Seismic Hazard Studies Research Articles

Combining borehole log-stratigraphies and ambient vibration data to build a 3D Model of the Lower Var Valley, Nice (France)

In seismic hazard studies, site effects assessment is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. The Lower Var Valley (Nice, southeastern France) is a 2 km wide fluvial sedimentary basin where site effects have been observed on earthquake recordings. In addition, this area is a significant economically developing zone including the second largest airport of France. Therefore, it is highly important to define a 3D sub-surface model of the basin to understand the ground motion amplifications. Based on the compilation of several years of geotechnical and geophysical studies, the Quaternary alluvial deposits of the Lower Var Valley were investigated using 331 borehole log-stratigraphies and 439 ambient vibration measurements. These data were combined in a commercial software that models and allows to visualize geoscientific data and geological models in 3D. The borehole log-stratigraphies helped to define a representative lithostratigraphic soil profile and the geometry of the different sedimentary layers. Only few boreholes are reaching the engineering bedrock (Pliocene conglomerates characterized by a S-wave velocity larger than 800 m.s−1). From the ambient vibration data, the fundamental resonance frequency identified from H/V curves indicates the bedrock depth using the well-known f0 = Vs/4H relationship. These H/V curves were interpreted using geological knowledge of the studied zone.Our results show that the bedrock depth varies, in the center of the valley, between 50 m in the northern part of the studied zone and more than 100 m on the river estuary. First, the average S-wave velocity in the alluvial deposits is estimated according to combined analysis of the borehole log-stratigraphies and H/V calculations on ambient vibration measurements. The spatial variability of this velocity has been translated into a velocity zonation of the valley. Second, this zonation allows us to extrapolate the Quaternary-Pliocene limit in the whole valley. The 3D model of the Lower Var Valley will be useful in future numerical simulations of seismic ground motions in order to evaluate and mitigate seismic risk in the area.

Interdisciplinary Dating of Utrish Seismic Dislocations: Localization of the Source of a Strong Historical Earthquake in the Western Caucasus

The results of paleoseismological, archeological, and dendrochronological studies and published materials have yielded the date of neotectonic Utrish seismic dislocations, one of the key components in seismic hazard studies of the Western Caucasus. According to previously obtained and newly acquired data, a destructive earthquake occurred here in the 12th–13th centuries. Based on seismotectonic fault data, the lower limit of its magnitude MW ≈ 6.9. The source is located in the Utrish Fault zone, along the coast of the Abrau Peninsula. The seismogenic role of this fault is established over the aggregate data.

Source parameters for small-moderate earthquakes in Marmara Region (Turkey)

The main aim of this study is to investigate the self-relation and self-similarity of earthquakes in and around the Marmara Sea (NW Turkey) by using these obtained source parameters. With this purpose, spectral source parameters for 77 small to moderate earthquake (3.5 ≤ ML ≤ 5.2) that occurred during 2004–2018 have been determined from P and S wave spectra according to Brune’s source model by using regional broadband seismograms. The average ratio of P/S wave corner frequency is found to be 1.3 that suggesting higher corner frequency for P wave. The static stress drops range from 0.1 and 136 MPa with a median value of 9.8 MPa (98 bars). The high stress drops for these events can indicate high frictional strength and low strain-rate of the faults. Similarly, the low values of the stress drop can indicate a general weakness of the faults in the study area. There is no clear dependence between the seismic moment and the static stress drop in the analyzed events but some events which have lower seismic moment value also have lower stress drop. Obtaining results indicated the corner frequency decreases with increasing of the seismic moment. Also, a slight depth dependence of the corner frequency has been observed for the analyzed events. Shallower events have larger corner frequency value than deeper events. Also, a clear depth dependence of the stress drop values has not been observed. However, the depth dependence of the seismic moment is seen more clearly. Our results indicated that the deeper events have larger seismic moment values in the study area. In spite of scattering in small events, a linear relationship between local magnitude (ML) and moment magnitude (MW) could be obtained as MW = 1.4261(± 0.31) + 0.6399(± 0.08)ML from P waves spectra and MW = 0.0136(± 0.21) + 0.9883(± 0.05)ML from S wave spectra and calculated MW values are consistent with waveform inversion (centroid moment tensor — CMT) results. These relationships may be useful for seismic hazard studies in the study area.

On the Identification of Industrial Components in the Horizontal-to-Vertical Spectral Ratio (HVSR) from Microtremors

In seismic hazard studies, the Horizontal-to-Vertical Spectral Ratio (HVSR) is nowadays routinely considered as a quick way to assess possible amplification effects. However, because of several issues that can affect the data, the HVSR cannot be considered as valid per se, and a careful data evaluation is necessary. In this study, a series of HVSR curves are evaluated in order to highlight industrial components that can significantly alter the natural HVSR. First, a controlled-source experiment is carried out in order to define the characteristics of spurious industrial signals. Data analysis shows that the coherence functions and the mildly smoothed amplitude spectra plotted with linear scales can help significantly in identifying industrial components that can be otherwise difficult to highlight. Coherence functions appear particularly interesting because of their ability to reveal the presence of industrial components independently of their amplitude. Field data from three sites are then analyzed on the basis of the evidence obtained through the controlled-source experiment. For the third site, data recorded on two different days are considered. While in the first data set no significant industrial component is present, in the second and third data sets, a series of remarkable industrial signals that severely alter the natural HVSR are identified. The assessment of the coherence functions and mildly smoothed amplitude spectra is therefore suggested as a valuable support to avoid pitfalls in the interpretation of the experimental HVSR. Finally, two quick and dirty procedures aimed at reducing the effect of industrial components are also presented.

Seismometers Within Cities: A Tool to Connect Earth Sciences and Society

The high degree of human activities in urban environments produces large background vibrations that makes it difficult to use data acquired in these areas for classical seismology. Seismometers installed within cities have been typically been used for the study of seismic hazard or for monitoring civil engineering problems. However, with the development of monitoring techniques based on the interpretation of the so-called seismic ambient noise, these data have gained scientific interest. Our objective is to discuss an additional utility of seismometers deployed within a city; its use as a tool to connect society with Earth sciences. Many citizen activities, from traffic to music concerts, produce vibrations that can be recorded seismically, and our experience shows that these records attract the attention of the media and social networks. With the emergence of low-cost and easy-to-use instruments in recent years, more citizens can now record ground motion and become interested in the interpretation of the recorded seismograms. The installation of permanent seismic networks in educational centers has proven to be a good approach to introduce students to Earth sciences at the national level and can also be developed at the urban scale using this new instrumentation. In this contribution we will first review the previous results related to the identification of the sources of vibration in urban areas and then present a new ongoing project based on the deployment of a seismic network in educational centers located in the city of Barcelona.

GROUND MOTION PREDICTION AT THE DAM SITES IN SW ROMANIA FOR MODERATE VRANCEA SUBCRUSTAL EARTHQUAKES

The seismic hazard studies of the last 30 years have been largely carried out taking into account the needs of the construction engineers, by linking the specific quantities of soil movement with the physical parameters determined instrumentally, namely, with the maximum values of soil acceleration. At present, interest in the results of hazard studies has increased and has extended to other areas such as insurance or design companies, environmental protection, etc. A fundamental element in the estimation of seismic hazard is the variation of the amplitude of the movement of the soil according to the distance, magnitude and local conditions. To understand and prevent the effects of the strong Vrancea earthquakes in the dam sites located in the South West of Romania, we study the seismic waves attenuation relations using the accelerations recorded by the national network of K2 accelerometers, following the moderate Vrancea intermediate earthquakes. The study area includes the largest agglomeration of dams in Romania, with almost 100 dams out of the 250 large dams. One of the most important specific requirements towards dams' safety is the seismic risk and hazard assessment and the computation of attenuation relationships is one of the most important steps of the work. The main objective of the present work is the evaluation of the specific attenuation relationships of the seismic wave propagating from Vrancea subcrustal focus toward south-west, with direct application for the dams situated in the area.

Study of the PGV, Strong Motion and Intensity Distribution of the February 1969 (Ms 8.0) Offshore Cape St. Vincent (Portugal) Earthquake Using Synthetic Ground Velocities

The 28 February 1969 (Ms 8.0) Cape St. Vincent earthquake is the largest shock to have occurred in the region after the Lisbon earthquake of 1755. However, the study of the rupture process has been limited due to the characteristics of the available seismic data which were analogue records that were generally saturated at both regional and teleseismic distances. Indeed, these data consist of just one accelerograph record at the 25th April Bridge in Lisbon (Portugal) and the observed intensities in the Iberian Peninsula and northern part of Morocco. We have used these data to simulate the distribution of PGV (Peak Ground Velocity) for the 1969 event at regional distances (less than 600 km) by using a 3D velocity model. The PGV values are very important in seismic hazard studies. The velocity model and the methodological approach were tested by comparing synthetic and observed ground velocities at regional distances for two recent, well-studied earthquakes that occurred in this region, namely, the 2007 (Mw = 5.9) and the 2009 (Mw = 5.5) earthquakes. By comparing the synthetic and observed PGA (Peak Ground Acceleration) at Lisbon, the focal depth was estimated equal to 25 km and the seismic moment equal to 6.4 × 1020 N m (Mw = 7.8) for 1969 earthquake. With these parameters, PGV values were obtained for 159 sites located in the Iberian Peninsula and northern region of Morocco where we have felt intensity values. Using different empirical relations, the instrumental intensity values were calculated and compared with the felt intensities. As a result, the synthetic PGV values obtained in this study for the 1969 earthquake could be used as reference values, and the methodological approach would allow the PGV and intensity to be simulated for other events in the region.

Probabilistic Seismic Hazard Assessment for the Shanxi Rift System, North China

ABSTRACTWe present the first probabilistic seismic hazard assessment (PSHA) specifically for the Shanxi rift system, north China, which has been defined as one of the areas of highest seismic hazard and risk in China in recent decades. We applied a Monte Carlo-based approach to PSHA, based on so far the most complete earthquake catalog available, a detailed zonation considering both seismicity distribution and local tectonic features, a logic tree of carefully selected ground-motion prediction equations, as well as a cautious consideration of actual local site effects for this region. Both areal sources (for Ms<6.0) and fault sources (for Ms≥6.0) were considered, and a synthetic earthquake catalog was generated through Monte Carlo simulation. A logic tree was applied to represent the epistemic uncertainty related to attenuation models for the rift system. Actual local site effects were incorporated and the stability of the results was also tested in this study. Our results show that nearly the entire rift system faces a significant seismic hazard and associated high seismic risk, as more than 80% of the population and the main economical infrastructure of Shanxi are concentrated here. The highest hazard is found in the areas around the north margin of Tianzhen fault and the north segment of Hengshan fault in the north, and in the Linfen basin and the area around Zhongtiaoshan fault in the south of the rift system. Our results are comparable to, but a refinement of, the results of previous probabilistic seismic hazard studies in the region. Deaggregation of seismic hazard for five large cities in the rift system indicates that the seismic hazard is most contributed by the nearby sources. Results obtained in this study provide a better understanding of the seismic hazard in the Shanxi rift system and can thereby help guiding earthquake risk mitigation in the future.

Ground motion prediction equation for West Sumatra due to distant shallow crustal, interface, andiIntraslab warthquakes

Ground motion prediction equation (GMPE) is an important component for seismic hazard study to minimize the casualty from an earthquake, especially in the building collapse. This study presents ground motion prediction equation for West Sumatra due to distant shallow crustal, interface, and intraslab earthquakes. The data sets consist of 375 strong motion data that are recorded by Meteorological, Climatological, and Geophysical Agency’s (BMKG) accelerograph sensors that located in West Sumatra, earthquake parameter data are from BMKG’s earthquakes catalog with the range moment magnitude of 4.0-6.4 and recorded at sites with hypocentral distance of 17 – 1000 km, focal mechanism data are from Global Centroid Moment Tensor (GCMT), and use site classification to enhance the quality of the results. This study shows good results with the standard deviation of residual value of 0.23 for shallow crustal, 0.29 for interface, and 0.49 for intraslab earthquakes.

Discussion of “Assessment of the Seismicity of Peshawar Region in Line with the Historical Data and Modern Building Codes (ASCE-07 and IBC3-2006)” by Shah et al

ABSTRACT This discussion presents review of the seismic hazard in Peshawar, performed by various national and international researchers and agencies in the past, which was critically compared with the proposed peak ground acceleration (PGA) of 0.06 g. The PGA = 0.06 g corresponds to seismic Zone 1, which is the lowest seismic hazard as per the Building Code of Pakistan. The building damages observed in Peshawar in the recent past earthquakes are revisited to judge the correctness of PGA = 0.06 g for Peshawar that pertain to its suitability for use in the seismic design of structures. The recent seismic hazard studies reported also by other researchers are presented. Seismic hazard studies of Peshawar report widely conflicting results. The placement of Peshawar in the lowest seismic hazard Zone 1 will have a number of socioeconomic implications. Therefore, a critical review of these studies was essential to recommend suitable design level ground motion for Peshawar.

Spatial and Temporal Variations of Gutenberg-Richter b-Values Using Classic Method of Moment (CMM) and Seismicity in and Around Van Lake

The aim of this study determined spatial-temporal variations of Gutenberg-Richter b-values using the Classic Method of Moment (CMM) and seismicity in and around Lake Van. We wrote a Matlab program for this method in the Matlab system. The faults for the Van region used as updated in the GIS system given the reference from the different sources by Bayrak and Türker (will be used in the doctoral thesis). Around Lake Van divided into 4 different seismic source regions. For each seismic source region, probabilities ranging from 0 to 1, the annual probability of exceedances and return periods were calculated for the next 25, 50 and 100 years. Van region determined the seismicity b value changes, β-values. We mapped spatial variations of the b-values using the GIS system. As a result, Lake Van region (region 2) occurred 7.1 magnitude an earthquake in the next 100 years with 31 % probability levels. If it occurred in the 50 years with 15 % probability levels, in the 25 years with 7.8 % probability levels. We estimated the low b-values in the Lake Van so, it has been the high seismicity and can be occurred an earthquake as the high magnitude in the next years. This study will be lead to earthquake hazard analysis and seismic hazard studies in and around Lake Van.

Simultaneous Dependence of the Earthquake‐Size Distribution on Faulting Style and Depth

AbstractWe analyze two high‐quality Southern Californian earthquake catalogues, one with focal mechanisms, to statistically model and test for dependencies of the earthquake‐size distribution, the b values, on both faulting style and depth. In our null hypothesis, b is assumed constant. We then develop and calibrate one model based only on faulting style, another based only on depth dependence and two models that assume a simultaneous dependence on both parameters. We develop a new maximum‐likelihood estimator corrected for the degrees of freedom to assess models' performances. Our results show that all models significantly reject the null hypothesis. The best performing is the one that simultaneously takes account of depth and faulting style. Our results suggest that differential stress variations in the Earth's crust systematically influence b values and that this variability should be considered for contemporary seismic hazard studies.

Potential Seismogenic Source Model for the Red Sea and Coastal areas of Saudi Arabia

Seismogenic source models are unavoidable and a crucial element in seismic hazard studies. A seismogenic source model for Red sea and surrounding areas was constructed. The source model was built up taking into consideration various scientific data sources and logical opinions. A comprehensive model for Red sea and coastal regions of western Saudi Arabia was generated. This model is based on regional geological and tectonic studies, already published seismogenic source models, historical seismicity, recent seismicity, and finally adding up gravity and magnetic studies to construct the final layout of our model. The evaluation of the resources available led to a conclusion for dividing the study area into 23 seismogenic source zones.

Potential Seismogenic Source Model for the Red Sea and Coastal areas of Saudi Arabia

Seismogenic source models are unavoidable and a crucial element in seismic hazard studies. A seismogenic source model for Red sea and surrounding areas was constructed. The source model was built up taking into consideration various scientific data sources and logical opinions. A comprehensive model for Red sea and coastal regions of western Saudi Arabia was generated. This model is based on regional geological and tectonic studies, already published seismogenic source models, historical seismicity, recent seismicity, and finally adding up gravity and magnetic studies to construct the final layout of our model. The evaluation of the resources available led to a conclusion for dividing the study area into 23 seismogenic source zones.

Repeating earthquakes and interplate coupling along the western part of the North Anatolian Fault

The Sea of Marmara accommodates segments of the North Anatolian Fault (NAF) in Turkey and remains the only part of the western NAF that has not ruptured during the last century. At its nearest, the segment is ~20 km from Istanbul, the largest city in Turkey. Thus, it is important to understand the locking state of the fault, since it illuminates the strain accumulation rate along the fault segments, which in turn is an important input parameter in seismic hazard studies. To infer the interplate locking state, we used repeating earthquakes that indicate fault creep in the surrounding area using long-term (April 2005 to May 2013) seismic observations at 12 broadband seismic stations operated by Bogazici University Kandilli Observatory and Earthquake Research Institute. We defined repeating earthquakes from waveform coherences that were >0.95 for 40-second-long waveforms of the vertical component. Using the selection procedure, we found 21 repeating earthquakes with magnitude 2.3 to 3.2 that are grouped into 9 sequences. They are distributed along the main NAF, comprising three groups of activity, one group in the Sea of Marmara and a group either side to the east and west. The three groups are located near the boundary of previous large earthquake ruptures, suggesting relatively weak coupling there. We also estimated the fault creep rate from the cumulative slip of the repeating earthquakes using a scaling relationship between repeating earthquakes' moment and slip. The slip rate for these three groups are similar (3–4 cm/yr) and comparable to, albeit slightly higher than, those expected from global plate models (~2.4 cm/yr). This suggests relatively weak locking around the groups. The relocation results of the repeating earthquake hypocenters in the Sea of Marmara suggest the creep is occurring at 10 to 20 km depth. These results suggest heterogeneous coupling in the segment.

Deterministic seismic hazard assessment for the Makkah region, western Saudi Arabia

Rapid social and industrial development of the Makkah and Jeddah regions emphasizes necessity of reassessment of seismic hazard, results of which are essential for aseismic design, emergency management, and insurance regulations. In this work, the deterministic seismic hazard studies are applied to evaluate level of seismic hazard for the Makkah region using the up-to-date geophysical, geological, and seismological database, and using different techniques. The assessment has been performed in terms of peak ground acceleration (PGA) and presumably active faults are considered as sources of earthquakes. Results of the study show that, in the context of the used models of seismic sources, the level of seismic hazard in the region is controlled by magnitude of earthquakes that may occur at the intersection of NE-SW faults (the Ad Damm fault zone and the Wadi Fatima Shear Zone) and the NNW-oriented faults that run parallel to the Red Sea coast (Red Sea Margin Faults group). It is important to make accurate mapping of faults using up-to-date data and to estimate if the faults are active at present. Particular attention should be given to careful estimation of maximum magnitude of possible earthquakes that can occur along the active fault.

Seismic hazard at a triple plate junction: the state of Chiapas (México)

The state of Chiapas (SE Mexico) conforms a territory of complex tectonics and high seismic activity. The interaction among the Cocos, North American and Caribbean tectonic plates, as well as the active crustal deformation inside Chiapas, determines a variety of seismogenic sources of distinct characteristics and particular strong ground motion attenuation. This situation makes the assessment of seismic hazard in the region a challenging task. In this work, we follow the methodology of probabilistic seismic hazard analysis, starting from the compilation of an earthquake catalogue, and the definition of seismogenic source-zones based on the particular seismotectonics of the region: plate-subduction-related sources (interface and intraslab zones), active crustal deformation zones and the shear zone between the North American and Caribbean plates formed by the Motagua, Polochic and Ixcan faults. The latter source is modelled in two different configurations: one single source-zone and three distinct ones. We select three ground motion prediction equations (GMPEs) recommended for South and Central America, plus two Mexican ones. We combine the GMPEs with the source-zone models in a logic tree scheme and produce hazard maps in terms of peak ground acceleration and spectral acceleration for the 500-, 1000- and 2500-year return periods, as well as uniform hazard spectra for the towns of Tuxtla Gutierrez, Tapachula and San Cristobal. We obtain higher values in comparison with previous seismic hazard studies and particularly much higher than the output of the Prodisis v.2.3 software for seismic design in Mexico. Our results are consistent with those of neighbouring Guatemala obtained in a recent study for Central America.

Analysis of the Key Features of the Seismic Actions Due to the three Main Earthquakes of 11th of May 2011 in Lorca, Spain

The seismic records are in general valuable information, especially in cases where damage in buildings has occurred. The main purpose of the present document is to describe the principal results of the analysis of features of ground motions due to the main three earthquakes that occurred in Spain on May 11, 2011. In this day the major earthquake hada magnitude of 5.1 Mw. This event triggered different levels of damage in numerous buildings in the city of Lorca located in southern Spain. Unfortunately, 9 persons died due mainly to the collapse of non-structural elements. We describe in the present paper the application of the software Seismograms Analyzer-e(SA-e) to perform the processing and the analysis of the seismic records obtained in five stations during the main three earthquakes on May 11 (the largest earthquake of magnitude 5.1 Mw, the precursor of magnitude 4.5Mw, and the after shock of magnitude 3.9 Mw). We also highlight the significant similitudes between the seismograms generated in the LOR station during these three earthquakes. Additionally, we determined the values of acceleration that occurred in the roof of the buildings of Lorca, because these values of acceleration contributed both to the damage of numerous buildings and the collapse of several parapets of some buildings. The analysis of these accelerations is relevant because the collapse of some parapets was the cause of the death of the 9 people that died during the main earthquake. For example, according to our study in the roof of a building with a fundamental period of 0.25 s the acceleration could have reached values near to 1.04 g. We also analyzed the potential of damage in function of the values of CAVSTD. Additionally, we determined hypothetical seismic forces for the design of parapets in buildings of Lorca considering the NCSE-02 normative, and the values of Sa based on the seismic records. We determined a significant difference between the seismic forces that could have been used to design the parapets of the buildings in the Lorca city and the forces determined according to the values of PGA that were proposed in a recent study of seismic hazard for Spain.

From fault creep to slow and fast earthquakes in carbonates

Abstract A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila (Mw 6.1) in 2009 and Norcia (Mw 6.5) in 2016 in central Italy. Surprisingly, within this region, fast (≈3 km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite its relevance for seismic hazard studies, the transition from fault creep to slow and fast seismic rupture propagation is still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures were obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3–5 km depth (i.e., P = 100 MPa and T = 100 °C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of seismicity in central Italy. The transition from slow to fast rupture is explained by an increase in the ambient temperature, which enhances the elastic loading stiffness of the fault, i.e., the slip velocities during nucleation, allowing flash weakening and, in turn, the propagation of fast ruptures radiating intense high-frequency seismic waves.

Seismic magnitude conversion and its effect on seismic hazard analysis

The aim of this study is to demonstrate the bias created in the seismic hazard studies due to the choice of magnitude scaling equations without any statistical basis. The earthquake catalogue of Tripura, India, has been used for the purpose of this study. The catalogue was homogenized using the various scaling equations suitable for the region. Then, the bias created on parameters, like the magnitude of completeness (Mc), a and b values of the Gutenberg–Richter recurrence relation, maximum magnitude (Mmax), and peak ground acceleration, was demonstrated. The standard deviations of Mc, a, and b parameters were observed to be 0.23, 0.27, and 0.037 respectively. The maximum variations in the Mmax and ground motion estimates were found to be 0.7 magnitude units and 0.2 g respectively. Then, the robustness of the regional rupture characters in overcoming the observed variations has been demonstrated. The trend of the rupture behavior of the seismic sources seems to be unaffected by the change in the magnitude scaling equations. The Mmax calculated from the rupture-based procedure was observed to be higher than that calculated from the probabilistic method. This variation in Mmax estimation has been utilized to critically assess the suitability of the magnitude scaling equations for the particular study area.