Application of simulated annealing to focal mechanism determination

Simulated annealing (SA) is known as one of the efficient methods for global optimization. It allows for getting an optimal solution by jumping out of local minimum. In this paper, we apply the simulated annealing to determination of focal mechanism solution. To investigate the feasibility of the application, we determine focal mechanism solution of the 14 March 2014 Iyo-Nada intermediatedepth earthquake. The computation of SA method is then 1278 times faster than that of the grid search method

Onset time and location of the main rupture of the 2018 Hokkaido Eastern Iburi, Japan, earthquake

The rupture process at the beginning stage of the 2018 Hokkaido Eastern Iburi, Japan, earthquake (MJ 6.7) is investigated by analyzing P-wave records from local strong-motion stations. The records show about 3 s of small amplitude arrival (so-called "initial rupture phase") followed by the onset of the main energy release ("main rupture phase"). In this paper we address the issue: where the seismic energy release corresponding to the main rupture phase started at the primary stage of the main rupture, by locating the main rupture onset position. For this aim we applied the 2D and the 3D search methods to estimate the relative location of the onset of the main rupture with respect to the hypocenter, and the time difference between them. The 2D method assumes a plausible initial fault plane and locates the main rupture onset position on the initial fault plane, while the 3D method does not require such assumed fault planes. In the 2D method we employed each of the nodal planes of the first-motion focal mechanism as the initial rupture plane. The 3D method was able to give a better solution than the 2D one. It suggests that the main rupture initiation point might not be on the same fault plane as the hypocenter is. The solution shows that the main rupture onset point is 5.8 km southward and 2.5 km upward from the hypocenter, with the rupture time of 3.3 s from the origin time. It is consistent with the previous studies on the relationship between the magnitude and the initial rupture duration

Estimation of displacement waveforms by baseline correction of near-fault acceleration records of the 2016 Kumamoto earthquake with median filter

The 2016 Kumamoto earthquake sequence occurred on April 14 (MJMA 6.5) and April 16 (MJMA 7.3). Seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale was observed in Mashiki Town, Kumamoto Prefecture for the both events and in Nishihara Village, Kumamoto Prefecture for the April-16 event. We estimate the displacement waveforms from these acceleration records. Since the acceleration seismograms include the long-period noise due to tilting of the ground and instrumental effects, the baseline corrections are required to derive the accurate velocity and displacement waveforms. We apply a median filter to the velocity waveforms to identify the linear trends on them due to the steplike noise on the acceleration records, and determine the time at which baseline shifts take place and the step value of each shift for the baseline correction through trial and error. Our baseline correction can successfully reconstruct the velocity and displacement waveforms from the acceleration records. The displacement waveforms show the static components consistent with the geodetic data

Computation of non-linear site response by the time-domain finite-difference method

We have developed a time-domain staggered-grid finite-difference code for modeling non-linear response of a one-dimensionally inhomogeneous subsurface structure to a SH plane-wave incidence. It employs the velocity-stress formulation of elastodynamic equation for the linear part, and adopts a elastoplastic rheology model for the non-linear relation between the stress and strain. In this paper, we apply this code to four constitutive models from linear-elastic to nonlinear: (1) linear elastic model, (2) linear viscoelastic model, (3) elastoplastic model, and (4) viscoelastoplastic model, which simulate shallow sand and clay structures and are vibrated by a vertically incident SH plane-wave of Ricker wavelet, to compare the linear and the non-linear soil behaviors including low strains damping (viscoelastic effect) and/or hysteretic attenuation (non-linear effect). We also apply it to a local strong-motion record of the 2000 Western-Tottori earthquake (MW6.8). We then simulate characteristics of non-linear site response such as reduction of the spectral amplitude in the high frequency band and shift of the peak frequencies to lower frequencies

FDM simulation of long-period ground motions around Oita Prefecture, Japan, using a land-ocean unified 3D structure model

Oita prefecture is located in northeastern part of Kyushu Island which is characterized by active subduction of the Philippine Sea plate (PHS) beneath the Eurasian plate and several active volcanoes along with the volcanic front. Oita area has frequently been damaged by large earthquakes and tsunamis since ancient times. From the point of view of disaster prevention, it is important to investigate the feasibility of strong ground motion prediction using realistic structural models. In this paper we use a land-ocean unified 3D (three-dimensional) structure model around Oita prefecture, which includes land and sea-floor topography and a seawater layer as well as subsurface structures of the arc side and the PHS slab to conduct the FDM (finite-difference method) simulations of strong ground motion in land and ocean areas for the 2015 Southern Oita, Japan, earthquake (MJMA5.7) whose hypocenter is located in the PHS slab. The simulated long-period (2–20 s) ground motions reproducing observed records demonstrate substantial contributions of thick low-velocity sediment layers in and around Beppu Bay and Oita basin to development of the motions. We also examine the topographic effects on the seismic motion by analyzing the simulation results to show the strong enhancement of the later phases

Three-dimensional P- and S-wave attenuation structures around the source region of the 2016 Kumamoto earthquakes

Abstract We investigate the three-dimensional P- and S-wave attenuation ( $$Q_{\text{P}}^{ - 1}$$ Q P - 1 and $$Q_{\text{S}}^{ - 1} )$$ Q S - 1 ) structures of the crust around the source region of the 2016 Kumamoto earthquakes, Japan. To estimate the attenuation structures, the path-averaged attenuation factor $$t^{*}$$ t ∗ is estimated from the amplitude decay rate of the P- and S-wave spectra corrected for the source spectrum. The $$Q_{\text{P}}^{ - 1}$$ Q P - 1 and $$Q_{\text{S}}^{ - 1}$$ Q S - 1 structures are estimated by tomography using $$t^{*}$$ t ∗ for the P- and S-waves, respectively. Several features are found in the attenuation structures as follows: In the source region, two high- $$Q_{\text{P}}$$ Q P and high- $$Q_{\text{S}}$$ Q S zones exist along the Futagawa and the Hinagu fault segments in the upper crust. The high- $$Q_{\text{P}}$$ Q P and high- $$Q_{\text{S}}$$ Q S zone along the Futagawa fault segment is found to include the large-slip area of the mainshock obtained from a source inversion study. In the lower crust, the low $$Q_{\text{P}}$$ Q P is distributed beneath the entire source region. A low- $$Q_{\text{P}}$$ Q P and low- $$Q_{\text{S}}$$ Q S zone also exists beneath the Kuju and Aso volcanoes, which is consistent with the shallow limited depth extent of the seismogenic zone due to high temperature. The western edge of this zone adjoins the eastern edge of the high- $$Q_{\text{P}}$$ Q P and high- $$Q_{\text{S}}$$ Q S area, including the large-slip area. Graphical Abstract