Sanriku-oki Earthquake Research Articles

The Ionospheric view of the 2011 Tohoku-Oki earthquake seismic source: the first 60\u2009seconds of the rupture

Using the specific satellite line of sight geometry and station location with respect to the source, Thomas et al. [Scientific Reports, https://doi.org/10.1038/s41598-018-30476-9] developed a method to infer the detection altitude of co-seismic ionospheric perturbations observed in Global Positioning System (GPS) – Total Electron Content (TEC) measurements during the Mw 7.4 March 9, 2011 Sanriku-Oki earthquake, a foreshock of the Mw 9.0, March 11, 2011 Tohoku-Oki earthquake. Therefore, in addition to the spatio-temporal evolution, the altitude information of the seismically induced ionospheric signatures can also be derived now using GPS-TEC technique. However, this method considered a point source, in terms of a small rupture area (~90 km) during the Tohoku foreshock, for the generation of seismo-acoustic waves in 3D space and time. In this article, we explore further efficacy of GPS-TEC technique during co-seismic ionospheric sounding for an extended seismic source varying simultaneously in space and time akin to the rupture of Mw 9.0 Tohoku-Oki mainshock and the limitations to be aware of in such context. With the successful execution of the method by Thomas et al. during the Tohoku-Oki mainshock, we not only estimate the detection altitude of GPS-TEC derived co-seismic ionospheric signatures but also delineate, for the first time, distinct ground seismic sources responsible for the generation of these perturbations, which evolved during the initial 60 seconds of the rupture. Simulated tsunami water excitation over the fault region, to envisage the evolution of crustal deformation in space and time along the rupture, formed the base for our model analysis. Further, the simulated water displacement assists our proposed novel approach to delineate the ground seismic sources entirely based on the ensuing ionospheric perturbations which were otherwise not well reproduced by the ground rupture process within this stipulated time. Despite providing the novel information on the segmentation of the Tohoku-Oki seismic source based on the co-seismic ionospheric response to the initial 60 seconds of the event, our model could not reproduce precise rupture kinematics over this period. This shortcoming is also credited to the specific GPS satellite-station viewing geometries.

Tsunami history over the past 2000 years on the Sanriku coast, Japan, determined using gravel deposits to estimate tsunami inundation behavior

At Numanohama Marsh on the rocky Sanriku coast in northeast Japan, we identified 17 sand layers with sedimentary structures characteristic of event deposits (i.e., a sharp erosional contact with the underlying unit, a normal and/or inverse grading structure, parallel/cross lamination, mud clasts, and/or a thin mud layer). Abundant marine nannofossils, coccoliths, and the small catchment area in the narrow valley suggest that these event layers were likely formed by inundations during tsunamis or storms. Among the 17 sand layers, nine contained gravels derived from the beach, riverbed, or slope areas. Six event layers contained all three of these gravels, suggesting a strong inflow current that transported round pebbles from the beach, erosion of wall rock in the valley during the inflow and outflow phases, and a backwash current that caused sub-angular riverbed gravel to accumulate in lowland areas. These six layers were traced up to 490 m inland from the coastline (present elevation: 7 m above mean sea level) and were several cm thick at almost every survey point. We concluded that these six event layers are tsunami deposits, and dating results revealed that they correlate with the 2011 Tohoku-oki earthquake; the 1933 and 1896 Sanriku-oki earthquakes; any of the 1763 Aomori-oki, 1793 Miyagi-oki, and 1856 Aomori-oki earthquakes; the 1611 Tohoku-oki earthquake; and the 869 Tohoku-oki (Jogan) earthquake. In addition, changes in the sedimentary environment inferred from diatom assemblages were well correlated with the deposition of three of the event layers. Two historical earthquakes — the 1454 and 1611 Tohoku-oki earthquakes — have been previously suggested as being the immediate predecessors of the 2011 Tohoku-oki earthquake, and our dating results favor the 1611 earthquake. Although the source region of the 869 Tohoku-oki (Jogan) earthquake has been estimated to have been off the Fukushima and Miyagi prefectures, our results suggest that the source region extended further north along the Japan Trench.

Revelation of early detection of co-seismic ionospheric perturbations in GPS-TEC from realistic modelling approach: Case study

GPS-derived Total Electron Content (TEC) is an integrated quantity; hence it is difficult to relate the detection of ionospheric perturbations in TEC to a precise altitude. As TEC is weighted by the maximum ionospheric density, the corresponding altitude (hmF2) is, generally, assumed as the perturbation detection altitude. To investigate the validity of this assumption in detail, we conduct an accurate analysis of the GPS-TEC measured early ionospheric signatures related to the vertical surface displacement of the Mw 7.4 Sanriku-Oki earthquake (Sanriku-Oki Tohoku foreshock). Using 3D acoustic ray tracing model to describe the evolution of the propagating seismo-acoustic wave in space and time, we demonstrate how to infer the detection altitude of these early signatures in TEC. We determine that the signatures can be detected at altitudes up to ~130 km below the hmF2. This peculiar behaviour is attributed to the satellite line of sight (LOS) geometry and station location with respect to the source, which allows one to sound the co-seismic ionospheric signatures directly above the rupture area. We show that the early onset times correspond to crossing of the LOS with the acoustic wavefront at lower ionospheric altitudes. To support the proposed approach, we further reconstruct the seismo-acoustic induced ionospheric signatures for a moving satellite in the presence of a geomagnetic field. Both the 3D acoustic ray tracing model and the synthetic waveforms from the 3D coupled model substantiate the observed onset time of the ionospheric signatures. Moreover, our simple 3D acoustic ray tracing approach allows one to extend this analysis to azimuths different than that of the station-source line.

The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

Historical tsunami and storm deposits during the last five centuries on the Sanriku coast, Japan

Abstract Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were correlated with historical tsunamis and storms that have been recorded in this region since the 15th century. We identified nine sandy layers in 15 geo-slices collected at distances ranging from 140 to 260 m from the coast in a lowland back marsh protected from the sea by a high sandy ridge. Based on their sedimentary characteristics, grain-size distribution, and marine microfossil assemblages, namely Coccoliths, these event layers, which were well preserved in their order of deposition, were probably either tsunami or storm deposits. Diatom analysis revealed four stages of paleo-environmental change from tidal marsh to wetland. The uppermost event layer covering the ground surface is probably the deposit from the 2011 Tohoku-Oki tsunami. Ages for other event deposits were radiometrically dated using 14C, 137Cs and 210Pb with Bayesian estimation. The second event layers can be correlated with the 1960 Chilean or 1968 Aomori-Oki earthquake tsunamis. The candidates for the third event layer are the 1947 Catherine and 1948 Ione typhoons as well as the candidates for the second event layer. The fourth and fifth event layers are likely associated with the 1933 and 1896 Sanriku-Oki earthquake tsunamis. Four lower event layers can be correlated with historical tsunamis and storms that occurred during the 15th to 18th centuries, such as the 1611 Sanriku-Oki earthquake tsunami.

Rupture behaviors of the 2011 Tohoku earthquake and its strongest foreshock through an empirical Green’s function deconvolution analysis

An empirical Green’s function (EGF) deconvolution analysis was applied to study the source characteristics of the 2011 Mw9.0 Tohoku and 2011 Mw7.4 Sanriku-Oki earthquakes. For the 2011 Tohoku earthquake, we demonstrate that nucleation released weak but high-frequency energy and that the rupture propagated downward and sped toward the deep region after the up-dip slip extended to the trench. Moreover, the 2011 Sanriku-Oki earthquake results and a previous study on the 1994 Sanriku-Oki earthquake suggest that large earthquakes in the subduction zone around the Tohoku area prefer to rapidly rupture toward the deeper (down-dip) region.

Spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 MW= 9.0 Tohoku-oki earthquake

[1] Imaging the extent to which the rupture areas of great earthquakes coincide with regions of pre-seismic interplate coupling is central to understanding patterns of strain accumulation and release through the earthquake cycle. Both geodetic and seismic estimates of the coseismic rupture extent for the March 11, 2011 MW = 8.9–9.0 earthquake Tohoku-oki earthquake may be spatially correlated (0.26 ± 0.05 to 0.82 ± 0.05) with a region estimated to be partially to fully coupled in the interseismic period preceding the earthquake, though there is substantial variation in the estimated distribution and magnitude of coseismic slip. The ∼400 km-long region estimated to have slipped ≥4 m corresponds to an area of the subduction zone interface that was coupled at ≥30% of long-term plate convergence rate, with peak slip near a region coupled ≥80%. The northern termination of rupture is collocated with a region of relatively low (<20%) interseismic coupling near the epicenter of the 1994 MW = 7.6 Sanriku-oki earthquake, and near a region of potential long-term low coupling or ongoing slow slip. Slip on the subduction interface beneath the coastline (40–50 km depth) is best constrained by the land-based GPS data and least constrained on the shallowest portion of the plate interface due to the ∼230 km distance between geodetic observations and the Japan trench.

Outer trench-slope faulting and the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

The 11 March 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) produced megathrust displacements of at least 40 m. The resulting tsunami devastated the Honshu coast southwest of regions struck by earthquake-generated tsunami in 1611, 1896 and 1933. The 1896 Meiji-Sanriku earthquake was also an underthrusting earthquake, but the 1933 Sanriku-oki earthquake was a trench-slope normal faulting event; both generated inundation heights of 10 to 25 m along the coast of Iwate prefecture. Possible occurrence of a great outer trench-slope earthquake seaward of the 2011 Tohoku Earthquake along a southwestward extension of the 1933 fault zone is a concern. The second largest 2011 aftershock, an outer rise Mw 7.7 normal faulting earthquake occurred near the southern end of the 1933 rupture. Additional aftershock activity has been distributed along a trend below the trench and diffusely spread in the outer rise, seaward of the megathrust region where the largest slip occurred. Coulomb stress perturbations of at least 5–10 bars are calculated for outer rise normal fault geometries for mainshock slip models. Whether a future great trench slope event will occur is uncertain, but the potential tsunamigenic hazard can be gauged by the huge inundations accompanying the 1933 rupture.

Rupture process of the 9 March, 2011 Mw 7.4 Sanriku-Oki, Japan earthquake constrained by jointly inverting teleseismic waveforms, strong motion data and GPS observations

[1] The slip history of the 2011 Mw 7.4 Sanriku-Oki, Japan earthquake, which occurred fifty-one hours before the Mw 9.1 Tohoku earthquake, is constrained by jointly inverting waveforms of teleseismic body waves, long period surface waves and local strong motions as well as GPS observations, after first relocating its hypocenter using a double difference approach and teleseismic P waves. The inverted results indicate that the rupture of this Sanriku-Oki earthquake was dominated by the failure of an elliptical shape asperity, elongating roughly along the plate motion direction. The rupture initiated at the southeast corner of this asperity and propagated mainly in the west-northwest direction with a rupture velocity of 3.1 km/s in the beginning 15 s and 1.1 km/s in the next 40 s. It released a total seismic moment of 1.6 × 1020 Nm, with 82% occurring in the first 25 s. The rupture had an average slip of 1 m and produced an average stress drop of 0.9 MPa. The Sanriku-Oki earthquake did not break the hypocenter region of the Mw 9.1 Tohoku earthquake but slightly increased the Coulomb stress there. A correlation between the high slip region and the high Vp/Vs ratio of the overriding plate right above the plate interface has been found, which suggests the Sanriku-Oki earthquake and its frequent predecessors might have broken a relatively weaker patch within a large strongly coupled asperity.

Possibility of recovery of slip deficit rate between the North American plate and the Pacific plate off Sanriku, northeast Japan

[1] The analysis of global positioning system (GPS) data shows an increase in the westward ground velocity in Iwate, northeast Japan, from around 2003, after a period of roughly constant crustal deformation rate. The spatiotemporal study of time series of displacements reveals that the westward slip deficit rate increased slightly from 2001 to 2003 at the deeper part of the source region of the 1994 Sanriku-oki earthquake. The increase has been considerable since January 2003, and the area of increased slip deficit rate has expanded, and now covers the source region of the 1994 Sanriku-oki earthquake. This indicates that the restoration of the slip deficit rate, which was affected by the 1994 earthquake, was not completed in the period from 1994 to 2003.

Fault plane geometry in the source region of the 1994 Sanriku-oki earthquake

We present the results of a seismic refraction experiment conducted off eastern Aomori, northeastern Japan forearc region, where the detailed crustal structure was poorly known. An objective analysis using high-quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth, and in particular, to obtain a well-constrained geometry of the plate boundary in the source region of the 1994 Sanriku-oki earthquake. Our results show that the subducting Pacific oceanic plate increases its dip from about 5° to approximately 15° around the longitude of 143.5° E, 80 km landward of the trench axis near the latitude of 40° N. The location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution. A cluster of aftershocks of the 1994 Sanriku-oki earthquake occurred around the bending point. This is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary. Furthermore, the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes. Our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies.

断層帯の物質科学と地震の発生過程 プレート境界面の摩擦パラメタの推定について―三陸沖のアスペリティを例として―

A numerical simulation of seismic cycles on a plate boundary is performed using a rate- and state-dependent friction law. The spatial distribution of frictional constitutive parameters is determined so that the simulated seismic cycles may mimic the occurrence of large earthquakes in the northern Sanriku area along the Japan trench. Two patches with velocityweakening frictional properties are assumed to represent two large asperities that were ruptured in the 1968 Tokachi-oki earthquake (Mw=8.2). By choosing appropriate friction parameters, I can simulate the recurrence of large earthquakes that break both the velocityweakening patches at a recurrence interval of 90 years. During each interseismic period of the simulated large earthquakes, one of the velocity-weakening patches is broken, resulting in the occurrence of a smaller earthquake. This may correspond to the occurrence of the 1994 Sanriku-oki earthquake of Mw=7.7. The coseismic slip amounts in the simulation are similar to those estimated for the 1968 Tokachi-oki and the 1994 Sanriku-oki earthquakes, and postseismic sliding is reproduced in the velocity-strengthening region. These simulation results for seismic cycles at the northern Sanriku subduction zone suggest that the frictional constitutive parameters on the plate boundary can be estimated to some extent. The model with realistic friction parameters is expected to be useful for forecasting the sliding process on the plate boundary.

Space–time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench

Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×10 20 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×10 20 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×10 20 N m, which was 20–35% of the accumulated moment (18.6–23.0)×10 20 N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×10 20 N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (∼10 2 s) for the 1968 Tokachi-oki earthquake to 10–20 min (∼10 3 s) for the 1896 Sanriku tsunami earthquake, to one day (∼10 5 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (∼10 7 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity.

三陸沖における再来大地震の震源過程の比較研究

三陸沖における再来大地震の震源過程の比較研究

A large flux of particulate matter in the deep Japan Trench observed just after the 1994 Sanriku-Oki earthquake

Three time-series sediment traps were deployed in the Japan Trench at 40°26′N, 144°28′E, from October 1994 to May 1995. The depths were approximately 1, 4.2 and 6.8 km and the water depth was 7150 m. There were large mass fluxes in spring at 1 and 4.2 km depths, whereas increased fluxes appeared from 27 December 1994 to 29 January 1995, at 4.2 and 6.8 km depths. The 1994 Sanriku-Oki earthquake ( M w=7.7) occurred on 28 December 1994, at 40°27′N, 143°43′E, adjacent to the study site. Distinct increases in non-biogenic material were observed at both 4.2 and 6.8 km just after the earthquake; the material seems to have originated from the surface sediments, though differing Mn/Al of particulate materials at the two depths imply a difference in their source areas. Analysis indicates that the main part of the increased particulate fluxes at 6.8 km depth derived from the sediment on the eastern slope of the Japan Trench.

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones

—We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10–40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.

時空間分布とサイスモ・ジオデティック・カップリング -中・長期予測の基礎-

We had the 1989, 1992 and 1994 Sanriku-Oki earthquakes in the region from 39. 0°N to 40. 6°N along the Japan trench. Summarizing the aseismic nature of the earthquakes, we propose two new concepts“seismo-geodetic coupling“and”a gap of the moment release”. The sum of their seismic moments is (4. 8-6. 6) ×1020Nm. These events were associated with aseismic ultra-slow fautings of source times of a day to a year following the respective main seismic shocks. The ultra-slow faultings respectively released interplate momens, 1. 4, (1-4), 4. 2×1020Nm, equivalent to those of Mw7. 3 to 7. 7 seismic events. The sum of the moments released by both seismic and aseismic faultings amounts to (11-16) ×1020Nm. The interplate moment is accumulated as (19-23) ×1020Nm for the area of 180 km (39. 0°N to 40. 6°N) 150 km (width of high seismicity region between the Sanriku coast and the Japan trench), with the relative plate motion of 8 cm/yr and the interval of 21-27 years from the 1968 Tokachi-Oki earthquake of Mw8. 2 to the 1989-1994 ultra-slow earthquakes. The ratio of (11-16) × l020Nm to (19-23) ×1020Nm leads to the rough estimate of “seismo-geodetic coupling coefficient”of 50-85%, where the ordinary seismic coupling coefficient has been estimated to be 20-30% between the Okhotsk and subducting Pacific plates. Based on the spatiotemporal distribution of the interplate moments including the ultra-slow earthquakes, three gaps of the moment release are suggested for the Sanriku-Oki region. This would serve as a basis for the mid-term/long-term prediction of future interplate earthquakes in the Sanriku-Oki region.

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

We investigated the rupture process of the 28 December 1994 Sanriku‐Oki earthquake (Mw = 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high‐frequency waves. The whole rupture time is about 55 sec. The three‐stage rupture process is very similar to the earlier‐stage rupture process of the 1968 Tokachi‐Oki earthquake (Mw = 8.2). The difference is that the rupture of the Tokachi‐Oki earthquake further propagated towards the north in the following stage instead of terminating at the western end. The occurrence of the Sanriku‐Oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the plate boundary.

Observation of Local Large Earthquakes by Broadband Seismographs: Implications for the Initial Rupture Mechanism of the 1989 November 1 Sanriku-Oki Earthquake.

During the earthquake swarm-like activity of Sanriku-Oki (offshore Iwate Prefecture, Japan) from October to November 1989, a relatively large earthquake (M 7.1 JMA) occurred on November 1. Streckeisen broadband seismometers installed in the Tsukuba Seismological Observatory in September 1989 successfully recorded the entire sequence of this activity, including the November 1 event. Due to the broadband characteristics of the seismometer and the high dynamic range 24-bit A/D converter, the recorded seismograms show various interesting features that might provide useful implications for both source and structure, even if seismograms from only one station are available. In the present paper we compare the broadband seismograms of the November 1 event recorded at Tsukuba to those of the second-largest event, which occurred on October 29 (M 6.5 JMA). We show that the first several seconds of both seismograms coincide with each other. This might imply that the November 1 event started as a much smaller event, similar to the October 29 event, but developed into a larger one.

Source Process of the 1989 Sanriku-Oki Earthquake, Japan: Source Function Determined Using Empirical Green Function.

Source Process of the 1989 Sanriku-Oki Earthquake, Japan: Source Function Determined Using Empirical Green Function.