Shigeru Nakao

Initial results from WING, the continuous GPS network in the western Pacific area

To investigate tectonic deformation in the western Pacific, a continuous GPS tracking network has been established, and named the Western Pacific Integrated Network of GPS (WING). Between 1995 and March 1997 we establised ten new sites. Data for the period July 1995 to October 1996 were analyzed, together with data from International GPS Service for Geodynamics (IGS) global sites, to estimate daily coordinates. A fiducial-free approach was used to obtain the most accurate baseline estimates. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS site is assumed to be moving westward relative to the stable Eurasian continent at ∼2 cm/yr according to Hekis [1996] estimate. We find that: (1) velocities of sites well within oceanic plates are in good agreement with rigid plate motion models; (2) sites close to plate boundaries are all affected by the deformation at those boundaries, among which back-arc rifting (spreading) is clearly visible at the Mariana and Okinawa troughs; (3) sites in eastern Asia are moving east to east-southeast relative to the stable Eurasian continent, suggesting long distance effects of the northward collision of India with Asia.

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region. However, the σ3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

Estimation and correction for the effect of sound velocity variation on GPS/Acoustic seafloor positioning : An experiment off Hawaii Island

A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5—4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4—7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.

Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption—Charging, discharging and recharging process inferred from GPS measurements

Using GPS data, we evaluate the volume change of the magma reservoir associated with the eruption of Kirishima Shinmoe-dake volcano, southern Kyushu, Japan, in 2011. Because ground deformation around Shinmoe-dake volcano is strongly affected not only by regional tectonic movement but also by inflation of Sakurajima volcano located approximately 30–40 km to the southwest, we first eliminate these unwanted contributions from the observed data to extract the signals from Shinmoe-dake volcano. Then, we estimate the source locations and volume change before, during, and after the highest eruptive activity occurring between January 26 and 31. Our model shows that the magma began to accumulate about one year prior to the sub-Plinian eruption, with approximately 65% of the accumulated magma being discharged during the peak of the eruptive activity, and that magma accumulation continued until the end of November 2011. An error analysis shows that the sources during the three periods indicated above are located in almost the same position: 5 km to the northwest of the summit at a depth of 8 km. The 95% confidence interval of the estimated source depth is from 7.5 to 13.7 km.

Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations

On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.

GPS observation of the first month of postseismic crustal deformation associated with the 2003 Tokachi-oki earthquake (MJMA 8.0), off southeastern Hokkaido, Japan

To investigate the postseismic crustal deformation associated with the Tokachi-oki earthquake (MJMA = 8.0) of 26 September 2003 in Japan Standard Time (JST), off southeastern Hokkaido, Japan, we newly established thirty GPS sites just after the mainshock in the eastern part of Hokkaido. Rapid data analysis for one month after the mainshock clearly indicated postseismic displacements only in the horizontal components. Observed maximum horizontal displacement was 6.6 cm from 28 September to 24 October, 2003. Absence of the vertical suggests that afterslip occurred in and around the coseismic fault rather than at downdip extension. Time series of coordinates are characterized by logarithmic decay functions with 4–11 days relaxation times. This suggests that postseismic deformation was due to afterslip on the fault following the large earthquake.

Postseismic deformation following the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) derived by GPS observation

We constructed a temporary GPS network around the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) in order to investigate the characteristics of its postseismic deformation. Our GPS network data, as well as the GEONET data, were analyzed using Bernese GPS software. We detected notable postseismic deformation in horizontal components close to the fault plane. The observed maximum displacement was 5.6 cm at the GNKI site on Genkaijima Island. A logarithmic law was adapted to the coordinate time series data, revealing decay times from 2 to 23 days, similar to those obtained for the 2003 Tokachi-oki Earthquake (M8.0). The amount of postseismic slips on the fault was assessed using the coseismic fault model proposed by the Geographical Survey Institute (GSI). We derived an optimum fault model of postseismic slip on the shallow (less than 3 km depth) portion of the fault. Our findings indicate that postseismic slip occurred only in shallow parts of the coseismic fault.

Unexpectedly frequent occurrence of very small repeating earthquakes (–5.1 ≤ MW ≤ –3.6) in a South African gold mine: implications for monitoring intraplate faults

We observed very small repeating earthquakes with −5.1 ≤ Mw ≤ −3.6 on a geological fault at 1 km depth in a gold mine in South Africa. Of the 851 acoustic emissions that occurred on the fault during the 2 month analysis period, 45% were identified as repeaters on the basis of waveform similarity and relative locations. They occurred steadily at the same location with similar magnitudes, analogous to repeaters at plate boundaries, suggesting that they are repeat ruptures of the same asperity loaded by the surrounding aseismic slip (background creep). Application of the Nadeau and Johnson (1998) empirical formula (NJ formula), which relates the amount of background creep and repeater activity and is well established for plate boundary faults, to the present case yielded an impossibly large estimate of the background creep. This means that the presently studied repeaters were produced more efficiently, for a given amount of background creep, than expected from the NJ formula. When combined with an independently estimated average stress drop of 16 MPa, which is not particularly high, it suggests that the small asperities of the presently studied repeaters had a high seismic coupling (almost unity), in contrast to one physical interpretation of the plate boundary repeaters. The productivity of such repeaters, per unit background creep, is expected to increase strongly as smaller repeaters are considered (∝ Mo −1/3 as opposed to Mo −1/6 of the NJ formula), which may be usable to estimate very slow creep that may occur on intraplate faults.

Dynamic Strain in a South African Gold Mine Produced by the 2011 Tohoku Earthquake

The 2011 Tohoku earthquake (2011/03/11 05:46:18 UT, M W 9.0) produced huge permanent displacements of up to 50 m and dynamic strain of up to 10−5 in the near-field. We observed dynamic strain (10−7) produced by this earthquake at a depth of 1 km in a gold mine in the Republic of South Africa at a distance of more than 14,000 km (125.25∘) from epicenter. The dynamic strain was observed by two Ishii-type borehole strainmeters about 30 m apart on opposite sides of a fault filled with gouge of several decimeters thickness, allowing the response of the fault to be investigated. The Tohoku earthquake seismic waves passed through the gold mine outside blasting hours, allowing us to analyze the tele-seismic body waves and the surface waves that circled the globe several times (R3 − R5 and G3 − G5, \(\sim 10^{-8}\)). We discuss the fault deformation associated with the dynamic strains by the wave packets. The seismograms of some sub-parallel components of the two strainmeters installed on the opposite sides of the fault appeared similar, although with differences on the order of \(\sim 10^{-8}\). These differences may imply a complex response of the fault (fault rheology; i.e. not only elastic, but perhaps plastic or even indicating the effects of fluid).