Activity in the Northeastern Japan Arc

J. Phys. Earth, 25, Suppl., S 95-S 104, 1977

SEISMIC ACTIVITY IN THE NORTHEASTERN JAPAN ARC

Akio TAKAGI, Akira HASEGAWA,and Norihito UMINO

Observation Center for Earthquake Prediction, Faculty of Science, Tohoku University,Sendai, Japan (Received June 1, 1977; Revised October 3, 1977)

The most recent data obtained from the high-gain seismograph network of To- hoku University are used to make an investigation on seismic activity and tectonics in the northeastern Japan arc. Seismic activity is closely related to the tectonic de- velopment of island arcs, and detailed analyses of the spatial distribution of microearthquakes in the northeastern Japan arc have led to conclusive evidence that almost all the shallow microearthquakes in the land area occur only in the layer with a P-wave velocity of 5.9 km/sec which was determined from explosion seismic observations. The configuration of the deep seismic zone in this region is discussed in relation to the plate subduction beneath the arc. The accurate determination of the hypocenters and focal mechanisms of microearthquakes has made it possible to investi- gate the subduction process beneath the arc.

1. Introduction

The detailed information on the relative motion between the oceanic and con- tinental plates in the subduction zone can be regarded as the first step for studying earthquake prediction in an island arc. The installation of a sufficient number of seismic stations for accurate observations is essential to the attainment of this purpose. In the Tohoku district, northeastern Honshu, a high-gain seismograph network of Tohoku University composed of 15 stations has been installed in order to investi- gate the detailed features of seismic activity in the northeastern Japan arc since the enforcement of the national program of earthquake prediction research in 1965. The network has been considerably improved with the provision of telephone telemetry during the third five-year plan of the program, allowing microearthquakes in this region to be located much more accurately than before. The adoption of this system has facilitated detailed studies of the spatial distribution of microearthquakes and provided some interesting facts in relation to seismic activity and tectonics beneath the northeastern Japan arc. These facts appear to be very important in under- standing the subduction process beneath the arc and probably also for the prediction of earthquakes which will occur in this region. In this paper, we report on the new results and the observation system of the seismic network.

S 95 S 96 A. TAKAGI, A. HASEGAWA, and N. UMINO

2 . Observation System of the Seismic Network The locations of observation stations of the seismic network of Tohoku Uni- versity are shown by the solid and double circles in Fig. 1. The output signals of three-component short-period seismometers are amplified and transmitted from each station (solid circle) to the three centers (double circles) using telephone telemetry. The open circles in the figure denote the temporary stations from which the signals are not sent to the centers. The seismic signals from OGA, FUT, NIB, SAW, and HOJ are collected at the Akita Sub-center, and those at HMK, HSK, MYK, and SNR are at the Kitakami Sub-center. From the southern stations, ATM, KWT, KMF, and KNK, the amplified signals are sent to the Observation Center for Earth- quake Prediction in Sendai. At the Observation Center and the Kitakami Sub-center, seismometers are also installed as with the case of their satellite stations. The signals in the vertical component received at the two Sub-centers from their satellite stations are transmitted again to the Observation Center, and thus the vertical data from all

Fig. 1. Locations of the observation stations of the seismic network of Tohoku University. Seismic Activity in the Northeastern Japan Arc S 97

Fig. 2. Block diagram of the observation system of the seismic network.

the stations are collected at the Observation Center. At the time when an earthquake is detected by three or more stations of the network, multi-channel pen recorders and 22-track FM magnetic tape recorders start to work and continue recording the delayed signals of the earthquake for 2min at these three centers. Figure 2 shows a block diagram of the observation system. The earthquakes analyzed in this study are all those detected at this seismic network.

3. Hypocenter DistributionShallow of Microearthquakes The microearthquakesobserved by the seismicnetwork of Tohoku University have been locatedby usingthe seismogramsrecorded on the multi-channelpen re- corderat the ObservationCenter. The method forthe determinationof hypocenters and the velocitystructure for P and S waves are givenin anotherpaper (HASEGAWA et al.,1977). The number of eventsthus locatedamounts to 4,500 or more during a periodof 18 months. S 98 A. TAKAGI, A. HASEGAWA, and N. UMINO

Fig. 3 Seismic Activity in the Northeastern Japan Arc S 99

Fig. 3. Epicenter distribution of shallow microearthquakes located in the Tohoku district during the period from April, 1975 to October, 1976. Bathymetric con- tours are at 500m intervals. Crosses denote the locations of the Stations.

Figure 3 shows the epicenter distribution of the shallow microearthquakes located in the Tohoku district during a period from April, 1975 to October, 1976.

The events with focal depths from 0 to 10km, 10 to 20km, and 20 to 30km are plotted in Figs. 3 (a), (b), and (c), respectively. The determined hypocenters within the network are accurate to within several kilometers in focal depth, which has been confirmed by using the data on quarry blasts. The seismic activity of shallow microearthquakes can regionally be divided into three groups: off the coast of Sanriku; along the volcanic front in the land area; and along the coast line of the Japan sea.

The clustered activity near 40°N off the coast of Sanriku has an interesting corre- lation to the sea-bottom topography as seen in Fig. 3. One of the present authors

(HASEGAWA, 1977) stated that this feature extends down to deeper seismic zone, and it may be caused by the distortion of plate in this region. A comparison of Figs. 3 (a) and (b) clearly indicates the existence of the regions in the land area where almost all the microearthquakes are occurring only in the depth range from 0 to 10km. In those regions earthquake swarms have occurred repeatedly in the past. On the other hand, microearthquakes are occurring even in the deeper parts as far as about 20km in the regions where destructive earthquakes have occurred in the past.

Shallow microearthquakes in the land area seldom occur at depths more than S 100 A. TAKAGI, A. HASEGAWA, and N. UMINO.

FR i E g S . E4 A R C H F o G c R a O l U P d e F Op t R h E d X i P s LO t - r S i I b O Nu t Si E o I n S M OL o O f G Y m , i c r o e a r th q u a k e s p r o j e c t e d o n t h e v e rt i c a l s e c t i o n a l o n g t h e O g a - K e s e n n u m a p r o f i l e a n d t h e c r u s t a n d u p p e r - m a n t l e s t r u c tu r e d e r i v e d f r o m e x p l o s i o n s e i s m i c o b se r v a t i o n s (

1 9 7 7 ) . Seismic Activity in the Northeastern Japan Arc S 101 about 20km as can be seenin Fig.3 (c). This phenomenon isidentical with thatfound in the Kinki district,southwestern Honshu (OIKE, 1975). Recently,the crustand upper-mantlestructure along the Oga-Kesennuma profilein the Tohoku districthas been determinedwith considerableaccuracy from explosionseismic observations (RESEARCH GROUP FOR EXPLOSION SEISMOLOGY, 1977). The crustand upper-mantle structurederived by them isillustrated in Fig. 4 (a). The horizontalscale is the same as the verticalone. Figure 4(b) isthat magnified in the depth directionfrom Fig. 4 (a). The bars acrossthe top of the figuresshow the land area. Numerals in the figuredenote the P-wave velocityin each layer. The microearthquakehypocenters locatedby our seismicnetwork areincluded in thefigure. The eventswithin a range of 60km width along the profile(those occurred in the rectangulararea illustrated in the insertedmap) are projectedon the verticalsection. It is worth noting that almost allthe microearthquakesin the land area are found only in the layerwith a P-wave velocityof 5.9km/sec. This factimplies that the so-calledbasaltic layer cannot produce earthquakes.This may be due to the differencein the propertyof materialsbetween the so-calledgranitic and basalticlayers in thatregion. The shallowearthquake occurringalong the coastof the Japan sea are not confinedto the graniticlayer as shown in Fig. 4. The complexityof the crustal structure,that is, the relativelyabrupt change in depth of Moho and Conrad dis- continuitiesnear thecoast line, may causethe microearthquakes in thebasaltic layer. However, the accuracy of theseearthquake hypocenters is not so high as thatof the eventsoccurring in the land area withinthe seismicnetwork. This problem cannot be solvedonly by usingthe data availableat present. The unusuallydeep microearthquakeswere observed in the lower crustand upper mantle in the land area. The epicenterof one of the earthquakesis shown in Fig. 3(c). These two earthquakesoccurred beneath Volcano Iwate, and the stress dropsof theseevents are inferred to be extremelylow from theseismograms in which relativelylong-period waves are predominantcompared with the microearthquakes occurringin the upper crustor in the deep seismiczone. The occurrenceof these eventsseems to be relatedto thevolcanic activity.

4. Deep Seismic Zone It iswidely known that there existsa deep seismic zone dipping downward from the vicinityof a trench beneath an island arc, and that deep and intermediate-depth earthquakesoccur only within thisdipping zone and not elsewhere. This deep seismic activityis relatedto the so-calledhigh-Q, high-velocitylithospheric slab descending into the mantle. A detailedstudy of the deep seismiczone beneath the northeastern Japan arc has been made by using the data obtained from the seismic network of Tohoku University, and a double-planed structure of the zone has been found in thisregion (UMINO and HASEGAWA, 1975; HASEGAWA et al.,1977). In this section, we give an outlineof our research on the deep seismic zone beneath the northeastern S 102 A. TAKAGI, A. HASEGAWA, and N. UMINO

Fig. 5. Focal depth distribution of microearthquakes located in the region from 39°N to 40°N projected on the vertical section in the E-W direction. The crustal structure is

also illustrated by solid line. Thick horizontal segment denotes the land area, and triangle and reverse triangle represent the positions of the volcanic front and the Japan trench, respectively.

Japan arc, the details of which have been described in previous work (HASEGAWA

et al., 1977; HASEGAWA, 1977).

The microearthquakes that occurred in the region from 39°N to 40°N during

a period from April, 1975 to October, 1976 are plotted on the vertical section in the

E-W direction as shown in Fig. 5. A pronounced separation of the deep seismic

zone into two planes is detected in this region. The same double-planed structure

has been ascertained in the other region from 37°N to 41°N in the Tohoku district.

TSUMURA (1973) also found a similar structure in the region from 35.5°N to 36.5°N

in the Kanto district, central Honshu, by using data from the seismic network of

Tokyo University. The region is situated at the southernmost part of the north-

eastern Japan arc and consequently the deep seismic zone forms a double-planed

structure throughout the arc having a length of about 600km or more parallel to

the Japan trench. Although somewhat scattered events are located beneath the Japan

trench, the accuracy in focal depth of these events is not so high and, therefore,

these are excluded from the present discussion.

The earthquake-generating stresses for events in the two planes have been in-

vestigated by superposing the initial motions of P waves. The deep seismic zone is

divided into 15 regions, and the initial motions of P waves for the events occurring Seismic Activity in the Northeastern Japan Arc S 103 in each regionare superposedand projectedon the same focalsphere to obtain the compositefocal mechanism solutionfor each group of earthquakes.The resultshows thatthe focalmechanisms forthe upper plane are characterizedby reversefaulting or down-dip compressionand thosefor the lower plane by down-dip extension. In orderto investigatethe subductionprocess beneath island arcs, we must have a betterunderstanding of where the deep and intermediate-depthearthquakes are actuallyoccurring within the descending slab. The geometricalrelationship between the deep seismiczone and the descendinghigh-Q, high-velocityslab beneath the northeasternJapan arc has been investigatedby usingthe arrivaltime of the fore- running phase of the ScS wave observedat some stationsof the network. Further- more, the resulthas been confirmedby usingthe travel-timeanomaly of intermediate-depthearthquakes observed at a small-scaleseismic array. ScS wave generatedby a largedeep earthquakeof June 29, 1975 in the western Japan sea was observed very clearlyat allthe stationsin the network. A distinct forerunningphase of the ScS phase was alsoobserved on the verticalcomponent at severalstations. The followingproperties of thisphase became evident;the arrival time isindependent of the epicentraldistance, and the differencein arrivaltime between thisphase and the ScS phase increaseswith the increasein depth to the deep seismiczone beneath each station.OKADA (1971)interpreted this kind of phase as a convertedScSp wave from the ScS wave at the boundary between the descending slaband the mantle above it. Accordingto his interpretation,we have determined the locationof the upper boundary of the descendinghigh-velocity slab from the observeddifference in arrivaltime between the two phases. An 80km thick plate model in which P and S wave velocitiesare 6% higherthan thosein the surrounding mantle isassumed here,and the travel-timedifference between the two waves iscal- culatedfor each stationby applyingthree-dimensional seismic ray tracingdeveloped by JACOB (1970).The positionof theupper boundary of theassumed dippingplate is

Fig. 6. Focal depth distributionof microearthquakesprojected on thevertical section in the E-W direction(open circles)and the locationof the boundary between the descending high-Q,high-velocity slab and the mantle above it (hatchedzone). Thick horizontal segment, triangleand reversetriangle are the same as in Fig. 5. S 104 A. TAKAGI, A. HASEGAWA, and N. UMINO determined in order that the calculated travel-time difference between the two phases may agree with the observed value at each station. The position of the boundary thus determined is illustrated by the hatched zone in Fig. 6. The upper boundary of the descending slab coincides with the upper plane of the double-planed deep seismic zone. The hypocenter distribution of intermediate-depth earthquakes located by the small-scale seismic array, which is situated at the central part of the network, is extremely different from that located by the network. This is due to the laterally heterogeneous structure in the upper mantle. The seismic rays of intermediate-depth events to the array will be bent at the upper boundary of the descending slab and, therefore, the apparent velocity and the direction of wave approach at the array will be altered from those of the laterally homogeneous structure. The hypocenters located by the array are obtained without taking this effect into consideration. The upper mantle structure can be estimated from the amount of this discrepancy. We have relocated these intermediate-depth earthquakes taking the lateral heterogeneity into account, and it becomes evident that the discrepancies between the hypocenters can be well explained by the inclined slab model derived from the difference in arrival time between ScS and ScSp waves. The occurrence of the earthquakes in the upper plane of the double-planed deep seismic zone can be interpreted as being the result of interaction between the descending slab and the mantle above it. The focal mechanisms for these events are consistent with this interpretation. The evidence obtained here will provide valuable information on the definition of the type of mechanism producing the plate motion beneath island arcs.

REFERENCES

HASEGAWA, A., A study of deep seismic zone in the Northeastern Japan Arc, Doctor' s thesis,Tohoku University, Sendai, 1977. HASEGAWA, A., N. UMINO, and A. TAKAGI, Double-planed structure of the deep seismic zone in the Northeastern Japan Arc, submitted to Tectonophysics,1977. JACOB, K.H., Three-dimensional seismic ray tracing in a laterallyheterogeneous spherical earth, J. Geophys. Res.,75, 6675-6689, 1970. OIKE, K., On a listof hypocenters compiled by the Tottori Microearthquake Observatory, Zisin (J. Seismol. Soe. Japan), 28, 331-346, 1975 (in Japanese). OKADA, H., Forerunners of ScS wave from nearby deep earthquakes and upper mantle structure in. Hokkaido, Zisin (J. Seismol. Soc. Japan), 24, 228-239, 1971 (in Japanese). RESEARCH GROUP FOR EXPLOSION SEISMOLOGY, Regionality of upper mantle around Northeastern Japan as derived from explosion seismic observations and its seismological implications, Tecto- nophysics,37, 117-130, 1977. TSUMURA, K., Microearthquake activityin the Kanto district,Publications for the 50th Aniversary of the Great Kanto Earthquake, 1923, pp. 67-87, 1973 (in Japanese). UMINO, N. and A. HASEGAWA, On the two-layered structure of deep seismic plane in Northeastern Japan Arc, Zisin (J. Seismol. Soc, Japan), 28, 125-139, 1975 (in Japanese).