We report on three intermediate depth earthquakes in the India‐Tibet collision zone, two under the Himalayan Thrust Belt (HTB) and one beneath the Indus Zangbo suture. The mb magnitudes of these three events are from 4.3 to 4.9, and are too small to be well located by conventional means. However, from modeling their broadband waveforms recorded at near‐regional distances on a temporary PASSCAL array, we can confidently confine the sources to be below the crust, between 70 and 80 km deep. The existence of these intermediate depth earthquakes in this area suggests relatively low temperatures in the mantle lithosphere. The two events under the HTB display strike‐slip mechanisms with some normal faulting component; this is quite different from the shallow thrust events typical of the same area. The principal P and T axes of all 3 fault plane solutions show roughly NS compression and EW extension, consistent with a regional stress field produced by the indenting of the India continent

Helmberger, Donald V.

Zhu, Lupei

Caltech Authors - Main

GEOPHYSICAL RESEARCH LE1TERS, VOL. 23, NO.5, PAGES 435-438, MARCH 1, 1996 
Intermediate depth earthquakes beneath the India-Tibet 
collision zone 
Lupei Zhu and Donald V. Heimberger 
Seismological Laboratory, California Institute of Technology, Pasadena 
Abstract. We report on three intermediate depth 
earthquakes in the India-Tibet collision zone, two un-
der the Himalayan Thrust Belt (HTB) and one beneath 
the Indus Zangbo suture. The mb magnitudes of these 
three events are from 4.3 to 4.9, and are too small 
to be well located by conventional means. However, 
from modeling their broadband waveforms recorded at 
near-regional distances on a temporary PASSCAL ar-
ray, we can confidently confine the sources to be below 
the crust, between 70 and 80 km deep. The existence of 
these intermediate depth earthquakes in this area sug-
gests relatively low temperatures in the mantle litho-
sphere. The two events under the HTB display strike-
slip mechanisms with some normal faulting component; 
this is quite different from the shallow thrust events 
typical of the same area. The principal P and T axes 
of all 3 fault plane solutions show roughly NS compres-
sion and EW extension, consistent with a regional stress 
field produced by the indenting of the India continent. 
Introduction 
Since the inception of the India-Eurasia collision at 
approximately 45Ma, the India plate has indented north-
wards about 2000 km, producing the highest continu-
ous topography on earth, the Himalayas and the Ti-
betan plateau. In the past two decades, considerable 
efforts were made to investigate the crustal and upper-
mantle structure of this area in order to understand the 
collision mechanism and formation of the plateau (see 
Molnar, 1988 for a review). 
Among various approaches, studying the source char-
acteristics and depth distribution of earthquakes pro-
vides unique information about the mechanical prop-
erties and stress orientation of the lithosphere. There 
have been numerous results published based on the re-
location and the source inversion of large events in the 
Tibetan Plateau and surrounding areas (Molnar and 
Chen, 1983; Baranowski et al., 1984; Ni and Barazangi, 
1984; Molnar and Lyon-Caen, 1989; Zhao and Helm-
berger, 1991). These studies conclude that seismicity 
beneath the Tibetan plateau is shallow, restricted to 
the upper 20 km of the crust. Only three intermediate 
depth earthquakes (85-90 km deep) located in southern 
Copyright 1996 by the American Geophysical Union. 
Paper number 96GL00385 
0094-8534/96/96GL-00385$03.00 
Tibet were reported (see Fig.1) (Chen et al., 1981; Mol-
nar and Chen, 1983; Ekstrom, 1987; Chen, 1988). Due 
to the lack of seismic stations on the plateau, almost all 
of above studies used records from stations outside the 
plateau, usually at teleseismic distances, thus limiting 
the number of events that could be investigated. How-
ever, from 1991 to 1992, a PASSCAL experiment was 
conducted on the Tibetan Plateau with a deployment of 
11 broadband stations. In this report, we present wave-
form modeling results of 3 events which show strong 
evidence of subcrustal focal depths. One of the events 
is located in the same area as two of previous interme-
diate depth events (Fig. 1), and the other two lie under 
the Himalayan Thrust Belt (HTB), where, to our knowl-
edge, few intermediate depth events have been reported 
to date. 
Data and Results 
The stations occupied during the 1991-1992 Tibet 
PASSCAL experiment were equipped with STS-2 Streck-
eisen seismometers which have a flat velocity response 
between 0.008 to 50 Hz. The details of the instrumen-
Figure 1. Map of southeastern Tibet showing ma-
jor tectonic boundaries and the station locations ( tri-
angles) of the 1991-1992 Tibet PASSCAL experiment. 
Shading indicates elevation above 3 km. Darkly shaded 
focal spheres are for subcrustal events analyzed in this 
study. Lightly shaded focal spheres are from Molnar 
and Chen (1983) and the Harvard CMT solution (for 
event on 1/10/86). All of them are located at interme-
diate depths. 
435 
436 ZHU AND HELMBERGER: INTERMEDIATE DEPTH EARTHQUAKES BENEATH INDIA-TIBET 
Table 1. Locations of the Events From PDE Catalog 
Event Date Origin time lat.flon.fdepth m• 
355 12-21-91 19h52m45.5 27.90N/88.14E/ 57 4.9 
061 03-07-92 22h41m50.8 29.44N /89.31E/113 4.3 
095 04-04-92 17h43m20.1 28.15N/87.98E/ 33 4.9 
tation can be found in Owens et al. (1993). During its 
one-year deployment, about 50 earthquakes within the 
plateau were recorded by the array with good signal-to-
noise ratios (Fig. 1). Randall et al. (1995) determined 
the focal depths and moment-tensors of 38 events in 
the north-central and southeastern part of the plateau 
by inverting long-period waveforms (T > 20s). Their 
results agree with previous studies that have indicated 
shallow seismicity. 
The PDE catalog locations and origin times of the 
three events investigated in this paper are listed in Ta-
ble 1. Events 355 and 095 are reported as crustal events 
and event 067 as subcrustal. A distinguishing charac-
ter of all 3 events recorded on the array is their sharp 
direct P and S arrivals, even at large distances (Fig. 2). 
This feature suggests a subcrustal focal depth because 
the first arrivals beyond 320-350 km for crustal events 
in this region are head-waves with emergent onsets on 
broadband displacement records. 
To test the sensitivity of waveforms to the focal 
depth, we have calculated the Green's functions for dif-
ferent source depths from 10 km to 90 km, using are-
Event355 
0 20 40 60 80 100 
t-x/4. 71 (sec) 
Figure 2. Examples of SH displacements of events 355 
and 095 (thickened traces). Note the sharp onsets of 
S waves. The corresponding synthetics are computed 
using the 1-D model (Table 2) with source mechanism 
and depth obtained from waveform inversion. 
Table 2. lD Velocity Model of the Tibetan Plateau 
layer Th. (km) Vp(km/s) V.(km/s) 
1 4 4.70 2.70 
2 60 6.20 3.50 
3 8.14 4.70 
:ftectivity code developed by Saikia and Burdick (1991). 
The velocity model consists of two layers over a half 
space with the Moho at 64 km (Table 2). It is very 
similar to model M45 (Romanowicz, 1982) which has 
been proved reasonably good for modeling events on 
the Tibetan Plateau (Randall et. al, 1995}. Fig. 3 
shows the SH component of the Green's functions at 
a distance of 350 km for various source depths, along 
with the observed SH displacements for Events 355 and 
095 recorded at the same distances. We focus on the 
SH displacement first because it is much simpler than 
P-SV displacement and has less dependence on source 
orientation. By comparing the waveforms for source 
depths of 60 km (above the Moho) and 70 km (below 
the Moho), one can see that the waveform shapes are 
quite diagnostic; the crustal synthetics predict a gen-
tly rising Sn phase whereas subcrustal events produce 
a sharp onset. Furthermore, the strong crustal phase 
Sd+SmS vanishes when the source is below the Moho. 
From the sharp S arrivals and lack of Sd/ SmS in the 
10 
20 
I 30 40 
1 eo 
~ t.!o 
:I 
i 70 
80 
90 
sSmS ;;zsz::. 
10sec 
Figure 3. SH components of the Green's functions of 
a strike-slip source at different depths. The velocity 
model is given in Table 2. Distance range is 350 km. 
Also shown in the figure are observed SH displacements 
of Events 355 and 095 recorded at station LHSA at the 
same distances. 
ZHU AND HELMBERGER: INTERMEDIATE DEPTH EARTHQUAKES BENEATH INDIA-TffiET 437 
Vertical 
Radial 
60km 
Data 
70km 
50 sec 
Figure 4. Comparison of vertical and radial displace-
ments at station LHSA with synthetics from sources 
above (60 km} and below (70 km} the Moho. The syn-
thetics are constructed using the source mechanisms ob-
tained from the waveform inversion at corresponding 
depths. 
observed waveforms, we conclude that Events 355 and 
095 are subcrustal events. The synthetics for sources at 
70, 80, and 90 km also show that for deeper source, the 
shape of first arrival is more impulsive. Accordingly, 
the step-like shape of first S arrivals indicates that the 
two events are confined to the uppermost mantle. 
Fig. 4 shows a comparison of vertical and radial dis-
placements for Event 355 with synthetics generated 
from a source above the Moho (60 km} and below it 
(70 km). Source mechanisms obtained from inversion 
of the whole array data are used when constructing 
the synthetics. Although the long period features of 
data and the two corresponding synthetics are similar, 
as is expected from theoretical arguments (Heimberger 
and Malone, 1975}, the synthetics for subcrustal focal 
depths match the broadband features of the data much 
better, with many individual phases aligned. 
To invert whole broadband seismograms for source 
depths and mechanisms, we apply a grid search in the 
source parameter space (strike, dip, rake, Mo, depth} 
to find the global minimum of misfit between the syn-
thetic and observed waveforms (Zhao and Heimberger, 
1994). An advantage of the method is that it allows 
time shifts between various portions of the observations 
and synthetics, thus relaxing the strong dependence of 
the inversion on the velocity model and the influence 
of lateral variation. Uncertainty of the solution can be 
estimated easily by quantifying the misfit error distri-
bution around the global minimum. 
The global minima of the 3 events as a function of 
depth are shown in Fig. 5. As expected, the wave-
form misfit errors of all3 events decrease rapidly as the 
70 
60 
50 
-~ 
I 
.5:40 
E 
Q) 
Ci> 3o 
-iC 0 
0 20 ~ 
10 
0 
0 20 40 60 80 100 
Depth (km) 
Figure 5. Waveform misfit as .function of foca;I ?epth. 
The misfit errors are normal1zed by the mm1mum. 
Curves for Event 095 and 067 are offset for displaying 
reason. 
sources drop below the Moho. The mechanisms show 
some depth dependence but remain mostly strike-slip. 
Table 3 lists the inversion results at the best-fit depths 
and the uncertainties estimated by examining all the 
solutions with misfit errors within 5% of the minimum. 
For all 3 events, the azimuths of the P and T axes are 
better constrained (within 10°}, while the plunges are 
relatively poorly constrained (within 10° - 20°). 
In Fig. 1 we have plotted the lower-hemisphere pro-
jection of the best fault plane solutions. The source 
mechanisms of the two events under the HTB display 
strong strike-slip behavior with some normal faulting. 
They are quite different from the mechanisms of shal-
low events in the same area, which show consistent low-
angle thrusting along the front of the Himalaya with 
slip vectors perpendicular to the mountain range (Mol-
nar and Chen, 1983; Baranowski et al., 1984; Ni and 
Barazangi, 1984; Molnar and Lyon-Caen, 1989}. The 
mechanism of the event near the suture zone is similar 
to the mechanism of the previous intermediate depth 
event nearby (Fig. 1}, except that it has more strike-
slip component. The T-axes are all horizontal in the 
Table 3. Focal Depths and Fault Plane Solutions From 
Waveform Modeling 
Event p axis a Taxis a h (km) Mo 
355 157±6/ 6±20 252±7/3±15 70 1.41 
067 210±6/27±11 303±8/3±11 80 0.25 
095 6±6/31±11 277±9/3±14 80 2.00 
a Azimuth/Plunge, all in degrees; Mo in 1016 Nm. 
438 ZHU AND HELMBERGER: INTERMEDIATE DEPTH EARTHQUAKES BENEATH INDIA-TIBET 
EW direction and the P-axes are roughly in the NS di-
rection. This is consistent with a regional stress field 
produced by the indenting of the India continent. 
Discussion 
Focal depth determinations based on global network 
traveltimes have always been difficult because of the 
trade-off between the depth and origin time (Zhao and 
Heimberger, 1991). This trade-off can be eliminated by 
the use of depth phases, pP and sP, but these phases 
become difficult to identify for magnitudes below 5, es-
pecially for strike-slip orientations. With broadband 
records at regional distances, the constraints on focal 
depths can be substantially improved. In particular, 
waveform modeling proves very effective in discrimi-
nating between crustal and subcrustal events, due to 
their fundamental waveform differences. Moreover, our 
success jn modeling the broadband waveforms up to 
1000 lqn <listance range with a shnple 1D velocity model 
shows th,t to ~he fi:rst o:rder t1J.Ei' plateau has a relatively 
simple crustal structure. But we also emphasize that 
lateral variation does exist. For example, the tangential 
displacements at stations WNDO, ERDO, and BUDO 
are delayed about 5 sec relative to the synthetics. These 
three station~ are located in the north-central plateau 
~here a 'mid.:~rustallow. velocity zone is report!'ld (Bran-
4on and ltomaD,owicz. 1986; ~h'!J et. al, 1995). ' 
In sumniarr, 'we have -d~tected ~ intermediate depth 
earthquake!! in the India-'Tibet collision zone in a one-
year time window. Ta!dng into account three previ-
ous reports, it is reason:al:)le to specul,.te that there are 
other unidentified intermediate depth earthquakes with 
moderate magnitudes in this area. The existence of in-
termediate depth earthquakes under the Himalaya and 
lij.dus Zangbo suture indicates the temperatures of the 
mantle lithosphere in southern Tibet are relatively low. 
The small number of these events studied to date limits 
our ability to explore the details of structure and rheol-
ogy of the lithospheric mantle, but we believe that with 
the installation and operation of the permanent CDSN 
station at Lhasa, more data will be available to shed 
light on questions about the tectonics in this continent-
contin~mt collision zone. 
Acknowlt~dgments. We thank 3 anonymous review-
ers, L. Burdick, C. Scrivner, T. Melbourne, and X. Song for 
their helpful comments on the manuscript. This work has 
been supported by the Department of Defense as monitored 
by the Air Force Office of Scientific Research under con-
tract F49620-92-J-0470. Contribution No. 5556, Division 
of Geolof$ical and Planetary Sciences, California Institute of 
Technology, Pasadena, California. 
References 
active tectonics of the Himalaya, J. Geophys. Res., 89, 
6919-6928, 1984. ' 
Brandon, C., and B. Romanowicz, A 'no-lid' zone in the cen-
tral Chang-Tang platform of Tibet: Evidence from pure 
path phase velocity measurement of long period Rayleigh 
waves, J. Geophys. Res., 91, 6547-6546, 1986. 
Chen, W.-P., J.L. Nabelek, T.J. Fitch, and P. Molnar, An in-
termediate depth earthquake beneath Tibet: Source char-
acteristics of the event of September 14, 1976, J. Geophys. 
Res., 86, 2863-2876, 1981. 
Chen, W.-P., A brief update on the focal depths of intracon-
tinental earthquakes and their correlation with heat ftow 
and tectonic age, Seiamo. Res. Lett., 59, 263-272, 1988. 
Ekstrom, G., A broadband method of earthquake analysis, 
Ph.D. Dissertation, 215 pp., Harvard Univ., Combridge, 
MA, 1987. 
Heimberger, D.V. and S.D. Malone, Modeling local earth-
quakes as shear dislocations in a layered half space, J. 
Geophys. Res., 80, 4881-4888, 1975. 
Molnar, P., A review of geophysical constraints on the deep 
structure of the Tibetan Plateau, The Himalaya and the 
Karakoram, and their tectonic implications, Phil. 1rarn. 
R. Soc. Lond., A3B6, 33-88, 1988. 
Molnar, P., and W.-P. Chen, Focal depths and fault plane 
solutions of earthquakes under the Tibetan Plateau, J. 
Geophys. Res., 88, 1180-1196, 1983. 
Molnar, P., and W.-P. Chen, S-P wave travel time residuals 
and lateral inhomogeneity in the mantle beneath Tibet 
and the Himalaya, J. Geophys. Res., 89, 6911-6917, 1984. 
Molnar, P., and H. Lyon-Caen, Fault plane solutions of 
earthquakes and active tectonics of the Tibetan Plateau 
and its margins, Geophys. J. Int., 99, 123-153, 1989. 
Ni, J ., and M. Barazangi, Seismotectonics of the Himalayan 
collision zone: geometry of the underthrusting Indian 
plate beneath the Himalaya, J. Geophys. Res., 89, 1147-
1163, 1984. 
Owens, T.J., G.E. Randall, F.T. Wu, and R.S. Zeqg, PASS-
CAL instrument performance during the Tibetan plateau 
passive seismic experiment, Bull. Seismol. Soc. Am., 83, 
1959-1970, 1993. 
Randall, G.E., C.J. Ammon, and T.J. Owens, Moment-
tensor estimation using regional seismograms from 
portable network deployments, Geophys. Res. Lett., BB, 
1665-1668, 1995. 
Romanowicz, B., Constraints on the structure of the Ti-
bet Plateau from pure path phase velocities of love and 
Rayleigh-waves, J. Geophys. Res., 87, 6865-6883, 1982. 
Saikia, C.K. and L.J. Burdick, Fine-structure of Pnl waves 
from explosion, J. Geophys. Res., 96, 14383~14401, 1991. 
Zhao, L. and D.V. Heimberger, Geophysical implication 
from relocation of Tibetan earthquakes - hot lithosphere, 
Geophys. Res. Lett., 18, 2205-2208, 1991. 
Zhao, L. and D.V. Heimberger, Source estimation from 
broadband regional seismograms, Bull. Seismol. Soc. 
Am., 84, 91-104, 1994. 
Zhu, L., T.J. Owens, and G.E. Randall, Lateral variation in 
crustal structure of the northern Tibetan Plateau inferred 
from teleseismic receiver functions, Bull. Seismol. Soc. 
Am., in press, 1995. 
Seismological Laboratory, California Institute of 
Technology, Pasadena, CA 91125 • 
Baranowski, H., H. Armbruster, L. Seeber and P. Molnar, (received June 5, 1995; revised October 19, 1995; 
· Focal depths and fault plane solutions of earthquakes and accepted December 19, 1995.) 


Intermediate depth earthquakes beneath the India-Tibet

collision zone

http://authors.library.caltech.edu/35705/1/96GL00385.pdf

Intermediate depth earthquakes beneath the India-Tibet collision zone

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main