The 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0) occurred on March 11, 2011, caused strong ground motion around northeastern Japan, and generated devastating tsunami, which killed more than 16,000 people. Before the strong ground motion hit cities, the Japan Meteorological Agency (JMA) issued Earthquake Early Warning (EEW) announcements to the general public of the Tohoku district and then the warning was automatically broadcast through TV, radios and cellular phone messages. JMA also issued the first tsunami warnings/advisories based on hypocentral parameters, i.e., location, focal depth and magnitude, at 14:49 (Japan Standard Time), which was about three minutes after the occurrence of the earthquake, and then upgraded them using sea-level observation data. This paper reports the performance of the EEW and the tsunami warnings/advisories, lessons learned from the earthquake, and direction for the improvement of the warning systems.
{"title":"Earthquake Early Warning and Tsunami Warning of JMA for the 2011 off the Pacific Coast of Tohoku Earthquake","authors":"M. Hoshiba, T. Ozaki","doi":"10.4294/ZISIN.64.155","DOIUrl":"https://doi.org/10.4294/ZISIN.64.155","url":null,"abstract":"The 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0) occurred on March 11, 2011, caused strong ground motion around northeastern Japan, and generated devastating tsunami, which killed more than 16,000 people. Before the strong ground motion hit cities, the Japan Meteorological Agency (JMA) issued Earthquake Early Warning (EEW) announcements to the general public of the Tohoku district and then the warning was automatically broadcast through TV, radios and cellular phone messages. JMA also issued the first tsunami warnings/advisories based on hypocentral parameters, i.e., location, focal depth and magnitude, at 14:49 (Japan Standard Time), which was about three minutes after the occurrence of the earthquake, and then upgraded them using sea-level observation data. This paper reports the performance of the EEW and the tsunami warnings/advisories, lessons learned from the earthquake, and direction for the improvement of the warning systems.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"15 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114515436","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We reported seismic waveform analysis results of the source process of the 2011 mega-thrust Tohoku-oki earthquake. The Tohoku-oki earthquake is the first mega-thrust earthquake in Japan since the initiation of modern and multi-channel seismic observation. Many researchers have performed source inversion using seismic waveforms observed at near-source or/and global seismic networks, and presented their seismic source models. As pointed out by some researchers, however, seismic source models for the 2011 Tohoku-oki earthquake are different from one another. The discrepancy has prevented us to understand the nature of mega-thrust earthquake in Tohoku-oki region, which may be originated from data processing manner, assumption of error structure, strength of constraints, and setting of source model. In this paper, we endeavor to describe detailed analysis procedure for each study. Common feature in slip distributions obtained by all studies is that the huge seismic slip located off the coast of Miyagi (Miyagi-oki) where huge slip deficit was detected by GPS studies. Many studies obtained the large rupture near Japan Trench or hypocenter during 45-70 sec after initial break, which may be a key of understanding this earthquake. At present, it is difficult to discuss the detailed rupture process of the 2011 Tohoku-oki earthquake. The future studies using multi-channel data analysis with proper error structure model will reveal the nature of mega-thrust earthquake.
{"title":"Seismic Source Process of the 2011 Tohoku-oki Earthquake","authors":"Y. Yagi","doi":"10.4294/ZISIN.64.143","DOIUrl":"https://doi.org/10.4294/ZISIN.64.143","url":null,"abstract":"We reported seismic waveform analysis results of the source process of the 2011 mega-thrust Tohoku-oki earthquake. The Tohoku-oki earthquake is the first mega-thrust earthquake in Japan since the initiation of modern and multi-channel seismic observation. Many researchers have performed source inversion using seismic waveforms observed at near-source or/and global seismic networks, and presented their seismic source models. As pointed out by some researchers, however, seismic source models for the 2011 Tohoku-oki earthquake are different from one another. The discrepancy has prevented us to understand the nature of mega-thrust earthquake in Tohoku-oki region, which may be originated from data processing manner, assumption of error structure, strength of constraints, and setting of source model. In this paper, we endeavor to describe detailed analysis procedure for each study. Common feature in slip distributions obtained by all studies is that the huge seismic slip located off the coast of Miyagi (Miyagi-oki) where huge slip deficit was detected by GPS studies. Many studies obtained the large rupture near Japan Trench or hypocenter during 45-70 sec after initial break, which may be a key of understanding this earthquake. At present, it is difficult to discuss the detailed rupture process of the 2011 Tohoku-oki earthquake. The future studies using multi-channel data analysis with proper error structure model will reveal the nature of mega-thrust earthquake.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129999424","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Aoi, T. Kunugi, W. Suzuki, N. Morikawa, H. Nakamura, S. Senna, H. Fujiwara
The 2011 M 9.0 Tohoku-Oki earthquake is the largest earthquake that occurred in and around Japan since the beginning of the recorded history. This megathrust event initiated approximately 100km off-shore Miyagi prefecture, in northeast Japan, and its rupture extended 400-500km along the subducting Pacific plate. This is the first M 9-class earthquake that has been closely recorded by a dense seismograph network. Strong motions of this earthquake are characterized by large seismic intensities and peak ground accelerations (PGA), long durations, and wideness of the area that experienced intense shaking. The ground motions were recorded by 1223 K-NET and KiK-net stations. The PGA exceeded gravity at 20 sites; the largest PGA, of 2933 gals, was observed at the K-NET Tsukidate station (MYG004). The attenuation of the recorded peak values shows a possibility of saturating strong ground motion amplitude with the magnitude. The complex features of the accelerograms and velocity waveforms are discussed in connection with the source processes estimated using long and short period waveform data. Due to the large ground motions and tsunamis associated with this event, more than 16 thousand people were killed and more than 360 thousand houses and buildings were totally or partially destroyed. Although the tsunamis were the primary cause of damage, the strong shaking, liquefaction and landslides also brought serious destruction. However, it was reported that the damage ratios of houses and buildings directly due to shaking were not as high as for the former earthquakes having comparable seismic intensities and PGA. The recorded ground motions at most stations where the seismic intensities and PGA were large had dominant periods shorter than 0.5s and relatively poor power in the 1-2s period range which has strong influence on the damage of few-stories wooden houses. The main reason for the short-period predominance is the amplification due to the low-velocity superficial layer and can be roughly explained by empirical amplification factors for 0.1-0.5s periods. Long-period ground motions were also observed. The velocity response spectra (5% dumping) for periods of 4-20s reached around 100cm/s at many stations, mainly in the Tohoku and Kanto regions. This level may be considered not very large taking into account that the Tohoku-Oki earthquake was an M 9-class event. In the Kanto district, at epicentral distances of 300-400km, liquefaction widely occurred at the artificially reclaimed land in Tokyo and Chiba bay areas and the basin of major rivers, such as the Tone and Ara. The damage (e.g., cutoff of lifelines and differential settlement of house-basements) due to liquefaction was very severe. In this paper we summarize the strong motion characteristics associated with the M 9.0 Tohoku-Oki earthquake and review the latest results from the viewpoint of strong motions.
{"title":"Strong Motion Characteristics of the 2011 Tohoku-Oki Earthquake","authors":"S. Aoi, T. Kunugi, W. Suzuki, N. Morikawa, H. Nakamura, S. Senna, H. Fujiwara","doi":"10.4294/ZISIN.64.169","DOIUrl":"https://doi.org/10.4294/ZISIN.64.169","url":null,"abstract":"The 2011 M 9.0 Tohoku-Oki earthquake is the largest earthquake that occurred in and around Japan since the beginning of the recorded history. This megathrust event initiated approximately 100km off-shore Miyagi prefecture, in northeast Japan, and its rupture extended 400-500km along the subducting Pacific plate. This is the first M 9-class earthquake that has been closely recorded by a dense seismograph network. Strong motions of this earthquake are characterized by large seismic intensities and peak ground accelerations (PGA), long durations, and wideness of the area that experienced intense shaking. The ground motions were recorded by 1223 K-NET and KiK-net stations. The PGA exceeded gravity at 20 sites; the largest PGA, of 2933 gals, was observed at the K-NET Tsukidate station (MYG004). The attenuation of the recorded peak values shows a possibility of saturating strong ground motion amplitude with the magnitude. The complex features of the accelerograms and velocity waveforms are discussed in connection with the source processes estimated using long and short period waveform data. Due to the large ground motions and tsunamis associated with this event, more than 16 thousand people were killed and more than 360 thousand houses and buildings were totally or partially destroyed. Although the tsunamis were the primary cause of damage, the strong shaking, liquefaction and landslides also brought serious destruction. However, it was reported that the damage ratios of houses and buildings directly due to shaking were not as high as for the former earthquakes having comparable seismic intensities and PGA. The recorded ground motions at most stations where the seismic intensities and PGA were large had dominant periods shorter than 0.5s and relatively poor power in the 1-2s period range which has strong influence on the damage of few-stories wooden houses. The main reason for the short-period predominance is the amplification due to the low-velocity superficial layer and can be roughly explained by empirical amplification factors for 0.1-0.5s periods. Long-period ground motions were also observed. The velocity response spectra (5% dumping) for periods of 4-20s reached around 100cm/s at many stations, mainly in the Tohoku and Kanto regions. This level may be considered not very large taking into account that the Tohoku-Oki earthquake was an M 9-class event. In the Kanto district, at epicentral distances of 300-400km, liquefaction widely occurred at the artificially reclaimed land in Tokyo and Chiba bay areas and the basin of major rivers, such as the Tone and Ara. The damage (e.g., cutoff of lifelines and differential settlement of house-basements) due to liquefaction was very severe. In this paper we summarize the strong motion characteristics associated with the M 9.0 Tohoku-Oki earthquake and review the latest results from the viewpoint of strong motions.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"21 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123626891","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In inversion analyses using ABIC, a non-full rank matrix for prior constraints is allowed in the formulation of Yabuki and Matsu'ura (1992), while Fukuda and Johnson (2008) claimed that the matrix must be full rank. This problem depends on consideration about the value of “zero” of zero eigenvalues contained in the prior constraint matrix. In actual inversion analyses, we must have some prior information about model parameters, even if it is not explicitly expressed. Therefore, the “zero” of the zero eigenvalues is considered to be not zero exactly. Based on this consideration, we can obtain the same inversion solution as Yabuki and Matsu'ura (1992). Mathematical difficulty in expressing the prior probability density function for a matrix with zero eigenvalues can be avoided by the use of non-informative prior.
{"title":"Inversion Analyses Based on ABIC with Non-full Rank Prior Information","authors":"Y. Fukahata","doi":"10.4294/ZISIN.64.91","DOIUrl":"https://doi.org/10.4294/ZISIN.64.91","url":null,"abstract":"In inversion analyses using ABIC, a non-full rank matrix for prior constraints is allowed in the formulation of Yabuki and Matsu'ura (1992), while Fukuda and Johnson (2008) claimed that the matrix must be full rank. This problem depends on consideration about the value of “zero” of zero eigenvalues contained in the prior constraint matrix. In actual inversion analyses, we must have some prior information about model parameters, even if it is not explicitly expressed. Therefore, the “zero” of the zero eigenvalues is considered to be not zero exactly. Based on this consideration, we can obtain the same inversion solution as Yabuki and Matsu'ura (1992). Mathematical difficulty in expressing the prior probability density function for a matrix with zero eigenvalues can be avoided by the use of non-informative prior.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"185 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"117130090","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We extracted unsteady vertical crustal deformations in the Shikoku region before GPS deployment using leveling and sea-level data. GPS-derived, steady-state vertical displacements related to ongoing subduction of the Philippine Sea plate were subtracted from the vertical displacements observed by leveling surveys. Monthly mean sea-level data were corrected for atmospheric pressure and hydrographic effects. As a result, we found an upheaval of 4-5cm near Kochi City from 1977 to 1980 in both the leveling and sea-level data. This may represent a crustal deformation caused by a long-term slow slip event on the plate boundary.
{"title":"Long-term Slow Slip Event around Kochi City from 1977 to 1980","authors":"A. Kobayashi","doi":"10.4294/ZISIN.64.63","DOIUrl":"https://doi.org/10.4294/ZISIN.64.63","url":null,"abstract":"We extracted unsteady vertical crustal deformations in the Shikoku region before GPS deployment using leveling and sea-level data. GPS-derived, steady-state vertical displacements related to ongoing subduction of the Philippine Sea plate were subtracted from the vertical displacements observed by leveling surveys. Monthly mean sea-level data were corrected for atmospheric pressure and hydrographic effects. As a result, we found an upheaval of 4-5cm near Kochi City from 1977 to 1980 in both the leveling and sea-level data. This may represent a crustal deformation caused by a long-term slow slip event on the plate boundary.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"30 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114413619","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Great earthquakes have historically occurred along the Nankai Trough. It has been said that they ruptured part or whole of characteristic fault planes A, B, C, D, and E repeatedly. However, there are a number of enigmas for their occurrence. Major ones are as follows. The 1944 Showa-Tonankai earthquake occurred only 90 years after the 1854 Ansei earthquakes. The 90-year period seems short compared with other time intervals of the historical earthquakes. The Tonankai earthquake did not rupture fault plane E west of the Suruga Trough, by some unknown reasons. The Tokai earthquake anticipated at fault plane E has not occurred yet since the Ansei-Tokai event even if a slow slip event occurred recently near the downdip end of its rupture zone. In this study, I propose a model to solve these enigmas. I characterize a fault plane of a great earthquake into a seismic-b.eq, a tsunami-b.eq, and a geodetic-b.eq, in which seismic waves, tsunamis, and crustal deformations are dominantly generated, respectively. I compare these different bands of rupture zones between the 1944 Showa-Tonankai and 1854 Ansei-Tokai earthquakes, the 1946 Showa-Nankai and 1854 Ansei-Nankai earthquakes, and the 1707 Hoei and other earthquakes, using seismic intensity data and previous studies on asperities, tsunamis, and crustal deformations. It is found that the Ansei-Tokai and Showa-Tonankai earthquakes scarcely shared their seismic-b.eqs. The tsunami- and geodetic-b.eqs of the Ansei-Tokai earthquake extended to the west of its seismic-b.eq, and was shared by, but did not cover the seismic-, tsunami- and geodetic-b.eqs of the Showa-Tonankai earthquake. It cannot thus be said that the Ansei-Tokai earthquake ruptured fault planes C+D+E or that fault plane E was left unbroken after the Showa-Tonankai earthquake. The occurrence of these two earthquakes is rather complementary from a viewpoint of the seismic-b.eq. The seismic-b.eq of the Ansei-Nankai earthquake also seems to have been different from and was located further north than that of the Showa-Nankai earthquake. On the other hand, the Hoei earthquake had a seismic-b.eq similar to those of the Showa earthquakes. I group historical great earthquakes into the Ansei-type or the Hoei-type, which has a seismic-b.eq similar to either of the Ansei or Hoei earthquake. It is likely that the Ansei-type earthquakes are the 684 Hakuho, 1096 Eicho-1099 Kowa, 1498 Meio, and 1854 Ansei earthquakes and recurred with a ∼400-year period, and that the Hoei-type earthquakes are the 887 Ninna, 1361 Shohei, 1707 Hoei, and 1944 Tonankai-1946 Nankai earthquakes and recurred with a ∼350-year period. Since the Showa-Tonankai earthquake was complementary to the Ansei-Tokai earthquake, the 90-year period between the two events is not a recurrence time and it is natural that the Showa-Tonankai did not rupture fault plane E. It is also natural that the next Tokai earthquake did not occur even if the slow slip event occurred at its downdip end, because it is
{"title":"Great Earthquakes along the Nankai Trough","authors":"T. Seno","doi":"10.4294/ZISIN.64.97","DOIUrl":"https://doi.org/10.4294/ZISIN.64.97","url":null,"abstract":"Great earthquakes have historically occurred along the Nankai Trough. It has been said that they ruptured part or whole of characteristic fault planes A, B, C, D, and E repeatedly. However, there are a number of enigmas for their occurrence. Major ones are as follows. The 1944 Showa-Tonankai earthquake occurred only 90 years after the 1854 Ansei earthquakes. The 90-year period seems short compared with other time intervals of the historical earthquakes. The Tonankai earthquake did not rupture fault plane E west of the Suruga Trough, by some unknown reasons. The Tokai earthquake anticipated at fault plane E has not occurred yet since the Ansei-Tokai event even if a slow slip event occurred recently near the downdip end of its rupture zone. In this study, I propose a model to solve these enigmas. I characterize a fault plane of a great earthquake into a seismic-b.eq, a tsunami-b.eq, and a geodetic-b.eq, in which seismic waves, tsunamis, and crustal deformations are dominantly generated, respectively. I compare these different bands of rupture zones between the 1944 Showa-Tonankai and 1854 Ansei-Tokai earthquakes, the 1946 Showa-Nankai and 1854 Ansei-Nankai earthquakes, and the 1707 Hoei and other earthquakes, using seismic intensity data and previous studies on asperities, tsunamis, and crustal deformations. It is found that the Ansei-Tokai and Showa-Tonankai earthquakes scarcely shared their seismic-b.eqs. The tsunami- and geodetic-b.eqs of the Ansei-Tokai earthquake extended to the west of its seismic-b.eq, and was shared by, but did not cover the seismic-, tsunami- and geodetic-b.eqs of the Showa-Tonankai earthquake. It cannot thus be said that the Ansei-Tokai earthquake ruptured fault planes C+D+E or that fault plane E was left unbroken after the Showa-Tonankai earthquake. The occurrence of these two earthquakes is rather complementary from a viewpoint of the seismic-b.eq. The seismic-b.eq of the Ansei-Nankai earthquake also seems to have been different from and was located further north than that of the Showa-Nankai earthquake. On the other hand, the Hoei earthquake had a seismic-b.eq similar to those of the Showa earthquakes. I group historical great earthquakes into the Ansei-type or the Hoei-type, which has a seismic-b.eq similar to either of the Ansei or Hoei earthquake. It is likely that the Ansei-type earthquakes are the 684 Hakuho, 1096 Eicho-1099 Kowa, 1498 Meio, and 1854 Ansei earthquakes and recurred with a ∼400-year period, and that the Hoei-type earthquakes are the 887 Ninna, 1361 Shohei, 1707 Hoei, and 1944 Tonankai-1946 Nankai earthquakes and recurred with a ∼350-year period. Since the Showa-Tonankai earthquake was complementary to the Ansei-Tokai earthquake, the 90-year period between the two events is not a recurrence time and it is natural that the Showa-Tonankai did not rupture fault plane E. It is also natural that the next Tokai earthquake did not occur even if the slow slip event occurred at its downdip end, because it is ","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125607325","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A double-planed shallow seismic zone has been found in the northeastern Japan forearc region. However, the characterizations of the focal mechanisms of earthquakes in this zone, especially the lower plane events, cannot be carried out adequately due to low seismicity and poor station coverage on the focal sphere of the onshore P-wave polarity data. In this study, we determine the focal depth using the sP depth phase and the focal mechanisms using the P-wave initial motions observed by the ocean bottom cabled seismic stations and temporary autonomous ocean bottom seismic networks off Miyagi, as well as the onshore seismic networks. Seven focal mechanism solutions of events were precisely determined. Using the classification based on the dip angles of the T, B and P axes, we classified 21 weakly constrained focal mechanisms of other events having poor station coverage. All the determined solutions were of the thrust-faulting type regardless of the focal depth. The offshore observations helped considerably in constraining the focal mechanisms of these far-offshore earthquakes, especially their rake angles. Although previous studies had reported that the focal mechanisms of the upper and lower plane events show predominantly normal and reverse faulting respectively, our result showed that the thrust events in the upper plane seem to occur on the plate boundary. We found that the focal mechanisms change from normal faulting in the upper plane to reverse faulting in the lower plane at a depth of 15km from the plate boundary, possibly defining the depth of the neutral plane in the northeastern Japan forearc region included in the source area of the 1933 Mw8.4 Sanriku earthquake. The western edge of the normal faulting events along the upper plane is located about 70km inward from the trench axis. These results suggest that the earthquake-generating stress field in the double-planed shallow and deep seismic zone in the northeastern Japan arc can be explained by the bending-unbending model of the subducting Pacific plate.
{"title":"Focal Mechanisms of Small Earthquakes within the Pacific Plate near the Japan Trench","authors":"S. Koga, Y. Ito, R. Hino, M. Shinohara, N. Umino","doi":"10.4294/ZISIN.64.75","DOIUrl":"https://doi.org/10.4294/ZISIN.64.75","url":null,"abstract":"A double-planed shallow seismic zone has been found in the northeastern Japan forearc region. However, the characterizations of the focal mechanisms of earthquakes in this zone, especially the lower plane events, cannot be carried out adequately due to low seismicity and poor station coverage on the focal sphere of the onshore P-wave polarity data. In this study, we determine the focal depth using the sP depth phase and the focal mechanisms using the P-wave initial motions observed by the ocean bottom cabled seismic stations and temporary autonomous ocean bottom seismic networks off Miyagi, as well as the onshore seismic networks. Seven focal mechanism solutions of events were precisely determined. Using the classification based on the dip angles of the T, B and P axes, we classified 21 weakly constrained focal mechanisms of other events having poor station coverage. All the determined solutions were of the thrust-faulting type regardless of the focal depth. The offshore observations helped considerably in constraining the focal mechanisms of these far-offshore earthquakes, especially their rake angles. Although previous studies had reported that the focal mechanisms of the upper and lower plane events show predominantly normal and reverse faulting respectively, our result showed that the thrust events in the upper plane seem to occur on the plate boundary. We found that the focal mechanisms change from normal faulting in the upper plane to reverse faulting in the lower plane at a depth of 15km from the plate boundary, possibly defining the depth of the neutral plane in the northeastern Japan forearc region included in the source area of the 1933 Mw8.4 Sanriku earthquake. The western edge of the normal faulting events along the upper plane is located about 70km inward from the trench axis. These results suggest that the earthquake-generating stress field in the double-planed shallow and deep seismic zone in the northeastern Japan arc can be explained by the bending-unbending model of the subducting Pacific plate.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"13 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2012-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125065918","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
K. Taniguchi, Mitsuhisa Watanabe, Yasuhiro Suzuki, H. Sawa
The 150 km long Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL) in central Japan is one of the most active fault systems in Japan. Paleoseismologcal studies 1980s have revealed that the most recent event and the average recurence interval of the ISTL. The approximately 7 km long portion of the fault system between Matsumoto and Okaya has been regarded as a gap without any active fault trace. The gap namely the “Shiojiri Pass Gap” has long been taken as a segment boundary owing to the geometric discontinuity. Recent geomorphological analyses of the gap have demonstrated a through-going left-lateral slip assocaited with recent earthquakes in this area, based on aerial photograph interpretation and excavation studies. Excavation study on this portion revealed that the latest faulting event occurred between 1,700 cal. B.P. to 1,310 cal. B.P. (255 A.D. -645 A.D.). The timing of the last faulting event at this study area coincides with the timing in the Gofukuji fault and Okaya fault. The active faults extending from the Matsumoto basin as far as the northwestern margin of the Suwa basin display the evidence for its recent reactivation at the same time.
{"title":"Newly-found Surface Faulting Event on the North-central Portion of the Itoigawa-Shizuoka Tectonic Line Active Fault System","authors":"K. Taniguchi, Mitsuhisa Watanabe, Yasuhiro Suzuki, H. Sawa","doi":"10.4294/ZISIN.64.11","DOIUrl":"https://doi.org/10.4294/ZISIN.64.11","url":null,"abstract":"The 150 km long Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL) in central Japan is one of the most active fault systems in Japan. Paleoseismologcal studies 1980s have revealed that the most recent event and the average recurence interval of the ISTL. The approximately 7 km long portion of the fault system between Matsumoto and Okaya has been regarded as a gap without any active fault trace. The gap namely the “Shiojiri Pass Gap” has long been taken as a segment boundary owing to the geometric discontinuity. Recent geomorphological analyses of the gap have demonstrated a through-going left-lateral slip assocaited with recent earthquakes in this area, based on aerial photograph interpretation and excavation studies. Excavation study on this portion revealed that the latest faulting event occurred between 1,700 cal. B.P. to 1,310 cal. B.P. (255 A.D. -645 A.D.). The timing of the last faulting event at this study area coincides with the timing in the Gofukuji fault and Okaya fault. The active faults extending from the Matsumoto basin as far as the northwestern margin of the Suwa basin display the evidence for its recent reactivation at the same time.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"130 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132184172","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A new method to estimate the velocity boundaries of real layered structure from only surface recordings was constructed by decomposing an SH-wave into instantaneous power of wave associated with rays in a homogeneous half space. The estimated results obtained by applying this method to seismograms were represented as a function of lapse-time (t) and depth-time (τ); i.e., travel time from surface toward depth-direction. We conducted the evaluation of the proposed method to the strong motion data recorded at the FCH array. The recordings were obtained using broadband velocity seismometers for the earthquakes that occurred in five source regions, the Eastern Yamanashi, East off Izu peninsula, in and around Tokyo, the Southwestern Ibaraki, and the Central Chiba regions. The estimated results of velocity boundaries obtained by this method were in good agreement with the velocity boundaries previously determined by means of down-hole method. The comparison study with seismic interferometry was also conducted, and yielded concordant results for the estimation of velocity boundaries.
{"title":"Estimation of Velocity Boundaries in a Sedimentary Layer-Basement System Using the Fuchu Array Recordings","authors":"Maki Takagishi, S. Kinoshita","doi":"10.4294/ZISIN.64.1","DOIUrl":"https://doi.org/10.4294/ZISIN.64.1","url":null,"abstract":"A new method to estimate the velocity boundaries of real layered structure from only surface recordings was constructed by decomposing an SH-wave into instantaneous power of wave associated with rays in a homogeneous half space. The estimated results obtained by applying this method to seismograms were represented as a function of lapse-time (t) and depth-time (τ); i.e., travel time from surface toward depth-direction. We conducted the evaluation of the proposed method to the strong motion data recorded at the FCH array. The recordings were obtained using broadband velocity seismometers for the earthquakes that occurred in five source regions, the Eastern Yamanashi, East off Izu peninsula, in and around Tokyo, the Southwestern Ibaraki, and the Central Chiba regions. The estimated results of velocity boundaries obtained by this method were in good agreement with the velocity boundaries previously determined by means of down-hole method. The comparison study with seismic interferometry was also conducted, and yielded concordant results for the estimation of velocity boundaries.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"58 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114724237","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The relationship between the days of earthquake in Japan and its vicinity and the Be-7 concentrations in the surface air at Takasaki were statistically analyzed. A day when one or more earthquakes of M 5 or greater occurred in Japan and its vicinity was defined as a “day of earthquake”. Earthquakes which were deemed aftershocks were not included in the analysis. In order to avoid the effect of seasonal variation, the residuals obtained by deducting 31-day moving averages from Be-7 concentrations and those obtained by deducting the ‘21-day’ moving averages of the periods from −15 to +0 days and from +11 to +15 days were analyzed. The ‘21-day’ moving averages were also calculated to avoid the effect of aftershocks. Rank-sum tests of the residuals showed that the Be-7 concentrations decreased from the 31-day and ‘21-day’ moving averages on days of earthquake. A binomial test showed that there were significantly more days of earthquake when the Be-7 concentrations were smaller than the medians of Be-7 concentrations for the 31-day periods (from 15 days before to 15 days after days of earthquake) than those when the Be-7 concentrations exceeded the medians. These suggest that the Be-7 concentrations decrease on days of earthquake. It is noted that the Be-7 concentrations on days of earthquake decreased on days of both precipitation and non-precipitation, respectively.
{"title":"A Statistical Analysis of the Relationship between Days of Earthquake in Japan and Its Vicinity and Beryllium-7 Concentrations in the Surface Air at Takasaki","authors":"Masaki Ichihashi","doi":"10.4294/ZISIN.64.23","DOIUrl":"https://doi.org/10.4294/ZISIN.64.23","url":null,"abstract":"The relationship between the days of earthquake in Japan and its vicinity and the Be-7 concentrations in the surface air at Takasaki were statistically analyzed. A day when one or more earthquakes of M 5 or greater occurred in Japan and its vicinity was defined as a “day of earthquake”. Earthquakes which were deemed aftershocks were not included in the analysis. In order to avoid the effect of seasonal variation, the residuals obtained by deducting 31-day moving averages from Be-7 concentrations and those obtained by deducting the ‘21-day’ moving averages of the periods from −15 to +0 days and from +11 to +15 days were analyzed. The ‘21-day’ moving averages were also calculated to avoid the effect of aftershocks. Rank-sum tests of the residuals showed that the Be-7 concentrations decreased from the 31-day and ‘21-day’ moving averages on days of earthquake. A binomial test showed that there were significantly more days of earthquake when the Be-7 concentrations were smaller than the medians of Be-7 concentrations for the 31-day periods (from 15 days before to 15 days after days of earthquake) than those when the Be-7 concentrations exceeded the medians. These suggest that the Be-7 concentrations decrease on days of earthquake. It is noted that the Be-7 concentrations on days of earthquake decreased on days of both precipitation and non-precipitation, respectively.","PeriodicalId":332254,"journal":{"name":"Journal of the Seismological Society of Japan","volume":"25 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116492253","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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