Characteristics of vertical land movement and microearthquake activity in the northeastern Japan arc

Tectonophysics, 77 (1981) 213-231 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

213

CHARACTERISTICS OF VERTICAL LAND MOVEMENT AND MICROEARTHQUAKE ACTIVITY IN THE NORTHEASTERN JAPAN ARC

HIRGSHI ISHII, YOICHIRO KOMUKAI and AK10 TAKAGI Observation Center for Earthquake Sendai 980 (Japan) (Received May 9,1979;

Prediction, Faculty of Science,

Tohoku

University,

revised version accepted January 19, 1981)

ABSTRACT Ishii, H., Komukai, Y. and Takagi, A., 1981. Characteristics of vertical land movement and microearthquake activity in the northeastern Japan arc. Tectonophysics, 77: 213231. Methods have been devised for analyzing vertical land movement and seismicity data using two-dimensional Chebychev functions and oblique projections. A filtering operation in the space domain is made possible by use of a two-dimensional Chebychev function. The oblique projections give an intuitive understanding of land deformation. Characteristic aspects of vertical land movement obtained by precise levelling and of the energy release of microe~thquakes with depths shallower than 20 km in the northeastern Japan are were investigated in detail applying these methods. Lineations with wavelength of about 20-60 km trending towards NE-SW were found for both the land deformation and the seismicity. It should be noted that this trend is almost perpendicular to the direction of the strain migration and is related to other geophysical information.

INTRODUCTION

The Tohoku district is of great geophysical interest because it overlies the sinking Pacific plate in the northeastern Japan arc. Recently, a strain wave migrating with a velocity of about 40 km/year in the no~he~~rn Japan arc was found by analyzing the data obtained from extensometers installed at the five crust&movement observatories in the region (Ishii et al., 1978a, 1979). Also in the northeastern Japan arc, a double-planed structure of the deep seismic zone was found (Takagi et al., 1977) and secular strain characteristics were studied in comparison with geophysical data (R.G.C.M., 1978). In this paper, we intend to investigate characteristics of vertical land movement in the Tohoku district and to discuss its relationship with microearthquake activity. 0040-1951/81/0000-0000/$

02.50 @ 1981 Elsevier Scientific Publishing Company

214

Kato and Kasahara (1977) introduced the time-space domain presentation for the analysis of levelling data along routes. Mizoue (1967) studied modes of secular vertical movements along several levelling routes in the Tohoku district. In the present paper, oblique projections of vertical movements are drawn utilizing a two-dimensional Chebychev approximation function to represent the overall movements in the Tohoku district. The technique is very useful because employment of a two-dimensional Chebychev function makes it possible to filter vertical movements in the space domain and oblique projections permit an intuitive understanding and a comprehensive pattern recognition of the land deformation.

a

SENDAI c

14tE

38N* UZE

215

/ t

b

-t IiOE

:‘\+4ON

38N \

142E

Fig. 1. Vertical land movement obtained by precise levellings in the Tohoku district. a. For the period of 1956 to 1966 (after G.S.I., 1971). b. For the period of 1966 to 1975 (after G.S.I., 1976). Contours are drawn by the author. c. Vertical land movement (1956-1966) in the Tohoku district approximated with a two-dimensional Chebychev function of 28 degrees.

DATA

The Geographical Survey Institute has repeated precise Ievellings in Japan. Since 1956, precise levelling has been carried out three times along all the levelling routes in the Tohoku district. Figure la shows vertical movement from 1956 to 1966 and Fig. lb, from 1966 to 1975, obtained by G.S.I. (1971, 1976). Levelling routes are shown by dotted lines in Fig. la. We employ these data in the following analyses for investigating characteristics of vertical land deformation in the Tohoku district. Errors in the data are considered to be several millimeters. METHOD OF ANALYSIS

In foregoing papers (R.G.C.M., 1978; Ishii et al., 1978b), it was revealed that Chebychev functions are very useful for secular strain analyses. Nabetani and Kano (1970) successfully applied two-dimensional Chebychev functions to the analysis of gravity survey data. In the present paper we employ two-dimensional Chebychev functions for

216

analyzing vertical land movement. The one-dimensional Chebychev polynomial was described in detail by Southworth and Deleeuw (1965). The twodimensional Chebychev approximation is presented in the Appendix. Data on vertical crustal movement for the last twenty years (Fig. 1) are analyzed by the use of a two-dimensional Chebychev approximation function. We define a 290 X 290 km square in the Tohoku region and partition it into 900 10 X 10 km squares. After constructing contours of the data by interpolation and extrapolation, we determine estimates of vertical movements at the grid points. Values at points in the sea are extrapolated by use of sea level data at tidal stations. First, we compute approximate Chebychev coefficients Ai,j from the values on the grid points and then investigate characteristics of vertical movements in the Tohoku district using a Chebychev approximation function produced from the coefficients. Figure lc shows vertical movements from 1956 to 1966, computed by approximate Chebychev coefficients. We can see a good approximation, comparing Fig. la with Fig. lc.

141E

142 E

Fig. 2. Oblique projection of vertical land movement in the Tohoku district approximated with a two-dimensional Chebychev function of 28 degrees. a. 1956-1966. b. 1966-1975.

OUTLINE

OF VERTICAL

MOVEMENTS

It is the purpose of this section to survey the general features of the vertical movements in the Tohoku district from 1956 to 1975. Using the twodimensional Chebychev coefficients computed by the method described in the Appendix, it is easy to calculate the Chebychev approximation function. We made oblique projections of the vertical movement from calculated values in order to examine features of the movement. Figures 2a and 2b are the figures obtained by Chebychev approximation functions of 28 degrees for the period 1956-1966 and 1966-1975, respectively. Coastlines are shown by the open circles. The features of vertical land deformation in the Tohoku district are apparent from Figs. 1 and 2.

218

Vertical mouemen t from 1956 to 1966 Figures la and 2a show the vertical movement in the above period. It is found that the northern part of the Tohoku district uplifted and the eastern coast subsided with maximum differences reaching 6-6 cm over a lo-year period. A subsidence is seen at the southern part of Morioka. An uplifted area in the southern part of the Tohoku district extends to the central part of Honshu.

Vertical movement from 1966 to 1975 Figures lb and 2b show the vertical movement for the period 1966-1975. A striking subsidence observed at the northern part and the southern part of the Tohoku district was caused by artificial pumping. The eastern coast continues to subside. Depression found in the central part resulted from the southeastern Akita earthquake (M = 6.2) of 1970. An uplifted area is also seen in the southern part of the district. CHARACTERISTICS

OF VERTICAL

MOVEMENT

AFTER

FILTERING

The purpose of this section is to investigate the characteristics of vertical land movement in more detail by filtering the data. As previously stated (Ishii et al., 1978b), filtering is performed by use of adequate Chebychev coefficients. First, we investigate long-wavelength vertical movement. Figures 3a and 3b are vertical movements approximated by a twodimension~ Chebychev function with O-7 degrees for the period 1956-1966 and 1966-1975, respectively. Wavelengths included in these presentations of vertical movement are approximately 100 km to infinity in length. It is evident that the Pacific coast in the Tohoku district continues to subside from 1956 to 1975. An uplifted region is seen in the southern part of the Tohoku district through this period, however, the uplift is larger over the first ten years than the second. The northern part of the Tohoku district is uplifted in the first ten years, while in the‘later ten years it is not found. It is seen that the subsidence due to pumping has almost been removed by filtering. Secondly, we examine medium-wavelength vertical movements. Figures 4a and 4b illustrate vertical movements filtered using the two-dimensional Chebychev function with 7-20 degrees for the period of 1956-1966 and 1966-1975, respectively. Wavelengths constructed for these presentations of vertical movement are approximately 30-100 km in length. It is of great interest to note that lineations in the direction NE-SW appeared during both periods with this band-pass filtering. Their wavelengths are about 5O-60 km and the wavelengths for the earlier 10 years are longer than for the later time interval. In order to see the lineations in more detail, contour maps are made for these vertical movements as shown in Figs. 5a and 5b for the period 1956-1966 and 1966-1975, respectively. White regions show up-

141E

b

14iE

lh

E

li2E

Fig. 3. Oblique projection of vertical land movement in the Tohoku district approx ima ted with a two-dimensional Chebychev function of 7 degrees. a. 1956-1966. b. 1966- ,197‘5.

220

Cl4

5

0

-5

a

b

140 E

140E

141 E

141 E

142 E

14’2E

Fig. 4. Oblique projection of vertical land movement in the Tohoku district a two-dimensional Chebychev function from 7 to 20 degrees. a. 1956-1966. 1975.

filtered using b. 1966-

39 N

140E

142E

40N

39 N

b

140E

14lE

142E

Fig. 5. a. Contour map obtained from Fig. 4a. b. Contour map obtained from Fig. 4b. The white areas show uplift and the lined areas, subsidence. Vertical movement of zero level is indicated by contours drawn at boundaries between horizontally lined areas and white areas, and contours in each area mean vertical movement of f2 mm,

4dN

V=38 km/yr . G=NSdW 38-N

100 km .

Fig. 6. Direction

and velocity

for migration

of maximum

shear strain (Ishii et al., 1978a).

lift and lined regions subsidence. The boundaries between lined regions and white regions are zero levels of the vertical movement. Contours in each region are for vertical movement of +2 mm. Lineations extending NE-SW are clearly recognized for both periods. A migrating strain wave, with a velocity of 38 km/yr in the N50” W direction, is illustrated in Fig. 6 (Ishii et al., 1978a; Ishii et al., 1979). It is found that the lineations for vertical movements are almost perpendicular to the direction of the migrating strain, comparing Fig. 5 with Fig. 6. It may be considered that the lineation of vertical movement is produced by propagating strain. ~ICROEARTIIQUAKE

ACTIVITY

In this section, patterns of shallow microearthquake epicenters are investigated. A high-gain seismograph network of Tohoku University consisting of

223

fifteen stations has been installed since 1975 to investigate in detail seismic activity in the Tohoku district and to determine hypocenters for microearthquakes. The detailed features of microearthquake activities have been investigated after the construction of the system. Takagi et al. (1977) found by the analysis of the data obtained by the system that almost all the shallow microearthquakes in the land area of the.Tohoku district occurred only in the layer with a P-wave velocity of 5.9 km/set, the depth of which is about 20 km.

1:IR E

139

140 i

!.

141 i

IYT t 142 t -------~42N

1YY L

4

41 N

N

39 N

38 N

37 N

Fig. 7. Epicenter distribution of shallow microearthquakes with depth less than 20 km in the Tohoku district for a year of 1977, observed by the seismic network of Tohoku University

224

Comparing a seismicity map of shallow small earthquakes from 1926 to 1967 by Japan Meteorological Agency with that of shallow microearthquake from 1975 to 1979 by Tohoku University, the pattern of seismic activity is similar. We, therefore, use an epicenter map of shallow microearthquakes of 1977 as an example in this paper. Figure 7 shows epicenters of shallow earthquakes with depths less than 20 km for the year 1977. The seismicity is active off the Pacific coast. Seismic activity is also seen in the land area and along the coastline of the Japan Sea. The seismic activity of shallow microearthquakes in the Tohoku district is regionally divided into these three groups. In order to analyze the seismic activity quantitatively and to recognize patterns of energy release, accumulated earthquake energy is calculated within 20 X 20 km areas. A summation of the logarithms of the seismic energy is used as the value representing the seismicity at the central point of each area, since a contrast of the seismicity pattern becomes too large by a few large events if the seismic energy itself is used. Figure 8 shows the seismicity in the Tohoku district computed by the use of the Chebychev approximation function of 24 degrees in the similar

Fig. 8. Oblique projection of seismic activity approximated chev function of 24 degrees for a year of 1977.

with two-dimensional

Cheby-

225

way as before. Features of the seismic energy release are easily observed from the height and width of the surface. Lineations of the seismicity are remarkable off the Pacific coast and in the central part of the land area. An isolated area of high seismicity is seen in the northern part, off the Pacific coast. In Fig. 9, contour lines are drawn for the value 50 of the seismic energy defined above since this value divides the active area clearly. It is seen that seismologically active areas are divided into several groups. Lineations in the direction NW-SE are strikingly pronounced.

138 E 139 E ,--__---------_

190 E

141 E

:4ii E lq3 E 142 E ____._-----\ ?----_--Y2N

39 N

Fig. 9. Contour map of the value 50 of seismic activity for a year of 1977 obtained from Fig. 8b.

226 VERTICAL

LAND MOVEMENT

AND MICROEARTHQUAKE

ACTIVITY

In the present section, we shall try to find a correlation between vertical land movement and microearthquake activity, In order to filter out an intermediate wavelength component in the seismic energy patterns, we computed a two-dimensional Chebychev function with 8-21 degrees as presented in Fig. 10. Lineations of the seismic activity found in the former section are revealed again. These kinds of lineations were found in the vertical movements, as already shown. Next, contour lines indicating the value 20 are made from Fig. 10 as illustrated in Fig. 11. The lineation of the seismic activity with wavelength 20-40 km has been well confirmed by these figures. As previously discussed, lineations in the direction NE-SW appeared as the result of band-pass filtering for both vertical land movement and microearthquake activity. It seems that microearthquake activity is related to vertical land movement. Though physical interpretation is not evident at the present stage, it is considered that the tectonic structure of the crust and upper mantle and aseismic plate motion may play an important role.

Fig. 10. Oblique projection of seismic activity for a year of 1977 filtered dimensional Chebychev function from 8 to 21 degrees.

using a two-

’

L--.-~-___-__

___~~_

a#

7-

f

~_

6@

fi.._._

/ :

El

8

: 20Kil

:

.._-___---.-

37 N

1977 Fig, 11. Contour Fig. 10.

map of the value 20 of seismic

activity

for a year of 1977 obtained

from

DISCUSSION

In the previous sections, it has been shown that vertical movements and microearthquake activity in the Tohoku district are lineated in a NE-SW direction. This direction is perpendicul~ to the direction of migration of crustal strains at a velocity of about 40 km/year, found by Ishii et al. (1978a, 1979). Although the lineations seen in the vertical movement may be a manifestation of propagating crustal movement, it is difficult to be certain of this significance because the interval of repeated levelling is only ten years. The

228

direction is also interesting from a geophysical point of view. Hasegawa et al. (1978) has found that the pressure axis of the average mechanism solution of microearthquakes in the upper plane of the double-planed deep seismic zone is nearly in the NW-SE direction. A. Takagi, A. Hasegawa and N. Umino (personal communication, 1978) also found that a trend of the fold formed by equal depth lines of microearthquakes in the upper plane of the double-planed deep seismic zone is also perpendicular to the migrating direction. It is also interesting to note that the propagation direction of the strain almost coincides with a direction of the motion of the Pacific plate derived by Morgan (1972) and Minster et al. (1974), while the direction obtained by Minster et al. is more E-W. Next, let us discuss a few points about strike of active faults, anticlinal and synclinal fold axes, and mechanism of shallow earthquakes, in the Tohoku district. From the distribution of active faults compiled by Matsuda et al. (1976), it is seen that the strike of the active faults is mostly in the N-S direction. Anticlinal and synclinal axes found in a geological map (G.S.J., 1958) also show a N-S direction for the most part. Directions of compression axes of earthquake generation stress obtained by using the first motion of P waves of the shallow earthquakes (Takagi et al., 1973) indicates largely an E-W direction. Furthermore, the principal strain axes observed by extensometers at crustal-movement observatories operated by Tohoku University (R.G.C.M., 1978) are mostly E-W compressional. These results give evidence that the E-W direction shows the direction of a typical stress field in the Tohoku district and it is related to the structure near the surface. On the other hand, it is reasonable to consider that the NW-SE direction found in this paper is related to plate subduction. As pointed out above, it is significant that various independent data show similar directional features. Therefore, it is reasonable to consider that these phenomena are produced by the motion of the sinking Pacific plate. If the relationship connecting the plate motion and these phenomena could be made clear, it would provide a powerful means for earthquake prediction. Therefore, further research for this problem is left for the future. SUMMARY

First, the characteristics of vertical movement in the Tohoku district was investigated using precise levelling data provided by G.&I. Two-dimensional Chebychev functions were applied for the analysis and oblique projections of vertical movement were employed for the comprehensive pattern recognition of crustal movements. Second, the characteristics of microearthquake activity in the Tohoku district were investigated by application of two-dimensional Chebychev functions to data from the seismic network of Tohoku University. Third, a comparison between vertical movement and microe~hqu~e

229

activity was performed, Related geophysical information was compared with these results. The main results of the present study are summ~~zed as follows: (1) Features of vertical land movement in the Tohoku district for the period of 10 years 1956-1966 and 1966-1975 were revealed by means of oblique projections. (2) Lineations of vertical land movements with wavelengths of about 4060 km extending from NE to SW were found by band-pass filtering of the data, (3) Features of the energy release of microearthquakes in the Tohoku district with depth less than 20 km for 1977 were manifested utilizing oblique projections similar to the projections of land movements. (4) Band-pass filtering of the energy release of microearthquakes also revealed lineations in a NE-SW direction. (5) The observed lineations have also been observed in other geophysical data, so that it is considered to be closely related to the motion of the Pacific plate, APPENDIX: THE TWO-DIMENSIONAL CHEBYCHEV APPROXIMATION

A Chebychev polynomial of n degrees is defined as: T,(X) = cos(n cos -I X) The orthogonality relation is:

0,

N-l

kqoTmfxk) T,(xk)

=

L

m#n

Nf2 , m = n f 0

N,

m=n=O

where:

m,

12 < N ,

xk

= COS

n(2k + 1) 2iV

and N is the number of samples. A two-dimensional Chebychev approximation function f(x, y) can be written as:

The Chebychev approximation coefficients A,,j are calculated using discrete data F(Q, yl) as:

230

Ai,j =

=

&

I$:yc: F(Xkr Yl)

M&zo’ ;ij

F(xk,

Ti(xk)

yr) cos

Tj(Yl)

ir’2E‘) COS~*(~& ‘)

where: X k=

cos[7r(2h + 1)] 2M

and yl=

cos[n(21+ 1)] m

The values of x and y are between -1 and +l. In order to obtain a value between -1 and +l from an arbitrary range of the independent variable z, we use the relation: 3c= 22 - (a + b) b-a

where a and b are the lower and upper limit of the independent variable z, respectively. ACKNOWLEDGEMENTS

We would like to express our appreciation to Dr. W. Thatcher and Dr. E.R. Engdahl for critically reviewing the manuscript and for giving useful suggestions. Acknowledgement is due to Prof. Z. Suzuki, Prof. T. Hirasawa and our colleagues for discussions and comments. Acknowledgement is also due to Miss N. Sakaki for assistance in preparing the manuscript.

231 REFERENCES Geographical Survey Institute, 1971. Crustal movements before and after Southeastern Akita Earthquake. Rep. Coord. Comm. Earthquake Predict., 5: 3-8 (in Japanese). Geographical Survey Institute, 1976. Tiltings of the crust in Tohoku region. Rep. Coord. Comm. Earthquake Predict., 15: 15-18 (in Japanese). Geological Survey of Japan, 1958. Geological Map of the Tohoku district. Hasegawa, A., Umino, N. and Takagi, A., 1978. Double-planed structure of the deep seismic zone in the northeastern Japan arc. Tectonophysics, 47: 43-58. Ishii, H., Sato, T. and Takagi, A., 1978a. Characteristics of strain migration in the northeastern Japanese arc. 1. Propagation characteristics. Sci. Rep. Tohoku Univ., Ser. 5, 25: 83-90. Ishii, H., Sato, T. and Tachibana, K., 1978b. Observation of crustal movements at the Akita Geophysical Observatory. 3. Application of Chebychev approximation function for data observed by extensometers and tiltmeter. J. Geod. Sot. Jpn., 24: 122-131 (in Japanese with English abstract). Ishii, H., Takagi, A. and Suzuki, Z., 1979. Characteristic movement of crustal deformation in northeast Honshu, Japan. Gerlands Beitr. Geophys., 88: 163-169. Ishii, H., Sato, T. and Takagi, A., 1980. Characteristics of strain migration in the northeastern Japan arc. 2. Amplitude characteristics. J. Geod. Sot. Jpn., 26: 17-25. Kato, T. and Kasahara, K., 1977. The time-space domain presentation of levelling data. J. Phys. Earth, 25: 303-320. Matsuda, T., Okada, A. and Huzita, K., 1976. Distribution of active faults in Japan, appendix to “faults and earthquake”. Mem. Geol. Sot. Jpn., 12. Mizoue, M., 1967. Modes of secular vertical movements of the Earth’s crust, 1. Bull. Earthquake Res. Inst., 45: 1019-1090. Minster, J.B., Jordan, T.H., Molnar, P. and Haines, E., 1974. Numerical modelling of instantaneous plate tectonics. Geophys. J.R. Astron. Sot., 36: 541-576. Morgan, W.J., 1972. Deep mantle convection plumes and plate motions. Bull. Am. Assoc. Pet. Geol., 56: 203-213. Nabetani, S. and Kano, H., 1970. Gravimetric investigation in the Takanuki district, Southern Abukuma Plateau. With special regards to the emplacement structure of granite pluton. Min. College of Akita University, Ser. A, 4: 89-106. Research Group for Crustal Movement of Tohoku University, 1978. Analysis of crustal movement in the northeastern Japanese arc observed by means of an array system. Sci. Rep. Tohoku Univ., Ser. 5, 25: 73-82. Southworth, R.W. and Deleeuw, S.L., 1965. Digital Computation and Numerical Methods. McGraw-Hill, New York. Takagi, A., Hasegawa, A. and Umino, N., 1973. Earthquake mechanisms of shallow earthquakes occurring in the Tohoku District. Abstract given at the meeting of the Seismological Society of Japan held in spring, 24. Takagi, A., Hasegawa, A. and Umino, N., 1977. Seismic activity in the northeastern Japan Island arc system. J. Phys. Earth (Special Issue), 25: 95-104.