Southernmost Ryukyu Subduction Zone Research Articles

The Seismogenic Potential of the Southernmost Ryukyu Subduction Zone as Revealed by Historical Earthquakes and Slow Slip events

The southernmost Ryukyu subduction zone may have a geodetically inferred Mw 7.5 to 8.7 megathrust earthquake in a shallow locked region, the Ryukyu fault. Paleoseismological evidence of historical earthquakes available from the last 417 years indicates that only a 1920 Mw 7.7 earthquake occurred within this magnitude range, near the downdip end of the Ryukyu fault. As slow slip events downdip the locked seismogenic zone may trigger a large subduction earthquake, we investigate how the first observed slow slip events in 2005, 2009, and 2015 initiated downdip in the Ryukyu fault interface affect the occurrence of a megathrust. We establish possible megathrust earthquake cycles from Mw 7.5 to 8.7 on the Ryukyu fault using constraints from the magnitude-frequency relation based on local historical earthquakes. This analysis shows a b value of 1.2 for magnitudes greater than Mw 7.0, which is higher than the empirical 1.0 value. This indicates that the recurrence of an event up to Mw 8.7 is longer than previously thought if the megathrust events follow the observed magnitude-frequency relation. Then, we quantify the influence of slow slip events on the triggering of a potential megathrust earthquake by calculating the static stress increase. We find that stress perturbations caused by the three slow slip events are generally consistent with the values that have triggered the large interplate earthquakes in several subduction zones. However, a large earthquake has not yet been triggered on the Ryukyu fault after a sequence of slow slip events. If the 1920 Mw 7.7 earthquake is the last rupture of the Ryukyu fault, the earthquake cycle on the Ryukyu fault is very likely in an early stage. However, this is not true if the slow slip events occur toward the end of the earthquake cycle and there has been no megathrust earthquake at the fault interface in the last 417 years, as the 2011 Mw 9.0 Tohoku earthquake. Thus, higher potential for a megathrust earthquake may occur in the southernmost Ryukyu subduction zone.

Source rupture characteristics and ground motion simulation of the southernmost Ryukyu subduction zone 31 March 2002 Mw7.1 earthquake

An Mw 7.1 Ryukyu subduction zone earthquake struck northern Taiwan in 2002. The joint source inversion result indicates that rupture mainly propagated toward the northwest and produced a significant directivity. The long-period strong ground motion observed in northern Taiwan was directly related to source rupture effects. To assess the seismic risk in Taiwan and even East Asia more generally, it is necessary to carefully consider the various types of megathrusts that occur in the southern Ryukyu subduction zone. • An Mw 7.1 Ryukyu subduction zone earthquake struck northern Taiwan in 2002. • Rupture mainly propagated toward the northwest resulted in a strong directivity. • Long-period strong ground motion was directly related to source rupture effects. • Megathrust in Ryukyu subduction needs to carefully consider to assess seismic risk. On March 31, 2002, a magnitude 7.1 earthquake occurred off the coast of eastern Taiwan. The Taipei 101 Financial Building was under construction at that time and due to extremely strong long-period shaking, two cranes on the top floor collapsed and fell to the ground, killing five people. This earthquake was caused by a rupture of the southern Ryukyu subduction zone. The result of a joint source inversion showed that the slip originated at the mid-depth subduction interface off the eastern coast and then propagated to the western fault plane with a rupture velocity of approximately 3.0 km/s. Forward ground motion simulation indicated that the main cause of the long-period strong motion in northern Taiwan was directly related to source effects, including long rupture lasting, rupture directivity and source radiation. The seismic waves were further amplified by the soft sediments in the Taipei basin which also resulted in longer vibration time in Taipei City.

Assessment of the peak tsunami amplitude associated with a large earthquake occurring along the southernmost Ryukyu subduction zone in the region of Taiwan

Abstract. The southernmost portion of the Ryukyu Trench near the island of Taiwan potentially generates tsunamigenic earthquakes with magnitudes from 7.5 to 8.7 through shallow rupture. The fault model for this potential region dips 10∘ northward with a rupture length of 120 km and a width of 70 km. An earthquake magnitude of Mw 8.15 is estimated by the fault geometry with an average slip of 8.25 m as a constraint on the earthquake scenario. Heterogeneous slip distributions over the rupture surface are generated by a stochastic slip model, which represents the decaying slip spectrum according to k−2 in the wave number domain. These synthetic slip distributions are consistent with the abovementioned identical seismic conditions. The results from tsunami simulations illustrate that the propagation of tsunami waves and the peak wave heights largely vary in response to the slip distribution. Changes in the wave phase are possible as the waves propagate, even under the same seismic conditions. The tsunami energy path not only follows the bathymetry but also depends on the slip distribution. The probabilistic distributions of the peak tsunami amplitude calculated by 100 different slip patterns from 30 recording stations reveal that the uncertainty decreases with increasing distance from the tsunami source. The highest wave amplitude for 30 recording points is 7.32 m at Hualien for 100 different slips. Compared with the stochastic-slip distributions, the uniform slip distribution will be highly underestimated, especially in the near field. In general, the uniform slip assumption only represents the average phenomenon and will consequently ignore the possibility of tsunami waves. These results indicate that considering the effects of heterogeneous slip distributions is necessary for assessing tsunami hazards to provide additional information about tsunami uncertainties and facilitate a more comprehensive estimation.

Strength of plate coupling in the southern Ryukyu subduction zone

Understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones. The interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy. Here, we calculate the mantle lithosphere buoyancy across the northern portion of the southern Ryukyu subduction zone based on gravity modeling with the constraints from a newly derived P-wave seismic velocity model. The result indicates that the strength of the plate coupling in the study area is relatively strong, which is consistent with previous observations in the southernmost Ryukyu subduction zone. Because few large earthquakes (Mw>7) have occurred in the southern Ryukyu subduction zone, a large amount of energy is locked and accumulated by plate coupling, that could be released in the near future.

Anisotropy in the subducting slab: Observations from Philippine Sea plate events in Taiwan

AbstractIn the southernmost Ryukyu subduction zone, slab‐guiding behavior from intermediate‐depth earthquakes is well documented with a low‐frequency (<2 Hz) first P arrival followed by sustained high‐frequency (3–10 Hz) wave trains. Such waves developed by propagating along a long path within the slab are expected to have high sensitivity to anisotropy within the slab. We determine shear wave splitting parameters from 178 intraplate events that are deeper than 100 km. The possible slab‐anisotropy‐associated polarization pattern shows the fast direction at N65°E and delay time of 0.13–0.45 s. This is stronger than the previously documented crust effect (<0.1 s), similar to the mantle wedge effect (0.28 s in average), but weaker than the upper mantle effect (1.3 s in average) in Taiwan. The fast axis reflects the fossil spreading direction of Philippine Sea plate with minor clockwise rotation due to the collision to Eurasian plate.

The potential for a great earthquake along the southernmost Ryukyu subduction zone

Interseismic GPS data along the Hualien‐Suao coast (NE Taiwan) shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost Ryukyu subduction zone. The measured shortening rate parallel to the Ryukyu Trench is 80 mm/yr, about twice of the shortening rate perpendicular to the Ryukyu Trench. We invert for slip‐deficit rates and the geometric configuration of the plate interface. Our preferred fault model dips 10° northward and extends about 70 km from the Ryukyu Trench to a depth of 13 km. The slip‐deficit rate exhibits a left‐lateral motion of 78 mm/yr and a normal motion of 36 mm/yr on a 290°‐trending fault. The slip rate budget of the southernmost Ryukyu subduction zone is close to the plate convergence rate, suggesting the plate interface is fully locked. Assessments of seismic hazard in this region need to consider the potential threat from Mw 7.5∼8.7 tsunami earthquakes generated by shallow ruptures.

Subducting oceanic high causes compressional faulting in southernmost Ryukyu forearc as revealed by hypocentral determinations of earthquakes and reflection/refraction seismic data

Absolute earthquake hypocenter locations have been determined in the area offshore eastern Taiwan, at the Southernmost Ryukyu subduction zone. Location process is run within a 3D velocity model by combining the Taiwanese and neighboring Japanese networks and using the 3D MAXI technique. The study focuses on the most active seismic cluster in the Taiwan region that occurs in the forearc domain offshore eastern Taiwan. Earthquakes distribute mainly along 2 active planes. The first one aligns along the subduction interface and the second one, shallower affects the overriding margin. Focal mechanisms within the shallow group indicate that nodal planes are either compatible with high-angle back-thrusts or low-angle thrusts. The active seismic deformation exclusively indicates reverse faulting revealing that the forearc basement undergoes trench-perpendicular strong compression. By integrating the seismological image into the regional context, we favor the hypothesis in which the dense seismicity occurring offshore marks the activity of en-échelon high-angle reverse faults accommodating the uplift of a broken piece of Ryukyu Arc basement, called Hoping Basement Rise. The uplift is inferred to be caused by the subduction of an oceanic relief, either exotic block, seamount or oceanic crust sliver. Our favored solution satisfies the narrowness of epicenter's cluster along the Hoping Canyon, and the observation of high-angle active faults on seismic lines crossing the area. Furthermore, this solution is compatible with the active uplift of the Hoping Rise demonstrated from morphological and sedimentological data. We do not exclude the branching of the high-angle reverse faults system onto a splay fault connected with the subduction interface but further investigations are needed to map precisely the 3D distribution of active faults that break the margin.

Inverse-ray imaging from triangulation of zero-offset reflection times

SUMMARY Imaging of subsurfaces based on 3-D inverse rays shooting from coincident sources and receivers along oblique profiles is presented. Two take-off angles of a 3-D inverse ray, one along the seismic profile and another orthogonal to the profile, are determined based on the common direction of inverse rays shooting from three non-collinear sources/receivers. Inverse-ray tracing from the triangulation of sources/receivers to the reflection points provides more objective imaging than does map migration, for which traveltimes along the oblique and/or sparse profiles must be interpolated into rectangular grids. Error analysis of 3-D inverse-ray imaging over a 2-D subduction model, with variation in spacing and orientation of profiles, indicates that the variation of the structural interface can be well resolved when spacing of the traveltime grids is less than the lateral scale of structural variation. All possible triangulations of reflection times along two neighbouring profiles are also implemented in inverse-ray imaging of a SEG/EAEG over-thrust model. The result shows that the coverage of inverse rays from triangulation of all reflection times is much denser than that from triangulation based on the three closest-spaced stations. Furthermore, imaging errors of the interface depth from most of the SEG/EAEG over-thrust model, where the imaging density is higher than 2000 points km−2, are less than 0.1 km even when the profile spacing is relatively large. In this study, migration velocity analysis of 2-D multi-channel seismic data and all possible triangulations of picked reflection times along oblique seismic profiles in the southernmost Ryukyu subduction zone are conducted. Application of 3-D inverse rays to a forearc basin in this subduction zone shows that the sediment thickness near the western end of the Ryukyu forearc region varies strongly with a maximum thickness greater than 6 km. Furthermore, a sediment subsidence greater than 1 km is found in the SE portion of the basin. Slab buckling and basement rise due to the oblique subduction of the Philippine Sea Plate may have resulted in this subsidence and induced strong and shallow earthquakes below the forearc basin.

Inverse-Ray Imaging of a 3D Homogeneous Layer Based on Common-Shot Gathers of Oblique Profiles

Imaging of 3D structural interfaces through reflected rays shooting from common-shot gathers is presented in this paper. First, by fitting the reflected arrivals picked from common-shot gathers, we calculate apparent clips and the shortest distances between sources and reflectors along two profiles. Then, based on the geometry of the profiles and a planar reflector, a unit normal vector of the reflector is determined from the apparent clips and the azimuths of two oblique profiles. A special case, when apparent clips are zero along two parallel profiles, for determining the reflector normal is also investigated. We propose three criteria to ensure the selected travel-times along two profiles resulting from the same planar reflector. These are that firstly, the same shortest distance from sources to the reflector is utilized; and secondly, we want to ensure the same normal of the reflector, and finally, the same ray distance. Prestack inverse-rays developed in this paper are applied to image the bathymetry of the Hoping Basin in the southernmost Ryukyu subduction zone and the fourth layer of the SEG/EAEG over-thrust model. Based on common-shot gathers along seven oblique profiles in the Hoping Basin, most of the reflection points are well imaged through inverse rays except when variation of the interface depth exceeds 300m across profiles with spacing greater than about 20km. Inverse-ray imaging of the over-thrust model also provides good agreement to its fourth interface except that imaging errors of about 1km in depth are found near the thrust faults. Inverse-ray imaging of 3D structures from 2D multi-channel seismic profiles is demonstrated if a pseudo-3D structure (a planar reflector) exists between profiles or if at least two profiles are within a Fresnel zone. Although this technique deals with a single-layered problem currently, it is fundamentally important when we extend it to image inhomogeneous multilayered media.

Crustal structure of the southernmost Ryukyu subduction zone: OBS, MCS and gravity modelling

SUMMARY In 1995, a combined ocean-bottom-seismometric (OBS) and multichannel seismic (MCS) survey with strong air-gun shots was carried out in the southernmost Ryukyu subduction zone. A crustal velocity structure constructed from the layer-stripping Monte Carlo inversion of three OBS/MCS profiles and the associated density models inverted from gravity data in the SW end of the Ryukyu arc-trench system are presented. Parallel to the arc in the southernmost Ryukyu subduction system, the OBS/MCS profiles show sedimentary layers of the Hoping, Nanao and East-Nanao forearc basins from west to east, warping of the arc basement and buckling of the subducted slab beneath the Hoping basement rise. The arc-parallel variation of the crustal structure may result from increasing lateral compression westward due to oblique subduction of the Philippine Sea plate and collision with the Luzon arc near the northwestern edge of the forearc region. Northward subduction and arc-parallel compression of the slab also have generated thrust faulting along the subduction interface and strike-slip faulting within the subducted slab, respectively. On 2002 March 31, an earthquake with a moment magnitude of 6.84 was induced by buckling of the subducted slab and strongly affected cities within an epicentral distance of 100 km. The velocity-interface models, the density models and the focal mechanisms presented in this paper therefore suggest that earthquakes induced by slab buckling or arc-parallel compression have been stronger but less frequent than those generated by northward subduction in the Ryukyu seismogenic zone off Taiwan.

Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling

Abstract The Okinawa Trough is a backarc basin located behind the Ryukyu arc–trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of ∼25–30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by present-day interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon arc with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic arc area, suggesting early arc magmatism in the SPOT area.

Transition between the Okinawa trough backarc extension and the Taiwan collision: New insights on the southernmost Ryukyu subduction zone

Located between the Okinawa trough (OT) backarc basin and the collisional zone in Taiwan, the southernmost Ryukyu subduction zone is investigated. This area, including the southwestern portions of the OT and Ryukyu island arc (RA) and located west of 123.5° E, is named the “Taiwan-Ryukyu fault zone” (TRFZ). West of 123.5° E, the OT displays NNW-SSE structural trends which are different in direction from the ENE-WSW trending pattern of the rest of the OT. Using joint analysis of bathymetric, magnetic, gravity and earthquake data, three major discontinuities, that we interpret as right-lateral strike-slip faults (Faults A, B and C), have been identified. These faults could represent major decouplings in the southern portion of the Ryukyu subduction zone: each decoupling results in a decrease of the horizontal stress on the portion of the RA located on the eastern side of the corresponding fault, which allows the extension of the eastern side of OT to proceed more freely.