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Site characterization is one of the most important components in seismic hazard analysis because it accounts for the important effects of near‐surface geology on ground motion. It is usually quantified based on the time‐averaged S‐wave velocity (VS) for the top 30 m of the profile (VS30). In this study, we estimate the site class in Jakarta based on VS structure estimated using microtremor array observations. The results show that microtremor‐derived VS profiles agree well with standard penetration test‐derived profiles at nine sites. The site‐class estimates in the Jakarta area can be divided into two National Earthquake Hazards Reduction Program classes: (a) site class E (soft soil) located in alluvium, beach ridge, and alluvial fan deposits in northern and western Jakarta, and (b) site class D (stiff soil) found mainly in alluvial fan deposits in southeastern Jakarta. The variation of VS30 in Jakarta leads to different soil amplification factors that will impact the seismic hazard at the surface. We show that the seismic hazard resulting from selected ground‐motion models (GMMs) illustrates a clear influence of site effects at long periods (>1 s). However, the effect on peak ground acceleration and response spectra for short periods (0.2 s) appear to be less pronounced, due to the GMMs’ treatment of basin effects and nonlinear soil behavior. Available GMMs may not accurately account for such effects in the Jakarta basin, and GMMs specific to Indonesia should be developed to accurately assess seismic hazard there.

On 27 August 1904, seismic stations from around the globe recorded an M >7 earthquake originating from central Alaska. Very little was known about this earthquake. One felt report from Rampart, Alaska, had been attributed to the notes of Harry Fielding Reid, yet its original source was unknown. Here, we present five felt reports for the 1904 earthquake that show evidence of felt shaking across most of central Alaska. Using the 1904 arrival‐time data, we estimate an epicentral location near Lake Minchumina at the northeastern extent of the Iditarod–Nixon fault. Our preferred fault for the 1904 earthquake is the right‐lateral Iditarod–Nixon fault, which, though relatively seismically quiet, generated an M 6.2 earthquake in 1935. Paleoseismic investigations are needed to search for evidence of fault activity, including the 1904 earthquake rupture, in the tectonically complex region of the 1904 earthquake. Electronic Supplement: Tables of arrival time, figures of station registers, visualization of NonLinLoc (NLL) solution for the 1904 Alaska earthquake, distribution of depths of the posterior probability for the 1904 and 1935 events, epicenter and samples of the posterior probability distribution for the 1904 and 1935 earthquakes, map of southward shift of epicenters, and estimated epicenters for the 3 February 2000 Kaltag earthquake.

The historical earthquake of the 1894 in Atalanti, Greece, produced widespread liquefaction effects on the broader coastal area. Although this earthquake has been extensively studied in the past, its magnitude still remains controversial. In the present study, we propose an indirect method for assessing its magnitude by simulating the ground motion and liquefaction effects, caused by a scenario earthquake with the characteristics of the 1894 event, at several sites located at the coastal zone of Atalanti Bay. For this purpose, we utilize information about the near‐surface soil conditions obtained by geophysical tools, data of recent seismic events, and results of stochastic simulation of strong ground motion for the scenario earthquake of the 1894 event. We confirm the high liquefaction potential of the investigated sites and establish that the low limit of magnitude ( M w ) for the 1894 earthquake is equal to 7.0. Electronic Supplement: Results of liquefaction analysis.

The Nakamura method, which involves horizontal‐to‐vertical spectral ratio (HVSR) analysis, is widely used for seismic microzonation studies. The noise from local traffic in city conditions presents a challenge for the application of HVSR analysis. This article presents a technique developed for separation of the transient noise due to local traffic (high‐level noise) and background ambient noise (low‐level noise) and the application of the HVSR analysis to both partitions of the noise. This approach is applied to identify the predominant frequencies for almost 200 noise samples from the Greater Toronto area. The results demonstrated that the developed technique is effective and allows estimation of the fundamental resonant frequency in the HVSR in urban environment, even in the presence of intensive nearby traffic. The interpretation of the obtained results showed that, most probably, the lower (fundamental) frequency appears due to multiple reflections from the overburden/bedrock boundary. In some cases, a resonance with higher amplitude is dominant, and it is due to a contrast boundary between soil layers in the overburden.

We study the source mechanism and depth of the recent moderate earthquake, which occurred on 1 May 2013 in the Kashmir seismic gap. The epicenter of the earthquake lies southeast of the Kashmir valley and close to the meisoseismal zone of the 1555 Kashmir earthquake (magnitude ∼7.6). This event provides an excellent opportunity in which to study the seismotectonics of the Kashmir Himalaya using global digital seismic data. We modeled the source parameters of the earthquake by least‐squares fitting of the teleseismic P ‐ and SH ‐waveform data. The minimum‐misfit solution reveals that the earthquake occurred on an oblique thrust fault with strike, dip, and rake of 346°, 26°, and 121°, respectively, and originated at a depth of 16±3 km. The strike of the fault plane matches that of the mapped Himalayan thrust faults in the region, and its depth puts it within the Himalayan wedge, close to the basal decollement (main Himalayan thrust [MHT]). However, the dip of the causal fault plane is larger than the inferred dip of the MHT and therefore requires the earthquake to have occurred on a ramp or a splay thrust. The most likely candidate is the main boundary thrust the downdip end of which matches with the geometry of the fault plane and coincides with the observed hypocentral distribution of small‐to‐moderate earthquakes in the region. We synthesize our findings with geodetic measurements from the Kashmir Himalaya to show that the hypocentral zone of this event marks a region of strain accumulation beneath the Kashmir seismic gap. This region could possibly be the point of initiation of a future great earthquake. We conclude by highlighting the outstanding questions in assessing seismic hazards in this region and designing a broadband seismological field experiment that is currently being undertaken to address these important issues.

Anomalous seismicity patterns in epicentral and surrounding areas have been shown to occur prior to large earthquakes, although the processes determining the spatial distribution and migration patterns of such seismicity are still poorly understood. We applied the improved pattern informatics (PI) method to earthquake data maintained by the Japan Meteorological Agency (JMA) for a broad region including northeastern inland Japan and the source area of the 2011 Tohoku earthquake, in order to reveal the precursory processes of the earthquake and its related statistical features. In particular, we focused on the spatial distribution and migration patterns of PI hotspots, which highlight areas of anomalous seismic activity. Our results show that such hotspots had been approaching the epicenter of the 2011 Tohoku earthquake from 2000 until the earthquake occurred. The possibility that this result was obtained by chance was rejected at the 95% confidence level based on Molchan’s error diagram. Our result supports the hypothesis that the preparatory processes of the 2011 Tohoku earthquake included anomalous seismic activity and its migration toward the epicenter of the earthquake.

The surficial geology of the city of Ottawa primarily consists of soft soil sediments with low shear‐wave velocities (![Graphic][1] ) underlain by hard bedrock with very large shear‐wave velocities (![Graphic][2] ). Earthquake recordings show unusually large seismic amplification values for weak motion. These unusually large seismic amplification factors were reconfirmed with the earthquake spectral ratio method using two stations, the horizontal‐to‐vertical earthquake spectral ratio method using a single station, and the horizontal‐to‐vertical spectral ratio technique using background noise. These findings were the motivation for carrying out an extensive site response analysis, using finite element modeling (FEM), as a part of the seismic microzonation studies for the city of Ottawa. The FEM results confirmed the large amplification ratios for weak‐motion recordings. FEM analysis was also carried out using a selection of strong‐motion time series for the study area. The combined effect of the soil–bedrock acoustic impedance contrast and the level of ground shaking on the variation of soil amplification factors for the fundamental frequency were investigated. The maximum value of the soil amplification factor for the fundamental frequency increased with increasing impedance contrast ratios until the soil/bedrock acoustic impedance contrast ratio reached values that were usually greater than 12; however, the change in peak amplification was much less with subsequent increases in the contrast ratio beyond that value. As expected, the value of the soil amplification factors for the fundamental frequency decreased with increasing peak ground acceleration (PGA) of the input motion due to nonlinear soil damping. Finally, for the Ottawa region, a mathematical model is suggested for soil amplification factors at the fundamental frequency, as a function of the soil/bedrock acoustic impedance contrast ratio and the PGA of the input motion. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

On 6 April 2009, an earthquake of Mw 6:13 (Herrmann et al., 2011) occurred in central Italy, close to the town of L'Aquila. Although the earthquake is considered to be a moderate-size event, it caused extensive damage to the surround- ing area. The earthquake is identified with significant directivity effects: high- amplitude, short-duration motions are observed at the stations that are oriented along the rupture direction, whereas low-amplitude, long-duration motions are observed at the stations oriented in the direction opposite to the rupture. The complex nature of the earthquake combined with its damage potential brings the need for studies that assess the seismological characteristics of the 2009 L'Aquila mainshock. In this study, we present the strong-ground-motion simulation of this particular earthquake using a stochastic finite-fault model with a dynamic corner frequency ap- proach. For modeling the resulting ground motions, we choose two finite-fault source models that take into account the source complexity of the L'Aquila mainshock. In order to test the sensitivity of ground-motion parameters to the seismic wave attenua- tion parameters, we use two different attenuation models obtained in the study region using weak-motion and strong-motion databases. Comparisons are made between the attenuation of synthetics and ground-motion prediction equations (GMPEs). Synthetic ground motions are further compared with the observed ones in terms of Fourier amplitude and response spectra at 21 strong-ground-motion stations that recorded the mainshock within an epicentral distance of 100 km. The spatial distribution of shaking intensity obtained from the "Did You Feel It?" project and site survey results are compared with the spatial distributions of simulated peak ground-motion intensity parameters. Our results show that despite the limitations of the method in simulating the directivity effects, the stochastic finite-fault model seems an effective and fast tool to simulate the high-frequency portion of ground motions.

This paper presents a new and comprehensive analysis of the magnitude of the 1663 Charlevoix, Quebec, earthquake. Based on a modified Mercalli intensity scale (MMI) of about VI from reports of damage to chimneys and a masonry wall in Roxbury and Boston, Massachusetts, the best estimate of the moment magnitude of this earthquake is M 7.3 to 7.9 from MMI attenuation relations. Using ground-motion attenuation relations and the threshold for chimney and masonry damage from fragility curves for seventeenth- and eighteenth-century dwellings in Boston, the magnitude of the 1663 earthquake is at least M 6.9 if the damage occurred on very soft soil conditions and is M 7.3 to M 7.7 if the damage took place on firm soil or bedrock. Both the best estimate of a length of 73 km for the most active section of the Charlevoix Seismic Zone and an estimated fault area of 73 km×25 km for the 1663 earthquake are consistent with an earthquake of M 7.3 to 7.6 based on scaling relationships. Also, in the 1811–1812 New Madrid earthquake sequence only the 7 February 1812 shock, the largest of the sequence, caused chimney damage beyond about 600 km. The 7 February New Madrid event is thought to have taken place on the Reelfoot fault, for which the length of the recent earthquake activity and of the suspected 1812 rupture is less than that of the active seismicity zone at Charlevoix. These observations suggest that the 1663 Charlevoix earthquake was approximately comparable in magnitude to or even larger than the largest of the 1811–1812 New Madrid earthquakes and therefore was at least M 6.8. When put together, the several lines of analysis in this study indicate that the best estimate of the size of the 1663 earthquake is M 7.5±0.45.

The investigated area, located around the Rio-Antirrio Strait, Central Greece, has been the target of a seismic microzonation campaign. Seventy seismic stations have been deployed for a period of 4 months, recording in continuous mode. Despite the high level of urban noise, we compiled a data set of 95 earthquakes recorded at most of the 70 sites. By employing the attributes of self-organizing maps (SOMs), a quality-control and signal-improving method is proposed. A SOM (Kohonen, 1997) is a type of unsupervised neural network. The main property of SOMs utilized is that while the competitive learning algorithm on whom this method is based maps the input data on an n -dimensional grid of neurons, the topological relations (proximity of patterns in input data) are preserved in the output space. SOM is applied to the horizontal-to-vertical spectral ratios (HVSR) of every weak event analyzed for each station separately and allows a better evaluation of the stability of the HVSR.

In 2006 we published an earthquake hazard model called LASSCI (LAyered Seismogenic Source model in central Italy). In October 2008 we began to update the model for use in 5- and 10-year forecasts. The LASSCI–2006 model is supported by good fault-based definitions of seismogenic sources and simple physically motivated models of earthquake occurrence; the LASSCI–2009 model has been improved by revision of the error propagation assumptions and increased accuracy of the earthquake probabilities. The 6 April 2009 earthquake that struck L’Aquila occurred on the model fault having the highest probability of occurrence in the 2009 revised LASSCI forecast: it is therefore consistent with our model assumptions. Furthermore, peak ground accelerations were in reasonable agreement with the values having 90% probability of not being exceeded in 50 yr. In the revised 2009 model, the aggregate probability of having a maximum-sized event in the next 5 yr on at least one of the neighboring sources (less than 25 km distance away) decreases in L’Aquila from 10% to 7% after the earthquake occurrence, but still remains a maximum there along the central Apennines. The LASSCI models 2006 and 2009, featuring characteristic fault sources and time dependence, seem to be suitable for guidance in reconstruction and seismic retrofit in the central Appenines.

Earthquake simulation on synthetic fault networks carries great potential for characterizing the statistical patterns of earthquake occurrence. I present an earthquake simulator based on elastic dislocation theory. It accounts for the effects of interseismic tectonic loading, static stress steps at the time of earthquakes, and postearthquake stress readjustment through viscoelastic relaxation of the lower crust and mantle. Earthquake rupture initiation and termination are determined with a Coulomb failure stress criterion and the static cascade model. The simulator is applied to interacting multifault systems: one, a synthetic two-fault network, and the other, a fault network representative of the San Francisco Bay region. The faults are discretized both along strike and along dip and can accommodate both strike slip and dip slip. Stress and seismicity functions are evaluated over 30,000 yr trial time periods, resulting in a detailed statistical characterization of the fault systems. Seismicity functions such as the coefficient of variation and a - and b -values exhibit systematic patterns with respect to simple model parameters. This suggests that reliable estimation of the controlling parameters of an earthquake simulator is a prerequisite to the interpretation of its output in terms of seismic hazard.

Most seismic codes adopt the average shear-wave velocity as a key parameter in the first 30 m of subsoil ( V S 30 ). Estimates of V S 30 are therefore required for both large- and small-scale seismic microzonation. We propose a technique to measure the V S 30 based on the horizontal to vertical spectral ratio (H/V) of microtremor recorded at a single station. The H/V is fitted with a synthetic curve using the independently known thickness of a superficial layer of the subsoil as a constraint. The proposed procedure consists of three steps: (1) identify the depth of a shallow stratigraphic horizon from independent geotechnical data, (2) identify its corresponding H/V marker, and (3) use it as a constraint to fit the experimental H/V curve with the theoretical one. The synthetic H/V curve is calculated by assuming a stratified one-dimensional (1D) soil model and a tremor wave field from distant random sources that are composed of Rayleigh and Love waves. This technique has been validated on different geological settings following a triple check procedure: (1) the theoretical Rayleigh-wave phase velocity dispersion curves calculated for the model derived from the H/V fit were compared to the experimental curves measured with the 2D extended spatial autocorrelation (ESAC) method and the 1D refraction microtremor (ReMi) array surveys; (2) the V S profiles and (3) the V S 30 estimates obtained by the proposed technique were compared to those obtained by using the latter array techniques. Finally, the inferred stratigraphy was compared to the geological model. The proposed technique is not meant to provide accurate V S profiles but has the potential to provide V S 30 estimates that are coherent with those measured through ESAC and ReMi and to satisfy law requirements. Furthermore, the H/V is found to be capable in detecting deviations from 1D subsoil geometry over the length of a few meters, the correctness of which was confirmed by direct drilling.

We estimate the ground motions produced by the 1906 San Francisco earthquake making use of the recently developed Song et al. (2008) source model that combines the available geodetic and seismic observations and recently constructed 3D geologic and seismic velocity models. Our estimates of the ground motions for the 1906 earthquake are consistent across five ground-motion modeling groups employing different wave propagation codes and simulation domains. The simulations successfully reproduce the main features of the Boatwright and Bundock (2005) ShakeMap, but tend to over predict the intensity of shaking by 0.1–0.5 modified Mercalli intensity (MMI) units. Velocity waveforms at sites throughout the San Francisco Bay Area exhibit characteristics consistent with rupture directivity, local geologic conditions (e.g., sedimentary basins), and the large size of the event (e.g., durations of strong shaking lasting tens of seconds). We also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern San Andreas fault, considering three additional hypocenters and an additional, random distribution of slip. Rupture directivity exerts the strongest influence on the variations in shaking, although sedimentary basins do consistently contribute to the response in some locations, such as Santa Rosa, Livermore, and San Jose. These scenarios suggest that future large earthquakes on the northern San Andreas fault may subject the current San Francisco Bay urban area to stronger shaking than a repeat of the 1906 earthquake. Ruptures propagating southward towards San Francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward.

We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli inten- sity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave- propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agree- ment with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area. Online Material: Modified Mercalli intensities and velocity waveforms, and a movie of simulated wave propagation.

The pilgrimage city of Lourdes in the French Pyrenees has been heavily damaged by earthquakes in the seventeenth and eighteenth centuries. Assessments of local seismic hazard have been performed through a seismic microzonation of the city. A campaign of urban noise recording has been conducted at about 180 points to determine seismic wave amplification by means of spectral ratios of the horizontal-to-vertical (H/V) components. A comparison of this ratio obtained from ambient noise measurements with spectral ratios obtained from earthquake S -wave recording (H/Href) reveals that the fundamental resonance frequency is well estimated but that the site amplification is generally underestimated. Resonance frequencies and amplitudes exhibit coherent patterns that relate to the geological structure. Bedrock sites are generally characterized by flat spectra whatever the geological nature of bedrock, unless topography or karst are present. Along the river, in particular at the pilgrimage site, thick sediments induce low resonance frequencies with large amplifications. Along and across valleys, north and east of the city, H/V spectra show systematic variations according to sediment filling properties. Finally, on the hill flanks east of the city, H/V spectra rather relate to topography. An additional issue with H/V spectral ratios based on ambient noise is whether this indicator reflects meaningful geological variations. This question is tested on sites where the S -wave ground velocity structure is known. A first test consists of estimating H/V spectral ratios by means of simulation of urban noise through the summation of synthetic seismograms with random properties. A second test consists of estimating H/Href spectral ratios by propagating an earthquake record through a 1D soil profile. These two modelings successfully predict most of the observed spectrum characteristics. In addition, they give some information on the nature of the urban noise, which may include a significant amount of shear energy.

In this article we propose an automatic procedure that performs thezonation of urban areas based on a set of horizontal-to-vertical ( H/V ) spectral ratiosfrom ambient noise recordings collected at many measurement points. The techniquesearches for the connected areas where the similarity among the spectral ratios ismaximized. The problem is posed as one of the optimal partitioning of the Delaunaytriangulation of the available measurement points. The technique explores and triesto partition some random variations of a Euclidean minimum spanning tree of thetriangulation. The optimization is performed using a genetic algorithm.Thetechniqueis applied to the zonation of the town of Vittorio Veneto in northeastern Italy and toa synthetic data set that tries to simulate the effects of some typical geological con-ditions.Introduction Seismic microzonation concerns the identification andmapping at local or site scales of areas having different po-tentials of hazardous earthquake effects, such as ground-shaking intensity, liquefaction, or landslide potential (Akiand Lee, 2003). It is commonly performed by experts takinginto account geophysical and geological information of dif-ferent natures. For ground-shaking intensity, a useful sup-port comes from the computation of horizontal-to-vertical(

This article presents an overview of the La Paca earthquake of magnitude mbLg 4.7, which occurred on 29 January 2005, with its epicenter located near the town of Avile´s in the Murcia region in southeast Spain. Despite its low magnitude, the earthquake caused important damage in two towns of the epicentral area, La Paca and Zarcilla de Ramos. These areas recorded intensities of VI–VII (European Macroseismic Scale, 1998) and sustained estimated economic losses amounting to 10 million €. Aftershocks continued for more than 2 weeks, producing considerable alarm in the population and mobilizing emergency services from the whole region. The La Paca seismic series is the third registered in the region in the past 8 years, being preceded by the Mula (1999) and southwest Bullas (2002) seismic series. These main events had also low magnitudes (mbLg 4.8) and caused damage levels similar to the 2005 earthquake. The case is an example of a moderate seismic zone where low-magnitude and frequent earthquakes have important implications on the seismic hazard and risk of the region. Although these are not the largest expected earthquakes, they have yielded important information for improving the knowledge of the seismic activity of the area. With this aim in mind, different topics have been analyzed from a multidisciplinary perspective, including seismicity, local tectonics and surface geology, focal mechanisms, macroseismic effects, and ground motion. Results indicate a local tectonic interpretation, consistent with a strike-slip focal mechanism, the confirmation of a triggering process between the 2002 and 2005 earthquakes, a geotechnical and ground-motion characterization for the damaged sites (supporting local amplification effects and estimated peak ground acceleration values of �0.1g), and an understanding of damage patterns in relation to local building trends. The results may be used as guidelines for future revisions of the Spanish Building Code (Norma de la Construccio´n Sismorresistente Espan˜ola [NCSE-02], 2002). The study results should contribute to risk mitigation in a region where strong-motion records from the maximum expected earthquakes are not available. This approach can be extended to other regions with similar seismic backgrounds and a lack of strong-motion records.

We present a high-resolution, three-dimensional P-wave seismic veloc- ity model of the sedimentary basin in the Salton Trough, southern California, and use the model for spectral-element method (SEM) wave propagation and ground- motion simulations to quantitatively assess seismic hazard in the region. The basin geometry is defined by a surface representing the top of crystalline basement, which was constrained by seismic refraction profiles and free-air gravity data. Sonic logs from petroleum wells in the Imperial Valley and isovelocity surfaces defined by seismic refraction studies were used to define P-wave velocity within the sedimentary basin as a function of two variables:(1) absolute depth and (2) depth of the underlying crystalline basement surface (CBS). This velocity function was used to populate cells of a three-dimensional spatial array (voxet) defining the P-wave velocity structure in the basin. The new model was then resampled in a computational mesh used for earthquake wave propagation and strong ground motion simulations based upon the SEM (Komatitsch et al., 2004). Simulation of the 3 November 2002 Mw 4.2 Yorba Linda earthquake demonstrates that the new model provides accurate simulation of strong ground motion amplification effects in the Salton Trough sedimentary basin, offering substantial improvements over previous models. A hypothetical Mw 7.9 earthquake on the southern San Andreas fault was then simulated in an effort to better understand the seismic hazard associated with the basin structure. These sim- ulations indicate that great amplification will occur during large earthquakes in the region due to the low seismic velocity of the sediments and the basin shape and depth.

Las Vegas Valley has a rapidly growing population exceeding 1.5 million, subject to significant seismic risk. Surveys of shallow shear velocity performed in the Las Vegas urban area included a 13-km-long transect parallel to Las Vegas Blvd. (The Strip), and borehole and surface-wave measurements of 30 additional sites. The transect was completed quickly and economically using the refraction microtremor method, providing shear velocity versus depth profiles at 49 locations. The lowest velocities in the transect, nehrp d class, are near intrabasin faults found near Interstate 15 and Lake Mead Blvd. Calcite cementation of alluvium (a.k.a. caliche) along the Las Vegas Strip elevates Vs30 values to 500–600 m/sec, nehrp c class. Our transect measurements correlate poorly against geologic map units, which do not predict the conditions of any individual site with accuracy sufficient for engineering application. Some usda soil map units do correlate, and Vs30 predictions based on measurements of soil units match transect measurements in the transect area. Extending soil-map predictions away from the area of dense measurement coverage generally failed to predict new measurements. Further, for several test sites the predictions were not conservative, in that the soil model predicted higher Vs30 than was later measured (predicting lesser potential ground motion). Subsurface information is needed to build a Vs30 model extending predictions throughout Las Vegas Valley. A detailed stratigraphic model built by correlating >1100 deep well logs in Las Vegas predicts Vs30 better than surface maps, but again only in parts of the Valley well-measured for velocity. The stratigraphic model yields good predictions of our transect Vs30 measurements. It is less accurate, although at least conservative, when extended to sites away from the transect.

The epicenter of the 2000 M L 5.2 Yountville earthquake was located 5 km west of the surface trace of the West Napa fault, as defined by Helley and Herd (1977). On the basis of the re-examination of geologic data and the analysis of potential field data, the earthquake occurred on a strand of the West Napa fault, the main basin-bounding fault along the west side of Napa Valley. Linear aeromagnetic anomalies and a prominent gravity gradient extend the length of the fault to the latitude of Calistoga, suggesting that this fault may be capable of larger-magnitude earthquakes. Gravity data indicate an ∼2-km-deep basin centered on the town of Napa, where damage was concentrated during the Yountville earthquake. It most likely played a minor role in enhancing shaking during this event but may lead to enhanced shaking caused by wave trapping during a larger-magnitude earthquake.

The surface rupture of the 1999 Chi-Chi earthquake ( M w 7.6) trendsmore than 100 km in a north–south direction. Surface deformation at the northernend stops abruptly at an area between the Tachia River and the Taan River where abroad pop-up structure with east to northeast strike can be found that has a trenddifferent from the north–south-striking main thrust. We combine the absolute ele-vation data before and after the Chi-Chi earthquake to obtain the regional verticaldisplacement and the magnitude of the pop-up structure. The greatest uplift couldreach as high as 15–16 m. Using deformation magnitude and the area-balancingmethod, we measure the depth of the detachment to show the subsurface geometryof the Chelungpu fault at its northern end. This shows that the geometry of theChelungpu fault controls termination of the surface rupture and the depth of thedetachment controls the amount of deformation.Introduction Perhaps the most unexpected geological feature asso-ciated with the Chi-Chi earthquake (

During a disastrous earthquake, the early assessment and timely reporting of the peak ground acceleration (PGA) and peak ground velocity (PGV) maps will be crucial in an effective emergency response operation. In this study, we first derive an empirical relationship between M L and M W. The PGA and PGV attenuation relationships are deduced with data from the Taiwan Strong Motion Instrumentation Program (TSMIP) and the Taiwan Rapid Earthquake Information Release System (TREIRS). Site corrections of the attenuation relationships for shallow and large earthquakes in Taiwan region are also obtained. Peak values of earthquake strong ground motion can be well determined in Taiwan as soon as the earthquake location is determined, and magnitudes are calculated by the TREIRS. This peak ground motion value information can be immediately turned into the calculated PGA and PGV maps that can be issued within two minutes of the earthquake origin time. During any disastrous earthquake, these maps are found to be very useful for immediate seismic damage assessment and dispatching of emergency response missions. Manuscript received 1 December 2000.

One of the most basic problems in seismology is earthquake location. In particular, the ability to quickly locate a large, potentially devastating earthquake is of fundamental importance in a real-time warning system where speed is a key factor in determining the level of success of such systems. We have developed a simple method that uses only the two earliest P -wave arrival times in a seismic array. Assuming a simple velocity model, these arrivals are used to construct a hyperbolic curve on which the approximate epicenter of the earthquake is expected to lie. Epicentral location along this hyperbola is further constrained by using the fact that the P waves arriving at the other stations in the array are not first arrivals. When applied to P -wave seismic data from the Hector Mine earthquake in California and a smaller event in the central United States, model results show agreement with actual earthquake locations. Although there is an inherent uncertainty in the subarray method of locating large earthquakes, this may be an acceptable trade-off in an early warning system in view of the time (a few to tens of seconds) saved by not waiting for other P arrivals. Whereas the main goal of this report is to present the location method, we also show that the station closest to the Hector Mine earthquake had recorded about 0.3 and 1 mm of ground motion within 2 and 3 sec, respectively, of the arrival of the P waves, thus indicating that a large event had occurred. Manuscript received 18 September 2003.

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high-quality data of three different periods, 1973–1981, 1984–1994, and 1995–2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of ∼2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

The 1975 Kalapana, Hawaii, earthquake occurred under the highly mobile south flank of Kilauea Volcano. It has been interpreted variously as a normal-faulting earthquake, a thrust-faulting earthquake, and a landslide. Primary evidence for the landslide model has been the failure of previous faulting models to explain the observed Love-wave radiation pattern and the tsunami amplitudes generated by the Kalapana event. Here, we present a reanalysis of the long-period, digital seismic data for this event. Centroid-moment-tensor analysis shows that the seismic radiation pattern can be explained well by thrust faulting on a plane dipping shallowly landward. The seismic moment of 3.8 × 10 20 N m ( M W 7.7) that we determine is approximately twice as large as earlier estimates. The geometry and seismic moment of the focal mechanism determined here are consistent with the observed tsunami amplitudes. Inversion of long-period body-wave waveforms shows that the earthquake source duration (∼72 sec) is unusually long for an earthquake of this size.

Located between the very active Japan and Ryukyu subduction zones and the northern China plate, the Korea Peninsula has been considered a part of the stable Eurasia continent and is very quiet in seismic and tectonic activity. Although there were many significant damaging earthquakes reported in historical times, seis- mic hazard in Korea has long been overlooked. Modern earthquake activity in the Korean Peninsula is very low and is not well recorded, at least until 1998 when the modernization of the Korean National Seismic Network was implemented. Thus, modern earthquake data are not adequate for evaluating seismic hazard in the Korean Peninsula. On the other hand, the historical earthquake catalog, which includes doc- umented earthquake information from around the Korean Peninsula and can be dated back to as early as A.D. 2, provides the only available long-term database for the investigation of temporal and spatial patterns of earthquake activity. The importance of seismic hazard assessment has significantly increased in modern times because of the recent construction of many critical facilities, such as nuclear power plants, super- computer centers, large hospitals, and high-technology centers, throughout the entire Korean Peninsula. Although uncertainties on the historical earthquake locations and their magnitudes are expected to be large, information obtained from this historical earthquake catalog can at least provide a long-term scientific basis for an estimation of seismic hazard in Korea. For the entire Korean Peninsula, seismic hazard is eval- uated in terms of the spatial distribution of seismicity and relative seismic energy release over the 2000 years of the historical record. Results from our preliminary analysis clearly demonstrate that seismic activity in the Korean Peninsula can be categorized into four prominent seismic zones, inside which seismic hazard is much higher than that in the surrounding regions. These four seismic zones include: (1) the western Korean seismic zone extending from Seoul to Pyongyang, which is char- acterized by a few concentrated regions of high seismicity and a high relative seismic energy release; (2) the eastern Korean seismic zone, which is characterized by a low seismic rate but a high relative seismic energy release from a few large historical events; (3) the northeastern Korean seismic zone, which is probably related to the deep Japan subduction-zone earthquakes underneath northeast China and has a very low seismicity but a very high relative energy release; and (4) the southern Korean seismic zone, which is characterized by many scattered patches of high seismicity and a few zones of high seismicity and high relative seismic energy release from a few large historical events. Among the three most seismically active regions near Pyongyang, Seoul, and Pusan, the probability of occurrence for an earthquake of magnitude greater than 5.0 is estimated to be about 1%, 2%, and 3% per year, respectively. Since significant damaging earthquakes (M 7.0) have occurred in these three regions in historical times, an effective assessment of seismic hazard potential in the Pyongyang, Seoul, and Pusan regions cannot be overlooked.

A temporary seismic network installed in the eastern part of the aftershock region of the 1999 Izmit, Turkey, earthquake revealed that aftershock epicenters were in a very linear alignment near the eastern end of the earthquake fault. The linear alignment extended over a length of about 15 km along the segment of the earthquake fault in an ENE-WSW direction. Hypocenters on the segment are confined to a narrow depth range near the bottom of the seismogenic region. It is inferred from these results that the afterslips occurred in the seismogenic region on the segment before the 1999 Duzce earthquake that occurred in the eastern extension of the Izmit earthquake fault nearly 3 months after the Izmit earthquake. It is possible that the afterslips concentrated larger stress on the fault of the 1999 Duzce earthquake and generated the earthquake earlier. Manuscript received 30 August 2000.

We examine the difference between expected earthquake rates inferred from the historical earthquake catalog and the geologic data that was used to develop the consensus seismic source characterization for the state of California [California Department of Conservation, Division of Mines and Geology (CDMG) and U.S. Geological Survey (USGS) Petersen et al., 1996; Frankel et al., 1996]. On average the historic earthquake catalog and the seismic source model both indicate about one M 6 or greater earthquake per year in the state of California. However, the overall earthquake rates of earthquakes with magnitudes ( M ) between 6 and 7 in this seismic source model are higher, by at least a factor of 2, than the mean historic earthquake rates for both southern and northern California. The earthquake rate discrepancy results from a seismic source model that includes earthquakes with characteristic (maximum) magnitudes that are primarily between M 6.4 and 7.1. Many of these faults are interpreted to accommodate high strain rates from geologic and geodetic data but have not ruptured in large earthquakes during historic time. Our sensitivity study indicates that the rate differences between magnitudes 6 and 7 can be reduced by adjusting the magnitude-frequency distribution of the source model to reflect more characteristic behavior, by decreasing the moment rate available for seismogenic slip along faults, by increasing the maximum magnitude of the earthquake on a fault, or by decreasing the maximum magnitude of the background seismicity. However, no single parameter can be adjusted, consistent with scientific consensus, to eliminate the earthquake rate discrepancy. Applying a combination of these parametric adjustments yields an alternative earthquake source model that is more compatible with the historic data. The 475-year return period hazard for peak ground and 1-sec spectral acceleration resulting from this alternative source model differs from the hazard resulting from the standard CDMG–USGS model by less than 10% across most of California but is higher (generally about 10% to 30%) within 20 km from some faults.

During the past decade, paleoseismic studies done by many researchers in the coastal regions of the Pacific Northwest have shown that regional downdropping and subsequent tsunami inundation occurred in response to a major earthquake along the Cascadia subduction zone. This earthquake occurred almost certainly in 1700 a.d., and is believed by many to have been of M 8.5–9 or perhaps larger. In order to characterize the severity of ground motions from this earthquake, we report on a field search and analysis of seismically induced liquefaction features. The search was conducted chiefly along the banks of islands in the lowermost Columbia River of Oregon and Washington and in stream banks along smaller rivers throughout southwestern Washington. To a lesser extent, the investigation included rivers in central Oregon. Numerous small- to moderate-sized liquefaction features from the earthquake of 1700 a.d. were found in some regions, but there was a notable lack of liquefaction features in others. The regional distribution of liquefaction features is evaluated as a function of geologic and geotechnical factors in different field settings near the coast. Our use of widely different field settings, each in which we independently assess the strength of shaking and arrive at the same conclusion, enhances the credibility of our interpretations. Our regional inventory of liquefaction features and preliminary geotechnical analysis of liquefaction potential provide substantial evidence for only moderate levels of ground shaking in coastal Washington and Oregon during the subduction earthquake of 1700 a.d. Additionally, it appears that a similar conclusion can be reached for an earlier subduction earthquake that occurred within the past 1100 years, which also has been characterized by others as being M 8 or greater. On the basis of more limited data for older events collected in our regional study, it appears that seismic shaking has been no stronger throughout Holocene time. Our interpreted levels of shaking are considerably lower than current estimates in the technical literature that use theoretical and statistical models to predict ground motions of subduction earthquakes in the Cascadia region. Because of the influence of estimated ground motions from Cascadia subduction-zone earthquakes on seismic hazard evaluations, more paleoliquefaction and geotechnical field studies are needed to definitively bracket the strength of shaking. With further work, it should be possible to extend the record of seismic shaking through much of Holocene time in large portions of Washington and Oregon.

Earthquakes in California's San Francisco Bay Area are likely to be more strongly affected by stress interaction than earthquakes in any other place in the world because of the region's closely spaced, subparallel distribution of faults. I believe, therefore, that meaningful quantification of earthquake probability and hazard in the Bay Area can be made only with the guidance provided by physically based and regionwide earthquake models that account for this interaction. This article represents a first step in developing a standard physical earthquake model for the San Francisco Bay Area through realistic, 3000-year simulations of earthquakes on all of the area's major faults. These simulations demonstrate that a standard physical earthquake model is entirely feasible, they illustrate its application, and they blueprint its construction. A standard physical earthquake model provides the mechanism to integrate fully the diverse disciplines within the earthquake research community. As a platform for data utilization and verification, a physical earthquake model can employ directly any earthquake property that is measurable in the field or in the laboratory to tune and test its seismicity products. As a platform for probability forecasts, a physical earthquake model can supply rational estimates of every imaginable earthquake statistic while simultaneously satisfying all slip and earthquake rate constraints. As a platform for hazard analysis, a physical earthquake model can compute earthquake shaking intensity from first principles by convolving a full suite of rupture scenarios with site-specific dislocation Green's functions. Physical earthquake models have advanced greatly in the last decade. Simulations of earthquake generation and recurrence are now sufficiently credible that such calculations can begin to take substantial roles in scientific studies of earthquake probability and hazard. Manuscript received 9 March 1999.

We have developed a Monte Carlo methodology for the estimation of seismic hazard at a site or across an area. This method uses a multitudinous resampling of an earthquake catalog, perhaps supplemented by parametric models, to construct synthetic earthquake catalogs and then to find earthquake ground motions from which the hazard values are found. Large earthquakes extrapolated from a Gutenberg-Richter recurrence relation and characteristic earthquakes can be included in the analysis. For the ground motion attenuation with distance, the method can use either a set of observed ground motion observations from which estimates are randomly selected, a table of ground motion values as a function of epicentral distance and magnitude, or a parametric ground motion attenuation relation. The method has been tested for sites in New England using an earthquake catalog for the northeastern United States and southeastern Canada, and it yields reasonable ground motions at standard seismic hazard values. This is true both when published ground motion attenuation relations and when a dataset of observed peak acceleration observations are used to compute the ground motion attenuation with distance. The hazard values depend to some extent on the duration of the synthetic catalog and the specific ground motion attenuation used, and the uncertainty in the ground motions increases with decreasing hazard probability. The program gives peak accelerations that are comparable to those of the 1996 U.S. national seismic hazard maps. The method can be adapted to compute seismic hazard for cases where there are temporal or spatial variations in earthquake occurrence rates or source parameters.

Reliable local earthquake locations depend on many factors including the quality of earthquake data, the spatial distribution of network stations, the location algorithms, and the local and regional seismic velocity models. The uncertainties of P- and S-wave arrival times at various seismic stations are, in large part, responsible for the uncertainty in the determination of crustal velocity models and earthquake locations. We took the advantage of the high-quality and high-dynamic range PANDA data from a 33-month seismic array experiment in the central United States to investigate the importance of crustal velocity models, especially the S-wave model, on earthquake locations. Several crustal models are used to relocate a group of selected earthquakes from the PANDA database. It is clear that earthquake depths are determined less reliably than the epicenters when the shear-wave velocity model is not properly determined. The mislocations in some examples are significant that the importance of determining reliable P and S velocity models independently cannot be overlooked. If the crustal P- and S-wave velocity structures in the region can be well determined, reliable earthquake locations are possible even with only a few accurate P and S readings.

The Northridge, California, earthquake of 17 January 1994 (Mw = 6.7) caused widespread damage in the San Fernando valley and adjacent areas. In many places, the earthquake motion was amplified or strongly modified by the local ground response, but the most localized and extreme site effect occurred at Tarzana, a small hill of gentle topography at the northern edge of the Santa Monica mountains where strong amplification (horizontal peak acceleration of 1.82g) and other unusual site effects were recorded at a CDMG accelerograph. The anomalous site effect was confined to a small area 50 m in radius around the station, beyond which the ground shaking was down to its theoretically expected level. If strong ground motion can vary so much in such a small scale, microzonation of urban areas may be an impossible goal at worst or, at best, only reliable at those sites where ground response has previously been measured. Hence, to clearly understand the dynamic response of the ground at the Tarzana site has become an important problem in earthquake hazard assessment and microzonation. This article reports results on the characteristics of the seismic response of the Tarzana site based on the investigation of aftershock data recorded on a local array and recorded accelerograms. Preliminary results strongly suggest that the Tarzana hill is formed by an old landslide of the Modelo formation. Such a structure is consistent with the anomalous resonant behavior at C00, the observed resonant frequencies, and the complex body wave recordings observed throughout the local array. The results also suggest that the 1.82g peak acceleration at C00 could have been caused by a small (Mw = 1.3) earthquake, located some 60 m from the station, generated by a downslope sliding of the hill with a maximum displacement of 4 cm over a circular area of nearly 50 m in radius. The slide was triggered by the Northridge mainshock.

The description of the great earthquake of 19 June 1858 is unusual: damage and high intensities were reported both in the state of Michoacan and in Mexico City. Although a coastal epicenter for this earthquake cannot be ruled out, the reports agree better with an intermediate-depth (about 50 km), normal-faulting event in the subducted Cocos plate. A careful examination of the reports of this event and other normal-faulting events below the Mexican altiplano suggests that a likely location is 18.0 °N, 100.8 °W, near the epicenter of the 6 June 1964 (M7.3, H = 55 km) event. This location is 220 km SW of the city. The magnitude of the earthquake is estimated to be about 7.7. We synthesize expected ground motions in CU, a hill-zone site in the city, from an event similar to that of 1858, using records from the 23 May 1994 earthquake (18.0 °N, 100.6 °W, H = 50 km, M5.7) as an empirical Green's function and stress parameter, Δσ, of 50, 160, and 300 bar. The expected peak horizontal acceleration in CU of Δσ = 160 bar is about 30 gals. Similar acceleration was recorded in CU during the 1985, Michoacan earthquake (M8.0). We compute expected ground motions at many sites in Mexico City using empirical transfer functions and random vibration theory and compare these motions and the expected damage in the city with those from the 1985 Michoacan earthquake. Results show that the overall expected damage during the postulated earthquake is ⅔ and 1⅓ of that during the Michoacan earthquake for Δσ = 160 and 300 bar, respectively. A greater percentage of low-rise construction, which constitute about 80% of the total in the city, will be damaged during the postulated earthquake than during the Michoacan earthquake. The expected ground motions for Δσ = 50 bar are smaller at all periods than those from the Michoacan earthquake. As the present building code for Mexico City contemplates coastal earthquakes of magnitude greater than 8.0, the case of Δσ = 50 bar is not of interest in this article. This preliminary study suggests a need for a more careful evaluation of expected ground motion in the Valley of Mexico from the postulated earthquake and its impact on the current design spectra of the city.

Probabilistic seismic hazard analysis (PSHA) is conducted because there is a perceived earthquake threat: active seismic sources in the region may produce a moderate-to-large earthquake. The analysis considers a multitude of earthquake occurrences and ground motions, and produces an integrated description of seismic hazard representing all events. For design, analysis, retrofit, or other seismic risk decisions a single “design earthquake” is often desired wherein the earthquake threat is characterized by a single magnitude, distance, and perhaps other parameters. This allows additional characteristics of the ground shaking to be modeled, such as duration, nonstationarity of motion, and critical pulses. This study describes a method wherein a design earthquake can be obtained that accurately represents the uniform hazard spectrum from a PSHA. There are two key steps in the derivation. First, the contribution to hazard by magnitude M, distance R, and ɛ must be maintained separately for each attenuation equation used in the analysis. Here, ɛ is the number of standard deviations that the target ground motion is above or below the median predicted motion for that equation. Second, the hazard for two natural frequencies (herein taken to be 10 and 1 Hz) must be examined by seismic source to see if one source dominates the hazard at both frequencies. This allows us to determine whether it is reasonable to represent the hazard with a single design earthquake, and if so to select the most-likely combination of M, R, and ɛ (herein called the “beta earthquake”) to accurately replicate the uniform hazard spectrum. This closes the loop between the original perception of the earthquake threat, the consideration of all possible seismic events that might contribute to that threat, and the representation of the threat with a single (or few) set of parameters for design or analysis.

During the 1990 Manjil, Iran, earthquake (MS = 7.7), an estimated 35,000 people lost their lives and more than 300,000 were left homeless. The earthquake ground shaking caused enormous destruction of unreinforced structures. In addition, widespread liquefaction contributed significantly to building damage in towns as far away as 85 km from the ruptured fault. Following the earthquake, the authors surveyed the liquefaction regions, conducted geotechnical field explorations, and documented case histories on liquefaction of level ground, liquefaction-induced building settlement, permanent ground displacement, and performance of piles and piers in liquefied soils. This article presents and discusses the data of the case histories investigated. In addition, based on the evidence from the Manjil earthquake, the importance of geologic input in mapping of liquefaction potential in earthquake-prone regions is demonstrated. In the companion article, the authors present the results of their analyses of liquefaction-related case histories.

Ground motion data recorded at 26 different locations in Las Vegas from six underground nuclear tests have been used to perform a preliminary microzonation of the city. The microzonation results are presented in the form of contour maps of the average observed seismic response throughout the city in 12 different frequency bands covering the period range from 0.16 to 6.0 sec. These data indicate that variations in long-period seismic response of as much as an order of magnitude can be expected at sites only a few kilometers apart in the city. Simple models of surface-wave propagation in the surface layer of alluvium of variable thickness can account for much of the observed variability. However, it appears that surface waves reflected back from the valley boundaries at the base of the surrounding mountain ranges also have a significant effect on the measured response.

The accuracy of stationary mathematical models of seismicity for calculating probabilities of damaging shaking is examined using the history of earthquakes in China from 1350 A.D. to 1949 A.D. During this time, rates of seismic activity varied periodically by a factor of 10. Probabilities of damaging shaking are calculated in 62 cities in North China using 50 yr of earthquake data to estimate seismicity parameters; the probabilities are compared to statistics of damaging shaking in the same cities for 50 yr following the data window. These comparisons indicate that the seismic hazard analysis is accurate if: (1) the maximum possible earthquake size in each seismogenic zone is determined from the entire seismic history rather than from a short-time window; and (2) the future seismic activity can be estimated accurately. The first condition emphasizes the importance of realistically estimating the maximum possible size of earthquakes on faults. The second indicates the need to understand possible trends in seismic activity where these exist, or to develop an earthquake prediction capability with which to estimate future activity. Without the capability of estimating future seismicity, stationary models provide less accurate but generally conservative indications of seismic ground-shaking hazard. In the United States, the available earthquake history is brief but gives no indication of changing rates of activity. The rate of seismic strain release in the Central and Eastern United States has been constant over the last 180 yr, and the geological record of earthquakes on the southern San Andreas Fault indicates no temporal trend for large shocks over the last 15 centuries. Both observations imply that seismic activity is either stationary or of such a long period that it may be treated as stationary for seismic hazard analyses in the United States.

An extension of the Aki-Larner technique to vertically inhomogeneous media is presented, so that vertical velocity gradients may be taken into account in two-dimensional models. Only SH waves are considered here, but the theory is valid for P and SV waves as well.This method is applied to investigate the seismic response of two-dimensional sedimentary deposits with large velocity gradients. Three different cases are considered: a shallow, high-contrast valley, a deep, high-contrast valley, and a deep, low-contrast valley. In each case, a comparison is performed with the results of a two-dimensional model taking into account only homogeneous sediments and a one-dimensional model taking into account the vertical inhomogeneity of sediments.The presence of a large velocity gradient does not change a lot the qualitative behavior of a two-dimensional deposit: local surface waves and/or two-dimensional resonance patterns are observed as in the case of homogeneous sediments. Nevertheless, the surface waves are much more dispersive with lower phase and group velocities and larger amplitudes. Only very deep valleys give rise to the development of two-dimensional resonance.On the other hand, the amplifications obtained in such deposits may reach much larger values than those predicted with either two-dimensional, homogeneous models or one-dimensional, inhomogeneous models. Since these results have been obtained for realistic values of valley geometrical and mechanical considerations, they should find some application in earthquake engineering or seismic microzonation studies.

A stochastic model is presented for estimating probabilities of exceeding site ground motions due to temporally dependent earthquake events. The model reflects the hypothesized dependence of the size of large earthquake events on the time of occurrence of the last major earthquake. An empirical attenuation relationship is used to describe the ground motion at a site originating from a well-defined fault system. The application of the model to the Middle America Trench is discussed. The seismic hazard potential in Mexico City is computed in terms of probabilities of exceeding peak ground acceleration levels. The results indicate that consideration of the seismic gap is important for estimating the seismic hazard at a site. It is also observed that site hazard estimates are greatly dependent on the specific attenuation relationship used. The need for other approaches of ground motion estimation is recognized.

The duration of ground acceleration, an important engineering seismology parameter, was determined for a subset of nuclear explosion and earthquake accelerograms. The nuclear-explosion, ground-motion data sample consisted of accelerograms derived from velocity recordings of high-yield events, MILROW and CANNIKIN, which have been assigned surface-wave magnitudes of 5.3 and 5.7, respectively. The earthquake-data sample consisted primarily of the Richter magnitude 6.6 San Fernando earthquake accelerograms. The criterion used to define duration of ground acceleration was the amount of time that the absolute acceleration is ≧ 5 per cent g, an approximate index of the strong phase of ground shaking. On the basis of this criterion and the data subset used, the duration of earthquake ground acceleration differs from that of nuclear explosions. The difference, which is not as great as has generally been thought in the past, is small inside 20 km. Earthquake durations are greater beyond 20 km. The scatter in the duration data was analyzed and found to be sensitive to local site amplification phenomena. These preliminary conclusions need to be validated by further research.

The city of Memphis, which is situated very close to the inferred epicenter of one of the three major 1811 to 1812 earthquakes, is in a potentially hazardous zone which will be susceptible to the usual seismic hazards. By recognizing the high level of seismicity in the New Madrid area, this study attempts to microzone the potential hazards by considering the following subjects: (i) the seismicity of the central United States; (ii) design earthquakes; and (iii) response analysis which allows construction of the necessary microzonation maps.The seismicity of the region is evaluated from state-of-the-art literature as there is no recorded strong-motion data available for the central United States. Synthetically generated accelerograms, simulating the design earthquakes, were used to represent the ground motions which were applied at a depth of 45 m, below ground surface, at numerous sites in Memphis. The soil stratigraphy was conceptualized from borehole data, made available by local sources, and dynamic soil properties estimated from available empirical correlations.The results of the response analysis were transformed into microzonation maps depicting: (i) zones showing qualitative estimates of ground response; (ii) zones showing the natural frequency of the soils; (iii) zones showing the peak spectral acceleration for 2 per cent damping ratio; and (iv) zones of liquefaction potential.These maps are useful for preliminary design and are not intended to be used on a quantitative basis. Further investigation is necessary in determining the stratigraphy and soil properties for a site-specific design and analysis.

Accelerograms recorded at six stations in the metropolitan Los Angeles area during the Borrego Mountain, 1968, the Lytle Creek, 1970, and the San Fernando, 1971, earthquakes in southern California have been studied. In comparing the ground motions recorded during different earthquakes at each of the six stations and in correlations of these motions recorded at different stations during the same earthquake, those aspects of the analysis which emerge from this study and are relevant for seismic zoning have been emphasized. It has been found that the patterns of strong ground shaking in this area depend predominantly on the mechanism and the distance of an earthquake source from a recording station and that the local soil conditions played only a minor role in modifying the ground motion at this particular area. It has been shown that gross spectral characteristics of ground motion recorded at various stations can be approximately related by the seismic moment at the low-frequency end and by the stress drop at the high-frequency end.

The Colima earthquake (magnitude 7.5) occurred just inland from the Middle America Trench, 110 km south of the Volcan de Colima and 160 km southeast of Manzanillo, Colima, Mexico. Damage at several cities and towns was severe, 30 people were killed, and hundreds were injured. Four days after the earthquake, a six-station portable seismograph array was set up in the epicentral area as part of a cooperative program between UCSD, the University of Mexico, and the Mexican Federal Power Commission. From about 330 aftershocks recorded in the following 212 weeks, accurate locations were obtained for 50. One large aftershock had a magnitude of 6.2, the others range in local magnitude from 1.5 to 4.5. The locations outline a region approximately 90 km long and 60 km wide, in nearly the same location as the aftershock zone inferred by Kelleher et al. (1973) for the 1941 earthquake. The focal depth of the aftershocks (ranging from 2 to 30 km) and the fault-plane solutions for the main event indicated a shallow dipping thrust plane (about 30°). The seismic moment estimated from mantle rayleigh waves is 3 × 1027 dyne-cm. The pattern of aftershocks was used to estimate the source dimensions. From the moment and source dimensions the average slip was estimated to be about 1.4 m, corresponding to a stress drop of about 8 bars. The occurrence of this earthquake is discussed in terms of the general seismicity of the Middle America Trench, the convergence rate predicted by plate tectonics, and the use of seismic gap theory for earthquake prediction. The fact that this earthquake may have been in the zone of the 1941 earthquake rather than in the adjacent seismic gap, suggests that caution must be taken in using seismic gap theory to predict earthquakes in the region. It further suggests that in the adjacent seismic gap a large earthquake may be eminent, and thus the gap may be an important area for deploying seismic instruments.

The strong earthquake ground motion recorded in the center of and above the fault plane is combined with field evidence of faulting and instrumental studies of aftershocks to deduce stresses during and after the San Fernando earthquake of February 9, 1971. Stress computations based on Brune's near-field, shear-wave spectra, peak velocity of ground motion, energy calculated from the strong-motion record, and a model of circular dislocation give mutually consistent stress estimates, which suggest that the effective stress operating during the earthquake was approximately 100 bars, while during the earthquake it dropped several tens of bars. The energy of the main event is estimated to be 1022 dyne cm. Thirteen aftershocks, recorded during the first 6 min, were associated with stress drops ranging from 10 to 500 bars, these events clustering along the north-eastern end of the dislocation surface. The strong-motion accelerograms provide invaluable data for detailed investigations of the pattern of earthquake energy release during and immediately after an earthquake. Used for the first time in this study, strong-motion accelerograms gave an excellent picture of stress history and migration of seismic activity during the first 6 min.

The problem of earthquake risk in Chile is discussed in terms of seismic history, geotectonic provinces and regional variations in earthquake incidence. Seismic zoning is impractical because all of Chile above Parallel 42° South has been within the epicentral area of some destructive earthquake. The probability concept of earthquake risk is discussed in reference to the seismicity of Santiago. It is found that the earthquake risk at Santiago for an acceleration of 0.1 g is 62.5% in any 10-year period. For the same interval, the risk of a “destructive” earthquake (intensity 8 1/2) is of the order of 20%.

The seismic responses of soil deposits are calculated at four sites where recordings were obtained during the 1957 San Francisco earthquake. The analyses are made utilizing an equivalent linear lumped-mass method of analysis incorporating the soil characteristics at the four sites and estimates of the rock motions at each site based on the record obtained on rock during this earthquake. Comparisons of the maximum amplitudes of ground motions, the time history of ground accelerations, velocity and acceleration spectra and spectral intensities for both computed and recorded ground accelerations indicate a reasonably high degree of agreement. The influence of the amplitude of base rock accelerations on the response of a soil deposit is also discussed and illustrated.