ALBERT

Description: Response spectra for California earthquakes of 3.0≤ M 〈7.5 are well described by a simple stochastic ground-motion model using an equivalent point-source concept. We determine the best-fit stress parameter of each California earthquake in the Next Generation Attenuation-West 2 database based on matching simulated to observed response spectral shapes over a wide frequency range, and we derive an expression for the mean stress as a function of magnitude and focal depth. A calibration factor is calculated for each event; this constant is the required amplitude adjustment in order that the simulations match the observed response spectral amplitudes with zero bias. The best-fit simulation model suggests that the attenuation in California can be modeled as R –1.3 at distances 〈50 km and R –0.5 at further distances; this does a better job at matching attenuation trends than the traditional model 1/ R model at distances 〈50 km, particularly for M 〈5.5 events. The model requires an overall multiplicative calibration factor of C sim =3.16 in order for the simulations to match the observed response spectral amplitudes, for all magnitudes and distances. The calibration constant could be attributed to simplifications inherent in the modeling of source, attenuation, and site processes, and the lack of consideration of multiple phases in stochastic simulations. We conclude that the equivalent point-source simulation method with the proposed modeling parameters can predict average ground motions in California, generally within a ±25% error band, for magnitudes up to M  7.5, distances 〈400 km, and frequencies 〉0.2 Hz. We finish the paper by providing a recipe for developing a simulation-based generic ground-motion prediction equation that can be adjusted for source and attenuation attributes in different regions by simple modifications to its key source and attenuation modeling parameters.

Description: We describe an efficient method to determine moment magnitude and stress parameter in near real time, in the immediate aftermath of a small-to-moderate earthquake ( M ~3–6), from ShakeMap ground-motion parameters (5% damped pseudospectral acceleration) at 1 Hz, 0.33 Hz, 10 Hz and/or peak ground acceleration. The methodology is based on relating ShakeMap parameters to source and attenuation parameters within the context of a generic stochastic point-source model, to provide an event-specific ground-motion prediction equation (GMPE) that will reliably predict amplitudes. It is an approach that has been optimized for applications in regions with sparse networks; an example is provided for southern Ontario. In brief, we initially develop a simulation-guided generic GMPE model based on the available database in the region. We derive the applicable regional anelastic attenuation, stress parameter, and site amplification models. Then, using the estimated regional models, we show how to invert ShakeMaps parameters to estimate moment magnitude and stress parameter in near real time, to define an event-specific GMPE for an event that just happened. The event-specific GMPE can be used to provide robust, calibrated ShakeMaps that are fully consistent with ground-motion observations.

Description: We develop a simple model to estimate moment magnitude for events of M 〈4 at distances out to ~300 km, based on readily available ShakeMap parameters and seismological scaling principles. Estimates of moment magnitude for such small events are not available from standard methods but are needed for local-network applications and for traffic light systems for induced-seismicity applications. This issue is currently of particular interest in central and eastern North America. The method takes advantage of the fact that for small events the response spectrum is well-correlated with seismic moment for periods greater than 0.3 s and can be predicted from a simple stochastic point-source model. We develop an equation by which we calculate M from the 1 s pseudoacceleration amplitudes (PSA) ( M ≥3) or the 0.3 s PSA ( M 〈3) at each station, using a simple linear equation that corrects for the effects of attenuation. We show that this method produces unbiased estimates of moment magnitudes in both eastern and western North America, for M ≤4 events recorded at distances 〈300 km.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Richter’s local magnitude distance correction terms may have a significant bias with respect to distance if applied in a region with attenuation properties different than southern California. Using data from a network of broadband seismometers installed in northern Oklahoma, we empirically constrain the attenuation properties based on the amplitudes of observed seismograms to determine a formula for local magnitude for this region that is unbiased with respect to distance. We use both the classic definition of local magnitude as proposed by Richter and a second method that calibrates local magnitude so that it agrees with moment magnitude where a regional moment tensor can be computed. For both methods, the new formula results in magnitudes systematically lower than the existing local magnitude model used by the Oklahoma Geological Survey. We compare the resulting magnitudes and discuss the benefits and drawbacks of each method. Our results highlight the importance of determining accurate distance correction terms for magnitude computation with regional network data.〈/span〉

Description: The accurate modeling of ground motion for induced-seismicity hazard estimation is critically dependent on how amplitudes scale with distance near the hypocenter. A rich database of ground motions from small events recorded at close distances in the Geysers region of California has been used to constrain the near-distance saturation effects that control the maximum observed ground motions and intensities for shallow-induced events. The results of this study support the modeling of these effects using an equivalent point-source concept, in which the effective source depth increases from a value near 1 km at moment magnitude ( M ) of 2 to a value near 3 km at M  4. This near-distance saturation behavior can be applied to the development of ground-motion models for induced seismicity in any region.

Description: A local magnitude ( M L ) relation for the western Canada sedimentary basin (WCSB) is developed using a rich ground-motion dataset compiled from local and regional networks in the area. The assessment of amplitude decay with distance suggests that joining direct waves with postcritical reflections from Moho discontinuity modifies the attenuation pattern in the 100–200 km distance range. The M L distance correction is parameterized using a trilinear function that accounts for the observed attenuation attributes. Regression of ground-motion amplitudes results in the following distance correction model (–log A 0 ) for earthquakes in WCSB: in which R is the hypocentral distance (km). The standard M L relations fail to capture the rates and shape of amplitude attenuation in the region, resulting in overestimated magnitudes by 0.3–0.6 units. The overestimation is larger for local networks due to the increased discrepancy between standard M L relations and actual attenuation properties at close distances. The derived relationship results in unbiased M L magnitude estimates in WCSB over a wide distance range (2–600 km), which ensures consistent magnitude estimates from local and regional networks.

Description: We examine ground motions for a sequence of earthquakes in Oklahoma, where assessment of hazard contributions from induced seismicity is of particular interest. We aim to empirically calibrate a model-driven equation that was derived for central and eastern North America (CENA), so that it will match the observed ground motions of the 2011 Prague, Oklahoma, sequence. We first show that ground motions in Oklahoma decay at a rate similar to the average attenuation observed in the stable continental region of CENA. We then search for any needed adjustments to the CENA source model to match ground motions from the induced seismicity sequence that occurred near Prague in 2011. An interesting feature noted in the ground-motion analysis is that the stress parameters ( ) for the Prague mainshock events ( M ≥5) are notably higher than those for aftershocks. Moreover, the stress parameter that characterizes the high-frequency ground-motion decays in both time and space relative to the three largest events. The largest events in the Prague sequence have similar source parameters to natural CENA events of the same magnitude and focal depth, but their aftershocks have weaker motions.

Description: We modeled the source and attenuation attributes of well-recorded M 6+ earthquakes based on the equivalent point-source approach, with the goal of determining how to treat ground-motion saturation effects within this context. We consider ground motions as originating from an equivalent point source and mimic finite-fault effects by treating the motion as emanating from a virtual point (not a real point on the fault rupture), such that ground motions are correctly predicted at close distances. This is achieved by using an effective distance metric , in which D rup is the closest distance to the rupture and h is a pseudodepth term that accounts for saturation effects. We found that the distance-saturation effect is magnitude dependent, extending to further distances with increasing magnitude. For earthquakes of M ≥6, we model the saturation term as log( h )=–1.72+0.43 M with a standard deviation of 0.19 in log10 units, based on the values obtained from the study earthquakes. The apparent source spectra of most M 6+ earthquakes can be modeled using a simple Brune point-source model. For a few of the M 6+ earthquakes, notably those in California, we observed a spectral sag at intermediate frequencies. For such earthquakes, a two-corner point-source model provides a better match than the Brune model. We conclude that an equivalent point-source model based on the effective distance concept can successfully predict the average ground motions from M 6+ earthquakes over a wide distance range, including close distances (〈20 km).

Description: We develop a generic ground-motion prediction equation (GMPE) that can be adjusted for use in any region by modifying a few key model parameters. The basis of the GMPE is an equivalent point-source simulation model whose parameters have been calibrated to empirical data in California, in such a way as to determine the decoupled effects of basic source and attenuation parameters on ground-motion amplitudes. We formulate the generic GMPE as a function of magnitude, distance, stress parameter, geometrical spreading rate, and anelastic attenuation. This provides a fully adjustable predictive model, allowing users to calibrate its parameters using observed motions in the target region. We also include an empirical calibration factor to account for residual effects that are different from and/or missing in simulations compared to observed motions in the target region. As an example, we show how the generic GMPE can be adjusted for use in central and eastern North America (CENA), and calibrated with the Next Generation Attenuation-East database. We provide median predictions of ground motions in CENA for average horizontal-component peak ground motions and 5% damped pseudospectral acceleration (periods up to T =10 s), for magnitudes M  3–8 and distances up to 600 km.