Geodetic measurement of horizontal crustal deformation in eastern Taiwan

Tectmphysics,

73

125 (1986) 73-85

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

GEODETIC

MEASUREMENT

OF HORIZONTAL

CRUSTAL

DEFORMATION

IN EASTERN TAIWAN

SHUI-BEIH YU and CHIUNGWU LEE Institute of Earth Sciences, Academia Sinica, Taipei (Replrblic of China)

(Revised version received June 11, 1985; accepted September 17, 1985)

ABSTRACT Yu, S.B. and Lee, C., 1986. Geodetic measurement of horizontal crustal deformation in eastern Taiwan. In: J. Angelier, R. Blanchet, C.S. Ho and X. Le Pichon (Editors), Geodynamics of the Eurasia-Philippine Sea Plate Boundary. Tectonophysics, 125: 73-85. Four medium-aperture trilateration networks in eastern Taiwan have been surveyed three to four times since 1981. One is the Ban network crossing an elongated active seismic zone, the others are the Hualien, Yuli and Taitung networks located at the northemmost, middle, and southernmost portions of the Lon~tudinal Valley, respectively. Based on changes of the observed line length, the three components of the surface strain rate tensor for each of the networks are obtained by a least squares adjustment technique. Then the principal strain rates are calculated. The Ilan network gives a principal strain rate of uniaxial extension at 2.3 pstrain/yr in the direction of N45”W. The Hualien network has a principal strain rate of 1.2 pstrain/yr extension in N62”E and 1.2 pstrain/yr contraction in N152OE. The Yuli network yields essentially a principal strain rate of uniaxial contraction at 8.4 ustrain/yr in N117’E, whereas the Taitung network has a principal strain rate of 1.5 pstrain/yr extension in N24”E and 3.9 gstrain/yr contraction in N114”E. The directions of contractionof both the Yuli and Taitung networks are consistent with the direction of the maximum compressive stress of this area. Furthermore, average velocities of the relative motion between geodetic stations in the Central Range and the Coastal Range are estimated from the average rates of changes in line length. Stations in the Hualien network show a left-lateral relative motion in a direction more or less parallel to the strike of the Longitudinal Valley, while stations in the Yuli and Taitung networks move toward each other in the direction appro~mately perpendicular to the trend of the Longitudinal Valley.

INTRODUCTION

Eastern Taiwan is marked by unusually high seismicity. Based on a large number of high-quality earthquake data located by means of the Taiwan Telemetered Seismographic Network (TTSN) during 1973-1980, Tsai et al. (1981) delineated the plate boundaries between the Eurasian and the Philippine Sea plates in eastern Taiwan. Due to active interaction between the plates one can expect that there should be very high crustal deformation activity in this area. As part of the Taiwan 0040-1951/86/$03.50

0 1986 Elsevier Science Publishers B.V.

L L

I -.A.

Fig. 1. The average eastern

Taiwan

Earthquake

Ustrg

-2

“,,F,.r

principal

strain

and the locations

Prediction

rates

(in pstrain/yr)

measured

at four

trilateration

networks

in

of the networks.

Research

Program

(Tsai et al., 1983)

four medium-aperture

trilateration networks were set up by the Institute of Earth Sciences, Academia Sinica (ASIES) in 1981 (Fig. 1). One network is located in the Ilan Plain, northeastern Taiwan.

The other

three

networks,

namely,

the Hualien,

Yuli,

networks, are located at the northernmost, middle, and southernmost Longitudinal Valley in eastern Taiwan, respectively. The Ilan network

is across an elongated

zone of intense

and portions

microearthquake

Taitung of the activity

lying between Ilan and Lotung (Fig. 2). This active seismic zone extends northeasterly offshore and passes by a few kilometers south of the Kueishan Island (Tsai et al., 1975).

The

elongated

seismic

zone

was suggested

to be the northeastward

continuation of the Niutou fault which is concealed underneath alluvium in the Ilan Plain. The 150 km long NNE-striking Longitudinal Valley is a distinctive geologic feature in eastern Taiwan. It separates two quite different geologic provinces, namely, the Central Range in the west and the Coastal Range in the east (see Fig. 1). The Longitudinal Valley is considered as an active collision boundary between the Eurasian and the Philippine Sea plates. The Longitudinal Valley fault is an active fault system in the valley and the fault plane strikes approximately N20”E to the valley) and dips 70” to the east (Barrier et al., 1982). It is a left-lateral thrust

fault.

(parallel oblique

75

Fig. 2. The Ilan trilateration

network.

The dashed

line indicates

the

seismic zone.

Part of the lines in the Ilan, Hualien, Yuli, and Taitung trilateration networks were surveyed three to four times by ASIES during 1981-1983. In this paper the changes of line length in the successive surveys are used to calculate the horizontal strain accumulation rates for each network. Furthermore, the average rate of change of line length (dL/dt) is utilized to estimate the average velocity of relative motion between the geodetic stations in the Central Range and the Coastal Range. The tectonic implications of the results from these preliminary t~lateration data are also discussed.

SURVEY

PROCEDURES

AND

DATA

REDUCTION

In each survey the distances between geodetic monuments of stations in each trilateration network are measured with a medium-range electronic distance meter (EDM, Hewlett-Packard 3808A). The EDM transmits a narrow, amplitude-modulated infrared laser beam to a set of retroreflector prisms which reflect the beam back to the EDM. Length of the optical path is determined by comparing the outgoing and incoming beams at successively higher modulation frequencies. Many phase comparisons are made at three frequencies, with the final output being an average of these comparisons. Each measured length value for each line in our trilateration networks is the average of more than thirty repeated measurements. Atmospheric measurements are monitored at each endpoint during the time of the distance measurement. Air temperatures are measured with digital readout temperature indicators which are connected with shielded thermistors mounted on top of 6-m masts. The accuracy of the temperature measurement is about fO.l”C. Atmospheric pressures are measured with precision aneroid barometers which are in

Fig. 3. The Hualien

trilateration

network.

intervals of 0.2 mm Hg and the water vapor pressures with dry-wet bulb psychrometers. The atmospheric refractive index along the optical path is calculated from the averages of these air temperature, atmospheric pressure, and water vapor pressure measurements at the instrument and the reflector stations. A 1 ppm error can be introduced

into measurements

of distances

about l°C, in average atmospheric pressure of 20 mm Hg.

pressure

by an error in average

temperature

of

of 2.6 mm Hg, or in the water vapor

After the meteorological corrections, the length of each line is reduced to the Kaula arc length. The nominal accuracy of the HP 3808A EDM is given as 5 mm + 1 mm/km by the manufacturer. A sophisticated calibration on the precision of our distance measurement system is not yet available. It seems reasonable to assume that the precision of our survey is described by a normal error distribution with a standard error u = (a2 + b2L2)1/2 where a = 5 mm, h = 10p6, and L is the line length (e.g., u = 11 mm for L = 10 km). A forty-hour experiment was conducted

in November

of 1983 to check

the

stability of our distance measurement system. The distances between the stations 131 and 132 in the Hualien network (Fig. 3) were measured every two hours in the experiment. Fig. 4 shows the variations of the corrected line lengths as well as the averages of temperature, atmospheric pressure, and water vapor pressure measured at two endpoints. The total scatter in the average of endpoint temperature was about 6°C. The 22 corrected values of line length ranged from 7408.020 m to 7408.038 m, a peak to peak variation of 1.8 cm, with an average distance of 7408.028 m and a standard deviation of 5 mm. Since neither the instrument nor the reflector were

.XxX

x Average

132 - 131

Temperature .-.,./ \.,y

* 1.

x

x x

..-.

.

Y: *

.

:I;

x

x -‘\. . .

J ,-.-_

7%

‘.

./

52

Average

m

;750 E1‘8 ,.-‘-._.

Atmosphers.?essure / k_.,.” /’

,.-.-.-.-

746i 1,

.‘--.-.

.-.-.

Y_./‘-“..,

f

9

Average

7t 70. 10.-./‘.._. 0. ,000 831116

Vapor

Average

q

\

Pressure 7LO8.028 .-.

m

lLO0 WCC 831117

Imoo

._C i

,.-‘\._,

__.A’

/

.~._._./“.__

sd=5mm

Line

Length

,200

,600

TIME

Fig. 4. Results of the 40-hr experiment

\/ DJo

?‘00 0‘00 831118

0000

on the effects of atmospheric

1100

conditions.

disturbed during the experiment, no setup error was involved. If we assume that no real change in line length occurred during the 40-hr period, then any apparent variations in line length are mainly due to instrumental and refractive index errors. Specifically, the error in estimating the refractive index was the dominant noise source. The second day was cloudy and windy. Therefore the atmospheric conditions along the optical path appeared to be rather uniform and the corrected line lengths were very stable. In favorable atmospheric

conditions,

the precision

of our distance

measurement

system is believed to be better than 1 ppm. We usually make the distance measurements at nighttime or on cloudy days. The lengths of lines in our trilateration networks STRAIN

range from 2 to 12 km. ACCUMULATION

Assuming the strain rate is uniform over the whole area covered by each network and over the time interval considered, the average annual strain rate of each network can be calculated. The strain rate for each line, i = L- ‘(d L/dt), where L is the line length and dL/dt is the average rate of change surface strain rate tensor, jr,, by:

of line length.

i is related

to the

6 = i,, sin*B + i,, sin 213+ i,, cos* 8 where 8 is the azimuth (measured clockwise from north) of the line and the strain rate tensor is referred to a geographic coordinate system with axis 1 directed east

TABLE

1

Strain rate ~~mp~flenls

of four trilat~ratlon

Network

networks

6,

in eastern Taiwan {in pstrain/yr,

Yuli

-6.8i_0.6

Taitung

-3.o;to.4

* The strain rates are referred The uncertainties

quoted

to a coordinate

are standard

1 .O ri_0.4

0.7 i 0.5

3.0 +_0.4

-- 2.5 t 0.4

2.0 :t 0.3

0.6 * 0.4

- 1.1 10.4

1.2 j, 0.6

llan

i,,

@I? 0.7 5 0.5

Hualien

1pstrain = 1O--o)*

1.7iCl.4

system with axis 1 directed

east and axis 2 directed

north.

deviations,

and axis 2 directed north. There is one such equation for each line in a network, and, if there are three or more lines in different orientations, three components of average strain

rate tensor

technique

i,,,

(Prescott

L,, and i,,

can be determined

by a least squares

adjustment

et al., 1979).

The components

of strain rate tensor calculated

using all of the surveys for each

of the networks are given in Table 1. The extension is reckoned as positive. The uncertainties quoted are standard deviations. The Yuli and Taitung networks show remarkable

east-west

contractions,

whereas

slight east-west extension. The north-south north-south extension in the Ilan network

the Hualien

and

Ilan

networks

show

contraction in the Yuli network and the are also significant, while the values of

Q,, for Hualien and Taitung networks do not differ significantly from zero at the 95% confidence level. The three networks across the Longitudinal Valley, that is, Hualien, Yuli, and Taitung networks, all show left-lateral slips across a north-south line, with, however, different strain accumulation rates. Average principal strain rates can be computed from the three components surface strain rate tensor by the following formula:

where

&, and

respectively. $=+

tan-‘-

i,

are the algebraically

The azimuth

larger

of i,, +, is found

and

smaller

principal

strain

of the

rates,

by:

2& %? - fli

The average principal strain rates for each of the networks in eastern Taiwan are shown in Fig. 1. The number of surveys and the time period spanned by the surveys are also shown. The Ilan network is marked by SE-NW extension of 2.3 pstrain/yr. This seems to be related to the back-arc spreading of the Okinawa Trough which may extend southwesterly into the Ilan plain. The Hualien network has the principal strain rates of 1.2 ~strain/yr extension in N62”E and 1.2 pstrain/yr contraction in N152”E. The principal strain rate of the Yuli network is essentially an uniaxial contraction of 8.4 pstrain/yr in the direction N117’E. Based on seventeen focal mechanisms in

19

0

, 1981

1982

Fig. 5. Strain accumulation

1983

curve of cl, for each of the

networks

in eastern

Taiwan.

the vicinity of the Yuli area (Yu and Tsai, 1982), Angelier (1984) determined the direction of maximum compressive stress of this area to be N130”E. It is very close to the direction of shortening in the Yuli network. The Taitung network has a principal strain rate of 1.5 ystrain/yr extension in N24”E (parallel to the strike of Longitudinal Valley) and 3.9 pstrain/yr contraction in N114”E. The direction of maximum compressive stress estimated from a tectonic analysis of numerous faults,

j

(IIyyI-1, 1SSl

1982

Fig. 6. Strain accumulation

1983

curve of qz.

i oL..._1. .~._

.A..

__L

--..l_

_______I

1983

1982

1981

Fig. 7. Strain accumulation curve of czz.

slickensides, tension gashes and folds at different scales in the Pinanshan Conglomerates (covered approximately the same area as the Taitung network) is N105’E (Barrier et al., 1982). It is also consistent with the direction of contraction in the Taitung network. The proportiona length changes in individual lines between two successive surveys yield a large number of strain increments for the various o~entations of the lines. The incremental strain tensor that best reproduces these strain increments as a function

of azimuth

of these strain

represents

increments

the strain change between

as a function

curve (Savage et al., 1981). The strain accumulation network

can be obtained

average

strain

accumulation

by essentially

rate previously.

surveys. A cumulative

of time represents between

the strain individual

the same scheme employed

But only the assumption

of spatially

plot

accumulation surveys of a

to calculate uniform

the strain

over the network

is necessary. Figures 5-7 show the strain accumulation curves of e,,, t12. and ez2 for all four networks in eastern Taiwan. The error bars represent one standard deviation on either side of the plotted points. The initial

level of strain for each network is arbitrary, and only changes from the initial value significant. The strain accumulation curves are reasonably consistent with a linear dependence upon time. it means that the assumption of the uniform accumuare

lation of strain in time is not unreasonable.

RELATIVE

MOTION

OF GEODETIC

STATIONS

Since all observations are made only between geodetic stations in the area of interest, that is, no tie to an external frame of reference in the trilateration networks, there is no way of detecting the motion of the network as a whole. In other words, only relative motion of stations within a network can be determined from our

81

E4 E g

T

13% 4302 5444.14m

yJ+x& AUG. FEB. 199l 19B2

-

in54302

q,J&Ig& 1982 Fig.

8. Line

least-squares

length

1993

as a function

.

I ”

AUG. 1961

JUN. KM. 1993

13l- 1215 9520.05m

I

I

I

FEB. 1992

of time for six lines in the Hualien

‘I 1 JGi3 N0v.

network.

Straight

lines are

fittings.

repeated trilateration surveys. The translation of the whole network does not affect the parameters of interest here, while the rotation of the entire network does. We specify the translation by arbitrarily assigning a zero velocity to the center of mass of the network. The rotational ambiguity is resolved by assuming no net rotation about the center of mass or by rotating the entire network as a whole into a configuration that minimizes the components of motion normal to the strike of a strike-slip fault. The former is called “inner coordinate” solution (Brunner, 1979); and the latter is called “outer coordinate” solution (Prescott, 1981). Observations of each line are plotted as a function of time, then a straight line is fitted to each of the plots (Fig. 8). The slope of this straight estimate of the average rate of length changes. The individual are weighted

by the reciprocal

square

of the standard

line (dL/dt) gives an values of average rate

deviation

in the rate. These

weighted rates are then used in a least squares adjustment to calculate the relative velocities of stations for three networks across the Longitudinal Valley as shown in Figs. 9-11. The error bars at the tip of each velocity vector denote the principal axes for the 95% confidence ellipse for each velocity. Only six lines connecting four stations in Hualien network (Fig. 8) have been surveyed more than three times, therefore only relative motion among these stations can be estimated (Fig. 9). The left-lateral relative motions between the stations in the Central Range (Stations 131,

Fig. 9. Relative

velocity

vectors

for stations

in the Hualien

network.

4302) and the Coastal Range (Stations 132, 1215) are more or less parallel to the strike of the Longitudinal Valley and their relative velocity is about 2 cm/yr. Chen (1974) indicated that all four triangulation stations located at the northernmost part of Coastal

Range

have consistently

(two of the stations moved northeastward

d

were also used as the trilateration relative

to the six stations

/

,

:

I

Scale

0

‘, KM

--\ ZENTRAL

p”

YULI

-5

0’ >

/’

RANGE/ !’ (. _

\

202

,/’

-\__ \ j_ I’

3L9

,i/’

,’ \. %-

, 1

,

COASTAL RANGE

,I

Fig. 10. Relative

velocity

vectors

for stations

in the Yuli network.

stations)

in the Longitudi-

83

Fig. 11. Relative velocity vectors for stations in the Taitung network.

nal Valley and Central Range for an average distance of 3.65 m during the time

period between 1909 to 1942 and 1971 when two triangulation surveys were conducted. If we assume that a 2-m slip of these stations was due to the major earthquake of 1951 (Hsu, 1962), this relative movement of the station was equivalent to an average left-lateral relative velocity of 3-6 cm/yr. The direction of the relative motion between the Coastal Range and the Central Range determined from these old triangulation data is in good agreement with the result from our trilateration data but the magnitudes of relative velocities are somewhat different. Figure 10 shows the relative motion between stations in the Yuli network. It is clear that the stations in the Coastal Range and those in the Central Range move toward each other in a direction appro~mately perpendicular to the ~n~tudinal Valley. The relative shortening is about 3.5 cm/yr. The relative motion between stations in the Taitung network is similar to that of the Yuli network (Fig. ll), but there is a little left-lateral component between the stations in the Pinanshan area and the Coastal Range.

DISCUSSION

The strain accumulation rates in the eastern Taiwan (on the order of several pustrain per year) appear to be much higher than in the western United States where

rates of shear @train/yr

strain

accumulation

(e.g. Prescott

the relatively

along

the San Andreas

fault average

small apertures

of our trilateration

networks.

The lines of each network

are mostly located in regions of the highest strain accumulation quite likely that the eastern

Taiwan

the western

as caused

Philippine

United

only 0.3

et al., 1979: Savage et al., 1981). This may be partly due to

States

rates. However.

has much higher strain accumulation

Sea plate and the Eurasian

by the very active

interaction

it is

rates than between

the

plate.

The extraordinarily high strain accumulation rate in the Yuli network was originally suspected to be partly due to local downhill movements of two stations situated at the hillsides of Central Range. Our geodetic station has one mark on the top of a prismatic granitic block and the other mark on a separated base plate. When the station horizontal position

was constructed. these two marks were aligned at the same by plumbing. We have checked the two marks of the station 202

located at the steeper hillside (see Fig. 10) and found the relative movement between the two marks being only 6 mm in the down-slope direction during the past three year period.

whereas

same period.

Thus,

station.

the length

We will establish

stability of geodetic Focal mechanism

changes

of some lines reached

there has been no appreciable several

reference

marks

superficial to further

monuments of the suspected stations. of an individual earthquake is usually

10 cm during movement

check

the

near the

the long-term

controlled

by a local

pre-existing fault plane. But the stress direction estimated from focal mechanisms of many earthquakes occurred at various depths and covered a large area can given an approximate direction of the present regional tectonic stress. The principal strain rate calculated from repeated trilateration survey data indicates the present near surface deformation due to the action of the tectonic stress in the vicinity of the trilateration network. The direction of the principal strain rate should be affected by faults across the network. In a homogeneous and isotropic medium the directions of the principal different. strain

stress and strain should

Taking

into account

the errors in estimating

rates and stress, the directions

pressive stress (N130”E) the major

faulting

be the same, otherwise of shortening

the directions (N117”E)

in the Yuli area are quite consistent.

in this area was caused

they will be somewhat

by the tectonic

of the principal

and maximum

com-

It may be inferred

that

stress

with the same

direction operative at the present. The compressive stress reconstructed from a tectonic analysis of the fault slip data in the Pinanshan Conglomerates is related to the thrust motion that occurred during the Quaternary and still continues at the present between the Coastal Range and the Central Range (Barrier et al., 1982). The estimated direction of maximum compressive stress (N105”E) for a long time period in the Pinanshan area is also in good agreement with the present direction of contraction (N114’E) inferred from repeated trilateration survey data. It suggested that the direction of tectonic stress in the southernmost portion of the Longitudinal Valley has not changed significantly for a long time period at least since the Quaternary.

85

It is concluded that the crustal deformation in the southern part of the Longitudinal Valley area is caused directly by the on-going collision between the Eurasian and the Philippine Sea plates. The horizontal crustal deformation in the Hualien network seems to differ appreciably from that in the Yuli and Taitung networks. This is probably due to the more complex tectonic situation in the Hualien area where the Philippine Sea plate starts to subduct northward under the Eurasian plate in the vicinity of Hualien. Precise trilateration and leveling survey data in the next few years hopefully will provide us a better understanding on the crustal deformation in eastern Taiwan.

ACKNOWLEDGEMENT

We thank our colleagues Messrs. S.Y. Chen, W.J. Lee, M.T. Lin and Y.C. Yang for their assistance in the field work and data reduction. We are greatly indebted to Drs. Y.B. Tsai, T.L. Teng and F.T. Wu for their valuable discussions and comments. This study was supported by the National Science Council of the Republic of China under Grant no. NSC73-0202-Mel-04.

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