Deformation of an Island Arc: Rates of Moment Release

Home , 1964 Niigata earthquake, Geology of Japan, Island arc, Nankai Trough, Northeastern Japan Arc, Sagami Trough

JOURNALOF GEOPHYSICALRESEARCH, VOL. 87, NO. B8, PAGES6829-6852, AUGUST10, 1982

DEFORMATION OF AN ISLAND ARC' RATES OF MOMENT RELEASE AND CRUSTAL SHORTENING IN INTRAPLATE JAPAN DETERMINED FROM SEISMICITY AND QUATERNARYFAULT DATA

Steven G. Wesnousky and Christopher H. Scholz

Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964

Kunihiko Shimazaki

Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, Japan

Abstract. The historical record of large (M) of relative plate motion takes place as a perma- 6.9) earthquakes and geologically determined nent strain of the overriding plate. In this rates of slip on Quaternary faults in intraplate study, seismicity and Quaternary fault data are Japan (Honshu and Shikoku) are used to estimate used to quantify the amount of horizontal defor- the average rate of seismic moment release mation that results from slip on faults within (•) for the last 400 years and durin• the late the crust of the islands of Honshuand Shikoku, Quaternary, respectively. Values of • estimated part of the Japanese island arc system. it is for from the two data sets are similar in regions this region that by far the most complete set of where seismic activity is concentrated on land: data concerning seismicity and Quaternary fault- We interpret this observation to suggest that • ing is available. in intraplate Japan has been constant during the The seismic moment tensor (M) is the most late Quaternary and is relatively free from direct measure of the elastic waves set up by an secular variation when averazed over periods of earthquake and the deformation resulting from its several hundreds of years. • in Shikoku may be occurrence [e.g., Aki, 1966' Aki and Richards, attributed almost solely to right-lateral slip of 1980]. For shearing on a fault, the scalar value the median tectonic line (MTL). The easterly of the seismic moment(M o) is •uA, where • is the strike of the MTL is consistent with a compres- shear modulus, u is the average slip on the sive stress field that trends northwest, Crustal fault, and A is the fault area. The moment shortening of the Izu Peninsula taken up on a set tensor (M) can be completely described when the of strike slip faults that shc,• north by north- strike, dip, and rake of the fault are known in west compression is =1 mm/yr. Northeast Honshu addition to Mo . In this study, the seismic is characterized by a set of reverse-.type faults momentsof large intraplate earthquakes(M) 6.9) that trend northerly. Conversion of • in north- in Japan are gathered from the literature or, in east Japan to strain rates suggests that about 5% the case of historical earthquakes, determined of the relative plate motion between Japan and from intensity data. Kostrov's [1974] formula the Pacific plate (=9.7 cm/yr) is accommodatedas relating seismic moment to strain is then used to a permanent east-west shortening (=5 mm/yr) of calculate the amount of strain that has resulted northeast Honshu. The predominant deformation in from slip during earthquakes over the last 400 central and western Honshu takes place as slip on years. Similar to earthquakes, active Quaternary a conjugate system of strike slip faults that faults with geologically estimated slip rates are strike northeast and southeast and shc,• right- described in terms of their average rate of seis- lateral and left-lateral motion, respectively. mic moment release and utilized to determine the Crustal shortening, resulting from slip on average rates of strain of Japan during the late faults, in central and western Honshu is 5 and Quaternary period. The values obtained for the 0.5 mm/yr, respectively, in an easterly direc- different periods of time are then used as the tion. Central and western Honshu are in closest basis for deciding whether or not the rate of proximity to the Nankai trough, and hence, the fault deformation in intraplate Japan has been stress field in these regions cannot simply be constant during the Quaternary period. attributed to the accommodation of the relative (northwesterly) convergence of the Philippine Sea Plate Tectonic Setting and Seismicity of Japan plate. The northward impingement of the Izu Peninsula into Honshu may influence stresses in The Japanese island arc is located along the central and western Japan, but a conclusive eastern border of the Eurasian plate and bounded explanation of the stress field in central and to the east and south by the Pacific and western Honshu remains an enigma. Philippine Sea plates, respectively (Figure 1). The majority of earthquakes in Japan, including Introduction the largest (Mo ) 1028 dyne cm), occur along the Japan trench and the Sagami and Nankai troughs The commonoccurrence of intraplate seismicity [e.g., Usami, 1966, 1975]. Focal mechanismsof and mountain belts adjacent to convergent plate large earthquakes along these features are usual- boundaries indicates that a significant portion ly of the low-angle thrust type and interpreted to indicate that the Pacific and Philippine Sea Copyright 1982 by the American Geophysical Union. plates are being subducted beneath northeastern and southwestern Japan, respectively [Fitch and Paper number 2B0732. Scholz, 1971; Kanamori, 1971, 1972; Ando, 1974b, 0148-0227/8 2/002B-0732505.00 1975; Abe, 1977; Scholz and Karo, 1978].

6829 6850 Wesnousky et al.' Deformation of an Island Arc

I$0 o 150 ø reverse on planes that strike north. Earthquakes in central and western Honshu generally exhibit strike slip movement of a left-or right-lateral nature on northwest or northeast striking fault planes, respectively. The area in and adjacent to the Izu Peninsula, at the junction of the Sagami and Nankai troughs, is also marked by strike slip faulting but in contrast to the remainder of Honshu, the P axes trend to the north. The Izu Peninsula appears to be a part of the Philippine Sea plate, and the northerly trend of P axes in this area is probably a mmnifestation of the recent collision of the Izu Peninsula with Honshu [Matsuda, 1978; Somerville, 1978]. The seismicity in this region also differs from the remainder of intraplate Japan in that it is not landward of a trench.

Quaternary Faulting

Fig. 1. Regional plate tectonic setting. The orientation and displacement of Quaternary faults closely mimics movement observed in recent earthquakes (Figure 4) [Research Group for Active Yamashinaet al. [1978] studied shallow seismici- Faults of Japan, 1980a, b]. Quaternary faults in ty in Japan and other island arcs and defined a northeastern Honshu are of the reverse type, zone extending along the frontal non volcanic strike northerly, and are distributed relatively arc, characterizedby a very low rate of seismic- uniformly throughthe region. The concentration ity, as the 'aseismic belt.' The shallow seis- of Quaternary faulting is greatest in central micity landwardof the aseismic belt is referred Honshu. The predominantdeformation in central to here as intraplate seismicity and generally Honshutakes place as slip on a conjugatesystem does not extend to depths greater than 15 km of strike slip faults that strike northeast and [e.g., Oike, 1975; Takagi et al., 1977; Watanabe northwest and show right- and left-lateral et al., 1978] (Figure 2). Theseismic moments of motion, respectively. This s•memotion of fault- the largest intraplate earthquakesare about 1027 ing is also seen in western Honshu,where the dyne cm, an order of magnitudeor more less than density of faulting is muchless. Faults in the seismic moments of the great interplate Izu Peninsularegion are also strike slip but are earthquakes. consistent with a compressivestress that trends Focal mechanism data have been used to delin- north to northwest [Somerville, 1978]. Hence, eate the ambient intraplate stress field in Japan [e.g., Honda, 1932; Honda and Masatsuka, 1952; Honda et al., 1956, 1967; Ichikawa, 1970, 1971; Nishimura, 1973; Yamashina, 1976; Shiono, 1977]. A summary of 44 intraplate events with magnitudes of about 6.0 and greater is presented in Table 1. Fault mechanisms of intraplate earthquakes are usually strike slip or high-angle reverse, and the distribution of P axes reveals that Honshu is, in general, subject to a regional maximum compressive stress that trends easterly (Fig- ure 3) . Normal faulting is found in Kyushu, adjacent to the junction of the Nankai Trough and 150- Ryukyu Trench. Near the junction of two arc systems, the state of stress within the earth's crust generally differs from the regional trend [Shimazaki et al., 1978]. Hokkaido, like Kyushu, 200 - is also located near the junction of two trenches (Figure 1). We limit the remainder of this study to the islands of Honshu and Shikoku, adjacent to 250 the Japan trench and the Nankai and Sagami Fig. 2. Focal depth distribution of microearth- troughs. For convenience, we will refer to this quakes projected on an east-west cross section region as intraplate Japan. (adapted from Hasegawa et al. [1978]). Seismic- The P axes of intraplate earthquakes in Honshu ity from 38ø-39øN is plotted. Arrow indicates are approximately aligned with the relative plate approximate location of the 'aseismic belt' velocity vector along the Japan trench (Figure [Yamashina et al., 1978]. Triangle and reverse 3). This alignment has been interpreted to arise triangle represent positions of volcanic front from the transmission of compressive stress and Japan trench, respectively. Location of across the Japan trench [e.g., Ichikawa, 1971; Japanese Islands is delineated by horizontal line Shiono, 1977; Nakamura and Uyeda, 1980] . Focal above cross section. Intraplate seismicity is mechanisms differ between northeast Honshu and located in upper 15 km of crust, west of the regions to the southwest (Figure 3). Earthquakes aseismic belt. The figure has a vertical exag- in northeastern Honshu commonly are high-angle geration of 2. Wesnousky et al.' Deformation of an Island Arc 6831

intraplate deformationin Honshutakes place as The mean rate of strain (•)of a seismogenic slip on a network of faults that accommodatea volume (V) resulting from earthquake dislocations regional compressive stress field that trends is proportional to the sum of the seismic moment east except for the Izu Peninsula, wheremaximum tensors (Z•) of all earthquakesoccurring in the horizontal stress is oriented north to northwest. volume (V) per unit time (T) [Kostrov, 1974]: Strain release in Shikoku occurs by slip along the median tectonic line (MTL), a major right- lateral fault that strikeseasterly, across the • = 1 (Z•) (3) northerncoast of Shikokuand parallel to the 2•VT Nankai trough (Figure 3). The orientation and slip of the MTL is consistent with a compressive Cumulative offsets on geologic faults may be as- stress that trends northwest, in contrast to an sumed as the sum of displacements resulting from easterly maximumcompression in Honshu. In sum- many earthquakes. Hence, the average rate of mary, geologic and seismologic data may be used slip of a geologic fault may be used to estimate to divide intraplate Japan into five regions of the average rate of moment release of the fault. distinct tectonic style: (1) northeast Honshu, We use formula (3) plus seismicity and Quaternary (2) central Honshu, (3) western Honshu, fault data to analyze crustal deformation in (4) Shikoku, and (5) the Izu Peninsula and intraplate Japan. adjacent environs (Figure 5). Deformation Rates From Seismicity Data The Seismic Moment Tensor Data The seismic moment tensor for a hypothetical shear dislocation at a point may be expressed as In this work, data for earthquakes prior to 1885 are from the 'Descriptive Catalogue of Disastrous Earthquakes in Japan' [Usami, 1975]. •-- Mo(bin• + bjni) (1) Datafrom Utsu's [1979] catalogue is usedfor the period 1885-1925. Magnitudes and epicentral data for events after 1925 are acquired from the where Mo is the scalar value of the momentand b SupplementarySeismological Bulletin of the Japan and n are unit vectors in the direction of slip Meteorological Agency (JMA) [1958, 1966, 1968] and normal to the fault plane, respectively. The (and monthly bulletins thereafter). Magnitudes P, T, and B axes of focal mechanism solutions in each catalogue are scaled to conform with the correspond to the principal axes of the moment definition of surface wave magnitude proposed by tensor [e.g., Aki and Richards, 1980]. Brune Gutenberg and Richter [1954]. [1968] showed that the slip rate along a fault Most of the cumulative deformation resulting may be expressed in terms of the seismic moment from seismicity occurs during the largest earth- as quakes [Brune, 1968]. We thus limit our concern to large earthquakes, defined here as those events with magnitudes greater than or equal to • = 1 (ZMo) (2) 6.9 (or Mo • 1026dyne cm). A plot of large •ST intraplate earthquakes versus time for the last 400 years shows that the frequency of occurrence where • is the average slip rate, • shear of large events has been higher for the period modulus, S surface area of fault plane, T time since 1880 than for the prior 300 years (Figure period over which seismicity occurred, and ZMo 6c). The greater frequency of events observed the cumulative sum of momentsof all earthquakes for the last 100 years reflects the initiation of occurring on the fault area S over time T. both instrumental recording [Kikuchi, 1904] in Brune [1968] and Davies and Brune [1971] used 1880 and the systematic collection of felt (2) to calculate average slip rates along major reports by the Central Meteorological Observatory plate boundaries, using a record of seismicity in 1884 [Usami and Hamamatsu,1967]. Prior to extending back as little as 50 years. Their this time, even though since the commencementof ability to do this was contingent on the high the 17th century it has been 'the rule of the slip rates (cm/yr) and short recurrence times government to have each feudal chief send in a (tens to hundreds of years) of large earthquakes detailed report of damage caused in his dominion along given segments of plate boundaries. The by an earthquake or other natural events' [Omori, style of seismicity within intraplate Japan is 1899], it appears that a numberof even the large distinctively different. Slip rates along given events may be missing from the historical record. faults are less (mm/yr), recurrence times greater The spatial distribution of seismicity discloses (several hundreds to thousands of years), and those areas that are deforming most intensely. seismic energy release is not concentrated along It is thus important to investigate whether data one fault but rather is divided among a large possibly missing from the earlier period (1581- network of faults (Figures 4 and 5). Hence, in a 1880) may account for any spatial distortion in region like this it is not appropriate to consid- the observed pattern of seismicity. er only a single fault. It is more suitable to The epicentral distributions of large earth- measure the total deformation resulting from the quakes during the periods 1581-1880 and 1881-1980 movements on all faults in the area. Kostrov are compared in Figure 5. Two major differences [1974] expanded the method proposed by Brune are immediately evident in the pattern of seis- [1968] to determine the average strain of a re- micity in the two periods. The first is the gion where seismicity is distributed uniformly in absence of seismicity on the Izu Peninsula during a volume rather than restricted to a given fault. the earlier period as compared to the occurrence 6832 Wesnousky et al.' Deformation of an Island Arc

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I •'m I Fig. 3. P axes of intraplate earthquakes with •gnitudes of about 6.0 and greater. Epicenters of strike slip and reverse-type faults are denoted by open and solid circles, respectively. •e horizontal projection of P axes are shwn by lines through epicenter s•bols. Numbers adjacent to earthquake s•bols correspond to the list of earthquake data for each event presented in Table 1. Events 38 and 39 were normal faults, and the T axes are represented by outward pointing arrows. •e Nankai Trough (N.T.), Japan Trench (J.T.), Sagami Trough (S.T.), and Izu-Bonin Trench (I-B.T.) are sho• schematically. Relative plate velocities (cm/yr) calculated from Seno [1977] are shwn by large hollw arrws. •e •dian tectonic line (•L) has right-lateral offset (half-sided arrows) and is sho• as a solid and dashed line where it has and has not, respectively, been active during the Quaternary. of three large events there in the last 100 cent to the Bungo Channel area in the earlier 300 years. The Izu region has been populated for at years in contrast to none during the more recent least the last 400 years. Furthermore, large period. This was previously noted by Shimazaki earthquakes in the Izu region produce shaking of [1976]. He concluded, on the basis of the JMA intensity IV as far away as Tokyo, a cultural historical developmentof Japanese civilization, center for well over 400 years. Hence, the that this must represent true secular variation quiescence of the earlier 300-year period appears of seismicity and is probably not a consequence to be real and not a consequenceof an inadequate of the population distribution. Aside from these historical record. The other major contrast is minor differences, this comparison of seismicity the occurrence of a number of earthquakes adja- patterns suggests that any data missing from the

Fig. 4. Distribution of active intraplate faults in Japan. Faults mappedby marine seismic reflection surveys are denoted by thinner lines. Adjacent plate boundaries are represented as thick stippled lines. (Data are adapted directly from Research Group for Active Faults of Japan [1980a, b]). 130OE 140 o 6835

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Fig. 5. Intraplate earthquakes in Japan (excluding Hokkaido) of M > 6.9 during the period 1881-1980 (Figure 5a) and 1581-1880 (Figure 5b). The radius of the epicenter symbols is proportional to the magnitude of the event. Numbers next to solid symbols in Figures 5a and 5b correspond to Tables 2 and 3, respectively. Numbers adjacent to open symbols in Figure 5a correspond to Table 3. Schematic plate boundaries are given and labeled as in Figure 3. Dashed line• are used to delineate the intraplate regions of Japan. Intraplate Japan is interpreted to be the area uorthwest of line ABCDE. FC divides northeast Japan from central Honshu. GD divides central Honshu from western Honshu. AB marks the aseismic front which generally parallels the eastern boundary of the aseismic belt [Yoshii, 1975; Yamashina et al., 1978]. The Izu Peninsula region south of BC is characterized by northward compressional tectonics reflecting the recent collision of the Izu Peninsula with Japan in the early Quaternary [Matsuda, 1978; Somerville, 1978]. CDE represents the landward extent of the nascent Wadati-Benioff zone of the Philippine Sea plate [Shiono, 1974]. The region south of ABCDEreflects plate subduction and collisional processes. 68S6 Wesnousky et al.' Deformation of an Island Arc

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• • I , ,• . I •oo •oo m'oo •9'oo 19'80 Fig. 6. Earthquakesof M ) 6.9 as a function of time for intraplate regions of Japan plotted beneath 50-year (saall circles) and 200-year (large solid circles) running averagesof the seismicmoment release rate (1025dyne cm/yr) for eachrespective region. Data points in the running average curves are placed in the middle of the time period sampled. Momentrelease rates calculated from the complete 400-year record are shown as horizontal dashed lines. (a) Earthquakes in central Japan (dashed vertical lines). (b) Earthquakesin northeast Japan (solid vertical lines). (c) Earthquakes from both Figures 6a and 6b. Wesnousky et al.: Deformation of an Island Arc 6837

,-• 0000• •'• 00 •' •'• 00000 ,•

ß . ß , . , , . . . , . . . , . 6838 Wesnouskyet al.' Deformation of an Island Arc

i S. I ZU EQ. 1934 GIFU EQ. 1969 N R]SZ-• 15Km .._ R]:7 =70 Km ,/• R•Z '= 7'Km

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N ? , , / o • ,O, •,00Km Fig. 7. Maps of JMAintensity from Usami [1975] for four of the earthquakesused in this study. Source parametersof these earthquakesare listed in Table 2. The radius of the circle used to approximate the area of intensity is given. Note that the scale differs for each map.

historical record do not introduce any spatial strong correlation between the areal distribution distortion into the observed pattern of of modified Mercalli (MM) intensity VI and seis- seismicity. sic moment(M o) [Hankset al., 1975; Herrmannet al., 1978]. In Japan, seismic intensity is gen- Seismic Momentof Historical Earthquakes erally described with the Japan Meteorological Agency's (JMA) intensity scale. The JMA scale It is necessaryto have a quantitative measure consists of only eight grades of intensity [see of both the moment(M o) and appropriate focal Usami, 1966] as comparedto the 12 of the MM mechanismfor each intraplate earthquake in order scale [Richter, 1958]. JMA IV is approximately to establish the resulting per•manentstrain equivalent to MMVI [Trifunac and Brady, 1975]. deformation. The moment tensors (M) of virtually JMA intensity maps are available for all but one all large earthquakes for the last 100 years have of the earthquakes in Table 2 [Usami, 1975; been determined with detailed study of instrumen- Kayano and Sato, 1974; Murai, 1977; Murai et al., tal, geodetic, and geologic data (Table 2). We 1978]. For each of these events, the areal dis- use information obtained from the areal distribu- tribution of JMA IV, V, and VI, as approximated tion of seismic intensity, focal mechanismsof by a circle around the epicentral area (Figure recent earthquakes, and observation of Quaternary 7), is compared to the event's Mo . Figure 8a geologic patterns to estimate the momenttensor showsa linear relationship between log (Mo) and of the remaining (historical) large earthquakes log (Area) of intensity IV over 4 orders of that occurred during the last 400 years. magnitude in Mo . All but two of the points plot Previous investigations have demonstrated a within a factor of 3.5 of the value of moment Wesnousky et al.' Deformation of an Island Arc 6839

>-•oo.o- t- EG Log (Mo)= 17,0 +l.3Mkw IO0- AREAOF INTENSITY •Z"• oz •- I.m.I MOMENT IL t_ o• •o • I0.0-

• 18 _ v•, I'+ 17 I0t"• • 4 ß dt/,-,, o •½6• LEGEND

0 .... sw

'•' 6e• • ....•zu ! ! i m 7:0 ' ' ½., .... 81.01 / •2o MAGNITUDE (From KAWASUMI) MOMENT(1026 Dyne Cm) Fig. 9. Magnitudes [from Kawasumi, 1951] versus seismic moment of historical earthquakes in Japan (solid circles). The empirical relations between AREAOF INTENSITY •--•' VS areal distribution of intensity and moment, MOMENT established in Figure 8, were used to determine the moments of the historical earthquakes. Values of moment were determined from areal dis-

I.O- tribution of intensity IV except where noted. The open circles are reestimates of magnitude by Usami [1975], based on intensity data not available to Kawasumi [1951].

predicted by the relation plotted in Figure $a. A similar relation results for intensities V and •6 ß ....NE VI, although not as many data are available and •/ •4•// LEGEND[].... sw the scatter is greater (Figures 8b and 8c). The empirical relation in Figure 8a is used to determine the Mo of historical earthquakes with published isoseismal maps by obtaining the areal xo••"20i []....I ZU .o, ., ,'.o ,'o distribution of intensity IV. Isoseismals of JMA MOMENT( 1026Dyne Cm) intensity V and VI (Figures 8b and 8c) are used only when data from JMA IV are not avaiable. Sufficient data are not available to formulate I0- AREAOF INTENSITYV[ '•' VS isoseismal maps for all of the historical earth- MOMENT quakes. Magnitudes of these events, however, have been estimated by Kawasumi [1951] from what seismic intensity data are available. He estimated magnitudes based on his estimate of the Log(Areo) = 2.10 Log(Mo)- 43.0 seismic intensity at a distance of 100 km from the epicenter. Since the areal distribution of intensity is seen to be a function of seismic LEGEND moment, it is reasonable to assume that the •,--- NE values of magnitude provided by Kawasumi [1951] A--_•zu should also be proportional to Mo . The magnitudes of large historical earthquakes given by .01- A_sw >• Kawasumi [1951] are compared to their seismic moments in Figure 9, when the latter may be .l ,'.o ,'o determined with the relations in Figure 8. A MOMENT(1026 Dyne cm) simple empirical relation between the two is shown and is used to estimate the moment of large Fig. 8. Plot of seismicmoment (1026 dyne cm) historical earthquakesfor whichisoseismal maps versus areal distribution (1014 cm2) of JMA in- are not available. tensity IV (Figure 8a), JMAintensity V (Figure Recentseismicity data and Quaternarygeologic 8b), and JMAintensity VI (Figure 8c) for intra- data maybe used to estimate the focal mechanisms plate earthquakes in Japan. Open, solid, and of the historical earthquakesfor which no in- half-filled symbolsrepresent earthquakeslocated strumentaldata are available. The stress system in the central, northeast, and Izu regions of reflected by recent focal mechanisms(Figure 3) intraplate Honshu,respectively. Numbersadja- and the mapof active faults of Japan(Figure 4) cent to data points correspondto Table 2. has been operative since the beginning of the Quaternary [Research Group for Quaternary Tectonic Map, 1973]. Hence, we assume that 6840 Wesnousky et al.' Deformation of an Island Arc

earthquakes during the last 400 years in north- of secular rates of moment release and crustal east Japan were of reverse type, similar to the shortening. Niigata earthquake of 1964, and those in central Crustal shortening in northeast Japan is and western Japan were of a strike slip nature, maximumin a direction about 100ø east of north similar to the Nobi earthquake of 1891. A (Figure 11a). •{o averaged over the last 400 general focal mechanismneed not be assumedin years, is 3.2 x 1025 dyne cm/yr (Table 4). The Shikoku because there is no historical record of maximum rate of horizontal compressive strain and large intraplate earthquakes during the last 400 equivalent crustal shortening computed with the years(see Figure 5). Similarly,the onlyknown 400-yearaverage of •o are 2.7 x 10-8/yrand large events in the Izu Peninsula occurred 5.3 mm/yr, respectively (Figure 11a). Similar to recently and their source mechanismshave been central Japan, the 50-year running average of •o studied in detail (Table 2). The moment(Mo) , shows large excursions from the 400-year average date, and epicentral data of all historical of •o (Figure 6b). In contrast to central Japan, intraplate earthquakes of M ) 6.9 are presented the 200-year running average of •{o is not flat in in Table 3. character but rather shows greater variation, ranging from 0.7 to 5 x 1025 dyne cm/yr as more Rates of Moment Release and Crustal Shortening recent time periods are sampled. Hence, 200 From Seismicity Data years appears to be too short a time period to estimate accurately secular No in northeast We treat the Izu Peninsula, and the northeast, Japan. The Niigata earthquake of 1964 (Mo = central, and western regions of Honshuseparately 3.2 x 1027 dyneca) accountsfor about 25%of the because of their distinct tectonic styles. The total moment release in northeast Japan during areal dimensions of the deforming volume in each the last 400 years. Though a 200-year period intraplate region (Figure 10a) are chosen as appears insufficient, hypothetical addition or follows' Northeast Honshu' 575 km north-south deletion of a Niigata-type earthquake to the and 190 km east-west; central Japan' 225 km historical record alters the 400-year average of north-south and 255 km east-west; western Honshu' •{o by only 25%. This lends confidence to the 125 km north-south and 340 km east-west; Izu hypothesis that the 400-year average of •o is Peninsula' 75 km north-south and 35 km east- representative of long-term rates. west. The thickness of the seismogenic volume is Only two intraplate earthquakes are histori- placed at 15 km. For each region, the moment cally recorded in western Honshu (Figure 5). The tensor (M) of all large earthquakes (Tables 2 and net moment release of these two earthquakes when 3) are rotated to a north-easts-verticalreference averaged over the last 400 years indicates •o frame and summedto obtain ZM. The strain rate• equal to 3.4 x 1024 dyne cm/yr. Rates of (•) of each region is found by substituting ZM horizontal strain and crustal shortening calcu- into equation (3). We consider only strains in lated from these data are maximumin an easterly the horizontal andvertical directions. An esti- direction and equal 0.6 x 10-8/yr and 2.0 mm/yr, mate of the rate of permanentcrustal shortening respectively. Estimates of secular •{o in a re- (or extension) is also computed by multiplying gion of such low seismic activity are susceptible rates of the maximum horizontal strain times the to large error. Hence, the above rates of strain distance across the deforming area in the respec- and strain shortening cannot with any confidence tive directions of maximum strain. Computations be considered representative of secular deforma- are performed with both the recent 100-year and tion rates. complete 400-year records of seismicity. The Shikoku has been aseismic during historic time results are summarized in Table 4 and discussed except for a group of earthquakes located in the below. Rates of horizontal deformation obtained vicinity of the Bungo Channel (Figure 5). Recent from the 400-year history of seismicity are also smaller earthquakes in this region are commonly shownin Figure 11a. located relatively deep (40-60 km), show normal The principal direction of shortening in cen- faulting, and have been interpreted to reflect tral Japan is about 95ø east of north. •{o corn- complex interplate processes associated with the puted from the 400-year record of seismicity is leading edge of the subducting Philippine Sea 2.1 x 1025 dyne cm/yr. Rates of maximumhori- plate [Shiono and Mikumo, 1975]. The large his- zontal compressive strain and crustal shortening, torical earthquakes located in the Bungo Channel calculated from the 400-year data set are region are not asssociated with any evidence of 2.6 x 10-8/yr and 6.7 mm/yr, respectively. The surface faulting andvery likely weresituated at release of seismic moment occurs as discrete depth. Hence, there is no evidence of intraplate units during earthquakes. If the average values type seismicity in Shikoku during the last 400 determined above are to be considered representa- years. t ive of secular rates, it is important that the Three large earthquakes have occurred in the time period sampled be of sufficient length. In Izu Peninsula this century' The North Izu event Figure 6a the historical record of large earth- (M -- 7.0) of 1930, the Izu-Hanto-Oki shock (M = quakes in central Japan is plotted beneath 50- 6.9) of 1974, and the Izu-Oshima (M = 6.8) earth- and 200-year running averages of •{o. The large quake of 1978. The only large historical earth- variation in values of No in the 50-year curve quake to be confidently located on the Izu clearly shows that a 50-year sample does not Peninsula is believed to have occurred on the provide an accurate estimate of the long-term Tanna fault in 841 A.D. [Usami, 1966; Somerville, average of No . The 200-year curve, in contrast, 1978]. The Tanna fault exhibited surface is much flatter and values of •{o are generally breakage during the 1930 North Izu earthquake. limited between1 to 2 x 1025 dyne cm/yr. This This suggests a 1000-year recurrence time for suggests that the 400-year record of seismicity earthquakes of about M • 7 on the Tanna fault provides a sufficient time window for assessment [Somerville, 1978]. Data from recent excavations Wesnousky et al.' Deformation of an Island Arc 6841

TABLE 3. Intraplate Earthquakes, 1580 to Present, of Magnitude Greater Than or Equal to 6.9 for Which Instrumentally Determined Source Paramerers are Not Available.

Epicentral Data

Latitude Longitude Moment%, No.t Date øN øE Magnitude 1026dyne cm

NortheastJapan- 1581-1880 4* 85 Sept.27, 1611 37.5 139.7 6.9 1.4, 102134 Oct.Jan. 18, 16441683 39.436.75 140.1139.65 6.97.3 4118 , .9* 144172 Aug.June 19,1, 17291694 40.237.6 140.2137.6 6.97.0 1 4* .4* 182195 MayMarch14, 8, 1746 1766 49.3(37) 140.6(140) 6.9 1 4* 216 Feb. 8, 1793 40.85 139.95 6.9 1.4' 223 July 10, 1804 39.05 139.95 7.1 2 6* 235 Dec. 18, 1828 37.6 138.9 6.9 3.6 (VI) 239 Dec. 7, 1833 38.9 139.15 7.4 32.0 248 May 8, 1847 36.7 138.2 7.4 6.5 (VI) sum -- 59.8 Northeast Japan: 1881-1980 311 Oct. 22, 1894 38.9 139.9 7.0 3.3 (IV) 338 May 12, 1900 38.7 141.1 7.0 6.9 (IV) 398 March 15, 1914 39.5 140.4 7.1 4.1 (IV) 494 May 1, 1939 40.0 139.8 7.0 1.3 (IV) sum-- 15.6 Central Japan: 1581-1880 78 Jan. 18, 1586 36.0 136.8 8.1 19.0 (VI) 82 Sept. 5, 1596 34.85 135.75 7.25 6.3 (VI) 115 June 16, 1662 35.25 136.0 7.8 15.0 (VI) 229 Aug. 2, 1819 36.2 136.3 7.4 1.3 (V) 254 July 9, 1854 34.75 136.0 7.6 8.5 (VI) 269 April 9, 1858 36.2 136.3 6.9 7.6 (VI) sum = 57.7

West Honshu: 1581-1880 81 ** Sept 4, 1596 33 3 1317 6 9 1 4* 225140 ** Jan...... 4, 1686 34.1 132.3 7.0 8.7(V) 259** AprDec.21, 26, 1812 1854 33.333.4 132.5132.1 6.97.0 1 4*9* 266 ** Oct. 12, 1857 33.9 132.7 6.9 1.4' 282 March 14, 1872 34.9 132.0 7.4 10.0 (VI) sum -- 24.8 sum (minus Bungo Channel events) -- 10.0 West Honshu: 1881-1980 358 June 2, 1905 34.2 132.3 6.9 5.3 (IV) sum = 5.3 sum (minus Bungo Channel events) = 0.0

*The relation betweenmagnitude (Mkw) and moment (Mo) in Figure9 wasused to •etermine the seismic moment. 'Number of each event corresponds to that listed in Usami's [1975] catalogue. The November 25, 1614 earthquake (M--7.7, lat--37.5N, long--138.1E) listed in Usami [1975] is not included because Hagiwara et al. [1980] re- examined the historical data and found that the event is probably much smaller wit• M=6.3. TSeismic moment is determined from the areal distribution of JMA Intensity with the relations shown in Figure 8 except for those values denoted by a single asterisk. The scale of intensity used is given in parentheses. We assume on the basis of similar intensity patterns that Event No. 239 produced the same seismic moment as the June 16, 1964 Niigata earthquake [see Hatori and,K,atayama, 1977]. Events located in or near the Bungo Channel. 6842 Wesnousky et al.' Deformation of an Island Arc

132' 134'

c.o. NE.

ZU A

Fig. t0. (a) Index map of regions discussed in this study: northeast Japan (NE), central Japan (C J), western Honshu (WH), Shikoku (SH), Izu Peninsula and adjacent environs (IZU). (b) Number is index location of maps referred to in Table 5 and the active faults book [Research Group for Active Faults of Japan, 1980a, b]. across the Tanna fault are in accord with this to a horizontal compressive strain rate of estimate [Matsudaet al., 1981]. If we assumea 1.4 x 10-$/yr. 1000-year recurrence time for each of the three earthquakes that occurred on the peninsula during Deformation Rates From Quaternary Fault Data this century, calculations produce an average momentrelease rate of 4.4 x 1023 dyne cm/yr. Data and Assumptions Converting• to rates of strain indicates that the Izu Peninsula is shortening in a northwest- The recent publication of 'Active Faults in erly direction at rates of 1.3 mm/yr, equivalent Japan' Sheet Maps and Inventories' [Research Wesnousky et al.' Deformation of an Island Arc 6843

TABLE 4. Summaryof Intraplate Deformation Rates Determined With Seismologic Data

Short ening Ext ens ion

Moment, Strain Strain Time 1025/yr Rate, Rate, Azimuth, Rate, Rate, Azimuth, Period Mechanism dynecm 10-8/yr mm/yr deg 10-8/yr mm/yr deg

Northeast Japan

1881-1980 Niigata (1964) 6.6 5.7 11.1 100 6.0 0.1 vertical 1581-1980 Niigata (1964) 3.2 2.7 5.3 100 2.9 0.4 vertical

Central Japan

1881-1980 Nobi 'a' (1891) 2.5 3.4 8.7 100 3.1 7.1 10 1581-1880 Nobi 'a' (1891) 2.1 2.6 6.7 95 2.6 5.8 5

West Honshu

1881-1980 Nobi 'a' (1891) 1.0 0.9 3.5 125 0.9 1.3 35 1581-1880 Nobi 'a' (1891) 0.34 0.6 2.0 105 0.6 0.1 15

Izu Peninsula

981-1980 none 0.044 1.4 1.3 150 1.4 0.6 60

*Thefocal mechanism assumed for historicalearthquakes. See Table 2.

Group for Active Faults of Japan, 1980a, b] 0ø (left lateral) or 180ø (right lateral). The (hereinafter referred to as the 'active faults parameter of fault width is assigned as follows' book') provides for the first time detailed data (1) faults of length )15 km are given a fault of uniform quality describing all active width of 15 kin/sin (8) (they break through the Quaternary faults in Japan. Faults listed and entire seismogenic thickness), (2) faults of mapped at 1:200,000 (Figure 10b) in the active length (15 km with total displacements (5 m are faults book were recognized primarily by inter- assumed to have equidimensional fault areas, pretation of aerial photographs supplemented by (3) faults of length (15 km with displacements of geological maps. Major faults were checked in >5 m are assigned a fault width of 15 km/sin (8) the field. Each map is supplemented by a table (they break through the entire seismogenic detailing, as available, the strike, dip, length, thickness). Earthquakes of length greater than total displacement, and average rate of slip of 15 kin, particularly those with slip greater than each fault in that area. Each fault is also 2-2.5 m, generally have a fault width approaching classified in terms of its 'certainty'' (I) Cer- 15 km (the seismogenic thickness) or greater rain beyond doubt that the fault was active (Table 2). Since all faults of length •15 km in during the Quaternary, (II) though not definitely the active faults book have displacements >5 m, certain of the age of the offset features, it is assumption 1 is reasonable. Assumption 2 is possible to infer the sense of displacement and justified by looking at the source dimensions of the fault is generally considered to have been smaller earthquakes (length (15 km) (Table 2). active during the Quaternary (T. Matsuda, per- The width of these smaller earthquake ruptures is sonal communication, 1981), (III) the fault is a usually about the same as the length. Hence, a mere lineament only suspected to be active during conservative manner in which to estimate the the Quaternary. source dimensions of those geologic faults, with The data in the active faults book provides length <15 km and u (5 m, •õ to assume(assump- most of the information necessary to describe. the tion 2) that they are equidimensional (width average rate of seismic moment release (•) of length). Many geologic faults of 1- to 15-km each Quaternary fault. Assumptions need only be length are listed in the active faults book with made when assigning a value for the shear modulus displacements of the order of tens of meters. (•-- 3 x 1011 dyne/cm2),the dimensionof fault Average displacementsassociated with even the width (W) and in those cases where inform•,tion on largest earthquakes in Table 2 reach to only a fault dip (8) is not available. Those faults few meters, and furthermore, these large earth- listed without information of the dip, but iden- quakes generally have fault lengths >20 km. tified as dip slip faults, are assumed to be Hence, it is not common for earthquakes with a reverse type (rake -- 90•) and to dip at 45 ø. fault length of <15 km to have displacements Similarly, strike slip faults without dips speci- >5 m. Instead it is more reasonable to assume fied are assumed to dip at 90• and have a rake of that the short fault segments with the larger 6844 0•,•, •'o"/"x,.A ^

6.7 -• m/yr) 1.3,.• (mm/yr) CRUSTAL SHORTEN ING CRUSTAL SHORTENING

26• 6• (10/yr) STRAINRATES STRAINRATES

• ' r.

'•• IO•yn••Yr •8• I0 dynecm•r MOMENT-RELEASERATE MOMENT-RELEASERATE Flg. 11a Flg. 11b •g. 11. (a) Regional (see •igu•e 10a) averageso• the •ates of mount •elease, ho•- go•tal compressire st•a•, and c•ustal shortening calculated •om the 400-yea• •eco•d o• seismicity. Suppos•io• that these values a•e •ep•ese•tat•ve o• secular •a•es dependenton the •ate o• seismicity as •ell as the qual•y o• the h•sto•c •eco•d earthquakes •n each •espect•ve •eg•o•. gach a•ea •s d•scussedsepa•atel• • the text. P•Qc•pal d•ect•o•s o• ho•igontal comp•ess•vest•a• a•d shortening a•e de•oted dashed ba•s. Rote that the values • the •gu •eg•on a•e co•uted o•ly •om seismicity •ith•n the small dashed box that e•closes the •gu •e•sula. Ro estimate •s g•ve• [o• the •gu •egio• as a •hole because p•ox•m•ty to plate boundaries •ega•ed co•de•t identification o• historical •t•aplate earthquakes. (b) Case 2 •eg•o•al (see •gu•e 10a) and g•d-b•-g•d (see •gu•e lob) averageso• the •ates o• moment•elease, ho•- go•tal comp•ess•vest•a•, aed c•ustal shortening calculated •om sl•p •ates o• active •aul•s o• land. Valid co•a•so• o• these values •th •ates obtained •om •he 400- •ea• •eco•d o• se•sm•c•t• p•ese•ed • •gu•e 1la •s co•di•o•al to the o• se•c•ty, the •eg•o•al geolo•, a•d the st•le a•d •a•e o• •ault•g • each •e- spective •egio•. See text •o• •u•the• d•scuss•o•. P•nc•pal d•ect•o•s o• ho•go•tal co•essive st•a• •o• the la•ge• •t•aplate •eg•ons a•d smalle• g•ds a•e dashed ba•s a•d th• so1•d l•nes, •espect•vel•. S•11er •ume•als •thi• the •eg•o• ce•al 3apa• g•ve average •a•es •o• •he local a•eas enclosedb• •he dashedboxes. Wesnousky et al.' Deformation of an Island Arc 684S

(>5 m) displacements listed in the active faults moment release, when cases 1 and 3 are assumed, book are the result of many earthquakes occurring do not vary by more than a factor of 2 from along a major fault of which only a small portion rates calculated assumingcase 2 (Table 5). The is presently visible at the surface. Faults with results thus appear quite stable and are not such large displacements probably cut through the greatly dependent on the estimates of slip rate entire seismogenic thickness of the crust assumed for those faults without specified slip (assumption3). Weuse these assumptionsand the rates but categorized only as degree 'A,' 'B,' or data provided in the active faults book to calcu- 'C.' late the averagerate of momentrelease (•) for each active Quaternary fault. Comparison of the Average Rates of Deformation Determined From Seismicity and Rates of Moment Release and Crustal Shortening Quaternary Fault Data Estimated from Slip Rates on Quaternary Faults A valid comparisonof values of • estimated Average rates of slip on a geologic fault may from the seismicity and geologic data sets is be estimated only when the age of geologic and strongly dependent on the quality of the geologi.c geomorphologicfeatures offset by the fault are record of offsets. In this section, values of • known. Slip on active faults in Japan is gener- calculated from the two data sets are compared ally evidenced by the displacement of low-relief and discussed with regard to the distribution of surfaces that formed by either fluvial or marine seismicity, the style and rate of faulting, and deposition and erosion during the Quaternary. the regional geology. The best estimates of sec- Tephra and organic matter in these terraces may ular •o, and the horizontal rates of strain and be dated with radiometric techniques, and hence crustal shortening are then provided for each the age of the displaced surfaces determined. region in Figure 12. The dates of fault offsets listed in the active In central Japan, the seismicity is concen- faults book are prevalently less than about trated on land (Figure 5), faulting is generally 200,000 years of age. Hence, values of slip rate strike slip (Figure 4), there is very little in the active faults book provide us directly sedimentary cover [Research Group for Quaternary with information of the average rate of deforma- Tectonic Map, 1973], and large earthquakes gener- tion in Japan during the late Quaternary. ally produce observable surface ruptures along Specific values of slip rates are given in mappable geologic faults [e.g., Matsuda, 1977]. the active faults book only when the ages of Both the seismicity and fault data indicate that displaced features have been quantitatively central Japan (grids 63-78) is being compressed determined. In many cases the precise age of in an easterly azin•th. •o estimated from displaced geologic markers is not known but can Quaternary fault data ranges from 0.6 to 1.9 x be constrainedwithin certain limits. In such 102õ dyne cm/yr (Table 5). •o (2.1 x 1025 dyne cases, the active faults book provides estimates cm/yr) computedfrom the 400-year record of seis- of the 'degree' of slip rate' degree 'A,' 1-10 micity lies just at the upper bound of the range mm/yr; degree 'B, ' 0.1-1 mm/yr; degree. 'C, ' of values constrained by the geologic data. 0.01-0.1 mm/yr. The moment release rate (•) may Similarly, seismologically determined rates of thus be defined for all faults listed in the horizontal compressive strain and crustal short- active faults book once a specific numerical ening (2.6 x 10-8/yr and6.7 mm/yr,respectively) value of slip rate is assigned to those faults are at the high end of the range of rates deter- with slip rates categorizedonly as degree'A,' minedgeologically (0.9 to 2.6 x 10-8/yrand 2.2 'B,' or 'C.' Three cases are considered here' to 6.6 mm/yr). Qualitative examination of data Case 1, faults of degree 'A ' 'B ' and 'C' slip in Figure 8 indicates that estimates of Mo for at 1 mm/yr, 0.1 mm/yr, and 0.01 mm/yr, respec- historical earthquakesare on average limited to tively; case 2, faults of degree 'A,' 'B,' and a factor of 2 to 3. Whensuch errors in the 'C' •lip at 5 mm/yr•0.5 mm/yr• and 0.05 mm/yr• computationof •o from the 400-year record of respectively' case 3, faults of degree 'A,' 'B,' seismicity are considered, the slightly larger and 'C' slip at 10 mm/yr, 1 mm/yr, and 0.1 mm/yr, rates determined seismologically should not be respect.ively. For each case, the momentrelease considered significant. We may arrive at a rate (•)of faults of certainty I and II are similar conclusion from geologic considerations. summed, and formula (3) is used to calculate the Aseismic creep has not been observed on any ac- principal directions and rates of horizontal tive faults in Japan except for relatively minor strain of each intraplate region. amounts of fault slip immediately after great The longer time base of the Quaternary fault earthquakes [Matsuda, 1977]. The geologic record data allows us to view the deformation process of fault offsets should then at best, if perfect- with greater detail than with historic seismicity ly preserved and recognized, yield estimates of data. The results of the deformation analysis deformation rates equal to those obtained from for faults of certainty I and II, for each indi- seismicity data. More likely is that fault off- vidual map (Figure 10b) and larger intraplate sets are not perfectly preserved and, hence, not regions (Figure 10a), are summarizedin Table 5. always recognized. The geologic data will thus •o for each intraplate region, whenonly faults probablyyield a minimumestimate of •o' Noting of certainty I are considered, is also for the that slip rates on Quaternary faults are gener- sake of comparison listed in Table 5. Confidence ally estimated from displacement of features limits on rate estimates for each region may be between 10,000 and 200,000 years old, the simi- obtained by comparingcase 2 results to the range larity of the values obtained from the two data of values defined by cases 1 and 3 in Table 5. sets strongly suggests that the mode and rate of Results for case 2 are pictorially presented in deformation in central Japan has been steady Figure l lb. In general, the average rates of through the late Quaternary. 6846 Wesnousky et al.: Deformation of an Island Arc

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offshore would not, of course, be recorded by active faults on land. Thus, the recent increase in the rate of deformation in northeast Japan, implied by the difference in •o obtained from the geologic and seismologic data, is probably not real but more likely reflects that the geologic

record of fault offsetsß in northeast Japan is 1.3 (mm/yr) incomplete. Hence, • in northeast Japan is best constrained by the seismologic data. CRUSTAL SHORTENING In western Honshu, • computed from Quaternary fault data(2 to 9 x 10•3 dynecm/yr) is anorder of magnitude less than •o computedfrom the seismicity data (3.4 x 1024 dyne cm/yr). In western Honshu, seismic activity is about an order of magnitude less than seen in central Honshu, a region of approximately equal area. Thus, in western Japan a proportionately greater period of time (>>400 years) as compared to central Honshu 6. (Id/yr) is probably needed to obtain an accurate estimate STRAIN RATES of secular•o from seismicity data. Hence •o computed from the Quaternary fault data presents a better constraint for estimates of secular •o. •'-'-'-5e Geologically determined values of •o (2 to 9 x 1023 dyne-cm/yr) convert to maximumrates of horizontal compressive strain and crustal shorteningof 0.4 to 1.9 x 10-9/yr and 0.1 to 0.7 mm/yr, respectively, oriented about 100ø east 28. I0 dyne-cm/yr of north. In Shikoku, •o estimated from the geologic MOMENT-RELEASE RATE data is about 5 to 7 x 1024 dyne cm/yr. Virtual- Fig. 12. An integration of data in Figures 11a ly all of this maybe attributed to slip along and 11b showingthe best estimatesof the long- the mediantectonic line (MTL). TheMTL has been term averagesof the rates of seismicmoment re- aseismicfor the last 1000years [Matsuda,1969, lease, horizontalcompressive strain, andcrustal 1977] and does not showevidence of aseismic shorteningof intraplate regionsof Japan. creep [Okada,1970]. This suggeststhat enough energy is now stored along the MTL to produce an earthquakewith seismicM o -- 5 to 7 x 1027 dyne In northeast Japan, both the geologic and cm if strain buildup in this region has proceeded seismicity data indicate that the average trend continuously through historic time [Shimazaki, of maximum horizontal shortening is about due 1976]. This is greater than the Mo of the great east. The rate of moment release, and hence 1891 Nobi earthquake. Figure 11b shows that the deformation, estimated from the fault record is, slip and orientation of the MTL are consistent however, 4 to 16 times less than estimates with being caused by a compressive stress field obtained with the seismicity data. The average that trends northwest. rate of moment release calculated from the Rates of deformation in the Izu Peninsula geologic record of fault displacements is 0.2 to determined from seismicity data are median to the 0.8 x 102S dyne cm/yr in contrast to 3.2 x 102S range of estimates obtained from fault data. The dyne cm/yr estimated from the 400-year historical moment release and crustal shortening rates record of seismicity. The large difference in obtained with the fault data range from 2.1 to •o estimatedfrom seismicity and the longergeo- 11.8 x 1023 dynecm/yr and 0.3 to 2.6 mm/yr,re- logic fault record at first sight implies that spectively (Table 5), in comparison to estimates the rate o• faulting in northeast Japan has of 4.4 x 1023 dyne cm/yr and 1.3 mm/yr assessed recently increased dramatically, but closer from the seismicity data (Figure 11a). The trend examination of the geologic data and distribution of maximum shortening indicated by each data set of seismicity indicates that this is probably not is also similar, trending about 30ø west of the case. north. The geologic data further show that the Northeast Japan, in contrast to central Japan, region immediately northward of the Izu Peninsula is largely overlain by Neogene and Pleistocene reflects similar rates of horizontal shortening sediments [Research Group for Quaternary Tectonic (0.9 to 1.7 mm/yr) in a northerly direction (Fig- Map, 1973] and is characterized by reverse-type ure 11b). The two data sets are thus consistent faulting. Many faults in northeast Japan may with the hypothesis that the rate and style of possibly remain unrecognized because they do not deformation in the Izu Peninsula has been completely break through the sedimentary cover. relatively constant during the late Quaternary. Furthermore, less than 50% of the moment release in northeast Japan during the last 400 years oc- Discussion curred onshore. Thus, a major amount of moment release and crustal shortening in northeast Japan The axes of maximum crustal shortening takes place offshore along the Japan Sea coast. obtained from recent triangulation measurements This is similarly reflected in the distribution [e.g., Nakane, 1973] showa pattern like that ob- of active faults (Figure 4) and folds [Huzita, tained in this study (Figure 11b). Geodetically 1980; Ishiwada and Ogawa, 1976]. Deformation measured rates of horizontal compressive strain Wesnousky et al.' Deformation of an Island Arc 6849

obtained by Nakane [1973], however, range up to analysis of the seismicity data in northeast 1 to 3 x 10-?/yr as comparedto 2 to 3 x Japanindicates that observedhorizontal rates of 10-8/yr determinedin this paper. Similar ob- shorteningin northeastJapan should be accore- servations have also been presented by Kaizuka panied by about 0.4 mm/yr thickening of the upper and Imaizumi [1981]. We have measured only the 15 km of the crust. Assuming that this is about permanent component of deformation as accommodat- 70% isostatically compensated, this thickening ed by slip on intraplate faults. The difference would result in only 0.1 to 0.2 mm/yr of observ- between our values and Nakane's [1973] m•st be able uplift. In central Japan, virtually no attributed to other strain processes. Folding uplift may be attributed to horizontal shortening may account for part of the difference. Elastic of the seismogenic layer because faulting is compressive strain increase during the inter- predominantly strike slip. Quaternary (last 1 to seismic periods of great interplate earthquakes 3 m.y.) uplift in northeast and central Japan is [e.g., Scholz and Kato, 1978; Shimazaki, 1974a, on average greater than 500 m. These observa- b; Yamashina et al., 1978] may also in large part tions thus indicate that processes responsible produce the discrepancy in values measured by the for most of the Quaternary uplift of the islands two techniques. This component of strain, must be seated beneath the upper 15 km seismogen- however, may not be considered permanent, since ic layer. Strain thickening of the lithosphere much of it may be released coseismically during beneath the seismogenic layer may play a signifi- the great interplate earthquakes [e.g., Scholz cant role in the uplift process. and Karo, 1978]. Strain release in Shikoku is predominantly Conclusion taken up as slip along one major fault, the median tectonic line (MTL). The easterly strike The seismicity and geologic data in Japan of the MTL is consistent with a model in which allow comparison of rates of seismic moment oblique plate subduction of the Philippine Sea release and, hence, crustal shortening over time plate in a northwest direction is partially taken periods ranging from hundreds to greater than up by right-lateral motion along the MTL (Figure about 100,000 years. The rate of seismic moment 3) [Fitch, 1972; Shimazaki, 1976]. Deformation release in intraplate Japan during the last 400 of Honshu,in contrast, is distributed as slip on years has averaged 5 to 6 x 1025 dyne cm/yr. In a system of many reverse and strike slip faults. central Japan, where seismic activity is predom- The relative plate velocity vector between the inantly located on land, the rate of seismic Pacific and Eurasia, along the Japan trench, is moment release calculated from recent seismicity about 9.7 cm/yr in an easterly azimuth (Figure data is in good agreement with the rate obtained 3). Maximum horizontal shortening in northeast from data describing the average slip rates of Honshu, as accommodated by slip on intraplate Quaternary faults. we interpret this result to faults, is also directed in an easterly direction suggest that the mode and rate of deformation we and averages about 5 mm/yr. This suggests that see in central Japan has been steady for at least about 5% of the relative plate velocity along the the last 10,000 to 200,000 years. The seismicity Japan trench is taken up as a relatively homo- and Quaternary fault data in the northeastern, geneous compression of northeast Japan. western, and Izu Peninsula regions of intraplate Interpretation of the intraplate stress field Japan are also consistent with this hypothesis. as being solely influenced by the transmission of Of direct consequence to the seismologist con- compressive stress across the plate boundaries cerned with assessing seismic risk, the results is, however, complicated by the observation that obtained in this study further suggest that the the central and western regions of Honshu are release of seismic moment in intraplate Honshu is also characterized by an eastward compression relatively free from secular variation when rather than a northwestward compression, as might averaged over time periods of several hundreds of be expected from the proximity of the two regions years. to the Nankai trough. One must thus appeal to other factors in attempting to explain the east Acknowledgments. The author (SGW) benefited trending stress field of central and western greatly from conversations with several Japanese Japan. The northward impingement of the Izu scientists when they visited the United States Peninsula into Honshu [Matsuda, 1978; Somerville, for the 1980 Ewing Symposium on Earthquake 1978] may affect the stress field in central Prediction. In particular, T. Matsuda provided Honshu similar to the way stresses in Asia are valuable discussions regarding the Quaternary influenced by the continuing northward encroach- geology of Japan; K. Ishibashi pointed out some ment of India [Tapponnier and Molnar, 1977]. new developments concerning the historical record This is, however, quite speculative and a satis- of seismicity in Japan; and K. Yamashina kindly fying explanation of the stress field in central provided a copy of his doctoral thesis. I (SGW) and western Japan remains a problem. would also like to thank Tom Hanks for suggesting The major portion of the Japanese Islands was that we use intensity data for estimating seismic uplifted during the Quaternary period [Research moments of historical earthquakes and Larry Group for Quaternary Tectonic Map, 1973]. Simi- Burdick, Art Frankel, Teruyuki Kato, Bill Menke, larly, the horizontal compressional tectonics of and Richard Quittmeyer for their constructive the Japanese Islands commenced at the onset of criticism. Edith Edsall provided assistance with the Quaternary period (1-3 m.y. B.P.) [Sugimura, seisinology computing facilities. Word processing 1967]. This suggests that the two processes are is by Erika Free. This paper was critically re- physically related. Northeast Japan is charac- viewed by Paul G. Richards and Terry Engelder. terized by reverse-type faulting. This geometry This research was supported by National Science of faulting constrains horizontal shortening to Foundation grant CME-80-22096. Lamont-Doherty be accompanied by vertical thickening. Our Geological Observatory contribution no. 3350. 6850 Wesnousky et al.: Deformation of an Island Arc

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