Seafloor Geodetic Observation Research Articles

Evidence of viscoelastic deformation following the 2011 Tohoku‐Oki earthquake revealed from seafloor geodetic observation

AbstractThe GPS/acoustic seafloor positioning has detected significant postseismic movements after the 2011 Tohoku‐Oki earthquake (M9.0), just above the source region off the Pacific coast of eastern Japan. In contrast to the coastal Global Navigation Satellite Systems sites where trenchward‐upward movements were reported, the offshore sites above the main rupture zone in the northern part of the source region exhibit landward displacements of tens of centimeters with significant subsidence from almost 3 years of repeated observations. At the sites above around the edge of the main rupture zone, smaller amount of trench‐normal movements was found. Although the terrestrial movements were reasonably interpreted by afterslip beneath the coastal area, these offshore results are rather consistent with effects predicted from viscoelastic relaxation in the upper mantle, providing definitive evidence of its occurrence. On the other hand, the results in the southern part of the source region imply superposition of effects from viscoelastic relaxation and afterslip.

Coseismic slip distribution of the 2011 off the Pacific Coast of Tohoku Earthquake (M9.0) refined by means of seafloor geodetic data

On 11 March 2011, the devastating M9.0 Tohoku Earthquake occurred on the interface of the subducting Pacific plate, and was followed by a huge tsunami that killed about 20,000 people. Several geophysical studies have already suggested that the very shallow portion of the plate interface might have played an important role in producing such a large earthquake and tsunami. However, the sparsity of seafloor observations leads to insufficient spatial resolution of the fault slip on such a shallow plate interface. For this reason, the location and degree of the slip has not yet been estimated accurately enough to assess future seismic risks. Thus, we estimated the coseismic slip distribution based on terrestrial GPS observations and all available seafloor geodetic data that significantly improve the spatial resolution at the shallow portion of the plate interface. The results reveal that an extremely large (greater than 50 m) slip occurred in a small (about 40 km in width and 120 km in length) area near the Japan Trench and generated the huge tsunami. The estimated slip distribution and a comparison of it with the coupling coefficient distribution deduced from the analysis of the small repeating earthquakes suggest that the 2011 Tohoku Earthquake released strain energy that had accumulated over the past 1000 years, probably since the Jogan Earthquake in 869. The accurate assessments of seismic risks on very shallow plate interfaces in subduction zones throughout the world can be obtained by improving the quality and quantity of seafloor geodetic observations.

Interseismic seafloor crustal deformation immediately above the source region of anticipated megathrust earthquake along the Nankai Trough, Japan

We monitored seafloor crustal deformation using the GPS/acoustic seafloor geodetic observation technique at three sites on the Kumano Basin, off southwestern Japan, which is located immediately above the source region of the anticipated Tonankai megathrust earthquake. We directly measured landward crustal movements of ∼40 mm/y in the ∼N80°W direction with respect to the Amurian Plate on the seafloor. The directions were found to be the same as those measured at the on‐land GPS stations. The magnitudes of the velocity vectors indicated significant crustal shortenings of approximately 10–20 mm/y between the Kumano Basin and the southeastern coast of the Kii peninsula of the Japanese Islands. The present observational results show strong and direct evidence for interplate locking during the interseismic period. Coupling ratios were roughly estimated at ∼0.6–0.8 on the plate interface up to at least 10 km in depth.

Plate coupling along the Manila subduction zone between Taiwan and northern Luzon

We use GPS data, trench parallel gravity anomaly (TPGA), and bathymetry to infer plate coupling patterns along the Manila subduction zone. Using a block model and a fault geometry constrained by seismicity, we simultaneously solve for the location of Euler pole and angular velocity between the Sunda and Luzon blocks as well as the slip-deficit rate on plate interface. Our estimates show that the Euler pole between the Sunda and Luzon blocks is situated at southern Palawan near 8.3°N and 119.4°E with the angular velocity of 4.6Myr−1. The estimated convergence rate along the Manila Trench continuously decreases southward from 91mm/yr at the northern tip of Luzon to 55mm/yr north of Mindoro. The inversion of GPS data reveals partially locked fault patches extending from the West Luzon Trough to the east of Scarborough Seamount chain. The slip-deficit rate in this region is in the range of 20–30mm/yr corresponding to a coupling ratio of 0.4. However, the fault slip behavior is not well resolved near the North Luzon Trough. Based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low, we investigate a variety of TPGA-based plate coupling models assuming different scaling between TPGA values and plate coupling ratios. The TPGA-based plate coupling models offer plausible rupture scenarios which are not constrained by current GPS data. The partially locked fault zone near 15–16.5°N may be associated with the subducted Scarborough Seamount wherein oceanic floor is highly fractured. The great subduction zone earthquake propagates beneath the Scarborough Seamount seems to be unlikely. The densification of GPS network in central Luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts.

2011年（平成23年）東北地方太平洋沖地震に伴う地震時および地震後の地殻変動と断層モデル

A great earthquake with a moment magnitude of 9.0 occurred on March 11, 2011, rupturing the plate boundary off the Pacific coast of the northeastern Japan. Large displacements induced by this great earthquake were observed by the GPS Earth Observation Network system (GEONET) over the entire Japanese Islands, and were also detected by the seafloor geodetic observation (SGO) along the Japan Trench. The maximum horizontal displacement observed by the GEONET reaches 5.4 m at the tip of the Oshika Peninsula, and exceeds 30 m at ocean bottom detected by SGO. The subsidence up to 1.1 m was observed by the land GPS along the Pacific coast, and the uplift up to 5 m was detected by the SGO near the epicenter. Based on these geodetic data, the area of coseismic slip is estimated to be ∼450 km long and ∼200 km wide along the Japan Trench with a maximum slip of larger than 50 m near the trench. Because no M≥8.5 earthquake was identified there during the historical period, how strains are accumulated and released is the most essential problem to be solved. The shallower portion of the plate interface has been considered to be decoupled, but such an idea needs to be revised after the Tohoku earthquake. The postseismic deformation exceeds 70 cm for 180 days after the main shock. The coseismic subsidence areas begin to uplift except for the area in the Iwate prefecture. Estimated afterslip exceeds 2.0 m and its area extends to the west, south and north of the coseismic slip area with a moment magnitude of 8.5 for 180 days. The area of the afterslip was extended westward and reaches a depth of approximately 90 km of the subducting plate. Northern and southern edges of the area of afterslip seem to be limited by the source region of the 1994 Sanriku-Haruka-oki earthquake and the north limit of the overriding Philippine Sea plates, respectively. Although the afterslip decayed rapidly with time, the area of afterslip is estimated to be ∼400 km long and ∼180 km wide with a maximum slip of larger than 0.15 m during the period from August 8 to September 7, 2011 that corresponds to a moment magnitude of 7.6. It is definitely important that we continue the observation of the postseismic deformation and carefully monitor the temporal and spatial change of the afterslip for the purpose of the mitigation of hazard due to incoming huge interplate earthquake and understanding the earthquake generation cycle.

Misattributed tsunami 2: The Tōhoku Japan (Mw 9.0) 2011.3.11 earthquake

Like the tsunamis that accompanied the 2010 Maule (Chile) and the 2004 Sumatra-Andaman and 2005 Nias earthquakes, the tsunamis associated with the Tōhoku, Japan (Mw 9.0), earthquake of 11 March 2011 may owe more to slip on superficial faults on the sea floor than to the megathrust itself. The pattern of shallow aftershocks during the 24h immediately following the main shock suggests that the crucial structures include faults in the upper plate, in the accretionary wedge landwards of the trench and on the outer rise. Seafloor geodetic observation could thus usefully contribute routinely to local hazard preparedness.

Research Spotlight: Tectonic plates relocked after 2005 earthquake in northeastern Japan

In August 2005 a magnitude 7.2 earthquake shook northeastern Japan. The quake occurred on the boundary of the North American plate and the Pacific plate off Miyagi prefecture. Earthquakes with a magnitude of about 7.5 have been occurring in this area approximately every 37 years. Sato et al. used seafloor geodetic observations from a combination of Global Positioning System (GPS) and acoustic measurements to detect seafloor movements during and after the earthquake. They focused on the movement of a seafloor reference point located about 10 kilometers east of the epicenter of the 2005 quake. (Geophysical Research Letters, doi:10.1029/2010GL045689, 2011)

Restoration of interplate locking after the 2005 Off-Miyagi Prefecture earthquake, detected by GPS/acoustic seafloor geodetic observation

[1] A 7.2-magnitude earthquake occurred on the plate boundary off Miyagi Prefecture, northeastern Japan, on 16 August 2005. From seafloor geodetic observations using the GPS/acoustic combination technique in this area, we have detected notable seafloor movements associated with, and subsequent to this event. The time series of horizontal coordinates obtained at the seafloor reference point MYGW, located about 10 km east of the epicenter, indicates that the strain accumulated for the interseismic period was partly released by the event and a constant strain accumulation restarted after 1–2 years of the post-seismic period with an erratic movement. The horizontal velocity of MYGW after the restart above is estimated to be about 5.7 cm/year toward WNW relative to the Eurasian plate. Our result is consistent with those derived from terrestrial GPS measurements and implies that the interplate locking was restored in the rupture area of the 2005 event around 2007.

Contributions to the Marine Geodesy with a Special Focus on the Development of Seafloor Geodetic Observation Technique

Contributions to the Marine Geodesy with a Special Focus on the Development of Seafloor Geodetic Observation Technique

Weak interplate coupling beneath the subduction zone off Fukushima, NE Japan, inferred from GPS/acoustic seafloor geodetic observation

Abstract We have been carrying out GPS/acoustic seafloor geodetic observations at several reference points situated along the Japan Trench, a major plate boundary of subduction. A time series of horizontal coordinates of one of the seafloor reference points, located off Fukushima, obtained from seven campaign observations for the period 2002-2008, exhibits a linear trend with a scattering root mean square of about 3 cm. A linear fit to the time series gives an intraplate crustal movement velocity of 3.1 cm per year in a westerly direction, which is significantly smaller than that at the other seafloor reference point 120 km away along the trench axis. This result implies weak interplate coupling in this region.

Development of New Seafloor Geodetic Observation System Based on AUV Technology

Development of New Seafloor Geodetic Observation System Based on AUV Technology

Undersea co-seismic crustal movements associated with the 2005 Off Miyagi Prefecture Earthquake detected by GPS/acoustic seafloor geodetic observation

Abstract We have been carrying out seafloor geodetic observations at two reference points situated off Miyagi Prefecture, northeastern Japan, using the GPS/Acoustic combination technique. Comparison of position estimates before and after the 2005 Off-Miyagi Prefecture Earthquake revealed a co-seismic crustal movement as large as 10 cm eastward at the site approximately 10 km from the epicenter, while no prominent movement was found at another site located 60 km away from the epicenter. The results at these two sites are consistent with crustal deformation calculated from the rectangular dislocation model on the fault derived from crustal movements observed at GEONET stations on land.

GPS/Acoustic seafloor geodetic observation: method of data analysis and its application

Abstract We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven campaign observations for the period 2002–2005 exhibits a linear trend with a scattering rms of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.

Combined GPS/Acoustic Seafloor Geodetic Observation System by Japan Coast Guard

Combined GPS/Acoustic Seafloor Geodetic Observation System by Japan Coast Guard

An application of numerical simulation techniques to improve the resolution of offshore fault kinematics using seafloor geodetic methods

Geodetic measurements reveal a number of tectonic phenomena, such as coseismic and postseismic displacements of earthquakes and interplate coupling on plate interfaces. However, since geodetic measurements are limited to land, slip distribution is poorly resolved offshore, though well constrained in the landward areas. Due to the poverty of offshore data, tectonic motion near trench axes has not been measured. Seafloor geodetic observations provide important information on offshore tectonics. Improved offshore resolution would allow determination of strain accumulation and release processes near trench axes. In this study, using numerical simulation, we discuss the potential for improvement of slip resolution in an offshore area using seafloor geodetic measurements. The plate interface along the Nankai trough is modeled by 36 planar fault segments, whose length and width, respectively, are set to 60 km and 50 km. Three hundred and seventy-five GPS observation sites on land and 10 seafloor sites aligned 60 km off the coast are used for the simulation. We carry out a checkerboard test and compare the estimated slip pattern with the given checkerboard pattern. Models that do not include seafloor sites generate large discrepancies in offshore deformation between the initial and estimated slip patterns, although there are similarities in coastal regions. This indicates poor resolution in offshore areas. When we apply our model to include seafloor sites, the difference between the initial and estimated slip patterns decreases for most of the modeled fault segments. Comparison between these two cases suggests the potential for use of seafloor geodetic techniques to improve offshore resolution.