Ryoya Ikuta

Variations of fluid pressure within the subducting oceanic crust and slow earthquakes

[1] We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes.

A subsurface structure change associated with the eruptive activity at Sakurajima Volcano, Japan, inferred from an accurately controlled source

Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We deconvolved 400 s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2–6 s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.

Interplate locking condition derived from seafloor geodetic data at the northernmost part of the Suruga Trough, Japan

We observed seafloor crustal deformation at two observation sites on opposite sides of the Suruga Trough off Japan from 2005 to 2011 to investigate the interplate locking condition at the source region of the anticipated great subduction earthquake, named Tokai earthquake. We estimated the displacement velocity vectors relative to the Amurian Plate on the basis of repeated observations. Our results at the two points, Suruga northeast and Suruga northwest (SNW) were 42 ± 8 mm/yr toward N94 ± 3°W and 39 ± 11 mm/yr toward N84 ± 9°W, respectively. These directions are the same as those measured at on-land GPS stations. The magnitudes of the velocity vectors indicate a significant shortening of approximately 4 mm/yr between SNW and on-land GPS stations located to the west of the Suruga Trough. The results show that the plate interface is strongly locked (no slip) shallower than the source region of the anticipated Tokai earthquake.

Using molluscan assemblages from paleotsunami deposits to evaluate the influence of topography on the magnitude of late Holocene mega-tsunamis on Ishigaki Island, Japan

AbstractFour ancient tsunami deposits were identified in a trench excavated on Ishigaki Island, Okinawa, Japan. Three of the tsunami deposits (T-I, T-II, and T-IV) consist of calcareous sand beds, whereas the other (T-III, located stratigraphically between T-II and T-IV) consists of boulders. Deposit T-I was caused by a tsunami in 1771. 14C dating, together with the elevations of the landward margins of these sandy tsunami deposits, suggests that tsunamis II and IV were similar in size to the 1771 tsunami, although the influence of local topographic features on the magnitudes of tsunamis has not yet been examined. This study reconstructs the local topographic features by comparing the molluscan assemblages incorporated within the tsunami deposits with those in recent beach deposits. The presence of species that inhabit the intertidal zone in lagoonal settings in all the assemblages indicates that the present-day shallow lagoon has been present off the study area since the occurrence of tsunami T-IV, which supports the previous hypothesis that the magnitudes of the 1771 tsunami and tsunamis II and IV were similar. These molluscan assemblages also suggest that a high relative abundance of large, heavy mollusc shells is a feature of the paleotsunami deposits in the coastal lowlands found along the shallow coral lagoons.

Secular and coseismic changes in S-wave velocity detected using ACROSS in the Tokai region

We discovered a secular change in the travel time of direct S-waves over a 10-year observation period by means of continuous operation of an artificial and stable seismic source, called Accurately Controlled Routinely Operated Signal System (ACROSS), which is deployed in the central part of Japan along the Nankai Trough. We used 13 High Sensitivity Seismograph Network Japan (Hi-net) stations around the ACROSS source to monitor the temporal variation in travel time. Green’s functions were calculated for each station daily from March 29, 2007, through October 30, 2017. Secular advance in the temporal variation in travel time was seen for the whole operation period, in addition to a steplike delay associated with the 2011 Tohoku earthquake. We estimated the rate of secular change and the amount of coseismic step by modeling the transfer function of S-waves with a linear trend and the coseismic step of the 2011 Tohoku earthquake. Distance dependences of the travel time changes can be explained as a combination of common bias and dispersion for each station, for both the secular and coseismic changes. This can be interpreted as a randomly distributed change in seismic velocity over the range of the observation region. An azimuthal dependence exists for both changes and shows larger changes in the NE–SW direction than in the NW–SE direction from the ACROSS source.

Development of Simulation Algorithm for Analyzing Temporal Stability of ACROSS Seismic Source

Abstract Temporal stability of the ACROSS seismic source is a very important feature for active geophysical monitoring to detect small changes related to the target region. The stability of the source depends on the shallow ground properties. Therefore, it is necessary to estimate the effects of the temporal changes of the shallow ground properties on the source’s motion and the excited waves. Then we try to estimate its effects by numerical simulation. In this paper, first, we review two examples of the temporal changes observed in the experiments and discuss the required properties of the simulation algorithm. According to the discussion, we develop a simulation algorithm for calculating the source’s rigid motion and its generated waves. In this algorithm, the ACROSS seismic source and the ground are modeled as a horizontally forced rigid circular disk and a multilayered media, respectively. The solution of displacement excited by a horizontally forced rigid circular disk in a multilayered media is derived by applying the explicit Green’s function approach to the solution of a horizontal point force. We also carry out simple numerical simulation to investigate the effects of the shallow ground properties on the excited waves and the source’s rigid motion in a simple case, using the program developed in this research.

Temporal Change in Reflected Waves from Deep Crust Possibly Synchronized with the Activity of DLFTs Detected Using ACROSS

ACROSS stands for Accurately Controlled, Routinely Operated Signal System. Repeated transmission of a controlled seismic signal and long-term continuous observation of the seismic waves enable precise monitoring of time-evolving physical properties of the Earth’s interior. An observation using the ACROSS source was conducted in the Tokai distinct, Japan to establish a method to monitor the state of coupling of the plate boundary and to understand the time-evolving phenomena such as slow-slip and deep low-frequency tremors (DLFTs). A seismic signal emitted from the ACROSS source was observed by a short-span surface seismometer array and a Hi-net station (N.HOUH) located at the distance of 57 km from the source for 10 months. The transfer function clearly shows the P-wave, S-wave and their later-phases. Coherent later phases were detected using the semblance analysis of the array of transfer functions. They were interpreted as the reflections from the Moho and the plate boundary. The temporal changes of these phases were examined using Hi-net data which were free from environmental effects. The temporal changes of these phases show correlation with the episodic activity of the DLFTs in 2005. This implies the possibility of seismic monitoring of seismogenic plate-subduction zone using ACROSS.