Seismic Source Zones Research Articles

Probabilistic Seismic Hazard Analysis in Thailand and Adjacent Areas by Using Regional Seismic Source Zones

We conducted probabilistic seismic hazard analysis for Thailand and adjacent areas using a method proposed by Cornell (1968). We produced seismic hazard maps showing peak ground acceleration (PGA). Twenty-one seismic source zones covering all of Thailand and extend into adjacent areas were employed. The seismicity data used in this study was a merged data set covering 1963-2007 from several international earthquake catalogues and a single Thai catalogue. We selected the strong ground-motion attenuation model for this study by applying several existing attenuation models to recorded strong groundmotion data and choosing the model that best fit our data. Seismic hazard analysis was carried out for 2521 grid points on a 0.25° × 0.25° mesh within a rectangle defined by longitudes 92-106°E and latitudes 0°-21°N. The resulting PGA maps for a 2% probability of exceedance for a 50-year time period suggest that ground motion of 0.3 to 0.4 g may occur in northern and western Thailand and from 0 to 0.2 g in other parts of Thailand. The seismic hazard analysis presented here is an important step toward an accurate evaluation of a seismic hazard potential in Thailand and adjacent areas. Further work is needed to refine the analysis. More observations of strong ground motion in the region are needed and further seismo-tectonic research should be encouraged.

Localization of Large Earthquakes in the Charlevoix Seismic Zone, Quebec, Canada, during the Past 10,000 Years

Earthquake-induced liquefaction features have been found in Holocene and Late Wisconsin sediments exposed along three rivers in the Charlevoix seismic zone. On the basis of their stratigraphic position and radiocarbon age constraints, the liquefaction features are thought to have formed during three or more earthquake episodes centered in Charlevoix during the past 10,000 years, including at least two prehistoric episodes approximately 5,000 and 10,000 years ago. The spatial distribution of liquefaction features coupled with liquefaction potential analysis suggests that the Charlevoix earthquakes were of moment magnitude ( M ) ≥ 6.2. Liquefaction features have not been found in similar sediments exposed along eight rivers in the Quebec City–Trois Rivieres area, 70 to 150 km from Charlevoix in the St. Lawrence River Valley. The apparent absence of liquefaction features in the Quebec City–Trois Rivieres area suggests that few, if any, large earthquakes have occurred here during the same time period. The geologic record of earthquakes may be incomplete in both areas due to fluctuations in Holocene sea level. Nevertheless, the rate of large earthquakes has apparently been much higher in the Charlevoix seismic zone than in adjacent areas of the St. Lawrence for thousands of years. These findings suggest that seismicity is localized in Charlevoix and that the presence of Iapetan rift faults that underlie the St. Lawrence Valley of southeastern Canada may not, in itself, indicate earthquake potential. These results may have important implications for other Iapetan rift faults in the eastern United States, as well as seismic source zone characterization and hazard assessment throughout eastern North America.

Seismicity of Kecskemét area

In the small seismic source zone of Kecskemet 203 earthquakes are known between 1739 and 2006, and about 90 percent of them have a magnitude value not more than 3.0, however the strongest event on July 8, 1911 has 5.6 surface-wave magnitude. Concerning the latter earthquake the maximum (epicentral) intensity I = VIII (EMS) was observed in the area enclosed by Kecskemet, Katonatelep and Hetenyegyhaza locations. The quake caused significant damage to buildings (I ≥ VI EMS) on about 6 thousands square kilometres and was felt (I ≥ III EMS) on some 85 thousands square kilometres. The focal depth is estimated as 11 km directly from the individual intensity data points. During the earthquake liquefaction (sand crater) occurred in the epicentral area and some electromagnetic effects were also observed. Studying the source dimensions we conclude the rupture area is between 40 and 67 square kilometres and the maximum displacement along the fault is estimated to 14–20 centimetres for the Kecskemet earthquake of July 8, 1911. A probabilistic seismic hazard assessment predicts 1.1–1.5 m/s2 peak ground accelerations, and 6.6–7.1 maximum (theoretical) earthquake intensity values with 10% chance of exceedance for an exposure time of 100 years in the studied area.

Probabilistic seismic hazard maps for Sinai Peninsula, Egypt

Sinai experienced the largest Egyptian earthquake with moment magnitude (Mw) 7.2 in 1995 in the Gulf of Aqaba, 350 km from Cairo. It is characterized by the presence of many tourist projects in addition to different natural resources. The aim of the current study is to present, for the first time, the probabilistic spectral hazard maps for Sinai. Revised earthquake catalogues for Sinai and its surroundings, from 112 BC to 2006 AD with magnitude equal or greater than 3.0, are used to calculate seismic hazard in the region of interest between 27°N and 31.5°N and 32°E and 36°E. We declustered these catalogues to include only independent events. The catalogues were tested for the completeness of different magnitude ranges. 28 seismic source zones are used to define the seismicity. The recurrence rates and the maximum earthquakes across these zones were also determined from these modified catalogues. Strong ground motion relations for rock are used to produce 5% damped spectral acceleration values for four different periods (0.2, 0.5, 1.0 and 2.0 s) to define the uniform response spectra at each site (grid of 0.2° × 0.2° all over the area). Maps showing spectral acceleration values at 0.2, 0.5, 1.0 and 2.0 s periods as well as peak ground acceleration (PGA) for the return period of 475 years (equivalent to 90% probability on non-exceedence in 50 years) are presented. In addition, Uniform Hazard Spectra (UHS) at 25 different periods for the four main cities (Hurghda, Sharm El-Sheikh, Nuweibaa and Suez) are graphed. The highest hazard is found in the Gulf of Aqaba with maximum spectral accelerations 356 cm s−2 at a period of 0.22 s for a return period of 475 years.

Seismicity in the North-West Frontier Province at the Indian-Eurasian Plate Convergence

In the last decade a number of large earthquakes have devastated the Asian continental region, including the 1998 MW 6.6 Afghanistan-Tajikistan, 1999 MW 7.6 Chi-Chi, 2001 MW 7.6 Gujarat, 2002 MW 6.1 Hindukush, 2004 MW 9.0 Sumatra-Andaman, 2005 MW 7.6 Kashmir, 2006 MW 6.3 Yogyakarta, and 2008 MW 7.9 Sichuan. Of these, the 1998 Afghanistan-Tajikistan, the 2002 Hindukush, and the 2005 Kashmir nucleated in the northwest region of Indian and Eurasian plate convergence. The earliest accounts of devastating earthquakes in this seismically vulnerable terrain can be traced back to the 8th century (Ambraseys and Bilham 2003). Recently, researchers have used chronological records of events, listed in earthquake catalogs, for statistical assessment of the seismicity in terms of frequency magnitude distributions, spatiotemporal behaviors, and maximum earthquake prognosis ( e.g. , Faenza and Pierdominici 2007; Thingbaijam et al. 2008; Thingbaijam and Nath 2008). The spatial seismicity patterns can be employed for classification of possible areal seismic source zones in the underlying seismotectonic framework of the region. Seismicity of a region encompasses the ongoing earthquake processes and long-term historical seismic activity as well as expectations for the occurrence of future large/great earthquakes. In the present work, we first compile a homogeneous earthquake catalog by converting different magnitude scales to a single scale; in this case, MW . Then we use this catalog to quantify seismicity patterns and to delineate broad seismic source zones and the potential maximum earthquakes in each zone. The quantification of the observed seismicity is achieved through the spatial distribution of b -value and correlation fractal dimension of the epicenters, DC . The tectonic framework of the study region extends from the Herat fault system on the west to northwest Himalaya on the east while touching the offshore boundary …

Fundamental Deformations in Asymmetric Continuum

Abstract The complexity of processes in a seismic source zone has inspired us to reconsider a class of basic motions and deformations in an asymmetric continuum, including simple motions (translation and rotation, named spin) and simple deformations. Simple deformations include the axial nuclei, that is, point extension/compression, and the shear nuclei related to string-string point deformations. The point deformation is further redefined as another kind of rotational motion, called twist, representing oscillations of the main shear axes and their amplitudes. Some remarks are added on the recording systems used to measure these motions and deformations.

Seismic source zone characterization for the seismic hazard assessment project PEGASOS by the Expert Group 2 (EG1b)

A comprehensive study of the seismic hazard related to the four NNP sites in NW Switzerland was performed within the project PEGASOS. To account for the epistemic uncertainties involved in the process of the characterization of seismic source zones in the frame of probabilistic seismic hazard assessments, four different expert teams have developed and defended their models in the frame of an intensive elicitation process. Here, the results of one out of four expert groups are presented. The model of this team is based first of all on considerations regarding the large scale tectonics in the context of the Alpine collision, and neotectonic constraints for defining seismic source zones. This leads to a large scale subdivision based on the structural ‘architectural’ considerations with little input from the present seismicity. Each of the eight large zones was characterized by the style of present-day faulting, fault orientation, and hypocentral depth distribution. A further subdivision of the larger zones is performed based on information provided by the seismicity patterns. 58 small source zones have been defined in this way, each of them characterized by the available tectonic constrains, as well as the pros and cons of different existing geologic views connected to them. Of special concern in this respect were the discussion regarding thin skinned vs. thick skinned tectonics, the tectonic origin of the 1356 Basel earthquake, the role of the Permo-Carboniferous graben structures, and finally the seismogenic orientation of faults with respect to the recent crustal stress field. The uncertainties connected to the delimitations of the small source zones have been handled in form of their regrouping, formalized by the logic tree technique. The maximum magnitudes were estimated as discretized probability distribution functions. After declustering the used ECOS earthquake catalogue and an analysis of data completeness as a function of time the parameters of the frequency-magnitude relations were derived within their uncertainties.

Preparing a seismic hazard model for Switzerland: the view from PEGASOS Expert Group 3 (EG1c)

The seismic hazard model used in the PEGASOS project for assessing earth-quake hazard at four NPP sites was a composite of four sub-models, each produced by a team of three experts. In this paper, one of these models is described in detail by the authors. A criticism sometimes levelled at probabilistic seismic hazard studies is that the process by which seismic source zones are arrived at is obscure, subjective and inconsistent. Here, we attempt to recount the stages by which the model evolved, and the decisions made along the way. In particular, a macro-to-micro approach was used, in which three main stages can be described. The first was the characterisation of the overall kinematic model, the “big picture” of regional seismogenesis. Secondly, this was refined to a more detailed seismotectonic model. Lastly, this was used as the basis of individual sources, for which parameters can be assessed. Some basic questions had also to be answered about aspects of the approach to modelling to be used: for instance, is spatial smoothing an appropriate tool to apply? Should individual fault sources be modelled in an intraplate environment? Also, the extent to which alternative modelling decisions should be expressed in a logic tree structure has to be considered.

Comments on “An investigation of seismicity for western Anatolia” by Sayıl and Osmanşahin (2008)

Numerous local or regional seismic hazard studies have been made to estimate the seismic hazard in and around Turkey using the statistical processing of historical and instrumental earthquake data. One of the most popular methods is the Gutenberg–Richter (G–R) law. Also, the knowledge of return period is of great importance in studying and analyzing earthquake hazard and/or seismicity. Such kind of reliable studies strongly depend on (i) selecting a studied area suitable with seismotectonics, (ii) preparing a homogenous catalogue including a large number of earthquakes, and (iii) making a comprehensive completeness analysis and selecting a suitable cut off magnitude, which should be done for each region separately if one studies several seismic zones whose seismotectonics characteristic are different, (iv) selecting a method suitable with data. The accurate estimation of a and b parameters of G–R is of primary importance because the evaluations of the seismicity depend on them. One of the most recent studies on statistical earthquake occurrence can be found in the paper of Sayil and Osmansahin (2008). In their study, Sayil and Osmansahin (2008) used G–R magnitude–frequency relation to investigate the seismicity of western Anatolia. They computed the recurrence periods for the 13 sub-regions of the limited area between 36 –40 N and 26 –32 E. Seismic source zones or sub-regions of an area are widely used to determine the earthquake hazard and earthquake risk of any area in the world. Seismic source zones are determined with two fundamental tools: a seismicity profile and the tectonic structure of the region under consideration (Erdik et al. 1999). Since Turkey is located in a very active seismic area, several studies have investigated the seismic hazard and seismic risk considering different seismic zones in and around Turkey (Alptekin 1978; Yaltirak et al. 1998; Erdik et al. 1985, 1999; Papaioannou and Papazachos 2000; Jimenez et al. 2001; Kayabali 2002; Karavos et al. 2003; Bayrak et al. 2005). Although the authors in all of these studies or other authors studying seismic hazard in different areas of the world use polygonal or rectangular seismic zones according to fault geometries and space distribution of earthquakes, Sayil and Osmansahin (2008) preferred to use elliptical seismic zones. I think that their subregions are not more realistic for studying seismicity of western Anatolia because the

Probabilistic seismic hazard in terms of intensities for Bulgaria and Romania – updated hazard maps

Abstract. Since 2007 Bulgaria and Romania are members of the European Union. All member states have to introduce the European earthquake building code EUROCODE 8 (EC 8) in the coming years. Therefore, new seismic hazard maps have to be calculated according to the recommendations in EC 8. Here the authors present a novel approach to compute such hazard maps. We prefer to use the macroseismic intensity as hazard parameter because of two reasons: - The irregular azimuthal attenuation pattern of the Vrancea intermediate depth earthquakes can be advantageously taken into account by using detailed macroseismic maps. - The intensity is directly related to the degree of damage and is the original information in the historical earthquake catalogues. The main base of our probabilistic analysis is the earthquake catalogue for SE-Europe (Shebalin et al., 1998) in combination with national and regional catalogues. Fore- and aftershocks were removed. Seismic source zones inside an area of about 200 km around Romania and Bulgaria were defined based on seismicity, neotectonics and geological development. For each seismic source the intensity-frequency relation was calculated and a maximum possible earthquake as well as a seismogenic depth was estimated. An appropriate attenuation law was assumed. To cope with the irregular isoseismals of the Vrancea intermediate depth earthquakes, a factor Ω was included in the macroseismic attenuation law. Using detailed macroseismic maps of three strong intermediate depth earthquakes, Ω was calculated for each observation. Strong local variations of Ω are avoided by averaging within grid cells of 0.5 degree in longitude and 0.25 degree in latitude. The contributions of all seismic sources, the crustal normal depth source zones and the Vrancea intermediate depth zone, were summed up and the annual probability of exceedance was calculated. The contribution of the Vrancea intermediate depth zone to each grid point was computed with the corresponding representative Ω of this point; a seismogenic depth of 120 km has been assumed. Each final seismic hazard map is a combination of two maps, the one for normal depth source zones and the one for the Vrancea intermediate depth zone. This is illustrated for a recurrence period of 475 years. Additional hazard maps were calculated for different recurrence periods.

Delineation of shallow seismic source zones usingK-means cluster analysis, with application to the Aegean region

The selection of specific uniform seismic source zones for use in probabilistic seismic hazard analysis is often controversial. Recognizing that a consistent approach to source model development is not always possible, as the information available relating to geology and seismotectonics can vary from region to region, the K-means algorithm for hierarchical cluster analysis can be used to partition regions based on observed seismicity. The Aegean [incorporating Greece, Albania, Former Yugoslav Republic of Macedonia (F.Y.R.O.M.), southern Bulgaria and western Turkey], with its varied seismotectonics and generally high seismicity, is used as an important area of seismicity in which to develop and demonstrate the application of K-means. Two types of algorithm are considered. The first is a point-source K-means that can be used to partition a catalogue of earthquake hypocentres. The second is a novel line-source development of the algorithm, appropriate in seismology as these are analogues for the traces of active faults, which is then applied to a catalogue of known fault ruptures in the Aegean. The common problems of the K-means methodology are also addressed. Ensemble analyses are used to identify better choices of initial estimates for the cluster centres. A cluster quality index is used to identify the optimum number of clusters, and its robustness assessed when considering different subsets of the observed earthquake catalogue. An alternative approach is also implemented: Monte Carlo seismic hazard analysis is used to compare models with different numbers of clusters with the observed seismicity of the 20th century. Considerable variation is found in the optimum number of clusters identified either by the quality index or by stochastic seismic hazard analysis. Ultimately the K-means partitions of seismicity are developed into source models and their representation of Aegean seismotectonics assessed. The result is that models containing between 20 and 30 clusters emerge as the most appropriate in capturing the spatial variation in hypocentral distribution and fault type in the Aegean.

Earthquake hazard in Northeast India — A seismic microzonation approach with typical case studies from Sikkim Himalaya and Guwahati city

A comprehensive analytical as well as numerical treatment of seismological, geological, geomorphological and geotechnical concepts has been implemented through microzonation projects in the northeast Indian provinces of Sikkim Himalaya and Guwahati city, representing cases of contrasting geological backgrounds — a hilly terrain and a predominantly alluvial basin respectively. The estimated maximum earthquakes in the underlying seismic source zones, demarcated in the broad northeast Indian region, implicates scenario earthquakes of M W 8.3 and 8.7 to the respective study regions for deterministic seismic hazard assessments. The microzonation approach as undertaken in the present analyses involves multi-criteria seismic hazard evaluation through thematic integration of contributing factors. The geomorphological themes for Sikkim Himalaya include surface geology, soil cover, slope, rock outcrop and landslide integrated to achieve geological hazard distribution. Seismological themes, namely surface consistent peak ground acceleration and predominant frequency were, thereafter, overlaid on and added with the geological hazard distribution to obtain the seismic hazard microzonation map of the Sikkim Himalaya. On the other hand, the microzonation study of Guwahati city accounts for eight themes — geological and geomorphological, basement or bedrock, landuse, landslide, factor of safety for soil stability, shear wave velocity, predominant frequency, and surface consistent peak ground acceleration. The five broad qualitative hazard classifications — ‘low’, ‘moderate’, ‘high’, ‘moderate high’ and ‘very high’ could be applied in both the cases, albeit with different implications to peak ground acceleration variations. These developed hazard maps offer better representation of the local specific seismic hazard variation in the terrain.

Estimation of Maximum Earthquakes in Northeast India

We attempt to estimate possible maximum earthquakes in the northeast Indian region for four seismic source zones, namely EHZ, MBZ, EBZ, and SHZ, which encapsulates the various seismogenic structures of the region and also for combined source zones taken as a single seismic source regime. The latter case exhibits a high maximum earthquake estimate of MW 9.4 (±0.85) through Bayesian interpretation of frequency magnitude distribution with Gamma function implicating a moderate deviation from the standard Gutenberg Richter model at the higher magnitudes. However, tapering Gutenberg Richter models with corner magnitudes at MW 8.01, 8.7 and 9.1, respectively indicated maximum values corresponding to MW 8.4, 9.0, and 9.3. The former approach was applied to each of the source zones wherein the data are presented in parts according to the data completeness, thereof. EHZ, MBZ, EBZ and SHZ are seen with maximum earthquakes of MW 8.35 (±0.59), 8.79 (±0.31), 8.20 (±0.50), and 8.73 (±0.70), respectively. The maximum possible earthquakes estimated for each individual zone are seen to be lower than that estimated for the single regime. However, the pertaining return periods estimated for the combined zone are far less than those estimated for the demarcated ones.

Seismic hazard estimation based on the distributed seismicity in northern China

In this paper, we have proposed an alternative seismic hazard modeling by using distributed seismicites. The distributed seismicity model does not need delineation of seismic source zones, and simplify the methodology of probabilistic seismic hazard analysis. Based on the devastating earthquake catalogue, we established three seismicity model, derived the distribution of a-value in northern China by using Gaussian smoothing function, and calculated peak ground acceleration distributions for this area with 2%, 5% and 10% probability of exceedance in a 50-year period by using three attenuation models, respectively. In general, the peak ground motion distribution patterns are consistent with current seismic hazard map of China, but in some specific seismic zones which include Shanxi Province and Shijiazhuang areas, our results indicated a little bit higher peak ground motions and zonation characters which are in agreement with seismicity distribution patterns in these areas. The hazard curves have been developed for Beijing, Tianjin, Taiyuan, Tangshan, and Ji’nan, the metropolitan cities in the northern China. The results showed that Tangshan, Taiyuan, Beijing has a higher seismic hazard than that of other cities mentioned above.

A seismic source zone model for the seismic hazard assessment of the Italian territory

We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.ZS9 is made out of 36 zones where earthquakes with Mw >=5 are expected. It also assumes that earthquakes with Mw up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates.

Recent seismicity in Northeast India and its adjoining region

Recent seismicity in the northeast India and its adjoining region exhibits different earthquake mechanisms – predominantly thrust faulting on the eastern boundary, normal faulting in the upper Himalaya, and strike slip in the remaining areas. A homogenized catalogue in moment magnitude, M W, covering a period from 1906 to 2006 is derived from International Seismological Center (ISC) catalogue, and Global Centroid Moment Tensor (GCMT) database. Owing to significant and stable earthquake recordings as seen from 1964 onwards, the seismicity in the region is analyzed for the period with spatial distribution of magnitude of completeness m t, b value, a value, and correlation fractal dimension D C. The estimated value of m t is found to vary between 4.0 and 4.8. The a value is seen to vary from 4.47 to 8.59 while b value ranges from 0.61 to 1.36. Thrust zones are seen to exhibit predominantly lower b value distribution while strike-slip and normal faulting regimes are associated with moderate to higher b value distribution. D C is found to vary from 0.70 to 1.66. Although the correlation between spatial distribution of b value and D C is seen predominantly negative, positive correlations can also be observed in some parts of this territory. A major observation is the strikingly negative correlation with low b value in the eastern boundary thrust region implying a possible case of extending asperity. Incidentally, application of box counting method on fault segments of the study region indicates comparatively higher fractal dimension, D, suggesting an inclination towards a planar geometrical coverage in the 2D spatial extent. Finally, four broad seismic source zones are demarcated based on the estimated spatial seismicity patterns in collaboration with the underlying active fault networks. The present work appraises the seismicity scenario in fulfillment of a basic groundwork for seismic hazard assessment in this earthquake province of the country.

Seismicity and Seismotectonics of the Western Lake Ontario Region

The western Lake Ontario region, a traditionally perceived area of low seismic risk, is densely populated and is home to, among other critical facilities, the nuclear reactors of Pickering and Darlington. These and other characteristics of the region call for improved estimates of seismic hazard. Due to a lack of understanding of the causative geological sources and recurrence characteristics of the reported seismic activity, there is considerable uncertainty regarding estimated ground motion parameters, a fundamental component of seismic hazard assessments. To attempt to improve the definition of the seismic source zones and, consequently, seismic hazard assessments, the hypocentres of about 30 local earthquakes were recomputed. A new data compilation, based on the revised locations or those with the least travel-time residuals, shows that local microearthquakes (ML"3.5) generally occur along, or at the intersection of, prominent aeromagnetic or gravity anomalies. A notable seismicity trend extends in a northeast-southwest direction between Toronto and Hamilton, and is bounded by magnetic lineaments. A major geological structure, the Central Metasedimentary Belt Boundary Zone (CMBBZ), coincides with a strong aeromagnetic anomaly which extends to the northeast into the Western Québec Seismic Zone. This magnetic lineament also extends to the south, across Lake Ontario, to join the Akron (Ohio) magnetic boundary that was associated with several historical earthquakes and with a mb=4.9 earthquake in 1986. Most of the seismic events recorded instrumentally in the 20th century have occurred within a depth range of 5 to 20 km. This observation supports the correlation of local earthquakes with deep geophysical and geological features, suggesting contemporary reactivation of basement structures. This may imply that a more conservative deterministic hazard estimate is needed to verify the probabilistic approach currently used to assess seismic hazard in southern Ontario.

Earthquake hazard zonation of Sikkim Himalaya using a GIS platform

An earthquake hazard zonation map of Sikkim Himalaya is prepared using eight thematic layers namely Geology (GE), Soil Site Class (SO), Slope (SL), Landslide (LS), Rock Outcrop (RO), Frequency–Wavenumber (F–K) simulated Peak Ground Acceleration (PGA), Predominant Frequency (PF), and Site Response (SR) at predominant frequencies using Geographic Information System (GIS). This necessitates a large scale seismicity analysis for seismic source zone classification and estimation of maximum earthquake magnitude or maximum credible earthquake to be used as a scenario earthquake for a deterministic or quasi-probabilistic seismic scenario generation. The International Seismological Center (ISC) and Global Centroid Moment Tensor (GCMT) catalogues have been used in the present analysis. Combining b-value, fractal correlation dimension (Dc) of the epicenters and the underlying tectonic framework, four seismic source zones are classified in the northeast Indian region. Maximum Earthquake of MW 8.3 is estimated for the Eastern Himalayan Zone (EHZ) and is used to generate the seismic scenario of the region. The Geohazard map is obtained through the integration of the geological and geomorphological themes namely GE, SO, SL, LS, and RO following a pair-wise comparison in an Analytical Hierarchy Process (AHP). Detail analysis of SR at all the recording stations by receiver function technique is performed using 80 significant events recorded by the Sikkim Strong Motion Array (SSMA). The ground motion synthesis is performed using F–K integration and the corresponding PGA has been estimated using random vibration theory (RVT). Testing for earthquakes of magnitude greater than MW 5, a few cases presented here, establishes the efficacy and robustness of the F–K simulation algorithm. The geohazard coverage is overlaid and sequentially integrated with PGA, PF, and SR vector layers, in order to evolve the ultimate earthquake hazard microzonation coverage of the territory. Earthquake Hazard Index (EHI) quantitatively classifies the terrain into six hazard levels, while five classes could be identified following the Bureau of Indian Standards (BIS) PGA nomenclature for the seismic zonation of India. EHI is found to vary between 0.15 to 0.83 quantitatively classifying the terrain into six hazard levels as “Low” corresponding to BIS Zone II, “Moderate” corresponding to BIS Zone III, “Moderately High” belonging to BIS Zone IV, “High” corresponding to BIS Zone V(A), “Very High” and “Severe” with new BIS zones to Zone V(B) and V(C) respectively.

Seismicity of the Bánát region

In the seismic source zone of Bánát more than 600 earthquakes are known since 1773 among them six events with magnitude of 5.0–5.7 measured on the surface magnitude scale. The macroseismic reinterpretation of the April 2, 1901 earthquake yields epicentral intensity of VII on the European Macroseismic Scale, and a focal depth value of 12 km. Based on empirical relations the maximum rupture area is estimated as 50–55 km 2 and the maximum displacement along the fault is about 16 cm in the Bánát seismic zone due to the MS = 5.7 event occurred on July 12, 1991. The average recurrence that we may expect an earthquake of M ≥ 3.4 every 1 year, an earthquake of M ≥ 4.3 every 10 years and an earthquake of M ≥ 5.3 every 100 years in the studied source zone. The probabilistic seismic hazard assessment predicts 1.3–2.1 m/sec 2 peak ground accelerations, and 6.7–7.3 maximum (theoretical) earthquake intensity values with 10% chance of exceedance for an exposure time of 100 years in the region.

Fine-scale partitioning of contemporary strain in the southern Walker Lane: Implications for accommodating divergent strike-slip motion

Treating the crust as a micropolar continuum and inverse modeling a high-quality catalog of earthquake focal mechanisms reveals that >km-scale heterogeneous strain is partitioned at ∼km-scales into pairs of plane strains with sub-perpendicular principal axes. These earthquakes occurred in a shallow crustal volume centered on the western Indian Wells Valley of eastern California, the site of divergent, right-lateral strike-slip motion along the boundary between the Sierran microplate and the southern Basin and Range province. The seismic events defining the plane strains occur in a diffuse seismic source zone and cannot be additionally subdivided on the basis of the type of strain they accommodate. The results indicate that seismic moment release for background earthquakes is partitioned into events associated with either fault-zone-perpendicular extension by normal faults or horizontal shearing by conjugate strike-slip faults. The spatial scales over which this partitioning occurs suggest that outcrop-scale brittle faults may in some cases record nearly perpendicular plane strains related to a single continuous deformation.