Earthquake-induced Ground Shaking Research Articles

The Importance of Assessing the Geological Site Effects of Ancient Earthquakes from the Archaeoseismological Point of View

Earthquakes have and continue to, occur worldwide, though some places are affected more than others by earthquake-induced ground shaking and the same earthquake can cause more damage in one area than in nearby locations due to site-specific geological site conditions, also known as local site effects. Depending on the chronology of the earthquakes, various disciplines of seismology include instrumental and historical seismology, archaeoseismology, palaeoseismology and neotectonics, each focusing on using specific sources of information to evaluate recent or ancient earthquakes. Past earthquakes are investigated to expand the pre-instrumental and instrumental earthquake catalog and better evaluate a region’s seismic hazard. Archaeoseismology offers a way to achieve these goals because it links how ancient civilizations and their environment might have interacted and responded to past earthquake-induced ground motion and soil amplification. Hence, archaeoseismology explores pre-instrumental (past) earthquakes that might have affected sites of human occupation and their nearby settings, which have left their co-seismic marks in ancient manufactured constructions exhumed by archaeological excavations. However, archaeoseismological observations are often made on a limited epicentral area, poorly constrained dated earthquakes and occasionally on unclear evidence of earthquake damage. Archaeological excavations or field investigations often underestimate the critical role that an archaeological site’s ancient geological site conditions might have played in causing co-seismic structural damage to ancient anthropogenic structures. Nevertheless, the archaeological community might document and inaccurately diagnose structural damage by ancient earthquake shaking to structures and even estimate the size of past earthquakes giving little or no consideration to the role of geological site effects in addressing the causative earthquake. This mixture of factors frequently leads to imprecise estimates of the size of ancient earthquakes and unlikely earthquake environmental impacts, leaving unexplained the location and the moment magnitude of the causative earthquake. Hence, it is essential not to rely solely on earthquake intensities based on archaeologically documented co-seismic damage without assessing the nature of the observed structural damage and the contribution of the geological site effects. This paper explains the geological site effects concept to archaeologists unfamiliar with the notion. It clarifies its role in assessing ground shaking, soil amplification and earthquake intensity by past earthquakes and how and why the geological site effects can be estimated when a site is thought to have been struck by an earthquake. Hence, the geological site effects must be considered when archaeological excavations describe and interpret destruction layers. Conversely, engineers and seismologists dealing with seismic hazard risk assessment must pay close attention to archaeological investigations assessing earthquake intensities and locations based on field evidence of damage to structures attributed to past earthquakes, because the geological site effects might have been factored in inaccurately or not at all.

A mixed-mode data collection approach for building inventory development: Application to school buildings in Central Sulawesi, Indonesia

Urban disaster risk management and reduction requires the development and periodic updating of regional building inventories. However, the development of such inventories can be very cost-intensive and time-consuming, making this a challenging task, particularly for low- and middle-income countries. This article discusses a mixed-mode building inventory data collection framework using a rapid and cost-effective remote survey technique that can be deployed in various geographic contexts. A key component of the proposed approach is an inter-rater reliability analysis of data collected from traditional sidewalk surveys and remote surveys for a small subset of buildings in the considered building portfolio, which is used to assess the suitability of the remote survey for the location(s) considered. The framework is demonstrated by developing a regional database of school buildings in the Central Sulawesi region of Indonesia. The database consists of 2536 school buildings from 454 elementary and high schools in the Palu, Sigi, and Donggala regions, susceptible to earthquake-induced ground shaking, tsunami, liquefaction, and landslides. The developed database can be used in pre-event/long-term risk analysis and management, post-event/near-real-time loss estimation, and regional-level decision-making on school assets and related policies. The database has been made available for public use and can be readily harmonized with similar databases for other regions.

Triggering processes of deep-seated gravitational slope deformation (DSGSD) in an un-glaciated area of the Cavargna Valley (Central Southern Alps) during the Middle Holocene

Triggering mechanisms and causative processes of deep-seated gravitational slope deformations (DSGSD) in Alpine and high mountain areas include, among others, post-glacial de-buttressing, earthquake-induced ground shaking, or co-seismic surface faulting. Distinguishing between climatic or tectonically driven factors is challenging since faults and fracture systems can play both an active and/or passive role in the process initiation. We applied an integrated approach, including morpho-structural analysis, geologic field survey, a paleoseismological approach applied to trenching, radiocarbon dating and detailed sedimentological analysis, to a DSGSD located in the Cavargna Valley (N Italy), an area that was never occupied by extensive ice tongues in the Upper Pleistocene and Holocene. We were able to document at least two phases of movement from the Middle Holocene onwards and related triggering factors. Thanks to the comparison with dated lacustrine turbidites in the Alpine Lakes, we argue a seismic triggering for the landslide onset, with a possible source located in southern Switzerland, close to the Insubric Line. The later evolution of the landslide was instead driven by climatic predisposing conditions (humid and cold), instead, corresponding to a phase of enhanced slope instability.

The controls on earthquake ground motion in foreland-basin settings: the effects of basin and source geometry

SUMMARYRapid urban growth has led to large population densities in foreland basin regions, and therefore a rapid increase in the number of people exposed to hazard from earthquakes in the adjacent mountain ranges. It is well known that earthquake-induced ground shaking is amplified in sedimentary basins. However, questions remain regarding the main controls on this effect. It is, therefore, crucial to identify the main controls on earthquake shaking in foreland basins as a step towards mitigating the earthquake risk posed to these regions. We model seismic-wave propagation from range-front thrust-faulting earthquakes in a foreland-basin setting. The basin geometry (depth and width) and source characteristics (fault dip and source-to-basin distance) were varied, and the resultant ground motion was calculated. We find that the source depth determines the amount of near-source ground shaking and the basin structure controls the propagation of this energy into the foreland basin. Of particular importance is the relative length scales of the basin depth and dominant seismic wavelength (controlled by the source characteristics), as this controls the amount of dispersion of surface-wave energy, and so the amplitude and duration of ground motion. The maximum ground motions occur when the basin depth matches the dominant wavelength set by the source. Basins that are shallow compared with the dominant wavelength result in low-amplitude and long-duration dispersed waveforms. However, the basin structure has a smaller effect on the ground shaking than the source depth and geometry, highlighting the need for understanding the depth distribution and dip angles of earthquakes when assessing earthquake hazard in foreland-basin settings.

Fragility assessment of non-ductile RC frame buildings exposed to combined ground shaking and soil liquefaction considering SSI

The recent strong seismic events have demonstrated that most damage to coastal buildings is often associated with liquefaction effects. The occurrence of earthquake-induced ground shaking and liquefaction may be dramatic for buildings and poses a considerable threat to people. However, the influence of the simultaneous action of ground shaking and soil liquefaction considering soil-structure interaction (SSI) effects on the structural behaviour of buildings has not been adequately studied yet. In this respect, we propose a methodology for the fragility assessment of non-ductile reinforced concrete (RC) moment-resisting frame buildings exposed to combined ground shaking and soil liquefaction, based on a fully coupled numerical approach considering SSI. A fully coupled (u-P) formulation is employed, which can capture in cohesionless soils the permanent shear-strain accumulation during dilation and soil liquefaction. Two-dimensional incremental dynamic analysis of the nonlinear SSI systems is conducted for the estimation of the building response. The proposed methodology yields either conventional fragility curves, or coupled fragility surfaces, which are a function of more than one intensity measures and provide useful and important information for an integrated fragility assessment. It is shown that the height of the building resting on liquefiable soil may significantly affect its vulnerability to combined ground shaking and soil liquefaction, with the non-ductile low-rise RC frame buildings suffering more structural damages compared to the mid- and high-rise ones. Also, the high-rise buildings, even susceptible to liquefaction, do not sustain severe structural damages but only sink or tilt. The proposed fragility models can be adapted to estimate the risk of typical non-ductile RC frame buildings exposed to ground shaking and soil liquefaction.

Estimation of the moment magnitude and local site effects of a postulated Late Bronze Age earthquake: Mycenaean citadels of Tiryns and Midea, Greece

Previously interpreted archaeological and geological field data from the Argive Basin, Greece, have been used to hypothesized that the nearby Late Bronze Age Mycenaean citadels of Tiryns and Midea, which settled on bedrock, might record synchronized co-seismic structural damage due to earthquake ground-shaking at ca. 1190 BCE, the end of the Mycenaean palatial period. However, from a quantitative archaeoseismological perspective, this hypothesis has flaws: (a) it overlooks that several structures excavated in the undeformed Holocene sediments lack evidence of co- seismic structural damage and damage is only documented for sturdier structures within the citadels, (b) it leaves unexplained the moment magnitude of the ‘causative’ surface-rupturing earthquake, and (c) disregards the role of local site effects on surface ground-motion. The plausibility of the previously proposed earthquake hypothesis is tested by estimating the earthquake magnitude of the alleged ancient earthquake and its local site effects at and around the citadels, assuming that the earthquake truly occurred. A retrospective geotechnical site microzonation coupled with the calculation of seismic amplification factors, surface ground-motions, and earthquake intensity account for the local site effects assessment. The present numerical modeling results indicate that the citadels and contemporaneous adjacent structures of the peasants had a lower and higher seismic hazard, respectively. Seismic amplification factors, earthquake-induced ground shaking, and seismic intensity are indeed lower for Tiryns than for Midea. Therefore, these results refute the idea of seismically induced destruction of the citadels and challenge the plausibility of the earthquake hypothesis. The previously archaeologically documented destruction patterns unlikely represent physical evidence of co-seismic damage by the archaeologically proposed earthquake. Other explanations ought to be sought to elucidate the interpreted destruction pattern seen in these Mycenaean centers.

Landslides triggered by the MW7.8 14 November 2016 Kaikōura earthquake: an update

In this study, we use version 2.0 of the digital inventory of landslides triggered by the MW 7.8 Kaikōura, New Zealand, earthquake of 14 November 2016 to revaluate some of the controls on their spatial distribution. Version 2.0 contains 29,557 hand-digitised landslides, whilst version 1.0 included only 10,195 mainly larger (> 10,000 m2) landslides. The main results from the re-analysis using version 2.0 of the inventory are the following: (1) the landslide frequency and density results indicate that the power laws fitted to version 1.0 and version 2.0 landslide inventories are similar; (2) the landslide densities decrease with increasing distance from the nearest fault that ruptured to the surface during the earthquake; (3) the larger landslides in the inventory occur on slopes with higher local slope relief; and (4) the results from logistic regression modelling using the version 2.0 inventory show that (a) the geological materials have a larger relative contribution to the fit of the overall model, when compared with the previous model based on the version 1.0 inventory, and (b) the “distance to fault” predictor variable continues to have more statistical power in predicting landslide probability than the modelled peak ground acceleration or peak ground velocity. These results suggest that future research on the Kaikōura earthquake landslide distribution—and the landslide distributions associated with other similar large earthquakes—might be directed towards investigating the interaction between surface fault rupture, fault damage-zone properties, earthquake-induced ground shaking, and the initiation of slope failures.

The impact of topography on seismic amplification during the 2005 Kashmir earthquake

Abstract. Ground surface topography influences the spatial distribution of earthquake-induced ground shaking. This study shows the influence of topography on seismic amplification during the 2005 Kashmir earthquake. Earth surface topography scatters and reflects seismic waves, which causes spatial variation in seismic response. We performed a 3-D simulation of the 2005 Kashmir earthquake in Muzaffarabad with the spectral finite-element method. The moment tensor solution of the 2005 Kashmir earthquake was used as the seismic source. Our results showed amplification of seismic response on ridges and de-amplification in valleys. It was found that slopes facing away from the source received an amplified seismic response, and that 98 % of the highly damaged areas were located in the topographically amplified seismic response zone.

The liquefaction potential of the upper quaternary sands of the Cape Flats, Western Cape, South Africa

Liquefaction of saturated cohesionless soils as a result of earthquake-induced ground shaking is a major concern for structures constructed within or on these soils. During an earthquake, shear waves propagate through the ground, generating shear stresses and strains that are cyclic in nature, causing the soil grains to rearrange into a denser state of packing. This process is too rapid for drainage to occur, which results in an increase in pore water pressure and a decrease in effective stress and shear resistance of the soil. The occurrence of a seismic event associated with the Milnerton Fault line extending below the Cape Flats, a low-lying sand-covered area connecting the Cape Peninsula with the southwestern Cape mainland of South Africa, can cause extensive damage to the largely inadequate infrastructure characterising the area. To evaluate the susceptibility of Cape Flats sand to undergo liquefaction during and after a seismic event, the empirical SPT-based method put forth by Idriss & Boulanger (2004) was used to assess the cyclic liquefaction potential of dense, dilative sands, whereas the CPT-based method published by Robertson (2016) was used to assess the volumetric response of the sands during undrained shear. Resistance of the Cape Flats sands to liquefy at probable earthquake moment magnitude and acceleration values of M = 6.0 and amax= 0.15 g respectively, was noted. Robertson's normalised Soil Behaviour Type (SBTn) chart revealed mostly dilative sands and silty sands susceptible to cyclic liquefaction. Loose, contractive sands prone to flow liquefaction are present, although limited in occurrence and extent.

An inerter‐equipped vibrating barrier for noninvasive motion control of seismically excited structures

The Vibrating Barrier (ViBa) is a large-scale oscillating mass-spring-damper unit contained in the ground and tuned to mitigate the motion of surrounding structures under earthquakeinduced ground shaking, without being directly in contact to them, through a structure-soilstructure interaction mechanism. Previous research showed that ViBa achieves significant seismic structural response reductions but, in doing so, it requires excessive vibrating mass, equal to the mass of the structure that aims to control or more, which hinders its practical applicability. This paper considers coupling ViBa with a grounded inerter acting as a mass amplifier to reduce ViBa mass/weight in suppressing seismically induced structural response. Attention is focused on structures amenable to modelling as single-degree-of freedom (SDOF) damped oscillators by establishing equations of motion of a 5-DOF dynamical system of a grounded inerter-equipped ViBa (IViBa) fused with a SDOF structure and accounting for soil structure interaction (SSI) effects due to soil compliance. Optimal closed-form H∞ and numerical H2 IViBa tuning are addressed minimizing the response of SDOF structure subject to harmonic resonant and to broadband/white base excitation, respectively. Numerical results pertaining to a small-scale physical ViBa prototype specimen are furnished quantifying the trade-off between IViBa mass and inertance considering non-rigid/compliant inerter-to-ground connectivity. Monotonic IViBa mass/weight reduction trend is found for fixed targeted structural performance under white stationary as well as colored non-stationary seismic excitation for increasing inertance and for sufficiently rigid inerter-to-ground connection. It is concluded that careful engineering design of the inerter-to-ground connection minimizing compliance is most critical in fully exploiting the beneficial effects of grounded inerter for mass/weight IViBa reduction facilitating, thus, its practical implementation.

Extreme Accelerations During Earthquakes Caused by Elastic Flapping Effect

Accurate recording of large, earthquake-induced ground shaking is critical for our understanding of earthquake physics as well as seismic hazard assessment. Extremely large accelerations with the peak value of 3.2 times the gravity acceleration were recorded at seismic station WTMC located in northern South Island of New Zealand during the recent magnitude 7.8 Kaikoura earthquake. However, the mechanisms responsible for the generation of such large accelerations are not well understood. Here we use numerical simulations to examine a range of physical models that can reproduce the observed characteristics of the acceleration record. We find that the record of the asymmetric, vertical accelerations, also observed during a magnitude 6.3 earthquake, can be explained by a flapping effect, that is, the local, elastic bouncing of a foundation slab on which the sensor is installed. Our results suggest that the extremely large accelerations recorded at seismic station WTMC do not reflect the actual ground shaking, but were caused by a local, system response around the sensor. Our finding has important implications for both the evaluation of future seismic hazard based on the waveform records of the Kaikoura earthquake and the installation methodology of strong-motion seismometers in all earthquake prone countries.

Field observations of seismic velocity changes caused by shaking-induced damage and healing due to mesoscopic nonlinearity

To investigate temporal seismic velocity changes due to earthquake related processes and environmental forcing in Northern Chile, we analyse 8 yr of ambient seismic noise recorded by the Integrated Plate Boundary Observatory Chile (IPOC). By autocorrelating the ambient seismic noise field measured on the vertical components, approximations of the Green's functions are retrieved and velocity changes are measured with Coda Wave Interferometry. At station PATCX, we observe seasonal changes in seismic velocity caused by thermal stress as well as transient velocity reductions in the frequency range of 4–6 Hz. Sudden velocity drops occur at the time of mostly earthquake-induced ground shaking and recover over a variable period of time. We present an empirical model that describes the seismic velocity variations based on continuous observations of the local ground acceleration. The model assumes that not only the shaking of large earthquakes causes velocity drops, but any small vibrations continuously induce minor velocity variations that are immediately compensated by healing in the steady state. We show that the shaking effect is accumulated over time and best described by the integrated envelope of the ground acceleration over the discretization interval of the velocity measurements, which is one day. In our model, the amplitude of the velocity reduction as well as the recovery time are proportional to the size of the excitation. This model with two free scaling parameters fits the data of the shaking induced velocity variation in remarkable detail. Additionally, a linear trend is observed that might be related to a recovery process from one or more earthquakes before our measurement period. A clear relationship between ground shaking and induced velocity reductions is not visible at other stations. We attribute the outstanding sensitivity of PATCX to ground shaking and thermal stress to the special geological setting of the station, where the subsurface material consists of relatively loose conglomerate with high pore volume leading to a stronger nonlinearity compared to the other IPOC stations.

The effects of normal and shear stress wave phasing on coseismic landslide displacement

Predictive models used to assess the magnitude of coseismic landslide strain accumulation in response to earthquake ground shaking typically consider slope-parallel ground accelerations only and ignore both the influence of coseismic slope-normal ground accelerations and the phase relationship between dynamic slope-normal and slope-parallel accelerations. We present results of a laboratory study designed to assess the significance of the phase offset between slope-normal and slope-parallel cyclic stresses on the generation of coseismic landslide displacements. Using a dynamic back-pressured shearbox that is capable of simulating variably phased slope-normal and slope-parallel dynamic loads, we subjected sediment samples to a range of dynamic loading scenarios indicative of earthquake-induced ground shaking. We detail the variations in strain accumulation observed when slope-normal and slope-parallel stresses occur independently and simultaneously, both in and out of phase, using a range of dynamic stress amplitudes. Our results show that the instantaneous phasing of dynamic stresses is critical in determining the amount of coseismic landslide displacement, which may vary by up to an order of magnitude based solely on wave-phasing effects. Instantaneous strain rate is an exponential function of the distance normal to the Mohr Coulomb failure envelope in plots of shear stress against normal effective stress. This distance is strongly controlled by the phase offset between dynamic normal and shear stresses. Our results demonstrate that conditions considered by conventional coseismic slope stability models can either overestimate or underestimate earthquake-induced landslide displacement by up to an order of magnitude. This has important implications for accurate assessment of coseismic landslide hazard.

Carbonate platform development in a Paleoproterozoic extensional basin, Vempalle Formation, Cuddapah Basin, India

Sedimentological investigation of the late Paleoproterozoic (Orosirian) Vempalle Formation of the Cuddapah Basin, Dharwar craton, India, reveals three facies association that range from supratidal to deep subtidal. Sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone, intraformational carbonate conglomerate, and a variety of columnar, domal, and stratiform microbialite facies. Deposition occurred in an extensional regime during development of a low-gradient ramp, where the distribution of microbialite facies is distinctly depth-partitioned. A gradual increase in synoptic relief of columnar stromatolites through the section, and the upward transition from stratiform to columnar microbialites, record a prolonged marine transgression with little or no influx of terrigenous detritus. Siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity. Redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory. Earthquake-induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep-seated mantle-plume that resulted in thermal doming of the Dharwar crust prior to the onset of Cuddapah deposition. Isotopic and elemental chemistry of a selection of Vempalle Formation carbonate rocks record elevated Mn2+ and Fe2+ concentrations and depleted carbon isotope values in inner ramp lagoonal facies, relative to more open marine stromatolitic facies. Patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses—either within the water column or within substrate pore fluids—that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments. These data suggest that, even in the aftermath of Early Paleoproterozoic oxygenation, that ocean chemistry was heterogeneous and strongly affected by local basin conditions.

Monofrequency in situ damping measurements in Ottawa area soft soils

In Ottawa, Canada, unusually high amplification ratios have recently been measured in clayey silts (called ‘Leda Clays’) at low levels of earthquake-induced ground shaking. However, the contribution of seismic Q, or material damping ( ξ=1/2 Q), to the overall ground motion at soft soil sites across the city is not well understood. This research investigates attenuation measurements in soft soils ( V s <250 m/s) for ongoing seismic hazard evaluation in the Ottawa area. The work focuses on in situ measurements of damping in two deep boreholes drilled into Leda Clay. To investigate the possibility of frequency-dependent dynamic properties of these materials at low strains, a new approach to the spectral ratio technique has been developed for the measurement of Q s in the field using a mono-frequency vibratory source (generating signals between 10 and 100 Hz), and two identical downhole 3-component geophones. Monofrequency signals also allowed for the measurement of dispersion (variation of velocity with frequency). Analysis of the data show that dynamic properties are, for the most part, independent of frequency in the homogenous silty soils, yielding negligible variation in shear wave velocity (<2 m/s) across the frequency test band, and small strain Q s 's ranging from 170 to 200 (damping of 0.25–0.30%) over soil thickness intervals ranging from 10 to 60 m. At intervals within 20 m of the ground surface, laminated silt and clay beds of elevated porosity are found to have slight influence on the frequency dependence of damping for frequencies greater than 70 Hz (damping increase to 0.6%).

Regolith modeling and its relation to earthquake induced building damage: A remote sensing approach

Regolith thickness is known as a major factor in influencing the intensity of earthquake induced ground shaking and consequently building damages. It is, however, often simplified or ignored due to its variable and complex nature. To evaluate the role of regolith thickness on earthquake induced building damage, a remote sensing based methodology is developed to model the spatial variation of regolith thickness, based on DEM derived topographic attributes and geology. Regolith thickness samples were evenly collected in geological formations at representative sites of topographic attributes. Topographic attributes (elevation, slope, TWI, distance from stream) computed from the ASTER derived DEM and a geology map were used to explore their role in spatial variation of regolith thickness. Stepwise regression was used to model the spatial variation of regolith thickness in erosional landscape of the study area. Topographic attributes and geology, explain 60% of regolith thickness variation in the study area. To test, if the modeled regolith can be used for prediction of seismic induced building damages, it is compared with the 2005 Kashmir earthquake induced building damages derived from high resolution remote sensing images and field data. The comparison shows that the structural damages increase with increasing regolith thickness. The predicted regolith thickness can be used for demarcating site prone to amplified seismic response.

Geological hazards in the Azores archipelago: Volcanic terrain instability and human vulnerability

The islands of the Azores archipelago are geologically young and located in a tectonically and volcanically active region. Not surprisingly, the islands are subject to many geological hazards, including earthquakes, landslides, and coastal erosion; some selected examples are discussed in this paper. As demonstrated by two recent earthquakes (1980, Terceira; 1998, Faial), the principal damage was related to one or more of these factors: (1) unsafe location of structures near faults; (2) unstable foundation soils; (3) poor quality of building materials and construction methods; (4) disregard of building codes; and (5) lack of building maintenance. Major landsliding events in the Azores (e.g., Ponta da Fajã, Ribeira Quente, and Fajã dos Cubres) typically are triggered by intense, long-duration precipitation and (or) earthquake-induced ground shaking. The loose, unconsolidated nature of the rocks and soils of these volcanic islands is another significant contributing factor, sometimes aggravated by ground instability caused by human activity. Coastal erosion is prevalent on the north coast of São Miguel and the south coast of Faial, mostly resulting from natural circumstances (e.g., steepness of cliffs, differential erosion, intense wave action during storms) and also from human activity (e.g., poorly engineered drainage works on cliff faces). Where severe, coastal erosion can pose a risk to populations and societal infrastructures situated near the tops of the seacliffs. To mitigate the risk of these and other geological hazards in the Azores, it is necessary to (1) prepare hazards and risks maps of the affected areas; (2) adopt prudent land-use planning that considers the hazards; (3) upgrade the building codes in the hazardous areas; (4) initiate slope-stabilization programs; (5) preserve the natural environmental integrity of the regions; and (6) educate the affected populace and governmental officials about the possibilities and consequences of hazardous processes.

Streamflow and water well responses to earthquakes.

Earthquake-induced crustal deformation and ground shaking can alter stream flow and water levels in wells through consolidation of surficial deposits, fracturing of solid rocks, aquifer deformation, and the clearing of fracture-filling material. Although local conditions affect the type and amplitude of response, a compilation of reported observations of hydrological response to earthquakes indicates that the maximum distance to which changes in stream flow and water levels in wells have been reported is related to earthquake magnitude. Detectable streamflow changes occur in areas within tens to hundreds of kilometers of the epicenter, whereas changes in groundwater levels in wells can occur hundreds to thousands of kilometers from earthquake epicenters.

Edge-Diffracted 1-Sec Surface Waves Observed in a Small-Size Intramountain Basin (Colfiorito, Central Italy)

During the Umbria-Marche, central Italy seismic sequence a small-aperture (≈200 m), four-station array was operating in the Colfiorito plain, a few kilometers away from the epicenters of the ML 5.6 and 5.8 mainshocks of 26 September 1997. The array was deployed approximately 500 m from the eastern edge of the basin. We analyze the three-component seismograms of 12 aftershocks, in a magnitude range of 2.5 to 4.1. Amplitudes of the horizontal components are systematically higher than those of the vertical component, with an average horizontal-to-vertical spectral ratio of about 3 at 1 Hz. In this frequency band, earthquake-induced ground shaking is highly coherent across the array. A 1-sec running-window zero-lag cross-correlation algorithm is used to compute apparent velocity and backazimuth of coherent wave trains in the frequency band 0.5 to 2 Hz. Apparent velocity and backazimuth show a different behavior in the first part of the seismograms compared to the late coda. The largest amplitude waves, that is, S waves and early coda, are characterized by low apparent velocities, mostly between 400 and 1200 m/sec. This suggests that, near the rock edge, the most significant part of seismic energy propagates horizontally in the basin. Backazimuth of these low-frequency, coherent wavetrains never coincides with the array-to-source direction. The predominant backazimuth is peaked around N110°, corresponding to the nearest, steep outcrop of the basin edge. The observed 1-sec coherent wave trains are interpreted as locally generated surface waves that are persistently diffracted from the nearby basin edge as long as a significant level of seismic radiation is incident to the bedrock. When the bedrock excitation decreases a much larger variability of both apparent velocity and backazimuth is observed, suggesting that, in the coda, randomly scattered waves within the basin and late arrivals of deeper origin become more important. Multipathing from the source to the site as well as multipathing within the basin are therefore interpreted as the main causes of the observed long-duration, coherent low-frequency basin shaking.

Liquefaction Potential for Sites in Manhattan and Buffalo, New York

Soil liquefaction, the transformation of a saturated, cohesionless soil into a liquefied state, occurs primarily as a result of earthquake-induced ground shaking. Liquefaction potential maps have been prepared for two urban areas with major concentrations of population: 1) the borough of Manhattan, and 2) the city of Buffalo, both in New York State. These maps show the probability of liquefaction for ground shaking induced by an earthquake having a peak ground acceleration of 0.15 g . Approximately 7,000 borehole logs from the two selected sites were collected and analyzed to determine the liquefaction potential of soils lying within 50 ft of the surface. Standard penetration test (SPT) values and water table elevation information from drilling logs were used as the primary data, and two different computational methods were used to estimate the liquefaction potential. The maps indicate the existence of several low lying areas in Manhattan which have a significant potential for liquefaction, while the potential for Buffalo is generally low. These differences correlate well with the geology of the two regions.