Takeshi Sagiya

Continuous GPS Array and Present-day Crustal Deformation of Japan

A GPS array with about 1,000 permanent stations is under operation in Japan. The GPS array revealed coseismic deformations associated with large earthquakes and ongoing secular deformation in the Japanese islands. Based on daily coordinate data of the GPS stations, strain rate distribution is estimated. Most regions with a large strain rate are related to plate boundaries and active volcanoes. In addition, the Niigata-Kobe Tectonic Zone (NKTZ) is recognized as a region of large strain rate along the Japan Sea coast and in the northern Chubu and Kinki districts. This newly found tectonic zone may be related to a hypothetical boundary between the Eurasian (or Amurian) and the Okhotsk (or North America) plates. Precise observation of crustal deformation provides important boundary conditions on numerical modeling of earthquakes and other crustal activities. Appropriate computation methods of continuous deformation field are directly applicable to data assimilation for such numerical simulations.

Evidence from the ad 2000 Izu islands earthquake swarm that stressing rate governs seismicity

Magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat. Such ‘swarm’ seismicity also migrates with time, and often exhibits a ‘dog-bone’-shaped distribution. The largest earthquakes in swarms produce aftershocks that obey an Omori-type (exponential) temporal decay, but the duration of the aftershock sequences is drastically reduced, relative to normal earthquake activity. Here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion. A 1,000-fold increase in seismicity rate and a 1,000-fold decrease in aftershock duration occurred during the two-month-long dyke intrusion. We find that the seismicity rate is proportional to the calculated stressing rate, and that the duration of aftershock sequences is inversely proportional to the stressing rate. This behaviour is in accord with a laboratory-based rate/state constitutive law, suggesting an explanation for the occurrence of earthquake swarms. Any sustained increase in stressing rate—whether due to an intrusion, extrusion or creep event—should produce such seismological behaviour.

Coseismic slip resolution along a plate boundary megathrust: The Nankai Trough, southwest Japan

Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (Mw=8.1) and 1946 Nankaido (Mw=8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span ∼50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post-1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satakes [1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high-slip regions, one off southeastern Shikoku Island (11 m maximum) and the other offshore of Kii Peninsula (3 m maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much longer than the average repeat time of ∼120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on trough-parallel splay faults could significantly decrease the maximum slip to about 6 m.

A decade of GEONET : 1994-2003 : The continuous GPS observation in Japan and its impact on earthquake studies

The dense continuous GPS network of Japan, now called GEONET, has been operated since 1994 by the Geographical Survey Institute. GEONET provides precise daily coordinates of all the stations, with which displacement rates and strain rates are calculated nationwide. Various characteristics of tectonic deformation in the Japanese Is-lands have been revealed. GEONET is also quite useful in earthquake studies, precisely detecting co-seismic, post-seismic, and inter-seismic deformation signals. These observations are utilized to infer physical processes at earthquake sources. Slow slip events on plate boundaries have been found from GPS data. Such slow events provide an important constraint on the mechanism of faulting. On the other hand, there has been no success in detecting pre-seismic deformation. Lack of a precursory signal before the 2003 Tokachi-Oki (M8.0) earthquake has posed a serious question to short-term earthquake prediction. GEONET enables a good linkage between monitoring and modeling studies, opening a possibility of practical data assimilation. For further contribution to earthquake studies, it is necessary to continue GEONET with high traceability on the details in observation and analysis.

Interplate coupling in the Tokai District, central Japan, deduced from continuous GPS data

I invert GPS displacement rates in the Tokai district to investigate strain accumulation associated with the anticipated Tokai earthquake. I apply a representation of interseismic strain accumulation at subduction zones by Savage [1983] and estimate a distribution of virtual normal fault slip (back-slip) on the plate boundary surface. Estimated back-slip takes its maximum value of 30–40mm/year under the Sea of Enshu. This value is nearly equal to the relative plate motion there, implying almost complete coupling. Back-slip is smaller under Suruga Bay, and its direction is different from the relative motion between the Eurasian and the Philippine Sea plates. Localized deformation at the northern tip of the Philippine Sea plate may be responsible for this discrepancy, partitioning tectonic strain due to the plate convergence into faults on both sides of the Izu peninsula and also distributed strain inside the peninsula.

Crustal deformation caused by magma migration in the northern Izu Islands, Japan

Intense crustal activity including earthquake swarms, eruptions, and a caldera formation in the northern Izu Islands started on June 26, 2000, accompanied with large crustal deformation. Permanent GPS data reveals the spatial pattern and time evolution of ground deformation. The observations reveal shrinking and subsidence of Miyakejima and extension between Kouzushima and Niijima. We constructed a source model to explain the observed displacements during the period between June 26 and the end of August. The model consists of a deflation source (0.12km³) beneath Miyakejima, tensile faults (1.04km³) located between Miyakejima and Kouzushima, and several shear faults. Mass balance considerations suggest that a large amount of magma migrated 30km from Miyakejima toward Kouzushima.

Water-weakened lower crust and its role in the concentrated deformation in the Japanese Islands

Abstract The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata–Kobe tectonic zone) was investigated by carefully analyzing the GPS data and qualitatively modeling the lower crust in NKTZ. It was concluded that this deformation zone is not a plate boundary between the Amurian plate (AMU) and the North America plate but is rather an internal deformation zone near the eastern margin of AMU. The data previously obtained on the conductivity anomalies in the lower crust and the 3 He/ 4 He ratios suggest that the concentrated deformation in NKTZ results from the lower crust in NKTZ being weakened by a high water content. The high water content is thought to result from the dehydration of subducting slabs. NKTZ has a higher water content in the lower crust than other regions do because there is no Philippine Sea plate (PHS) seismic slab beneath NKTZ. In other regions, it is estimated that the mantle wedge above the seismic Philippine Sea slab prevents the water dehydrated from the slab from rising to the lower crust, and that the lithosphere within PHS itself prevents the water dehydrated from the Pacific plate from rising up through it.

Coseismic crustal deformation from the 1994 Hokkaido‐Toho‐Oki Earthquake Monitored by a nationwide continuous GPS array in Japan

Using a brand-new nationwide continuous GPS array, we monitored coseismic displacements from the October 4, 1994 Hokkaido-Toho-Oki (Kurile islands) earthquake (MJMA=8.1). Based on 2-week time series of site coordinates of 21 GPS stations, we present a coseismic deformation field of whole Hokkaido within 1 cm precision. For example, the station at Nemuro, 170 km west of the epicenter, moved 44 cm to the east and subsided 10 cm. Even stations in southern Hokkaido, 600 km apart from the epicenter, moved a few cm horizontally toward the epicenter. We compare the GPS result with displacements calculated from two preliminary models, assuming fault planes parallel or perpendicular to the Kurile trench. Since the observed were far-field displacements on land, both models can explain the GPS result.

Reply to comment from Blanco et al. (2015) on “Evidence from acoustic imaging for submarine volcanic activity in 2012 off the west coast of El Hierro (Canary Islands, Spain) by Pérez et al. [Bull. Volcanol. (2014), 76:882–896]

We begin by noting our appreciation for the comment from Blanco et al. (2015) on BEvidence from acoustic imaging for submarine volcanic activity in June 2012 off the west coast of El Hierro (Canary Islands, Spain)^ by Pérez et al. (2014) because it provides the opportunity to maintain an open scientific debate on this issue within the right framework. This is especially important because one of the co-authors of the comment from Blanco et al. (2015) had previously made a suggestion to us that we should not send the acoustic imaging data taken on June 28, 2012, for publication. In our opinion, this recommendation was detrimental to open scientific debate, which is always tremendously beneficial for the development of science. Secondly, the comment from Blanco et al. (2015) suggests that readers may have been confused; we emphasize that the submarine volcanic activity in 2012 off the west coast of El Hierro described by Pérez et al. (2014) was not, as inferred by Blanco et al. (2015), a volcanic eruption. It has been well documented (e.g., Italiano and Nuccio 1991; Caracausi et al. 2005; García et al. 2006; Pérez and Hernández 2007) that new and/or sporadic volcanic activities, such as relatively weak or significant visible degassing processes during volcanic unrest, have commonly occurred both in subaerial and submarine environments of volcanic systems. Such activity includes things that are not a volcanic eruption, which implies release of juvenile volcanic material and not just the sudden release of steam/gas. Pérez et al. (2014) used only acoustic imaging data taken on June 28 as evidence for submarine volcanic activity in 2012 off the west coast of El Hierro. Without this data, it would have been impossible for us to submit our scientific contribution for publication.

Co-seismic slip, post-seismic slip, and aftershocks associated with two large earthquakes in 1996 in Hyuga-nada, Japan

We analyzed continuous GPS data to investigate the spatial distribution of post-seismic slip associated with two large earthquakes of October 19 and December 2, 1996, in Hyuga-nada, Japan. We found that the moment release due to post-seismic events was comparable to the co-seismic moment release during the two earthquakes. The source parameters of the first post-seismic event are as follows: the moment release = 1.7 × 1019 Nm; the maximum slip = 0.06 m at about 50 km northwest from the epicenter of the first earthquake; the characteristic decay time (= final slip/initial slope) = 15 days. For the second post-seismic event, the moment release = 2.0 × 1019 Nm; the maximum slip = 0.13 m at about 15 km northwest from the epicenter of the second earthquake; the characteristic time = 100 days. In both events, the slip vectors of the downgoing Philippine Sea (PHS) Plate on the SW-striking interplate boundary are directed west, in accordance with the co-seismic slip. It is also shown that the sites for co-seismic slip, post-seismic slip, and aftershocks do not overlap but complementarily share the plate boundary. This suggests that individual sites are characterized by their own constitutive laws, which may control modes of moment release as well as the entire sequence.