ALBERT

Description: 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. Online 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.

Description: 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 w  7.1 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 0.7 s, 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. Online 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.

Description: We presented a set of ground-motion prediction equations (GMPEs) for the horizontal components of strong-motion records from subduction interface events in Japan. We assembled and processed in a consistent manner a large set of strong-motion records from reliably identified subduction interface events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, and site class was based on site period as the site parameter. We adopted a bilinear magnitude-scaling function hinged at M w  7.1 and found that the magnitude-scaling rate for large events is much smaller than that for smaller events. To reliably determine the magnitude-scaling rate for events with M w ≥7.1, we used a set of strong-motion records obtained since 1968 to increase the number of records from large events. A small number of strong-motion records are from recording stations with inferred site classes using the response spectral ratio of the horizontal-to-vertical components or a geological description of the surface soil layers. The effect of site information quality for subduction interface events was examined using a goodness-of-fit parameter from a dataset with or without the sites having an inferred site class. The site information quality made a significant difference at all spectral periods, because the model fit improved significantly when the sites with inferred classes were excluded. We modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to the horizontal portion of the seismic-wave travel distance within a set of assumed volcanic zones. The within-event residuals were approximately separated into within-site and between-site components, and the corresponding standard deviations were calculated using a random effects model. The between-site standard deviations vary significantly among site classes and with spectral periods. Online Material: Figures showing distributions of between-event and within-event residuals.

Description: The recording on high-resolution broadband seismic networks of several great interface subduction earthquakes during the last decade provide an excellent opportunity to extend source-scaling relations to very large magnitudes and to place constraints on the potential range of source parameters for these events. At present, there is a wide range of uncertainty in the median rupture areas predicted for any given seismic moment by current relationships between magnitude and rupture area for subduction interface earthquakes. Our goal is to develop an updated set of earthquake source-scaling relations that will reduce this current large degree of epistemic uncertainty and improve the accuracy of seismic-hazard analysis and the prediction of the strong-motion characteristics and tsunamis of future subduction earthquakes. To achieve this goal, we compiled a database including slip models of interface earthquakes that occurred worldwide with moment magnitudes ranging from M  6.75–9.1. We characterized the seismic sources based on well-established criteria to estimate the asperity areas as well as the average slip on the faults, and we used these parameters to compute an updated set of magnitude scaling relations of the various characteristics of the fault. Additionally, we followed an alternative approach to quantifying slip models for use in developing characteristic slip models of future earthquakes. This involved analyzing the 2D Fourier transforms of the slip functions in the compiled database and deriving a wavenumber spectral model of the slip distribution.

Description: The great subduction earthquakes that occurred recently in Peru, Chile, and Japan have provided unprecedented information about the ground motions generated by such earthquakes. The 23 June 2001 M  8.4 Peru earthquake was recorded at eight strong-motion stations; the 27 February 2010 M  8.8 Maule, Chile, earthquake was recorded at over 10 strong-motion stations; and the 11 March 2011 M  9.0 Tohoku, Japan, earthquake was recorded at more than a thousand stations and produced the most extensive dataset of recordings for any earthquake. For the first time, data are available to guide the generation of ground-motion simulations from great subduction earthquakes. Broadband ground-motion simulations can enhance the usefulness of the recordings of these earthquakes by providing a means of interpolating and extrapolating the recorded data. Once they have been validated, broadband ground-motion simulations can be used for forward predictions of the ground motions of great subduction events in regions such as Cascadia, in which there are no strong-motion recordings of large subduction earthquakes. In this study, we test our ability to use a hybrid method to simulate broadband strong-motion recordings of megathrust earthquakes by demonstrating that our simulations reproduce the amplitudes of the recorded ground motions without systematic bias. We use simulations to study the distribution of various intensity measures of ground motion caused by these earthquakes and to validate our ground-motion simulation method by comparing the simulated ground motions with recorded ground motions as well as with empirical ground-motion prediction models.

Description: We have simulated accelerograms from many of the strong motion stations close to the mainshock of the 1987 Whittier Narrows earthquake using a semi-empirical Green's function summation technique. This method allows gross aspects of the source rupture process to be treated deterministically using a kinematic model based on first motion studies, teleseismic modeling and the distribution of aftershocks. Stochastic aspects of the rupture process are then included to simulate irregularity in both rupture and slip velocity. Gross aspects of wave propagation are modeled using theoretical Green's functions calculated with generalized rays. Detailed aspects of the source radiation at high frequencies, as well as unmodeled propagational aspects such as scattering, are included empirically by using multiple recordings of a smaller Imperial Valley earthquake as empirical source functions. Our main objective is to see how well we can predict the peak ground accelerations, time histories and response spectra of ground motions of a moderate sized earthquake within the Los Angeles Basin having limited detailed source information. We find that the simulations predict the observations accurately enough to identify which phases and amplitudes in the observed data may be due to local site response rather than source or radiation effects. Comparisons between observed and simulated accelerograms for all the stations modeled are made using peak ground acceleration, and using time histories and response spectra for the stations that have been hand-digitized to date. The Bright Avenue Whittier station has the largest simulated peak acceleration, in agreement with the recorded peak acceleration data.

Description: In current ground-motion models, the uncertainty in predicted ground motion is usually modeled with a lognormal distribution. One consequence of this is that predicted ground motions do not have an upper limit. In reality, however, there probably exist physical conditions that limit the ground motion. Applying the usual uncertainty distribution in probabilistic seismic hazard analysis may lead to ground-motion estimates that are unrealistically large, especially at the low annual probabilities considered for important structures, such as dams or nuclear reactors. A recently proposed statistical procedure to compare the actual and expected numbers of predicted spectral accelerations exceeding a given value gives clear results when applied to a ground-motion model developed for Japan from a very large strong-motion data set. It shows that, for increasingly large spectral accelerations, the actual number of exceedances becomes progressively less than the expected number of exceedances. The pattern of this discrepancy depends on the site class and the earthquake tectonic category. These results suggest that assuming a normal distribution for the prediction errors of an attenuation model (empirical ground-motion prediction equation) is likely to result in overestimation of the extreme values of spectral accelerations.