Long-term Crustal Deformation in and around Japan, Simulated by a 3-D Plate Subduction Model Chihiro Hashimoto (1) and Mitsuhiro Matsu ura (2) (1) Institute of Frontier Research for Earth Evolution, Japan Marine Science and Technology Center, Yokosuka, Japan (e-mail: hashi@jamstec.go.jp). (2) Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan (e-mail: matsuura@eps.s.u-toyko.ac.jp). Abstract We constructed a 3-D simulation model for long-term crustal deformation due to steady plate subduction in and around Japan by incorporating viscoelastic slipresponse functions into a realistic 3-D structure model. The lithosphereasthenosphere system is modelled by an elastic surface layer overlying a Maxwellian viscoelastic half-space. Kinematic interaction at plate interfaces is rationally represented by the increase of tangential displacement discontinuity (fault slip) across the interfaces. With this model, giving the steady slip rates at plate interfaces calculated from NUVEL-1A, we simulated long-term crustal deformation caused by steady plate subduction in and around Japan. The simulated crustal deformation pattern is characterized by steep uplift at island arcs, sharp subsidence at ocean trenches and gentle uplift at outer rises. The results of numerical simulation well explain the characteristic pattern of observed free-air gravity anomalies; island-arc high, trench low and outer-rise gentle high. Introduction At subduction zones oceanic plates descend beneath continental plates at constant rates on a long-term average. The occurrence of large interplate earthquakes can be regarded as its perturbation. Thus, change in tectonic stress and crustal deformation during one earthquake cycle can be decomposed into two parts; the steady increase or decrease due to steady slip motion over the whole plate interface and the cyclic change due to stick-slip motion in a seismogenic region. The steady slip motion brings about steady stress accumulation on a curved plate interface and steady crustal deformation characterized by island-arc uplift and trench subsidence (Matsu'ura & Sato, 1989[1]; Sato & Matsu'ura, 1993[2]). The stick-slip motion brings about cyclic change in tectonic stress and crustal deformation. In realistic numerical simulation of the complete earthquake generation 111
cycle, we cannot discard the contributions from steady slip motion over the whole plate interface. A 3-D plate subduction model The model region extends from 125 E to 155 E in longitude and from 20 N to 50 N in latitude (Fig. 1a). The lithosphere-asthenosphere system is modelled by a 40 km-thick elastic surface layer overlying a Maxwellian viscoelastic half-space. The geometry of plate interfaces is represented by the superposition of bicubic B-spline functions with equally spaced local supports (the knot intervals are 0.1 both in longitude and latitude). In this region the lithosphere is divided into four plates; the Pacific (PA), the North American (NA), the Philippine Sea (PH), and the Eurasian (EU) plates. These four plates are in interacting with each other at four plate interfaces. We constructed a 3-D simulation model for long-term crustal deformation due to steady plate subduction in and around Japan by incorporating viscoelastic slip-response functions into a realistic 3-D structure model (Fukui, Sato & Iwasaki, 2001[3]). Kinematic interaction at the plate interfaces is rationally represented by the increase of tangential displacement discontinuity (fault slip) across the interfaces. The relative plate motion at the plate interfaces is calculated from NUVEL-1A (DeMets et al., 1994[4]). The boundary between the NA and the EU plates is ignored in this model. With this model, given the 3-D geometry of plate interfaces and the steady slip rates v pl there, we can calculate surface deformation rates due to steady plate subduction. Figure 1: A 3-D structural model and relative plate motion in and around Japan. The lithosphere is divided into four plates; the Pacific (PA), the North American (NA), the Philippine Sea (PH), and the Eurasian (EU) plates. These four plates are in interacting with each other at four plate interfaces; Σ 1 (PA-NA), Σ 2 (PA-EU), Σ 3 (PH-NA), and Σ 4 (PH-EU). The relative plate motion at the plate interfaces is calculated from NUVEL-1A. Calculated deformation patterns The 3-D plate subduction model realizes a uniform horizontal velocity field for each plate, indicating simple convergent block motion, except for deformation zones near plate boundaries. Figure 2a shows the simulated horizontal velocities of the oceanic plates relative to the Eurasian plate. The convergence rate between the North American and the 112
Pacific plates is about 8 cm/yr in the northeast Japan, and that between the Eurasian and the Philippine Sea plates is about 4 cm/yr in the southwest Japan. The vertical crustal motion caused by the steady plate subduction is characterized by steep uplift at island arcs, sharp subsidence at ocean trenches, and gentle uplift at outerrises (Figs 2b and 2c). The maximum subsidence rate is about 4 mm/yr at the Kuril-Japan trench and 2.5 mm/yr at the Nankai trough. The maximum uplift rates are about 2.5 mm/yr in the northeast Japan and 1.5 mm/yr in the southwest Japan. The calculated deformation patterns show the significant effects of 3-D geometry of plate interfaces. For example, the large-scale horizontal bends of plate interfaces in the northeast Japan and the southwest Japan bring about subsidence in broad regions, which are caused by the horizontal extension of the overlying plates due to steady subduction along the curved plate interfaces. Figure 2: (a) Simulated horizontal velocities of the oceanic plates relative to the Eurasian plate. The convergence rate between the North American and the Pacific plates is about 8 cm/yr in the northeast Japan, and that between the Eurasian and the Philippine Sea plates is about 4 cm/yr in the southwest Japan. (b) The calcurated vertical motion. The vertical crustal motion caused by the steady plate subduction is characterized by steep uplift at island arcs, sharp subsidence at ocean trenches, and gentle uplift at outer-rises. (c) The contours of vertical crustal motion projected on the map. Free-air gravity anomalies The accumulation of vertical displacements changes surface height from an unknown surface configuration at an initial isostatic state. Therefore, we cannot directly compare the theoretical result with observed surface topography. On the other hand, the free-air gravity anomalies calculated from the vertical displacement may be directly compared with observations, since the gravity anomalies are zero at the initial isostatic state. The pattern 113
of free-air gravity anomaly rates calculated from the uplift rates is similar to that of uplift rates shown in Fig. 2, and its profile across the island arc-trench system indicates the characteristic pattern consisting of island-arc high, trench low and outer-rise gentle high (Fig. 3b). This pattern is in accord with that of observed free-air gravity anomalies (Fig. 3a). We can find the large-scale negative gravity anomaly regions in the Tokachi-oki (TO) and the Bungo channel (BC). These broad subsidence regions are caused by steady slip along the large-scale horizontal bends of the plate interfaces there (Fig. 3b). We can also find a characteristic pattern associated with the triple junction off the Boso peninsula, where the Philippine Sea plate is subducting beneath the North American plate and is running on the subducting Pacific plate. The landward-high of gravity anomalies along the Japan trench is interrupted at the Sagami trough (ST), because of the subsidence due to steady subduction of the Philippine Sea plate at the ST (Fig.3b). Figure 3: (a) The map of free-air gravity anomalies by Sandwell & Smith [5] from satellite altimetry. Marine gravity anomalies are constructed from along-track sea surface slope profiles. (b) Calculated pattern of free-air gravity anomaly rates. We can find the large-scale negative gravity anomaly regions in the Tokachi-oki (TO) and the Bungo channel (BC). The landward-high of gravity anomalies along the Japan trench is interrupted at the Sagami trough (ST). References [1] Matsu'ura, M. & Sato, T., 1989, A dislocation model for the earthquake cycle at convergent plate boundaries, Geophys. J. Int., 96, 23-32. [2] Sato, T. & Matsu ura, M., 1993, A kinematic model for evolution of island arc-trench systems, Geophys. J. Int., 114, 512-530. [3] Fukui, K., Sato, T. & Iwasaki, T., 2001, Modeling of 3-D configuration of plate boundaries in and around Japanese Islands, 2nd ACES Workshop Proceedings, ed. 114
M. Matsu ura, K. Nakajima & P. Mora, ACES Cooperation for Earthquake Simulation, 301-303. [4] DeMets, C., Gordon, R., G., Argus, D. F. & Stein, S., 1994, Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions, Geophys. Res. Lett., 21, 2191-2194. [5] Sandwell, D. F. & Smith, W. H. F., 1997, Marine gravity anomaly from Geosat and ERS 1 satellite altimetry, J. Geophys. Res., 102, 10039-10054. 115