Amplification Of Strong Ground Motion Research Articles

Multimodal Rayleigh and Love Wave Joint Inversion for S‐Wave Velocity Structures in Kanto Basin, Japan

AbstractPrevention of earthquake disaster is critical in the densely populated Kanto Basin, Japan, because of the presence of thick sediments that amplify strong ground motion. For this, a high‐resolution three‐dimensional S‐wave velocity model is essential to understand the complex amplification of strong ground motion. We constructed a three‐dimensional S‐wave velocity model using the joint inversion of multimodal dispersion curves of Rayleigh and Love waves. Inclusion of higher modes significantly improved the accuracy and precision of the surface‐wave inversion by more than 50% compared to using only fundamental mode. Our proposed model revealed a low‐velocity anomaly of sediments toward the east and a curved velocity contour of the basement rock, thereby better explaining the observed complex surface‐wave dispersion curves that have been unexplained in past homogeneous multilayered models. Estimated S‐wave velocity model better reflected subsurface heterogeneity, providing vital information for hazard assessment in a metropolitan area where huge earthquakes are expected.

Ensemble ShakeMaps for Magnitude 9 Earthquakes on the Cascadia Subduction Zone

AbstractWe develop ensemble ShakeMaps for various magnitude 9 (M 9) earthquakes on the Cascadia megathrust. Ground-shaking estimates are based on 30 M 9 Cascadia earthquake scenarios, which were selected using a logic-tree approach that varied the hypocenter location, down-dip rupture limit, slip distribution, and location of strong-motion-generating subevents. In a previous work, Frankel et al. (2018) used a hybrid approach (i.e., 3D deterministic simulations for frequencies <1 Hz and stochastic synthetics for frequencies >1 Hz) and uniform site amplification factors to create broadband seismograms from this set of 30 earthquake scenarios. Here, we expand on this work by computing site-specific amplification factors for the Pacific Northwest and applying these factors to the ground-motion estimates derived from Frankel et al. (2018). In addition, we use empirical ground-motion models (GMMs) to expand the ground-shaking estimates beyond the original model extent of Frankel et al. (2018) to cover all of Washington State, Oregon, northern California, and southern British Columbia to facilitate the use of these ensemble ShakeMaps in region-wide risk assessments and scenario planning exercises. Using this updated set of 30 M 9 Cascadia earthquake scenarios, we present ensemble ShakeMaps for the median, 2nd, 16th, 84th, and 98th percentile ground-motion intensity measures. Whereas traditional scenario ShakeMaps are based on a single hypothetical earthquake rupture, our ensemble ShakeMaps take advantage of a logic-tree approach to estimating ground motions from multiple earthquake rupture scenarios. In addition, 3D earthquake simulations capture important features such as strong ground-motion amplification in the Pacific Northwest’s sedimentary basins, which are not well represented in the empirical GMMs that compose traditional scenario ShakeMaps. Overall, our results highlight the importance of strong-motion-generating subevents for coastal sites, as well as the amplification of long-period ground shaking in deep sedimentary basins, compared with previous scenario ShakeMaps for Cascadia.

Analysis of Intraslab Predigital Earthquakes of the South-Central Alaska Region

Abstract I compare intensity data from 17 predigital intraslab earthquakes in south-central Alaska and digitized waveforms for nine of these events to similar data for the 24 January 2016 Iniskin and 30 November 2018 Anchorage intraslab events. Both the 2016 and 2018 events were associated with amplification of strong ground motion at distances of over 100 km from their epicenters, likely due to the velocity structure associated with flat slab subduction in the region as well as the thick sedimentary sequence found within the Cook Inlet basin. Six of the predigital events also appear to have similar amplification of strong ground motion. Waveform comparisons suggest the 4 May 1934 Prince William Sound and 3 October 1954 Kenai Peninsula events had similar focal mechanisms and depths to the 30 November 2018 Anchorage event. Earthquakes occurring in regions where the P-wave velocity of the slab exceeds 7.5 km/s tend to be associated with higher strong ground motions.

Earthquake environmental effects produced by the Mw 6.1, 20th May 2016 Petermann earthquake, Australia

Earthquake Environmental Effects (EEEs) identified in the source region of the 20th May 2016 intraplate moment magnitude (Mw) 6.1 Petermann earthquake in Central Australia are described and classified using the Environmental Seismic Intensity (ESI-07) scale. EEEs include surface rupture, ground fissures and cracks, vegetation damage, rockfalls, and displaced (jumped) bedrock fragments. The maximum ESI intensity derived from EEEs is X, consistent with previous observations from some moderate Mw crustal earthquakes. Maximum ESI isoseismals correlate with the location of the surface rupture rather than epicentre area due to the dipping geometry of the reverse source fault. ESI isoseismals encompass a larger area of the hanging-wall than the footwall, indicating stronger ground motions on the hanging-wall due to increased proximity to the rupture source and ground motion amplification effects. The maximum areal extent of secondary (seismic shaking-induced) EEEs (300 km2) is significantly smaller than expected using the published ESI-07 scale (approx. 5000 km2). This relates to the low topographic relief and relatively homogeneous bedrock geology of the study region, which (i) reduced the potential for site response amplification of strong ground motions, and (ii) reduced the susceptibility of the landscape to EEE such as landsliding and liquefaction. Erosional degradation of the observed EEE features and decreasing confidence with which they can be uniquely attributed to a seismic origin with increasing time since the earthquake highlight challenges in using many of the natural features observed herein to characterise the locations and attributes of paleo-earthquakes.

Amplification of Strong Ground Motions at Heathcote Valley during the 2010–2011 Canterbury Earthquakes: The Role of 2D Nonlinear Site Response

Research Article| September 25, 2017 Amplification of Strong Ground Motions at Heathcote Valley during the 2010–2011 Canterbury Earthquakes: The Role of 2D Nonlinear Site Response Seokho Jeong; Seokho Jeong aQuakeCoRE, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, seokho.jeong@canterbury.ac.nz Search for other works by this author on: GSW Google Scholar Brendon A. Bradley Brendon A. Bradley bDepartment of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, brendon.bradley@canterbury.ac.nzcCurrently at Department of Civil and Environmental Engineering, Stanford University, Stanford, California. Search for other works by this author on: GSW Google Scholar Author and Article Information Seokho Jeong aQuakeCoRE, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, seokho.jeong@canterbury.ac.nz Brendon A. Bradley bDepartment of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, brendon.bradley@canterbury.ac.nzcCurrently at Department of Civil and Environmental Engineering, Stanford University, Stanford, California. Publisher: Seismological Society of America First Online: 27 Sep 2017 Online Issn: 1943-3573 Print Issn: 0037-1106 Bulletin of the Seismological Society of America (2017) 107 (5): 2117–2130. https://doi.org/10.1785/0120160389 Article history First Online: 27 Sep 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Seokho Jeong, Brendon A. Bradley; Amplification of Strong Ground Motions at Heathcote Valley during the 2010–2011 Canterbury Earthquakes: The Role of 2D Nonlinear Site Response. Bulletin of the Seismological Society of America 2017;; 107 (5): 2117–2130. doi: https://doi.org/10.1785/0120160389 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract This article presents a quantitative case study on the site amplification effect observed at Heathcote Valley, New Zealand, during the 2010–2011 Canterbury earthquake sequence for 10 events that produced notable ground acceleration amplitudes up to 1.4g and 2.2g in the horizontal and vertical directions, respectively. We performed finite‐element analyses of the dynamic response of the valley, accounting for the realistic basin geometry and the soil nonlinear response. The site‐specific simulations performed significantly better than both empirical ground‐motion models and physics‐based regional‐scale ground‐motion simulations (which empirically account for the site effects), reducing the spectral acceleration prediction bias by a factor of 2 in short vibration periods. However, our validation exercise demonstrated that it was necessary to quantify the level of uncertainty in the estimated bedrock motion using multiple recorded events to understand how much the simplistic model can overestimate or underestimate the ground‐motion intensities. Inferences from the analyses suggest that the Rayleigh waves generated near the basin edge contributed significantly to the observed high‐frequency (⁠f>3 Hz⁠) amplification, in addition to the amplification caused by the strong soil–rock impedance contrast at the site fundamental frequency. Models with and without considering soil nonlinear response illustrate, as expected, that the linear‐elastic assumption severely overestimates ground motions in high frequencies for strong earthquakes, especially when the contribution of basin‐edge‐generated Rayleigh waves becomes significant. Our analyses also demonstrate that the effect of pressure‐dependent soil velocities on the high‐frequency ground motions is as significant as the amplification caused by the basin‐edge‐generated Rayleigh waves. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Amplification of strong ground motions at Heathcote Valley during the 2010–2011 Canterbury earthquakes: Observation and 1D site response analysis

The strong motion station at Heathcote Valley School (HVSC) recorded intense ground accelerations (peak value of 1.4g in horizontal and 2.2g in vertical component) during the 22 February 2011 Christchurch earthquake. More importantly, ground motions recorded at HVSC in numerous other events during the 2010–2011 Canterbury earthquake sequence also exhibited consistently larger peak ground accelerations compared with nearby strong motion stations, which suggests significant near-surface site amplification effects.This paper presents a quantitative case study of near-surface site effects of Heathcote Valley during the 2010–2011 Canterbury earthquake sequence, by means of one-dimensional nonlinear dynamic finite element analyses based on the recorded ground motions and a rigorous site characterisation study. Various geophysical and geotechnical in-situ tests are performed to establish a simplified three-dimensional representation of wave velocities, mass densities and the geological structure (i.e. layering) of Heathcote Valley. Simulations are performed using the velocity profile at the location of station HVSC with the finite element analysis program OpenSees. Overall, simulations agree well with the recorded motions and suggest that ground motions at HVSC are amplified in a wide band of frequencies. However, the one-dimensional simulations tend to underestimate the site response at frequencies higher than the site fundamental frequency, likely due to its inability of modelling surface waves caused by the inclined soil-rock interface. Comparison between the nonlinear and the equivalent linear model shows that, although both approaches produce similar level of peak amplitude, the equivalent linear model significantly underestimates the high frequency motions.

Application and evaluation of a rapid response earthquake-triggered landslide model to the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal

The 25 April 2015 Mw 7.8 Gorkha earthquake produced strong ground motions across an approximately 250km by 100km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (>60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides≥0.0162km2; observed n=2214, modeled n=2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Effects on amplification of strong ground motion due to deep soils

Many seismically vulnerable regions in India and worldwide are located on deep soil deposits which extend to several hundred meters of depth. It has been well recognized that the earthquake shaking is altered by geological conditions at the location of building. As seismic waves propagates through uppermost layers of soil and rock, these layers serve as filter and they can increase the duration and amplitude of earthquake motion within narrow frequency bands. The amplification of these waves is largely controlled by mechanical properties of these layers, which are function of their stiffness and damping. Stiffness and damping are further influenced by soil type and thickness. In the current study, an attempt has been made to study the seismic site response of deep soils. Three hypothetical homogeneous soil models (e.g., soft soil, medium soil and hard soil) lying on bedrock are considered. Depth of half space is varied from 30 m to 2,000 m in this study. Controlled synthetic motions are used as input base motion. One dimensional equivalent linear ground response analyses are carried out using a computer package DEEPSOIL. Conventional approach of analysing up to 30 m depth has been found to be inadequate for deep soil sites. PGA values are observed to be higher for deeper soil profiles as compared to shallow soil profiles indicating that deeper soil profiles are more prone to liquefaction and other related seismic hazards under earthquake ground shaking. The study recommends to deal the deeper soil sections more carefully for estimating the amplification factors for seismic hazard assessment at the surface.

Dynamic analysis of ground with rigorous use of strain dependency and its application to seismic microzonation of alluvial plane

Seismic microzonation is one of the most important measures to mitigate earthquake hazards in urban areas. Because the ground motion varies significantly with the subsurface geology, it is needed for microzonation to account as much as possible for the local soil conditions. Noteworthy is that nonlinear deformation properties of soil play essential roles in amplification of strong ground motion. It is desired furthermore to focus on the expected damage extent in addition to the calculated maximum acceleration and/or velocity. The present study first developed a computer code for one-dimensional response analysis of ground that reasonably takes into account nonlinear dynamic soil properties. Second, correlations between the calculated ground motion and damage extent were obtained by examining seismic damages during the past earthquakes. By combining these two issues, seismic microzonation was carried out, and detailed damage distribution was assessed. The product of this study covers not only the damage caused by ground shaking but also liquefaction problem and lifeline damage.

Spectral element modeling of spontaneous earthquake rupture on rate and state faults: Effect of velocity‐strengthening friction at shallow depths

We develop a spectral element method (SEM) for simulating dynamic rupture on rate and state faults and use it to study how the rupture is affected by a shallow fault region of steady state velocity‐strengthening friction. Our comparison of the developed SEM and a spectral boundary integral method (BIM) for an antiplane (two‐dimensional) test problem shows that for the finest resolution that we use, the two methods produce virtually identical solutions, with negligible differences in rupture arrival times and peak slip velocities (less than 0.05% of their absolute values). The convergence with grid reduction of the developed SEM is comparable to that of BIM. We also use the test problem to compare numerical resolution required for different state evolution laws and for linear slip‐weakening friction. Using our three‐dimensional implementation of the methodology, we find that a shallow velocity‐strengthening fault region can significantly alter dynamic rupture and ground motion. The velocity‐strengthening region suppresses supershear propagation at the free surface occurring in the absence of such region, which could explain the lack of universally observed supershear rupture near the free surface. In addition, the velocity‐strengthening region promotes faster falloff of slip velocity behind the rupture front and decreases final slip throughout the entire fault, causing a smaller average stress drop. The slip decrease is largest in the shallow parts of the fault, resulting in a depth profile of slip qualitatively consistent with observations of shallow coseismic slip deficit. The shallow velocity‐strengthening region also reduces the amplification of strong ground motion due to a low‐velocity bulk structure.

Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California

The 1994 Northridge earthquake ( M w = 6.7) triggered extensive rock slope failures in Pacoima Canyon, immediately north of Los Angeles, California. Pacoima Canyon is a narrow and steep canyon incised in gneissic and granitic rocks. Peak accelerations of nearly 1.6 g were recorded at a ridge that forms the left abutment of Pacoima Dam; peak accelerations at the bottom of the canyon were less than 0.5 g, suggesting the occurrence of topographic amplification. Topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 ( M w = 6.7) San Fernando earthquake. Furthermore, high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones. We conducted field investigations and slope stability back-analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake. Our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions, which would have generated a significantly lower number of areas affected by landslides. In contrast, modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994. This observation confirms a significant role for topographic amplification on the triggering of landslides at the site, and emphasises the need to select carefully the inputs for seismic slope stability analyses.

Empirical Relations between Elastic Wavespeeds and Density in the Earth's Crust

A compilation of compressional-wave ( V p) and shear-wave ( V s) velocities and densities for a wide variety of common lithologies is used to define new nonlinear, multivalued, and quantitative relations between these properties for the Earth's crust. Wireline borehole logs, vertical seismic profiles, laboratory measurements, and seismic tomography models provide a diverse dataset for deriving empirical relations between crustal V p and V s. The proposed V s as a function of V p relations fit V s and V p borehole logs in Quaternary alluvium and Salinian granites as well as laboratory measurements over a 7-km/sec-wide range in V p. The relations derived here are very close to those used to develop a regional 3D velocity model for southern California, based on pre-1970 data, and thus provide support for that model. These data, and these relations, show a rapid increase in V s as V p increases to 3.5 km/sec leading to higher shear-wave velocities in young sedimentary deposits than commonly assumed. These relations, appropriate for active continental margins where earthquakes are prone to occur, suggests that amplification of strong ground motions by shallow geologic deposits may not be as large as predicted by some earlier models.

山地形における地震動の増幅特性

山地形における地震動の増幅特性

Amplification of strong ground motion in the city of Zagreb, Croatia, estimated by computation of synthetic seismograms

A hybrid technique consisting of modal summation and subsequent finite differences modelling is applied for the computation of synthetic accelerograms along a profile crossing the city of Zagreb, the capital of Croatia. Assuming the source geometry is known, the amplification properties of the underlying soil may be determined by comparison of synthetics and their response spectra computed for a bedrock model with the ones obtained under the assumption of a realistic laterally varying local model. The peak ground acceleration is larger by a factor of up to 3.5 than the value obtained for the bedrock model. The amplification of the response spectra is most prominent for frequencies below 2 Hz, and increases sharply to the SW from the mapped fault running through the centre of the city.

Peak Horizontal-to-Vertical Acceleration Ratio and Local Amplification of Strong Ground Motion

Residuals of the peak horizontal acceleration for site-independent attenuation relations are shown to have weak positive correlation with the average observed peak horizontal/vertical acceleration ratio (q) at the recording site. This ratio systematically increases as the hardness of the soil decreases. Taking the average observed q at the recording station into account reduces the scatter of data slightly in fitting the assumed peak acceleration attenuation law. If the soil classification of the data is available, soil dependence is found to be weaker if the data are corrected for the effect of q prior to regression. This correction reduced the mean value of q = 1.85 for the Dinarides area to q = 1.52, which, on the average, is characteristic of rock sites.

Determination of site amplification in the Los Angeles urban area from inversion of strong-motion records

AbstractThe amplification of strong ground motion at sites in the greater Los Angeles, California, region is determined using the generalized-inverse method of Andrews (1986). Site-amplification estimates are determined at 281 strong-motion sites that provided horizontal-component accelerograms from the 1971 San Fernando, 1987 Whittier Narrows, 1991 Sierra Madre, or 1994 Northridge mainshocks. The estimates are determined relative to the spectral level recorded at a single reference site. In a second inversion, a source-site interaction term is added to Andrews's (1986) model to quantify the effect selected mainshock records have on site-amplification estimates. The source-site interaction term is applied to the San Fernando Valley sites' records of the Northridge earthquake and to three Los Angeles basin sites' records of the Whittier Narrows mainshock.Site-amplification spectra are averaged within two frequency bands: the intermediate-frequency band (IFB) from 0.5 to 1.5 Hz and the high-frequency band (HFB) from 2 to 6 Hz. Results are displayed on maps of surficial geology. Average spectral levels are correlated with average shear-wave velocity in the uppermost 30 m, a geotechnical parameter used by the National Earthquake Hazards Reduction Program (NEHRP) to characterize site amplification. Statistically significant correlation is found in both frequency bands. Average spectral amplification levels for NEHRP class B, C, and D sites in the LA urban area are determined. These averages display an expected increase in spectral amplification with category, with a more pronounced variation in the IFB than in the HFB. Considerable overlap in the one-standard-deviation range of the C and D site levels is found in both IFB and HFB, suggesting that site-specific spectral amplification is influenced by more than just near-surface shear-wave velocity. Average site-amplification levels are compared with those obtained from Northridge aftershock records at 28 collocated sites and are found to be in reasonably good agreement in both frequency bands. Some interesting outliers, that is, sites for which the mainshock- and aftershock-determined amplifications differ significantly, are identified as sites that experienced ground failure during the Northridge mainshock.

Effects of an alluvial basin on strong ground motions

It has been reported that structures on loose soil sustain more damage during earthquakes than those on solid rock. This phenomenon has long been ascribed to the influence of local site geology. Gutenberg (1956) observed stronger shaking and longer duration at sites on alluvium than at those on crystalline rock, and large variations in ground shaking even at sites only 1000 feet apart. During the recent Michoacafi earthquake of 1985, most of the damages occurred in Mexico City more than 300 km from the epicenter (Anderson et al., 1986). The city is situated atop very soft sediments that caused dramatic amplification of strong ground motion, severe damage to the structures, and considerable loss of life within the area of the city located on top of the soft sediments. On 20 May 1986 a magnitude M L = 6.5 earthquake occurred near Hualien in eastern Taiwan. The epicenter was located at 24°4.9'N and 121o35.49 ' E, and the focal depth was 15.8 kin. This earthquake caused some damage in the Hualien and Taipei areas (Tsai et al., 1986). Accelerographs at 37 sites of the Strong Motion Accelerographs Array in Taiwan phase 1 (SMART1 array, Fig. 1; Bolt et al., 1982) were triggered. The Lotung Large Scale Seismic Test Array (LSST Array, Figs. 1 and 2; Wen et al., 1986) is situated within the southwestern quadrant of the SMART1 array. Surface recordings and two groups of downhole recordings of the LSST array were also obtained. In this study, we use the frequency-wavenu mber (f-k) analysis to study the variations in wave propagation direction and velocity. The change in peak acceleration, velocity, and displacement with respect to depth and the frequency-dependent ground motion amplification are studied by using the downhole and surface recordings of the LSST array.

Wave propagation in a three-dimensional circular basin

AbstractWave propagation in a three-dimensional circular basin model is studied by using a ray technique which employs the method of principal curvature for geometric spreading calculation. Three components of ground motion are calculated at surface stations assuming incident plane P and SH waves with various incidence angles. Computed results show that seismic energy is substantially enhanced in the central region of the basin while it diverges rapidly toward the edge of the basin. Compared to a two-dimensional basin with the same cross section, amplitudes are larger for stations in the central region and smaller for stations near the edge. Waveforms and amplitudes can change drastically between two nearby stations due to caustics and geometric focusing for different paths. It is found, for the particular basin geometry and velocity contrast used, that the first multiple P wave and other particular phases experience two caustics near the basin center which are associated with the principal curvatures of the wave front. This results in the reversal of the polarity of the waveform. As incidence angle increases, later arrivals at near-edge stations and the stations in the diagonal direction are more strongly developed from the curved boundary of the basin, but amplitudes of ground motions generally decrease as incidence angle increases. In general, wave responses strongly depend on the incidence angle and station location. These results have important implications for site amplification of strong ground motions since wave behavior is so complex even for this simple idealized structure.