John X. Zhao

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.

The Mw 6.2 Christchurch earthquake of February 2011: preliminary report

Abstract A moment magnitude (Mw) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealands second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 Mw 7.8 Hawkes Bay earthquake and the most expensive in New Zealands recorded history. The effects of the earthquake on the regions population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the Mw 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeast–southwest-striking fault dipping southeast at c. 69°, with maximum slip at 3–4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

A Comparison of VS30 and Site Period as Site‐Effect Parameters in Response Spectral Ground‐Motion Prediction Equations

Abstract V S 30 , the shear‐wave travel time averaged soil shear‐wave velocity of the top 30xa0m, has been used to represent site effects in many recent ground‐motion prediction equations (GMPEs). However, while V S 30 has been found to be a reasonable parameter to represent site effects in some studies, other studies provide contradictory evidence. In the present study, a systematic comparison between the predictive capabilities of these two site‐effect parameters is carried out using a large ground‐motion dataset from Japan. The basis of the adopted approach is to compare the standard deviations and amplitudes of amplification ratios in empirically modeling site effects by using either site period ( T S , four times the shear‐wave travel time from the bedrock to the ground surface) or V S 30 . The site effects modeled specifically include site amplification ratios between surface and borehole records from KiK‐net, in addition to the site‐effect terms from a GMPE. For KiK‐net data, T S is determined to be a better predictive parameter than V S 30 for soil sites with T S >0.6u2009u2009s, while the two parameters lead to a similar variability in amplification ratios for sites with T S V S 30 and T S are statistically equal for all site classes at most periods, while V S 30 leads to smaller variability than T S at some spectral periods. The conflict between the KiK‐net surface–borehole records, and the results from the GMPE is likely to be a result of large variability in the GMPE, containing source‐, path‐, and site‐variability, as compared with the reduced variability in the surface–borehole KiK‐net data pairs. Although V S 30 and T S lead to statistically similar standard deviations for the data from a GMPE, T S still leads to better median amplification ratios than V S 30 .

Geometric Spreading Functions and Modeling of Volcanic Zones for Strong-Motion Attenuation Models Derived from Records in Japan

Abstract Attenuation models derived from recorded ground motions are still important elements of probabilistic seismic hazard studies. Engineers use empirical attenuation models to derive the displacement demand for a site of interest from an earthquake at a given location. Many attenuation models have been published for different parts of the world and for different types of earthquakes. Most models have a simple function of constant or magnitude-dependent geometric spreading, and seldom consider well-known seismological effects such as Moho reflection for shallow crustal earthquakes, multiple travel paths and constructive interference for subduction earthquakes, and special characteristics of volcano zones. The reason for not accounting for such effects may be the desire for simplicity in the attenuation functional forms for engineering applications and a lack of records from which to reliably identify these effects quantitatively. In this article, a large set of strong-motion records obtained from dense recording networks in Japan is used to derive geometric attenuation functional form and a possible manner to model the effect of volcanic zones. A liberal approach is taken to introduce a relatively large number of parameters that can account for known seismological effects while retaining a fairly simple attenuation functional form, based on analyses of residuals from simple models similar to those published previously. Preliminary results are reported here, together with the proposed geometric attenuation function forms and plausible explanation of the physical process that leads to the proposed geometric attenuation functions. The proposed model shows a large increase in the maximum likelihood from the random effects methodology, the elimination of bias in the distribution of residuals with respect to source distance, and much improved fitting for well-recorded earthquakes.

A Preliminary Study on the Near-Source Strong-Motion Characteristics of the Great 2008 Wenchuan Earthquake in China

Abstract The great 2008 Wenchuan earthquake (12 May 2008) with a moment magnitude of 7.9 and a surface-wave magnitude of 8.0 in Shichuan, China, caused unprecedented loss of human life and widespread severe damage to many types of structures. Thirty-two strong-motion records were obtained within a source distance of 300xa0km, and three near-source records were obtained within a source distance of 20xa0km. We present the preliminary results on the characteristics of the near-source records and the strong-motion aspects of this great earthquake. This earthquake may be divided into four subevents, according to the rupture time history and the final slip distribution. Three of the four subevents have large surface fault displacement, and we consider the subevents that generated the three near-source records as surface-rupture earthquakes, supported by the comparisons made to records from other surface-rupture or buried-fault earthquakes. One station recorded strong ground motions from two subevents in two well-separated time windows, and this allows us to examine the effect of earthquake parameters for each of the subevents. We find that, in the spectral period range of 0.5–2xa0s, the response spectra of the near-source records from the Wenchuan earthquake are significantly less than those of buried-fault earthquakes, such as the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake that have a much smaller moment magnitude than the Wenchuan earthquake. In the fault-normal direction the displacement spectra at long period for the closest station are similar to those of the Lucerne record from the 1992 Landers earthquake but significantly smaller than those of the TCU052 and TCU068 records from the 1999 Chi-Chi, Taiwan, earthquake. At short and intermediate period, the near-source spectra are much larger than the design spectra in the previous version of the Chinese design code for the heavily damaged area, but they are comparable at long spectral periods.

ESTIMATING MODAL PARAMETERS FOR A SIMPLE SOFT-SOIL SITE HAVING A LINEAR DISTRIBUTION OF SHEAR WAVE VELOCITY WITH DEPTH

Simple approximate expressions are derived for estimating equivalent modal parameters of a single soft-soil layer that has a straight line distribution of shear wave velocities and is on a flexible half space which is more rigid than the overlying soil layer. It is shown that radiation damping from the flexibility of the half space can be converted approximately into an equivalent material damping and that a site of a single soft-soil layer on a flexible half space can be replaced by an equivalent single layer, with the same shear wave velocity distribution, on a rigid bedrock. The implementation for modal analyses and non-linear analyses of the equivalent site under earthquake excitation is discussed.

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.

Nonlinear Site Models Derived from 1D Analyses for Ground‐Motion Prediction Equations Using Site Class as the Site Parameter

Nonlinear site models are an important part of ground‐motion prediction equations (GMPEs) and can be constructed in a number of ways. If numerous soil‐site strong‐motion records contain the effect of strong nonlinear soil response, the parameters for the nonlinear model can be a part of the regression parameters for GMPEs. It is also possible to derive nonlinear site models by numerical simulation. However, the number of strong‐motion records from Japan that contain the effect of strong nonlinear soil response is still too small to derive nonlinear site terms. We present a model of nonlinear site terms using site class as the site parameter in GMPEs based on a 1D equivalent linear model. The 1D model was constructed based on the shear‐wave velocity profiles from the KiK‐net strong‐motion stations with a wide range of site periods, soil depth, and impedance ratios. The rock‐site strong‐motion records were from different earthquake categories in Japan and the Pacific Earthquake Engineering Research Center dataset. Those records had a wide range of earthquake magnitudes, source distances, and peak ground accelerations. A random effects regression model was fitted to the calculated spectral amplification ratios, accounting for the effect of site impedance ratios, earthquake magnitudes, and source distances of the rock‐site records. We also designed a method to adjust the 1D model so it can be used in a GMPE, accounting for the fact that a 1D model is an overly simplistic assumption for many real strong‐motion recording stations in many parts of the world.nnOnline Material: Tables of nonlinear site model parameters.

Magnitude‐Scaling Rate in Ground‐Motion Prediction Equations for Response Spectra from Large Subduction Interface Earthquakes in Japan

Magnitude‐scaling rates (MSRs; the rates of increase in predicted response spectra with increasing moment magnitude) are evaluated for three ground‐motion prediction equations for response spectra from subduction interface earthquakes, including two empirical models developed for data from Japan and a model based on synthetic records generated by using a stochastic finite‐fault model. MSRs vary significantly among the three models, and the difference between the two empirical models is unacceptably large. A set of 2100 strong‐motion records from subduction interface events with a magnitude of 6.5 or larger from Japan, including the 11 March 2011 magnitude 9 earthquake, were compiled. The earthquakes were grouped according to magnitude, so that the magnitude spread in each group is less than 0.2 magnitude units. Each earthquake group was treated as a single event with magnitude equal to the average magnitude for the group. A simple attenuation model was fitted to the normalized and grouped data. The model has a constant term for each earthquake group to represent the effect of magnitude. Three separate functions of magnitude (a linear model for events with a magnitude greater than 7, a bilinear model, and a curved model) were then fitted to the constants, and MSRs were derived from these functions. At short periods, the derived MSRs are only a fraction of those from two of the three attenuation models. At spectral periods greater than 0.5xa0s, the derived MSRs are between about 1/3 and 1/2 of those of the two empirical models but are closer to those based on a set of synthetic records.