Kojiro Irikura

Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period

A spectral acceleration attenuation model for Japan is presented in the present study. The data set includes a very large number of strong ground-motion records up to the 2003 Off Tokach main and aftershocks. Site class terms, instead of individual site correction terms, are used. The site classes of recording stations are from a recent study on site classification for strong-motion recording stations in Japan according to a classification scheme that has been used in Japanese engineering design. The use of site class terms enables tectonic source-type effects to be identified and accounted for in the present model. The effects of a faulting mechanism for crustal earthquakes also are accounted for. For crustal and interface earthquakes, a simple form of an attenuation model (with respect to distance) is able to capture the main strong-motion characteristics and achieves unbiased estimates. For subduction slab events, a simple distance modification factor is employed to achieve plausible and unbiased predictions. The effects of source depth, tectonic source type, and faulting mechanism of crustal earthquakes are significant. The need for magnitude- squared terms is evaluated, and the use of magnitude-squared terms reduces the interevent error further.

An Empirical Site-Classification Method for Strong-Motion Stations in Japan Using h/v Response Spectral Ratio

Having a reliable site classification scheme is vital for the development of robust strong-motion attenuation models. We discuss a promising empirical site- classification scheme based on strong-motion data from Japan. We assigned site classes, according to the site classification defined in engineering design codes in Japan, for those K-net sites where boreholes reached either to rock or to stiff soils with shear-wave velocity of 600 m/sec or larger, using four site classes defined by dominant site period. The average response spectral ratios of the horizontal and vertical components (h/v) of earthquake records for all site classes were found not to be strongly affected by Japan Meteorological Agency (jma) magnitude, hypocentral distance, and focal depth for all site classes. We used h/v ratios for records from the classified K-net sites to establish a site classification index using the mean spectral ratios over a wide range of spectral period. Using the index, we were able to classify both K-net stations with soil layers thicker than 20 m and other strong-motion stations in Japan. The peak period of the h/v spectral ratio can also be used to identify soft soil sites. The site amplification factors calculated from the site class terms based on the new site classification are consistent with the period bands defined for these site classes.

Surface Rupturing and Buried Dynamic-Rupture Models Calibrated with Statistical Observations of Past Earthquakes

In the context of the slip-weakening friction model and simplified asper- ity models for stress state, we calibrate dynamic rupture models for buried and surface-rupturing earthquakes constrained with statistical observations of past earth- quakes. These observations are the kinematic source models derived from source in- versions of ground-motion and empirical source models of seismic moment and rupture area. The calibrated parameters are the stress-drop distribution on the fault and average stress drop. We develop a set of dynamic rupture models that consist of asperities and surrounding background areas. The distribution of dynamic stress drop outside the asperity is characterized by a fraction of the stress drop on the as- perity. From this set of models, we identify dynamic fault models with defined stress- drop characteristics that satisfy the observations. The selected dynamic fault models show that surface-rupturing earthquakes are characterized by a large area of negative stress-drop surrounding the asperities, while buried earthquakes present positive or zero stress drop. In addition, the calibrated fault models that match the observations show that the average stress drop is independent of earthquake size for buried earth- quakes, but scale dependent for surface-rupturing earthquakes. This suggests that, in the context of our parameterization, buried earthquakes follow self-similarity scaling, and surface-rupturing earthquakes break this self-similarity. We apply the calibrated dynamic models to simulate near-source ground motion consistent with observations that suggest that buried earthquakes generate stronger ground motion than surface- rupturing earthquakes at high frequency. We propose possible mechanisms that satisfy this observation, as follows: buried rupture has a hypocenter location below the as- perity; this can produce strong directivity of the slip velocity function toward the free surface. That effect, in addition to a reduced fault area and low fracture energy during rupture, may be significant in enhancing high-frequency ground motion. On the other hand, surface-rupturing earthquakes have a shallow hypocenter, large fracture energy on the asperities, and enhanced energy absorption due to large areas of negative stress drop in the background area. These characteristics of large earthquakes inhibit severe directivity effects on the slip velocity function directly toward the free surface, redu- cing the high-frequency ground motion.

Short‐Period Source Model of the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake

We have constructed a short‐period source model for the 2011 M wxa09.0 Off the Pacific Coast of Tohoku earthquake using strong‐motion records at stations near the source fault. The observed strong motions contain five wavepackets that correspond to specific strong‐motion generation areas (SMGAs). The origins of the wavepackets were retrieved from the original seismograms using a semblance analysis. We determine the locations of the SMGAs based on sources extracted from the corresponding wavepackets. The short‐period source model consists of five SMGAs (SMGA1–5), which are located west of the hypocenter and along the down‐dip edge of the source fault. SMGA1 is located in the Miyagi‐Oki source region west of the hypocenter, SMGA2 in the middle Sanriku‐Oki source region north of the hypocenter, and SMGA3 in the southern Sanriku‐Oki source region west of the hypocenter. SMGA4 and SMGA5 are located near the down‐dip edge of the mainshock source fault, extending from offshore Fukushima prefecture to offshore Ibaraki prefecture. At some stations near the source fault, impulsive waves are also seen on the recorded seismograms. Ground motions corresponding to these impulsive waves cannot be accounted for using the conventional uniform SMGA model. We attempt instead to simulate the observed ground motions at the Onagawa Nuclear Power Plant at a depth of 128xa0m very near the source fault, using a heterogeneous source model. The impulsive waves are well simulated using the heterogeneous model with higher stress parameters within a small subarea inside the SMGAs.

On Scaling of Fracture Energy and Stress Drop in Dynamic Rupture Models: Consequences for Near‐Source Ground‐Motions

We calculate spontaneous dynamic rupture models for several well-recorded moderate to large earthquakes and analyze the scaling properties of fracture energy and stress drop. Among the set of 12 source models for 9 different earthquakes, the large events did break the surface while the moderate-size events occurred as completely buried ruptures (i.e. no surface faulting). We find that dynamic and static stress drop differ by only about 10%. Fault-averaged stress drop increases with increasing earthquake magnitude, while also fault-averaged (or maximum) fracture energy grows with magnitude. The scaling of fracture energy with the stress intensity factor appears to be sensitive to whether or not the earthquake rupture broke the surface, indicating that large earthquakes consume more fracture energy as the rupture expands and reaches the surface. This scaling of fracture energy may shed light on the recent observation that large, surface breaking earthquakes apparently generate lower near-source ground motions than buried ruptures in a certain period range of engineering interest. The derived empirical scaling relations for fracture energy may help to constrain the initial conditions for future dynamic rupture modeling, but can also be used in physics-based source characterization for near-source ground-motion calculations.

Strong-Motion Generation Areas of a Great Subduction-Zone Earthquake: Waveform Inversion with Empirical Green’s Functions for the 2003 Tokachi-oki Earthquake

A waveform inversion with empirical Greens functions was conducted to estimate strong-motion generation areas (SMGAs) of the 2003 Tokachi-oki, Japan, earthquake (Mw 8.3). Although the rupture process of this great subduction-zone earthquake has been revealed with waveform inversions based on low-frequency (lower than 0.2 Hz) ground motions, it is much more important from an engineering point of view to investigate how strong ground motions with higher frequencies were generated from the earthquake. Waveform data with higher frequencies have been excluded from the conventional inversions because of the difficulty in computing realistic theoretical Greens functions at higher frequencies. In this study, with the aid of empirical Greens functions, we extended the applicability of the waveform inversion to higher-frequency ground motions up to 1 Hz. We selected three after- shocks for use in the analysis, referring to the similarity of the group delay time be- tween the mainshock and the aftershock records. The resultant slip model has three SMGAs, each of which is close to the hypocenter of one of the aftershocks used. Gen- erally speaking, locations of the SMGAs identified in this study agree well with as- perities identified from the inverted results using low-frequency (lower than 0.2 Hz) ground motions plus geodetic data and tsunamis. It implies that strong ground motions up to 1 Hz were generated from almost the same asperities producing lower-frequency ground motions. In terms of the complexity of slip models, although our analysis is focused on high frequencies, our slip model is at least as simple as conventional low- frequency slip models. Such results would be useful in constructing source models of future great subduction-zone earthquakes for strong-motion prediction.

Characterized Source Model for Simulating Strong Ground Motions during the 2008 Wenchuan Earthquake

Extremely valuable strong-motion records during the 2008 Wenchuan earthquake were obtained at a number of stations close to the source region. We estimate the characterized source model for simulating ground motions using the empirical Green’s function (EGF) method and the hybrid method using strong-motion data from the earthquake. The characterized source model consists of several asperities with large slip in the entire rupture area of the earthquake. The locations of the asperities that generate strong motions were determined from the timing of pulse arrivals in observed records near the source fault. Because aftershock records of the earthquake had not been released yet, observed records from an aftershock ( M wxa04.9) of the 2008 Iwate–Miyagi Nairiku earthquake ( M wxa06.9), which occurred in the shallow inland crust of the northeastern region in Japan, are used as substitutes for the EGFs. The synthetic motions obtained by the EGF method agree well with the observed records with respect to acceleration and velocity waveforms in the period range of less than 2xa0s. Broader-band strong motions are simulated by the hybrid method, which combines the numerical method for periods longer than 2xa0s with the EGF method for periods shorter than 2xa0s. The best-fit source model was obtained with stress drop on each asperity of approximately 13xa0MPa. The ground motions numerically simulated at the Bajiao station (SFB) near an asperity and in the forward rupture direction show directivity effects that are too strong in comparison with the observed motions. We examined the influence of rupture-velocity fluctuation on the synthetic waveforms in the forward rupture direction near the asperity, finding that the forward directivity effects decrease with increase in velocity variances.

Source model of the 2007 Noto-Hanto earthquake (Mw 6.7) for estimating broad-band strong ground motion

A source model for estimating broad-band ground motions from the 2007 Noto-Hanto earthquake (Mw 6.7) is estimated from a comparison of the observed records of the mainshock and synthesized motions based on the characterized asperity model using the empirical Green’s function method. The observed records of aftershocks used as the empirical Green’s functions are carefully selected to have almost the same radiation characteristics and source distance as the asperities of the mainshock. The best-fit source model consists of two asperities of different size. A large one is located just above the hypocenter, with an area of 6.3×6.3 km2 and stress drop of about 26 MPa. A smaller one is located north-east of the large one, with an area of 3.6×3.6 km2 and stress drop of about 10 MPa. The stress drop of the large one is about twofold higher than the average values of inland crustal earthquakes so far estimated, while that of smaller one is almost average. We found that the remarkable directivity pulses from the source model struck the northern part of the Noto peninsula, causing heavy damage in some towns there.

Ground‐Motion Prediction Equations for Shallow Crustal and Upper‐Mantle Earthquakes in Japan Using Site Class and Simple Geometric Attenuation Functions

In this article, ground‐motion prediction equations (GMPEs) based on the horizontal components of the strong‐motion records from shallow crustal and upper‐mantle earthquakes in Japan are presented. We assembled a large dataset from earthquakes with a moment magnitude ( M w) over 4.9 and a reliable earthquake category (the tectonic location of earthquakes) up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions. A bilinear magnitude‐scaling function hinged at M wxa07.1 was adopted, with the scaling rates for large events being much smaller than those for the smaller events. Site classes based on site period were used as site terms, and nonlinear site terms were included. We modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within volcanic zones. Most strong‐motion records in our dataset are from stations with a measured shear‐wave velocity profile down to engineering bedrock. A small number of records are from stations with inferred site classes using the response spectral ratio of the horizontal‐to‐vertical components or geologic description of the surface soil layers. We tested the effect of site information quality by comparing the goodness‐of‐fit parameters from the model with and without the sites with inferred site classes. Our results suggest that the site information quality made a significant difference for spectral periods over 0.7xa0s, that is, the exclusion of sites with inferred site classes improves the model fit significantly. The within‐event residuals were approximately separated into within‐site and between‐site components, and the corresponding standard deviations were calculated. The approximate separation allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired.nnOnline Material: References for fault rupture plane models, earthquake records and volcanic zones information, illustration of site information quality effect, standard deviations for between‐event, within‐event, between‐site and within‐site residual, and the distribution of between‐event and within‐event residuals.

Ground‐Motion Prediction Equations for Subduction Interface Earthquakes in Japan Using Site Class and Simple Geometric Attenuation FunctionsGMPEs for Subduction Interface Earthquakes in Japan

The frequency content of strong ground motions from subduction slab earthquakes differs significantly from that of ground motions produced by other categories (tectonic locations: shallow crustal, upper mantle, and subduction interface) of earthquakes in subduction zones. In the last two decades, a large number of records from subduction slab events have been obtained in Japan. We present a ground‐motion prediction equation (GMPE) for this category of earthquakes. We used a large dataset from reliably identified slab events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, site classes were used as site terms, and nonlinear site amplification ratios were adopted. A bilinear magnitude‐scaling function was adopted for large earthquakes with moment magnitude M w≥7.1, with the scaling rates for large events being much smaller than for the smaller events. A magnitude‐squared term was used for events with M w<7.1 as well as the bilinear magnitude‐scaling function. We also modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within possible volcanic zones. We found that excluding the records from sites with inferred site classes improved the model goodness of fit. The within‐event residuals were approximately separated into within‐site and between‐site residuals, and the corresponding standard deviations were calculated using a random effects model. The separation of within‐event residuals into within‐site and between‐site components allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired.nnOnline Material: Figures showing the distribution of between‐event residuals with respect to magnitude and fault‐top depth and the distribution of within‐event residuals with respect to magnitude and source distance.