Observed Peak Ground Acceleration Research Articles

Performance and Next-Generation Development of the Finite-Fault Rupture Detector (FinDer) within the United States West Coast ShakeAlert Warning System

ABSTRACTRapid fault rupture information is important to estimate seismic ground motions and damage in large earthquakes, and is, therefore, of great value for earthquake early warning (EEW) and rapid response. The Finite-Fault Rupture Detector (FinDer) algorithm computes earthquake line-source models by comparing spatial distributions of high-frequency seismic amplitudes with precomputed template maps. FinDer is one of two seismic EEW algorithms currently adopted by the United States West Coast ShakeAlert EEW system. Between March 2018 and October 2022, FinDer detected 1048 earthquakes (2.3 ≤ M ≤ 7.1) inside the FinDer-reporting region in California, Oregon, and Washington with a median detection time of 8.5 s (75th and 95th percentile: 11.5 s, 38.9 s) after event origin and median errors (first report) of 6.7 km (75th and 95th percentile: 10.5 km, 25.5 km) in location, −0.45 s (mean ± st. dev.: 0.1 ± 5.9 s) in origin time, and 0.33 units (mean ± st. dev.: 0.33 ± 0.31 m.u.) in magnitude. Ground motions estimated using FinDer source parameters are in excellent agreement with observed peak ground accelerations, and residuals are, on average, 30% smaller than if predicted from catalog source parameters. This suggests that FinDer’s simple source parameter terms are accounting for more complex high-frequency source characteristics. This article summarizes the performance of FinDer in ShakeAlert and describes the recent improvements to the algorithm addressing issues encountered during real-time operation. This includes the handling of latent seismic data, robust event detection in regions with sparse instrumentation, enabling faster magnitude convergence in large earthquakes, use of fault- and scenario-specific earthquakes (e.g., along the Cascadia subduction zone or San Andreas fault), as well as increased robustness of FinDer in complex earthquake sequences. We demonstrate the performance of the new FinDer version 3 algorithm using waveform playbacks of selected events along the U.S. West Coast, Japan, and China, including both historic and synthetic earthquakes.

Rupture directivity effect on strong ground motion during the 12 May 2008 MW7.9 Wenchuan earthquake

We focus here on the rupture directivity effect on the spatial distribution and attenuation characteristics of near-field ground motions during the 2008 MW7.9 Wenchuan earthquake. We examine the difference between the observed ground motions in and opposite the rupture directions and compare them with Next Generation Attenuation-West2 (NGA-West2) ground motion prediction models. The isochrone directivity predictor is used to quantify the band-limited nature of the rupture directivity effect on strong ground motion. Our results show that the observed peak ground velocity (PGV) and spectral accelerations of periods of 1.0 s and longer are significantly amplified in the rupture direction, but de-amplified in the opposite direction affected by rupture directivity effect of this event. In contrast, the effect of rupture directivity on the observed peak ground acceleration (PGA) and periods of shorter than 1.0 s are relatively weak. The rupture directivity of this event shows clear period dependent and band limited characteristics with the strongest effect occurring around the period of 7.5 s.

A Shape of the Response Spectrum for Evaluation of the Ultimate Seismic Capacity of Structures and Equipment including High-frequency Earthquake Characteristics

In 2016, an earthquake occurred at Gyeongju, Korea. At the Wolsong site, the observed peak ground acceleration was lower than the operating basis earthquake (OBE) level of Wolsong nuclear power plant. However, the measured spectral acceleration value exceeded the spectral acceleration of the operating-basis earthquake (OBE) level in some sections of the response spectrum, resulting in a manual shutdown of the nuclear power plant. Analysis of the response spectra shape of the Gyeongju earthquake motion showed that the high-frequency components are stronger than the response spectra shape used in nuclear power plant design. Therefore, the seismic performance evaluation of structures and equipment of nuclear power plants should be made to reflect the characteristics of site-specific earthquakes. In general, the floor response spectrum shape at the installation site or the generalized response spectrum shape is used for the seismic performance evaluation of structures and equipment. In this study, a generalized response spectrum shape is proposed for seismic performance evaluation of structures and equipment for nuclear power plants. The proposed response spectrum shape reflects the characteristics of earthquake motion in Korea through earthquake hazard analysis, and it can be applied to structures and equipment at various locations.

Peak ground motions and characteristics of nonlinear site response during the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake

The observed peak ground accelerations and peak ground velocities (PGVs) of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake generally followed the median values from ground motion prediction equations with reasonable errors at fault distances ≥ 50 km. However, at smaller distances, the equations significantly underestimated the peak ground motions, and it was eminent for PGVs. A comparison of surface-to-borehole spectral ratios of S-waves during the mainshock and other events revealed that the sites at smaller distances experienced various degrees of nonlinear site response. The two most widely known characteristics of nonlinear site response are the weakening of higher-frequency components and shifting of predominant frequencies to lower ones in comparison with the linear site response. At one of the sites that recorded the largest intensity of 7 in JMA scale of 0–7, the latter nonlinear effect was so dominant that the ground motions around the new predominant frequency got intensified by one order of magnitude in comparison with that during the weak-motions. Two sites, which were closely located, recorded vertical peak ground accelerations exceeding 1 g for the up-going motions. The recordings showed asymmetric waveforms and amplitudes characteristics of the nonlinear site response in extreme vertical ground motions recorded during a few earthquakes in the past. Few sites having lower vertical peak ground accelerations were also suspected of being experienced nonlinear site response on vertical motions. These findings suggest taking a cautious approach to enumerate the reduction in amplification at higher frequencies using the single-station horizontal-to-vertical (H-to-V) spectral ratio technique. However, we found that the H-to-V technique was still useful to detect nonlinearity. Finally, an ad hoc equation was derived to correct the nonlinear site amplification in predicting horizontal PGVs with respect to one of the most widely used attenuation models in Japan. The results indicated that the effect was much stronger for a larger input motion than that for a proportional change in the Vs30 values.

Comparison of Soil Nonlinearity (In SituStress–Strain Relation and G/Gmax Reduction) Observed in Strong‐Motion Databases and Modeled in Ground‐Motion Prediction Equations

Research Article| November 20, 2018 Comparison of Soil Nonlinearity (In Situ Stress–Strain Relation and G/Gmax Reduction) Observed in Strong‐Motion Databases and Modeled in Ground‐Motion Prediction Equations Philippe Guéguen; Philippe Guéguen aISTerre, Université Grenoble Alpes, CNRS/IFSTTAR, 1381 rue de la piscine ‐ CS40700, 38058 Grenoble, Cedex 9, France, philippe.gueguen@univ-grenoble-alpes.fr Search for other works by this author on: GSW Google Scholar Luis Fabian Bonilla; Luis Fabian Bonilla bDépartement GERS (Géotechnique, Environnement, Risques naturels et Sciences de la terre), Laboratoire Séismes et Vibrations, Université Paris Est ‐ IFSTTAR, 14‐20 Boulevard Newton, Cité Descartes, 77447 Marne‐la‐Vallée, Cedex 2, France Search for other works by this author on: GSW Google Scholar John Douglas John Douglas cDepartment of Civil and Environmental Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2019) 109 (1): 178–186. https://doi.org/10.1785/0120180169 Article history first online: 20 Nov 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Philippe Guéguen, Luis Fabian Bonilla, John Douglas; Comparison of Soil Nonlinearity (In Situ Stress–Strain Relation and G/Gmax Reduction) Observed in Strong‐Motion Databases and Modeled in Ground‐Motion Prediction Equations. Bulletin of the Seismological Society of America 2018;; 109 (1): 178–186. doi: https://doi.org/10.1785/0120180169 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Earthquake ground motions are strongly affected by the upper tens of meters of the Earth’s crust and consequently local site effects need to be included in any ground‐motion prediction. In ground‐motion prediction equations (GMPEs), it is increasingly common to account for possible nonlinear behavior of near‐surface materials (soil). These nonlinear site terms adjust observations made on soft soil sites to the ground motion expected on bedrock and hence allow these abundant soil records to be used within the regression analysis for the derivation of empirical GMPEs. These nonlinear site terms also allow rapid predictions of the expected ground motions on soil rather than requiring a site‐response analysis to be conducted. In this study, we compare the signature on observed peak ground acceleration as a function of a strain proxy of nonlinear soil behavior within four large strong‐motion databases to the predicted signature from four recent GMPEs, three of which explicitly include nonlinear site terms. We find that observed nonlinearity in the databases, interpreted in terms of strain–stress relationships and reduction of shear modulus, is limited, but even this limited effect is underestimated by the nonlinear site terms of the considered GMPEs, which suggests that predictions from these GMPEs could be biased for soft soil sites but also on bedrock. Some of this mismatch could be explained by the use of the average shear‐wave velocity in the top 30 m (⁠VS30⁠) to characterize sites as well as errors in these values. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The 2011 Sikkim earthquake ([formula omitted] 6.9) of the Himalaya: Estimates of earthquake source parameters and representability of attenuation characteristics

The empirical accelerograms of the September 18, 2011, Sikkim earthquake (MW=6.9) have been used in the present study to estimate its source parameters and to identify a representative relation for the attenuation of peak ground acceleration with distance. The spectra of high energy packets observed in the SH waves (approximated by transverse components of accelerograms) has been modeled using Brune’s model. A two-step procedure has been adopted for this purpose. The estimated source parameters of the 2011 Sikkim earthquake have been found to be as - corner frequency (fc): 0.09 Hz, seismic moment (M0): (2.43 ± 0.5) × 1026 dyne-cm, stress drop (Δσ): 50.4 ± 9.28 bars, and the moment magnitude (MW): 6.87 ± 0.05. These estimated values have been found to be consistent with those of reported values obtained using different data sets and procedures. The high-frequency earthquake ground motions are governed by stress drop at the source. Therefore a value of stress drop estimated here is useful for the simulation of earthquake strong ground motions in the region. The attenuation characteristics of observed peak ground acceleration as a function of distance have been compared with those of obtained using five empirical relations. It has been found that a relation given by McGuire (1978) represents the attenuation characteristics of peak ground acceleration is relatively better. This relation may be used for estimating the peak ground acceleration of similar magnitude for future earthquakes in the region. This relation is also useful for the simulation of earthquake strong ground motions in the region using envelope technique. The outcome of the present study is useful for the estimation of the seismic hazard of the Sikkim region of the Himalaya.

Rupture Directivity Effects on Strong Ground Motion during the 15 April 2016Mw 7.0 Kumamoto Earthquake in Japan

Abstract The M w 7.0 Kumamoto, Japan, earthquake occurred on 15 April 2016 at 16:25 UTC. Using ground accelerations recorded by 104 near‐field stations, we investigate spatial variability of observed ground motions, apparent period dependence, and azimuthal variation, as well as rupture directivity effects on various intensity measures. We develop a simplified ground‐motion model that includes both geometric and anelastic attenuation terms. Comparisons of observed and predicted ground motions suggest that predictions from the Next Generation Attenuation‐West2 models provide good fits for the overall observation. Analysis of spatial distribution of the residuals shows that observed peak ground velocity (PGV) and long‐period spectral accelerations (SAs) in the 150°–180° azimuth range along the rupture backward direction (southwest of the fault) can be as low as 0.3–0.8 times the average observation of this event. Long‐period ground motions on the northeast side of the fault in the forward direction are much higher than average, with PGV and long‐period SAs ranging from 1.2 to 1.5 times the average. There is clear period dependence of the strong ground motion variation. The biases due to directivity generally decrease with decreasing period for all azimuth ranges. On the distance dependence of directivity effects, our study shows that directivity effects can be considered practically nonsignificant for stations close to the hypocenter. We also perform a log–linear regression of the residuals, using a new directivity predictor. Our results show that for the 2016 M w 7.0 Kumamoto earthquake, rupture directivity produces significant amplifications in the rupture forward direction, whereas deamplification effects are observed in the rupture backward region. Directivity effects are particularly relevant for PGV and long‐period SA (i.e., SA at periods ≥2.0 s). Such effects do not have systematic influence on peak ground acceleration and short‐period ground motions (i.e., SA at periods Electronic Supplement: Figures of variation of regression residuals with R rup for observed peak ground acceleration (PGA), peak ground velocity (PGV), and spectral accelerations (SAs) and of regression residuals versus V S 30 .

Characteristic ground motions of the 25th April 2015 Nepal earthquake (Mw 7.9) and its implications for the structural design codes for the border areas of India to Nepal

The 25th April 2015 Nepal Earthquake was found associated with a series of aftershocks, and the mainshock rupture propagated predominantly towards SE direction where a major aftershock (Mw 7.3) rocked on 12th May 2015 to the east of the mainshock that enhanced the rate of occurrence of aftershocks in the affected region. We conducted a rigorous analysis of strong motion data to understand the characteristics of ground motion and their bearing on the structural design codes, responsible for the damage to the structures in the border area of India to Nepal. The effect of ground geology on the acceleration response spectra are also evaluated using main shock and its associated strong earthquakes. All the sites used in the present analysis are located on alluvium deposits showing a predominant period of 0.242sec for horizontal components and at 0.193sec for vertical components. Our results demonstrated that observed Peak Ground Acceleration (PGA) has prominent distribution in the border cities of UP and Bihar. PGA ranges from 3 to 80cm/sec2 in the study region for the epicentral distance varying from 120km to 495km with respect to the source zone (mainshock). The Peak Ground Velocity (PGV) varies from 1 to 16cm/sec while the Peak Ground Displacement (PGD) lies in between 1cm and 20cm for the same area. Our study shows that variation of PGD, PGV, and PGA are controlled and dictated by the geo-morphological constraints, besides the nature and extent of structural heterogeneities of the sub-surface geological formation materials. The obtained normalised spectral amplifications are compared with the Bureau of Indian Standard code for construction of buildings which shows that the current Indian building design code is within the structural limits proposed for the seismic forces at all periods for alluvium sites, suggesting that the structural heterogeneity has the strong role contributing towards the intrinsic attenuation in the seismic wave propagating medium. Our analysis also shows a good correspondence with the nonlinearity of the seismic waves, which in turn controls the degree of damage in an area. We infer that extent of damage to the structures in the border cities of India, vicinity to the rupture zone can be avoided if the existing building design code could have been implemented as the earthquake risk resilient mitigation plan.

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.

Investigating the role of source mechanism, surface topography, and attenuation on the observed PGA pattern in May 28, 2004, Mw 6.2 Baladeh earthquake (Iran)

In this paper, we use seismic waveform simulation to investigate the influence of source mechanism complexity, surface topography, and quality factor on the observed peak ground motions in May 28, 2004, moment magnitude (Mw) 6.2 Baladeh earthquake. The observed peak ground acceleration (PGA) pattern in this event, which is the biggest earthquake to hit the Central Alborz Mountains of Iran in modern instrumental era, is irregular in some respects. First, the observed PGA contours are elongated toward north-west and, second, the maximum observed PGA value of 1049 cm/s2 on the horizontal component of Hasan Keyf station 50 km away from the epicenter is quite high and irregular for an earthquake of this magnitude, at such long distance. In this study, we employ the spectral element method, implemented in SPECFEM3D software package to simulate the 3D wave propagation from several source models in the area. Our results suggest directivity effect is the main cause of the anomalous observations in this earthquake and could account for the elongation of PGA contours and also the anomalous maximum PGA value observed at Hasan Keyf strong motion station. We show that the surface topography has minor effect on the observed peak ground acceleration and the resulting PGA maps. Also by finding the bounds of seismic quality factor effect on the peak ground acceleration values, we show that this factor could not account for the elongation of iso-acceleration contours in the north-west direction.

Hybrid attenuation model for estimation of peak ground accelerations in the Kutch region, India

The Kutch region of Gujarat in India is the locale of one of the most devastating earthquake of magnitude (M w) 7.7, which occurred on January 26, 2001. Though, the region is considered as seismically active region, very few strong motion records are available in this region. First part of this paper uses available data of strong motion earthquakes recorded in this region between 2006 and 2008 years to prepare attenuation relation. The developed attenuation relation is further used to prepare synthetic strong motion records of large magnitude earthquakes using semiempirical simulation technique. Semiempirical simulation technique uses attenuation relation to simulate strong ground motion records of any target earthquake. The database of peak ground acceleration obtained from simulated records is used together with database of peak ground acceleration obtained from observed record to develop following hybrid attenuation model of wide applicability in the Kutch region: $$ \begin{aligned} \ln \left( {\text{PGA}} \right) & = - 2.56 + 1.17 \, M_{\text{w}} - \, 0.015R - 0.0001\ln \left( {E + 15} \right) \\ &\quad 3.0 \le M_{\text{w}} \le 8.2;\quad 12 \le R \le 120;\quad {\text{std}} . {\text{ dev}}.(\sigma ): \pm 0.5 \\ \end{aligned} $$ In the above equation, PGA is maximum horizontal ground acceleration in gal, M w is moment magnitude of earthquake, R is hypocentral distance, and E is epicentral distance in km. The standard deviation of residual of error in this relation is 0.5. This relation is compared with other available relations in this region, and it is seen that developed relation gives minimum root mean square error in comparison with observed and calculated peak ground acceleration from same data set. The applicability of developed relation is further checked by testing it with the observed peak ground acceleration from earthquakes of magnitude (M w), 3.6, 4.0, 4.4, and 7.7, respectively, which are not included in the database used for regression analysis. The comparison demonstrates the efficacy of developed hybrid attenuation model for calculating peak ground acceleration values in the Kutch region.

Critical Parameters Affecting Bias and Variability in Site-Response Analyses Using KiK-net Downhole Array Data

Because of the limited number of strong‐motion records that have measured ground response at large strains, any statistical analyses of seismic site‐response models subject to strong ground motions are severely limited by a small number of observations. Recent earthquakes in Japan, including the M w 9.0 Tohoku earthquake of March 2011, have substantially increased the observations of strong‐motion records that can be used to compare alternative site‐response models at large strains and can subsequently provide insight into the accuracy and precision of site‐response models. Using the Kiban‐Kyoshin network (KiK‐net) downhole array data in Japan, we analyze the accuracy (bias) and variability (precision) resulting from common site‐response modeling assumptions, and we identify critical parameters that significantly contribute to the uncertainty in site‐response analyses. We perform linear and equivalent‐linear site‐response analyses at 100 KiK‐net sites using 3720 ground motions ranging in amplitude from weak to strong; 204 of these records have peak ground accelerations greater than ![Graphic][1] at the ground surface. We find that the maximum shear strain in the soil profile, the observed peak ground acceleration at the ground surface, and the predominant spectral period of the surface ground motion are the best predictors of where the evaluated models become inaccurate and/or imprecise. The peak shear strains beyond which linear analyses become inaccurate in predicting surface pseudospectral accelerations (PSA; presumably as a result of nonlinear soil behavior) are a function of vibration period and are between 0.01% and 0.1% for periods 0.5 s do not display noticeable effects of nonlinear soil behavior. Online Material: Site‐specific information and model residuals at 100 KiK‐net stations. [1]: /embed/inline-graphic-1.gif

Nonlinear Soil Behavior Examined on the Basis of Propagation Time Observed at the KiK‐net Ichinoseki‐Nishi Vertical Array

Abstract The Iwate‐Miyagi Nairiku earthquake (14 June 2008, M 7.2, depth 8 km) caused severe damage in the southern part of Iwate and the northern part of Miyagi Prefectures in Japan. The KiK‐net Ichinoseki‐Nishi vertical array site (IWTH25), operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), is situated near the epicenter. The observed peak ground accelerations at the surface in the up–down, east–west, and north–south directions were, respectively, 3866 cm/s 2 , 1432 cm/s 2 , and 1143 cm/s 2 . The extreme ground motions recorded at this site provide valuable information about the wave propagation and soil degradation at a large strain level. We examined the nonlinear soil behavior at the site due to the earthquake on the basis of normalized input‐output minimization (NIOM) analysis. First, we analyzed 279 records observed before the Iwate‐Miyagi Nairiku earthquake and estimated the vertical propagation time (initial value) between the bottom of the well and the ground surface by taking into account incidence angles. Then we analyzed the records of the Iwate‐Miyagi Nairiku earthquake and 179 events that occurred after it (from 14 June 2008 to 19 October 2009). The results revealed that (1) the S ‐wave propagation time increased from 0.265 s to about 0.35 s because of the nonlinear behavior of the soil; (2) this propagation time change corresponded to a 60% reduction in the shear moduli of the upper soil layers (surface to 64‐m depth) and a strain level of 1×10 −3 ; (3) a gradual decrease in the propagation time with decreasing seismic intensity was observed soon after the principal motions; however, (4) the propagation times in the coda part and for the events after the Iwate‐Miyagi Nairiku earthquake were still larger than the initial value, indicating that the shear rigidities of soils remained less than their values before the earthquake for more than a year.

２００３年宮城県沖スラブ内地震における表層地盤の非線形応答とその強震動への影響

We investigate nonlinear soil responses based on strong motion records at a large number of observation sites during the 2003 Miyagi-oki intraslab earthquake (Mw 7.0). First, we examine the efficiency of S-H/V (horizontal-to-vertical spectral ratio for the S-wave portion at the ground surface) method to identify the nonlinearity by comparing the results with those by the standard S-wave surface-to-borehole spectral ratio method. In the examination we propose a new quantitative index to measure the degree of the nonlinear soil response, DNL, which evaluates the gap between the spectral ratio for strong ground motion and that for weak ground motion. The DNL values by the S-H/V method as well as those by the surface-to-borehole spectral ratio method increase with observed peak ground acceleration (PGA) values at surface (100∼1000 cm/s2), reflecting the increase of the degree of nonlinearity. However, both DNL values at sites with large S-wave velocity of the surface layer (>300 m/s) do not show any increase even for large PGA values (∼800 cm/s2), indicating linear site response. From these facts we conclude that the S-H/V spectral ratio method is also efficient to identify the nonlinearity. Second, in order to examine the effects of nonlinear soil response on strong ground motions, we make the broadband strong motion simulation for the 2003 Miyagi-oki earth.quake by means of the empirical Green’s function (EGF) method. The synthetic waveforms for the horizontal components at the ground surface significantly overestimate the observed ones at stations with large DNL values; the synthetic PGA values are about two times greater than the observed ones. We confirm that the overestimation is attributed to the ignorance of the nonlinear soil response in the EGF simulation. Finally we briefly discuss the potential of the DNL value for studying the nonlinear soil response.

Near-Source Strong Ground Motion Characteristics of the 2008 Wenchuan Earthquake

The free-field strong ground motions recorded at 64 sites within 200 km from the ruptured Longmen Shan fault during the 12 May 2008 M w 7.9 Wenchuan earthquake were used to examine the near-source strong ground motioncharacteristics. The magnitude of vertical and horizontal peak ground accelerations (PGAs), frequency content, and duration of acceleration histories are analyzed. The observed peak ground accelerations at distances exceeding 60 km from the ruptured fault were significantly higher than those predicted by the Next Generation Attenuation (NGA) model, whereas, for periods of larger than 1 s, the spectral accelerations are much smaller than those of the NGA model. The same is also true for the observed peak ground velocities (PGVs). The duration of ground shaking is longer than for most earthquakes of comparable similar magnitude. For sites within 60 km from the ruptured fault, the accelerations observed on the hanging wall are larger than those observed on the footwall. The duration of ground shaking along the rupture direction (or the forward direction) is shorter than that opposite to the rupture direction (or the backward direction). There is also a systematic increase of PGA and PGV at sites located in the direction of the rupture propagation, whereas no such clear trend was found in the stations located along the backward direction. From the observed acceleration time histories at various locations, the rupture process of the 2008 Wenchuan appears to be more complicated than the focal mechanism interpreted from far-field seismic records. For example, the generation mechanism of the two distinct wave trains observed at Wolong and other stations need more detailed studies.

Statistical Features of Short-Period and Long-Period Near-Source Ground Motions

Abstract This study collects recorded ground motions from the near-source region of large earthquakes and considers to what extent this historic record can inform expectations of future ground motions at similar sites. The distribution of observed peak ground acceleration (PGA) is well approximated by the lognormal distribution, and we expect the observed distribution to remain unchanged with the addition of data from future earthquakes. However, the distribution of peak ground displacements (PGD) will likely change after a well-recorded large earthquake. Specifically we expect future observations of PGD greater than those previously recorded. We use seismic scaling relations to motivate the expected distribution of PGD as uniform on the logarithmic scale, or at least fat-tailed. Because PGA does not scale with fault rupture area or slip on the fault, there are no such scaling relations to predict the observed distribution of PGA. The observed records show that there is essentially no correlation between PGD and PGA for near-source ground motions from large events. The large uncertainty in a future value of PGD in the near-source region of a large earthquake exists despite the ability of Earth scientists to accurately model long-period ground motions. In contrast, the relative certainty in a future value of PGA exists despite the inability to model short-period ground motions reliably. The stability of the observed distribution of PGA with respect to new ground-motion records enables us to predict the distribution of future PGA and to calculate the probability of exceeding the largest recorded PGA.

An Analysis of the Mw 4.7 Jabalpur, India, Earthquake of 16 October 2000: Toward Ground-Motion Estimation in the Region from Future Events

With the aim of ground-motion estimation in the region of Jabalpur, India, which suffered serious damage during the 21 May 1997 earthquake ( M w 5.8), we analyze an event that occurred on 16 October 2000. This earthquake was recorded by broadband seismic stations of Jabalpur (jpbl, Δ = 45 km) and Rewa (rewa, Δ = 173 km), and a few other stations at regional distances. Seismic phases constrain the depth of the focus to 27 km. Regional centroid moment tensor inversion of long- period waves at farther stations (Δ > 250 km) yields a focal mechanism with one of the nodal planes characterized by: strike φ = 57°, dip δ = 43°, and rake λ = 22°. A similar focal mechanism is obtained from the inversion of filtered JBPL and rewa data. Compared with the earthquake of 2000, the 1997 event was deeper (36 km) and the corresponding nodal plane ( φ = 61°, δ = 64°, and λ = 74°) involved less strike-slip component. The seismic moment obtained from S -wave spectra is 1.36 × 10 16 N m ( M w 4.7). Within the framework of the ω 2 -source model, the high-frequency level of the source spectrum of the 2000 event requires Δ σ = 42 MPa. The same Δ σ was estimated for the 1997 earthquake, suggesting that the stress drop of subcrustal earthquakes in the Jabalpur region may, in general, be high. We simulate ground motions at JBLP site assuming a ω 2 -source model and Δ σ = 42 MPa and using the random vibration theory (rvt) and the empirical Green’s function technique. The predicted attenuation curves from rvt fit observed peak ground acceleration (pga) and peak ground velocity (pgv) data from the events of 2000 and 1997 (45 R 2 and 4.5 cm/sec, respectively, in the epicentral region during the 1997 Jabalpur earthquake. The corresponding values for the proposed maximum credible earthquake in the Jabalpur area ( M w 6.5, h = 30 km, Δ = 20 km) are 235 cm/sec 2 and 13 cm/sec, respectively. The simulated ground motions at JBPL, with Δ σ = 42 MPa, along with the transfer functions now available from an extensive microzonation study of the area, may be used to predict ground motions in the Jabalpur area during future earthquakes.

Ground-Motion Predictions from Empirical Attenuation Relationships versus Recorded Data: The Case of the 1997-1998 Umbria-Marche, Central Italy, Strong-Motion Data Set

We evaluate the goodness of fit of attenuation relations commonly used for the Italian national territory (Sabetta and Pugliese, 1996) by using the maximum likelihood approaches of Spudich et al. (1999) and Scherbaum et al. (2004). According to the classification scheme proposed by Scherbaum et al. (2004), the Sabetta and Pugliese (1996) relationships show consistent discrepancies between the predicted and the observed peak ground acceleration (pga) at rock sites in the Umbria- Marche region, central Italy; however, at soft sites the agreement between observations and prediction is satisfactory. The bias of the residuals, computed with the Sabetta and Pugliese (1996) models for pga, peak ground velocity, (pgv) and pseudovelocity response spectrum (psv) (for M l = 4–6 and epicentral distances up to 100 km) is negative. This means that on the average, the predictions overestimate the observations, but the overestimation decreases with increasing magnitude. Then, we present regional predictive relations (UMA05) for maximum horizontal pga, pgv, and 5%-damped psv, derived from the strong-motion data recorded in the Umbria-Marche area and classified as to four site categories. The UMA05 attenuation relationships for rock sites are log 10 (PGA) = −2.487 + M 1 − 1.280 log 10 ( R 2 + 3.94 2 ) 0.5 ± 0.268 log 10 (PGV) = −1.803 + 0.687 M 1 − 1.150 log 10 ( R 2 + 2.74 2 ) 0.5 ± 0.300 and log 10 (PGA) = −2.500 + 0.544 M 1 − 1.284 log 10 R h ± 0.292 log 10 (PGV) = −1.752 + 0.685 M 1 − 1.167 log 10 R h ± 0.297, where pga is measured in fraction of g and pgv in centimeters per second, M l is the local magnitude in the range 4–6, R is the epicentral distance in the range 1–100 km, and R h is the hypocentral distance in kilometers. We used the random effect model (Brillinger and Priesler, 1985; Abrahamson and Youngs, 1992; Joyner and Boore, 1993; Joyner and Boore, 1994) to estimate the component of variance related to the earthquake-to-earthquake, station-to-station, and record-to-record variability, and to quantify the benefit of introducing a site classification in the attenuation model to reduce the variance. The introduction of the site classification in the attenuation model allows a reduction of the station-to-station component of variability (from 0.19 to 0.14 for pga, and from 0.21 to 0.18 for pgv). We also found that the record- to-record component represents the largest contribution to the model uncertainty.

The Dudley earthquake of 2002: A moderate sized earthquake in the UK

The 4.7 ML Dudley earthquake on 22 September 2002 at 23:53 (UTC) was widely felt throughout England and Wales, and was the largest earthquake to occur onshore in the United Kingdom (UK) since the magnitude 5.1 ML Bishop's Castle earthquake in 1990. The earthquake hypocentre, determined from inversion of observed P- and S-wave travel-time data, suggests a source depth of 14 km and this depth estimate is supported by forward modelling of observed waveforms. Focal mechanisms obtained from both first motion polarities of local observations and moment tensor inversion of regional observations show left-lateral, strike-slip faulting along a near vertical, near north–south striking fault plane whose orientation is in good agreement with the surface expression of the observed faults in the region. Two aftershocks were recorded within the location error ellipsoid of the mainshock. Comparison of the waveform signals revealed that the mainshock and aftershocks were nearly co-located and possibly had the same source mechanism. The observed peak ground acceleration is found to be less than that predicted using empirical relations, which have been considered applicable in the UK. Seismic moment M 0 and stress drop Δ σ were measured from on-scale records where L g arrivals were clear, and then used to give better estimates of the peak ground accelerations using a stochastic approach.

Attenuation characteristics of peak ground acceleration from fault trace of the 1999 Kocaeli (Turkey) earthquake and comparison of spectral acceleration with seismic design code

The 17 August 1999 Kocaeli earthquake in Turkey produced a majorsurface rupture. We traced this surface rupture from Golcuk toDuzce and located it accurately by using GPS. The closest distancefrom the surface rupture to the strong motion observation sites weredetermined. Then the attenuation characteristics of the observed peakground acceleration were compared with the attenuation relation given byFukushima and Tanaka (1992), which is suitable for the near-fault zone inJapan and gives results that closely match data recorded during the 1995Hyogo-ken Nanbu earthquake in Japan. Although this attenuation relationwas developed for Japan, we found that it agreed well with the KOCAELIearthquake. Furthermore, the observed spectral acceleration of 5%damping was compared with the building design code of Turkey and theobserved level was lower than the code.