Horizontal Ground Acceleration Research Articles

Liquefaction hazard zonation mapping of the Saitama City, Japan

Liquefaction hazard zonation mapping of the Saitama City targeted on the Kanto Plain NW Edge Fault is described in this paper. The study involves the geotechnical properties of the alluvial soil of the city including Standard Penetration Test (SPT), shear wave velocity and other geological data analysis. The city being highly urbanized is situated on the soft soil (alluvial deposits) at the proximity of an active seismic fault that has increased the possibility of liquefaction hazard in the area. Kanto Plain NW Edge Fault is an active fault that lies very near to the Saitama City having the estimated possible earthquake magnitude of 7.4. The possible peak horizontal ground acceleration (amax) from this earthquake is calculated as from 0.15 g to 0.30 g. By considering all possible acceleration values the liquefaction potential maps were prepared and presented in this paper. Additionally, the shear wave velocity is very low and amplification ratio is very high at the marshy deposit but it has comparatively high velocity and low amplification ratio at the marine loam deposit area of the Omiya Plateau. In this paper the liquefaction potential of the area is expressed in terms of liquefaction potential index (PL). The PL value for the clayey silt deposit in the marshy area with shallow water table is very high. In addition, the PL value in the marine loam deposit of the Omiya Plateau is less which indicates that loam deposit has less liquefaction potential than marshy deposit. The map obtained from this study was validated with the field condition of the study area. Hence, it is expected that this study will assist in characterizing the seismic hazards and its mitigation and will provide valuable information for urban planning in the study area in future.

Strong motion attenuation relationship for Turkey—a different perspective

For Anatolian earthquakes, there are insufficient strong motion data from rock sites to model an attenuation relationship for Turkey. This necessitates the use of records from soil sites, which are significantly affected by amplification. In order to include soil site data in the attenuation analyses, boreholes were drilled at 64 recording stations on soil sites. After removing the effects of soil amplification, rock site and soil site data were combined to establish an attenuation relationship. Various models were tested through regression analyses using moment magnitude, epicentral distance and threshold peak horizontal ground acceleration. A new attenuation relationship is modeled for Turkey.

Vector-Valued Probabilistic Seismic Hazard Assessment for the Effects of Vertical Ground Motions on the Seismic Response of Highway Bridges

The vertical ground motion component is disregarded in the design of ordinary highway bridges in California, except for the bridges located in high seismic zones (sites with design horizontal peak ground acceleration greater than 0.6 g). The influence of vertical ground motion on the seismic response of single-bent, two-span highway bridges designed according to Caltrans Seismic Design Code (SDC-2006) is evaluated. A probabilistic seismic hazard framework is used to address the probability of exceeding the elastic capacity for various structural parameters when the vertical component is included. Negative mid-span moment demand is found to be the structural parameter that is most sensitive to vertical accelerations.A series of hazard curves for negative mid-span moment are developed for a suite of sites in Northern California. The annual probability of exceeding the elastic capacity of the negative mid-span moment is as large as 0.01 for the sites close to active faults when the vertical component is included. Simplified approaches based on the distance to major faults or the median design peak acceleration show that there is a large chance (0.4 to 0.65) of exceeding the elastic limit if the current 0.6 g threshold is used for the consideration of vertical ground motions for ordinary highway bridges. The results of this study provide the technical basis for consideration of a revision of the 0.6 g threshold.

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

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

Evaluation of the dynamic response of the Damlapinar CFR dam

Concrete faced rock-fill (CFR) dams are now very popular. The structural complexity of these dams and the high risk associated with the social and environmental consequences of failures require very reliable analyses of their performance, especially as regards seismic loading. In this paper, a dynamic analysis procedure for CFR dams is proposed and described, using the Damlapinar CFR dam as a case study. The acceleration time history and maximum horizontal peak ground acceleration values were developed based on maximum credible earthquakes over 50 years from nearby seismic sources. Numerical studies showed that the Damlapinar CFR dam will remain stable under the maximum credible earthquake with a peak ground acceleration of 0.15 g. According to the numerical results, earthquake-induced deformations of the dam are expected to be within the safety margins determined by the Ministry of Public Works and Settlement, Turkey. The results also indicate that a non-linear analysis capable of capturing dominant non-linear mechanisms can be used to assess the stability of CFR dams.

Large scale landslides triggered by Quaternary tectonics in the Acambay graben, Mexico

AbstractDetailed analysis was conducted on large‐scale gravitational‐tectonic deformations and landslides in the Acambay graben, an intra‐arc basin in the trans‐Mexican volcanic belt (TMVB). Field mapping and remote sensing revealed the slope instability of the northern graben boundary induced by the Acambay‐Tixmadejé fault. Two major landslides of 0·1 km3 and 0·05 km3 in volume were identified and their characteristics were analyzed according to the role of tectonics, mechanism of slope failure, and possible triggering factors. Quaternary faulting played a major role in increasing the local relief, and the activity of the Acambay‐Tixmadejé fault represents the main geomorphic factor conditioning the gravitational movements. Moreover, displacements along this fault generated sliding surfaces and reduced the strength of the rock mass. The two landslides are classified as large‐scale rotational slides involving volcanic rocks of late Miocene‐Pleistocene age. Since the Acambay graben is a seismogenic area with a known maximum horizontal ground acceleration of 0·5 g, a strong earthquake could be ascribed as the possible triggering mechanism of the landslides. Our work represents the first analysis of large gravitational slope movements in tectonically active regions in Mexico, a process that can be common in the intra‐arc basins of the TMVB, where active tectonic, seismicity, weak altered volcanic rocks, and heavy rains affect the slope stability. Copyright © 2010 John Wiley & Sons, Ltd.

Dynamic behavior of reinforced slopes: horizontal acceleration response

ABSTRACT: Shaking-table tests on plane-strain geosynthetic-reinforced model slopes using stepwise intensified sinusoidal pulse loads with specific frequencies were conducted to investigate the influence of peak ground accelerations, wave frequencies and plastic slope displacements on the horizontal acceleration response of the slope. A comparative study on the crest response behavior suggested that the tests using pulse loads give useful insights into the dynamic behavior of soil structures subjected to earthquake loads. Test results indicate that both the horizontal acceleration response at the crest and the lateral slope displacement are influenced by the horizontal peak ground acceleration, the wave frequency, and the cumulative permanent displacement of the slope. Amplification factors at the crest of the slope are functions of input wave frequencies; they decrease exponentially with increasing input horizontal peak ground acceleration. Transitions from an amplification state to a de-amplification state at the slope crest occur at certain levels of input ground accelerations, which are associated with the development of major slip planes in the slope mass (or with noticeable slope displacement). It was found that a ratio between the plastic slope displacement and the total slope height (Dmax/Ht) of the order of 0.42–5.9 × 10−2 is relevant to this amplification–deamplification transition. A practical implication of these observations is that the resonance acceleration of the slope is a major factor that needs to be considered in seismic stability analyses of reinforced slopes with a small plastic slope displacement of Dmax/Ht ≈ 2.5 × 10−3; it does not need to be considered in the seismic stability evaluation for reinforced slopes with plastic slope displacements of Dmax/Ht = 5.0–5.9 × 10−2.

복합소재패널을 이용한 반강접 중저층 강골조의 내진성능평가

본 연구의 목적은 복합소재패널 즉 FRP 내력패널을 이용한 반강접 강골조의 내진성능평가이다. 강골조의 최적내진보강을 제안하기 위하여 중약지진 규모의 지진자료를 이용하여 모멘트골조에 복합소재패널의 보강위치를 바뀌어가며 해석을 수행하였다. 복합소재패널이 보강된 강골조의 지진해석은 시간이력해석을 이용하여 수행하였으며, 활용된 지반운동자료는 미국 NEHRP 프로젝트의 모멘트저항 골조에 활용한 자료 중, 50년 동안 지진이 일어날 가능성 10%, 50%의 확률을 가진 지진 20개씩을 활용하였다. 접합부는 현장에 많이 쓰이고 있는 반강접 접합부를 고려하였으며, 복합소재패널 연결부는 비선형 연결요소는 GAP을 사용하였다. 복합소재패널을 이용한 강골조가 패널을 보강하지 않은 무보강에 비해 보강패턴에 따라 보강효과를 검증해 보았으며, 보강 후 구조물응답에 대하여 고찰하였다. This paper presented regarding an parametric study to investigate seismic capacity evaluation of semi-rigid steel frame infilled with composit panel. In order to propose the optimum retrofit of the steel frame, we analysed the various pattern of retrofitted steel frame subjected to weak/medium earthquake. Steel frame with composit panel was analysed by Time history analyses analysis. The model were analysed using the suites of ground motion developed by NEHRP project on steel moment resisting frame. These earthquakes consist of 20 horizontal ground acceleration record each, i.e., a 10%, 50% probability of accidence in a 50 year period. We considered the semi-rigid connection which are commonly used in field, and modeled the nonlinear connection element (GAP) between panel and frame. It was shown that how is the steel frame with composit panel effected. We also examined the response of retrofitted frame.

Probability of Peak Ground Horizontal and Peak Ground Vertical Accelerations at Tehran and Surrounding Areas

The active faults near Tehran are capable of producing Mw magnitudes of 6.62–7.23; at epicentral distances of 25 km from the active faults, and Mw = 7.23, the peak ground horizontal acceleration, PGH, is between 386 and 730 cm/s2 and peak ground vertical acceleration, PGV, is between 192 and 261 cm/s2; the historic earthquake of the fourth century BC, Mw = 7.16 produced the highest estimated PGH acceleration in Tehran, between 254 and 479 cm/s2 and PGV acceleration between 127 and 173 cm/s2. Earthquakes from 1909 to 2008, within 300 km from Tehran, are used for calculation of magnitude frequency relation, and results are applied to estimate PGH and PGV accelerations as a function of magnitudes for a set of fixed epicentral distance and site conditions; also as a function of epicentral distances for a set of fixed magnitudes and site conditions. Poisson’s distribution is used for probability calculation of PGH and PGV accelerations for several exposure times, site conditions and epicentral distances; accelerations with very high probability, near 1, are presented. At an epicentral distance of 10 km and exposure time of 450 years, in the northern part of Tehran, close to Mosha and the Northern Tehran faults, PGH acceleration is 800–420 cm/s2 and PGV is 400–220 cm/s2 with high probability. At an epicentral distance of 25 km and 1,000 years exposure time, PGH is 610–320 cm/s2 and PGV is 310–160 cm/s2 with high probability, where larger values are for soft soil and smaller values are for hard rock.

A Comparative Study of the Least Squares Method and the Genetic Algorithm in Deducing Peak Ground Acceleration Attenuation Relationships

In engineering applications, the development of attenuation relationships in a seismic hazard analysis is a useful way to plan for earthquake hazard mitigation. However, finding an optimal solution is difficult using traditional mathematical methods because of the nonlinearity of many relationships. Furthermore, using unweighted regression analysis in which each recording carries an equal weight is often problematic because of the non-uniform distribution of the data with respect to distance. In this study, the least squares method (LSM) and a genetic algorithm (GA) were employed as optimization methods for an attenuation model to compare the robustness and prediction accuracy of the two methods. Different (equal and unequal) weights of each recording were used to compare the adaptability of the weighting for practical application. The unequal weights of each recording were defined as functions of the hypocentral distance or the shortest distance from a station to the fault on the earth’s surface. Finally, regression analysis of horizontal peak ground acceleration (PGA) attenuation model in southwest Taiwan was shown.

Preliminary re-evaluation of probabilistic seismic hazard assessment in Chile: from Arica to Taitao Peninsula

Abstract. Chile is one of the most seismically active countries in the world; indeed, having witnessed very large earthquakes associated with high horizontal peak ground accelerations, the use of probabilistic hazard assessment is an important tool in any decision-making. In the present study, we review all the available information to improve the estimation of the probabilistic seismic hazard caused by two main sources: shallow interplate, thrust earthquakes and intermediate depth, intraplate earthquakes. Using previously defined seismic zones, we compute Gutenberg-Richter laws and, along with appropriate attenuation laws, revaluate the probabilistic seismic hazard assessments in Chile. We obtain expected horizontal peak ground acceleration with a 10% of probability of being exceeded in 50 years, reaching from 0.6 g up 1.0 g in the coast and between 0.4 g and 0.6 g towards the Andes Mountains, with larger values in Northern part of the country. The present study improves our knowledge of geological hazards in Chile, enabling the mitigation of important human and material losses due to large earthquakes in the future.

The April 2009 Abruzzi, Italy, Earthquake: Impact on Site Seismic Hazard Estimation

At 0132 UTC (0332 local time) on 6 April 2009, a major earthquake (ML = 5.8, Mw = 6.3) occurred a few kilometers southwest of the city of L'Aquila in the Abruzzi region of central Italy (http://earthquake.usgs.gov/eqcenter/eqinthenews/2009/us2009fcaf/). The quake was preceded by a long and often intense seismic sequence, and it was followed by a series of aftershocks. The extensive damage from the major earthquake resulted in about 300 fatalities, 1200 injuries, and 40,000 evacuees, suggesting widespread ground motioneffects and Mercalli‐Cancani‐Sieberg (MCS) scale intensities of up to level IX (http://portale.ingv.it/ primo‐piano‐1/news‐archive/2009‐news/april‐6‐earthquake/copythe‐l‐aquila‐seismic‐sequence‐april‐2009/view?set_language=en). Such intensity levels correspond to an impact on buildings, in terms of maximum ground horizontal acceleration, as great as 0.6 g, where g is the acceleration of gravity. An acceleration of about 0.6 g actually was measured by accelerometric stations in the L'Aquila valley (http://mceer.buffalo.edu/infoservice/disasters/ground‐motion‐summary.pdf).

Methodology to evaluate rock slope stability under seismic conditions at Solà de Santa Coloma, Andorra

Abstract. An analytical methodology is presented to evaluate rock slope stability under seismic conditions by considering the geomechanical and topographic properties of a slope. The objective is to locate potential rockfall source areas and evaluate their susceptibility in terms of probability of failure. For this purpose, the slope face of a study area is discretized into cells having homogenous aspect, slope angle, rock properties and joint set orientations. A pseudostatic limit equilibrium analysis is performed for each cell, whereby the destabilizing effect of an earthquake is represented by a horizontal force. The value of this force is calculated by linear interpolation between the peak horizontal ground acceleration PGA at the base and the top of the slope. The ground acceleration at the top of the slope is increased by 50% to account for topographic amplification. The uncertainty associated with the joint dip is taken into account using the Monte Carlo method. The proposed methodology was applied to a study site with moderate seismicity in Solà de Santa Coloma, located in the Principality of Andorra. The results of the analysis are consistent with the spatial distribution of historical rockfalls that have occurred since 1997. Moreover, the results indicate that for the studied area, 1) the most important factor controlling the rockfall susceptibility of the slope is water pressure in joints and 2) earthquake shaking with PGA of ≤0.16 g will cause a significant increase in rockfall activity only if water levels in joints are greater than 50% of the joint height.

Geoengineering and Seismological Aspects of the Niigata-Ken Chuetsu-Oki Earthquake of 16 July 2007

The M6.6 Niigata-Ken Chuetsu-Oki earthquake of 16 July 2007 occurred off the west coast of Japan with a focal depth of 10 km, immediately west of Kashiwazaki City and Kariwa Village in southern Niigata Prefecture. Peak horizontal ground accelerations of 0.68 g were measured in Kashiwazaki City, as well as at the reactor floor level of the world’s largest nuclear reactor, located on the coast at Kariwa Village. Liquefaction of historic and modern river deposits, aeolian dune sand, and manmade fill was widespread in the coastal region nearest the epicenter and caused ground deformations that damaged bridges, embankments, roadways, buildings, ports, railways and utilities. Landslides along the coast of southern Niigata Prefecture and in mountainous regions inland of Kashiwazaki were also widespread affecting transportation infrastructure. Liquefaction and a landslide also damaged the nuclear power plant sites. This paper, along with a companion digital map database available at http://walrus.wr.usgs.gov/infobank/n/nii07jp/html/n-ii-07-jp.sites.kmz, describes the seismological and geo-engineering aspects of the event.

Disaggregation of Probabilistic Ground-Motion Hazard in Italy

Abstract Probabilistic seismic hazard analysis is a process that integrates over aleatory uncertainties (e.g., future earthquake locations and magnitudes) to calculate the mean annual rate of exceedance (MRE) of given ground-motion parameter values at a site. These rates reflect the contributions of all the sources whose seismic activity is deemed to affect the hazard at that site. Seismic hazard disaggregation provides insights into the earthquake scenarios driving the hazard at a given ground-motion level. This work presents the disaggregation at each grid point of the Italian rock ground-motion hazard maps developed by Gruppo di Lavoro MPS (2004), Meletti and Montaldo (2007), and Montaldo and Meletti (2007). Disaggregation is used here to compute the contributions to the MRE of peak ground horizontal acceleration (PGA) and 5%-damped 0.2, 1.0, and 2.0 sec spectral acceleration values corresponding to different mean return periods (MRPs of 475 and 2475 yr) from different scenarios. These scenarios are characterized by bins of magnitude, M , source-to-site distance, R , and number, e , of standard deviations that the ground-motion parameter is away from its median value for that M - R pair as estimated by a prediction equation. Maps showing the geographical distribution of the mean and modal values of M , R , and e are presented for the first time for all of Italy. Complete joint M – R – e distributions are also presented for selected cities. Except for sites where the earthquake activity is characterized by sporadic low-magnitude events, the hazard is generally dominated by local seismicity. Moreover, as expected, the MRE of long-period spectral accelerations is generally controlled by large magnitude earthquakes at long distances while smaller events at shorter distances dominate the PGA and short-period spectral acceleration hazard. Finally, for a given site, as the MRP increases the dominant earthquakes tend to become larger and to occur closer to the site investigated.

Probabilistic Seismic Hazard Model for Vanuatu

Abstract This study develops a preliminary probabilistic seismic hazard model for Vanuatu. The area of investigation, formerly referred to as the New Hebrides, lies in the center of a chain of partly vulcanologically active islands that mark the present-day boundary between the Australia-India plate and the microplate of the North Fiji basin. The seismicity of the Vanuatu arc is dominated by an east-dipping subduction zone, which shows striking structural anomalies in the central part between 14° and 18° S. Our historical catalog contains 7519 events within the Vanuatu region for the period from 1964 to 2003, drawn from the global teleseismic catalogs by the United States Geological Survey/National Earthquake Information Center (USGS/NEIC, see Data and Resources section) and Engdahl et al. (1998). As a measure of seismic hazard, we use horizontal peak ground acceleration (PGA) and horizontal spectral ground acceleration (SGA) at a period of 1 sec. The hazard estimates are based on a logic-tree approach to account for the epistemic uncertainties associated with our analysis. Our results suggest that the entire island arc experiences a high and uniform seismic hazard. Typical values for PGAs range from 0.65 g to 0.77 g with a 10% probability of exceedence in 50 yr. For Port Vila, the capital and largest city in Vanuatu, we additionally present a PGA hazard curve and a uniform hazard spectrum over the period range 0.1–2 sec.

Reassessment of Probabilistic Seismic Hazard in the Marmara Region

Abstract In 1999, the eastern coastline of the Marmara region (Turkey) witnessed increased seismic activity on the North Anatolian fault (NAF) system with two damaging earthquakes ( M 7.4 Kocaeli and M 7.2 Duzce) that occurred almost three months apart. These events have reduced stress on the western segment of the NAF where it continues under the Marmara Sea. The undersea fault segments have been recently explored using bathymetric and reflection surveys. These recent findings helped scientists to understand the seismotectonic environment of the Marmara basin, which has remained a perplexing tectonic domain. On the basis of collected new data, seismic hazard of the Marmara region is reassessed using a probabilistic approach. Two different earthquake source models: (1) the smoothed-gridded seismicity model and (2) fault model and alternate magnitude-frequency relations, Gutenberg–Richter and characteristic, were used with local and imported ground-motion-prediction equations. Regional exposure is computed and quantified on a set of hazard maps that provide peak horizontal ground acceleration (PGA) and spectral acceleration at 0.2 and 1.0 sec on uniform firm-rock site condition (760 m/sec average shear wave velocity in the upper 30 m). These acceleration levels were computed for ground motions having 2% and 10% probabilities of exceedance in 50 yr, corresponding to return periods of about 2475 and 475 yr, respectively. The maximum PGA computed (at rock site) is 1.5 g along the fault segments of the NAF zone extending into the Marmara Sea. The new maps generally show 10% to 15% increase for PGA, 0.2 and 1.0 sec spectral acceleration values across much of Marmara compared to previous regional hazard maps. Hazard curves and smooth design spectra for three site conditions: rock, soil, and soft-soil are provided for the Istanbul metropolitan area as possible tools in future risk estimates.

Probabilistic seismic hazard assessment of Sanandaj, Iran

In this paper, the peak horizontal ground acceleration over the bedrock (PGA) is calculated by a probabilistic seismic hazard assessment (PSHA). For this reason, at first, all the occurred earthquakes in a radius of 200 km of Sanandaj city have been gathered. After elimination of the aftershocks and foreshocks, the main earthquakes were taken into consideration to calculate the seismic parameters (SP) by Kijko (2000) method. The seismotectonic model of the considered region and the seismic sources of the region have been modeled. In this research, Sanandaj and its vicinity has been meshed as an 8 (vertical lines) * 10 (horizontal lines) and the PGA is calculated for each point of the mesh using the logic tree method and the five attenuation relationships (AR) with different weighted coefficient. These calculations have been performed by the Poisson distribution of four hazard levels. Then by using it, four regional maps of the seismic hazard regions have been provided for Sanandaj and its vicinity. The results show that the maximum and minimum value of PGA for the return periods of 75, 225, 475, 2475 years are (0.114, 0.074) (0.157, 0.101), (0.189, 0.121) and (0.266, 0.170), respectively.

Horizontal and vertical ground motion prediction equations derived from the Italian Accelerometric Archive (ITACA)

A set of Ground Motion Prediction Equations (GMPEs) for the Italian territory is proposed, exploiting a new strong-motion data set become available since July 2007 through the Italian Accelerometric Archive (ITACA). The data set is composed by 561 three-component waveforms from 107 earthquakes with moment magnitude in the range 4.0–6.9, occurred in Italy from 1972 to 2007 and recorded by 206 stations at distances up to 100 km. The functional form used to derive GMPEs in Italy (Sabetta and Pugliese in Bull Seismol Soc Am 86(2):337–352, 1996) has been modified introducing a quadratic term for magnitude and a magnitude-dependent geometrical spreading. The coefficients for the prediction of horizontal and vertical peak ground acceleration, peak ground velocity and 5% damped acceleration response spectra are evaluated. This paper illustrates the new data set, the regression analysis and the comparisons with recently derived GMPEs in Europe and in the Next Generation Attenuation of Ground Motions (NGA) Project.

Source Parameters for the 28 April 2007 Mw 4.0 Earthquake in Folkestone, United Kingdom

A moderate size earthquake ( M L 4.3, M w 4.0) occurred in southeastern England on 28 April 2007. The earthquake was of some significance as it caused damage in the town of Folkestone and produced the largest peak horizontal ground acceleration (PGA, 0.1 g ) measured in the United Kingdom to date. It was followed by 12 aftershocks between M L 0.8 and 1.7. The earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the United Kingdom. The hypocenter of the event was at a depth of about 5 km beneath Folkestone, with an error ellipse indicating horizontal errors in a location of about 5 km. The depth was well constrained using a number of techniques, of which local travel-time inversion and teleseismic depth phase modelling are most reliable. A stress drop of 28.6 bars and a source radius of 0.5 km were determined from the analysis of displacement source spectra. We derived a near-surface attenuation factor κ =0.02 from the aftershock data that were used in the spectral analysis of the mainshock. Applying the horizontal to vertical (H/V) spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0.4 and 3.9 Hz. This amplification is likely to have contributed to the mainshock PGA of 0.1 g measured at the same site. Similar site conditions may have been responsible for the damage in parts of Folkestone. The moment tensor computed from regional broadband data showed a strike-slip mechanism with a normal component and either right-lateral movement on a west-southwest–east-northeast-striking or left-lateral movement on a north-northwest–south-southeast-striking nodal plane. The north-northwest–south-southeast-striking nodal plane matches the trend of the main faults affecting the Kent coalfield and also possibly the Variscan front. It is thus possible that the causative fault was associated with the Variscan front, a major structural boundary at the northern limit of late Carboniferous folding and thrusting.