The observation of seafloor crustal deformation is very important to understand plate motions, nu... more The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its "true" position. reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.
We have developed an observation system for ocean bottom crustal deformation using an acoustic ra... more We have developed an observation system for ocean bottom crustal deformation using an acoustic ranging-GPS link. A seafloor-positioning test at Suruga Bay revealed that our system can obtain the horizontal location of a sea-bottom station within 5 cm (95 % confidence interval) using accurate GPS positioning data. GPS positioning error is a major source of error in our whole system. We carried out experiments on kinematic GPS positioning with a long base line and a moving antenna to investigate its accuracy. The results of the experiments suggest that data with PDOP (Position Dilution of Precision) of 2 or less can reduce the GPS positioning error to 7 cm. The network of the observation system for ocean bottom crustal deformation may lead to advances in tectonic studies along subduction zones and contribute to predictions of interplate large earthquakes in the future.
We have investigated the spatial distribution of the static stress drop of similar aftershocks of... more We have investigated the spatial distribution of the static stress drop of similar aftershocks of the 2007 Noto Hanto earthquake, central Japan, and have examined the relationship between the coseismic slip of the mainshock and the static stress drop of the similar aftershocks. The static stress drop is estimated by using the empirical Green's function method for P-and S-waves. The estimated static stress drop approximately ranges from 5 to 20 MPa, which is a typical range of values for tectonic earthquakes. The static stress drops of the aftershocks in a large slip area of the mainshock tend to be larger than those in a small slip area. This suggests a large difference between the strength and the dynamic stress level in the large slip area and a small difference in the small slip area if the final stress level is equal to the dynamic stress level.
In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 2... more In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P-and T-axes were N97°± 23°E and N6°± 32°E, and the averages of the dips of the P-and T-axes were 11°± 10°a nd 32°± 25°, respectively. These variations in the P-and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1 ) is oriented E-W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1 -σ 2 )/(σ 1 -σ 3 ), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3 , and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1 . The direction of the σ 1 -axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.
We investigated stress drops during an earthquake swarm in northeastern Noto Peninsula, central J... more We investigated stress drops during an earthquake swarm in northeastern Noto Peninsula, central Japan, which is characterized by ongoing seismic activity in four clusters. We focused on the spatiotemporal distribution of the static stress drop and its relationship with the source faults of the earthquake swarm. Employing the empirical Green's function method, we estimated static stress drops for 90 earthquakes of MJMA 3.0-5.4. We obtained logarithmic mean stress drops of 13 MPa and 19 MPa from Pwave and S-wave analyses, respectively, typical values for crustal earthquakes. We comprehensively analyzed the spatiotemporal distribution of static stress drops in the northern cluster due to the abundance of available data and clarity of fault structures there. We observed larger static stress drops for earthquakes along shallow portions of the source faults, as defined by the hypocentral distribution during a given period. Conversely, we observed smaller static stress drops for earthquakes at medial depths along the faults. These results suggest higher fault strength at shallower depths along the faults and reduced fault strength at medial depths. We attribute the high fault strength at shallow depths to low pore fluid pressure after only limited fluid diffusion near the fault terminus. In contrast, we attribute the reduction in fault strength at greater depths to high pore fluid pressure within the fault following penetration by migrating fluids.
The 2011 off the Pacific coast of Tohoku earthquake (the 2011 Tohoku earthquake) caused a strong ... more The 2011 off the Pacific coast of Tohoku earthquake (the 2011 Tohoku earthquake) caused a strong shaking in the eastern part of Honshu Island, Japan. We calculated the autocorrelation functions (ACFs) of ambient-noise waveforms in Miyagi prefecture and investigated the velocity change associated with the 2011 Tohoku earthquake. Seismic Interferometry enables us to extract Green’s functions of between two stations using correlation functions of ambient noise or coda waves (e.g., Campillo and Paul, 2003; Shapiro et al ., 2005). In other words, an ACF of a waveform gives us the Green’s function in the case that the source and receiver are collocated and let us detect reflected waves beneath the receiver. This method allows for the constant monitoring of the location of underground reflectors and the temporal change on subsurface velocity structure without artificial sources. We investigated the temporal change in ACFs of the ambient noise observed at 10 Hi-net stations in Miyagi prefec...
Intense swarm‐like seismicity associated with shallow normal faulting was induced in Ibaraki and ... more Intense swarm‐like seismicity associated with shallow normal faulting was induced in Ibaraki and Fukushima prefectures, Japan, following the 2011 Tohoku‐Oki earthquake. This seismicity shows a systematic spatiotemporal evolution, but little is known of the heterogeneity in crustal structure in this region, or its influence on the evolution of the seismicity. Here, we elucidate a high‐resolution model of crustal structure in this region and determine precise hypocenter locations. Hypocenters in Ibaraki Prefecture reveal a planar earthquake alignment dipping SW at ~45°, whereas those in Fukushima Prefecture show a more complex distribution, consisting of conjugate sets of aligned small earthquakes. On the north of the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), we imaged a high‐velocity body at shallow depths that lacks aftershock seismicity. Based on fault source models, the large‐slip region of the Iwaki earthquake is situated along a zone that ...
The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the ... more The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the 2011 M9 Tohoku earthquake. We analyzed 1142 earthquakes with$$4.4 \le M_{W} \le 5.0$$4.4≤MW≤5.0that occurred in 2003 through 2018 and obtained spatio-temporal pattern of stress drop on the Pacific Plate that subducts beneath the Okhotsk Plate. Here we show that small earthquakes at edges of a region with a large slip during the 2011 Tohoku earthquake had high values of stress drop, indicating that the areas had a high frictional strength and suppressed the coseismic slip of the 2011 Tohoku earthquake. In addition, stress drops of small earthquakes in some of the areas likely decreased after the 2011 Tohoku earthquake. This indicates that the frictional strength decreased at the areas due to the following aftershocks of the 2011 Tohoku earthquake, consistent with a high aftershock activity. This also supports that the frictional properties on a subducting plate interface can be monitored ...
We show that the spatial heterogeneity in the coseismic displacement of large earthquakes likely ... more We show that the spatial heterogeneity in the coseismic displacement of large earthquakes likely reflects the spatial characteristics of the frictional properties and that it can be inferred from the stress drop of moderate-sized earthquakes. We analyzed stress drops of 686 earthquakes with magnitudes of 4.0 to 5.0 off the southeast of Hokkaido, Japan, and investigated the spatial heterogeneity between the difference of shear strength and dynamic stress level on the Pacific Plate. We deconvolved observed P and S waves with those of collocated small earthquakes and derived the source effect of the earthquakes. We then estimated the corner frequencies of the earthquakes and calculated stress drops using a circular fault model. The values of stress drops showed a spatial pattern consistent with slip distributions of historical large earthquakes. Earthquakes that occurred in the area with a large coseismic slip during the 1968 Tokachi-oki (M W 8.2) and the 2003 Tokachi-oki (M W 8.0) earthquakes had large values of stress drop, whereas earthquakes in the afterslip area of the 2003 Tokachi-oki earthquake showed smaller values. In addition, an area between coseismic ruptures of the 1973 Nemuro-oki (M W 7.8) and the 2003 Tokachi-oki earthquakes had a large value of stress drop. Ruptures occurred in this area during the 1952 Tokachi-oki earthquake (M W 8.1), and the area acted as a barrier during the 2003 Tokachi-oki earthquake. These facts suggest that the frictional properties of the plate interface show little temporal change, and their spatial pattern can be monitored by stress drops of moderatesized earthquakes. The spatial heterogeneity provides important information for estimating the slip pattern of a future large earthquake and discussing a policy for disaster mitigation, especially for regions in which slip patterns of historical large earthquakes are unclear.
A cluster of earthquake activity took place beneath the Tanzawa Mountains at a depth of 20 km dur... more A cluster of earthquake activity took place beneath the Tanzawa Mountains at a depth of 20 km during the end of January 2012. The activity began at 22:39 UT on January 27 and included 78 earthquakes with magnitudes of 2.0 and greater within the span of 50 h. Five of them had magnitudes greater than 4.0, and the largest one was a M5.4 earthquake. We relocated the hypocenters by using the double-difference method and characterized their migrations away from the first earthquake of the cluster activity. The migration was consistent with fluid diffusion and had a similar speed to that of non-volcanic tremors and of induced earthquakes caused by water-injection experiments. We then analyzed stress drops for 16 earthquakes of M3.5 and greater that occurred from July 2003 to June 2012 in the area of the cluster activity. Earthquakes that occurred before and after the cluster activity had typical and stable values of stress drop. This is consistent with structural studies indicating the existence of little fluid in the region. In contrast, the cluster activity included earthquakes with significantly small stress drops. The leading hypothesis is that the cluster activity was associated with a decrease in the shear strength due to an increase in pore pressure, and this can explain both the migration of hypocenters and the small stress drops associated with the cluster activity. This hypothesis is also supported by the fact that earthquakes before and after the cluster activity had similar values of stress drop, thus suggesting that the activity was triggered by a different mechanism from the other earthquakes in the same region. The most plausible explanation is that there is a little fluid in the closed system beneath the Tanzawa Mountains that is undetectable by structural observations.
We have investigated the spatial distribution of the static stress drop of similar aftershocks of... more We have investigated the spatial distribution of the static stress drop of similar aftershocks of the 2007 Noto Hanto earthquake, central Japan, and have examined the relationship between the coseismic slip of the mainshock and the static stress drop of the similar aftershocks. The static stress drop is estimated by using the empirical Green's function method for P-and S-waves. The estimated static stress drop approximately ranges from 5 to 20 MPa, which is a typical range of values for tectonic earthquakes. The static stress drops of the aftershocks in a large slip area of the mainshock tend to be larger than those in a small slip area. This suggests a large difference between the strength and the dynamic stress level in the large slip area and a small difference in the small slip area if the final stress level is equal to the dynamic stress level.
In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 2... more In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P-and Taxes were N97°± 23°E and N6°± 32°E, and the averages of the dips of the P-and Taxes were 11°± 10°a nd 32°± 25°, respectively. These variations in the P-and Taxes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1) is oriented E-W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1-σ 2)/(σ 1-σ 3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3 , and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1. The direction of the σ 1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.
The observation of seafloor crustal deformation is very important to understand plate motions, nu... more The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its "true" position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.
The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthq... more The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M > 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro-oki and 2003 Tokachi-oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of $3 km/s, has been ruptured by the 1952 Tokachi-oki earthquake. The seismic gap has also slipped incidental to neighboring asperities. The difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface, such as a reflectivity of the plate interface. We estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun-ocean bottom seismometer experiment on the along-trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi-oki source area including the seismic gap, rather than in the 1973 Nemuro-oki source area. The strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi-oki and 1973 Nemuro-oki earthquakes. Our results suggest that the 1952 Tokachi-oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake.
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Papers by Takuji Yamada