Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
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Volume 59, Issue 4
Displaying 1-12 of 12 articles from this issue
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ARTICLE
  • Presumed Asperities for the Anticipated Tokai Earthquake (Seismic Activity Change in the Tokai Region: Part 4)
    Shozo MATSUMURA
    2007 Volume 59 Issue 4 Pages 271-284
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.271
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    The slow-slip event in the area under Lake Hamana, which may have been triggered by the 2000 Miyake seismo-volcanic activities, lasted for over five years and has finally ceased at the end of 2005. Within the same period, changes in seismic activity were detected in central Shizuoka prefecture, which is assumed to be the main locked zone for the anticipated Tokai earthquake. It is considered that the slow-slip released part of the accumulated stress around the western edge of the locked zone, and that, once released, the stress was loaded on the rest of the locked zone, producing these seismicity changes. This study is concerned with locating the essentially locked part within the focal zone of the future Tokai earthquake, that is, distinguishing the asperities from the seismicity changes. Knowing the position of the asperity prior to the final breakage of the anticipated earthquake is beneficial for both hazard estimation and earthquake prediction. Analysis of the spatial pattern of the seismicity changes was based on the following assumption. The seismicity change was caused by a quasi-static slip that progress on the weakly locked zone of the focal area and eventually result in stress concentration on the asperities. The spatial pattern of the seismicity change is classified into two categories, activation and quiescence, and the asperities could be assigned to the former. As a result, three activated zones identified in central Shizuoka prefecture could be considered candidates for the asperities. Five evidences reported by several researchers were used to verify that the assumed asperities are actually stress-concentrated. These evidences are b-value change, tidal dependence of seismic activity, large stress drop, radiation of short-period seismic waves, and transition aspects of the slow-slip distribution. All these evidences supported our basic assumption.
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Dedicated Section: The M7.2 Miyagi-oki Earthquake of 16 August 2005
ARTICLES
  • Interplate Quasi-Static Slip off Miyagi and Fukushima Prefectures Estimated from Small Repeating Earthquake Data
    Naoki UCHIDA, Toru MATSUZAWA, Satoshi MIURA, Satoshi HIRAHARA, Akira H ...
    2007 Volume 59 Issue 4 Pages 287-295
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.287
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    Spatio-temporal distribution of quasi-static slip on the plate boundary east off Miyagi and Fukushima prefectures, NE Honshu, Japan is estimated by using small repeating earthquake data. The analysis of small repeating earthquakes has advantages of relatively high spatial resolution, especially near the trench, and the availability of long-term data (22 years) compared to GPS data. The results show that the repeating earthquakes are distributed outside the coseismic slip areas (asperities) of large earthquakes, showing that fault creep is dominant outside the asperities. The cumulative slip (slip histories) of small repeating earthquake groups reveal the existence of many non-steady aseismic slip events. Most of the episodic quasi-static slip events are associated with M≥ 6 earthquakes and they are frequently seen in the areas near the Japan trench in particular. Minor afterslip (∼15cm) of the 2005 Miyagi-oki earthquake (M7.2) is also estimated in the area which encompasses the coseismic slip area of the 2005 earthquake.
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  • Aftershock Distribution of the 2005 Miyagi-oki Earthquake (M 7.2) Revealed by Ocean Bottom Seismological Observation: Preliminary Results
    Ryota HINO, Kensuke SUZUKI, Yojiro YAMAMOTO, Minoru NISHINO, Toshihiko ...
    2007 Volume 59 Issue 4 Pages 297-308
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.297
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    Hypocenter distribution of the 2005 Miyagi-oki earthquake (M 7.2) and its aftershocks were relocated by using ocean bottom seismographic (OBS) data. The mainshock was observed by 14 OBSs which were deployed in the focal area about one month prior to the occurrence. Additional 16 OBSs were deployed for aftershock observations. Relocated hypocenters concentrate along a landward dipping plane whose location and dip angle correspond well to the plate boundary defined by a previous study. Although the most significant aftershock activity concentrates around the rupture area of the mainshock where substantial amount of coseismic slip was retrieved, two moderate (M 6.3) earthquakes with numerous aftershocks were observed in the trenchward region. The hypocenters of those earthquakes were also located along the plate boundary and the epicenter locations correspond to the aftershock area of the 1981 inter plate earthquake (M 7.0). Most of the aftershocks near the rupture area form a very thin plane and have focal mechanism solutions of a thrust fault type almost identical to that of the mainshock, suggesting these aftershocks occurred along the rupture plane of the mainshock, the plate boundary. There is a conspicuous linear cluster of the aftershocks at the eastern edge of the rupture area. The focal depth distribution of this cluster is slightly thicker than those of the aftershocks around the mainshock hypocenter. The cluster contains a number of events with focal mechanisms other than the thrust type. These observations suggest that a number of aftershocks occurred not only along but also off the plate interface at the up-dip end of the mainshock rupture. The b-value of the aftershocks in the linear cluster is significantly larger than that of the aftershocks near the mainshock hypocenter. This difference may reflect the difference in characteristic scales of structural heterogeneity along the plate interface and that within the plates.
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  • Interaction between Fault Segments on a Subducting Plate Interface
    Keisuke ARIYOSHI, Toru MATSUZAWA, Yasuo YABE, Akira HASEGAWA, Naoyuki ...
    2007 Volume 59 Issue 4 Pages 309-324
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.309
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    A two-dimensional (2D) numerical simulation of seismic cycles on the plate boundary in a subduction zone is performed based on a rate- and state-dependent friction law to obtain insight into complicated cycle of interplate earthquakes off Miyagi Prefecture and to investigate possible precursory sliding behavior. Assuming that the complexity of seismic cycle such as the off Miyagi Prefecture Earthquake is mainly due to the heterogeneity of frictional properties, we divide the plate boundary into five segments along dip direction: three frictionally stable segments and two (shallower and deeper) seismogenic segments are placed alternately. From many trial simulation results, we find that the frictional parameters (b-a)and dc in the deeper seismogenic segment should be smaller than those in the shallower segment in order to generate earthquakes in the deeper segment more frequently than in the shallower segment. The simulation results show that preseismic, coseismic, and postseismic slip in a seismogenic segment tends to become larger when it occurs just after the earthquake in the other seismogenic segment, because the previous earthquake in the other seismogenic segment keeps on forward slip through the intermediate aseismic segment. We also find that the stress rate in the down-dip direction in the vicinity of the lower edge of the deeper seismogenic segment is compressional for the period of about one year prior to the occurrence of the earthquake in the segment while it is tensional prior to the event in the shallower segment. This result suggests that seismic activity change in a slab can be a clue to the intermediate-term prediction of large interplate earthquakes.
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  • Relocation of the M∼7 Miyagi-oki Earthquakes in the 1930s: Seismic Slips of Asperities that Were Re-ruptured during the 1978 M7.4 Miyagi-oki Earthquake ?
    Norihito UMINO, Toshio KONO, Tomomi OKADA, Junichi NAKAJIMA, Toru MATS ...
    2007 Volume 59 Issue 4 Pages 325-337
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.325
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    Hypocenters of the main shocks and aftershocks of the 1933 Mjma 7.1, 1936 Mjma 7.4, 1937 Mjma 7.1, 1939 Mjma 6.9, 1978 Mjma 7.4, and 1981 Mjma 7.0 Miyagi-oki, NE Japan, earthquakes are relocated by using S-P times reported in the Seismological Bulletin of the Japan Meteorological Agency (JMA) and those re-read from original smoked-paper seismograms observed at Mizusawa station of National Astronomical Observatory of Japan (NAOJ) and at Mukaiyama station of Tohoku University. In order to avoid large errors caused by inaccuracies in the arrival times of P- and S-waves and the limited number of observation stations, we determined hypocenters by using a grid search method based on the assumption that these events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of the 1933, 1936, 1937, and 1978 earthquakes are determined close to each other, and distributions of their aftershocks show that aftershock areas of 1933, 1936, and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central, and westernmost portions, respectively. It is likely that the 1933, 1936, and 1937 events possibly ruptured a part of the source area of the 1978 event, that is the eastern, central, and western portions, respectively. Locations of the main shock and aftershock area of the 1939 event are adjacent to the eastern edge of the source area of 1978 event. After the 1978 event, the 1981 earthquake had occurred there following the slip on the asperities in the presumed Miyagi-oki earthquake source area.
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  • Distributions of Seismic Intensities of the 1930s M∼7 Miyagi-oki Earthquakes
    Toshio KONO, Norihito UMINO, Akira HASEGAWA
    2007 Volume 59 Issue 4 Pages 339-353
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.339
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    Distributions of seismic intensities of the 1933 MJMA 7.1, 1936 MJMA 7.4 and 1937 MJMA 7.1 Miyagi-oki earthquakes are investigated and compared with that of instrumental seismic intensities of the 2005 M 7.2 Miyagi-oki earthquake. Residuals of seismic intensities are estimated from an empirical regression equation of seismic intensity attenuations in NE Japan. Large positive residuals of seismic intensities, namely overdetermined seismic intensities, are deduced at several commission stations of the Central Meteorological Observatory (CMO) in the central and western portions of Fukushima prefecture and in northern Kanto District. Based on local newspaper articles reporting seismic damages by the 1930s M∼7 Miyagi-oki earthquakes, seismic intensities are estimated in Iwate, Miyagi, Fukushima, Ibaraki, Tochigi, and Gunma prefectures. Since there were no newspaper articles of seismic damages in the central and western portions of Fukushima prefecture and in northern Kanto District, seismic intensities there are estimated to be less than 4. Locations with seismic intensity 5 are estimated at several places close to the main shock of the 1930s M∼7 Miyagi-oki earthquakes. Distribution of seismic intensity 5 estimated from the seismic damages of the 1936 M 7.4 Miyagi-oki earthquake are very similar to that of the instrumental seismic intensity of the 2005 M 7.2 Miyagi-oki earthquake: these two earthquakes seem to be caused by an slip of the same asperity on the upper surface of the Pacific plate.
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  • Estimation of Source Spectrum of the 2005 Miyagi-oki Earthquake and Attenuation and Site Amplification Factors around Miyagi Prefecture
    Yutaka MAMADA, Genyuu KOBAYASHI, Hideo TANAKA
    2007 Volume 59 Issue 4 Pages 355-370
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.355
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    We estimated source spectra of the 2005 Miyagi-oki earthquake (MJMA 7.2) and 11 events (4.0< MJMA<5.0) consisting of inter-plate and inland earthquakes occurred around Miyagi region applying the spectral inversion method to seismic records observed at KiK-net stations in Miyagi prefecture. Attenuation and site amplification factors were also estimated. Before applying the method, geometrical spreading factor was estimated as a function of frequency using the twofold spectral ratios. It was successfully estimated at a frequency range from 3 to 20 Hz. The average over this frequency range is represented as 1.12±0.27. Applying this average value to the geometrical spreading factors for all frequency ranges (0.5 to 20 Hz), we performed the spectral inversion for the data recorded at 12 KiK-net stations in Miyagi prefecture. As a result the attenuation factor (1/Qs) of shear waves was successfully estimated at frequency ranges above 3 Hz. The frequency dependent 1/Qs can be approximated by Qs(f)-1=(5.0×10-3)ƒ-0.8 at a frequency range of 3 to 12 Hz. We found that total attenuation effect including geometrical spreading and attenuation factor contribute little to the observed strong acceleration at high frequency range. Short-period spectral level of the 2005 Miyagi-oki earthquake estimated from the source spectrum is roughly two times as large as that expected from the empirical rule for the event with same seismic moment. These results indicate that high level of high frequency source spectrum of the 2005 Miyagi-oki earthquake contributed to the observed strong acceleration at high frequency range in Miyagi region.
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  • Interplate Slip Distribution Inferred from Co- and Post-seismic Deformation Associated with the 2005 Miyagi-oki Earthquake (M7.2)
    Satoshi MIURA, Takeshi IINUMA, Satoshi YUI, Toshiya SATO, Kenji TACHIB ...
    2007 Volume 59 Issue 4 Pages 371-379
    Published: March 25, 2007
    Released on J-STAGE: August 05, 2013
    DOIhttps://doi.org/10.4294/zisin.59.371
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    A typical interplate earthquake with M7.2 occurred on August 16, 2005 on the plate boundary off Miyagi Prefecture. Co- and post-seismic deformations associated with this event were investigated to reveal the causal interplate slips using continuous GPS data and geodetic inversion. The co-seismic slip distribution shows good agreement with that estimated by seismic waveform inversions. The major slip area is limited to the southeastern part of the rupture area of the 1978 Miyagi-oki earthquake (M7.4). The post-seismic slip extended uni-laterally to the southwest of the co-seismic slippage. These distinctive features both of the co- and post-seismic slips might be caused by the existence of the locked plate interface in the northern and southwestern parts of the 1978 rupture area, where seismogenic stress has not released yet.
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LETTERS
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