Stress and slip on a fault during seismic cycle crucially depend on frictional properties on the fault. The parameter estimation of frictional properties may be achieved by fitting data to a numerical model. This procedure is known as “data assimilation”. One of diffculties in our problem is that measurements are strictly limited in time compared with earthquake recurrence periods. However, whole process of transient slip events such as afterslip and slow slip events have been recently observed by, for example, dense GPS networks because the duration of them are at most several years. Thus we first intend to estimate frictional parameters in afterslip and slow slip segments. Once we have parameters there, it might be possible to constrain frictional parameters in asperities by considering interactions among asperities, afterslip segments and other segments. We use two-degree-of-freedom fault cell model as the simplest one that contains interaction of stress changes due to fault slips. We define time series of cumulative displacement at fault cells as data. Then, using the sum of the squared residuals between the data and the calculated values for a frictional parameter set, we evaluate how good the parameter set is. The results are; 1) in the case of data with two coseismic terms or more, interseismic slip does not fully contribute to the residual estimation, because residual is small, if recurrence times of earthquakes are the same even for the case of different parameter set, 2) in the case without coseismic terms, the amount of displacement and the duration of afterslip or slow slip events are useful to constrain the possible parameter ranges except for the critical slip distance in rate- and state-friction law, L, in the case of afterslip at steady state, and 3) even one afterslip or slow slip event is useful. Moreover we find out; 4) data without any clear unstable slip can exclude the parameter range that produce unstable slip in the limited terms of used data, and 5) slip history on the another area can be useful to constrain the possible parameter range, because the fault slip with frictional parameters can change another fault slip behaviors. On the estimation of frictional parameters using large-degree-of-freedom continuum-approximated fault cell models, results 4) and 5) suggest that A) the number of possible solutions do not increase as many as that of the degree-of-freedom, and stress responses to fault slip among the fault cells become to be strong constraint of frictional parameters, and that B) every data on all areas and for all terms can be useful for the estimation of frictional parameter in the system that fault slip causes stress change everywhere, except the case that data have no information about the frictional parameter. Results from A) and B) suggest that realistic estimation of interplate frictional parameters will be possible through appropriate processes according to individual problems.

View full abstract

We studied the source model and the underground velocity structure model for quantitative estimation of long-period (3-20s) ground motion in the Oita Plain due to a hypothetical Nankai earthquake. First, the deep subsurface velocity structure of the Oita sedimentary basin was validated by a long-period ground motion simulation of the 2000 western Tottori earthquake records. The simulated waveforms reproduced the amplification and duration of the observed waveforms and peak periods of pseudo response spectra at strong motion stations both in and out the Oita Plain reasonably well. Then we combined the subsurface velocity structure with a crustal velocity structure and performed long-period ground motion simulations of a Nankai earthquake by a three-dimensional finite-difference method. The source model has a total area of 34,000 km² with total seismic moment of 6.24×10²¹ Nm (M_W 8.5). In addition to the scenario in which rupture propagates from the east (east-hypo model) that have been generally accepted, we investigated another from the west (west-hypo model). For the east-hypo model, the maximum amplitudes of the simulated horizontal ground motion in the Oita plain was three to four times as large as that on a rock site beside the plain. Especially in the bay area, the maximum ground motion reached 100cm/s and the pseudo velocity response spectra at period 6-8s were more than 400cm/s. On the other hand, the ground motion simulated by the west-hypo model was roughly one fifth smaller than that by the east-hypo model. It is also pointed out that in the east-hypo model simulation, the seismic waves generated by the two asperities near to the Oita plain are enlarged due to the directivity effect and amplified and prolonged by the sedimentary basin structure.

View full abstract

Kego Fault is a 50km-long northwest-trending left-lateral active fault in north Kyushu, Japan. 2005 West off Fukuoka Prefecture Earthquake was generated from the north part of its submarine extent. We conducted single-channel seismic profiling and coring of sediments in Hakata bay in order to reveal Holocene paleoseismicity for seismic risk evaluation of the unbroken southern part of the fault, on which highly-populated Fukuoka city is developing. The active fault traces in the bay extend continuously from land, and keep same mode and sense of faulting shown on land as left-lateral strike slip with downthrown to the east. Our high-resolution seismic profile records show consistent vertical displacement down to the east along the fault traces in the Holocene sediments. Horizontally deposited layers below 2.1m from the bay floor were successively faulted, and the layers on down-thrown side between 2.1 to 2.6m were faulted by 0.3m, and 3.8 to 6.2m by 0.6m, and the acoustic basement by 3m, suggesting that there were three or more seismic events during Holocene. We collected four piston-core samples from both sides of the fault traces. The gray colored fossiliferous Holocene mud shows no evidence of hiatus in the 8m-long cored layers on the downthrown side. The carbon-14 dates of molluscs and echinoderms fossils suggest that the latest two events took place between 4,500 yBP to 4,000 yBP, and 8,500yBP to 6,500 yBP, respectively. Considering the interval of the events and the timing of the latest event, we positively warn that the southern half of Kego Fault has high probability in generating a large earthquake anytime in near geological future.

View full abstract

S-wave velocity structures were explored in Niigata and Shonai plains from long-period micro-tremor array measurements to estimate long-period earthquake ground motion for disaster prevention. The Rayleigh wave phase velocities in a period range from 0.3 to 5.0 seconds were estimated by a frequency-wavenumber spectral analysis of vertical microtremors at 9 sites in Niigata plain and 4 sites in Shonai plain. The phase velocities at each site were inverted to a 1-D S-wave profile with 5 layers using a heuristic search technique by very fast simulated annealing. In the northwestern part of Niigata plain along the coast of the Sea of Japan, the profiles of sedimentary layers are characterized by S-wave velocities of about 0.4 km/s, 0.7 km/s, 1.0 km/s and 2.0 km/s, respectively. On the contrary, the profiles in the southeastern part are characterized by S-wave velocities of about 0.4 km/ s, 1.2 km/s, 1.5 km/s and 1.8 km/s, respectively. These results show that compositions of S-wave velocity of sedimentary layers vary with the area in Niigata plain. However, compositions of S-wave velocities of sedimentary layers are similar in Shonai plain and the profiles of sedimentary layers are characterized by S-wave velocities of about 0.4 km/s, 1.0 km/s, 1.2 km/s and 1.8 km/s, respectively. The estimated depths of the top of the basement layer in Niigata plain agree well with those of the top of the Green tuff formation. In Shonai plain, the estimated depths to the basement layer correspond to those of the top of the pre-Tertiary basement. The Bouguer anomaly distribution that was low cut filtered at 150 km can be explained by the distribution of the estimated depths of the top of the basement layer. Dispersion characteristics derived from the observed transverse component records during the Mid Niigata Prefecture earthquake in 2004 and the Niigataken Chuetsu-oki earthquake in 2007 are well explained by the theoretical dispersion curves of Love wave group velocity calculated from the estimated S-wave velocity structures in two plains.

View full abstract