of focus of
580 k m .
Some of t h e observed points on these curves were checked against the
Jeffreys and Bullen tables (1948), using the times of 8c8
foci and the corresponding depth allowances.
f o r a surface
A t first, Gutenberg and
Kichter travel-time curves for h — 600 k m wen» used; however, the observed t r a v e l times f r o m this earthquake at epicentral distances of 90° to
approximately 125° were earlier than the times given b y their curves.
I n this figure, the n(ScS)
n =
4.
and the -»(sScS)
series are plotted up to order
T h e identification of order n — 5 was possible in many of the
records, but the scatter of the data readings was large; hence, these
readings were omitted.
by
the
USC&GS
the data
the
in
T h e recently installed long-period instruments
South America
at close epicentral
travel
distance of
time
observations
made
distances,
of
the
possible the gathering of
and hence, the extension
2(8cS)
down
to
an
of
epicentral
7°.
I n F i g u r e 7 are shown the t r a v e l times of some of the phases Avhich
have arrived through the m a j o r circle path.
to the travel-time curves for the 3(8c8)~,
T h e solid lines correspond
and the 4(ScS)~,
lines to their corresponding surface images.
and the dashed
T h e observations of the
earlier arrivals in this figure f i t those of the 3(8cS)~
and the latter ones
fit those of their surface image. A f t e r the arrival times of the
3(sSc8)~,
there are a number of observations which do not fit the 4(8c8)~
travel-
time curve, and the latter arrivals scatter v e r y much, so one cannot
identify, with
certainty,
these phases as being of
the 4(8c8)~
series.
These phases in question are clearly recorded on some of the seismograms (see F i g u r e 5) with
amplitudes larger than those phases which
have arrived through the minor circle path.
Figure
8 shows the travel-time
" E l e m e n t a r y Seismology
curves given in Eichter's
book,
on page 684, and on i t a dashed travel-
time ciu've has been drawn which corresponds to the correct travel-time
curve for the
2(8cS)
phase obtained in this study.
The
travel-time
curve published earlier b y Gutenberg and Richter (1937) f o r the same
phase, f o r a depth of focus of 600 k m , should be redrawn as the dashed
travel-time curve shown in Figures 6 and 8.
OBSERVED
TRAVEL
TIMES
FOR
MU LTI I ' L Y - R EFLECTElJ.
ETC.
425
REFERENCES
BOLT B. A., The Revision of Earthquake Epicenters, Focal Depths and Origin
Times Using a High-Speed
Computer. Geophysical Journal, Vol. 3,
433-440 (1960).
GUTENBERG B. and RICHTER C. F., Données Relatives a L'étude Des Tremblements de Terre a Foyer Profond.
Publications du Bureau Central
Séismologique International, Ser. A , Travaux Scientifiques, 15, 1-70
(1937).
JEFFREYS H . and BULLEN K . E., Seismological
Association (1948).
RICHTER C. F., Elementary
Tables.
London,
British
Seismology. Freeman & Co., San Francisco (1958).