Aftershock Ground Motions Research Articles

Assessment of Seismic Performance of Buildings with Incorporation of Aftershocks

Aftershock ground motions are essential for the study of aftershock synthesis into seismic risk. However, the number of available aftershock records is limited, which necessitates that the methods for synthesizing aftershocks to be developed. This paper aims to propose a methodology to synthesize aftershocks based on the information of the mainshocks, which is able to incorporate the uncertainties of ground motions and can be utilized in seismic risk analysis. The proposed method is illustrated by application to risk analysis for two nonductile RC frame buildings and is validated by comparing with the results obtained from using as-recorded mainshock-aftershock (MS-AS) sequences. The results calculated when only mainshocks are considered are also studied to investigate the MS-AS effects. The paper shows that the synthesized MS-AS sequences using the proposed method can yield results statistically close to the as-recorded sequences. The results also reveal that only considering the mainshock will underestimate the seismic risk but the underestimation is limited.

The damage investigation of inelastic SDOF structure under the mainshock–aftershock sequence-type ground motions

Structures located in seismically active regions may be subjected to mainshock–aftershock sequence-type ground motions, which are characterized by the presence of strong aftershock ground motions after the mainshock and separated by short intervals of time. This manuscript investigates the damage of inelastic SDOF structure with four hysteretic models and three response demand parameters. The relative intensity of the aftershock ground motion to the mainshock ground motion is defined, denoted by PGAas/PGAms. The aftershock ground motions are scaled to have different levels of PGAas/PGAms. The results indicate that the effect of aftershock on ductility demand is slight for PGAas/PGAms≤0.5 and can be ignored in engineering practice. The aftershock ground motion has more significant influence on the normalized hysteretic energy and damage index than on ductility demand. The effect of aftershock ground motion with larger PGAas/PGAms on the response demand is generally more obvious for non-degrading system than for degrading system.

Shaking table test on the seismic failure characteristics of a subway station structure on liquefiable ground

SUMMARYIn order to investigate the seismic failure characteristics of a structure on the liquefiable ground, a series of shaking table tests were conducted based on a plaster model of a three‐story and three‐span subway station. The dynamic responses of the structure and ground soil under main shock and aftershock ground motions were studied. The sand boils and waterspouts phenomena, ground surface cracks, and earthquake‐induced ground surface settlements were observed in the testing. For the structure, the upward movement, local damage and member cracking were obtained. Under the main shock, there appeared longer liquefaction duration for the ground soil while the pore pressure dissipated slowly. The acceleration amplification effect of the liquefied soil was weakened, and the soil showed a remarkable shock absorption and concentration effect with low frequency component of ground motion. However, under the aftershock, the dissipation of pore pressure in the ground soil became obvious. The peak acceleration of the structure reduced with the buried depth. Dynamic soil pressure on the side wall was smaller in the middle and larger at both ends. The interior column of the model structure was the weakest member. The peak strain and damage degree for both sides of the interior column exhibited an ‘S’ type distribution along the height. Moreover, the seismic response of both ground soil and subway station structure exhibited a remarkable spatial effect. The seismic damage development process and failure mechanism of the structure illustrated in this study can provide references for the engineers and researcher. Copyright © 2013 John Wiley & Sons, Ltd.

2011 年東北地方太平洋沖地震で被災した盛土での余震観測と地震動の評価

2011 年東北地方太平洋沖地震で被災した盛土での余震観測と地震動の評価

Seismic damage of long span steel tower suspension bridge considering strong aftershocks

The residual capacity against collapse of a main shock-damaged bridge can be coupled with the aftershock ground motion hazard to make an objective decision on its probability of collapse in aftershocks. In this paper, a steel tower suspension bridge with a main span of 2000 m is adopted for a case-study. Seismic responses of the bridge in longitudinal and transversal directions are analyzed using dynamic elasto-plastic finite displacement theory. The analysis is conducted in two stages: main shock and aftershocks. The ability of the main shock-damaged bridge to resist aftershocks is discussed. Results show that the damage caused by accumulated plastic strain can be ignored in the long-span suspension bridge. And under longitudinal and transversal seismic excitations, the damage is prone to occur at higher positions of the tower and the shaft-beam junctions. When aftershocks are not large enough to cause plastic strain in the structure, the aftershock excitation can be ignored in the seismic damage analysis of the bridge. It is also found that the assessment of seismic damage can be determined by superposition of damage under independent action of seismic excitations.

Ground Motion-Based Testing of Seismic Hazard Models in New Zealand

Abstract We develop a testing methodology for the New Zealand national probabilistic seismic hazard (PSH) model that builds on the groundwork of previous studies. Our fundamental approach is to test the full model, or in other words, the final output of the model (ground-motion exceedance for a given return period). Our results show that the PSH model is rejected as underpredicting the historical number of exceedances for specific peak ground acceleration (PGA) levels obtained directly from instrumental strong-motion data over the last 1–4 decades. However, when aftershock ground motions are removed from the strong-motion data, the model is not inconsistent with the observations. The implications for the PSH model are that the lack of aftershocks in the model led to initial model rejection and that the model may perform better for short (decadal) time periods if aftershocks are included in the PSH model. The results are different from those of earlier Modified Mercalli Intensity (MMI)-based studies that suggested the PSH model was predicting hazard slightly higher than that of the historical record. Our new test dataset has the advantage of using observed PGA rather than PGA inferred from MMI. Establishment of a protocol for formally testing future versions of the New Zealand PSH model within a testing center such as those using the Collaboratory for the Study of Earthquake Predictability protocol will require consideration of the fact that the tests are limited by the available datasets of strong earthquake shaking.

SITE AMPLIFICATION IN SHALLOW SOILS IN KASHIWAZAKI CITY, ESTIMATED FROM GROUND MOTION DATA FROM AFTERSHOCKS OF THE 2007 CHUETUOKI EARTHQUAKE AND MICROTREMOR EXPLORATIONS

Observations of earthquake ground motion due to aftershocks of the 2007 Chuuetsu-oki earthquake were conducted in Kashiwazaki city, Niigata prefecture, by installing 10 seismometers with different geological conditions for aiming at understanding local site amplification in the area. The site amplification shows large peaks around one second at the site in the central parts of the city. We also conducted microtremor array explorations at the observation sites. The shallow Vs-profiles were estimated from inversions of Rayleigh wave phase velocities obtained from array analysis of vertical microtremors. The amplifications of S-waves for the inverted profiles are similar to those from the aftershock ground motions. The average S-wave velocity for the top 30 meters of the S-wave profiles is calculated at each site. The average velocities at the sites with damage of wooden houses are less than 130 m/s.

A probabilistic framework for quantification of aftershock ground‐motion hazard in California: Methodology and parametric study

AbstractThis paper presents a proposed method of aftershock probabilistic seismic hazard analysis (APSHA) similar to conventional ‘mainshock’ PSHA in that it estimates the likelihoods of ground motion intensity (in terms of peak ground accelerations, spectral accelerations or other ground motion intensity measures) due to aftershocks following a mainshock occurrence. This proposed methodology differs from the conventional mainshock PSHA in that mainshock occurrence rates remain constant for a conventional (homogeneous Poisson) earthquake occurrence model, whereas aftershock occurrence rates decrease with increased elapsed time from the initial occurrence of the mainshock. In addition, the aftershock ground motion hazard at a site depends on the magnitude and location of the causative mainshock, and the location of aftershocks is limited to an aftershock zone, which is also dependent on the location and magnitude of the initial mainshock. APSHA is useful for post‐earthquake safety evaluation where there is a need to quantify the rates of occurrence of ground motions caused by aftershocks following the initial rupture. This knowledge will permit, for example, more informed decisions to be made for building tagging and entry of damaged buildings for rescue, repair or normal occupancy. Copyright © 2008 John Wiley & Sons, Ltd.

Annual limit-state frequencies for partially-inspected earthquake-damaged buildings

A procedure is presented for estimating an “annual limit-state frequency”, that is, the mean annual frequency of exceeding a (e.g. seismic-drift) limit state, for a partially-inspected earthquake-damaged SMRF building with fractured beam-column connections. Typically, inspection of all the moment-resisting connections in a building for fractures is prohibitively expensive. Motivated by this reality, the proposed procedure (developed as part of the SAC Steel Project) accounts for the uncertainty, due to incomplete inspection, in the total number and locations of fractured connections. Since the procedure can also take into account the aftershock ground motion hazard, an annual limit-state frequency estimated for a damaged building can serve as a basis for deciding, for example, whether to permit occupancy soon after the main-shock. Conversely, the estimated annual limit-state frequency can be used to guide inspection decisions, such as whether to inspect more connections and thereby reduce the uncertainty in the state of damage.

Site Effects in Avcilar, West of Istanbul, Turkey, from Strong- and Weak-Motion Data

Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during the 17 August 1999 Izmit earthquake, whose epicenter was roughly 90 km east of the city. Most of the fatalities and damage occurred in the suburb of Avcilar that is 20 km further west of the epicenter than the city proper. To investigate this pattern of damage, the U.S. Geological Survey, in cooperation with the Kandilli Observatory & Earthquake Research Institute (KOERI), deployed portable digital seismographs at seven free-field sites in western Istanbul, to record aftershocks during the period from 24 August to 2 September. The primary objective of this deployment was to study the site effects by comparing the aftershock ground motions recorded at sites inside and outside the damaged area, and to correlate site effects with the distribution of the damaged buildings. In addition to using weak-motion data, mainshock and aftershock acceleration records from the KOERI permanent strong-motion array were also used in estimating the site effects. Site effects were estimated using S waves from both types of records. For the weak-motion data set, 22 events were selected according to the criteria of signal-to-noise ratio (S/N ratio) and the number of stations recording the same event. The magnitudes of these events ranged from 3.0 to 5.2. The acceleration data set consisted of 12 events with magnitudes ranging from 4.3 to 5.8 and included two mainshock events. Results show that the amplifying frequency band is, in general, less than 4 Hz, and the physical properties of the geologic materials are capable of amplifying the motions by a factor of 5-10. In this frequency band, there is a good agreement among the spectral ratios obtained from the two mainshocks and their aftershocks. The damage pattern for the 17 August Izmit earthquake is determined by several factors. However, our study suggests that the site effects in Avcilar played an important role in contributing to the damage. Manuscript received 30 August 2000.

System identification estimation of soil properties at the Lotung site

Dynamic properties of the soils at the Lotung test site, Lotung, Taiwan, are estimated from seismic vertical array measurements (input–output data sets) using both time-invariant and time-variant parametric modeling methods (system identification). Soil properties are directly mapped from model parameters to an equivalent lumped mass model of the soil interval. Shear stiffness and damping ratios were calculated for 8 events with M L ranging from 4.5 to 7.0. Shear stiffness ranged between 0.5 and 6 MN/m, inversely proportional to PGA. The equivalent viscous damping ratio varied from 2 to 30% of critical damping, proportional to PGA. Degradation of soil behavior, while less pronounced with increasing depth, consistently occurs above a peak input acceleration of 0.07 g. Although “non-linear” behavior is evident above 0.17 g, Event 7 (0.21 g) is accurately predicted using a linear constant parameter model estimated from the smaller Event 8 aftershock ground motions.

Modelling of ground motion in the Higashinada (Kobe) area for an aftershock of the 1995 January 17 Hyogo-ken Nanbu, Japan, earthquake

We model the ground motion from an aftershock of the 1995 January 17 Hyogo-ken Nanbu (Kobe) earthquake to investigate basin edge effects on wave propagation in Higashinada ward, downtown Kobe. Point-source finite-difference seismograms calculated using a double-couple solution and a 2-D basin structure are compared with the ground-motion velocity seismograms recorded in a small station array deployed at sites within and outside the heavily damage zone in Higashinada ward. The comparison suggests that in the frequency range 0.1–2 Hz that was analysed, the observed spatial amplitude variation of the aftershock ground motion is attributable mainly to the basin edge effects. We found that the basin edge effect, caused by the superposition of the direct S wave and the basin-edge-diffracted waves, amplified the ground motion in a narrow zone that is offset by about 0.7 km from the basin edge.

Basin structure effects on long-period strong motions in the San Fernando Valley and the Los Angeles Basin from the 1994 Northridge earthquake and an aftershock

Abstract The strong ground motions recorded within the San Fernando Valley and in the Los Angeles Basin during the 17 January 1994 Northridge earthquake show complex waveforms and complicated patterns of peak acceleration and velocity. This complexity persists even at long periods (1 to 10 sec). We investigate basin structure effects on long-period wave propagation in the San Fernando Valley and in the Los Angeles Basin by the two-dimensional finite-difference modeling of the mainshock and one selected aftershock with M = 4.1. The aftershock ground motion records in the San Fernando Valley and in the Los Angeles Basin can be explained by basin structure effects, assuming a simple point source model. The structural effects and a source model composed of two subevents can explain the most prominent characteristics of the strong-motion waveforms observed during the mainshock at sites south of the epicenter, while such a combination is not enough to explain the large strong motions observed in the north. This result suggests that the rupture propagation effects significantly amplified the ground motions at sites north of the epicenter.

Basin Structure Effects in the Kobe Area Inferred from the Modeling of Ground Motions from Two Aftershocks of the January 17, 1995, Hyogo-ken Nanbu Earthquake.

Ground motion records from aftershocks of the January 17, 1995, Hyogo-ken Nanbu earthquake provide a good opportunity for evaluating the Osaka basin edge effects on wave propagation in the Kobe area. Aftershock ground motion recorded in a small array deployed at sites within and outside the heavily damaged zone, east of Kobe City, show very large amplification at the sedimentary sites located close to the basin edge. The peak velocities at sediment sites were up to 15 times larger than those at the rock site. To model the effect of the basin edge structure and the source radiation in the frequency range 0.1-2.5 Hz, we calculate double-couple point source SH seismograms using two-dimensional finite difference techniques. By comparing the synthetic seismograms with the transverse component of the observed waveforms from two selected aftershocks, we find that the northern edge of the Osaka basin has a fault-like structure, dipping north. The rather complex basin edge structure causes seismic energy to focus at sites close to the edge. The constructive interference of different basin generated waves amplifies the ground motion. The focal area and the amplification due to the basin structure depend on the source location, i.e. the arrival direction of seismic waves. Our results indicate that the basin edge effect is one of the reasons why the zone heavily damaged during the main shock lies along a narrow belt. They also indicate that the thin surface sediments in the Kobe region amplify the ground motion at frequencies around 2 Hz considerably.

On the variability of aftershock ground motions in the San Fernando Valley : S. E. Hough, C. Dietel, G. Glassmoyer & E. Sembera, Geophysical Research Letters, 22(6), 1995, pp 727–730

On the variability of aftershock ground motions in the San Fernando Valley : S. E. Hough, C. Dietel, G. Glassmoyer & E. Sembera, Geophysical Research Letters, 22(6), 1995, pp 727–730

On the variability of aftershock ground motions in the San Fernando Valley

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake recorded at a 3‐element small‐aperture array within the town of Northridge, above the mainshock rupture plane. Many of the M4‐5 aftershocks are observed to have a prolonged shaking duration, up to ˜8 seconds, with conspicuous longer period (≈1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock that occurred at 23:49 GMT on 1/17 show the origin of these waves. A slant‐stack cross‐correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities and a back‐azimuth that is approximately 10 degrees off that of the direct arrivals. Based on the inferred apparent velocities and consideration of studies in other sedimentary basins, we conclude that these later arrivals consist of surface waves generated within the San Fernando Valley. Similar results are obtained for a M3.4 event recorded across the array. The surface waves are not, however, a ubiquitous feature of the aftershock recordings. We show that other M˜4 events recorded at the same site are characterized by simple displacement pulses and durations that are typical for their magnitude, suggesting that 3‐dimensional site response may be difficult to predict in cases where the sources are close to a valley or basin and/or the basin structure is complex.

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.