Keiichi Tadokoro

Induced earthquakes accompanying the water injection experiment at the Nojima fault zone, Japan: Seismicity and its migration

The 1995 Hyogo-ken Nanbu (Kobe) earthquake of M7.2 occurred on January 17, 1995. After the earthquake, a scientific drilling program called the Nojima Fault Zone Probe was carried out at the Nojima fault which ruptured during the mainshock. Water was injected during two periods, February 9–13 and March 16–25, 1997. The pumping pressure at the surface was about 4 MPa. Pressurized water was injected into a 1800-m-deep borehole and supplied to the surrounding rock at depths between 1480 and 1670 m. The total amount of injected water was 258 m3. An increase in earthquake activity was observed 4 or 5 days after the beginning of each water injection. The seismicity increased in the region around 3 or 4 km from the injection point. This suggests that these earthquakes are likely to have been induced by the water injections. The induced earthquakes were located between 2 and 4 km in depth and had magnitudes ranging from −2 to +1. The hypocenters of the induced earthquakes migrated with speeds of ∼2–40 m/h. The speeds decreased with time, suggesting a relationship with the diffusion of water. Values of intrinsic permeability of 10−14–10−15 m2 were obtained from the time dependency of induced seismicity change. The coefficient of friction in the area where the induced earthquakes occurred was estimated to be less than 0.3. Twenty earthquake clusters were found, and cross-spectrum analysis was applied to them. We could distinguish between the induced and noninduced earthquakes from the analysis. The induced earthquakes forming each cluster migrated with speeds of 20–80 m/h, which means microscale water migration or permeation. The present water injection experiment revealed that the Nojima fault zone was highly permeable and could slip with small (∼10% or less) increases in pore fluid pressure or shear stress.

S wave splitting in the aftershock region of the 1995 Hyogo-ken Nanbu earthquake

The 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake; M7.2) occurred on January 17, 1995. A temporary aftershock observation was carried out by the Geophysical Research Group Organized by Universities for Prediction Seismology in 1995 from October 1995 to January 1996. A dense seismic network was deployed in and around the aftershock region, and some seismic stations were set directly on the active fault traces. We analyze the S wave splitting in and around the aftershock region of the Hyogo-ken Nanbu earthquake. We detect a spatial variation of S wave splitting which can be related to crack orientations in an area where a large earthquake has occurred. At stations more than 1 km away from the Hyogo-ken Nanbu earthquake fault zone, the leading shear wave polarization directions (LSPD) are found to be parallel to the axis of the regional maximum horizontal compressional stress (E-W), suggesting that cracks caused by regional tectonic stress exist in the area. However, at stations within 500 m of the earthquake fault zones, the LSPDs are parallel to the fault strikes (N45°–50°E). This suggests that new fractures were produced parallel to the faults by shear faulting of the Hyogo-ken Nanbu earthquake and resulted in S wave splitting along the earthquake fault zone. Nevertheless, shear fault origin anisotropy was not evident at stations on the active faults which ruptured in 1596. This implies that healing processes have already closed the fractures produced by shear faulting in 1596. Therefore we can expect that S wave splitting can be a useful tool for monitoring healing processes of active faults.

Is the Ryukyu subduction zone in Japan coupled or decoupled? —The necessity of seafloor crustal deformation observation

The 2004 Sumatra-Andaman earthquake of Mw 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake (Mw>8) could potentially occur there in the future.

Error evaluation in acoustic positioning of a single transponder for seafloor crustal deformation measurements

The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

Precise, three-dimensional seafloor geodetic deformation measurements using difference techniques

AbtractCrustal deformation on land can now be measured and monitored routinely and precisely using space geodetic techniques. The same is not true of the seafloor, which covers about 70 percent of the earth surface, and is critical in terms of plate tectonics, submarine volcanism, and earthquake mechanisms of plate boundary types. We develop new data processing strategies for quantifying crustal deformation at the ocean floor: single- and double-difference methods. Theoretically, the single difference method can eliminate systematic errors of long period, while the double difference method is able to almost completely eliminate all depth-dependent and spatialdependent systematic errors. The simulations have shown that the transponders on the seafloor and thus the deformation of the seafloor can be determined with the accuracy of one centimeter in the single point positioning mode. Since almost all systematic errors (of temporal or spatial nature) have been removed by the double difference operator, the double difference method has been simulated to be capable of determining the threedimensional, relative position between two transponders on the seafloor even at the accuracy of sub-centimeters by employing and accumulating small changes in geometry over time. While the surveying strategy employed by the Scripps Institution of Oceanography (SIO) requires the ship maintain station, our technique requires the ship to move freely. The SIO approach requires a seafloor array of at least three transponders and that the relative positions of the transponders be pre-determined. Our approach directly positions a single transponder or relative positions of transponders, and thus measures deformation unambiguously.

Monitoring of fault healing after the 1999 Kocaeli, Turkey, earthquake

The North Anatolian fault zone that ruptured during the mainshock of theM 7.4 Kocaeli (Izmit) earthquake of 17 August 1999 has beenmonitored using S wave splitting, in order to test a hypothesisproposed by Tadokoro et al. (1999). This idea is based on the observationof the M 7.2 1995 Hyogo-ken Nanbu (Kobe) earthquake, Japan.After the Hyogo-ken Nanbu earthquake, a temporal change was detectedin the direction of faster shear wave polarization in 2–3 years after the mainshock (Tadokoro, 1999). Four seismic stations were installed within andnear the fault zone at Kizanlik where the fault offset was 1.5 m, about80 km to the east of the epicenter of the Kocaeli earthquake. Theobservation period was from August 30 to October 27, 1999. Preliminaryresult shows that the average directions of faster shear wave polarization attwo stations were roughly parallel to the fault strike. We expect that thedirection of faster shear wave polarization will change to the same directionas the regional tectonic stress reflecting fault healing process. We havealready carried out a repeated aftershock observation at the same site in2000 for monitoring the fault healing process.

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.

Recent Developments of GPS Tsunami Meter foraFar Offshore Observations

A new tsunami observation system using GPS buoys has been developed, which employs the RTK-GPS technique to detect and monitor tsunamis in real-time before they reach the coast. A series of experimental GPS buoys succeeded in detecting three tsunamis with amplitudes of about 10 cm. Following this success, since 2007, the Japanese government has established GPS buoy systems for monitoring sea waves at 19 sites around the Japanese coast. These systems succeeded to detect 11th March 2011 Tohoku-Oki earthquake tsunami. Through these experiences, we recognized two problems that need to be solved in order to deploy buoys at farther distances from the coast: one is that positioning accuracy decreases as the distance increases and the data transmission by radio becomes difficult for a long distance. In order to overcome these difficulties, first, a new algorithm of PPP-AR for real-time application was employed. The test analysis showed that the positioning accuracy may attain a few centimeters even if the reference GPS network that generates precise orbits and clocks is farther than 1,000 km. Then, a satellite communication system was experimentally used to send data in both directions between the land base and buoy. The data that was obtained on the buoy was transmitted to the land base and was shown on a webpage in real-time, successfully. This kind of system may have further applications of earth science; for example, we are trying to implement GPS/acoustic apparatus for continuous monitoring of ocean floor crustal deformations.

Anomalous seismograms generated by an intermediate‐depth earthquake: Unusual scattering sources in the upper mantle of central Japan

[1] Strong scattering sources were detected in the upper mantle beneath central Japan from seismic data recorded by the high-sensitivity seismograph network (Hi-net). Anomalous seismograms, consisting of many strong later phases whose amplitudes did not decay significantly with time, were observed from an intermediate-depth earthquake. A comparison of those anomalous seismograms with others generated by other deep earthquakes shows that major scattering sources may exist predominately in the upper mantle, not in the crust. Three possible candidates have been considered as the sources of scattering in the upper mantle including magma conduits, the previous collision zone and slab melting. Although seismic waves may be scattered strongly from each potential candidate, a region associate with slab melting might be more suitable for generating the anomalous seismogram due to the existence of abundant melting spots within the subducted Philippine Sea slab that is extremely young and hot beneath central Japan. INDEX TERMS: 0669 Electromagnetics: Scattering and diffraction; 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology:Seismicityandseismotectonics.Citation: Lin, C.-H., M. Ando, N. Fujii, K. Yamaoka, K. Tadokoro, A.-S. Jin, K. Obara, and M. Ishida, Anomalous seismograms generated by an intermediate-depth earthquake: Unusual scattering sources in the upper mantle of central Japan, Geophys. Res. Lett., 30(11), 1586, doi:10.1029/2002GL016837, 2003.

Strain partitioning and interplate coupling along the northern margin of the Philippine Sea plate, estimated from Global Navigation Satellite System and Global Positioning System-Acoustic data

Southwest Japan is located in the subduction margin between the continental Amurian and oceanic Philippine Sea plates. Recent land GNSS (Global Navigation Satellite System) and offshore Global Positioning SystemAcoustic geodetic measurements were used to clarify the deformation in and around these plate margins. We examined strain partitioning and interplate coupling using a block modeling approach on the observed velocities. Although the main plate boundaries are the Nankai Trough and Sagami trough, our results suggest that one-third of the relative plate motion between the two plates is accommodated by several block boundaries in the southeastern margin of the Amurian plate. The most active boundaries, with a slip rate of ≥8 mm/yr, cross southwest Japan from the Okinawa Trough through the Median Tectonic Line and Niigata Kobe tectonic zone, to the eastern margin of the Japan Sea. A subparallel boundary with a slip rate of 4–5 mm/yr is along the coastline of Japan. These two boundaries have a right-lateral shear motion that accommodates part of the interplate motion, with a boundary across the Korean Peninsula and Japan Sea. The slip partitioning results in an eastward decrease of relative block motion from 78 to 4 mm/yr along the Nankai Trough and Suruga trough. Interplate coupling is moderate to strong at 10–25 km depth along the Nankai Trough, but it is lower at ~132°E, ~136°E, and ~137°E than in the surrounding regions, corresponding to the segment boundaries of past megathrust earthquakes, suggesting that regions of insufficient strain accumulation act as a barrier for earthquake rupture. INTRODUCTION Southwest Japan is situated in a subduction zone, where the northern margin of the Philippine Sea plate subducts beneath the southeastern margin of the continental Amurian plate. Large M ≥ 8 megathrust earthquakes have ruptured shallow parts of the plate interface with a recurrence interval of 100–200 yr along the Nankai Trough, the main boundary between these plates (Ando, 1975; Ishibashi, 2004). Many slow earthquakes, including slow slip events (SSEs), low-frequency tremors, and very low frequency earthquakes (VLFEs), also occur around the rupture area of the megathrust earthquakes on the plate interface (cf. Obara, 2010). However, previous studies suggest that the relative plate motion between the Amurian and Philippine Sea plates is not accommodated by a single boundary. Numerous active faults and high seismicity, including M ~7 earthquakes on the islands of Japan, indicate strain partitioning in the continental margin (e.g., Shen-Tu et al., 1995). The most famous feature of the strain partitioning is the fault system of the Median Tectonic Line (MTL), which accommodates part of the margin-parallel component of the oblique relative motion between the Amurian and Philippine Sea plates (i.e., Fitch, 1972). Its right-lateral strike-slip motion was proposed based on geological studies (Kanaori, 1990; Tsutsumi et al., 1991), and has been confirmed by geodetic observations (Miyazaki and Heki, 2001; Tabei et al., 2003). Another dextral strike-slip fault system subparallel to the MTL has been proposed 400 km from the Nankai Trough (Gutscher and Lallemand, 1999; Gutscher, 2001). Contemporary deformation in southwest Japan has been monitored by dense, continuous GNSS (Global Navigation Satellite System) networks since the mid-1990s. The largest network is the GEONET (GNSS Earth Observation Network System), operated by the Geospatial Information Authority of Japan (Sagiya, 2004). The GEONET showed that high strain rates are found not only GEOSPHERE GEOSPHERE; v. 14, no. 2 doi:10.1130/GES01529.1 11 figures; 3 tables; 1 supplemental file CORRESPONDENCE: nishimura .takuya .4s@ kyoto -u .ac.jp CITATION: Nishimura, T., Yokota, Y., Tadokoro, K., and Ochi, T., 2018, Strain partitioning and interplate coupling along the northern margin of the Philippine Sea plate, estimated from Global Navigation Satellite System and Global Positioning System-Acoustic data: Geosphere, v. 14, no. 2, p. 535–551, doi: 10 .1130 /GES01529.1. Science Editor: Shanaka de Silva Guest Associate Editor: Laura M. Wallace Received 15 March 2017 Revision received 16 November 2017 Accepted 19 January 2018 Published online 16 February 2018