Showing papers in "Bulletin of the Seismological Society of America in 2008"

Displacement and Geometrical Characteristics of Earthquake Surface Ruptures: Issues and Implications for Seismic-Hazard Analysis and the Process of Earthquake Rupture

Abstract: There now exist about three dozen historical earthquakes for which in- vestigators have constructed maps of earthquake rupture traces accompanied by de- scriptions of the coseismic slip observed along the fault strike. The maps and slip distributions are compiled here to place observational bounds on aspects of seismic- hazard analysis and fault mechanics. Analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults. The observations also give support to the ideas that there exists a process zone or volume of about 3-4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults, and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes. To this may be added the observation that the form of earthquake surface-slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike. Online Material: Maps of surface ruptures, digitized values and curve fits to surface-slip distributions, and notes and references for Tables 1 and 2.

Spatial correlation of peak ground motions and response spectra

Abstract: The intensities of ground motions and structural responses at two sites are correlated. The magnitude of the correlation depends on the distance between the sites and the natural vibration periods of the structures. This study investigates the spatial correlation of the peak ground motions and the pseudospectral acceleration (PSA) responses using the California records and the Chi-Chi records. Because the correlation arises from interevent and intraevent variability, the correlations for individual variability alone and for the combined variability are assessed. The analysis results indicate that the spatial intraevent correlation decreases as the separation distance increases and that the magnitude of the correlation of the PSA responses depends on the considered natural vibration periods. The results also indicate that the spatial intraevent correlation of the PSA responses for the California records decays more rapidly than that for the Chi-Chi records. Based on the analysis results, a simple empirical equation to predict the spatially varying correlation coefficient of the PSA responses, which can be employed in seismic-hazard and seismic-risk assessments, is proposed.

Effects of Love Waves on Microtremor H/V Ratio

Abstract: The horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green’s functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S -wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak.

Surface Rupture of the 2005 Kashmir, Pakistan, Earthquake and Its Active Tectonic Implications

Abstract: To provide a detailed record of a relatively rare thrust surface rupture and examine its active tectonic implications, we have conducted field mapping of the surface rupture associated with the 2005 M w 7.6 Kashmir earthquake. Despite the difficulty arising from massive earthquake-induced landslides along the surface rupture, we found that typical pressure ridges and warps extend northwestward for a distance of ∼70 km, with a northeast-side-up vertical separation of up to ∼7 m. Neither the main frontal thrust nor the main boundary thrust is responsible for the earthquake, but three active faults or fault segments within the Sub-Himalaya, collectively called the Balakot–Bagh fault, compose the causative fault. Although the fault exhibits substantial geomorphic expression of repeated similar surface ruptures, only a part of it had been mapped as active before the earthquake. The location of the hypocenter suggests that the rupture was initiated at a deep portion of the northern–central segment boundary and propagated bilaterally to eventually break all three segments. Our obtained surface rupture traces and the along-strike-slip distribution are both in good agreement with results of prompt analyses of satellite images, indicating that space geodesy can greatly aid in time-consuming field mapping of surface ruptures. Assuming that the extensive fill terrace in the meizoseismal area was abandoned during 10–30 ka, we tentatively estimate the earthquake recurrence interval and shortening rate on the Balakot–Bagh fault to be 1000–3300 yr and 1.4–4.1 mm/yr, respectively. These estimates indicate that the Balakot–Bagh fault is not a main player of Himalayan contraction accommodation. ![Graphic][1] Selected field photographs and ArcGIS files of the mapped surface rupture traces and measured vertical separations are available in the electronic supplement to this article. [1]: /embed/inline-graphic-1.gif

A Nonconvolutional, Split-Field, Perfectly Matched Layer for Wave Propagation in Isotropic and Anisotropic Elastic Media: Stability Analysis

Abstract: A nonconvolutional, split-field, perfectly matched layer, referred to as the multiaxial perfectly matched layer (M-PML), is proposed and implemented. The new formulation is obtained by generalizing the classical perfectly matched layer (PML; as originally proposed by Berenger, [1994]) to a medium in which damping profiles are specified in more than one direction. Under the hypothesis of small damping and using an eigenvalue sensitivity analysis based on first derivatives, we propose a method to study the stability of the M-PML. With this method we demonstrate that the stability of the M-PML is related to the ratios of the specified damping profiles. Recognition of this fact leads to a general procedure for constructing robust, stable M-PML models for anisotropic media. It is also demonstrated that for any anisotropic medium the classical PML exhibits instabilities related to an eigenvalue with zero real part of multiplicity higher than one. Furthermore, we show that exponential growth due to eigenvalues with positive real part can be present in the classical PML for some orthotropic media. The effectiveness of the proposed M-PML and its advantages relative to the classical PML are demonstrated by constructing stable terminations for the aforementioned anisotropic media. The method of stability analysis is developed and demonstrated for two-dimensional elastodynamics problems, but its extension to three-dimensional configurations is straightforward.

Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan

Abstract: Subduction zone earthquakes have not been taken into special consid- eration in most previous probabilistic seismic hazard analyses (PSHA) in Taiwan. However, they may be critical to properly analyze the earthquake hazard in metro- politan Taipei, so they need to be studied. Strong-motion data from subduction zone earthquakes, of both interface and intraslab types, obtained by the TSMIP and SMART1 arrays in northeastern Taiwan, are used to establish the attenuation equa- tions for peak ground acceleration (PGA) and response spectral acceleration (SA). The resultant PGA and SA attenuation equations include two site classes and two earth- quake source types. The ground-motion values predicted by these attenuation equa- tions are higher than those obtained from the crustal earthquake attenuation equations previously used in Taiwan but are lower than those predicted by the attenuation equa- tions for worldwide subduction zone earthquakes.

Statistical Tests of the Joint Distribution of Spectral Acceleration Values

Abstract: Assessment of seismic hazard using conventional probabilistic seismic hazard analysis (PSHA) typically involves the assumption that the logarithmic spectral acceleration values follow a normal distribution marginally. There are, however, a variety of cases in which a vector of ground-motion intensity measures are considered for seismic hazard analysis. In such cases, assumptions regarding the joint distribution of the ground-motion intensity measures are required for analysis. In this article, statistical tests are used to examine the assumption of univariate normality of logarithmic spectral acceleration values and to verify that vectors of logarithmic spectral acceleration values computed at different sites and/or different periods follow a multivariate normal distribution. Multivariate normality of logarithmic spectral accelerations are verified by testing the multivariate normality of interevent and intraevent residuals obtained from ground-motion models. The univariate normality tests indicate that both interevent and intraevent residuals can be well represented by normal distributions marginally. No evidence is found to support truncation of the normal distribution, as is sometimes done in PSHA. The tests for multivariate normality show that interevent and intraevent residuals at a site, computed at different periods, follow multivariate normal distributions. It is also seen that spatially distributed intraevent residuals can be well represented by the multivariate normal distribution. This study provides a sound statistical basis for assumptions regarding the marginal and joint distribution of ground-motion parameters that must be made for a variety of seismic hazard calculations.

Probability of Detecting an Earthquake

Abstract: We present a new method for estimating earthquake detection proba- bilities that avoids assumptions about earthquake occurrence, for example, the event-size distribution, and uses only empirical data: phase data, station infor- mation, and network-specific attenuation relations. First, we determine the detection probability for each station as a function of magnitude and hypocentral distance, using data from past earthquakes. Second, we combine the detection probabilities of sta- tions using a basic combinatoric procedure to determine the probability that a hy- pothetical earthquake with a given size and location could escape detection. Finally, we synthesize detection-probability maps for earthquakes of particular magnitudes and probability-based completeness maps. Because the method relies only on detec- tion probabilities of stations, it can also be used to evaluate hypothetical additions or deletions of stations as well as scenario computations of a network crisis. The new approach has several advantages: completeness is analyzed as a function of network properties instead of earthquake samples; thus, no event-size distribution is assumed. Estimating completeness is becoming possible in regions of sparse data where meth- ods based on parametric earthquake catalogs fail. We find that the catalog of the Southern California Seismic Network (SCSN) has, for most of the region, a lower magnitude of completeness than that computed using traditional techniques, although in some places traditional techniques provide lower estimates. The network reliably records earthquakes smaller than magnitude 1.0 in some places and 1.0 in the seis- mically active regions. However, it does not achieve the desired completeness of mag- nitude ML 1:8 everywhere in its authoritative region. A complete detection is achieved at ML 3:4 in the entire authoritative region; thus, at the boundaries, earthquakes as large as ML 3:3 might escape detection.

Analysis of Rock-Fall and Rock-Fall Avalanche Seismograms in the French Alps

Abstract: This study reviews seismograms from 10 rock-fall events recorded between 1992 and 2001 by the permanent seismological network Sismalp in the French Alps. A new seismic magnitude scale was defined, which allowed us to compare and classify g round-motion vibrations generated by these Alpine rock-falls. Each rock-fall has also been cha racterized by its ground-motion duration t30 at an epicentral distance of 30 km. No relation was found between rock-fall parameters (fall height, runout distance, volume, potential energy) and rock-fall s eismic magnitudes derived from seismogram amplitudes. On the other hand, the signal duration t30 shows a rough correlation with the potential energy and the runout distance, highlighting the co ntrol of the propagation phase on the signal length . The signal analysis suggests the existence of at le ast two distinct seismic sources: one corresponding to the initial rupture associated with an elastic r ebound during the detachment and the other one generated by the rock impact on the slope. Although the fall phenomenon includes other complex processes (fragmentation of the block, interaction with topography, plastic deformation during and after impact) 2D finite-element simulations of thes e two seismic sources are able to retrieve the main seismogram characteristics.

M-logA Observations for Recent Large Earthquakes

Abstract: Using a magnitude ( M )-log area ( A ) dataset augmented with seven large ( M >7.0) earthquakes occurring since Wells and Coppersmith (1994), this short note assesses the current validity of the bilinear M -log A relations for continental, strike-slip earthquakes proposed by Hanks and Bakun (2002), in particular the L -model scaling at M >7. The relations determined by Hanks and Bakun (2002) are only insignificantly altered, leaving these bilinear M -log A relations as valid now as when first proposed.

A Fully Bayesian Inversion for Spatial Distribution of Fault Slip with Objective Smoothing

Abstract: In inversion of geodetic data for spatial distribution of fault slip under smoothing and positivity constraints, subjective or incorrect techniques are often employed to select a smoothing parameter that determines the relative weight placed on fitting the data versus smoothing the slip distribution. A popular objective method based on Akaike’s Bayesian information criterion (ABIC) is incorrect if positivity constraints are employed. We introduce a fully probabilistic inversion method to simultaneously estimate the slip distribution and smoothing parameter objectively and correctly in a Bayesian framework. The complete solution to the inverse problem, the joint posterior probability density function of slip and smoothing parameter, is formulated using Bayes’ theorem and sampled with a Markov chain Monte Carlo method. We apply the new method to coseismic displacement data from the 1999 Chi-Chi, Taiwan, earthquake and compare the results of our method with conventional methods. The slip distribution estimated with our method is significantly smoother than that estimated by the ABIC-based method, demonstrating that the ABIC-based method may in general generate significantly undersmoothed or oversmoothed slip distributions under positivity constraints. On the other hand, the estimated slip distribution is similar to that estimated using another conventional method based on cross validation. Multiple trade-off curve plots between slip roughness and data misfit show that trade-off curves provide no basis for selecting a single smoothing parameter. The new method is easily generalized to slip inversions of multiple data sets with unknown relative weighting and to slip inversions with unknown fault geometry parameters.

Dynamic Stresses, Coulomb Failure, and Remote Triggering

Abstract: Dynamic stresses associated with crustal surface waves with 15–30-sec periods and peak amplitudes 5 km). The latter is consistent with the observation that extensional or transtensional tectonic regimes are more susceptible to remote triggering by Rayleigh-wave dynamic stresses than compressional or transpressional regimes. Locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal/volcanic systems.

A Comprehensive Relocation of Earthquakes in Taiwan from 1991 to 2005

Abstract: We have carried out a comprehensive relocation of a total of 267,210 earthquakes in Taiwan that occurred during the past 15 yr. We based our relocation process on the earthquake catalog of the Taiwan Central Weather Bureau Seismic Network (CWBSN) and made improvements in three aspects. First, we incorporated a large dataset of the S-P times from 680 Taiwan Strong-Motion Instrumentation Pro- gram (TSMIP) stations distributed throughout the island of Taiwan to improve the coverage of earthquakes on the island. Secondly, we added 18 Japan Meteorological Agency (JMA) stations in the southern Ryukyu Island chain to enhance the station coverage for eastern offshore events, especially around the subduction zone northeast of Taiwan. Thirdly, we adopted 3D VP and VP=VS models in predicting the travel times of P and S waves. The effectiveness of these improvements in earthquake re- location can be seen in three aspects: (1) the reduction in the residuals of P-wave arrival times and S-P times, (2) a better understanding of the attenuation relationship between the peak-ground acceleration and epicentral distance, and (3) the geologically meaningful patterns of station corrections to P-wave arrival times and S-P times. Online Material: Catalog of relocated earthquakes in Taiwan from January 1991 to

Real-time evolutionary earthquake location for seismic early warning

Abstract: An effective early-warning system must provide probabilistic estimates of the location and size of a potentially destructive earthquake within a few seconds after the event is first detected. In this work we present an evolutionary, real-time location technique based on an equal differential time (EDT) formulation and a probabilistic approach for describing the hypocenter estimation. The algorithm, at each timestep, relies on the information from triggered arrivals and not-yet-triggered stations. With just one recorded arrival, the hypocentral location is constrained by the Voronoi cell around the first triggering station constructed using the travel times to the not-yet-triggered stations. With two or more triggered arrivals, the location is constrained by the intersection of the volume defined by the Voronoi cells for the remaining, not-yet-triggered stations and the EDT surfaces between all pairs of triggered arrivals. As time passes and more triggers be- come available, the evolutionary location converges to a standard EDT location. Synthetic tests performed using the geometry of the Irpinia seismic network, southern Italy (ISNet), and the simulation of an evolutionary location for the 2000 Mw 6:6 Western Tottori, Japan, earthquake indicate that when a dense seismic net- work is available, reliable location estimates suitable for early-warning applications can be achieved after 1-3 sec from the first event detection. A further simulation with an Mw 6:7 southern Greece earthquake shows that at a regional scale, the real-time location can provide useful constraints on the earthquake position several seconds before a non-real-time algorithm. Finally, we show that the robustness of the algorithm in the presence of outliers can be effectively used to associate phase arrivals coming from events occurring close in time, and we present a preliminary algorithm for event detection.

Ground-Motion Prediction Equations for Eastern North America from a Referenced Empirical Approach: Implications for Epistemic Uncertainty

Abstract: I outline a referenced empirical approach to the development of ground-motion prediction equations (GMPEs). The technique is illustrated by using it to develop GMPEs for eastern North America (ENA). The approach combines the ENA ground-motion database with the empirical prediction equations of Boore and Atkinson (2008) for the reference region of western North America (WNA). The referenced empirical approach provides GMPEs for ENA that are in agreement with regional ground-motion observations, while being constrained to follow the overall scaling behavior of ground motion that is observed in better-instrumented active tectonic regions. They are presented as an alternative to the commonly used stochastic ground-motion relations for ENA. The motivation of the article is not to supplant stochastic GMPEs but is rather to consider other approaches that might shed light on their epistemic uncertainty. Differences between the referenced empirical GMPEs of this study and the stochastic GMPEs of Atkinson and Boore (2006), along with inconsistencies between both of these studies and inferences based on intensity observations, suggest that uncertainty in median ENAGMPEs is about a factor of 1.5–2 for M ≥5 at distances from 10 to 70 km. Uncertainty is greater than a factor of 2 for large events ( M ≥7) at distances within 10 km of the source. It may be that saturation effects not modeled in the stochastic predictions, but inferred from observations in other regions, cause overestimation of near-source amplitudes from large events in Atkinson and Boore (2006). On the other hand, these saturation effects cannot be directly verified in ENA data. Differences in predictions according to the approach taken are also significant at distances from 40 to 150 km, due to uncertainty in the shape of the attenuation function that will be realized in future earthquakes.

Three-Dimensional Simulations of Seismic-Wave Propagation in the Taipei Basin with Realistic Topography Based upon the Spectral-Element Method

Abstract: We use the spectral-element method to simulate strong ground motion throughout the Taipei metropolitan area. Mesh generation for the Taipei basin poses two main challenges: (1) the basin is surrounded by steep mountains, and (2) the city is located on top of a shallow, low-wave-speed sedimentary basin. To accommodate the steep and rapidly varying topography, we introduce a thin high-resolution mesh layer near the surface. The mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave-speed contrasts. Variations in Moho thickness beneath Northern Taiwan are also incorporated in the mesh. Spectral-element simulations show that ground motion in the Taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains. The amplification of ground motion is mainly controlled by basin depth and shallow shear-wave speeds, although surface topography also serves to amplify and prolong seismic shaking.

Quantifying Measurement Uncertainty of Thirty-Meter Shear-Wave Velocity

Abstract: Thirty-meter shear-wave velocity ( V S 30) is commonly used to estimate near-surface conditions for site classification, seismic site response, liquefaction analysis, and other geotechnical earthquake engineering application. Quantifying the uncertainty in V S 30 measurements is important for determining the accuracy and precision of this geophysical test. This study gathers existing available comparative and blind shear-wave velocity tests to evaluate the apparent or observable intra- and intermethod variability. Presented in this article are estimates of V S 30 uncertainty for the invasive methods of downhole, suspension logging, and seismic cone penetration testing, the noninvasive method of spectral analysis of surface waves, and the method of shear-wave velocity correlated geologic units. Discussed is the issue of soil disturbance with respect to invasive methods and how this may result in measurement bias. Results of this study indicate that uncertainty of shear-wave velocity measurements are on the order of 1%–3% coefficient of variation (standard deviation/mean) for downhole, suspension logging, and seismic cone penetration testing, 5%–6% coefficient of variation for spectral analysis of surface waves, and 20%–35% coefficient of variation for shear-wave velocity correlated geologic units. Presented here are procedures for propagating the uncertainty and/or bias in forward analyses.

Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone

Abstract: We relate the late Holocene northern San Andreas fault (NSAF) paleo- seismic history developed using marine sediment cores along the northern California continental margin to a similar dataset of cores collected along the Cascadia margin, including channels from Barclay Canyon off Vancouver Island to just north of Mon- terey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseis- mic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earth- quakes, including the land paleoseismic record. Along the NSAF during the last ∼2800 yr, 15 turbidites, including one likely from the great 1906 earthquake, establish an average repeat time of ∼200 yr, similar to the onshore value of ∼240 yr. The combined land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar number of events during the same period. While the average recurrence interval for full-margin Cascadia events is ∼520 yr, the southern Cascadia margin has a repeat time of ∼220 yr, similar to that of the NSAF. Thirteen of the 15 NSAF events were preceded by Cascadia events by ∼0-80 yr, averaging 25-45 yr (as compared to ∼80-400 yr by which Cascadia events follow the NSAF). Based on the temporal association, we model the coseismic and cumulative post- seismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earth- quakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb fail- ure stress (CFS) resulting from viscous deformation related to a Cascadia earthquake over ∼60 yr does not contribute significantly to the total CFS on the NSAF. However, the coseismic deformation increases CFS on the northern San Andreas fault (NSAF )b y up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.

Ground-Motion Modeling of the 1906 San Francisco Earthquake, Part II: Ground-Motion Estimates for the 1906 Earthquake and Scenario Events

Abstract: We estimate the ground motions produced by the 1906 San Francisco earthquake making use of the recently developed Song et al. (2008) source model that combines the available geodetic and seismic observations and recently constructed 3D geologic and seismic velocity models. Our estimates of the ground motions for the 1906 earthquake are consistent across five ground-motion modeling groups employing different wave propagation codes and simulation domains. The simulations successfully reproduce the main features of the Boatwright and Bundock (2005) ShakeMap, but tend to over predict the intensity of shaking by 0.1–0.5 modified Mercalli intensity (MMI) units. Velocity waveforms at sites throughout the San Francisco Bay Area exhibit characteristics consistent with rupture directivity, local geologic conditions (e.g., sedimentary basins), and the large size of the event (e.g., durations of strong shaking lasting tens of seconds). We also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern San Andreas fault, considering three additional hypocenters and an additional, random distribution of slip. Rupture directivity exerts the strongest influence on the variations in shaking, although sedimentary basins do consistently contribute to the response in some locations, such as Santa Rosa, Livermore, and San Jose. These scenarios suggest that future large earthquakes on the northern San Andreas fault may subject the current San Francisco Bay urban area to stronger shaking than a repeat of the 1906 earthquake. Ruptures propagating southward towards San Francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward.

Ambient noise measurements for preliminary site-effects characterization in the urban area of Florence, Italy

Abstract: Environmental noise measurements and borehole data were used for a preliminary seismic characterization of the Plio-Quaternary sedimentary basin underlying the city of Florence (Italy). An extensive survey of ambient vibrations was conducted by using the seismic noise horizontal-to-vertical (H/V) spectral ratio technique (HVSR) to map the fundamental resonance frequency of the sedimentary cover. In order to use this information to infer main features of the basin subsurface geometry, an estimate of the S -wave velocity profile in the sediment layer was obtained from array noise recordings along with information on resonance frequency and direct measurements of the sediment thickness available at 23 borehole sites. A first-order reconstruction of the seismic bedrock topography in the area was provided, which shows a good consistency with available geological/log data. From these pieces of information, a preliminary soil classification of the whole area was attempted in order to identify situations where possible seismic amplification effects are expected during future earthquakes and where more accurate investigations are needed to parameterize the local seismic response.

Nonlinear Ground-Response Analysis of Turkey Flat Shallow Stiff-Soil Site to Strong Ground Motion

Abstract: Blind predictions of ground response at the Turkey Flat vertical array site during the 2004 Parkfield earthquake were performed using a series of nonlinear and equivalent-linear ground-response analysis codes. In this article, we report the out- come of the blind predictions, compare the predictions to measurements (released after the predictions were made), and provide discussion on residuals between the data and model predictions. The prediction exercise considered various sources of material variability and model variability. Specifically, these analyses utilized five dif- ferent nonlinear analysis codes, each exercised with a suite of small-strain shear-wave velocity profiles and modulus reduction/damping curves that encompass the range of reported material properties. Below the site frequency of about 5 Hz, velocity profile variability dominates the computed response variability. Upon full release of strong- motion data from the site, the predictions were generally found to be biased towards underprediction at high frequencies. Sensitivity analyses and results of recent geophy- sical testing at the site suggest this underprediction bias is most likely a result of in- accuracies in prior estimates of near-surface shear-wave velocities. Analyses performed with the recent profile show no significant bias except at the site frequency, where the response is overpredicted.

Variability of Near-Field Ground Motion from Dynamic Earthquake Rupture Simulations

Abstract: This study investigates near-field ground-motion variability due to dynamic rupture models with heterogeneity in the initial shear stress. Ground velocity seismograms are synthesized by convolving the time histories of slip velocity obtained from spontaneous dynamic rupture models with Green’s functions of the medium calculated with a discrete wavenumber/finite-element method. Peak ground velocity (PGV) estimated on the synthetics generally matches well with an empirically derived attenuation relation, whereas spectral acceleration (SA) shows only an acceptable match at periods longer than 1 sec. Using the geometric mean to average the two orthogonal components leads to a systematic bias for the synthetics, in particular at the stations closest to the fault. This bias is avoided by using measures of ground motion that are independent of the sensor orientation. The contribution from stress heterogeneity to the overall ground-motion variability is found to be strongest close to the fault and in the backward directivity region of unilaterally propagating ruptures. In general, the intraevent variability originating from the radiation pattern and the effect of directivity is on the same order or larger than the interevent variability. The interevent ground-motion variability itself is dominated by the hypocenter-station configuration and is influenced only to a lesser extent by the differences in the dynamic rupture process due to the stress heterogeneity. In our modeling approach the hypocenter location is not picked arbitrarily but is determined to be mechanically consistent with the stress heterogeneity through a procedure emulating tectonic stress loading of the fault and nucleation. Compared to the peak ground motion recorded during the 2004 Parkfield, California, earthquake our simulated seismograms show enhanced spatial correlation that may be attributed to the simplicity of the assumed crustal model or to an incomplete representation of the spatial heterogeneity of dynamic rupture parameters. Nevertheless, the intraevent PGV variability in the near-fault region determined for the Parkfield dataset is of the same order of magnitude as for our simulations.

On the Discrepancy of Recent European Ground-Motion Observations and Predictions from Empirical Models: Analysis of KiK-net Accelerometric Data and Point-Sources Stochastic Simulations

Abstract: Ground-motion amplitudes from recent European earthquakes of moderate magnitudes have been observed to be systematically smaller than the values expected from a number of popular empirical ground-motion prediction equations used for seismic hazard analysis. It has been suggested that these discrepancies are caused by regional variations in seismotectonic character and as a consequence that seismic hazard estimates using these ground-motion models would be too high. In the present article, we explore the hypothesis that the discrepancy can simply be explained by the fact that these models adopt magnitude-independent functional forms (letting Y designate the response spectral acceleration M designate the magnitude, then d log Y / dM is assumed independent of both magnitude and distance). The data collected by the KiK-net array (see the [Data and Resources][1] section) provides a unique opportunity to test this hypothesis and to analyze some of the pitfalls of deriving magnitude-independent functional form models and applying them for predictions of ground motion from smaller events (and vice versa). Borehole rock KiK-net ground-motion data (337 events, 3894 records) have been used to derive empirical ground-motion models with magnitude-independent functional forms for various magnitude ranges. By using these new ground-motion models and stochastic simulations, we discuss the ground-motion distance decays and magnitude effects for ground-motion models obtained with different magnitude range datasets. This analysis clearly indicates that response spectral amplitudes of ground motions from large earthquakes decay slower with distance than those from small earthquakes and confirms that the magnitude scaling of ground motion decreases as earthquake magnitude increases. Using stochastic simulations, we demonstrate that the observed decay in scaling could be a mixture of geometrical decay from extended sources and the fact that response spectral values instead of Fourier spectral values are considered. New ground-motion models (with functional forms including coefficients to model the observed magnitude-dependent scaling and decay rate) have finally been calculated for both surface and borehole site conditions of the analyzed dataset. These models show similar decays for intermediate period or moderate magnitude earthquakes. Our site classifications remove most of the statistical trend of the site effect and suggest that source and path effects could dominate the aleatory variability. [1]: #sec-16

Probability of Occurrence of Velocity Pulses in Near-Source Ground Motions

Abstract: Near-source ground-motion records affected by directivity may show unusual features in the signal resulting in low-frequency cycle pulses in the velocity time history, especially in the fault-normal component. Such an effect causes the seis- mic demand for structures to deviate from that of so-called ordinary records. This circumstance may be particularly hazardous for structural engineering applications if it is not properly accounted for. In fact, current attenuation laws are not able to capture such effects well, if at all, and therefore current probabilistic seismic hazard analysis (PSHA) is not able to predict this peculiar spectral shape. This failure may possibly lead to an underestimation of, in particular, the nonlinear demand. Account- ing for pulse-type records in earthquake engineering practice should be reflected both in the PSHA and in the record selection for seismic assessment of structures. These applications require a model for the probability of occurrence of pulselike records. Herein such a model is proposed on an empirical basis. A set of pulselike fault-normal ground motions from the Next Generation Attenuation of Ground Motions (NGA) Project dataset, as systematically identified by Baker (2007), is used. The independent variables studied are chosen from those considered by seismologists to affect the am- plitude of directivity pulses. Issues related to the dataset and the explanatory power of the proposed models are also discussed.

One Hundred Years of Earthquake Recording in Australia

Abstract: A comprehensive new catalog of Australian earthquakes has been compiled and used to review the seismicity of Australia. The catalog contains 27,000 events, of which 17,000 are considered to be mainshocks. The catalog is complete for all Australian events with a magnitude greater than M L 5.5 since 1910, M L 5 since 1960, M L 4 since 1970, and M L 3.5 since 1980. It is complete for events in southern Australia above M L 3.5 since 1965 and M L 2 since 1980. Before the development of local magnitude scales for Australia, around 1990, the Richter magnitude scale was generally used. At 600 km (a typical hypocentral distance for much of Australia) the Richter formula overestimates magnitude by around 0.5 units. Thus, the results in the catalogs before and after the early 1990s are potentially discrepant. Most well-located Australian earthquakes are in the southern areas of the continent, where the seismometer density is greatest. In general, the location uncertainty of Australian earthquakes is high. Only 60% of events are located with an uncertainty of 10 km or less. This percentage is smaller for earthquakes before 1980, and before 1960 very few events were located to within 10 km. The hypocentral depths of Australian earthquakes range mostly between 8 and 18 km, except for the southwest corner of the continent where they are typically shallower than 5 km. The seismicity in some areas of Australia has been steady for at least 100 yr (including the southeast corner, the Flinders Ranges, and the northwest corner). In contrast, seismicity in the southwest corner jumped by at least a factor of 6 in the 1940s and has been steady since then. Much of the rest of Australia is characterized by episodic seismicity. These episodes begin with a period of high activity lasting 1–10 yr and they are normally associated with a large ( M >6) earthquake. Following the large earthquake, there is often a period of moderate activity lasting from a few years to several decades. Before and after each episode is a quiescent period of low activity lasting 0.1–10 ka, during which the seismicity is more than an order of magnitude lower than during the period of high activity. Frequency-magnitude relations were calculated using events since 1970 from the new catalog. Gutenberg–Richter a - and b -values were calculated on an 85-km grid, and maps of the probability of an earthquake of M ≥4.9 occurring per year were derived. These results are very similar to the Global Seismic Hazard Assessment Program (GSHAP) map for Australia. The results were used to define four large (>20,000 km 2 ) seismogenic zones (Fig. 1). There are also several other small zones, some of which appear to reflect recent episodes, while others appear to be long-lived. The expected number of earthquakes M ≥5 and M ≥6, strain rate, and deformation rate is given for the four large zones, the remainder of Australia, and the whole Australian continent. The combined estimates of strain using seismic Global Positioning System (GPS) and Satellite Laser Ranging (SLR) data suggest east–west compressive deformation across southern Australia of 0.65±2.0 mm per year, likely to be in the 0.5–1.0-mm-per-year range.

Compressional and Shear-Wave Velocity versus Depth Relations for Common Rock Types in Northern California

Abstract: This article presents new empirical compressional and shear-wave velocity ( Vp and Vs ) versus depth relationships for the most common rock types in northern California. Vp versus depth relations were developed from borehole, laboratory, seismic refraction and tomography, and density measurements, and were converted to Vs versus depth relations using new empirical relations between Vp and Vs . The relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales, such as lithology, consolidation, induration, porosity, and stratigraphic age. Standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in Vp and Vs caused by these other factors. Because gabbros, greenstones, basalts, and other mafic rocks have a different Vp and Vs relationship than sedimentary and granitic rocks, the differences in Vs between these rock types at depths below 6 or 7 km are generally small. The new relations were used to derive the 2005 U.S. Geological Survey seismic velocity model for northern California employed in the broadband strong motion simulations of the 1989 Loma Prieta and 1906 San Francisco earthquakes; initial tests of the model indicate that the Vp model generally compares favorably to regional seismic tomography models but that the Vp and Vs values proposed for the Franciscan Complex may be about 5% too high.

Frequency-Domain Elastic Full Waveform Inversion Using the New Pseudo-Hessian Matrix: Experience of Elastic Marmousi-2 Synthetic Data

Abstract: A proper scaling method allows us to find better solutions in waveform inversion, and it can also provide better images in true-amplitude migration methods based on a least-squares method. For scaling the gradient of a misfit function, we define a new pseudo-Hessian matrix by combining the conventional pseudo-Hessian matrix with amplitude fields. Because the conventional pseudo-Hessian matrix is assumed to neglect the zero-lag autocorrelation terms of impulse responses in the approximate Hessian matrix of the Gauss–Newton method, it has certain limitations in scaling the gradient of a misfit function relative to the approximate Hessian matrix. To overcome these limitations, we introduce amplitude fields to the conventional pseudo-Hessian matrix, and the new pseudo-Hessian matrix is applied to the frequency-domain elastic full waveform inversion. This waveform inversion algorithm follows the conventional procedures of waveform inversion using the backpropagation algorithm. A conjugate-gradient method is employed to derive an optimized search direction, and a backpropagation algorithm is used to calculate the gradient of the misfit function. The source wavelet is also estimated simultaneously with elastic parameters. The new pseudo-Hessian matrix can be calculated without the extra computational costs required by the conventional pseudo-Hessian matrix, because the amplitude fields can be readily extracted from forward modeling. Synthetic experiments show that the new pseudo-Hessian matrix provides better results than the conventional pseudo-Hessian matrix, and thus, we believe that the new pseudo-Hessian matrix is an alternative to the approximate Hessian matrix of the Gauss–Newton method in waveform inversion.

Empirical Relationships between Modified Mercalli Intensity and Engineering Ground-Motion Parameters in Greece

Abstract: New relationships between modified Mercalli intensity (MMI) and engineering ground-motion parameters are developed for Greece. The ground-motion parameters investigated were peak ground acceleration (PGA), velocity, displacement, Arias intensity, and cumulative absolute velocity. The observed earthquake intensity is quantified in terms of the observed MMI at the recording station and the data set consists of 310 time histories recorded from 89 Greek earthquakes. The selected records were found to be characterized by high-frequency, low-energy content and short duration. Two sets of empirical relationships between MMI and the selected ground-motion parameters were derived. The first set of MMI predictive equations are independent of magnitude and epicentral distance, and they were derived by fitting the mean values of the ground-motion parameters using a weighted least-squares regression technique. The influence of magnitude, epicentral distance, and the local site conditions were incorporated into the second MMI predictive model, resulting in a decrease of the model variance. The lowest standard deviation observed for the first MMI predictive model was for PGA, while for the second MMI predictive model, Arias intensity exhibited the smallest variability. Another finding of the present study was that the local site effect has a little influence on the MMI predictive model for peak ground velocity (PGV). The proposed predictive equations are valid for MMI values IV–VIII, and some of them might be used for rapid assessment of the ground shaking and mapping damage potential.

The Acquisition of Source, Path, and Site Effects from Microearthquake Recordings Using Q Tomography: Application to the United Kingdom

Abstract: Source, site, and propagation parameters are inverted from a U.K. database of weak-motion events (2.0> M L>4.7). This results in the complete spectral parameterization of over 3200 velocimetric records of 273 events from the year 1992 to 2006. The S wave is extracted from the vertical records and is processed using a multitaper Fourier transform. We initially use a nonlinear least-squares log-space optimization to obtain estimates of the attenuation parameter for each spectrum. The estimates of t * are then used to geometrically constrain a depth-dependent Q model using a technique adapted from velocity tomography. We then invert for the remaining frequency-dependent parameters and a collective amplitude parameter from the velocity spectra while fixing the newly computed attenuation parameters based on raytracing through our Q model. The resultant amplitude parameters are then split into source moments, apparent geometrical spreading, and site correction factors. We find a frequency-independent depth-dependent Q structure. A linear relationship proportional to 0.7 M L between moment magnitude ( M w) and local magnitude ( M L) is found in the range of 2–4.7 M L. The majority of stress drops are found to range on the order of 0.1–10 MPa. A multiple segment apparent geometrical spreading model is found to best describe the amplitude decay with distance, accounting for factors such as geometrical spreading and scattering, along with multiple phase interference in the analysis window. Site response functions are found to broadly correlate with regional geology, mean amplification occurring in the Cenozoic sedimentary rock sites to the southeast of England relative to the harder Palaeozoic rock sites of Wales and Scotland. We use a bootstrap analysis technique to analyze the dependence of our results on the data in order to estimate the variance of the results and check the robustness of different inversions. Synthetic spectra are also computed in order to obtain minimum variance and bias of model parameters associated with the method. In applying a geometrical Q constraint, through the use of Q tomography, we find that the robustness of the results is significantly increased. A thorough analysis of the trade-offs involved in the inversion is performed using synthetic datasets. We find strong trade-offs between the parameters, but we are able to show that this covariance is reduced when adopting the Q -tomography approach.

GPS Seismology: Application to the 2002 Mw 7.9 Denali Fault Earthquake

Abstract: The 2002 Denali fault, Alaska, earthquake ( M w 7.9) caused one or more components of most broadband seismometers in western Canada to clip, yet did not trigger strong-motion instruments, thus leaving a substantial gap in the seismic record of this event. However, the large-amplitude surface waves generated by this event were well recorded by the Global Positioning System (GPS) in the same region, out to epicentral distances of more than 3000 km. In this article, we explore the capabilities of GPS seismology, specifically how the relative strengths of GPS and seismic data sets can be exploited in order to more effectively study earthquake source characteristics and wave-propagation effects. High-rate (1-Hz) GPS data from 23 stations throughout western North America have been analyzed to derive displacement waveforms for this event, and the impact of instrumentation (GPS receiver model) and error-reduction strategy (modified sidereal and spatial filtering) on the noise characteristics of displacement time series at each GPS site was assessed. After applying error-reduction methods to GPS displacements, the final average noise floors of 0.5 cm in the horizontal and 1.5 cm in the vertical indicate that large dynamic displacements are observable by GPS. We validate the GPS displacements by comparing broadband seismic recordings (integrated to displacement) with GPS recordings for four effectively colocated sets of instruments. We show excellent agreement between the unclipped seismic and GPS recordings of the surface waves from the Denali earthquake over the period range of 10–50 sec and for ground displacements exceeding about 1.0 cm. Thus, a large GPS displacement data set is now available for western North America, an area where records of this event were previously missing or incomplete. The final GPS seismograms are archived at Incorporated Research Institutions for Seismology (IRIS) for public use in future studies of the 2002 Denali earthquake.