Interpretation of Seismic and Volcanic Activities in the Izu Block in Relation to Collision Tectonics

Transcription

1 J. Phys. Earth, 39, , 1991 Interpretation of Seismic and Volcanic Activities in the Izu Block in Relation to Collision Tectonics Yoshiaki Ida Earthquake Research Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Distinctive seismic and volcanic activities in the Izu block are interpreted as mechanical processes responding to the subduction and collision of the Philippine Sea plate. Systematic propagation of seismicity and interconnection between some seismic and volcanic events confirm that the activities are of a mechanical origin. It is inferred that the western part of the Izu block deforms so as to adjust the difference of plate motion between subducting and colliding segments along the Suruga trough, while the eastern part involves a seismo-volcanic activitiy like the earthquake swarms east off the Izu Peninsula described by Hill's model. In this seismo-volcanic activity, ascending magma intrudes into parallel fissures and induces seismic slips on the connected faults. Such system of fissures and faults is able to yield a NW-SE contraction in response to the collision of the Izu Peninsula and a NE-SW extension that gives a downgoing component of the plate motion along the Sagami trough. The remarkable crustal inflation observed in recent activities may reflect the volume expansion of ascending magma associated with the vesiculation of dissolved volatiles. The monogenetic volcanoes in the eastern Izu block have been progressively constructed by the seismo-volcanic processes repeated in the late Quaternary. 1. Introduction The tectonic area containing the Izu Peninsula, Japan, which is called here the Izu block, had significant seismic and volcanic events since the 1974 Izu-hanto-oki earthquake of magnitude 6.9. Seismicity systematically migrated northeastward for the earlier period of this activation. The later period is characterized by earthquake swarms that have been repeated east off the Izu Peninsula. Finally in July 1989, the earthquake swarm was accompanied by a submarine eruption that generated a new volcanic crater named Teisi Knoll. Significant crustal deformation was also observed in this area sometimes with and sometimes without seismicity. The Izu block is a tectonically complicated place (Fig. 1). The Philippine Sea plate is subducting along the Suruga and Sagami troughs, but the Izu Peninsula that has been carried on this plate is colliding with the Honshu mainland at its junction between these troughs (Sugimura, 1972; Matsuda, 1978). On the other hand, the Pacific plate that has sunk down from the Japan and Izu-Ogasawara trenches supplies magma to Received September 14, 1990; Accepted January 25,

2 422 Y. Ida Fig. 1. Tectonic circumstances of the Izu block that contain the Izu Peninsula and its vicinity. The Philippine Sea plate is moving northwestward at about 3 to 5cm/year relative to the Honshu continent that is assumed here to sit on the Eurasian plate following the conventional concept (Seno, 1977). The Philippine Sea plate is sinking down from the Suruga and Sagami troughs but colliding with the continent at the junction of the Izu Peninsula (Sugimura, 1972; Matsuda, 1978). The volcanic chain from Fuji to Hachijo-jima Volcanoes is supplied with magma from the deep-seated Pacific plate. the volcanic chain with Fuji, Izu-Oshima, Miyake-jima, and Hachijo-jima Volcanoes, as well as small monogenetic volcanoes within this block. Several models have been proposed for the understanding of these activities in relation to the tectonic framework of the Izu block. For instance, Nakamura (1980) maintained that the deformation of this block should be attributed to the bending of the Philippine Sea plate along the Suruga and Sagami troughs. Ishibashi (1988) examined some great earthquake occurrences adjacent to the peninsula on the basis of this bending model. On the other hand, Mogi (1981a) emphasized the role of subducting plate motion along the Suruga trough in the seismic activation of the Izu block. Ishibashi (1978) and Somerville (1978) analyzed the deformation in more detail from the viewpoint of the accommodation of the plate collision. Ukawa (1991) evaluated the effect of the collision on the deformation and seismicity. These models, however, did not sufficiently consider magmatic or volcanic phenomena in the tectonic processes. The east part of the Izu block is a volcanically J. Phys. Earth

3 Interpretation of Seismic and Volcanic Activities 423 distinctive area in which Quaternary monogenetic volcanoes are widely distributed (Aramaki and Hamuro, 1977; Hamuro et al., 1983; Koyama et al., 1991). The 1989 volcanic eruption at Teisi Knoll is regarded as one of the activities of the monogenetic volcano group (Soya et al., 1989). Simultaneously this eruption has showed magmatic origin of recent earthquake swarms east off the Izu Peninsula without doubt, even if it was suggested earlier by Mogi (1989) and Tada and Hashimoto (1989). In this context, the present paper tries to construct a comprehensive tectonic picture, particularly reexamining the role of magmatic activities in the tectonic processes of the Izu block. 2. Fundamental Tectonic Structure The tectonic circumstances of the Izu block are summarized in Fig. 2 with recent seismic and volcanic activities. As is shown in this figure, let us begin our study with a simple assumption that the Philippine Sea plate is moving approximately northwestward at the velocity about 3 to 5cm/year relative to Honshu (Seno, 1977). Concerning the plate boundaries, it is assumed that the Philippine Sea plate is subducting on the lines connecting the deepest points along the Sagami and Suruga troughs and colliding with Honshu continent somewhere to the north of the Izu Peninsula (Sugimura, 1972). In fact, the exact location and nature of the plate boundaries in this region are the problems under debate (Nakamura and Shimazaki, 1981; Kaizuka, 1984; Ishibashi, 1978, 1985; Ishida, 1990; Yoshida, 1991; Koyama et al., 1991). The situation has been made more complicated by the proposal that central Honshu should be separated in the Fossa Magna region adjacent to the Izu Peninsula by a nascent plate boundary between the Eurasian and North American plates (Kobayashi, 1983; Nakamura, 1983). Some of these problems will be examined later in connection with our tectonic scheme. Most of the known focal mechanism solutions in the Izu block are of strike-slip type. The location, length and orientation of the fault with the slip direction are shown in Fig. 2 for each of major earthquakes (Abe, 1978; Shimazaki and Somerville, 1978; Ukawa, 1991). The dominant orientations of the strike-slip faults that are noticed in this figure enable us to divide the Izu block into the eastern and western parts, as follows. In the eastern part of the Izu block, a right-lateral fault is directed east to west and a left-lateral fault is perpendicular to it, making a conjugate fault system that corresponds to a northwest to southeast compression with an extension normal to it. Since the compression axis is parallel to the motion of the Philippine Sea plate relative to Honshu, the compression must originate from the collision of the Izu Peninsula (Fig. 3). A simple compression, however, should rather produce a thrust type of faulting, and the observed strike-slip focal mechanisms require an additional force that may cause a northeast to southwest extension. The northeast to southwest extension can be attributed there to the plate subduction along the Sagami trough. Since the relative plate motion is almost parallel to the Sagami trough and keeps the bending geometry of the plate from developing successively, however, it is unlikely that the bending stress proposed by Nakamura (1980) is effective to produce extensive deformation continuously. The origin of the extension should be rather assigned to a mechanical effect, such as a slab pull force. Vol. 39, No. 1, 1991

4 424 Y. Ida Fig. 2. Seismicity, volcanism and tectonics of the Izu block. The lines connecting the deepest points along Sagami and Suruga troughs are shown as the consuming plate boundaries, while the collision boundary (thick dashed line) is tentative (Sugimura, 1972). The locations of individual monogenetic volcanoes are shown by dots (Aramaki and Hamuro, 1977; Hamuro et al., 1983; Koyama et al., 1991), and other Quaternary volcanoes are distributed in the hatched areas. Most of recent major, earthquakes are of strike-slip type, and the fault arid slip directions are shown with the year and magnitude for each of the major events (Abe, 1978; Shimazaki and Somerville, 1978; Ukawa, 1991). For instance, 74 M 6.9 means the earthquake of magnitude 6.9 that occurred in See text for the middle Izu line. The orientation of the conjugate fault system is turned by about 45 in the western part of the Izu block. In particular, some faults are well developed parallel to the plate motion. The 1974 Izu-hanto-oki earthquake happened on one of these faults at the southern border of the Izu Peninsula (Fig. 2). Since some other earthquakes are distributed along a southeast extension of the same line, this linear structure may be regarded as a preexisting tectonic line. It has been argued (e.g., Mogi, 1986; Ukawa et al., 1988) that this tectonic line may also stretch northwestward into Honshu mainland over the Suruga trough. The 1978 Izu-Oshima-kinkai earthquake of magnitude 7.0 presumably shows the J. Phys. Earth

5 Interpretation of Seismic and Volcanic Activities 425 Fig. 3. Principal deformation scheme of the Izu block. The western part (left half) relaxes the velocity difference between the subducting and colliding segments of the Philippine Sea plate along the Suruga trough. The eastern part (right half) that contains the monogenetic volcanoes is subject to the compression due to the collision of the Izu Peninsula and the extension associated with the subduction along the Sagami trough. location of another tectonic line, which is tentatively called the middle Izu line in this paper. This tectonic line, which is traced from Izu-Oshima Island to the inside of the Izu Peninsula is composed of two broken segments (Fig. 2). The western segment is parallel to the plate motion and the eastern segment runs east to west, consistently with other major faults in the both regions. It is inferred that the western segment of the middle Izu line further stretches northwestward over the Suruga trough to Honshu mainland. The first reason for this idea is that a step of topography with the lower altitude on the southwestern side of this line is recognized on the sea bottom of Suruga Bay as well as on the western shoreline of the Izu Peninsula. In the second place, microseismicity (Matsu'ura et al., Vol. 39. No

6 426 Y. Ida 1988; Ishida, 1990) and the occurrences of larger earthquakes (Kayano and Utsu, 1987) around the Suruga trough are significant on the southwestern side of the middle Izu line with the other side left seismically inactive. Furthermore, the distribution of Quaternary volcanoes in the western Izu Peninsula is confined to the northeastern side of the tectonic line. These pieces of evidence also suggest that the middle Izu line may give a boundary to the colliding area of the Philippine Sea plate (Figs. 2 and 3). The tectonic lines parallel to the plate motion are considered to participate mainly in adjusting shear strain due to the collision of the Izu Peninsula (Fig. 3), as was suggested by Mogi (1981a). Namely slips along these tectonic lines are able to relax effectively the velocity difference between the southwestern subduction and the northeastern collision. Actually this relaxation process may require more complicated deformation involving some slips on conjugate faults, other smaller seismicity and aseismic creep (Ishibashi, 1978; Somerville, 1978). Anyway the process finally brings some lithospheric material from the Suruga trough down below the Honshu continent. Such material flow makes the eastern part subject to an extension from the western side. In short, deformation of the Izu block can be broadly interpreted as a result of the subduction of the Philippine Sea plate with the collision. Major consumption of the plate occurs along the Suruga trough, and the western part of the Izu block accommodates shear deformation between the subducting and colliding segments of the plate. On the other hand, the eastern part is subject to compression due to the collision as well as perpendicular extension that mainly arises from the subduction along the Sagami trough. 3. Propagation of Deformation Recent seismicity of the Izu block has two periods of significant activities. The old activity began with the earthquake swarm near Ito City in The 1930 Kita-Izu earthquake of magnitude 7.3 happened on Tanna fault a half year after this swarm and another smaller event followed in the middle Izu Peninsula in 1934 (Fig. 2). The new activity is for the period from the 1974 Izu-hanto-oki earthquake to the earthquake swarms east off the Izu Peninsula. Between these two activations, there was a dormancy for about 40 years with little seismicity except some earthquakes near Izu-Oshima Island particularly in In addition, some old documents recorded swarm-like earthquakes near Ito City in 1868 (Kayano and Utsu, 1987), but the detail about the activity is not known. Prior to the old activity starting in 1930, several large earthquakes took place to the northeast of the Izu Peninsula, including the great Kanto earthquake of magnitude 7.9 in 1923 (Kayano and Utsu, 1987). This suggests that deformation was transmitted to the Izu block from the northeastern region. On the other hand, high seismicity to the southeast of the Izu Peninsula preceded the new activation (Mogi, 1981b; Yoshida, 1982), and so deformation may have propagated from the south at this time, even if observed crustal deformation may be interpreted alternatively as a propagation from the northeast similar to the old activity (Fujii, 1977). In the new activation period, more systematic migration of seismicity was observed within the Izu block (e.g., Mogi, 1985). Taking earthquake swarms as well as major J. Phys. Earth

7 Interpretation of Seismic and Volcanic Activities 427 Fig. 4. Recent seismic and volcanic activities in the Izu block. The fault and slip directions are shown with the year/month for each of the major earthquakes (Abe, 1978; Shimazaki and Somerville, 1978; Ukawa, 1991). The areas of earthquake swarms are dotted with their active periods (Matsu'ura et al., 1988). The volcanic activity is represented simply by opening of a volcanic fissure. earthquakes into account (Matsu'ura et al., 1988), the location of seismicity is traced with time in Fig. 4. Let us assume here that the earthquake swarms are characterized by same overall shear faulting that is expected from that of the major earthquake with similar fault orientation. In the seismic sequences it is then found for instance that the seismic slip at the 1974 Izu-hanto-oki earthquake was transmitted to the second conjugate fault (actually fault group associated with an earthquake swarm) like Fig. 5(b) and then to other faults (or fault groups) like Fig. 5(b) and (c). The 1978 Izu-Oshima-kinkai earthquake involves the migration steps of Fig. 5(a) and (b) between the faults with same orientation, in addition to the steps of Fig. 5(b) and (c). In order to understand intuitively how each elementary step in Fig. 5 is able to transmit the deformation and slip, it is useful to consider the intermediate stress state that results from the first slip and induces the second. Namely, compressed and expanded areas denoted by P and T, respectively, in the same figure give an idea of the causality between the two slips. In fact, such slip transmission implicitly assumes preexisting weak zones, which allow slips more easily there under relatively small stress changes than in the ambient area. Vol. 39, No. 1, 1991

8 428 Y. Ida Fig. 5. Elementary steps of slip propagation from the first fault S1 to the second S2. Compressed (P) and expanded (T) areas that have resulted from the first slip and induce the second are shown for intuitive understanding of the causality between the two slips. (a) Propagation of the fault on the same line. (b) Slip transmitted to a conjugate fault in the middle. (c) Slip transmitted to a conjugate fault near the end of the first. (d) Slip transmitted to a fault parallel to the first. Fig. 6. Mechanical interaction between the seismic slip and the magma intrusion into a fissure. (a) A seismic slip S1 produces an extension T and induces a magma intrusion V1. (b) A magma intrusion V1 produces a compression P and induces a seismic slip S2. (c) A magma intrusion V1 induces another magma intrusion V2 through the seismic slip S2. For example, V1 and V2 may represent the magmatic activities of Izu-Oshima Volcano and the aligned submarine monogenetic volcanoes, respectively, and S1 and S2 may correspond to the slip along the middle Izu line and an earthquake swarm to the south of Izu-Oshima Volcano, respectively (See Figs. 2 and 4). Some mechanical interaction is also probable between seismic and volcanic activities. For instance, let us consider the 1986 eruption of Izu-Oshima Volcano that generated a new fissure aligned northwest to southeast (e.g., Ida, 1989). The volcanic fissure has such geometrical relation to the seismic slip of the 1978 Izu-Oshima-kinkai earthquake as in Fig. 6(a), and so the seismic slip may have caused extension T and promoted opening of the fissure. As a reverse process, magma intrusion can produce compression P and induce seismic slip like Fig. 6(b). The seismicity that was activated J. Phys. Earth

9 Interpretation of Seismic and Volcanic Activities 429 to the south of Izu-Oshima Volcano after this eruption (Fig. 4) is probably an example of such process. Coupling of the two steps in Fig. 6(a) and (b) yields the interaction between the volcanic activities in Fig. 6(c). Some old submarine volcanoes align parallel to the fissures of Izu-Oshima Volcano in contact with the fault of the 1978 Izu-Oshima-kinkai earthquake (Fig. 2), suggesting this type of interaction. It has been shown above that seismic and volcanic events in and around the Izu block are connected to one another by some mechanical interactions. This confirms that individual events are parts of the processes that may relax the overall deformation associated with the subduction and collision of the Philippine Sea plate. It is emphasized that volcanic activities also participate in the relaxation process, simultaneously releasing the magmatic energy. 4. Seismo-Volcanic Process and Monogenetic Volcanoes The eastern part of the Izu block has distribution of small monogenetic volcanoes (Aramaki and Hamuro, 1977; Hamuro et al., 1983; Koyama et al., 1991) over relatively wide area bounded by the middle Izu line and Tanna fault (Fig. 2). Such volcanic activity contrasts with concentrated magma effusion at a single poligenetic volcano like Fuji, Izu-Oshima, and Miyake-jima Volcanoes on the same volcanic belt. Recent earthquake swarms east off the Izu Peninsula have occupied the easternmost area of the monogenetic volcano group. The earthquake swarms have been repeated almost every year after the occurrence of the 1978 Izu-Oshima-kinkai earthquake. These activities were accompanied by remarkable crustal uplift and elongation, centered in the focal areas (Ishii, 1989; Geographical Survey Institute, 1989b). The presence of such a crustal inflation suggested magmatic origin of the earthquake swarms (Mogi, 1989), and more quantitative analyses showed that the crustal inflation can be explained by magma intrusion into a fissure aligning approximately northwest to southeast (Tada and Hashimoto, 1989; Shimazaki, 1989). It is added in Fig. 4 that the predicted volcanic activities there may be mechanically correlated with the 1980 Izu-toho-oki, 1989 Ito-oki and 1990 Izu-Oshima-kinkai earthquakes in the manner of Fig. 6(a) or (b). The submarine eruption at Teisi Knoll in 1989 revealed explicitly that the earthquake swarms in question surely involved magmatic activity. Furthermore the volcanic products of this eruption resemble some of the older monogenetic volcanoes (Soya et al., 1989). It is thus inferred that the seismo-volcanic activities similar to the recent earthquake swarms east off the Izu Peninsula have constructed the monogenetic volcano group for a long time interval. It has been pointed out that seismic and magmatic activities may have the mutual interactions like Fig, 6. Similar interactions are also realizable in smaller scales within the earthquake swarm that may correspond to the seismo-volcanic process generating monogenetic volcanoes. Namely, occurrence of many small earthquakes can be interpreted as a result of seismic slips along small faults connecting magma intrusions in fissures. Such a model of the earthquake swarm was originally proposed by Hill (1977), and applied to the earthquake swarm in this region by Ishida (1984). An idealized configuration of Hill's model is shown in Fig. 7(a). It is assumed there that all the volcanic fissures have a constant length L, separated by a constant distance Vol. 39, No. 1, 1991

10 430 Y. Ida Fig. 7. System of magma intrusion into fissures with cooperative seismic slips. The model was originally proposed by Hill (1977) for a mechanism of the earthquake swarm. (a) Idealized configuration of fissures and faults. (b) The effect of the fissure opening u on the displacements parallel and perpendicular to the fissures. (c) Strain component CT perpendicular and ap parallel to the fissures, in which e=2lu/h(h+2l) is the volume of intruded magma per a unit horizontal area. H, and that seismic faults are connected at a 45 angle to the fissures. In this model, a uniform opening u of the fissures can be adjusted by a uniform slip u/ ã2 on the seismic faults, so that no inhomogeneous stress may be left in the solid medium as well as in the magma. When the uniform fissure opening u occurs in this system, the two blocks at the both ends of a fissure approach each other by the distance u as a result of the cooperative seismic slip (Fig. 7(b)). Taking such shortening also into account, let us evaluate the distance changes between geometrically equivalent points in the system, for one separated by H perpendicular to the fissures, and the other separated by H+2L parallel to them (Fig. 7(a)). The result, which is given in Fig. 7(b), leads to the expression of the bulk strain component ƒãt perpendicular and cp parallel to the fissures in Fig. 7(c), where the displacement u is replaced by the corresponding volume e of intruded magma per a unit area. Magma intrusion naturally produces extension with positive ƒãt responding to the fissure opening. It is rather unexpected, however, that such a contraction as is given by negative ƒãp arises parallel to the fissures. The capability of contraction is important as a mechanism to accommodate the system to the shortening associated with the collision. Simple thrust faulting is also able to shorten the system, but simultaneously produces crustal thickening. Therefore the gravitational effect against the thickening J. Phys. Earth

11 Interpretation of Seismic and Volcanic Activities 431 tends to hinder the thrust faulting from accumulating contractive strain unlimitedly. It is thus proposed that the effect of the collision has been accommodated in the eastern Izu block mainly by the deformation process in Fig. 7 that allows unlimited contraction as far as magma is available. Here small volcanic fissures are assumed to be aligned northwest to southeast in the direction of the collision. The actual sizes of fissures and faults may be variable, but the angles between them cannot deviate too much from 45. A dominant phenomenon usually observed in this process is an earthquake swarm with magma intrusion only in the lithospheric interior but the process sometimes exposes magma effusion to the surface. Since such seismo-volcanic process requires sufficient supply of magma from below, the process is considered to have taken place only in small restricted area for a specific time period like the recent earthquake swarms east off the Izu Peninsula. The activity must cover the entire area for a longer time interval, however, to relax the overall deformation. Therefore the seismo-volcanic activity must have been spatially shifted from time to time and progressively constructed the monogenetic volcanoes over wide area. According to Koyama et al. (1991), ages of the monogenetic volcanoes tend to be concentrated in some narrow region for a specific period, consistently with the migration process proposed here. 5. Crustal Deformation Associated with Magmatic Processes Crustal deformation induced by the seismo-volcanic processes in the eastern Izu block has several problems to be examined more physically. As has been already mentioned, the crustal inflation east off the Izu Peninsula accumulated for the period of the earthquake swarms was modeled by the opening of a single fissure due to magma intrusion (Tada and Hashimoto, 1989; Shimazaki, 1989). According to the deformation scheme in Fig. 7, however, magma intrusion should involve many small fissures connected with seismic faults, and the net deformation should contain not only extension normal to the fissures but also contraction parallel to them. In fact, there is some misfit of the EDM data to a single crack model (Tada and Hashimoto, 1989) near the end of the assumed crack, which may be explained as a consequence of the contraction. The most recent swarm with the submarine eruption at Teisi Knoll has been studied in more detail. The eruption contained three or more explosive events with strong volcanic tremors for the period of July 11 to 13, even if the explosion was observed above the sea only once on July 13 (Ida, 1990a) The seismicity was most dominant for July 4 to 10 before the eruption, and relatively large earthquakes, such as that of magnitude 5.5 on July 9, happened frequently for this period. Substantial uplift and elongation of the ground observed in this swarm was centered in the focal area (Fig. 8). The continuous EDM (Tsuneishi, 1990) and tilt (Yamamoto et al., 1991) measurements confirm that the ground deformation was almost completed during the preeruptive period with dominant seismicity. A simple model that magma volume of about 107m3 should have intruded into a spherical cavity about 2km deep below Teisi Knoll fits the data of the ground deformation semi-quantitatively. Models that have one or more sheets of magma Vol. 39, No. 1, 1991

12 432 Y. Ida Fig. 8. Crustal deformation associated with the seismo-volcanic activity near Ito City in July Dominant seismicity occurred in the dotted area prior to the submarine eruption at Teisi Knoll, and was accompanied by the crustal inflation shown here. The result of leveling (Geographical Survey Institute, 1989 a) is shown by contours of uplift along with the main leveling points denoted by squares, in which the zelo-level is fixed at Higashi-Izu with relatively stable deformation. The distance changes of EDM baselines (Geographical Survey Institute, 1989a; Tsuneishi, 1990) are given by numerical values (positive extensive). The downward direction of a tilt change (Yamamoto et al., 1991) is shown by the arrow with the intensity by a numeric value. distributed along the high seismicity zones were proposed for better fitting (Tada and Hashimoto, 1989; Okada and Yamamoto, 1991). Probably the configuration of interconnected faults and fissures like Fig. 7 would give a more realistic model, even if it has not yet been tested. In the eruptive stage for July 11 to 13, seismicity was much lower and deformation J. Phys. Earth

13 Interpretation of Seismic and Volcanic Activities 433 Fig. 9. Model of the eruptive process that generated Teisi Knoll in July 1989 (Ida, 1990a). In the preeruptive stage, magma intruded into shallow crust, producing the earthquake swarm and the crustal inflation. Only a minor part of the magma further ascended up to the surface and caused explosions on the ocean bottom. was almost stabilized. This indicates that a minor part of the magma further moved up to the surface and participated in the surface eruption. Actually only small amounts of volcanic products were available from the eruption, even if their volume has not yet been quantitatively estimated. In other words, major magma volume stayed in the crustal interior in spite of the eruptive activity on the crustal surface (Fig. 9). Such sequence of the volcanic process differs thoroughly from the concept of the inflation-deflation theory that predicts an accumulation and release of the magma pressure in the eruptive process. The presence of the significant crustal inflation means that the magma intrusioirr is not a passive process to fill simply the space that has been made by an extensive force. The crustal inflation should reflect a more active mechanism involving an increase of the magma pressure or a volume expansion of ascending magma. In the present seismo-volcanic process, however, the magma migration was not caused by a preexisting magma pressure, because the crustal inflation was observed not before but during the magma migration process. A more likely driving mechanism of magma motion is probably a buoyancy force. It was pointed out that the vesiculation of volatiles in the magma should be effective for the magma to gain a buoyancy force (Ida, 1990b). The vesiculation can be initiated by a pressure decrease associated with the fissure opening. The resultant bubble inclusion Vol. 39, No. 1, 1991

14 434 Y. Ida Fig. 10. Magma ascent- process involving the vesiculation of dissolved volatiles. A pressure release in the magma induces the vesiculation, makes new bubble inclusions and reduces the bulk magma density. Expansion of the ascending magma contributes to the crustal inflation. Magma may either move up to the surface or be stabilized in the crustal interior, depending on the shallower density contrast between the magma and the crust. in the magma is able to reduce the bulk magma density so effectively that the magma may begin to move upward subject to buoyancy (Fig. 10). The ascent motion and the vesiculation process can be accelerated with decreasing confining pressure at shallower depth. In this process, the ascending magma may have an increased volume of the bubble inclusion and expand enough to cause a remarkable crustal inflation. In the magma ascent process, whether or not magma can attain to the surface depends on the density contrast between the magma and the ambient crust (Fig. 10). If the crustal density is lowered enough near the surface, magma may be gravitationally stabilized there. It is thus physically realizable that most magma that had ascended below Teisi Knoll stayed stably in the shallow crustal interior, as has been predicted in Fig Tectonic Model of the Izu Block It has been proposed that the western part of the Izu block deforms so as to adjust the velocity difference between the subducting and colliding segments of the Philippine Sea plate along the Suruga trough, while the eastern part is subject to the contraction due to the collision of the Izu Peninsula and the extension associated with the subduction along the Sagami trough. The model is schematically displayed in Fig. 11. In this figure, the seismo-volcanic activity that produces monogenetic volcanoes in the eastern part is simplified in configuration and exaggerated in size. Here the volcanic fissures do not always extend throughout the entire thickness of the lithosphere. J. Phys. Earth

15 Interpretation of Seismic and Volcanic Activities 435 Fig. 11. Tectonic model of the Izu block. The western part (left half) adjusts the velocity difference between the subducting and colliding segments of the Philippine Sea plate along the Suruga trough. In the eastern part (right half), monogenetic volcanoes are progressively generated by the seismo-volcanic process that involves magma intrusion into fissures with cooperative seismic slips. This seismo-volcanic process shortens the area in response to the collision, and simultaneously elongates it toward the Sagami trough, giving a downgoing component of the slab motion. For the convenience of the display, the seismo-volcanic activity is simplified in configuration and exaggerated in size. According to this model, the monogenetic volcano group in the eastern part have been progressively constructed by the seismo-volcanic processes similar to the recent activities east off the Izu Peninsula. The area occupied by these volcanoes is bounded by the two major strike-slip faults, i.e., the middle Izu line and Tanna fault (Fig. 2) These faults are characterized by the orientations and slip directions consistent with those for the inside seismo-volcanic processes, so that the interior deformation can be terminated smoothly by them. The tectonic stress has slightly different orientation across these boundary faults (Tsukahara and Ikeda, 1983), and so the stress condition favorable to the formation of the monogenetic volcanoes seems to be confined within this area (Yoshida, 1991). For instance, Quaternary volcanoes east of Tanna fault are no longer monogenetic. The seismo-volcanic processes produce a non-rigid intraplate deformation that consists of the northeast to southwest extension and the northwest to southeast contraction. It has been pointed out that the contraction responds to the collision. The extension also has an important tectonic implication, because it gives a northeastward velocity component of the plate motion toward the Sagami trough (Fig. 11). For instance, the 1923 Great Kanto earthquake of magnitude 7.9 involved a significant reverse slip component dipping down from the Sagami trough, in addition to the strike-slip motion across the plate boundary (Kanamori, 1971; Ando, 1971; Matsu'ura and Iwasaki, 1983). The reverse slip is explained most simply as a result of the northeastward displacement expected from our model, because the relative plate motion is almost parallel to the Sagami trough (Seno, 1977) without producing significant downgoing motion. Vol. 39, No. 1, 1991

16 436 Y. Ida In addition to the focal mechanism of the Great Kanto earthquake, some other deformation around the Sagami trough is also inconsistent with the motion of the entire Philippine Sea plate. To be free from such difficulty, Ishibashi (1985, 1988). assumed that the plate motion should be directed more northward. In fact, the relative plate motion may differ between at the Suruga and Sagami troughs, if the nascent plate boundary along the Fossa Magna region divides central Honshu into the Eurasian and North American plates (Nakamura, 1983; Kobayashi, 1983). The presumed plate boundary may be inactive, however, because no significant seismicity nor clear deformation has been currently observed there (Ishida, 1990). An alternative solution of the problem is to separate the Philippine Sea plate into two parts, one subducting along the Sagami trough and the other along the Suruga trough. Ishida (1990) proposed that the two divided parts of the plate may be seismologically recognized below the Honshu continent as separated portions of the subducting slab with aseismic gap between them. Yoshida (1991) gave a similar idea containing some additional cuts of the Philippine Sea plate. The present model in Fig. 11 gives an idea of how the eastern part of the Philippine Sea plate that subducts along the Sagami trough can be separated from the main plate on the occasion of the collision. Namely, the intraplate deformation caused by the seismo-volcanic process is participating in separating the plate without producing any discontinuity of material on the surface lithosphere. According to this idea, the eastern Izu block may be subject to a strong extensional force associated with the tearing of the plate. In this meaning, monogenetic volcanoes are generated under an extensional stress field (Nakamura, 1986) even in the Izu block. 7. Discussion Some geological evidence suggests that the monogenetic volcanoes began to be generated about 100 to 150 thousand years ago (Takahashi, 1986; Koyama et al., 1991). On the other hand, it is presumed that the Izu Peninsula had the first contact with central Honshu in the early Quaternary and that serious influence of the collision on various tectonic phenomena started 70 to 300 thousand years ago (Huchon and Kitazato, 1984; Ito et al., 1989). Therefore the proposed tectonic scheme has been established in the Izu block rather recently. Such evolution of the tectonic feature must have mainly responded to the change of the motion of the Philippine Sea plate (Seno and Maruyama, 1984; Kaizuka, 1984), which itself was affected by the collision of the Izu Peninsula. The contact of the Izu Peninsula with central Honshu must have first bent the convergent plate boundary in a cusp form, and then deformed the south Fossa Magna region in various ways (Ito et al., 1989). It is quite likely that the collision boundary has not yet been fixed completely but is still migrating landward even at present. Therefore the relative motion between the Philippine Sea and Eurasian plates may have only to be relaxed partly by the deformation in the colliding Izu block. Ishibashi (1978) actually estimated that recent major earthquakes in the western Izu block should have released only a minor part of the relative plate motion. In the present paper, two types of earthquake swarms, one volcanic and the other J. Phys. Earth

17 Interpretation of Seismic and Volcanic Activities 437 tectonic, have been considered. Namely, the seismo-volcanic process in Figs. 7 and 11 contains the first type, while the other nonvolcanic earthquake swarm appears in discussing the propagation of deformation in Fig. 4. The two types have been tentatively distinguished by the elongation direction of the seismic area, but our general knowledge about an earthquake swarm is not enough to justify this treatment. It is hoped that a future study will give a better understanding on the nature of the earthquake swarms in this tectonic province. Some crustal deformation in the Izu block took place without significant seismicity. For instance, an inflation centered near the Hiekawa pass (marked ~ in Fig. 8) preceded the activities east off the Izu Peninsula (Geographical Survey Institute, 1989b). Aseismic crustal uplift was also observed near Ito City after the 1930 Kita-Izu earthquake (Tsuboi, 1933; Fujii, 1977). It may be probable for these crustal deformations to reflect some magmatic process in a deep lithosphere. On the other hand, Kuno (1954) proposed in his classic paper a magma reservoir that had supplied magma to the Omuro-yama Volcano group. A more detailed structure of magma transport system is another important problem to examine in future. I would like to thank Dr. K. Ishibashi and Dr. A. Yoshida for many helpful comments to improve the paper. The discussion with Dr. M. Takahashi and Dr. M. Koyama was helpful fox me to find the meaning of the monogenetic volcano group in relation to the collision tectonics. Dr. M. Ukawa kindly sent me a copy of his preprint on the mechanical structure of the Izu block. Mrs. T. Matsumoto helped with the preparation of the manuscript. REFERENCES Abe, K., Dislocations, source dimensions and stresses associated with earthquakes in the Izu Peninsula, Japan, J. Phys. Earth, 26, , Ando, M., A fault-origin model of the Great Kanto earthquake of 1923 as deduced from geodetic data, Bull. Earthq. Res. Inst., Univ. Tokyo, 49, 19-32, Aramaki, S. and K. Hamuro, Geology of the Higashi-Izu monogenetic volcano group, Bull Earthq. Res. Inst., Univ. Tokyo, 52, , 1977 (in Japanese with English summary and captions). Fujii, Y., Greep dislocation propagation along the east-off-izu tectonic line, Japan, around the year of 1930 and 1976, J. Seismol. Soc. Jpn., 30, , 1977 (in Japanese with English summary and captions). Geographical Survey Institute, On the results of the geodetic surveys in the eastern part of Izu Peninsula, Rep. Coord. Comm. Predict. Volc. Erupt., 44, 68-77, 1989a (in Japanese with English captions). Geographical Survey Institute, Graphs of Height Changes in Bench Marks in and around Izu Peninsula, Geographical Survey Institute, Tsukuba, 240 pp., 1989b. Hamuro, K., S. Aramaki, K. Fujioka, T. Ishii, and T. Tanaka, The Higashi-Izu-oki submarine volcanoes, Part 2, and the submarine volcanoes near the Izu shoto Islands, Bull. Earthq. Res. Inst., Univ. Tokyo, 58, , 1983 (in Japanese with English summary and captions). Hill, D. P., A model for earthquake swarms, J. Geophys. Res., 82, , Vol. 39, No. 1, 1991

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20 440 Y. Ida Tsukahara, H. and R. Ikeda, State of stress in the Kanto-Tokai area, J. Seismol. Soc. Jpn., 36, , 1983 (in Japanese with English summary and captions). Tsuneishi, Y., Automatic electronic distance measurement on the off-ito swarm earthquakes and submarine eruption of July, 1989, Chigaku-Zashi, 99, 31-43, 1990 (in Japanese with English summary and captions). Ukawa, M., Collision and fan-shaped compressional stress pattern in the Izu block at the northern edge of the Philippine Sea plate, J. Geophys. Res., 96, , 1991 Ukawa, M., T. Eguchi, and Y. Fujinawa, Seismic activity in the subducting Philippine Sea plate along the Suruga trough revealed by OBS observation, J. Phys. Earth, 36, 69-87, Yamamoto, E., Y. Okada, and T. Ohkubo, Ground tilt changes preceding the 1989 submarine eruption off Ito, Izu Peninsula, J. Phys. Earth, 39, , Yoshida, A., Recent seismic activity and its characteristics in the region in and around the Izu Peninsula, J. Seismol. Soc. Jpn., 35, , 1982 (in Japanese with English captions). Yoshida, A., On the tectonic cause of the recent activities in the northeastern Izu Peninsula, J. Phys. Earth, 39, 1991 (in press). J. Phys. Earth