Horizontal Ground Velocity Research Articles

Ground motion models for shallow crustal and deep earthquakes in Hawaii and analyses of the 2018 M 6.9 Kalapana sequence

The damaging 4 May 2018 M 6.9 Kalapana earthquake and its aftershocks have provided the largest suite of strong motion records ever produced for an earthquake sequence in Hawaii exceeding the number of records obtained in the deep 2006 M 6.7 Kiholo Bay earthquake. These records provided the best opportunity to understand the processes of strong ground shaking in Hawaii from shallow crustal (< 20 km) earthquakes. There were four foreshocks and more than 100 aftershocks of M 4.0 and greater recorded by the seismic stations. The mainshock produced only a modest horizontal peak ground acceleration (PGA) of 0.24 g at an epicentral distance of 21.5 km. In this study, we evaluated the 2018 strong motion data as well as previously recorded shallow crustal earthquakes on the Big Island. There are still insufficient strong motion data to develop an empirical ground motion model (GMM) and so we developed a GMM using the stochastic numerical modeling approach similar to what we had done for deep Hawaiian (>20 km) earthquakes. To provide inputs into the stochastic model, we performed an inversion to estimate kappa, stress drops, Ro, and Q(f) using the shallow crustal earthquake database. The GMM is valid from M 4.0 to 8.0 and at Joyner–Boore (RJB) distances up to 400 km. Models were developed for eight VS30 (time-averaged shear-wave velocity in the top 30 m) values corresponding to the National Earthquake Hazards Reduction Program (NEHRP) site bins: A (1500 m/s), B (1080 m/s), B/C (760 m/s), C (530 m/s), C/D (365 m/s), D (260 m/s), D/E (185 m/s), and E (150 m/s). The GMM is for PGA, peak horizontal ground velocity (PGV), and 5%-damped pseudo-spectral acceleration (SA) at 26 periods from 0.01 to 10 s. In addition, we updated our GMM for deep earthquakes (>20 km) to include the same NEHRP site bins using the same approach for the crustal earthquake GMM.

Simulation of near-fault ground strains and rotations from actual strike-slip earthquakes: case studies of the 2004Mw 6.0 Parkfield, the 1979Mw 6.5 Imperial Valley and the 1999Mw 7.5 Izmit earthquakes

SUMMARYPrevious studies have demonstrated that finite-fault simulations of actual or hypothetical earthquakes using deterministic, physics-based simulation techniques constitute an effective tool for characterizing near-fault ground strains and rotations in the low-frequency range. The characteristics of these motions are further investigated in this study by performing forward ground-motion simulations of three well-documented strike-slip earthquakes (i.e. 2004 Mw 6.0 Parkfield, 1979 Mw 6.5 Imperial Valley and 1999 Mw 7.5 Izmit) using models of the seismic source and crustal structure available in the literature. Time histories of ground strains and rotations are numerically generated at near-fault stations and at a dense grid of observation points extending over the causative fault. This is achieved by finite differencing translational motions simulated at very closely spaced stations using a kinematic modelling approach. The simulation results show that the three strike-slip earthquakes produce large-amplitude pulse-like shear strain and torsion in the forward direction of rupture propagation. The time histories of specific components of displacement gradient, strain and rotation at near-fault stations can be estimated from those of ground velocities using a phase velocity, whereas peak ground torsions in the near-fault region can be reasonably estimated from peak horizontal ground velocities using a scaling factor. However, both the phase velocity and the scaling factor exhibit significant variability in the near-fault region of the considered earthquakes. The concept of isochrones is also utilized to associate fault rupture characteristics with near-fault ground strains and rotations. The results indicate that the seismic energy radiated from the high-isochrone-velocity region of the fault—which encompasses areas of large slip locally driven by high stress drop—arrives at a near-fault station in a short time interval that coincides with the time window of the large-amplitude pulse-like shear strain and torsion.

Observational evidence of non-extensive behavior of seismic coda waves

The last decades a lot of progress has been made in studying Earth and planetary Physics based on the science of complexity and the application of generalized formalisms of statistical physics and entropy. Towards this direction, investigation of the dynamical features of seismic waves is of fundamental importance since they could provide information on the statistical properties of the lithosphere.The non extensive statistical mechanics (NESM) seems to be a promising framework for studying complex earthquake systems that exhibit long-range interactions and memory effects. In this work, coda waves are studied from the point of view of non extensive statistical mechanics. Our aim is to demonstrate the applicability of Non extensive statistical mechanics to the fluctuations of coda amplitudes and to define the entropic q parameters obtained in the coda waves recorded by seismological stations in the South Aegean, and generated by three strong earthquake events in the front of the Hellenic Arc.Coda waves are interpreted as scattered seismic waves by heterogeneities in the earth (cracks, faults, velocity and/or density anomalies, etc.). Due to the nature of those scatterers distribution we follow the NESM approach and study the probability distribution functions of the increments of seismic coda waves generated by seismic events recorded in South Aegean and generated by three strong events.We defined the increment function X(t)=V(t+1)−Vt, where V(t) is the measured horizontal ground velocity. We proceed by analyzing the normalized increments X(t) constructing the probability density function PDFp(x), where x=X−XσX, σX being the standard deviation of X(t) and <X> the observed mean value on the same data set. The results of the present work suggests that the seismic coda waves increments deviate from the Gaussian shape and their respective probability density function px could be adequately described by a q-Gaussian with entropic q-values in the range of1.35<q<2.02 with an error of δq=±0.05.This observation implies that seismic coda wave complexity originating by multiscattering effects, can be described by Tsallis entropy terms. The entropic q-values estimated vary along each seismic path for each of the three strong earthquake events analyzed. This observation implies that the value of q is determined by the particular nature of path between the source and the recorded station.

A New Uniform Moment Tensor Catalog for Yunnan, China, from January 2000 through December 2014

AbstractBecause of the collision of the Indian and Eurasian tectonic plates, the Yunnan Province of southwestern China has some of the highest levels of seismic hazard in the world. In such a region, a catalog of moment tensors is important for estimating seismic hazard and helping understand the regional seismotectonics. Here, we present a new uniform catalog of moment tensor solutions for the Yunnan region. Using a grid-search technique to invert seismic waveforms recorded by the permanent regional network in Yunnan and the 2 yr ChinArray deployment, we present 1833 moment tensor solutions for small-to-moderate earthquakes that occurred between January 2000 and December 2014. Moment magnitudes in the new catalog vary from Mw 2.2 to 6.1, and the catalog is complete above Mw∼3.5–3.6. The moment tensors are constrained to be purely double-couple and show a variety of faulting mechanisms. Normal faulting events are mainly concentrated in northwest Yunnan, while farther south along the Sagaing fault the earthquakes are mostly thrust and strike slip. The remaining area includes all three styles of faulting but mostly strike slip. We invert the moment tensors for the regional stress field and find a strong correlation between spatially varying maximum horizontal stress and Global Positioning System observations of horizontal ground velocity. The stress field reveals clockwise rotation around the eastern Himalayan syntaxis, with northwest–southeast compression to the east of the Red River fault changing to northeast–southwest compression west of the fault. Almost 88% of the centroid depths are shallower than 16 km, consistent with a weak and ductile lower crust.

Low‐Frequency Seismic Noise Characteristics From the Analysis of Co‐Located Seismic and Pressure Data

AbstractWe analyze the characteristics of seismic noise generated by surface pressure changes, using co‐located seismic and pressure data from the USArray Transportable Array for the entire year of 2014. We compare pressure and seismic signals at frequencies of 0.01 and 0.02 Hz and model their main characteristics. At these frequencies, vertical and horizontal seismic noise can be explained by the formulas SZ = A * Sp + NZ and SH = B * Sp + NH, where SZ and SH are the power spectral densities of vertical and horizontal ground velocities (sum of two horizontal components), Sp is the power spectral density of pressure, and NZ and NH are constant terms. We first determine A and B from the high pressure range (Sp &gt; 10 Pa2/Hz) in which NZ and NH are small. From A and B, we then determine the near‐surface rigidity and pressure wave speed. This rigidity is modified from true rigidity by a factor (λ + μ)/(λ + 2 μ); this factor is about 0.7 for typical crustal rocks but approaches 1 near the Earth's surface. To estimate true rigidity, empirical formulas that relate density and seismic velocities are needed. Once A and B are determined, NZ and NH are determined. NZ and rigidity of the subsurface determine a threshold pressure in vertical data. NH is generally small but at some stations it changes seasonally and creates separate branches in plots of pressure versus seismic velocity. This may explain why seismic‐noise reduction approaches using co‐located pressure data often fail to work for horizontal data but usually work well for vertical data.

Rotational Ground‐Motion Records from Induced Seismic Events

Research Article| December 07, 2016 Rotational Ground‐Motion Records from Induced Seismic Events Zbigniew Zembaty; Zbigniew Zembaty aOpole University of Technology, Faculty of Civil Engineering, ul. Prószkowska 76, 45‐758 Opole, Polandz.zembaty@po.opole.plp.bobra@po.opole.pl Search for other works by this author on: GSW Google Scholar Grzegorz Mutke; Grzegorz Mutke bGłówny Instytut Górnictwa, Department of Geology and Geophysics, Plac Gwarków 1, 40‐166 Katowice, Polandgmutke@gig.katowice.pl Search for other works by this author on: GSW Google Scholar Dariusz Nawrocki; Dariusz Nawrocki cSilesia University, Faculty of Earth Science, 41‐200 Sosnowiec, ul.Będzińska 60, Polanddariusz.nawrocki.wnoz@gmail.com Search for other works by this author on: GSW Google Scholar Piotr Bobra Piotr Bobra aOpole University of Technology, Faculty of Civil Engineering, ul. Prószkowska 76, 45‐758 Opole, Polandz.zembaty@po.opole.plp.bobra@po.opole.pl Search for other works by this author on: GSW Google Scholar Author and Article Information Zbigniew Zembaty aOpole University of Technology, Faculty of Civil Engineering, ul. Prószkowska 76, 45‐758 Opole, Polandz.zembaty@po.opole.plp.bobra@po.opole.pl Grzegorz Mutke bGłówny Instytut Górnictwa, Department of Geology and Geophysics, Plac Gwarków 1, 40‐166 Katowice, Polandgmutke@gig.katowice.pl Dariusz Nawrocki cSilesia University, Faculty of Earth Science, 41‐200 Sosnowiec, ul.Będzińska 60, Polanddariusz.nawrocki.wnoz@gmail.com Piotr Bobra aOpole University of Technology, Faculty of Civil Engineering, ul. Prószkowska 76, 45‐758 Opole, Polandz.zembaty@po.opole.plp.bobra@po.opole.pl Publisher: Seismological Society of America First Online: 14 Jul 2017 Online Issn: 1938-2057 Print Issn: 0895-0695 © Seismological Society of America Seismological Research Letters (2017) 88 (1): 13–22. https://doi.org/10.1785/0220160131 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Zbigniew Zembaty, Grzegorz Mutke, Dariusz Nawrocki, Piotr Bobra; Rotational Ground‐Motion Records from Induced Seismic Events. Seismological Research Letters 2016;; 88 (1): 13–22. doi: https://doi.org/10.1785/0220160131 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 SocietySeismological Research Letters Search Advanced Search ABSTRACT Surface rotational ground‐motion records that were collected by a network of deep mining exploration monitoring in the Polish region of the Upper Silesian Coal basin are reported. A set of 51 rotation records acquired during the first two months of the recording program is analyzed. The three strongest records are discussed in detail in this article. Thus far, the maximum measured rotation velocity has reached 0.03°/s, whereas the respective maximum rotational acceleration was equal to 2.53°/s2. Based on the maximum horizontal ground velocity of 2 cm/s, this corresponds to the modified Mercalli intensity (MMI = IV). The recorded “time history” records of ground rotations have similar durations to those of their respective translational records. However, their Fourier spectra are shifted to higher frequencies in a characteristic way, as predicted by earlier, theoretical considerations. The long‐term objective of the research program is to acquire robust surface rotation records that correspond to MMIs VI or even VII, since mine tremors with a magnitude of 4 or higher are expected to occur in this area in the near future. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake

On 25 April 2015, a large earthquake of Mw 7.8 occurred along the Main Himalayan Thrust fault in central Nepal. It was caused by a collision of the Indian Plate beneath the Eurasian Plate. The epicenter was near the Gorkha region, 80 km northwest of Kathmandu, and the rupture propagated toward east from the epicentral region passing through the sediment-filled Kathmandu Valley. This event resulted in over 8000 fatalities, mostly in Kathmandu and the adjacent districts. We succeeded in observing strong ground motions at our four observation sites (one rock site and three sedimentary sites) in the Kathmandu Valley during this devastating earthquake. While the observed peak ground acceleration values were smaller than the predicted ones that were derived from the use of a ground motion prediction equation, the observed peak ground velocity values were slightly larger than the predicted ones. The ground velocities observed at the rock site (KTP) showed a simple velocity pulse, resulting in monotonic-step displacements associated with the permanent tectonic offset. The vertical ground velocities observed at the sedimentary sites had the same pulse motions that were observed at the rock site. In contrast, the horizontal ground velocities as well as accelerations observed at three sedimentary sites showed long duration with conspicuous long-period oscillations, due to the valley response. The horizontal valley response was characterized by large amplification (about 10) and prolonged oscillations. However, the predominant period and envelope shape of their oscillations differed from site to site, indicating a complicated basin structure. Finally, on the basis of the velocity response spectra, we show that the horizontal long-period oscillations on the sedimentary sites had enough destructive power to damage high-rise buildings with natural periods of 3 to 5 s.

Real-time infrasonic monitoring of the eruption at a remote island volcano using seismoacoustic cross correlation

Abstract On 2013 November 20, a submarine eruption started close to Nishinoshima island, which lies ∼1000 km south of Tokyo. Real-time monitoring of the eruption is crucial for understanding the formation processes of the new volcano island and related disaster prevention. In situ monitoring, however, is difficult in practice because the closest inhabited island, Chichijima, is 130 km away from Nishinoshima. This study presents an infrasonic monitoring method that uses cross-correlating records at a pair of online stations on Chichijima. One is the horizontal ground velocity recorded at a permanent seismic station operated by the Japan Meteorological Agency (JMA). The seismic records were corrected for atmospheric pressure using an empirical ground response to infrasound. The other is the air pressure recorded at the JMA Meteorological Observatory. For each station, we divided the whole records into 3600-s segments. To suppress outliers, each segment was normalized by the envelope function. We then calculated cross-correlation functions between the pair of stations using the fast Fourier transform. They present clear successive arrivals of infrasound coming from Nishinoshima. We also conducted an offline tripartite-array observation using three low-frequency microphones with a station spacing of ∼50 m installed in 2013 May. The array analysis supports the results obtained from the online stations. The typical root-mean-squared amplitude is on the order of 0.01 Pa, and the typical duration is several days. The amplitudes were primarily controlled by the effective sound velocity structure from Nishinoshima to Chichijima. The infrasonic observations together with the meteorological observation at Chichijima suggest that infrasonic activity was not present in the first two weeks in 2015 January. With the help of a more quantitative estimation of the meteorological effect, we could infer eruptive activity in real time. Now many online seismic stations are available worldwide. This study shows that installation of another infrasonic sensor close to the existing seismic station (∼1 km) is useful for monitoring a remote island volcano in real time.

Seismic hazard across Bulgaria and neighbouring areas: extreme magnitude recurrence and strong ground shaking

A contemporary probabilistic seismic hazard assessment (PSHA) study for Bulgaria and the surrounding Balkan area is performed under constraints of a newly developed, fit-for-purpose historical earthquake catalogue and the theory of extreme values. Sensitivity analyses are first adopted as preparatory reviews on subsets of the adopted data to determine suitable values for the constraints of cut-off magnitude threshold, sample extreme interval and start year of catalogue data to impose on the parent database for both the full region considered as well as significant urban centres within it. Maximum estimates are then determined for magnitude recurrence hazard using Gumbel’s third asymptotic extreme values distribution for return periods of 50 and 100 years, and also these time intervals at 90 % probability of not being exceeded (PNBE). Gumbel’s first asymptotic extreme values distribution is also used with carefully selected, geographically relevant ground motion models for peak horizontal ground acceleration, PGA(h), and peak horizontal ground velocity, PGV(h), for the same return periods. The former provides direct comparison with the current EUROCODE 8 anti-seismic building code standard promoted across Europe, the previous GSHAP and SESAME hazard mapping projects as well as a number of recent studies. Sofia is forecast an upper bound magnitude of 7.33 M w (±0.78) compared with 7.31 M w (±0.55) for the full Balkan extent and 7.24 M w (±0.70) for the political triple junction area of southwest Bulgaria, viz., Bulgaria, Greece and The Former Yugoslav Republic of Macedonia. Sofia is also forecast a 475-year return period (equivalent to a 50-year return period at 90 % PNBE) magnitude of 7.27 M w, with an equivalent PGA (the standard EUROCODE 8 metric) of 156 cm s−2 and PGV of 13 cm s−1.

Seismic hazard across Bulgaria and neighbouring areas: regional and site-specific maximum credible magnitudes and earthquake perceptibility

A probabilistic seismic hazard assessment is developed here using maximum credible earthquake magnitude statistics and earthquake perceptibility hazard. Earthquake perceptibility hazard is defined as the probability a site perceives ground shaking equal to or greater than a selected ground motion level X, resulting from an earthquake of magnitude M, and develops estimates for the most perceptible earthquake magnitude, MP(max). Realistic and usable maximum magnitude statistics are obtained from both whole process and part process statistical recurrence models. These approaches are extended to develop relationships between perceptible earthquake magnitude hazard and maximum magnitude recurrence models that are governed by asymptotic and finite return period properties, respectively. Integrated perceptibility curves illustrating the probability of a specific level of perceptible ground motion due to all earthquakes over the magnitude range extending from −∞ to a magnitude Mi are then developed from reviewing site-specific magnitude perceptibility. These lead on to achieving site-specific annual probability of exceedance hazard curves for the example cities of Sofia and Thessaloniki for both horizontal ground acceleration and ground velocity. Both the maximum credible earthquake magnitude M3 and the most perceptible earthquake magnitude MP(max) are of importance to the earthquake engineer when approaching anti-seismic building design. Both forms of hazard are illustrated using contoured hazard maps for the region bounded by 39°–45°N, 19°–29°E. Patterns are observed for these magnitude hazard estimates—especially MP(max) specific to horizontal ground acceleration and horizontal ground velocity—and compared to inferred patterns of crustal deformation across the region. The full geographic region considered is estimated to be subject to a maximum credible earthquake magnitude M3—estimated using cumulative seismic moment release statistics—of 7.53 Mw, calculated from the full content of the adopted earthquake catalogue, while Bulgaria’s capital, Sofia, is estimated a comparable value of 7.36 Mw. Sofia is also forecast most perceptible earthquake magnitudes for the lowest levels considered for horizontal ground acceleration of MPA(50) = 7.20 Mw and horizontal ground velocity of MPV(5) = 7.23 Mw for a specimen focal depth of 15 km.

Insights on the kinematics of deep-seated gravitational slope deformations along the 1915 Avezzano earthquake fault (Central Italy), from time-series DInSAR

Among the causes of deep-seated gravitational slope deformations (DGSD), the most important is relief energy, which is closely related to the intensity of the active tectonic deformations, either at the regional scale or at the scale of a single active fault. We analyzed some examples of DGSD from the Eastern border of the Fucino basin, in the Central Apennines, where extensional tectonics has been active since the late Pliocene. Photogeological and field geomorphological analysis was performed to identify landforms typically associated with DGSD, such as counterslope scarps, double crests, trenches, and bulging slopes. These features are located on a mountain range at less than 1 km from the causative fault of the 1915 Avezzano earthquake. We used the SBAS Differential SAR Interferometry technique to measure the slow movements of the surface, and calculated differential vertical and horizontal ground velocities of 2–4 mm yr − 1 during the period spanning from 1992 to 2001. The quantitative information on the kinematics of the deformation provided some inferences on the different processes responsible for the evolution of the observed DGSD. The displacement time series shows non-linear deformation trends at some locations, possibly correlated with a strong meteorological event. We speculate that DGSD in this area are normally subject to slow deformation, and that sudden slip along sliding surfaces (observed in excavations) may sporadically be triggered off by extreme meteorological or seismic events. Evidence of catastrophic collapse of previous DGSD along the same mountain slope reinforce this hypothesis.

Reduction of Structural Damage by Nonlinear Soil Response

Following the 1994 Northridge earthquake, the areas with a high density of reported breaks in water pipes in typical residential areas in San Fernando Valley and in Los Angeles often did not coincide with the areas having a high density of severely damaged (red-tagged) buildings. As the former is an indicator of large strains and nonlinear soil response, this observation suggests that the damage to buildings in some areas may have been smaller then expected because the soil dissipated part of the energy of the ground motion by nonlinear response. This paper presents an attempt to quantify this relationship between the density of red-tagged buildings, \iN (per km²), and the severity of shaking (via peak horizontal ground velocity, υ\dm\da\dx, or modified Mercalli intensity, \II\N\DMM\N), including the density of breaks in water pipes, \in (per km²), as a variable specifying the level of strain in the soil Approximate empirical relationships for \iN = \if(υ\dm\da\dx,\in) and \iN = \if(\iI\dM\dM,n) are presented. The trends in the data indicate that, for υ\dm\da\dx in the range from ∼35 to ∼125 cm/s, the rate of growth of \iN versus υ\dm\da\dx tends to decrease at sites with large strain in the soil (i.e., large \in). For υ\dm\da\dx beyond ∼150 cm/s, the beneficial effects of nonlinear soil response seem to fade out, as large differential motions associated with soil failure begin to contribute to the damage of structures. Assuming fairly uniform density and quality of building stock and of water pipes in the areas studied, the derived relationships are then used to map υ\dm\da\dx and \II\N\DMM\N in San Fernando Valley and in Los Angeles. The resulting maps are more detailed than what could be obtained from the density of strong motion stations and of sites with reports on felt intensity.

The Northridge, California, earthquake of 1994: fire ignition by strong shaking

The Northridge earthquake contributed unprecedented detail and quality of data on strong ground motion and on its effects on man-made structures. About 110 fires have been attributed directly to the effects of this earthquake. Two hypotheses for the principal causative agents leading to fire ignition were examined: differential motion and strains in the soil, and inertial forces. The fire-ignition frequency is described with respect to: (1) simple measures of strain in the soil (via density of water pipe breaks, n), (2) occurrence of severely damaged buildings (via density of red-tagged buildings, N), (3) site intensity of shaking, ( I MM), and (4) inertial forces (via peak horizontal ground velocity, v m ). It is shown that the rate of fires (per unit area) ignited by earthquake shaking can be predicted by several empirical equations of comparable accuracy and in terms of common scaling parameters of strong ground motion.

A SIMPLIFIED METHOD FOR PREDICTION OF KINEMATIC SOIL–FOUNDATION INTERACTION EFFECTS ON PEAK HORIZONTAL ACCELERATION OF A RIGID FOUNDATION

The kinematic soil–foundation interaction changes the free field ground motion to a different motion at the foundation of a structure. This interaction effect may be expressed by the ratio of the peak horizontal acceleration of a rigid and relatively lightweight foundation to the peak horizontal acceleration at the ground surface in the free field. It is found that the interaction effect can be defined by a simple function of the ratio of the peak horizontal ground velocity and ground acceleration in the free field, the length of the foundation and the shear wave velocity in the soil. Predictive equations for the kinematic soil foundation effect are derived using 350 strong motion records generated by 114 earthquakes world-wide. At the same time, an attenuation relationship is derived for the ratio of the peak horizontal ground velocity and acceleration from the same set of data. Ten case histories are studied; the interaction effects are calculated by using the predictive equations and then compared with measured field values. The results of the comparison illustrate the degree of predictive capability of the method when the foundation mass and the inertial soil–foundation interaction are not considered.

A SIMPLIFIED METHOD FOR PREDICTION OF KINEMATIC SOIL-FOUNDATION INTERACTION EFFECTS ON PEAK HORIZONTAL ACCELERATION OF A RIGID FOUNDATION

The kinematic soil–foundation interaction changes the free field ground motion to a different motion at the foundation of a structure. This interaction effect may be expressed by the ratio of the peak horizontal acceleration of a rigid and relatively lightweight foundation to the peak horizontal acceleration at the ground surface in the free field. It is found that the interaction effect can be defined by a simple function of the ratio of the peak horizontal ground velocity and ground acceleration in the free field, the length of the foundation and the shear wave velocity in the soil. Predictive equations for the kinematic soil foundation effect are derived using 350 strong motion records generated by 114 earthquakes world-wide. At the same time, an attenuation relationship is derived for the ratio of the peak horizontal ground velocity and acceleration from the same set of data. Ten case histories are studied; the interaction effects are calculated by using the predictive equations and then compared with measured field values. The results of the comparison illustrate the degree of predictive capability of the method when the foundation mass and the inertial soil–foundation interaction are not considered.

Ground motion at the San Francisco international airport from the Loma Prieta earthquake sequence, 1989

AbstractFollowing the Loma Prieta earthquake, the U.S. Geological Survey installed four portable digital seismic recorders at the San Francisco International Airport (SFO) for one week to study aftershock ground motion at this important Bay area “lifeline.” This study was motivated largely by the need to anticipate strong ground motion from future major earthquakes affecting the Bay area and, to a lesser extent, by the fact that SFO was shut down for 13 hours owing to damage from the Loma Prieta shock. Accordingly, the recording sites were chosen so as to elucidate the effects of varying thicknesses of low-velocity surficial alluvium on the ground motion. Three large aftershocks with magnitudes ranging from 4.2 to 4.5 each produced ground motion that was recorded at all four SFO stations. One of our stations was collocated with a permanent ground motion recorder that indicated a peak horizontal velocity of 29 cm / sec and a peak horizontal acceleration of 0.33 g during the 18 October mainshock. From the aftershock data and one mainshock record, it is possible to extrapolate approximately the mainshock ground motion to other locations at SFO and, more generally, to assess the effects of low-velocity sedimentary cover, including artificial fill material, on the character of the ground motion. The main-shock ground motion recorded at the permanent station was apparently typical for most of SFO where the near-surface alluvium resulted in peak horizontal ground velocity, in the frequency band 0.1 to 3 Hz, amplified by a factor of about 2.5 relative to that recorded at bedrock sites. Observations, in the epicentral distance range 59 to 95 km, including SFO, of the moho-reflected phases PmP and SmS from the aftershocks support the hypothesis, presented elsewhere, that the phase SmS accounted for much of the peak ground motion throughout most of the San Francisco Bay area.

Seismic loading provision changes in National Building Code of Canada 1985

This paper describes the process by which Canadian seismic loading provisions are developed and then details the primary changes being introduced in the 1985 edition of the National Building Code of Canada. The most significant change is the inclusion of new seismic zoning maps, based on a new seismic risk methodology, a new probability level, and additional seismic zones, and incorporating both horizontal ground velocity and acceleration as zoning parameters. The format of base shear calculation is revised to incorporate these changes, including the specification of a new seismic response parameter. The base shear formula is calibrated to ensure that, on a cumulative basis throughout the country, the level of seismic loading remains unchanged. Additional changes discussed in the paper include the removal of dynamic analysis as a specific option in the base shear calculation and some significant changes in the calculation of torsional effects.

Use of seismoscope records to determineMLand peak velocities

AbstractMore information about ground motion can be extracted from seismoscope records than a single point on a response spectrum. To demonstrate this, the relation between seismoscope response and Wood-Anderson instrument output and peak horizontal ground velocity has been studied by simulating the various responses for a range of distances and magnitudes. The simulations show that the relation used by Jennings and Kanamori (1979) to convert from peak seismoscope readings to the peak response of a Wood-Anderson instrument has a distance- and magnitude-dependent systematic error. The error is negligible, however, for modern seismoscopes at distances of a few tens of kilometers. At several hundred kilometers, the relation underestimates the Wood-Anderson response by as much as a factor of two. The spread in Jennings and Kanamori's estimate of ML for the 1906 San Francisco earthquake, recorded on seismoscopes having relatively low natural frequencies (0.26 and 0.5 Hz), is reduced by the results in this paper—the upper value, from a seismoscope in Carson City, Nevada, at 290 km from the fault, going from ML = 7.2 to ML = 7.0 and the lower value, from Yountville, California (R ≈ 60 km), going from about 6.3 to 6.4. About 0.3 units of the remaining spread may be due to local geologic site conditions. If the 0.3 units is distributed equally between the Yountville and Carson City recordings, the estimates of ML for the San Francisco earthquake then become 6.5 and 6.8, somewhat lower than Jennings and Kanamori's final estimates of 634 to 7. Although the error in using the relation of Jennings and Kanamori to estimate Wood-Anderson response was at most a factor of 1.6 for the 1906 earthquake, the error can be substantially larger for smaller earthquakes recorded on similar low frequency seismoscopes.The relation between Wood-Anderson and seismoscope response used by Jennings and Kanamori can be combined with an empirical relation between peak horizontal velocity and Wood-Anderson response to predict peak velocity from seismocope recordings. The simulations show that this relation (vmax = 8.1Awa, where vmax is the peak horizontal velocity in centimeters/second and Awa is one-half the range of the Wood-Anderson motion in meters) forms a lower bound for estimates of peak velocity from seismoscope recordings. The relation is good for stations within about 100 km of earthquakes with moment magnitudes of about 4.5 to 6.5, and it underestimates peak velocity by factors up to 2 or 3 for larger earthquakes at distances within 100 km. An application of the simulation method to the 1976 Guatemala earthquake (moment magnitude = 7.6) results in 37 cm/sec as a lower bound to vmax, with 66 cm/sec as a more likely value, from the seismocope recording in Guatemala City (approximately 25 km from the Motagua fault).

Effects of local geology on ground motion near San Francisco Bay

abstract Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, California. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of March 22, 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. For sites underlain by a layer of younger bay mud or artificial fill, maximum horizontal ground velocities generally increased with thickness of the layer and were as much as ten times greater than those recorded on nearby bedrock. The maximum vertical velocities for these sites were between 1 and 3.5 times greater. Spectral amplification curves clearly define a “dominant ground period” of about 1 second for sites underlain by younger bay mud. For sites underlain by older, more consolidated sediments, no clearly defined “dominant ground period” was found. Maximum ground velocities for the older bay sediment sites were about twice those recorded on bedrock. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.