Chatelain, Jean-Luc

Hardjawidjaksana, K.

Joyodiwiryo, Y.

Kartusuwita, J.

Kertapati, E.

Kuntoro, T.

Lion-Caen, H.

McCaffrey, R.

Molnar, P.

Nasution, J.

Patmowidodo, S.

Prih Haryadi, P.Y.

Reed, D.

Riyadi, M.

Roecker, S.

Sukarna, A.

Horizon / Pleins textes

1- 9 
BulL Geol. Res. and Dev. Centre, No. 10, 1984 
Preliminary Results of the 1982 Microearthquake 
Survey in Nusa Tenggara Timur, Indonesia 
R. McCaffrey, H. Lion-Caen, P. Fllolnar, S. Roecker 
Massachusetts Institute of Technology, Cambridge MA, USA 
J.L. Chatelain 
Office de la Recherche Scientitifique et Technique d'outre Mer, Noumea 
K. Hardjawidjaksana, Y. Joyodiwiryo, E. Kertapati, T. Kuntoro, J. Nasution, A. Sukarna 
Geological Research and Development Centre, Bandung, Indonesia 
J. Kartusuwita, S. Patmowidodo, P.Y. Prih Haryadi, M. Riyadi 
Institute of Meteorology and Geophysics, Jakarta, Indonesia 
D. Reed 
Scripps Institute of Oceanography, La Jolla, CA, USA 
ABSTRACT 
Froin late June until the middle of September, 1982, we operated portable seismographs on the islands of Flores, 
Timor, Adonara, Lembata, Pantar, Alor, and Savu in Nusa Tenggara Timur, eastern Indonesia, During this period, 
approximately 2000 earthquakes were recorded of which 460 were locatable and occurred near the seismograph 
network. Many of the events were shallow (less than 50 km depth) although there was a concentration of activity 
in the 70 - 200 km depth range at  about latitude 9's and longitude 124' E. This cnvcentration forms a 45' 
northwest dipping seismic zone and occurs within the lithosphere of the Indian Ocean. - Australian platc subducted 
at the Java trench - Tiinor trough subduction system. Levels of seismic activity east of about 124.5' E seem to 
be depressed possibly owing to the partial subduction of continental crust in that area. Seven earthquakes of 
magnitude 5.0 or greater occurred near the network during the survey period. Three of these occurred at  shallow 
depth beneath western Flores including the two events of August 6 that caused ccnsiderable damage. Other large 
events included two near go S, 124O E, one north of Savu Island and one deep (570 km) cvcnt about 100 km 
north of western Flores. 
INTRODUCTION 
Most of the present tectonic complexity observed 
in southeast Indonesia is due to the northward 
movement of the Australian continental mass rela- 
tive to southeast Asia as part of the Indian Ocean 
plate and its encroachment on the Indonesian 
islands. The presence of continental crust beneath 
the Sunda forearc region north of the Timor trough 
produces a marked change in the distribution of 
earthquakes, volcanoes, and geologic structures 
along this portion of the convergent margin. West 
of the Savu Sea, where oceanic crust is being sub- 
ducted, tectonic features such as earthquakes and 
volcanoes lie in belts parallel to the Java trench. 
East of the Savu Sea where continental crust is 
underthrusting the forearc, however, the line of 
active volcanoes is interrupted, sismicity occurs 
at very low levels, and the width of the forearc 
varies drastically along strike. 
In this paper we present locations of microearth- 
quakes recorded by a 10 week long temporary 
seismograph network around the Savu Sea during 
1982. @ur aims are to characterize the process 
of collision of a continental margin with an island 
arc and to describe more accurately the distribu- 
tion of earthquakes in the Nusa Tenggara Timur 
region. 
-I- 
-% 
2 
THE FIELD PROGRAM 
&, a cooperative effort between the Massachusetts 
Institute of Technology (USA), Geological Research 
and Development Centre (Bandung), and Institute 
of Meteorology and Geophysics (Jakarta), a network 
of portable seismographs was installed in Nusz 
Tcnggara Timur for approximately 10 weeks in 
1982. The first station was installed on June 25 
and the last removed on September. 16. 
tions of the recording sites are given in Figure l 
and Table 1 and the operation dates are‘displayed 
in Figure 2. All instruments used short-period 
vertical seismometers and, in addition, station 
Adonara operated one horizontal component (N-S). 
Portable instruments used include models Spreng- 
nether MEQ-800, Kinemetrics PS-IA, anc! Geo- 
tech Portacorder. Additional arrival time data were 
acquired at the permanent Indonesian seismograph 
stations at Kupang, Timor (KUG), Waingapu, 
Sumba (WSI), Ujung Pandang, Sulawesi (MKS), and 
The loca- 
Table 1. Coordinates of Temporary Stations 
Station Latitude Longitude 
os ’ OE ’ 
Ende 
Maumere 
Larantuka 
Adonara 
Hadekawa 
. Pantar 
Mali 
Dili 
Atambua 
Kefamenanu 
Niki-niki 
Soe 
Camplong 
Baun 
Savu 
8 49.1 121 38.7 
8.38.0 122 12.5 
8 16.2 122 59.5 
8.23.1 123 9.4 
8 23.0 123 32.2 
8 23.7 124 4.9 
8 8.6 124 34.9 
8 34.9 125 34.5 
9 5.5 124 53.3 
9 26.5 124 28.5 
9 48.5 124 28.0 
9 54.4 124 18.4 
10 3.0 123 55.0 
10 18.5 123 40.0 
10 29.1 121 52.2 
Rg. 1. Geographic and tectonic map of southeast Indonesia. Thrust faults are shown as solid lines with 
barbs on the hanging wall, Position of temporary seismograph stations are shown by solid circles 
and permanent stations by solid squares. Solid triangles show active volcanoes. X’s and associated 
numbers show epicenters and depths of events of magnitude 5.0 or greater. Bathymetric contours 
are in metres. 
3 
25-2 
July 
I -  
- 
IO- 
- 
20 - 
- 
August 
I -  
- 
10- 
- 
20- 
- 
i t ember  
I -  
- 
I O -  
- 
June 
' 
o) L
o) 
E m  
L 
Co 
c, 
E 
m 
O- .- 
e
=I 
E 
a 
Fig, 2. Bar chart showing operation dates for all 
temporary seismograph stations 
Ambon (AA1 at 3.9% and 128.1°E). Time syn- 
chronization was obtained by marking the broad- 
casted Coordinated Universal Time on the seismo- 
gram either electronically or manually via the 
instrument. 
DATA ANALYSIS. 
Arrival times were picked from the seismograms 
with the aid of a digitizing table which introduced 
an error of no more than 0.10 s. Additional errors 
are in the clock correction (0.05 to 0.3 s) and in 
determining the onset of the phases (probably less 
than 0.10 s for local events). 
Earthquakes were located with the computer 
program HYPOINVERSE (Klein, 1978) which uses 
a layered, flat earth velocity model. We assumed the 
P wave velocities of Herrin (1968) and a P to S wave 
velocity ratio of 1.75 . All locations were graded A, 
By or C (in order of decreasing reliability) according 
to root-mean-square error in residuals (RMS), resolu- 
tion of epicenter and depth, and the distribution 
and number of stations recording the event, as out- 
lined in Table 2. The uncertainties in the hypocen- 
tral locations associated with the above classifica- 
tions were evaluated by examining the variations in 
locations of many events (of each classification) due 
to perturbations in the assumed crustal structure, 
ratio of P t t ?  wave velocities, and group of 
stations recording the event. The tests indicate that, 
for an A classification, the depth is within 8 km 
and the epicenter is within 5 km. A B rating infers 
a depth within 15 km'and an epicenter within 
10 km. The uncertainty associated with the C 
classification is more difficult to assess because most 
events of this type fail the requirement that the 
nearest station be within one or two focal depths 
of the epicenter. 
Nevertheless, these events can have well determined 
epicenters when they are within the network or 
located by many stations. In general, the uncer- 
tainty in the location increases with decreasing 
depth and with the distance to the nearest station 
when the event is outside the area enclosed by 
the network. 
RESULTS 
The locations of all epicenters are shown on 
Figure 3. Clearly, a large concentration of recorded 
events occurred south of Pantar island (9OS, 124OE) 
.J 
- 
- 
4 
t 
' *  
* *  * 
O O t- * * o  O t  . o 
O 
- 
* O O . O 0  O o o  O @  o o O 0  o . o r  . -  + O 0  
o DEPTHS .o 
O 
0 0 - 5 0 k m  
- 50- 100 
0 0  
. o  
& O 0  0 0  
0 100- 150 
0 150- 200 
4 > 2 0 0 k m  
. a 
O *  O 0  
O o 0  0 O 
o O Seismograph 
3 station 
I I I 
at depths of 70 - 200 km. Figure 4 shows a view 
of all hypocenters projected onto an east-west 
trending vertical plane. East of 1 19OE, the seismic 
zone is fairly well defined to  a depth of about 200 
lan and to 300 km between 1 2 2 O  and 124OE. 
East of 124OE, however, the seismic zone changes 
shallow and there are only 2 events of class B 
deeper than '00 km. Note that the volcanic chain 
dS0 terminates at 124OE. The Seismic CpieSCenCe 
at intermediate depths in the vicinity of Alor and 
Wetar is also observed in the long term SeiSmiCity 
Pattern described by earthquake locations deter- 
mined by the International Seismological Centre 
using data from the World Wide Standardized 
Seismograph Network (see Cardwell and Isacks, 
1978). This quiescence in seismicity and vol- 
canism is probably related to the presence of.the 
Australian continental mass in the subduction 
zone but the mechanism is not clear. 
character as the majority Of events appear to be Hypocenters beneath the eastern Savu Sea reveal 
' 
a trend of deepening to the northwest (Figure 3). 
BY plotting cross-sectional projections at various 
digmeht occurs at about ~ 2 5 0 ~  (Figure 5). %is 
projection includes all earthquakes within the area 
enclosed in brackets in Figure 3. In this projection, 
we observe a dense zone of seismicity dipping to 
the northwest at an angle, of 4S0 between 70 and 
we have determined that dle best 
Table 2. Rating Criteria for Hypocentral Locations. 
Gap Rating RMS ERH ERZ # Phases #s. Nearest s .  km km Stations 
-_-----I_ 
A > 0.1,Gl.O <IO GI0 2 7  3 1  <depth < 180° . 
B < 2.0 <I o 430 > 5  - G2 x depth < 270° 
C < 3.0 <99 <99 2 4  - I - 
I18'E I20 I22 I24 I26 128' 
6" S 
8 
IO 
12" s 
I I I I 
l -
o . 0 -  
Fig. 3. Epicenter map for all earthquakes located by the temporary network. Brackets outline area of 
projection of Figure 5. 
J 
5 
f ig .  4. Projection of all earthquake hypocenters onto an east-west vertical plane. Large dpts represent class A 
events, intermediate size dots are class B, and small dots are class C. Triangles show active volcanoes at  
surface. No vertical exaggeration. 
m. 
Pantar  OS NW 
A A 4 
1 . .  p I .  . i -  ... . . . ... .. . 0 .  0 . 8 . .  - . .. 
. . . . '3 . OØ . 
I . 1 . 
I O0 
km 
209 
300 1 
5. Projection of hypocenters in bracketed area in Figure 3 onto a vertical plane sttiking N25OW. 
Symbols are as in Figure 4. 
6 
200 km depth and continuing more sparsely to 
300 km. At shallower depths, a few better located 
events (class A) form a zone that appears to flatten 
out at about 50 km depth beneath Timor. These 
events do not project towards the Timor trough but 
rather occur in the lower crust or upper mantle 
beneath Timor since the crust of Timor is probably 
about 40 km thick (Chamalaun er a l ,  1976). 
One striking feature of the seismic zone between 
100 and 200 km depth is the presence of two 
separate lineations about 30 km apart that converge 
at about 200 km depth. This feature of the seismic 
zone is not an artifact oftlie projection as the 
events in the upper zone are at the same longitude 
as those that form the lower zone. It is difficult to 
decide whether or not this truly constitutes a double 
seismic zone, as is observed beneath Japan 
(Hasegawa er a l ,  1978), because the locations of 
most of tlie earthquakes in the upper zone are of 
class B, representing less well constrained locations. 
The majority of earthquakes occurring at less than 
50 km beneath Timor depth are classified as C due 
to the requirement that the nearest station be within 
one or two focal depths of the epicenter. The depths 
of these shallower events are in general not as well 
constrained as those of the deeper events. Neverthe- 
less, ,we suggeststhat many of these earthquakes 
occurred within the crust for two reasons. First, 
stations operating on Timor recorded many events 
with S-P times of less than 5 s, implying a focal 
depth of less than 40 or 50 km. Secondly, seismo- 
grams of earthquakes that occurred near the 
stations on Timor often displayed a strong degree of 
ringing probably due to scattering aiid indicative of 
a source within the crust. 
Figure 6 is a projectior. of all earthquakes shown in 
Figure 3 west of 123.3OE on to a north trending 
vertical plane. The better located events (A and B) 
outline a seismic zone that dips to the north to 
almost 300 km depth. This seismic zone projects 
upward towards the Java trench at l loS and 
represents the subducted portion of the Indian 
Ocean plate. The scattering of hypocenters in this I 
projection is probably due to the large width of the 
projection area. 
Java Trench s 
A m  
I I I '  I 
\ 
I ' -. 
N 
Fig. 6. Projection of all hypocenters between 118.75"E and 123.25'E onto a north-south striking vertical plane. 
Symbols are as in Figure 4. 
7 
A large number of shallow events occurred just 
south of 8OS (Figure 6). In map view (Figure 3), we 
see that most of these occurred beneath western 
Flores and are associated with the sequence on 
August 6 and 7. We infer that these events probably 
result from deformation of the island arc during 
collision because a preliminary fault plane solution 
for the main event of August 6 shows a’strike-slip 
mechanism (from the National Earthquake Informa- 
tion Service). 
EVENTS OF MAGNITUDE 5.0’OR GREATER 
. During the survey period, seven earthquakes of 
magnitude 5.0 or greater (as reported in the 
Preliminary Determination of Epicenters Bulletin) 
occurred within the field area (Figure 1 and Table 3). 
The largest of these occurred north of western 
Flores at 2&40m UCT on August 6 and was of 
magnitude 5.9. This event was followed by a 
mb = 5.5 earthquake in the same region 5 minutes 
later. Both events were felt by members of the 
survey team in Ende and Maumere. Another, 
smaller (mb = 5.3) shock occurred in the same 
region on August 21 at 05h06m. All of these 
events were shallow but the depths are poorly 
constrained by our data. 
Two large events occurred within the seismic zone 
at 9OS and 124OE. These occurred at depths of 
46 km (August 2, 10h49m, mb= 5.4) and 86 km 
(August 11, 10h28m, mb = 5.5). We have no 
reports of either event being felt. One very shallow 
event of magnitude 5.0 occurred on August 9 at 
03h15m beneath the central Savu Sea. Finally, 
a very deep shock (579 km) occurred north of 
western Flores on August‘ 9 (21h09mY nib = 5.2). 
’ 
Table 3. Earthquakes of. Magnitude 5.0 
DlSTRIBUTION OF S - P TIMES 
The difference between the arrival times of the P 
and S wave at  a seismograph station is a direct 
measure of the distance between the station and the 
hypocenter. By examining the frequency of occur- 
rence of 3 - P times at a single station, we can 
often gain insights into the distribution of earth- 
quakes not apparent from the pattern of locatable 
events. In Figure 7, S - P times are plotted against 
the number of events for many of the temporary 
stations. 
The stations on Timor (Figure 7a) show a large 
number of S - P times of less than about 20 s and in 
the range 30 - 70 s.. (The S - P times can be multi- 
plied by 10 to make an approximate conversion to 
distance in kilometers). The presence of events with 
S - P times of less than 5 s (especially at Kefame- 
nanu) indicate that the crust of Timor is involved in 
the deformation associated with the collision process. 
The peaks of 14 s at Atambua, Soe, and Hadakewa, 
10 s at Kefamenanu, and 13 s at Pantar, (Figure 7 b) 
are due to earthquakes frbm the intermediate depth 
seismic zone at 9OS. and 124OE. Both Hadakewa 
and Pantar show peaks at 4 - 5 s indicating shallow 
earthquakes along the volcanic arc. The arc in this 
region is cut by numerous shallow faults (Silver 
et aL, 1983) which may be active but the earth- 
quakes may also be due to volcanic activity. Ende 
and Maumere show strong peaks at 19 and 24 s, 
respectively; that are due to the swarm of activity 
north of westem Flores. 
The broad peak between 30 and 70 s at the Timor 
stations (Figure 7a) is conspicuously absent from 
the stations along the volcanic chain (Figure 7b). 
or Greater 
J 
Date Origin Time Latitude Longitude. Depth . Magnitude 
1982 H r h h s  O S ’ O E  ’ km 
August 2 10 49 36.7 9 3.4 124 2.0 46 5.4 
6 20 40 56.0 8 6.1 120 31.3 25 5.9 
6 20 46 18.4 8 10.8 120 38.6 ‘26 5.5 
9 3 15 47,l 9 44.5 121 20.5 5 5.0 
9 21 9 35.9 7 24.8 120 27.1 579 5.2 
11 10 28 52.5 8 56.3 124 6.0 86 5.5 
21 5 6 7.7 8 25.1 120 20.9 O 5.3 
t Il 
N 
20 - - Ende 
O II I ,  , I I ~ . I .  I III. I I , ,  * I . .  
- Maumere 
I I I I  , , , 
20 - 
Dili 
O 
- 
- 
20 - - Atambua 
O I I I 1  , , ,  , , 
Kef amenanu 
Soe 
O 
10 
O 1 1 , 1 1 1  I I .  I ,  
I O  Camplong 
O I I I ,  yILLLILLI,I.<,II!,. , , 
r 
N 
40 L I  L 
- 
- 
Hadakewa 
I,-, ,, 
Mali 
O 
1 
60 80 I O0 20 4 0  
' o ~ & l , l  I , I , ,  , I .  , ,, 
Savu 
O 
O 20 4 0  60 80 I O0 
'o!LuL&ddhL,ll .II a .yLu, III I I  I I l  ,,I ,, , ,, I 
O 
S - P Time (SI S -P Time (SI 
Rg. 7. Plot of S - P times (rounded to nearest second) against number of occurrences N. 
9 
It is likely that most of these earthquakes occurred 
near 7OS and 129OE within a zone of extreme 
activity along the eastern Banda Arc (Cardwell and 
Isacks, 1978). Apparently, shear waves from that 
area propagate poorly to stations on the volcanic 
arc but well enough to the southwest to be 
observed at the stations on Timor. This simple 
observation suggests preliminarjly that :the subducted 
slab north of the Timor trough is continuous beneath 
the eastern Banda Arc and north of Timor. 
SUMMARY 
A 10 week microearthquake survey in Nusa 
Tenggara Timur during 1982 has provided a more 
detailed picture of the distribution of earthquakes in 
this tectonically active region. Many of the locatable 
earthquakes occurred in a dense seismic zone at I 
intermediate depths beneath 9OS and 124OE. These 
earthquakes are part of a steeply north to northwest 
dipping seismic zone that projects upward toward 
Timor and probably represents subducted lithosphere 
of the Indian Ocean - Australian plate. East of this 
active region, where continental crust of Australia is 
involved in the subduction zone, earthquake 
activity is at relatively low levels and mostly at 
shallow depths. To the west, however, an inter- 
mediate depth seismic zone is observed to dip to 
the north from the Java trench to slightly less than 
300 km depth. A swarm of earthquakes occurred 
near western Flores starting with a magnitude 5.9 
event on August 6. Many of the shocks were felt 
shock caused damage to several buildings. 
the Kabupaten Manggarai area and the main 
RE FERENCES 
Anonym, 1982. Preliminary Determination of Epi- 
centers, National Earthquake Information 
Service, June-Septhber. 
Cardwell, R.K. and B.L. Isacks, 1978. Geometry of 
subducted lithosphere beneath the Banda Sea 
in eastern Indonesia from seismicity and fault 
plane solutions. Jouriial of Geophysical 
Research, 87, 2825-2838. 
Climalaun, F.H., K. Lockwood and A. White, 1976. 
The Bouguer gravity field of eastem Timor, 
Tec tonophysics, 30, 24 1 -259. 
planed structure of the deep seismic zone in 
the northeastern Japan Arc. Tectonophysics, 
Hasegawa, A., N. Umino and A. Takagi, 1978. Dbubl 
47,43-58. 
Herrin, E., 1968. Introduction to ”1968 Seismologi- 
cal Tables for P Pha~es”. Bulletin of the Seis- 
mological Society of America, 58, 1193-1 195. 
Klein, F.W., 1978. Hypocenter Location Program 
HYPOINVERSE, U.S.G.S. Open-file Report 
78-694. 
Silver, E.A., D. Reed, P.. McCaffrey and Y .  Joyodi- 
wiryo, 1983. Back arc thrusting in the eastern 
Sunda arc, Indonesia: A consequence of arc- 
continent collision, Journal of Geophysical 
Research, 88, 7429-7448. 
\ 


Preliminary results of the 1982 microearthquake survey in Nusa Tenggara Timur, Indonesia

Preliminary results of the 1982 microearthquake survey in Nusa Tenggara Timur, Indonesia

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