FIG 5 - uploaded by Kunihiko Shimazaki
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The spatial distribution of intraplate earthquakes which followed the 1896 interplate earthquake. The shaded area shows a portion of the plate boundary which probably slipped in 1896 to 1897. The area encloses the epicenter of the 1896 Sanriku earthquake (closed square), earthquake faults of the 1896 event and its largest aftershock (open rectangles; Aida, 1977), and epicenters of large aftershocks (closed triangles; Richter, 1958).
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All of the active periods of intraplate earthquakes for the past 100 years in Tohoku, northeast Japan correlate with the occurrence of great interplate earth-quakes along the Japan trench. The seismically active periods are preceded by a period of quiescence lasting 10 to 15 years. One-fourth of intraplate earth-quakes of magnitude 5.8 and above pr...
Contexts in source publication
Context 1
... shaded areas are aftershock areas of the large earthquakes along the Japan trench listed in Table i. (The shaded area indicated as "1896-18977" shows an area which probably slipped in 1896 to 1897; see Figure 5.) The solid squares indicate epicenters of large interplate earthquakes. ...
Context 2
... on historical tsunamis also suggest that the southern part of the aftershock area of the 1896 event slipped previously in 1793. In particular, the slip occurred on a portion of the plate boundary which ruptured during the largest aftershocks of the 1896 earthquake in 1897 (Aida, 1977;tIatori, 1975; see Figure 5). Although the portion of the plate boundary which slips at the same time appears to change in time, each segment probably slips roughly once in 100 years. ...
Context 3
... Tohoku, the most enhanced postseismic activity followed the 1896 interplate earthquake. Figure 5 shows epicenters of intraplate earthquakes which occurred within 20 years after the 1896 Sanriku earthquake (most of them occurred within 10 years after the earthquake). The activity concentrates in an area to the west of the area which probably slipped in 1896 to 1897. ...
Context 4
... activity concentrates in an area to the west of the area which probably slipped in 1896 to 1897. This suggests that the underthrusting drag shifted from the shallower part to the deeper part of the interface, an area be- tween the aseismic front and the western boundary of the shaded area in Figure 5, and brought about the high intraplate seismicity. If this is the case, the seismically active area to the west of the slipped plate boundary ( Figure 5) should have been compressed by the underthrusting oceanic plate after the interplate earthquake. ...
Context 5
... suggests that the underthrusting drag shifted from the shallower part to the deeper part of the interface, an area be- tween the aseismic front and the western boundary of the shaded area in Figure 5, and brought about the high intraplate seismicity. If this is the case, the seismically active area to the west of the slipped plate boundary ( Figure 5) should have been compressed by the underthrusting oceanic plate after the interplate earthquake. This possibility is supported by the following evidence. ...
Context 6
... the intraplate earthquakes plotted in Figure 5 is the Rikuu earthquake of 1896 (M = 7.2) which took place about 2 months after the 1896 interplate earth- quake. A report on foreshocks in Imamura's (1913) paper suggests that minor fore- shock activity started only a few days after the Sanriku interplate event. ...
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Citations
... Jones and Hauksson (1997) examined seismicity rates for events of M ≥ 3.0 in southern California from 1945 to 1996 in terms of the seismic cycle concept (Imamura, 1937;Fedotov, 1965;Mogi 1969Mogi , 1981Shimazaki, 1978;Reasenberg and Simpson, 1992;Sykes, 1996 were also performed. They used data from two independent catalogs and the quantitative analysis they carried out showed that the precursory quiescence and rate increase is not unique, since changes in occurrence rates of this duration and significance often occur in both datasets. ...
Seismicity rate changes in selected regions of the broader Aegean area were studied by application of the Dieterich (1994) Rate/State formulation. The coseismic slip of the strongest events (Μw≥5.8) that occurred during selected ‚study‛ periods was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just before and after each strong event and their influence was then examined in connection with the occurrence rate of the smaller magnitude events above the individually determined magnitude of completeness in each sub-area and for the respective time intervals, named as ‚study‛ or ‚forecasting‛ periods. After defining the probability density function (PDF) of seismicity distribution, a Rate/State model was used to combine static Coulomb stress changes (ΔCFF) with seismicity rates and to compare the observed with the expected rates of earthquake production for each time period and sub-area. Different parameter values combinations were tested in order to evaluate the model sensitivity. Qualitative and quantitative correlation between the observed and the expected seismicity rates provide a test for the validity and sufficiency of the model. Εarthquake probabilities for exceedance of magnitudes M=6.0 and M=6.5 during the next decade were finally illustrated. After deriving seismicity evolution from stress changes, the inverse method was attempted. Spatial and temporal evolution of the stress field in well monitored areas of Aegean were carried out. The highest accuracy and large sized regional catalogues were utilized in order to invert seismicity rate changes into stress variation through a Rate/State dependent friction model. After explicitly determining the physical quantities incorporating in the modeling (characteristic relaxation time, fault constitutive parameters, reference seismicity rates) stress changes in both space and time were derived and their possible connections with earthquake clustering and fault interactions were evaluated. The forward modeling approach resulted to satisfactory correlation between real and synthetic seismicity rates and is expected to constitute a useful mean for the time dependent seismic hazard assessment. The inverse method yielded promising results in the cases where the available data were sufficient and should provide a powerful tool for future research as the earthquake data becomes enriched and more precise.
... A very interesting question arises as to why two large earthquakes, the 1994 Shikotan Earthquake ( M , = 8.2) and the 1969 Shikotan Earthquake ( M , = 8.21, occurred in nearly identical locations within a short period (25 years). If the potential to generate large earthquakes in this region continuously increased with the accumulation of strain due to plate motion since the 1969 Shikotan Earthquake, the 25-year period between the two large earthquakes seems t o be too short to build up sufficient potential to generate an M, = 8.2 earthquake (Utsu 1972;Kanamori 1977;Shimazaki 1978). Several authors (Katsumata et Kikuchi & Kanamori 1995;Tanioka et al. 1995) interpreted the short recurrence of large earthquakes in the same region as intraplate earthquakes. ...
Abstract Bathymetric data from south of Hokkaido obtained during a cruise of R/V Hakuho-Maru are summarized, and their correlation with earthquake occurrence is discussed. There are structural lineations on the seaward slope of the Kuril Trench, oblique to the Kuril Trench axis and parallel to the magnetic lineations in the Pacific plate. The structural lineations comprise horst-grabens generated by normal faulting. This suggests that Cretaceous tectonic structures originating at the spreading centre affect present seismotectonics around the trench axis. The structural-magnetic relation is compared to the case of the Japan Trench. North-east of the surveyed area, there are two major fracture zones (Nosappu Fracture Zone and Iturup Fracture Zone) that divide the oceanic plate into three segments. If the fracture zones (FZ) and the zone of paleo-mechanical weakness, represented by magnetic lineations, can control the direction of normal faults at a trench, the extent of the resulting topographic roughness on the seaward slope of the trench would be different across an FZ because of the differences in ages. By studying recent large earthquakes occurring in the south Kuril region, it is shown that several main-aftershock distributions for large earthquakes in this region are bounded by the Nosappu FZ and the Iturup FZ. Two models (Barrier model and Rebound model) are presented to interpret earthquake occurrence near the south Kuril Islands. The Barrier model explains seismic boundaries seen in several examples for earthquake occurrence in the south Kuril regions. The fracture zone forming the boundary of two segments with different magnetic lineations is also the boundary of two different normal fault systems on their ocean bottom, and the difference in sea-bottom roughness between two normal fault systems should affect the seismic coupling at a plate interface. Due to the difference of seismic coupling, earthquake occurrence is controlled by an FZ and then the FZ acts as a seismic boundary (Barrier model). Existing normal faults created by plate bending of subducting oceanic plate should rebound after its subduction (Rebound model). This rebound of normal faults may cause intraplate earthquakes with a high-angle reverse-fault mechanism such as the 1994 Shikotan Earthquake. The energy released by an intraplate earthquake generated by normal-fault rebounding is not directly related to that of interplate earthquakes such as low-angle thrust earthquakes. It is a reason why large earthquakes occurred in the same region during a relatively short period.
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A new criterion is introduced to judge if the vicinity of the source region of a great interplate earthquake is in an active period. It is based on the stress change caused by the great earthquake. A region is regarded as being in an active period of seismicity if the occurrence rate of earthquakes on faults in the stress shadow of the great earthquake is significantly higher than in the early stage of the seismic cycle, and if the stressing rate of these faults is sufficiently low. This criterion was applied to the seismicity in the central part of southwest Japan before and after the 1944 Tonankai and 1946 Nankai earthquakes. The results show that before the 1944 Tonankai earthquake, the region was in an active period from at least 1927.The region was in a quiet period for almost50 years after the 1946 Nankai earthquake.Data after 1995 show that the region is once more in an active period of seismicity preceding the next great interplate earthquakes along the Nankai trough,although the total number of earthquakes has not yet significantly increased. Our results indicate that earthquake probability in the central part of southwest Japan will become high in the coming decades until the next great interplate earthquakes along the Nankai trough.
... It is the intraplate motion in NE Japan that sets up interplate interaction. Shimazaki (1978), Seno (1979) and Ryedelek and Sacks (1990) reported that intraplate earthquakes precede interplate events. Some elastic deformation is, nevertheless, probably supplied from the Japan trench to northern NE Japan as the initial (Neogene) movement of northern Tohoku towards the Paci®c plate off-Sanriku resulted in the increasing of the coupling there, along the subduction thrust plane. ...
An existing geodetic flow velocity model, obtained by using an internal free network adjustment technique, is used to derive estimates for various strain rates parameters in NE Japan. The greatest shortening rates of the principal strains, trending ∼E–W, are located in regions where much steady, internal, frame-invariant plastic flow deformation is observed to be taking place. The internal geodetic adjustment technique yielded the internal deformation in the Tohoku arc; most of the intraplate deformations, including the much folding deformations observed in the inner zone, are produced from within. An interseismic transient elastic loading at a strongly coupled/locked Japan trench would not be needed. The observed ongoing extensive ductile folding deformation in the inner zone of Tohoku may mean that the geodetic strain rates, causing shortening at ∼2–3 cm/yr, probably reflect the more correct level of the deformation, which is steady/permanent, in NE Japan as compared with seismic/faulting data, which indicate ∼0.5 cm/yr shortening.
... The land earthquakes are more likely to occur at any time when the probability is high; therefore, the time interval between land and sea earthquakes will exhibit a broad-band distribution rather than the correlation peak observed. Nevertheless, this simple model does show why the land quakes precede the sea quakes; Shimazaki (1978) and Sen0 (1979) pointed this out from a study of the more recent seismicity (1850-present) in Tohoku, NE Japan. ...
A significant correlation is found, in both space and time, between the intraplate (land) and interplate (sea, thrust zone only) earthquakes in Tohoku, NE Japan that has persisted since the times of reliably reported events in AD 1600. the correlation peaks at a land-lead of about 36 yr with an average correlation distance of 200km, with the implication of an average strain migration rate of 5.6 km yr-1. the correlation is highly significant (> 99 per cent), both from formal statistics and from tests of random shuffles of the data. Additional analysis of the data, as a point process, confirms the results of the correlation analysis. the sharpness of the correlation peak, when compared to the individual times of occurrence of the land and sea events suggests a trigger mechanism.
To explain the correlation, the general model ofsubduction-rupture-rebound is extended to include additional features; the buckling of the land plate from the force of the subducting slab, and the viscoelastic coupling of the plate to the underlying asthenosphere. A buckle produces a high-stress region in the continental plate where earthquakes are more prone to occur, thus producing the spatial correlation in the data. This may also explain the preferred location on land for the smaller modern-day seismic events in NE Japan. the viscoelastic coupling controls the interaction between the land and sea events, resulting in the temporal correlation in the data. Because of viscosity, the model equations are diffuse-like with strain pulses as solutions; thus from the inferred strain migration rate it is possible to estimate asthenospheric viscosity (η=7 × 1018Pa s) using this model. A large land shock generates a strain pulse that affects the locked fault at the thrust zone several decades later. As the continental plate tends to pull away from the subducting slab, the frictional force arising from the overburden pressure is reduced, thus unlocking the fault and triggering a sea earthquake.
The viscoelastic model is also used to explain surface deformations measured by triangulation surveys in Japan in 1904 and 1964. Horizontal displacements, which we believe are surface manifestations of the strain pulse from the large 1896 Riku-U land shock (M= 7.5) in NE Japan, are fit well by the model and provide a viscosity estimate η= 13 × 1018 Pa s.
... The land earthquakes are more likely to occur at any time when the probability is high; therefore, the time interval between land and sea earthquakes will exhibit a broad-band distribution rather than the correlation peak observed. Nevertheless, this simple model does show why the land quakes precede the sea quakes; Shimazaki (1978) and Sen0 (1979) pointed this out from a study of the more recent seismicity (1850-present) in Tohoku, NE Japan. ...
The viscosity of the Earth's mantle has been estimated from studies of post-glacial rebound1,2, post-seismic deformations of the ground following large earthquakes3,4, and aftershock sequences5–7. Here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction-induced seismicity between the intraplate (land) and interplate (sea) earthquakes in north-east Japan. The correlation persists since the time of reliably reported earthquakes in AD 1600; land events precede sea events by ~36 yr, with a mean distance between land–sea pairs of ~200 km. Because of the visocoelastic coupling of the lithosphere to the asthenosphere, a plausible mechanism to explain the correlation is stress migration, governed by the viscosity of the asthenosphere. Large land shocks generate diffuse-like stress pulses which sweep past, and unlock, the thrust fault in the subduction zone, thus triggering the sea events. The correlation time and distance provide a measure of the speed of diffusion (5.6 km yr−1) and hence an estimate of the viscosity (7 × 1018 Pa s).
Statistical methods in Earth Sciences using useful models of time series, point processes and space-time processes are developping in conjunction with a variety of concrete applications. In the present paper, such models and methods successfully applied in seismology and related science of solid earth are reviewed. Particular emphasis is place don point process models in application to the analysis of seismic activity. Also, some developping models for analyzing various aspect of seismic wave data are intensively reviewed in section 9. Further, a number of remakable contributions of Japanese statisticians to some areas in earth sciences are also introduced in section 10.
A large Mw=8.2 earthquake occurred off Shikotan Is., one of the Kurile Is., on October 4, 1994. We inverted 32 body-wave records to determine the rupture pattern using an iterative deconvolution method. The mechanisms of the subevents were allowed to vary during rupture. The source parameters obtained are: the location of the initial break = (43.48°N, 147.40°E); the centroid depth = 56 km; (strike, dip, rake) = (49°, 75°, 125°) for the total source; the seismic moment Mo = 2.6×1021 Nm (Mw=8.2); source time duration T = 42 s; the average rupture velocity ν = 2.5 km/s. We also determined the mechanism using long-period Love and Rayleigh waves from 14 stations. The solution for a finite source distributed over a depth range from 0 to 90 km is (strike, dip, rake) = (54°, 76°, 129°) with Mo = 2.3×1021 Nm, in good agreement with that from body waves. Referring to the extent of the aftershock area and the subevent distribution, we estimated the fault area S = 120 × 60 km², the average slip D=5.6m, and the stress drop Δσ=11 MPa. We computed synthetic waveforms as well as static displacements using either the steep or the low-angle plane as the fault plane, and found that the steep-dip fault model fits the data better. Our result (the mechanism, large centroid depth, high stress drop) strongly suggests that the 1994 Shikotan earthquake is a lithospheric earthquake: an intra-plate event that ruptures through a substantial part of the subducting oceanic lithosphere. This type of lithospheric earthquake is relatively common.
The seismicity rate (M>=3.0) in southern California shows two cycles with periods of high activity (90 events/year), from 1945-1952 and 1969-1992, and lower activity (60-70 events/year) from 1952-1969 and 1992-present. Abrupt drops in the seismicity rate occur after the 1952 Kern County (M7.5) and the 1992 Landers (M7.3) earthquakes. The sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake. This temporal correlation with the preceding earthquake suggests that the seismic cycle (lower seismicity after a major earthquake and higher seismicity before the next major earthquake) should be interpreted as a response to the first earthquake rather than a precursor to the second. Southern California is now at a rate of seismicity as low as it experienced in the 1950s and 1960s.
Fractal properties of active fault systems in Japan are evaluated and compared to Gutenberg-Richter b values. Properties of the active fault network and seismicity were evaluated at 20 km intervals along three lines oriented along the length of Honshu, the main island of Japan. Fractal dimensions for the active fault network are calculated using the box counting method. The box curves often reveal the presence of an abrupt transition in slope at ∼8 km scales. This transition separates linear regions in the box curves that span box sizes of 17.5 to 8.5 km and 7.75 to 2 km. Power law coefficients were computed for the 17.5 to 8.5 km (DL) and 7.75 to 2 km (DS) range of box sizes from overlapping 70×70 km regions of the active fault complex. The maximum likelihood method is used to estimate b value from earthquakes occurring in the seismogenic zone (upper 20 km). The correlation between D and b value are found to vary considerably throughout Japan. In general, D and b are negatively correlated, which suggests that increased complexity in the active fault network accommodates rupture along fault planes of relatively larger surface area. Areas of significant positive correlation are also observed in Japan. The positive correlations arise through joint increases in b and D. In such areas the probability of large magnitude earthquakes decreases in response to increased fragmentation of the active fault network and increased possibility that stress release will take place along faults of smaller surface area. Negative correlations between b and D generally occur when increases in D are paralleled by decreases in b. Negative correlation areas bound the intensely faulted region of central Japan. An area of negative correlation also occurs in the intensely faulted high D area of central Japan where continued increases in D are paralleled by decreases in b. In general, these observations suggest that as fault complexity increases (as measured by D), interconnected planes of larger and larger surface area will accommodate strain release. However, with continued rise in fault complexity, strain release begins to occur on smaller fragments resulting in higher b and lower-magnitude seismicity. The analysis may provide insights into the relationship between seismicity associated with large-scale faults populations and earthquake hazard assessment.
