Miyagi-oki Earthquake Research Articles

Introduction to the special section for the 2005 M7.2 Miyagi-oki earthquake

An M=7.2 earthquake occurred on the plate boundary off Miyagi Prefecture, northeastern Japan, at 11:46 a.m. Japanese Standard Time on 16 August 2005, when the number of people returning to their homes after the holidays was at a peak. This earthquake resulted in the Japanese bullet train of Tohoku/Akita/Yamagata/Shinkansen falling out of service for half a day, and it took a whole day for the service to get back to normal. A suspended ceiling also fell at a public facility in Sendai city, and many people were injured. Overall, this earthquake injured 79 people, and caused partial damage to 339 buildings. Large earthquakes with a magnitude of ∼7.5 have occurred repeatedly at a recurrence interval of ‘37 years along the plate boundary off Miyagi Prefecture. When the M=7.2 event occurred in the source area of the anticipated Miyagioki earthquake, more than 27 years had passed since the last event, and the possibility of an earthquake occurrence within the next 10 years exceeded 50% according to the evaluation report by the Headquarters for Earthquake Research Promotion (2003) of the Japanese government. Among all of the anticipated earthquakes in Japan, this Miyagi-oki earthquake had the highest probability of occurrence. Although the August 2005 Miyagi-oki earthquake occurred within the source area of the anticipated Miyagioki earthquake, the magnitude was slightly smaller than M=7.5 that had been anticipated. For this reason, the Headquarters for Earthquake Research Promotion predicted that this earthquake is not the anticipated Miyagi-oki earthquake. However, if this 2005 earthquake is not the one anticipated—subsequently denoted here as the “coming” earthquake—when are we to expect the Miyagi-oki earthquake? Moreover, what is happening within the source area of the coming Miyagi-oki earthquake at this moment? It is critical for us to provide answers to those questions in order to get a clear picture of the coming Miyagi-oki earthquake. Dense nationwide seismic and GPS networks, which were recently constructed as the “Kiban” networks [in Japanese, this means the basic observation network (Okada et al., 2004; Sagiya, 2004)], were successful in recording the mainshock rupture, aftershock activity and post-

Coseismic slip distribution of the 2005 off Miyagi earthquake (M7.2) estimated by inversion of teleseismic and regional seismograms

Abstract A large earthquake (M7.2) occurred along the plate boundary off Miyagi Prefecture (Miyagi-Oki), northeastern Japan, on August 16, 2005. In this area, large earthquakes (~M7.5) have occurred repeatedly at intervals of about 37 years, and more than 27 years have passed since the last event occurred. To estimate the relationship between this earthquake and the previous events, we determined coseismic slip distribution by this 2005 Miyagi-Oki earthquake by adopting the seismic waveform inversion method of Yagi et al. (2004) and compared it with that of the previous 1978 Miyagi-Oki earthquake. We performed two cases of the inversions; inversion using only far-field seismograms and that using far-field seismograms and local seismograms simultaneously. Both results show that a large slip occurred near the hypocenter and rupture extended to the westward deeper portion. Considering that the rupture area of the 2005 event partly overlapped with the southeastern part of that of the 1978 event, suggests this result the possibility that plural asperities exist which cause the sequence of Miyagi-Oki earthquakes and that the 2005 event ruptured one of such asperities, although the previous 1978 event ruptured all the asperities at one time.

Revisiting the three M~7 Miyagi-oki earthquakes in the 1930s: possible seismogenic slip on asperities that were re-ruptured during the 1978 M=7.4 Miyagi-oki earthquake

Abstract Hypocenters of main shocks and aftershocks of the 1933 M=7.1, 1936 M=7.4, 1937 M=7.1 and 1978 M=7.4 Miyagi-oki earthquakes are relocated 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 the Mizusawa station of the National Astronomical Observatory of Japan (NAOJ) and the Mukaiyama station of Tohoku University. In order to reduce the error caused by inaccuracies of the arrival times and the small number of seismic observation stations, we determined the hypocenters by using a grid search method that assumed that the events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of these four earthquakes were determined to be close to each other, while the distributions of their aftershocks seem to disperse on the upper boundary of the Pacific plate. These distributions show that aftershock areas of the 1933, 1936 and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central and westernmost portions, respectively. This result suggests that the 1933, 1936 and 1937 events possibly ruptured a part of the source area of the 1978 event, i.e., its eastern, central and western portions, respectively.

Nonlinear site response from the 2003 and 2005 Miyagi-Oki earthquakes

Abstract We estimate nonlinear site response by comparing site response estimates from the 16 August 2005 Mj=7.2 and 26 May 2003 Mj=7.0 Miyagi-Oki earthquakes with site response estimates from aftershocks of the 2003 event. Site response is solved by a spectral inversion technique to separate source, path, and site components. The constraint motion in the inversion is a regional attenuation model derived from fitting the spectra of data recorded at borehole KiK-net stations in the region and a theoretical source spectrum for each event determined using the same borehole stations. Site response is calculated at the surface of the KiK-net and K-NET stations. In general, the average aftershock site response is larger than for the two mainshocks, especially at a higher frequency. When comparing site response with input ground motion level, the predominant frequency and the site response values tend to decrease as the level of input ground motion increases.

Co- and post-seismic slip associated with the 2005 Miyagi-oki earthquake (M7.2) as inferred from GPS data

Abstract A large earthquake with M7.2 occurred on August 16, 2005 along 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 coseismic 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 previous 1978 event. The post-seismic slip extended to the southwest of the co-seismic slip area. These distinctive features of both the co- and post-seismic slips might be caused by the existence of the locked plate interface, where seismogenic stress has not released yet, in the northern part of the 1978 rupture area.

Small repeating earthquakes and interplate creep around the 2005 Miyagi-oki earthquake (M=7.2)

AbstractTaking advantage of the feature that creep around an asperity is necessary for the recurrent rupture of the same small asperity (small repeating earthquakes), we have estimated the spatio-temporal distribution of quasistatic slip (creep) around the 2005 Miyagi-oki earthquake (M=7.2) using the distribution of small repeating earthquakes. The creep was detected mainly outside of the coseismic slip areas for the 2005 Miyagi-oki, 1978 Miyagi-oki (M=7.6) and 2003 Fukushima-oki (M=6.8) earthquakes. The creep rates estimated from the recurrence intervals and slip amounts of small repeating earthquakes for 21 years were almost constant for the areas near the western limit of the interplate earthquakes but they varied temporally in the areas nearer to the Japan trench. The changes in the creep rates before and after the 2005 Miyagi-oki earthquake were not significant with the exception of small slip accelerations in some areas near the Japan trench. These results suggest that the plate boundary around the source area for the 2005 earthquake is still mostly locked.

Site-Response Estimation for the 2003 Miyagi-Oki Earthquake Sequence Considering Nonlinear Site Response

The M w 7.0 Miyagi-Oki earthquake, which occurred on 26 May 2003, was well recorded by the KiK-net and K-net networks. A large number of stations recorded very high peak ground accelerations above 0.5g and large peak ground velocities above 0.5 m/sec. These high ground-motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high-frequency ground-motion content associated with this intraslab earthquake. The objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration ! 0.3g. Linear site response is first estimated from observed weak motion (aftershock) records. In this case, we use a spectral inversion method, without reference stations, to separate the source, path, and site-response effects. The resulting weak motion analysis for the source, path, and site response agree with other previous studies. The mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency-dependent radiation pattern. The comparison of the site am- plification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the Mw 7.0 event. The results also suggest a correlation between lownear-surface materialvelocity and thedegreeofnonlinearity.

津波水門の提案

The present study shows the importance of the flood gate performance during severe earthquake accompanied by tsunami. One of the major concerns for mitigation of disastrous earthquake was the occurrence of Tokai, Tonankai and Nankai, Miyagi-oki earthquake which are expected to be accompanied by huge tsunami. Because tsunami source of those earthquakes locates very near to the coast of Japan, quick operation of gate-closing is necessary. Exiting flood gates are recently renewed for automatic operation but it takes much time to close the gate and damage during earthquake and resultant incapability of gating are worried. Considering those situation, new type of flood gate which was designated as a special countermeasure for tsunami was presented. This new tsunami gate gives us the reliable gating capability and safe system at emergency situation by employing the self-gate-closing system.

Afterslip of the plate interface following the 1978 Miyagi–Oki, Japan, earthquake, as revealed from geodetic measurement data

From the analysis of the tide gauge records and precise leveling data, we found a postseismic deformation following the 1978 Miyagi–Oki earthquake (M=7.4), Japan, on June 12, 1978. The postseismic deformation in the vicinity of the focal region lasted at least four years. By assuming that the deformation was caused by an afterslip on the subducting plate interface, we estimated the area and amount of the afterslip by the inversion analysis. The result of the inversion analysis indicates that, in about two years following the mainshock, the afterslip was restricted to the mainshock fault and its vicinity on the plate interface. In the following years, the afterslip ceased on the mainshock fault, and it migrated towards the down-dip extension of the mainshock fault. The total amount of aseismic moment release was 80–140% of that of the coseismic moment release. Similar afterslip has been reported for the 1994 Sanriku–Haruka–Oki earthquake (M=7.7) that occurred about 280km north of the 1978 Miyagi–Oki earthquake. The occurrence of the large afterslip on the mainshock fault and its migration towards the deeper portion of the plate boundary may be a common feature for the interplate earthquakes occurring in the northeast Japan along the Japan Trench.

Deconvolution of squared velocity waveform as applied to the study of a noncoherent short-period radiator in the earthquake source

We consider an inverse problem of determination of short-period (high-frequency) radiator in an extended earthquake source. This radiator is assumed to be noncoherent (i.e., random), it can be described by its power flux or brightness (which depends on time and location over the extended source). To decide about this radiator we try to use temporal intensity function (TIF) of a seismic waveform at a given receiver point. It is defined as (time-varying) mean elastic wave energy flux through unit area. We suggest estimating it empirically from the velocity seismogram by its squaring and smoothing. We refer to this function as “observed TIF”. We believe that one can represent TIF produced by an extended radiator and recorded at some receiver point in the earth as convolution of the two components: (1) “ideal” intensity function (ITIF) which would be recorded in the ideal nonscattering earth from the same radiator; and (2) intensity function which would be recorded in the real earth from unit point instant radiator (“intensity Green's function”, IGF). This representation enables us to attempt to estimate an ITIF of a large earthquake by inverse filtering or deconvolution of the observed TIF of this event, using the observed TIF of a small event (actually, fore-or aftershock) as the empirical IGF. Therefore, the effect of scattering is “stripped off”. Examples of the application of this procedure to real data are given. We also show that if one can determine far-field ITIF for enough rays, one can extract from them the information on space-time structure of the radiator (that is, of brightness function). We apply this theoretical approach to short-periodP-wave records of the 1978 Miyagi-oki earthquake (M=7.6). Spatial and temporal centroids of a short-period radiator are estimated.

耐震設計用地震動予測モデルの有効性の検証 (1)

Theoretical ground motion model presented here is tested against Miyagi-oki earthquake of June 12, 1978 observed at the stations Ofunato and Shiogama. The Green's functions of Ofunato and Shiogama are calculated exactly using the reported epicentral distance and focal depth. The theoretical ground motion models are estimated by the convolution of their Green's functions and the one source function derived by Inversion method. The wave form and Fourier spectra functions of the theoretical ground motion models agree well with the corresponding ones of Ofunato and Shiogama.

Rupture extent of the 1978 Miyagi‐Oki, Japan, earthquake and seismic coupling in the northern Honshu Subduction Zone

Underthrusting at subduction zones can cause large earthquakes at shallow depths, but is always accommodated by aseismic creep below a certain depth. This transition depth is referred to as the depth of seismic coupling and can be directly observed in a subduction zone as the lower depth extent of significant moment release of the deepest large underthrusting earthquakes. In 1978, a large (Ms=7.5) earthquake occurred off the coast of Miyagi Prefecture in northern Honshu. Its focal mechanism represents underthrusting of the Pacific plate beneath Honshu. Since the hypocenter is located 150 km landward from the trench and there are no other large interplate earthquakes further landward from the trench axis, this event defines the maximum depth of the coupled zone.The lower limit of significant moment release of the Miyagi‐Oki earthquake is obtained by analysis of the long‐period P waves. The deconvolved source time function consists of a dominant single pulse with peak moment release at 12 s and a total duration of 18 s. The rupture extent of this dominant pulse does not extend deeper than 40 km, thus the transition from coupled to uncoupled in northern Honshu occurs at or above 40 km depth.

Broadband analysis of the extended foreshock sequence of the Miyagi-Oki earthquake of 12 June 1978

abstract The Miyagi-Oki earthquake of 12 June 1978, a large (Ms 7.8) interplate thrust event, occurred in a region which had not experienced earthquakes of magnitude greater than 7 since 1938. A sequence of four moderate-sized (5.4 < mb < 6.1) earthquakes encircled the rupture zone of the Miyagi-Oki earthquake over a period of 2 yr before the main shock. Broadband displacement and velocity records of body waves recorded digitally by stations of the Global Digital Seismograph Network are analyzed to determine the static and dynamic characteristics of the sequence. These characteristics include moment, radiated energy, dynamic and static stress drop, and apparent stress. Inversions of duration measurements made on the velocity waveforms permit quantifying the complexity of an event as well as constraining its rupture geometry. Intervals of 7 to 8 months separated the first three events; the main shock occurred 4 months after the third event. The rupture process of the third event was relatively complex; the event also had a substantially higher dynamic stress drop (175 bars) than did the stress drops of the first two events (9 and 10 bars, respectively). These observations suggest that the third event was an interme-diate-term precursor to the main shock. The fourth event, a short-term precursor to the main shock, occurred about 8 min before the main shock. Its dynamic stress drop (20 bars) was lower than that of the third event but higher than that of the first two events.

A statistical model for prediction of quasirealistic strong ground motion.

The total power, the duration, and the starting time of band-pass filtered accelerograms are chosen as model parameters to express amplitude variations of ground acceleration in both the time and frequency domains. In order to estimate model parameters of certain earthquakes and site conditions, a probabilistic source model is used and accelerograms recorded during the Matsushiro earthquake swarm are analyzed statistically. The duration and the starting time of the strong ground motion are found to have close relations to the amplification characteristics of surface layers at the sites. As an example, model parameters are estimated by assuming the source parameters and the site conditions of the Miyagi-Oki earthquake of June 12, 1978. Wave-forms, running spectra, and velocity response spectra of synthetic accelerograms resemble those of observed accelerograms.

Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978

The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 10 27 dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km 2, respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment M 0 = 1.7 × 10 27 dyne-cm, slip dislocation u = 1.9 m, Δσ = 95 bar, rupture velocity ν = 3.2 km s −1, rise time τ = 2 s , for the first event; M 0 = 1.4 × 10 27 dyne-cm, u = 2.4 m, Δσ = 145 bar , for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 10 10 erg cm −2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.