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A triple-planed structure of seismicity and earthquake mechanisms at the subduction zone off Miyagi Prefecture, northern Honshu, Japan
Earth and Planetary Science Letters, 55 (1981) 25-36 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
25
[5]
A triple-planed structure of seismicity and earthquake mechanisms at the subduction zone off Miyagi Prefecture, northern Honshu, Japan Tetsuzo Seno * and Bubpha Pongsawat ** International Institute of Seismology and Earthquake Engineerin~ Building Research Institute, Ministry of Construction, Tsukuba, Ibaraki Prefecture, 305 (Japan) Received November 4, 1980 Revised version accepted March 10, 1981
A detailed cross-section of seismicity combined with types of earthquake mechanisms was constructed for the subduction zone off Miyagi Prefecture, northern Honshu, Japan, where a large Ms = 7.5 earthquake occurred on June 12, 1978. Nodal-plane solutions were determined for fourteen earthquakes with body wave magnitudes I> 5.4 using data recorded at WWSSN and Japanese stations. For smaller-magnitude events, the types of focal mechanism were analysed using P-wave first-motion data recorded at regional stations. The results obtained are: (1) For the area 150-200 km landward from the trench axis, the cross-section suggests a triple-planed seismic zone; that is, the zone of thrusting overlaps the so-called double-planed seismic zone. The upper and lower planes of the double-planed seismic zone are characterized by down-dip compression and down-dip tension, respectively. (2) Thrusting terminates at a depth of 60 km, where the aseismic front in the area is located, and it merges into the zone of down-dip compression in the deeper part, in a down-dip direction. In contrast, the depth of the zone of down-dip compression appears to shift by 10 km at the aseismic front. (3) The extension of the zone of down-dip compression seaward beyond the aseismic front and the shift of the depth of the zone may be caused by loading within the descending slab through a mechanical coupling between the plates at the focal area of the 1978 earthquake prior to its occurrence.
1. Introduction T h e spatial d i s t r i b u t i o n of earthquakes b e n e a t h t r e n c h - i s l a n d arc systems provides some basic i n f o r m a t i o n o n the k i n e m a t i c a n d d y n a m i c processes occurring there. Studies of the geometry of the W a d a t i - B e n i o f f zone, c o m b i n e d with focalm e c h a n i s m solutions, c o n t r i b u t e to o u r u n d e r s t a n d i n g of the state of the stress a n d the rheological properties w i t h i n d e s c e n d i n g slabs b e n e a t h isl a n d arcs [1-8]. N o r t h e r n H o n s h u , Japan, is a * Present address: Department of Geophysics, Stanford University, Stanford, CA 94305, U.S.A. ** Present address: Meteorological Department, Bangkok 11, Thailand.
u n i q u e island arc u n d e r which a d o u b l e - p l a n e d seismic zone has b e e n u n a m b i g u o u s l y d e m o n strated n o t only b y the d e t e r m i n a t i o n of hypocenters using a large, local n e t w o r k over T o h o k u [5,9] b u t also b y the selection from p u b l i s h e d d e t e r m i n a t i o n s of events with well c o n s t r a i n e d hyp o c e n t r a l p a r a m e t e r s [6,10]. I n the present study, we c o n s t r u c t a detailed cross-section of seismicity a n d e a r t h q u a k e m e c h a n i s m s along the s u b d u c t i o n zone off a n d b e n e a t h Miyagi Prefecture, situated i n the southeastern part of n o r t h e r n H o n s h u (see Fig. 1). T h e focal m e c h a n i s m s of earthquakes along the seismic zone u n d e r n o r t h e r n H o n s h u have b e e n studied o n the basis of composite focal mechan i s m s for small a n d m i c r o - e a r t h q u a k e s [5,9,11];
"0012-82IX/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing company
26
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)
Miyagi Pref
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.
the purpose because they lack resolution among the different types of mechanism which occur close together in space. On the other hand, the number of large events which can produce reliable solutions is so limited [6,13] that the authors could not describe every portion of the seismic zone by characteristic types of mechanism. In this study, we use smaller-magnitude (mb~>4) events and classify their types of mechanism based on the P-wave first motions recorded at regional stations in northern Honshu. Another concern of this study is to produce basic data in order to examine any changes of seismicity and focal mechanism that m a y have occurred in connection with the Miyagi-Oki ( M s = 7.5) earthquake of June 12, 1978. The earthquake ruptured the thrust zone down to a depth of 50-60 km, with dimensions of approximately 30 × 80 km 2 near the coast of Miyagi prefecture [14,15] The rupture zone of this earth-quake is
,
,, 1964 - 1975
1360E
140 °
144 °
Fig. 1. Map of northeastern Japan. Miyagi Prefecture is indicated.
focal mechanisms for larger ( m b 1> 6) events have been derived from data recorded at the World W i d e S t a n d a r d i z e d Seismograph N e t w o r k (WWSSN) [1,6[. The upper and lower planes of the double seismic zone are characterized by down-dip compression and down-dip tension, respectively [5-7,9,11]. Yoshii [6] presented a schematic representation of the type of focal mechanism beneath northern Honshu, in which he proposed that an aseismic front which is the seaward edge of a low-Q and low-V zone in the mantle between the continental plate and the descending slab beneath northern Honshu [ 12], may mark the position where the predominant types of mechanism changes from thrusting to down-dip compression. The present study focusses attention on the transition zone of earthquake mechanisms near the aseismic front. Studies based on the composite focal mechanism solution are not appropriate for
'
'
Trench Axis
,i;
39 °
3S'
37°N
141'
142°
1/.3'
144"
Fig. 2. Epicentra]distributionof earthquakesoff MJyagiPrefectureduring the period 1964-1975. Selectionof the events is based on data file EQ-l [13]; see the text for details. Only the events landward of the dashed line are used for the analysis of mechanism type. The rupture zone of the 1978 Miyagi-Oki earthquake [15] is indicated by the chain line.
27
indicated by the chain line in Fig. 2. It seems likely that the portion of the descending slab near the rupture zone of the Miyagi-Oki earthquake had been stressed through a mechanical coupling between the plates and it may be reflected in seismicity and earthquake mechanisms prior to the occurrence of this earthquake. The seismicity and earthquake mechanisms connected with the Miyagi-Oki earthquake will be briefly discussed in the present study; the details of space-time patterns of seismicity and earthquake mechanisms during a period of about fourteen years preceding the Miyagi-Oki event will be presented in another paper [ 16].
the following condition: the focal depth is determined by p P - P time intervals, or it is 50 km or larger and its standard error is not greater than 5 km. From the list of file EQ-1, we selected the events with at least twenty observations of P arrival times. Fig. 2 shows the distribution of epicenters of the selected events. Only those located within the area indicated by the rectangle are treated in this study. The area has dimensions of 180 km along strike of, and 280 km perpendicular to the trench axis.
Fig. 3 shows earthquake loci plotted in a vertical cross-section perpendicular to the trench axis. Closed circles indicate foci with pP-depths and open circles those without pP-depths. For the offshore events, we used pP-depths corrected for the water depth, following Yoshii's [6] assumption that the pP phases reported in the bulletins are those reflected from the surface of the seawater (see Yoshii [6]). The cross-section represent a doubleplaned seismic zone under the area more than 160 km from the trench axis. Beneath the sector from 150 to 200 kin, the activity of the upper zone is diffuse and appears to be constituted by two zones of activity. In the next section, we will see that each of these zones is specified by a different type of focal mechanism.
2. Data In order to elucidate a fine structure of seismicity along the subduction zone, it is essential to select events with well-constrained hypocentral parameters [6-8,10,17]. We follow Yoshii's [6] method which mainly uses the events with focal depths controlled by pP phases. We have used earthquake data file EQ-1 compiled by Yoshii I13]. EQ-1 contains the events during the period 1964-1975 taken from the Bulletins of the International Seismological Center (ISC) which satisfy 1964
I 975 2S,Ok~
-
_
Trench Axis
200
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s,0
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U
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0°
o
50
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0
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•~ o
o
'
o
I1%
0
o
100kin
cb
0
•
0
•
pP-depth
o
S . D . ~g5 k m
Fig. 3. A cross-section of earthquake foci perpendicular to the trench axis. Closed circles represent foci determined from the p P - P time intervals and open circles those without pP-depths which are 50 k m and larger and whose standard error is no greater than 5 km.
28
3. Focal-mechanism analysis
1964 -1975( ISC ) , ~
,
MbzS.4
,
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For earthquakes of m b ----5.4 and above, we determined the focal-mechanism solutions. The events analysed are listed in Table 1 and their epicenters are shown in Fig. 4. One event of August 12, 1969 (m b = 5.3) was added because it is the largest event which was located in the lower plane of the double-seismic zone. The focal mechanism solution of event E was already determined by Yoshii [13], thus we refer to his result for this event. Initial motions of P-waves and polarization angles of the initial half cycles of S-waves were read from film chips of the long-period records at WWSSN stations. When the long-period data were not sufficient, first motions of short-period P-waves were used as supplements. P-wave data recorded at the stations of the Japan Meteorological Agency (JMA) and reported in the bulletins of the ISC, "were added to the WWSSN data. Focal mechanism diagrams obtained are shown in Appendix 1. The parameters of the solutions are listed in Table 1.
It
Trench
..?
Axis
t
39 =
3 8°
I
:'i• M
r
/ / I
37~1
141OE
i
1/.,2 °
/
143 °
I
14/,°
Fig. 4. Epicentral distribution of earthquakes of m b = 5 . 4 and above off Miyagi prefecture. The order of occurrence of the events is denoted alphabetically except for event O.
Out of the fifteen earthquakes in Table 1, twelve events have shown thrust-type mechanisms, two events (C and O) normal faulting, and one event (G) strike-slip faulting. Events C and G are located
TABLE 1
Solutions of focal mechanism for earthquakes off Miyagi prefecture during the period 1964-1975 Solution
Date
Lat.
Long.
(°N)
(°E)
H * (km)
mb
A
(o) A B C D E ** F G H I J K L M N O
Feb. 16, 1965 Jan. 17. 1967 Apr. 21, 1968 M a y 01, 1968 Jul. 05, 1968 Aug. 16, 1968 Mar. 16, 1969 Sep. 14, 1970 Mar. 25, 1971 Apr. 04, 1971 Jul. 04, 1972 Nov. 19, 1973 May 05, 1974 Apr. 08, 1975 Aug. 12, 1967
38.97 38.33 38.68 38.65 38.54 38.57 38.57 38.77 38.49 38.41 38.55 38.99 37.78 37.75 38.39
142.01 142.20 142.99 143.22 142.14 143.39 142.83 142.27 142.15 142.18 142.08 141.93 141.77 141.75 142.02
55 40 41 16 44 13 37 43 39 40 40 49 42 43 77
5.4 5.9 5.4 5.4 6.0 5.4 5.5 5.6 5.5 5.8 5.4 6.1 5.7 5.7 5.3
120 117 111 120 130 109 58 118 103 122 122 115 116 108 100
B
P
T
8
q~
8
AZ
PL
AZ
(o)
(o)
70 76 06 72 70 76 60 78 74 74 80 80 70 70 33
285 285 291 285 320 285 159 285 285 285 285 265 285 285 296
21 14 84 18 20 14 73 12 16 17 10 12 20 20 58
115 115 111 116 133 107 197 116 105 118 119 110 I 13 107 138
24 30 51 26 25 30 06 32 28 28 34 34 24 24 75
308 300 291 306 305 290 292 302 283 307 305 301 303 289 290
64 58 39 62 65 59 34 56 60 60 55 55 65 64 12
(o)
(o)
(o)
(o)
(o)
PL
A and B: dip directions (q~) and dip angles (8) of the two nodal planes; P and T: azimuths (AZ) and plunges (PL) of P and T axes. * H is the corrected pP-depths after Yoshii [13]. ** Solution for event E is after Yoshii [13].
29
about 100 km landward from the trench axis and at a depth of about 40 km. The T-axes of these events trend normal to the trench axis and dip 39 ° and 34 ° , respectively, which are slightly larger than the dip of the seismic zone there. Event O, located 170 km from the trench axis and at a depth of 77 km, shows a down-dip tension type of mechanism at the lower plane of the double-seismic zone. It is clear that the data from the focal mechanisms of large (mb I> 5.4) events are not sufficient adequately to describe the mode of deformation along and especially within, the descending slab. Thus, we will now try to extract information on the types of faulting from smaller-magnitude events. For events with m b = 5.3 and less, it is usually difficult to read first motions of long-period P-waves at teleseismic stations without ambiguity. Although first motions of short-period P-waves are
C
DEC.30 1967
JUN.20 1974
often readable at teleseismic stations for larger events (mb ,-~ 5), they are not reliable. We have used P-wave first-motion data recorded at JMA stations located in northern Honshu, for which large amplitudes of first motions are to be expected. Because it is impossible to determine nodal -plane solutions only from the first motion data at regional stations, we classified types of focal mechanism based on the assumption that a given mechanism in the region belongs to one of three types, viz., thrusting, the fault strike of which is roughly parallel to the trench axis in the region, down-dip compression, and down-dip tension. The criterion of the classification is that the first-motion data are consistent with only one of these three types. For the analysis we used only those events located more than 140 km landwards from the trench axis, i.e. events located landward of the dashed line in Fig. 2. This was because the data
D
NOV. 19 1973
/
o\
\
o
\,
o/ o~
T
JUN.23 1966
SER03 1968
N
o
°2
Tt)t D
Fig. 5. Examples of the classification of focal-mechanism types. C, T and D denote the events classified into down-dip compression, down-dip tension, and thrusting types, respectively. Typical focal mechanism diagrams of the above three types are shown at the bottom right.
30 from events located remoter from the coast cover only a small solid angle of the focal sphere and thus the type of mechanism difficult to identify. Another reason was that the uncertainty of a focus with respect to the Moho or Conrad discontinuity affects the computed take-off angles more seriously for shallower events. In the computation of take-off angles, we assumed a spherically, symmetrically layered Earth and used the crustal structure revealed by seismic refraction studies off the Pacific coast of northern Honshu [6,18]. The structure used is a representative one for the offshore region. Generally, in northern Honshu, the crustal structure and Pn velocity vary from sources to stations (see Yoshii [6]); however, this does not cause much difference in the value of take-off angles from those computed in this study because their foci are mostly more than 15 km below the Moho depth. The effects of the descending lithospheric plate on the ray paths from the intermediate events in northern Honshu are not serious [10,19]. Fig. 5 shows examples of the classification. Thrusting,down-dip compression, and down-dip tension type are represented by the symbols "D", "C", and "T", respectively. Typical mechanism diagrams of the three specific types are shown in the inset at bottom right. We referred to as many mechanism diagrams belonging to the specific faulting types as possible, in addition to the focal mechanisms derived from previous studies [5,6,9,11-13,16,20] in the region. We know from these studies that, in the region more than 140 km from the trench axis, the major types of focal mechanism are only thrusting (which represents underthrusting of the Pacific plate beneath northern Honshu), down-dip compression, and downdip tension; thus we feel that the basic assumption that we have made for the classification is, at least partly, justified. Because of the scarcity of P-wave first motion data, there remain events which could not bc classified definitely into one of the three types. For these events, a further weak classification was made as follows: Type A represents events consistent with thrusting or down-dip compression and not with down-dip tension. Type B represents events consistent with down-dip compression or down-dip
tension but not with thrusting. After this, a few events remained unclassified into any group. 4. Cross-section of focal-mechanism type
Figs. 6 and 7 show the epicentral distribution and the cross-section of the types of focal mechanism obtained, respectively. Symbols "D", "C", and "T" are the same as in Fig. 5. The open and closed circles represent types A and B, respectively. Crosses are the events unclassified into any group or which were not analysed. The results for the larger events supported by the WWSSN data are represented by the larger symbols. Two events (C and G in Fig.4), 100 km from the trench axis, were classified into T type for convenience. The cross-section of types of faulting is almost consistent with previous studies of focal mechanisms of earthquakes on double-planed seismic zones under island arcs [3,5,6,8,9,11]; that is, the upper plane is characterized by down-dip compression (C) type and the lower plane by down-dip tension (T) type. The present study has given a better resolution for the upper seismic zone under
1964--1975
'/
/
:
Trench iI "~t ?; Axis
39°
o
~ c o
38"
37 N°
r
.
.oi
O l 0km iS, 141"E
' 142"
, 143"
/
, 144"
Fig. 6. Epicentral distribution of the types of focal mechanism. C, T, and D denote the events classified into down-dip compression, down-dip tension and thrusting types, respectively.
Open and closed circles denote the events of type A and B, respectively. Crossesdenote the eventswhich are not analysed or cannot be classifiedinto any type. Larger symbolsrepresent the results for larger (m b >~5.4) events.
31 1975 250km
1964
Trench
15,o
200
loo X
A.F
D
x
Axis
5bO D
X 0
* co o o o
r~ C C
C
•
C •
o•co
T 50
o
T
•
T
T
IT,
I
C
100 krn
cc C
T
T
T O
Type
A
•
Type
B
Fig. 7. A cross-section of types of focal mechanism, pP-depths and well-constrained depths are used for this section. The symbol notation is the same as in Fig. 6. A.F. denotes the aseismic front [13].
the offshore area from 150 to 200 km landwards from the trench axis. Beneath this area, the seismic activity presents a triple-planed structure; the upper seismic zone is decomposed into the layer of D-type events and that of C-type events, although the number of C-type events is limited. The distance between the zones of D and C types is 10-15 km and that between the zones of C and T types is 15-20 km. The location of the aseismic front in the region [13] is indicated by the broken line in Fig. 7. D-type events are distributed down to a depth of 60 km, that is, approximately to the location of the aseismic front. Yoshii [12] proposed that thrusting occurs only seaward of the aseismic front; the present study supports this. The zone of C-type events appears to shift upward at the aseismic front from seaward to landward. Under the area landward of the aseismic front, the distance between the zones of C- and T-type events is 25-30 km. The distribution of type A and B events is consistent with the distribution of those of D, C and T type. 5. Discussion and conclusion
The triple-planed structure seaward of the aseismic front is easily understood if a thrust zone
along the plate interface overlaps the doubleplaned seismic zone which represents fractures within the descending slab. This feature is not so prominent in Fig. 7 because of the sparsity of C-type events under the zone of D type; however, it hecomes very distinct in the study including the more recent period form 1976 through June 1978 [16], during which many C-type events occurred seaward of the aseismic front (see Appendix 2). The location of C events relative to the zone of D-type events will give an estimate for the location of C events relative to the plate interface, because D events are most likely to occur at the plate interface. Shimamura [21] estimated the locations of earthquakes within the descending slab relative to the plate interface in the Kuril arc by interpreting the secondary phase between P and S arrivals observed at a station in Hokkaido as the P wave which is reflected and converted from the S wave at the upper boundary of the descending plate. He relocated the hypocenters 10 km below the plate interface in the depth range of 20-50 km. The distance between the zones of D and C types is concordant with his results. Another interesting feature in the cross-section of Fig. 7 is the apparent shift of the zone of C events at the aseismic front. In contrast, the zone of D events appears to merge into the zone of C
32 events deeper than 70 km in a down-dip direction. It seems necessary to examine whether the apparent shift of the C zone is due to systematic errors of the hypocentral determinations. Focal depths of many of the events seaward of the aseismic front are controlled by pP readings but, in contrast, few are controlled landward of the front (see Fig. 3). Thus, this might have produced a systematic bias for the depth of C events between the landward and seaward parts of the aseismic front. However, the shift of the zone can also be seen in the cross-section which does not use pP-depths, but uses instead the usual routinely processed hypocenters (Appendix 2). On the other hand, in this case, the distinct change of Pn velocity from 8.1 to 7.5 k m / s at the aseismic front in northern Honshu [6,22,23] could be a cause of the systematic bias between hypocenters seaward and landward of the aseismic front. The systematic errors in hypocenters due to the Pn velocity change at the front are on the order of 10 km [24], which is comparable with the apparent shift of the C zone. It is not a straightforward matter to estimate the sense and extent of the systematic errors due to Pn velocity change for the locations of the events in Fig. 7, because the number and azimuthal distribution of the regional and teleseismic stations used to locate the events vary considerably. Considering the accuracy of the data used in the present study, it seems difficult to decide whether the apparent shift of the C zone is real or not. Using ScSp waves converted from ScS waves at the boundary of a sharp velocity contrast between the descending slab and the mantle above it, Hasegawa et al. [19] located the boundary exactly on the upper plane of the double-planed seismic zone under northern Honshu, in the depth range greater than 60 kin. Thus the spatial distribution of D and C events shown in Figs. 7 and A-2, if we interpret D events as located at the plate interface, is in agreement with the result by Hasegawa et al. [19], within the accuracy both of the hypocentral determinations and the estimation of the boundary of ScSp conversion. Yoshii [6] proposed that the aseismic front may mark the change in type of faulting between thrusting and down-dip compression. In the crosssection in the present study, the zone of C events
extends further seaward beyond the aseismic front by 50 km. The cross-section of seismicity, combined with focal mechanisms in the region just north of the area in this study (39-40.5°N) [6,16], does not present a triple-planed structure seawards of the aseismic front, at least during the period from 1964 through June 1978. This suggests that the occurrence of C events seaward of the aseismic front may be a regional characteristic of the area studied and possibly be a premonitory phenomenon prior to the 1978 Miyagi-Oki earthquake. Many C and D events, along with a few T events, occurred within and near the rupture zone of the Miyagi-Oki earthquake (see Fig. 6). The continental plate would have loaded this part of the descending slab, through a mechanical coupling between the plates at the rupture zone of the MiyagiOki event, before its occurrence. This may have induced the activity of C- and T-type events within the slab in the vicinity of the rupture zone of the Miyagi-Oki earthquake. The shift of the C zone at the aseismic front, that is, the shift of the neutral surface between the down-dip compressional and down-dip tensional stresses, may also have been caused by this loading which is likely to have put an additional longitudinal force on the part of the descending slab. Goto and Hamaguchi [25,26] have explained the double seismic zone beneath northern Honshu by a thermal stress within the slab caused by heating from the surrounding mantle as it descends. Recently they have shown that, when the plate interface seaward of the aseismic front is fixed, that is, when the oceanic plate is coupled with the continental plate there, the zones of down -dip compression and down-dip tension become distinct under the fixed interface and shift downward, in contrast to the case when the interface is set free [26]. Their result may support the above speculation as to the cause of the extension and shift of the C zone seaward of the aseismic front. In order to be certain of the uniqueness of the triple-planed structure in the vicinity of the rupture zone of the 1978 Miyagi-Oki earthquake, more close studies of seismicity and focal mechanisms are necessary in other regions along island arcs, and in the area of the present study for the period after the Miyagi-Oki earthquake; they should in-
33 c l u d e p r e c i s e r e l o c a t i o n s o f e a r t h q u a k e s r e l a t i v e to e a c h other, to the p l a t e i n t e r f a c e , a n d to t h e e a r t h ' s surface.
Acknowledgements W e w o u l d like to t h a n k M. O t s u k a , D i r e c t o r at the International Institute of Seismology and E a r t h q u a k e E n g i n e e r i n g ( I I S E E ) for his e n c o u r a g e m e n t a n d c r i t i c a l r e v i e w o f ttie m a n u s c r i p t . T h e m a j o r p a r t o f this s t u d y was c a r r i e d o u t d u r i n g t h e i n d i v i d u a l s t u d y b y o n e o f t h e a u t h o r s (B.P.) at t h e I I S E E . W e t h a n k T. Y o s h i i for p r o v i d i n g us w i t h the d a t a file E Q - 1 a n d for d i s c u s s i o n a n d P. S o m e r v i l l e for c r i t i c a l r e v i e w o f t h e m a n u s c r i p t . D i s c u s s i o n s w i t h S. M i y a m u r a a n d w i t h t h e staffs at t h e I I S E E , a n d a s s i s t a n c e b y M. T a n a k a in t y p i n g the m a n u s c r i p t w e r e h e l p f u l . T h i s w o r k was partly supported by the National Science Foundation under grant EAR81-08718.
References I B. Isacks and P. Molnar, Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes, Rev. Geophys. Space Phys. 9 (1971) 103-174. 2 K.F. Veith, The nature of the dual zone of seismicity in the Kuril arc, EOS 58 (1977) 1232. 3 E.R. Engdahl and C.H. Scholz, A double Benioff zone beneath the central Aleutians: an unbending of the lithosphere, Geophys. Res. Lett. 4 (1977) 473-476. 4 B.L. Isacks and M. Barazangi, Geometry of Benioff zones: Lateral segmentation and downwards bending of the subducted lithosphere, in: Island Arcs, Deep Sea Trenches and Back-Arc Basins, Am. Geophys. Union, Maurice Ewing Ser. 1 (1977) 99-114. 5 A. Hasegawa, N. Umino and A. Takagi, Double-planed structure of the deep seismic zone in the northeastern Japan arc, Tectonophysics 47 (1978) 43-58. 6 T. Yoshii, A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan, Tectonophysics 55 (1979) 349-360. 7 K. Fujita and H. Kanamori, double seismic zones and stresses of intermediate depth earthquakes, preprint (1981). 8 I.R. Samowitz and D.W. Forsyth, Double seismic zone beneath the Mariana island arc (submitted to J. Geophys. Res.). 9 N. Umino and A. Hasegawa, On the two-layered structure of deep seismic plane in northeastern Japan arc, Zisin 27 (1975) 125-139 (in Japanese).
10 M. Barazangi and B.L. Isacks, A comparison of the spatial distribution of mantle earthquakes determined from data produced by local and by teleseismic networks for the Japan and Aleutian arc, Bull. Seismol. Soc. Am. 69 (1979) 1763-1770. 11 A. Hasegawa and N. Umino, Spatial distribution of hypocenters and earthquake mechanisms under northern Japan, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1978) 36 (in Japanese). 12 T. Yoshii, Proposal of the "aseismic front", Zisin 28 (1975) 365-367 (in Japanese). 13 T. Yoshii, Compilation of geophysical data around the Japanese islands, I, Bull. Earthq. Res. Inst. 54 (1979) 75-117 (in Japanese). 14 Tohoku University, Earthquake off Miyagi prefecture, June 12, 1978, Rep. Coord. Comm. Earthq. Predict. 21 (1979) 55-59 (in Japanese). 15 T. Seno, K. Shimazaki, P. Somerville, K. Sudo and T. Eguchi, Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978, Phys. Earth Planet. Inter. 23 (1980) 39-61. 16 T. Seno, Seismicity and focal mechanisms prior to the Miyagi-Oki, Japan, earthquake of June 12, 1978, preprint (1980). 17 W.M. Chapple and D.W. Forsyth, earthquakes and bending of plates at trenches, J. Geophys. Res. 84 (1979) 6729-6749. 18 S. Asano, T. Yamada, K. Suyehiro, T. Yoshii, Y. Misawa and S. Iizuka, Crustal structure off northeastern Japan as revealed from refraction method using ocean bottom seismographs, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1979) 104 (in Japanese). 19 A. Hasegawa, N. Umino and A. Takagi, Double-planed deep seismic zone and upper mantle structure in the northeastern Japan arc, Geophys. J. R. Astron. Soc. 54 (1978) 281 - 296. 20 T. Nakajima, Spatial and sequential distribution of focal mechanisms before and after the Tokachi-Oki earthquake of May 16, 1968, Geophys. Bull. Hokkaldo Univ. 32 (1974) 25-42 (in Japanese). 21 H. Shimamura, Distribution of thickness of the Kuril plate and hypocentral distribution of earthquakes within the plate, Abstr. Annu. Meet. Seismol. Soc. Jpn. 1 (1973) 118 (in Japanese). 22 Research Group for Explosion Seismology, Regionality of the upper mantle around northeastern Japan as derived from explosion seismic observations and its seismological implications, Tectonophysics 37 (1977) 117- 130. 23 S. Suzuki, Lateral variation of P-wave velocity in the upper mantle beneath northern Japan, as derived from travel times of earthquakes, Zisin 29 (1976) 99- 116 (in Japanese). 24 S. Suzuki, Determination of earthquake hypocenters in consideration of the lateral variation of velocity in the upper mantle beneath the island arcs of Japan--on the Nemuro-Oki earthquake of 1973, Zisin 28 (1975) 181-199 (in Japanese). 25 K. Goto and H. Hamaguchi, Double-planed structure of the intermediate seismic zones--thermal stress within the
34
Appendix I
A N
Focal mechanism solutions of the earthquakes listed in Table 1 are presented as equal-area projections of the lower hemisphere of the focal sphere in Fig. A-I. Large circles indicate WWSSN long-period P-wave first-motion data and small circles WWSSN short-period and JMA data. Polarization angles of long-period S-waves are indicated by the arrows. Nodal lines are indicated by thin lines in this figure and their parameters are listed in Table 1. For thrust-type mechanisms, it i s often difficult to constrain the shallower dipping nodal planes. For such cases, we assumed that the strike of the shallower dipping plane is parallel to the trend of the trench axis in the region, i.e., ~ N I 5 ° W , following Seno et al. [15].
B
¢
( l) N
N
6
F N
H N
14
35
I
J N
K N
i . .
. o
L
M
N N
0 N
o
o
•
•
Fig. A-I. Focal mechanism solutions represented as equal-area projection of the lower hemisphere of the focal sphere, Closed and open circles represent P-wave first motion compression and dilatation, respectively. Arrows indicate S-wave polarization direction. The letters beside each subfigure denote earthquake events in Fig. 4 and Table I.
36
Appendix 2. Seno [16] analysed focal mechanisms of earthquakes from 1976 to June 11, 1978, that is, during a two and a half year period just prior to the Miyagi-Oki earthquake of June 12, 1978, in the same region and using the same method as in this study. Fig. A-2 shows the cross-section of focal mechanism
1964
-
JUN.11,1978 ;~Owrn
2q0
,s,o
types which incorporate the results for this period. The pPdepths are not used in this figure in order to examine the systematic bias of the focal depth distribution due to pP-depths (see discussion in the last section). The zone of C-type events seaward of the aseismic front becomes more distinct and the shift of the zone of C events at the aseismic front can also be seen in this figure.
! 010
A.F.
Trench Axis 0
510 D x x
o
D
o
o0 50
C C
C~
C
C
T
C
T •
e
ccc
~.
~.co,.. To
Q • •
T
100km
•
c
T
T o
Type A
•
Type
B
Fig. A-2. A cross-section of the focal-mechanism types incorporating the results during the period from 1976 through June 11, 1978 [16]. The focal depths from the routine determinations of the ISC are used instead of pP-depths. The symbol notation is the same as in Fig. 7.
lithosphere, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1978) 20 (in Japanese). 26 K. Goto and H. Hamaguchi, Distribution of thermal stress within the descending lithospheric slab, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1980) 48 (in Japanese).
25
[5]
A triple-planed structure of seismicity and earthquake mechanisms at the subduction zone off Miyagi Prefecture, northern Honshu, Japan Tetsuzo Seno * and Bubpha Pongsawat ** International Institute of Seismology and Earthquake Engineerin~ Building Research Institute, Ministry of Construction, Tsukuba, Ibaraki Prefecture, 305 (Japan) Received November 4, 1980 Revised version accepted March 10, 1981
A detailed cross-section of seismicity combined with types of earthquake mechanisms was constructed for the subduction zone off Miyagi Prefecture, northern Honshu, Japan, where a large Ms = 7.5 earthquake occurred on June 12, 1978. Nodal-plane solutions were determined for fourteen earthquakes with body wave magnitudes I> 5.4 using data recorded at WWSSN and Japanese stations. For smaller-magnitude events, the types of focal mechanism were analysed using P-wave first-motion data recorded at regional stations. The results obtained are: (1) For the area 150-200 km landward from the trench axis, the cross-section suggests a triple-planed seismic zone; that is, the zone of thrusting overlaps the so-called double-planed seismic zone. The upper and lower planes of the double-planed seismic zone are characterized by down-dip compression and down-dip tension, respectively. (2) Thrusting terminates at a depth of 60 km, where the aseismic front in the area is located, and it merges into the zone of down-dip compression in the deeper part, in a down-dip direction. In contrast, the depth of the zone of down-dip compression appears to shift by 10 km at the aseismic front. (3) The extension of the zone of down-dip compression seaward beyond the aseismic front and the shift of the depth of the zone may be caused by loading within the descending slab through a mechanical coupling between the plates at the focal area of the 1978 earthquake prior to its occurrence.
1. Introduction T h e spatial d i s t r i b u t i o n of earthquakes b e n e a t h t r e n c h - i s l a n d arc systems provides some basic i n f o r m a t i o n o n the k i n e m a t i c a n d d y n a m i c processes occurring there. Studies of the geometry of the W a d a t i - B e n i o f f zone, c o m b i n e d with focalm e c h a n i s m solutions, c o n t r i b u t e to o u r u n d e r s t a n d i n g of the state of the stress a n d the rheological properties w i t h i n d e s c e n d i n g slabs b e n e a t h isl a n d arcs [1-8]. N o r t h e r n H o n s h u , Japan, is a * Present address: Department of Geophysics, Stanford University, Stanford, CA 94305, U.S.A. ** Present address: Meteorological Department, Bangkok 11, Thailand.
u n i q u e island arc u n d e r which a d o u b l e - p l a n e d seismic zone has b e e n u n a m b i g u o u s l y d e m o n strated n o t only b y the d e t e r m i n a t i o n of hypocenters using a large, local n e t w o r k over T o h o k u [5,9] b u t also b y the selection from p u b l i s h e d d e t e r m i n a t i o n s of events with well c o n s t r a i n e d hyp o c e n t r a l p a r a m e t e r s [6,10]. I n the present study, we c o n s t r u c t a detailed cross-section of seismicity a n d e a r t h q u a k e m e c h a n i s m s along the s u b d u c t i o n zone off a n d b e n e a t h Miyagi Prefecture, situated i n the southeastern part of n o r t h e r n H o n s h u (see Fig. 1). T h e focal m e c h a n i s m s of earthquakes along the seismic zone u n d e r n o r t h e r n H o n s h u have b e e n studied o n the basis of composite focal mechan i s m s for small a n d m i c r o - e a r t h q u a k e s [5,9,11];
"0012-82IX/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing company
26
~--'~ Sea of Okhotsk
40° Japan Sea
~
Pacific Ocean "-
~:~
;: ,~~
)
Miyagi Pref
;
Ojika Pen in .,i
~
,
;
( )
.,.o
.
Trench
axis
/ ,
f
,
.
the purpose because they lack resolution among the different types of mechanism which occur close together in space. On the other hand, the number of large events which can produce reliable solutions is so limited [6,13] that the authors could not describe every portion of the seismic zone by characteristic types of mechanism. In this study, we use smaller-magnitude (mb~>4) events and classify their types of mechanism based on the P-wave first motions recorded at regional stations in northern Honshu. Another concern of this study is to produce basic data in order to examine any changes of seismicity and focal mechanism that m a y have occurred in connection with the Miyagi-Oki ( M s = 7.5) earthquake of June 12, 1978. The earthquake ruptured the thrust zone down to a depth of 50-60 km, with dimensions of approximately 30 × 80 km 2 near the coast of Miyagi prefecture [14,15] The rupture zone of this earth-quake is
,
,, 1964 - 1975
1360E
140 °
144 °
Fig. 1. Map of northeastern Japan. Miyagi Prefecture is indicated.
focal mechanisms for larger ( m b 1> 6) events have been derived from data recorded at the World W i d e S t a n d a r d i z e d Seismograph N e t w o r k (WWSSN) [1,6[. The upper and lower planes of the double seismic zone are characterized by down-dip compression and down-dip tension, respectively [5-7,9,11]. Yoshii [6] presented a schematic representation of the type of focal mechanism beneath northern Honshu, in which he proposed that an aseismic front which is the seaward edge of a low-Q and low-V zone in the mantle between the continental plate and the descending slab beneath northern Honshu [ 12], may mark the position where the predominant types of mechanism changes from thrusting to down-dip compression. The present study focusses attention on the transition zone of earthquake mechanisms near the aseismic front. Studies based on the composite focal mechanism solution are not appropriate for
'
'
Trench Axis
,i;
39 °
3S'
37°N
141'
142°
1/.3'
144"
Fig. 2. Epicentra]distributionof earthquakesoff MJyagiPrefectureduring the period 1964-1975. Selectionof the events is based on data file EQ-l [13]; see the text for details. Only the events landward of the dashed line are used for the analysis of mechanism type. The rupture zone of the 1978 Miyagi-Oki earthquake [15] is indicated by the chain line.
27
indicated by the chain line in Fig. 2. It seems likely that the portion of the descending slab near the rupture zone of the Miyagi-Oki earthquake had been stressed through a mechanical coupling between the plates and it may be reflected in seismicity and earthquake mechanisms prior to the occurrence of this earthquake. The seismicity and earthquake mechanisms connected with the Miyagi-Oki earthquake will be briefly discussed in the present study; the details of space-time patterns of seismicity and earthquake mechanisms during a period of about fourteen years preceding the Miyagi-Oki event will be presented in another paper [ 16].
the following condition: the focal depth is determined by p P - P time intervals, or it is 50 km or larger and its standard error is not greater than 5 km. From the list of file EQ-1, we selected the events with at least twenty observations of P arrival times. Fig. 2 shows the distribution of epicenters of the selected events. Only those located within the area indicated by the rectangle are treated in this study. The area has dimensions of 180 km along strike of, and 280 km perpendicular to the trench axis.
Fig. 3 shows earthquake loci plotted in a vertical cross-section perpendicular to the trench axis. Closed circles indicate foci with pP-depths and open circles those without pP-depths. For the offshore events, we used pP-depths corrected for the water depth, following Yoshii's [6] assumption that the pP phases reported in the bulletins are those reflected from the surface of the seawater (see Yoshii [6]). The cross-section represent a doubleplaned seismic zone under the area more than 160 km from the trench axis. Beneath the sector from 150 to 200 kin, the activity of the upper zone is diffuse and appears to be constituted by two zones of activity. In the next section, we will see that each of these zones is specified by a different type of focal mechanism.
2. Data In order to elucidate a fine structure of seismicity along the subduction zone, it is essential to select events with well-constrained hypocentral parameters [6-8,10,17]. We follow Yoshii's [6] method which mainly uses the events with focal depths controlled by pP phases. We have used earthquake data file EQ-1 compiled by Yoshii I13]. EQ-1 contains the events during the period 1964-1975 taken from the Bulletins of the International Seismological Center (ISC) which satisfy 1964
I 975 2S,Ok~
-
_
Trench Axis
200
is0
s,0
,oo
?
U
I •
if':~¢
0 o
0°
o
50
•
•
o
•
0
.
ioo@O 0
•~ o
o
'
o
I1%
0
o
100kin
cb
0
•
0
•
pP-depth
o
S . D . ~g5 k m
Fig. 3. A cross-section of earthquake foci perpendicular to the trench axis. Closed circles represent foci determined from the p P - P time intervals and open circles those without pP-depths which are 50 k m and larger and whose standard error is no greater than 5 km.
28
3. Focal-mechanism analysis
1964 -1975( ISC ) , ~
,
MbzS.4
,
!)~':
For earthquakes of m b ----5.4 and above, we determined the focal-mechanism solutions. The events analysed are listed in Table 1 and their epicenters are shown in Fig. 4. One event of August 12, 1969 (m b = 5.3) was added because it is the largest event which was located in the lower plane of the double-seismic zone. The focal mechanism solution of event E was already determined by Yoshii [13], thus we refer to his result for this event. Initial motions of P-waves and polarization angles of the initial half cycles of S-waves were read from film chips of the long-period records at WWSSN stations. When the long-period data were not sufficient, first motions of short-period P-waves were used as supplements. P-wave data recorded at the stations of the Japan Meteorological Agency (JMA) and reported in the bulletins of the ISC, "were added to the WWSSN data. Focal mechanism diagrams obtained are shown in Appendix 1. The parameters of the solutions are listed in Table 1.
It
Trench
..?
Axis
t
39 =
3 8°
I
:'i• M
r
/ / I
37~1
141OE
i
1/.,2 °
/
143 °
I
14/,°
Fig. 4. Epicentral distribution of earthquakes of m b = 5 . 4 and above off Miyagi prefecture. The order of occurrence of the events is denoted alphabetically except for event O.
Out of the fifteen earthquakes in Table 1, twelve events have shown thrust-type mechanisms, two events (C and O) normal faulting, and one event (G) strike-slip faulting. Events C and G are located
TABLE 1
Solutions of focal mechanism for earthquakes off Miyagi prefecture during the period 1964-1975 Solution
Date
Lat.
Long.
(°N)
(°E)
H * (km)
mb
A
(o) A B C D E ** F G H I J K L M N O
Feb. 16, 1965 Jan. 17. 1967 Apr. 21, 1968 M a y 01, 1968 Jul. 05, 1968 Aug. 16, 1968 Mar. 16, 1969 Sep. 14, 1970 Mar. 25, 1971 Apr. 04, 1971 Jul. 04, 1972 Nov. 19, 1973 May 05, 1974 Apr. 08, 1975 Aug. 12, 1967
38.97 38.33 38.68 38.65 38.54 38.57 38.57 38.77 38.49 38.41 38.55 38.99 37.78 37.75 38.39
142.01 142.20 142.99 143.22 142.14 143.39 142.83 142.27 142.15 142.18 142.08 141.93 141.77 141.75 142.02
55 40 41 16 44 13 37 43 39 40 40 49 42 43 77
5.4 5.9 5.4 5.4 6.0 5.4 5.5 5.6 5.5 5.8 5.4 6.1 5.7 5.7 5.3
120 117 111 120 130 109 58 118 103 122 122 115 116 108 100
B
P
T
8
q~
8
AZ
PL
AZ
(o)
(o)
70 76 06 72 70 76 60 78 74 74 80 80 70 70 33
285 285 291 285 320 285 159 285 285 285 285 265 285 285 296
21 14 84 18 20 14 73 12 16 17 10 12 20 20 58
115 115 111 116 133 107 197 116 105 118 119 110 I 13 107 138
24 30 51 26 25 30 06 32 28 28 34 34 24 24 75
308 300 291 306 305 290 292 302 283 307 305 301 303 289 290
64 58 39 62 65 59 34 56 60 60 55 55 65 64 12
(o)
(o)
(o)
(o)
(o)
PL
A and B: dip directions (q~) and dip angles (8) of the two nodal planes; P and T: azimuths (AZ) and plunges (PL) of P and T axes. * H is the corrected pP-depths after Yoshii [13]. ** Solution for event E is after Yoshii [13].
29
about 100 km landward from the trench axis and at a depth of about 40 km. The T-axes of these events trend normal to the trench axis and dip 39 ° and 34 ° , respectively, which are slightly larger than the dip of the seismic zone there. Event O, located 170 km from the trench axis and at a depth of 77 km, shows a down-dip tension type of mechanism at the lower plane of the double-seismic zone. It is clear that the data from the focal mechanisms of large (mb I> 5.4) events are not sufficient adequately to describe the mode of deformation along and especially within, the descending slab. Thus, we will now try to extract information on the types of faulting from smaller-magnitude events. For events with m b = 5.3 and less, it is usually difficult to read first motions of long-period P-waves at teleseismic stations without ambiguity. Although first motions of short-period P-waves are
C
DEC.30 1967
JUN.20 1974
often readable at teleseismic stations for larger events (mb ,-~ 5), they are not reliable. We have used P-wave first-motion data recorded at JMA stations located in northern Honshu, for which large amplitudes of first motions are to be expected. Because it is impossible to determine nodal -plane solutions only from the first motion data at regional stations, we classified types of focal mechanism based on the assumption that a given mechanism in the region belongs to one of three types, viz., thrusting, the fault strike of which is roughly parallel to the trench axis in the region, down-dip compression, and down-dip tension. The criterion of the classification is that the first-motion data are consistent with only one of these three types. For the analysis we used only those events located more than 140 km landwards from the trench axis, i.e. events located landward of the dashed line in Fig. 2. This was because the data
D
NOV. 19 1973
/
o\
\
o
\,
o/ o~
T
JUN.23 1966
SER03 1968
N
o
°2
Tt)t D
Fig. 5. Examples of the classification of focal-mechanism types. C, T and D denote the events classified into down-dip compression, down-dip tension, and thrusting types, respectively. Typical focal mechanism diagrams of the above three types are shown at the bottom right.
30 from events located remoter from the coast cover only a small solid angle of the focal sphere and thus the type of mechanism difficult to identify. Another reason was that the uncertainty of a focus with respect to the Moho or Conrad discontinuity affects the computed take-off angles more seriously for shallower events. In the computation of take-off angles, we assumed a spherically, symmetrically layered Earth and used the crustal structure revealed by seismic refraction studies off the Pacific coast of northern Honshu [6,18]. The structure used is a representative one for the offshore region. Generally, in northern Honshu, the crustal structure and Pn velocity vary from sources to stations (see Yoshii [6]); however, this does not cause much difference in the value of take-off angles from those computed in this study because their foci are mostly more than 15 km below the Moho depth. The effects of the descending lithospheric plate on the ray paths from the intermediate events in northern Honshu are not serious [10,19]. Fig. 5 shows examples of the classification. Thrusting,down-dip compression, and down-dip tension type are represented by the symbols "D", "C", and "T", respectively. Typical mechanism diagrams of the three specific types are shown in the inset at bottom right. We referred to as many mechanism diagrams belonging to the specific faulting types as possible, in addition to the focal mechanisms derived from previous studies [5,6,9,11-13,16,20] in the region. We know from these studies that, in the region more than 140 km from the trench axis, the major types of focal mechanism are only thrusting (which represents underthrusting of the Pacific plate beneath northern Honshu), down-dip compression, and downdip tension; thus we feel that the basic assumption that we have made for the classification is, at least partly, justified. Because of the scarcity of P-wave first motion data, there remain events which could not bc classified definitely into one of the three types. For these events, a further weak classification was made as follows: Type A represents events consistent with thrusting or down-dip compression and not with down-dip tension. Type B represents events consistent with down-dip compression or down-dip
tension but not with thrusting. After this, a few events remained unclassified into any group. 4. Cross-section of focal-mechanism type
Figs. 6 and 7 show the epicentral distribution and the cross-section of the types of focal mechanism obtained, respectively. Symbols "D", "C", and "T" are the same as in Fig. 5. The open and closed circles represent types A and B, respectively. Crosses are the events unclassified into any group or which were not analysed. The results for the larger events supported by the WWSSN data are represented by the larger symbols. Two events (C and G in Fig.4), 100 km from the trench axis, were classified into T type for convenience. The cross-section of types of faulting is almost consistent with previous studies of focal mechanisms of earthquakes on double-planed seismic zones under island arcs [3,5,6,8,9,11]; that is, the upper plane is characterized by down-dip compression (C) type and the lower plane by down-dip tension (T) type. The present study has given a better resolution for the upper seismic zone under
1964--1975
'/
/
:
Trench iI "~t ?; Axis
39°
o
~ c o
38"
37 N°
r
.
.oi
O l 0km iS, 141"E
' 142"
, 143"
/
, 144"
Fig. 6. Epicentral distribution of the types of focal mechanism. C, T, and D denote the events classified into down-dip compression, down-dip tension and thrusting types, respectively.
Open and closed circles denote the events of type A and B, respectively. Crossesdenote the eventswhich are not analysed or cannot be classifiedinto any type. Larger symbolsrepresent the results for larger (m b >~5.4) events.
31 1975 250km
1964
Trench
15,o
200
loo X
A.F
D
x
Axis
5bO D
X 0
* co o o o
r~ C C
C
•
C •
o•co
T 50
o
T
•
T
T
IT,
I
C
100 krn
cc C
T
T
T O
Type
A
•
Type
B
Fig. 7. A cross-section of types of focal mechanism, pP-depths and well-constrained depths are used for this section. The symbol notation is the same as in Fig. 6. A.F. denotes the aseismic front [13].
the offshore area from 150 to 200 km landwards from the trench axis. Beneath this area, the seismic activity presents a triple-planed structure; the upper seismic zone is decomposed into the layer of D-type events and that of C-type events, although the number of C-type events is limited. The distance between the zones of D and C types is 10-15 km and that between the zones of C and T types is 15-20 km. The location of the aseismic front in the region [13] is indicated by the broken line in Fig. 7. D-type events are distributed down to a depth of 60 km, that is, approximately to the location of the aseismic front. Yoshii [12] proposed that thrusting occurs only seaward of the aseismic front; the present study supports this. The zone of C-type events appears to shift upward at the aseismic front from seaward to landward. Under the area landward of the aseismic front, the distance between the zones of C- and T-type events is 25-30 km. The distribution of type A and B events is consistent with the distribution of those of D, C and T type. 5. Discussion and conclusion
The triple-planed structure seaward of the aseismic front is easily understood if a thrust zone
along the plate interface overlaps the doubleplaned seismic zone which represents fractures within the descending slab. This feature is not so prominent in Fig. 7 because of the sparsity of C-type events under the zone of D type; however, it hecomes very distinct in the study including the more recent period form 1976 through June 1978 [16], during which many C-type events occurred seaward of the aseismic front (see Appendix 2). The location of C events relative to the zone of D-type events will give an estimate for the location of C events relative to the plate interface, because D events are most likely to occur at the plate interface. Shimamura [21] estimated the locations of earthquakes within the descending slab relative to the plate interface in the Kuril arc by interpreting the secondary phase between P and S arrivals observed at a station in Hokkaido as the P wave which is reflected and converted from the S wave at the upper boundary of the descending plate. He relocated the hypocenters 10 km below the plate interface in the depth range of 20-50 km. The distance between the zones of D and C types is concordant with his results. Another interesting feature in the cross-section of Fig. 7 is the apparent shift of the zone of C events at the aseismic front. In contrast, the zone of D events appears to merge into the zone of C
32 events deeper than 70 km in a down-dip direction. It seems necessary to examine whether the apparent shift of the C zone is due to systematic errors of the hypocentral determinations. Focal depths of many of the events seaward of the aseismic front are controlled by pP readings but, in contrast, few are controlled landward of the front (see Fig. 3). Thus, this might have produced a systematic bias for the depth of C events between the landward and seaward parts of the aseismic front. However, the shift of the zone can also be seen in the cross-section which does not use pP-depths, but uses instead the usual routinely processed hypocenters (Appendix 2). On the other hand, in this case, the distinct change of Pn velocity from 8.1 to 7.5 k m / s at the aseismic front in northern Honshu [6,22,23] could be a cause of the systematic bias between hypocenters seaward and landward of the aseismic front. The systematic errors in hypocenters due to the Pn velocity change at the front are on the order of 10 km [24], which is comparable with the apparent shift of the C zone. It is not a straightforward matter to estimate the sense and extent of the systematic errors due to Pn velocity change for the locations of the events in Fig. 7, because the number and azimuthal distribution of the regional and teleseismic stations used to locate the events vary considerably. Considering the accuracy of the data used in the present study, it seems difficult to decide whether the apparent shift of the C zone is real or not. Using ScSp waves converted from ScS waves at the boundary of a sharp velocity contrast between the descending slab and the mantle above it, Hasegawa et al. [19] located the boundary exactly on the upper plane of the double-planed seismic zone under northern Honshu, in the depth range greater than 60 kin. Thus the spatial distribution of D and C events shown in Figs. 7 and A-2, if we interpret D events as located at the plate interface, is in agreement with the result by Hasegawa et al. [19], within the accuracy both of the hypocentral determinations and the estimation of the boundary of ScSp conversion. Yoshii [6] proposed that the aseismic front may mark the change in type of faulting between thrusting and down-dip compression. In the crosssection in the present study, the zone of C events
extends further seaward beyond the aseismic front by 50 km. The cross-section of seismicity, combined with focal mechanisms in the region just north of the area in this study (39-40.5°N) [6,16], does not present a triple-planed structure seawards of the aseismic front, at least during the period from 1964 through June 1978. This suggests that the occurrence of C events seaward of the aseismic front may be a regional characteristic of the area studied and possibly be a premonitory phenomenon prior to the 1978 Miyagi-Oki earthquake. Many C and D events, along with a few T events, occurred within and near the rupture zone of the Miyagi-Oki earthquake (see Fig. 6). The continental plate would have loaded this part of the descending slab, through a mechanical coupling between the plates at the rupture zone of the MiyagiOki event, before its occurrence. This may have induced the activity of C- and T-type events within the slab in the vicinity of the rupture zone of the Miyagi-Oki earthquake. The shift of the C zone at the aseismic front, that is, the shift of the neutral surface between the down-dip compressional and down-dip tensional stresses, may also have been caused by this loading which is likely to have put an additional longitudinal force on the part of the descending slab. Goto and Hamaguchi [25,26] have explained the double seismic zone beneath northern Honshu by a thermal stress within the slab caused by heating from the surrounding mantle as it descends. Recently they have shown that, when the plate interface seaward of the aseismic front is fixed, that is, when the oceanic plate is coupled with the continental plate there, the zones of down -dip compression and down-dip tension become distinct under the fixed interface and shift downward, in contrast to the case when the interface is set free [26]. Their result may support the above speculation as to the cause of the extension and shift of the C zone seaward of the aseismic front. In order to be certain of the uniqueness of the triple-planed structure in the vicinity of the rupture zone of the 1978 Miyagi-Oki earthquake, more close studies of seismicity and focal mechanisms are necessary in other regions along island arcs, and in the area of the present study for the period after the Miyagi-Oki earthquake; they should in-
33 c l u d e p r e c i s e r e l o c a t i o n s o f e a r t h q u a k e s r e l a t i v e to e a c h other, to the p l a t e i n t e r f a c e , a n d to t h e e a r t h ' s surface.
Acknowledgements W e w o u l d like to t h a n k M. O t s u k a , D i r e c t o r at the International Institute of Seismology and E a r t h q u a k e E n g i n e e r i n g ( I I S E E ) for his e n c o u r a g e m e n t a n d c r i t i c a l r e v i e w o f ttie m a n u s c r i p t . T h e m a j o r p a r t o f this s t u d y was c a r r i e d o u t d u r i n g t h e i n d i v i d u a l s t u d y b y o n e o f t h e a u t h o r s (B.P.) at t h e I I S E E . W e t h a n k T. Y o s h i i for p r o v i d i n g us w i t h the d a t a file E Q - 1 a n d for d i s c u s s i o n a n d P. S o m e r v i l l e for c r i t i c a l r e v i e w o f t h e m a n u s c r i p t . D i s c u s s i o n s w i t h S. M i y a m u r a a n d w i t h t h e staffs at t h e I I S E E , a n d a s s i s t a n c e b y M. T a n a k a in t y p i n g the m a n u s c r i p t w e r e h e l p f u l . T h i s w o r k was partly supported by the National Science Foundation under grant EAR81-08718.
References I B. Isacks and P. Molnar, Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes, Rev. Geophys. Space Phys. 9 (1971) 103-174. 2 K.F. Veith, The nature of the dual zone of seismicity in the Kuril arc, EOS 58 (1977) 1232. 3 E.R. Engdahl and C.H. Scholz, A double Benioff zone beneath the central Aleutians: an unbending of the lithosphere, Geophys. Res. Lett. 4 (1977) 473-476. 4 B.L. Isacks and M. Barazangi, Geometry of Benioff zones: Lateral segmentation and downwards bending of the subducted lithosphere, in: Island Arcs, Deep Sea Trenches and Back-Arc Basins, Am. Geophys. Union, Maurice Ewing Ser. 1 (1977) 99-114. 5 A. Hasegawa, N. Umino and A. Takagi, Double-planed structure of the deep seismic zone in the northeastern Japan arc, Tectonophysics 47 (1978) 43-58. 6 T. Yoshii, A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan, Tectonophysics 55 (1979) 349-360. 7 K. Fujita and H. Kanamori, double seismic zones and stresses of intermediate depth earthquakes, preprint (1981). 8 I.R. Samowitz and D.W. Forsyth, Double seismic zone beneath the Mariana island arc (submitted to J. Geophys. Res.). 9 N. Umino and A. Hasegawa, On the two-layered structure of deep seismic plane in northeastern Japan arc, Zisin 27 (1975) 125-139 (in Japanese).
10 M. Barazangi and B.L. Isacks, A comparison of the spatial distribution of mantle earthquakes determined from data produced by local and by teleseismic networks for the Japan and Aleutian arc, Bull. Seismol. Soc. Am. 69 (1979) 1763-1770. 11 A. Hasegawa and N. Umino, Spatial distribution of hypocenters and earthquake mechanisms under northern Japan, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1978) 36 (in Japanese). 12 T. Yoshii, Proposal of the "aseismic front", Zisin 28 (1975) 365-367 (in Japanese). 13 T. Yoshii, Compilation of geophysical data around the Japanese islands, I, Bull. Earthq. Res. Inst. 54 (1979) 75-117 (in Japanese). 14 Tohoku University, Earthquake off Miyagi prefecture, June 12, 1978, Rep. Coord. Comm. Earthq. Predict. 21 (1979) 55-59 (in Japanese). 15 T. Seno, K. Shimazaki, P. Somerville, K. Sudo and T. Eguchi, Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978, Phys. Earth Planet. Inter. 23 (1980) 39-61. 16 T. Seno, Seismicity and focal mechanisms prior to the Miyagi-Oki, Japan, earthquake of June 12, 1978, preprint (1980). 17 W.M. Chapple and D.W. Forsyth, earthquakes and bending of plates at trenches, J. Geophys. Res. 84 (1979) 6729-6749. 18 S. Asano, T. Yamada, K. Suyehiro, T. Yoshii, Y. Misawa and S. Iizuka, Crustal structure off northeastern Japan as revealed from refraction method using ocean bottom seismographs, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1979) 104 (in Japanese). 19 A. Hasegawa, N. Umino and A. Takagi, Double-planed deep seismic zone and upper mantle structure in the northeastern Japan arc, Geophys. J. R. Astron. Soc. 54 (1978) 281 - 296. 20 T. Nakajima, Spatial and sequential distribution of focal mechanisms before and after the Tokachi-Oki earthquake of May 16, 1968, Geophys. Bull. Hokkaldo Univ. 32 (1974) 25-42 (in Japanese). 21 H. Shimamura, Distribution of thickness of the Kuril plate and hypocentral distribution of earthquakes within the plate, Abstr. Annu. Meet. Seismol. Soc. Jpn. 1 (1973) 118 (in Japanese). 22 Research Group for Explosion Seismology, Regionality of the upper mantle around northeastern Japan as derived from explosion seismic observations and its seismological implications, Tectonophysics 37 (1977) 117- 130. 23 S. Suzuki, Lateral variation of P-wave velocity in the upper mantle beneath northern Japan, as derived from travel times of earthquakes, Zisin 29 (1976) 99- 116 (in Japanese). 24 S. Suzuki, Determination of earthquake hypocenters in consideration of the lateral variation of velocity in the upper mantle beneath the island arcs of Japan--on the Nemuro-Oki earthquake of 1973, Zisin 28 (1975) 181-199 (in Japanese). 25 K. Goto and H. Hamaguchi, Double-planed structure of the intermediate seismic zones--thermal stress within the
34
Appendix I
A N
Focal mechanism solutions of the earthquakes listed in Table 1 are presented as equal-area projections of the lower hemisphere of the focal sphere in Fig. A-I. Large circles indicate WWSSN long-period P-wave first-motion data and small circles WWSSN short-period and JMA data. Polarization angles of long-period S-waves are indicated by the arrows. Nodal lines are indicated by thin lines in this figure and their parameters are listed in Table 1. For thrust-type mechanisms, it i s often difficult to constrain the shallower dipping nodal planes. For such cases, we assumed that the strike of the shallower dipping plane is parallel to the trend of the trench axis in the region, i.e., ~ N I 5 ° W , following Seno et al. [15].
B
¢
( l) N
N
6
F N
H N
14
35
I
J N
K N
i . .
. o
L
M
N N
0 N
o
o
•
•
Fig. A-I. Focal mechanism solutions represented as equal-area projection of the lower hemisphere of the focal sphere, Closed and open circles represent P-wave first motion compression and dilatation, respectively. Arrows indicate S-wave polarization direction. The letters beside each subfigure denote earthquake events in Fig. 4 and Table I.
36
Appendix 2. Seno [16] analysed focal mechanisms of earthquakes from 1976 to June 11, 1978, that is, during a two and a half year period just prior to the Miyagi-Oki earthquake of June 12, 1978, in the same region and using the same method as in this study. Fig. A-2 shows the cross-section of focal mechanism
1964
-
JUN.11,1978 ;~Owrn
2q0
,s,o
types which incorporate the results for this period. The pPdepths are not used in this figure in order to examine the systematic bias of the focal depth distribution due to pP-depths (see discussion in the last section). The zone of C-type events seaward of the aseismic front becomes more distinct and the shift of the zone of C events at the aseismic front can also be seen in this figure.
! 010
A.F.
Trench Axis 0
510 D x x
o
D
o
o0 50
C C
C~
C
C
T
C
T •
e
ccc
~.
~.co,.. To
Q • •
T
100km
•
c
T
T o
Type A
•
Type
B
Fig. A-2. A cross-section of the focal-mechanism types incorporating the results during the period from 1976 through June 11, 1978 [16]. The focal depths from the routine determinations of the ISC are used instead of pP-depths. The symbol notation is the same as in Fig. 7.
lithosphere, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1978) 20 (in Japanese). 26 K. Goto and H. Hamaguchi, Distribution of thermal stress within the descending lithospheric slab, Abstr. Annu. Meet. Seismol. Soc. Jpn. 2 (1980) 48 (in Japanese).