National Earthquake Hazards Reduction Program Research Articles

Evaluation of local site effect in the western side of the Suez Canal area by applying H/V and MASW techniques

The soft sediments are one of the most important factors responsible for the amplification of the seismic ground motion in an area of study. Three components, single-station microtremor measurements were performed at 61 sites along the Suez Canal to estimate the fundamental frequencies of the soil and corresponding H/V amplitude ratios by using the horizontal-to-vertical spectral ratio (HVSR) method. We have applied the investigations of the shear wave velocity for supplementing the existing seismic microzonation of the Suez Canal. The multichannel analysis of surface wave (MASW) tests were done along the Suez Canal in the three cities, Suez, Ismailia, and Port Said using 24 channels digital engineering seismograph with 4.5 Hz geophones from September 2014 to January 2015 to get the shear wave velocity VS30. The SeisImager/SW software was used for analyzing the data, and 1D-shear wave velocity model have achieved for each site. The HVSR curves show that the fundamental frequency values are ranging from 0.57 to 1.08 Hz, and H/V amplitude ratios are ranging from 4.05 to 6.46. The average values of VS30 are (548, 301), (241, 319), (194, 110, 238) for Suez, Ismailia, and Port Said respectively. The average of shear wave velocity up to 30 m depth is estimated and used for site classification based on the National Earthquake Hazard Reduction Program (NEHRP) classification. The majority of the sites was classified as Class D (stiff soil) except one site at Port Said city is classified as Class E (soft soils), and another site in the Suez city is classified as Class C (hard rock).

Estimation ofκ0Implied by the High‐Frequency Shape of the NGA‐West2 Ground‐Motion Prediction Equations

The spectral‐decay parameter κ is often used to account for the reduction of the high‐frequency amplitude of ground motion caused by attenuation within the site profile. In this study, we used the inverse random vibration theory approach described by Al Atik et al. (2014) to calculate Fourier amplitude spectra from predicted values of response‐spectral acceleration for all five Next Generation Attenuation (NGA)‐West2 ground‐motion prediction equations (GMPEs). We used these spectra to estimate κ using the spectral‐decay method. Each GMPE was evaluated for a National Earthquake Hazard Reduction Program B/C site condition and for default estimates of depth to the top of rupture, hypocentral depth, and sediment (basin) depth. We derived estimates of κ for magnitudes ranging from 3.5 to 8.0 and distances ranging from 5 to 20 km and used a mixed‐effects model to derive equations for these estimates as a function of magnitude. We also calculated κ from the geometric mean of the response‐spectral accelerations of the GMPEs to check the sensitivity of the results to the two approaches and found that the values of κ derived in this study using a mixed‐effects model are in good agreement with these estimates. The values of κ obtained in this study do not necessarily represent the physical high‐frequency damping within the site profiles used to develop the NGA‐West2 GMPEs. Instead, they are intended to represent the high‐frequency shape of the median predicted spectral accelerations from the GMPEs. The κ model developed in this study can be used in inversions to develop stochastic models that are intended to mimic the predictions from the NGA‐West2 GMPEs. Online Material: Tables of random effects result on coefficients κ , and figures of Fourier amplitude spectrum.

Seismic site conditions in Gangneung, Korea, based on Rayleigh-wave dispersion curves and topographic data

To determine the near-surface shear wave velocities (Vs) and characterize seismic site conditions in Gangneung, on the east coast of South Korea, passive and active surface waves were recorded at 136 sites. Dispersion images of the Rayleigh waves were obtained by the spatial autocorrelation method and inverted to Vs models. From these one-dimensional Vs models, the depth to the soft bedrock, average Vs at the top of the bedrock, average Vs of the overburden layer, average Vs in the top 30-m interval (Vs30), and average resonance frequency were estimated to be 16 ± 1 m, 481 ± 14 m/s, 254 ± 5 m/s, 374 ± 16 m/s, and 5.5 ± 0.3 Hz, respectively. A lower estimate for Vs30 and a relatively thick overburden layer make the downtown and agricultural areas in the region more prone to significant ground amplifications. Multiple regression analysis of Vs30 yielded a correlation coefficient of 0.79 with respect to the variables of elevation and topographic slope. Using this proxy-based empirical relationship, Vs30 values were computed at 13,583 sites covering the entire city. A microzonation map, constructed based on the Vs30 data, indicates that the ground in Gangneung is categorized mainly into B (44%), C (39%), and D (15%) National Earthquake Hazards Reduction Program site classes, with minor constituents assigned to the A and E classes.

New methods to improve the assessment of shear wave velocities and seismic hazard parameters in Jeddah city, western Saudi Arabia

Seventy-five geotechnical boreholes were drilled in Jeddah city to identify the soil characteristics and calculate the shear wave velocity. The depth of boreholes extends up to 40 m in some sites, where the standard penetration test (SPT) was assigned at each 1.5 m depth interval. The groundwater table fluctuated between 0.5 and 9 m below the ground level. In addition, the multichannel analysis of surface waves (MASW) method was applied at the same borehole locations to estimate the shear wave velocity of the subsurface layers. 1D shear velocity profiles were processed using SeisImager software. The calculated shear wave velocity at the mid-point of each surveying line was compared with that of the borehole because the mid-point of the surveying line coincides with the borehole location. In addition, horizontal–vertical-spectral ratio inversion (INV_HVSR) of microtremor measurements recorded at the borehole locations represents a third method to estimate the shear wave velocity of subsurface layers. Results of these three methods are then compared and, finally, the average shear wave velocity values, up to a depth of 30 m, were assessed for Jeddah city. According to Vs (30) values, the soil classes for Jeddah city can be classified into three classes as “site class D,” “site class C,” and “site class B,” according to the National Earthquake Hazard Reduction Program (NEHRP). These results should be added to the Saudi Building Code (SBC) to improve the requirements for the design of earthquake-resistant structures in Jeddah city.

Evaluation of Six NEHRP B/C Crustal Amplification Models Proposed for Use in Western North America

We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)‐West2 project. A third model is based on a near‐surface velocity profile developed from the NGA‐West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation ( κ ) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013, 2016, unpublished manuscript, see [Data and Resources][1]) and Yenier and Atkinson (2015), the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty. [1]: #sec-9

Near-surface geophysical investigation of the 2010 Haiti earthquake epicentral area: Léogâne, Haiti

The [Formula: see text] Léogâne fan delta in southwestern Haiti borders the epicentral region of the devastating magnitude 7.0 Haiti earthquake of 12 January 2010. The flat plain of the Léogâne area experienced some of the worst shaking, destruction of buildings, and loss of life caused by the Haiti earthquake. This intense shaking was attributed by previous workers to either activation of a blind (no surface expression) thrust fault some 4 km beneath the Léogâne fan delta or to strike-slip motion along a shallow, ground-breaking fault that ruptured the uppermost part of the fan delta. Our research team from the University of Houston and the Haiti Bureau of Mines and Energy collected shallow seismic and gravity data in the fan delta where previous studies of earthquake aftershocks, coastal uplift of coral reefs, and radar interferometry all indicated a maximum amount of coseismic uplift. Our objective was to acquire geophysical information on the subsurface stratigraphy, structure, and material properties of the fan. S-wave seismic studies revealed an average velocity of [Formula: see text] for the first 30 m. These velocity values suggest that the near-surface sediments at Léogâne are a seismically hazardous class D sediment type (National Earthquake Hazard Reduction Program). Interpretation of our various seismic data sets has indicated prolonged sedimentary environments of fluvial channeling and channel migration to a depth of approximately 350 m as expected in this fan delta setting. There was no clear evidence on our seismic reflection lines for substantial faulting in the seismically slow, shallow fan delta sediments. Integrated geophysical data analyses indicated south-dipping seismically slow layers on the southern end of the Léogâne fan with a less well defined northward dip with the broad, anticlinal axis aligned with the area of maximum coseismic uplift at the coast. The sudden, coseismic upheaval of the ground surface above the proposed blind thrust combined with extreme shaking of the seismically weak sediments contributed to the destructiveness of the earthquake on the Léogâne fan delta.

On the selection of ground–motion attenuation relations for seismic hazard assessment of the Peninsular Malaysia region due to distant Sumatran subduction intraslab earthquakes

The assessment of seismic hazard from seismicity especially those induced by either subduction interface or intraslab earthquakes requires appropriate ground-motion attenuation relations that are tuned to a suitable magnitude–distance range. The Peninsular Malaysia region could be subject to a remarkable seismic hazard due to the large-magnitude, long-distance earthquakes from the Sumatran subduction zone. The earthquakes from Sumatran subduction intraslab zone have not been taken into special consideration in terms of applying the applicable ground-motion relations in the most previous probabilistic seismic hazard assessment (PSHA) studies for the region. This study has attempted first to derive response spectral ground-motion attenuation relations for distant subduction intraslab earthquakes based on the ground-motion data recorded in Peninsular Malaysia, Japan, and Iran. The response spectra database has been compiled from hundreds of ground motions from subduction intraslab events of moment magnitude (M) 5.0–7.7 recorded at sites with hypocentral distance (Rhyp) of about 120 up to 1400km. The proposed empirical relations are for peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped pseudo-acceleration (PSA) for four site classes (i.e., National Earthquake Hazards Reduction Program (NEHRP) site classes B, C, D, and E). Second, an evaluation of the applicability of the present study, the global and overseas regional ground-motion relations to the Sumatran subduction intraslab earthquakes has been presented in order to develop ground-motion logic tree for seismic hazard assessment in the Peninsular Malaysia region.

Site specific design response spectrum proposed for the capital city of Agartala, Tripura

ABSTRACTThe design response spectrum is smooth in shape compared to other response spectra. The objective of design spectra is to estimate the possible earthquake lateral loads which may experience on a particular structure during its design life. In this study, an attempt has been made to develop, design response spectra for the Agartala city, Tripura which is one of the North Eastern states in India considered at the highest level of seismic activity in the country having a zone factor 0.36g as per Indian seismic code (BIS 1893-2002). Based on the present data set collected from 1731-2011, the region is characterized and the seismicity parameters estimated separately for each source zone by Gutenberg and Richter (G-R) relationship. The two ground motion models were used for the hazard prediction. However, hazard curve, and uniform hazard response spectra (UHRS) (2% and 10% probability level for 50 years) has been developed for the Agartala at seismic bedrock level condition. Further, the NEHRP (National Earthquake Hazards Reduction Program) site classes D and E-types have been identified based on the average shear wave velocity at an upper 30 m depth of the subsurface soil profile. The direct shear wave velocity profiles were obtained through multi channel analysis of surface wave tests conducted at 27 locations at Agartala city. The design response spectra have been developed at the surface level for both the site classes. The results could be used as direct inputs for earthquake resistant design of civil engineering infrastructures of the study area.

Updates to Building-Code Maps for the 2015 NEHRP Recommended Seismic Provisions

With the 2014 update of the U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) as a basis, the Building Seismic Safety Council (BSSC) has updated the earthquake ground motion maps in the National Earthquake Hazards Reduction Program (NEHRP) Recommended Seismic Provisions for New Buildings and Other Structures, with partial funding from the Federal Emergency Management Agency. Anticipated adoption of the updated maps into the American Society of Civil Engineers Minimum Design Loads for Building and Other Structures and the International Building and Residential Codes is underway. Relative to the ground motions in the prior edition of each of these documents, most of the updated values are within a ±20% change. The larger changes are, in most cases, due to the USGS NSHM updates, reasons for which are given in companion publications. In some cases, the larger changes are partly due to a BSSC update of the slope of the fragility curve that is used to calculate the risk-targeted ground motions, and/or the introduction by BSSC of a quantitative definition of “active faults” used to calculate deterministic ground motions.

Multichannel analysis of surface-waves and integration of downhole acoustic televiewer imaging, ultrasonic Vs and Vp, and vertical seismic profiling in an NEHRP-standard classification, South of Concordia, Kansas, USA

Seismic site characteristics, as pertaining to earthquake hazard reduction, are a function of the subsurface elastic moduli and the geologic structures. This study explores how multiscale (surface, downhole, and laboratory) datasets can be utilized to improve “constrained” average Vs30 (shear-wave velocity to a 30-meter depth). We integrate borehole, surface and laboratory measurements for a seismic site classification based on the standards of the National Earthquake Hazard Reduction Program (NEHRP). The seismic shear-wave velocity (Vs30) was derived from a geophysical inversion workflow that utilized multichannel analysis of surface-waves (MASW) and downhole acoustic televiewer imaging (DATI). P-wave and S-wave velocities, based on laboratory measurements of arrival times of ultrasonic-frequency signals, supported the workflow by enabling us to calculate Poisson's ratio, which was incorporated in building an initial model for the geophysical inversion of MASW. Extraction of core samples from two boreholes provided lithology and thickness calibration of the amplitudes of the acoustic televiewer imaging for each layer. The MASW inversion, for calculating Vs sections, was constrained with both ultrasonic laboratory measurements (from first arrivals of Vs and Vp waveforms at simulated in situ overburden stress conditions) and the downhole acoustic televiewer (DATV) amplitude logs. The Vs30 calculations enabled categorizing the studied site as NEHRP-class “C” — very dense soil and soft rock. Unlike shallow fractured carbonates in the studied area, S-wave and P-wave velocities at ultrasonic frequency for the deeper intact shale core-samples from two boreholes were in better agreement with the corresponding velocities from both a zero-offset vertical seismic profiling (VSP) and inversion of Rayleigh-wave velocity dispersion curves.

Development of Earthquake Energy Demand Spectra

Current seismic codes are generally based on the use of response spectra in the computation of the seismic demand of structures. This study evaluates the use of energy concept in the determination of the seismic demand due to its potential to overcome the shortcomings found in the current response spectra–based methods. The emphasis of this study is placed on the computation of the input and plastic energy demand spectra directly derived from the energy-balance equation with respect to selected far-field ground motion obtained from Pacific Earthquake Engineering Research (PEER) database, soil classification according to National Earthquake Hazards Reduction Program (NEHRP) and characteristics of the structural behavior. The concept and methodology are described through extensive nonlinear time history analyses of single-degree-of-freedom (SDOF) systems. The proposed input and plastic energy demand spectra incorporate different soil types, elastic perfectly plastic constitutive model, 5% viscous damping ratio, different ductility levels, and varying seismic intensities.

Imaging evidence for Hubbard Glacier advances and retreats since the last glacial maximum in Yakutat and Disenchantment Bays, Alaska

AbstractHigh‐resolution 2‐D multichannel seismic data, collected during the 2012 UTIG‐USGS National Earthquake Hazards Reduction Program survey of Disenchantment and Yakutat Bays in southeast Alaska, provide insight into their glacial history. These data show evidence of two unconformities, appearing in the form of channels, and are interpreted to be advance pathways for Hubbard Glacier. The youngest observable channel, thought to have culminated near the main phase of the Little Ice Age (LIA), is imaged in Disenchantment Bay and ends at a terminal moraine near Blizhni Point. An older channel, thought to be from an advance that culminated in the early phase of the LIA, extends from Disenchantment Bay into the northeastern edge of Yakutat Bay, turning southward at Knight Island and terminating on the southeastern edge of Yakutat Bay. Our interpretation is that Hubbard Glacier has repeatedly advanced around the east side of Yakutat Bay in Knight Island Channel, possibly due to the presence of Malaspina Glacier cutting off access to central Yakutat Bay during times of mutual advance. We observe two distinct erosional surfaces and retreat sequences of Hubbard Glacier in Yakutat Bay, supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations. Interpretation of chaotic seismic facies between these two unconformities suggests that Hubbard Glacier exhibits rapid retreats and that Disenchantment Bay is subject to numerous episodes of outburst flooding and morainal bank collapse. These findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances.

Estimation of the maximum credible hazard in Kuala Lumpur and Singapore due to gigantic Sumatran megathrust earthquakes: based on a comparative study on attenuation laws

This paper has classified nine attenuation laws derived for subduction interface earthquakes through a comparative study based on the peak ground accelerations (PGAs) recorded in Peninsular Malaysia and Singapore. The goal of classification was to identify the attenuation laws that were the most compatible with the region. The PGAs predicted by the relations provided by Nabilah and Balendra (Earthq Eng 16:1076–1094, 2012) and Zhao et al. (Bull Seismol Soc Am 96:898–913, 2006) were found to correlate well with the recorded PGAs. The maximum credible hazard in Kuala Lumpur and Singapore was obtained by calculating the acceleration response spectra caused by three great Sumatran megathrust earthquakes (the worst possible earthquake scenarios) using Zhao et al. spectral attenuation law. Current building code in the region requires that buildings should be capable of resisting a notional ultimate lateral design load equals to 1.5 % of the characteristic dead weight, simultaneously applied at each floor (i.e., the minimum design level of buildings equals to ~0.15 m/s2). The results show that the risk level for structures of the two cities with natural period Tn = 1–1.5 s corresponding to 10- and 15-story buildings located on hard soil sites [National Earthquake Hazards Reduction Program (NEHRP) site class C] is high as the demand spectrum due to a worst possible earthquake scenario with moment magnitude (MW) 9.5 and an epicentral distance (Repi) of 600 km from Singapore was noticeably higher than their design level. The results of the present study are applicable in seismic hazard assessment projects in Peninsular Malaysia and Singapore. The most interesting conclusion from the earthquake engineering perspective, however, is that the design spectral shapes specified in IBC 2012 and Eurocode 8 (type 2) codes may not be particularly appropriate for sites that could be affected by large-magnitude, distant earthquakes.

Shear wave velocity mapping of Hat Yai district, southern Thailand: implication for seismic site classification

Soil characteristics play an important role in the degree of ground shaking due to local site amplification during an earthquake. The objectives of this work are to study shear wave velocity (Vs) distribution in the near surface, and to develop a seismic site classification map for soil effect characterization and seismic hazard assessment in Hat Yai district, southern Thailand. The Vs determination based on the multichannel analysis of surface waves technique, has been carried out and analyzed at 70 measuring sites throughout the district. On the basis of the weighted-average Vs in the upper 30 m depth (Vs30), a seismic site classification map, based on the National Earthquake Hazards Reduction Program (NEHRP) standard has been developed. It is found that the NEHRP site class in Hat Yai can be classified into four groups in accordance with the value of Vs30 within the range of about 150 to 1160 m s−1. Most parts of the study area are typically classified as site class C and D. Site class C is mostly found within the colluvial and terrace deposits in the western and eastern part of the area, whereas site class D is concentrated in the alluvial sediment of the middle and northern flood plain areas. A small portion of site class B is observed in the western mountain ranges, where there is a thin overburden on the firm rock. There is a remarkably low Vs30 value at only one site, located near the main stream in the northern part of the study area. The results imply that the soil characteristics in the central and northern Hat Yai district pose a medium to high amplification rate with respect to the other regions. Although Vs data alone are insufficient to verify the potential of the amplification of ground shaking, this study provides an initial attempt to understand seismic hazards in the study area.

NGA-West2 Research Project

The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.

Seismic microzonation for Penang using geospatial contour mapping

Shear wave velocity (V s) and the fundamental site period of the subsurface condition are the primary parameters that affect seismic soil amplification in particular sites. Within the topmost layer of the soil, which measures 30 m, the average shear wave velocity V s30 is commonly used to build codes for site classification for the design of earthquake-resistant structures and to conduct microzonation studies. In this study, the development of a microzonation map for V s30 distribution, National Earthquake Hazard Reduction Program V s30 site classification, and a fundamental site period for Penang are presented. The multichannel analysis of surface wave (MASW) test was conducted for more than 50 sites with available borehole data to develop the microzonation maps. The ten selected V s profiles measured by MASW show a good correlation with the data obtained using empirical correlations in a previous study. The highest V s values were identified at the northeastern and southeastern parts of Penang Island, corresponding to the shallow bedrock and the outcrop zone. Conversely, the lowest V s values were found in the northwestern and southwestern parts of the Penang mainland owing to the thick layer of soft clay and silt deposits. The site period map shows the variation in site periods, with the highest value of 1.03 s at the western part of the Penang mainland and the lowest value of 0.02 s at the eastern part of the Penang Island. The microzonation maps developed in this study are vital to studies on seismic hazard and earthquake mitigation programs in Malaysia.

Dynamic Site Characterization and Correlation of Shear Wave Velocity with Standard Penetration Test ‘N’ Values for the City of Agartala, Tripura State, India

Seismic site characterization is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to gauge the average dynamic properties of soil deposits and thus helps to evaluate the surface level response. This paper presents a seismic site characterization of Agartala city, the capital of Tripura state, in the northeast of India. Seismically, Agartala city is situated in the Bengal Basin zone which is classified as a highly active seismic zone, assigned by Indian seismic code BIS-1893, Indian Standard Criteria for Earthquake Resistant Design of Structures, Part-1 General Provisions and Buildings. According to the Bureau of Indian Standards, New Delhi (2002), it is the highest seismic level (zone-V) in the country. The city is very close to the Sylhet fault (Bangladesh) where two major earthquakes (M w > 7) have occurred in the past and affected severely this city and the whole of northeast India. In order to perform site response evaluation, a series of geophysical tests at 27 locations were conducted using the multichannel analysis of surface waves (MASW) technique, which is an advanced method for obtaining shear wave velocity (V s) profiles from in situ measurements. Similarly, standard penetration test (SPT-N) bore log data sets have been obtained from the Urban Development Department, Govt. of Tripura. In the collected data sets, out of 50 bore logs, 27 were selected which are close to the MASW test locations and used for further study. Both the data sets (V s profiles with depth and SPT-N bore log profiles) have been used to calculate the average shear wave velocity (V s30) and average SPT-N values for the upper 30 m depth of the subsurface soil profiles. These were used for site classification of the study area recommended by the National Earthquake Hazard Reduction Program (NEHRP) manual. The average V s30 and SPT-N classified the study area as seismic site class D and E categories, indicating that the city is susceptible to site effects and liquefaction. Further, the different data set combinations between V s and SPT-N (corrected and uncorrected) values have been used to develop site-specific correlation equations by statistical regression, as ‘V s’ is a function of SPT-N value (corrected and uncorrected), considered with or without depth. However, after considering the data set pairs, a probabilistic approach has also been presented to develop a correlation using a quantile–quantile (Q–Q) plot. A comparison has also been made with the well known published correlations (for all soils) available in the literature. The present correlations closely agree with the other equations, but, comparatively, the correlation of shear wave velocity with the variation of depth and uncorrected SPT-N values provides a more suitable predicting model. Also the Q–Q plot agrees with all the other equations. In the absence of in situ measurements, the present correlations could be used to measure V s profiles of the study area for site response studies.

Regional Differences in Ground-Motion Amplitudes of Small-to-Moderate Earthquakes across North America

Abstract We studied the response spectral amplitudes and peak ground acceleration (PGA) of earthquakes with moment magnitude ( M ) from 3.0 to 6.0 to investigate regional differences in ground‐motion amplitudes across North America on a nonparametric basis, for a common National Earthquake Hazards Reduction Program B/C site condition (e.g., shear‐wave velocity of 760 m/s in the top 30 m of the soil column). We use southern California as the reference region because it has the most plentiful data against which to compare other regions. Amplitudes of response spectra in eastern Canada/northeastern United States become increasingly larger than those in southern California as distance, frequency, and magnitude increases. In the central United States, ground‐motion amplitudes are larger than those in southern California at distances more than 100 km (and lower at distances less than 100 km), over all frequency and magnitude ranges studied. Amplitudes for crustal events in the Pacific Northwest/British Columbia are lower than those in southern California at frequencies ≥1 Hz, over all magnitude and distance ranges (but have similar amplitudes below 1 Hz). Amplitudes in northern California are similar to those in southern California at low frequencies (below 3.3 Hz) but have lower amplitudes at high frequencies (PGA) over all distances and magnitudes. We conclude that regional amplitude differences are largely attributable to complex frequency‐dependent differences in regional attenuation. For example, the slope of apparent geometric spreading at distances less than 100 km is significantly steeper (more negative) in eastern Canada/northeastern United States than other regions and may be frequency dependent. The regional attributes of the attenuation comparisons appear to be the same for both vertical and horizontal components, at least for B/C site conditions. Online Material: Figures of region‐dependent ground‐motion amplitudes, apparent geometric spreading, statistical tests of the ground motions, and figures and table of ratios of ground motions relative to those from southern California.

Development of a NEHRP site classification map of Chiang Mai city, Thailand, based on shear-wave velocity using the MASW technique

To account for site amplification and seismic hazard mapping, the shear-wave velocity (Vs) profile to a depth of 30 m (Vs (30)) is an important parameter and can be used to calculate the ground motion for specific site conditions. In this study, the near-surface Vs profiles of soils were collected at 44 sites in Chiang Mai city using the multi-channel analysis of surface-wave technique. The Vs of each tested location was average weighted to Vs (30) based on the National Earthquake Hazards Reduction Program (NEHRP) criteria. The average Vs (30) value of the alluvium soils was about 362 m s−1, which falls between NEHRP site classes C and D. The average Vs (30) values of flood plain, fluvial clay and natural levee soils (at 300, 299 and 311 m s−1, respectively) all equated to NEHRP class D. The colluvial deposits in the north-western part of the city were mainly composed of gravel, coarse sand and rock fragments, and were assigned to class C (average Vs (30) of 412 m s−1). Soils with lower Vs values will experience higher earthquake ground shaking than those of the bedrock. Accordingly the major part of Chiang Mai city may experience substantial ground shaking due to the amplification in the soft soils.

Determination of Site Amplification from Regional Seismicity: Application to the Swiss National Seismic Networks

Online Material: Supplemental figures and tables. The propagation of energy released by an earthquake through the uppermost crust has a significant impact on the ground motion that is observed at the surface. Knowledge of this site amplification effect can significantly reduce the uncertainty involved in the determination and application of stochastic or empirical predictive ground‐motion equations and therefore reduce the uncertainty in subsequent hazard calculations. However, due to the heterogeneity of the upper crust, the site amplification effect is highly variable over scales of kilometers or less (e.g., Boore, 2004). It is important to characterize site amplification at seismic instrument locations in order to correct or account for data from several recording sites, such that a common reference (e.g., Poggi et al. , 2011) is defined in ground‐motion prediction equations (GMPEs). State of the art GMPEs take advantage of site characterization based on broad classes, such as National Earthquake Hazards Reduction Program (NEHRP) soil class (BSSC, 2003; Akkar and Bommer, 2010), or the average shear‐wave velocity of the uppermost 30 m ( V S 30; e.g., Abrahamson and Silva, 2008). However, despite the use of this simple characterization, recent work has shown the importance of considering site‐to‐site amplification variability in the estimation of ground‐motion prediction uncertainty (Atkinson, 2006; Al Atik et al. , 2010). This highlights that the strong variability of site amplification, even within a single NEHRP or V S 30 class, contributes significantly to the uncertainty of ground‐motion prediction. Developers of GMPEs typically make an assumption of ergodicity; that is, that the variability over space is treated as an uncertainty in time. Spatial variability (beyond simple site classification) is therefore assumed to be zero. In order to minimize the influence of site‐to‐site variability in single‐site hazard analyses, Atkinson (2006) introduced the concept of single‐site sigma (uncertainty). Its use …