Main Seismic Sources Research Articles

Correlations between the Topography-Induced Gravity, Terrain Structure and the Seismicity in the Gulf of Panama

This study presents new maps of the topographic and geophysical setting and seismicity in the region of the Gulf of Panama. The spatial analysis is based on the comparative analysis of the datasets on geoid, free-air gravity anomaly, topography and earthquakes. The cartographic framework is developed using the Generic Mapping Tools (GMT) scripting toolset. Seismic activity in the Central America is high due to the complex geologic setting, tectonic activity and lithosphere plate subduction. The data include the Earth Gravitational Model (EGM2008), the General Bathymetric Chart of the Oceans (GEBCO) and gravity grids. The seismicity data were collected from the Incorporated Research Institutions for Seismology (IRIS) catalogue on 1970–2021. The variations in data were compared to analyse correlations between the geophysical, seismic and topographic parameters. Free-air gravity, geoid and topographic data derived from the high-resolution datasets were used to investigate their effects on the main seismic sources in the region. The comparison of the maps showed that the distribution of the shallow earthquakes in the Pacific segment of Panama coincides with negative free-air anomalies and lower geoid values. The results revealed high values of geoid in the high mountainous regions of Panama (Cordilliera de Talamanca, southern coast of Peninsula de Azuero and eastern Panama, 77.5–78.5°W), which correspond to the topographic roughness in the highlands. Negative values of geoid are found over the Caribbean Sea basin (−4 to 0 m). The analyses of seismicity showed 1740 earthquake events varying by magnitudes from 2.9 to 7.8 at the depths up to 225 m (near the west coast of Colombia). A high concentration of the earthquakes is in the western region of the Panama’s shelf waters (~82–83.5°W), and on the border with Colombia (~77–78.5°W). High gravity anomalies (over 220 mGal) are found in the mountainous regions which match the geodynamic processes associated with the Earth structure and geodetic and geophysical effects. The regions of the high seismicity were defined in the Gulf of Chiriqui and eastern part of the Gulf of Panama.

Amplification effect of near-field ground motion around deep tunnels based on finite fracturing seismic source model

Dynamic failure of rock masses around deep tunnels, such as fault-slip rockburst and seismic-induced collapse, can pose a significant threat to tunnel construction safety. One of the most significant factors that control the accuracy of its risk assessment is the estimation of the ground motion around a tunnel caused by seismicity events. In general, the characteristic parameters of ground motion are estimated in terms of empirical scaling laws. However, these scaling laws make it difficult to accurately estimate the near-field ground motion parameters because the roles of control factors, such as tunnel geometry, damage zone distribution, and seismic source parameters, are not considered. For this, the finite fracturing seismic source model (FFSSM) proposed in this study is used to simulate the near-field ground motion characteristics around deep tunnels. Then, the amplification effects of ground motion caused by the interaction between seismic waves and deep tunnels and corresponding control factors are studied. The control effects of four factors on the near-field ground motion amplification effect are analyzed, including the main seismic source wavelength, tunnel span, tunnel shape, and range of damage zones. An empirical formula for the maximum amplification factor ( α m ) of the near-field ground motion around deep tunnels is proposed, which consists of four control factors, i.e. the wavelength control factor ( F λ ), tunnel span factor ( F D ), tunnel shape factor ( F s ) and excavation damage factor ( F d ). This empirical formula provides an easy approach for accurately estimating the ground motion parameters in seismicity-prone regimes and the rock support design of deep tunnels under dynamic loads.

Permanent, seasonal, and episodic seismic sources around Vatnajökull, Iceland from the analysis of correlograms

In this study, we locate and characterise the main seismic noise sources in the region of the Vatnajökull icecap (Iceland). Vatnajökull is the largest Icelandic icecap, covering several active volcanoes. The seismic context is very complex, with glacial and volcanic events occurring simultaneously and the classification between the two can become cumbersome. Using seismic interferometry on continuous seismic data (2011–2019), we calculate the propagation velocities and locate the main seismic sources by using hyperbolicas geometry and a grid-search method. We identify and characterise permanent oceanic sources, seasonal glacial-related sources, and episodic volcanic sources. These results give a better understanding of the background seismic noise sources in this region and could allow the identification of seismic sources associated with potentially threatening events in real-time.

Feasibility Study of an Earthquake Early Warning System in Eastern Central Italy

An earthquake early warning system (EEWS) is a monitoring infrastructure that allows alerting strategic points (targets) before the arrival of strong shaking waves during an earthquake. In a region like Central Italy, struck by recent and historical destructive earthquakes, the assessment of implementation of an EEWS is a significant challenge due to the proximity of seismic sources to many potential targets, such as historical towns, industrial plants, and hospitals. In order to understand the feasibility of an EEWS in such an area, we developed an original method of event declaration simulation (EDS), a tool for assessing the effectiveness of an EEWS for existing seismic networks, improving them with new stations, and designing new networks for EEW applications. Values of the time first alert (TFA), blind zone radius (BZ), and lead time (LT) have been estimated with respect to selected targets for different network configurations in the study region. Starting from virtual sources homogeneously arranged on regular mesh grids, the alert response was evaluated for actual and improved seismic networks operating in the area, taking into account the effects of the transmission and acquisition systems. In the procedure, the arrival times of the P wave picks, the association binder, the transmission latencies, and the computation times were used to simulate the configuration of PRESTo EEWS, simulating both real-time and playback elaborations of real earthquakes. The NLLOC software was used to estimate P and S arrival times, with a local velocity model also implemented in the PRESTo EEWS. Our results show that, although Italy’s main seismic sources are located close to urban areas, the lead times calculated with the EDS procedure, applied to actual and to improved seismic networks, encourage the implementation of EEWS in the study area. Considering actual delays due to data transmission and computation time, lead times of 5–10 s were obtained simulating real historical events striking some important targets of the region. We conclude that EEWSs are useful tools that can contribute to protecting people from the harmful effects of earthquakes in Italy.

The assessment of local site effects and dynamic behaviour in Nicosia, Cyprus

Single-station microtremor measurements were conducted to investigate earthquake and soil behaviour for the first time in Nicosia, Cyprus. Cyprus is located in a tectonically complex area in the Eastern Mediterranean where three plates meet. The study area was chosen to cover the areas to be opened for new development. Nicosia, the capital of Cyprus, is also the island's most important cultural, industrial, commercial, and transportation centre. The study creates base maps for the soil to assess earthquake resistance crucial for construction. Microtremor Method was applied at 100 stations and the Multi-Channel Analysis of Surface Waves (MASW) method was used at 52 stations. Also, RefractionMicrotremor (Re-Mi) and L-Shaped Spatial Autocorrelation (L-SPAC) methods were carried out at 17 stations to substantiate the research. The results of the microtremor method indicate that the predominant soil period values have an average of 1 second and pre-dominant peak period values are generally found between 0.1 to 5 s at the study area. Peak amplitude values are observed between 1 and 2.4. The Vulnerability Index Parameter (Kg) exceeded 20 at the central and the southern stations, and Kg values change between 7 and 54 units. The Kg values were found to be higher than 20 in soils where shear wave velocity is lower than 760 m/s. At the same time, the values of the predominant peak period were greater than 1 second. Cyprus is located in the Alpine Himalayan earthquake zone. The Cyprus Arc is known as the main seismic source of the island, It constitutes the tectonic border among African and Eurasian lithospheric plates in the region. During an earthquake in Nicosia, seismic waves will be amplified by an average of 1.5 times and soil deformation will occur due to the exceeding elastic limits. The results provided important insight into soil behaviour and indicated its reactions in a potential earthquake.

Fault modelling, seismic sequence evolution and stress transfer scenarios for the July 20, 2017 (MW 6.6) Kos–Gökova Gulf earthquake, SE Aegean

The July 20, 2017, MW 6.6 Kos–Gokova Gulf earthquake occurred offshore, near Bodrum of Turkey and Kos of Greece. It was one of the strongest in the broader area during the last many decades causing two deaths, many injuries and extensive damages. We investigated the evolution of the seismic sequence using seismological and geological tools. The aftershock sequence was relocated mainly in order to define the geometry of the main seismic source, depicting a NNW-dipping fault plane. It also revealed significant clustering, associated with other nearby faults, and asymmetric spatio-temporal evolution. Along with morphotectonic analysis on Kos Island, and other published seismological information (e.g. focal mechanisms), we modelled the seismic source of the mainshock, as well as the one of the strongest aftershocks (August 8, MW 5.3). We applied the Coulomb failure criterion in order to investigate the effect of the mainshock on the strongest aftershock, and the rest of the sequence as well. Using the same method, we also investigated the stress changes of both strongest shocks for the prevailing E–W-trending normal faults in this area. Among other conclusions and implications, we deduce that the prevailing tectonic setting of the Gokova Gulf consists of roughly E–W-striking normal faults forming inner horsts and grabens.

Interaction between fault and off-fault seismic sources in hazard analysis – A case study from Israel

In this study, seismic source characterization (SSC) for purpose of seismic hazard analysis (SHA) is explored, with a focus on the moment balance and interaction between on-fault and off-fault seismicity. For the purpose of this study, a homogenous, uniform, earthquake catalog for Israel is first compiled and de-clustered. Gridded seismicity is calculated based on the de-clustered catalog, using a fixed kernel, as well as an adaptive kernel. The main faults are represented as planar features and their magnitude-frequency distribution is adjusted to match both the geological and the seismological available data. A full probabilistic seismic hazard analysis (PSHA) is then conducted for the entire state of Israel. Results are presented both in terms of the full spatial distribution, in the form of hazard maps, as well as at specific point locations - near and far from the main sources. Our results show that, based on the available data and balancing seismic moment, the characteristic portion of the main seismic sources in Israel is larger than the classical 94% (Suggested by Youngs and Coppersmith 1985) and is about 96%–98%, depending on the fault. This suggests that the exponential model, as currently used in the building code, leads to a significant underestimation of hazard. We further show that as long as the moment is balanced, hazard results are only mildly sensitive to model choices, such as size of kernel, or type of cutoff between on-fault and off-fault sources. Finally, we suggest using the adaptive-kernel with the Fault-extract interaction, which we believe is the best representation of the currently available geological, seismological and geodetic information.

The effect of alternative seismotectonic models on PSHA results – a sensitivity study for two sites in Israel

Abstract. We present a full probabilistic seismic hazard analysis (PSHA) sensitivity analysis for two sites in southern Israel – one in the near field of a major fault system and one farther away. The PSHA analysis is conducted for alternative source representations, using alternative model parameters for the main seismic sources, such as slip rate and Mmax, among others. The analysis also considers the effect of the ground motion prediction equation (GMPE) on the hazard results. In this way, the two types of epistemic uncertainty – modelling uncertainty and parametric uncertainty – are treated and addressed. We quantify the uncertainty propagation by testing its influence on the final calculated hazard, such that the controlling knowledge gaps are identified and can be treated in future studies. We find that current practice in Israel, as represented by the current version of the building code, grossly underestimates the hazard, by approximately 40 % in short return periods (e.g. 10 % in 50 years) and by as much as 150 % in long return periods (e.g. 10E−5). The analysis shows that this underestimation is most probably due to a combination of factors, including source definitions as well as the GMPE used for analysis.

Transposing an active fault database into a fault-based seismic hazard assessment for nuclear facilities – Part 2: Impact of fault parameter uncertainties on a site-specific PSHA exercise in the Upper Rhine Graben, eastern France

Abstract. We perform a fault-based probabilistic seismic hazard assessment (PSHA) exercise in the Upper Rhine Graben to quantify the relative influence of fault parameters on the hazard at the Fessenheim nuclear power plant site. Specifically, we show that the potentially active faults described in the companion paper (Jomard et al., 2017, hereafter Part 1) are the dominant factor in hazard estimates at the low annual probability of exceedance relevant for the safety assessment of nuclear installations. Geological information documenting the activity of the faults in this region, however, remains sparse, controversial and affected by a high degree of uncertainty. A logic tree approach is thus implemented to explore the epistemic uncertainty and quantify its impact on the seismic hazard estimates. Disaggregation of the peak ground acceleration (PGA) hazard at a 10 000-year return period shows that the Rhine River fault is the main seismic source controlling the hazard level at the site. Sensitivity tests show that the uncertainty on the slip rate of the Rhine River fault is the dominant factor controlling the variability of the seismic hazard level, greater than the epistemic uncertainty due to ground motion prediction equations (GMPEs). Uncertainty on slip rate estimates from 0.04 to 0.1 mm yr−1 results in a 40 to 50 % increase in hazard levels at the 10 000-year target return period. Reducing epistemic uncertainty in future fault-based PSHA studies at this site will thus require (1) performing in-depth field studies to better characterize the seismic potential of the Rhine River fault; (2) complementing GMPEs with more physics-based modelling approaches to better account for the near-field effects of ground motion and (3) improving the modelling of the background seismicity. Indeed, in this exercise, we assume that background earthquakes can only host M < 6. 0 earthquakes. However, this assumption is debatable, since faults that can host M > 6. 0 earthquakes have been recently identified at depth within the Upper Rhine Graben (see Part 1) but are not accounted for in this exercise since their potential activity has not yet been described.

April 2016 Ecuador Earthquake of Moment Magnitude M<sub>w</sub>7.8: Overview and Damage Report

The northern coastal region of Ecuador was affected by a moment magnitude Mw7.8 strong earthquake on April 16, 2016, with a recorded PGA of 1.41 g close to the epicenter. The earthquake was named “Pedernales” because the epicenter was near this small town located in the coastal province of Manabí, with a hypocentral depth of 21 km. Two strong aftershocks of Mw6.7 and Mw6.9 were felt a month later and they continued the destruction caused by the main event. Ecuador is located in a seismic zone with a long history of major earthquakes that have caused a lot of destruction in the last six centuries, the main seismic source comes from the subduction of the Nazca Plate under the South American Plate, but there are also crustal earthquakes and volcanism on the Andes Mountains that cross the country from north to south. In the northern coastal province of Esmeraldas a powerful earthquake of moment magnitude Mw8.8 was recorded in 1906, with a rupture area that has caused strong earthquakes in the 20th and 21st centuries. This paper describes the seismological aspects and the recorded strong ground motions, the geotechnical and infrastructure performance, site amplification and liquefaction on a few places, structural damage and collapse of buildings, extensive non-structural damage (mainly large damage to masonry walls) at high-rise buildings in cities near (100 km) and far away (270 km) from the epicenter, due to local soil conditions and flexible framed reinforced concrete structures.

Seismic sources and stress transfer interaction among axial normal faults and external thrust fronts in the Northern Apennines (Italy): A working hypothesis based on the 1916–1920 time–space cluster of earthquakes

In this study we analyse the main potential seismic sources in some axial and frontal sectors of the Northern Apennines, in Italy. This region was hit by a peculiar series of earthquakes that started in 1916 on the external thrust fronts near Rimini. Later, in 1917–1921, seismicity (up to Mw≈6.5) shifted into the axial zone and clearly migrated north-westward, along the belt of active normal faults. The collection of fault-slip data focused on the active normal faults potentially involved in this earthquake series. The acquired data allowed us to better characterize the geometry and kinematics of the faults. In a few instances, the installation of local seismic networks during recent seismic sequences allowed the identification of the causative faults that are hinted to be also responsible for past earthquakes, particularly in the Romagna region and north-eastern Mugello. The Coulomb stress changes produced by the historical earthquakes generally brought closer to failure all the faults that supposedly caused the main seismic events of 1916–1921. However, the stress change magnitude is generally small and thus the static stress interaction among the main seismic sources is not supported by a significant seismic correlation. Significant stress change loading may be instead inferred for the triggering of a number of seismic events on neighbouring normal faults by the Garfagnana 1920 earthquake. In addition, the computation of the seismic stress changes suggests that seismic events with magnitude≥6 may transmit stresses from the axial normal faults to specific external thrusts and vice versa. It is possible that a correlation may be made between loading applied by the major 1917–1920 extensional ruptures and the increased seismicity on the distal external thrusts.

Scenario-based assessment of buildings' damage and population exposure due to earthquake-induced tsunamis for the town of Alexandria, Egypt

Abstract. Alexandria is the second biggest city in Egypt with regards to population, is a key economic area in northern Africa and has very important tourist activity. Historical records indicate that it was severely affected by a number of tsunami events. In this work we assess the tsunami hazard by running numerical simulations of tsunami impact in Alexandria through the worst-case credible tsunami scenario analysis (WCTSA). We identify three main seismic sources: the western Hellenic Arc (WHA – reference event AD 365, Mw = 8.5), the eastern Hellenic Arc (EHA – reference event 1303, Mw = 8.0) and the Cyprus Arc (CA – hypothetical scenario earthquake with Mw = 8.0), inferred from the tectonic setting and from historical tsunami catalogues. All numerical simulations are carried out in two sea level conditions (mean sea level and maximum high-tide sea level) by means of the code UBO-TSUFD, developed and maintained by the Tsunami Research Team of the University of Bologna. Relevant tsunami metrics are computed for each scenario and then used to build aggregated fields such as the maximum flood depth and the maximum inundation area. We find that the case that produces the most relevant flooding in Alexandria is the EHA scenario, with wave heights up to 4 m. The aggregate fields are used for a building vulnerability assessment according to a methodology developed in the framework of the EU-FP6 project SCHEMA and further refined in this study, based on the adoption of a suitable building damage matrix and on water inundation depth. It is found that in the districts of El Dekhila and Al Amriyah, to the south-west of the port of Dekhila, over 12 000 (13 400 in the case of maximum high tide) buildings could be affected and hundreds of them could sustain damaging consequences, ranging from critical damage to total collapse. It is also found that in the same districts tsunami inundation covers an area of about 15 km2, resulting in more than 150 000 (165 000 in the case of maximum high tide) residents being exposed.

Landslides and other geomorphologic and hydrologic effects induced by earthquakes in Portugal

A geodatabase of landslides and other geomorphologic and hydrologic phenomena, triggered by earthquakes in Portugal, was built based on the analysis of historical records. As a result, 28 earthquake-triggered landslides were identified; the landslides were associated with 10 earthquakes that occurred between 382 and 1969. However, the landslide recognition and the definition of its precise location were difficult due to the time elapsed since the occurrence of most of the landslides and to the changes in the morphologic features of the affected slopes. Despite these limitations, it was possible to establish that most landslides were disrupted landslides that affected jointed rocks. The earthquake-triggered landslides are concentrated in the southern and central areas of Portugal, close to the main seismic sources, as opposed to the recurrent landslides induced by precipitation in the northern and central parts of the country. Other identified geomorphologic and hydrologic phenomena induced by earthquakes in mainland Portugal include: ground cracks (48 cases), liquefaction (75 cases) and water spring and fountains anomalies (262 cases). These secondary earthquake effects were reported for 5 earthquakes, but these may be considered representative only in two cases: the earthquakes of 1 November 1755 and of 23 April 1909. The distance between the site of the above-mentioned phenomena and the corresponding earthquake epicentre varies according to the earthquake magnitude and the type of phenomena. The 1755 earthquake (Mw 8.7) induced geomorphologic and hydrologic effects farther from the epicentre (maximum distance of 658 km) than the 1909 earthquake (Mw 6), whereas the effects associated with the latter were felt much closer to the earthquake epicentre (maximum distance of 172 km). The hydrologic effects brought about by the 1755 earthquake probably occurred throughout the country (distance of at least 658 km from epicentre) in contrast to ground cracks (526 km) and liquefaction (504 km).

PROBABILISTIC SEISMIC HAZARD ASSESSMENT FOR YANGON REGION, MYANMAR

According to the seismicity and the records of the previous considerably high magnitude earthquakes, Yangon Region can be regarded as the low to medium seismicity region. Moreover, tectonically the region is surrounded by the subduction zone between the Indian Plate and Burma Plate to the west and the right lateral Sagaing fault to the east. The most significant earthquake happened around this region is the Bago earthquake of 5th May, 1930 with the magnitude of 7.3. This earthquake caused 500 casualties and great destruction in Bago. However, considerable damage and 50 deaths were recorded in Yangon. It was originated from the Sagaing fault. The seismic hazard analysis is performed for Yangon Region by applying the probabilistic way. In conducting seismic hazard analysis, firstly the most possible seismic sources are identified and the seismic source parameters are then determined for each sources. Based on the seismicity, focal mechanism study of the previous events, and the geological data, the main seismic sources to cause the earthquake potentials for this region are subduction zone of Indian Plate beneath Burma Plate, Sagaing fault, and Kyaukkyan fault. Including those sources, thirteen areal seismic sources and two fault sources are identified. After that the seismic source parameters such as the seismicity parameters of a- and bvalues, maximum magnitude of earthquake potentials and earthquake recurrence parameters for certain magnitude of each seismic source are estimated. The seismic hazard analysis is carried out for 10% and 2% probabilities of exceedance in 50 years and the seismic hazard maps are represented in terms of peak ground acceleration and spectral acceleration at the periods of 0.2s and 1.0s for those recurrence intervals.

The primary role of the Paganica-San Demetrio fault system in the seismic landscape of the Middle Aterno Valley basin (Central Apennines)

The Paganica normal fault, that caused the April 9, 2009, L'Aquila earthquake (Central Apennines), is part of the Paganica-San Demetrio fault system (PSDFS). All available data useful to constrain the Quaternary activity of the PSDFS were reviewed, with the aim to point out its role in the landscape evolution of the L'Aquila region and particularly of the Middle Aterno Valley basin. Uplifted remnants of a low relief landscape (Summit Paleosurface, SP) constrain to about 800 m the total offset of the PSDFS since the late Lower Pleistocene. Also, a sedimentary cycle ended at the beginning of Middle Pleistocene is now displaced about 550 m along the PSDFS, whereas alluvial deposits containing tephra layers (560–360 ka) have been displaced more than 150 m by the Paganica fault segment.The long term slip rate values based on this evidence (ranging between 0.4 and 0.9 mm/y), together with the short term slip rate values based on displacements along Late Pleistocene geomorphic features and deposits, have shown that the PSDFS i) has played a primary role in the landscape evolution of the area, despite its minor geomorphic appearance, ii) is linked at depth with other important listric faults located in the PSDFS footwall (Assergi and Campo Imperatore faults) that in this paper are interpreted as secondary faults.This result has relevant seismotectonic implications for the Middle Aterno Valley basin. The PSDFS is the main seismic source of the basin and controls also the seismic potential of other linked structures, including the Assergi and Campo Imperatore faults.

Los temblores sentidos en Costa Rica durante: 1973-1983, y su relación con la sismicidad del país

Earthquakes in che metropolitan arca of San José, felt with intensity equal or higher than III (MM) for che period 1973-1983, mere localized by means of a local network of stations and portable seismographs, combined with information talen from international bulletins (ISC, PDE, EDR). The distribution of seismi-city and its relation with morphotectonic units allowed a geografic zonification of the country, and mas found that the main seismic sources were associated with subduction and local faults. The last eme proved lo be the most dangerous seismic source and the borders of the internal basins the ones more potentially risky, as evidenced by the Tilarán (1973) and the Pérez Zelechán (1983) earthquakes. The central pacific coast represents a seismic gap and aleo has a high seismic poten-tial. Seismic sequences of the "swarm" type or precursors, main quake and after-shocks hace been observed in the Central Valley, mainly oriented toman d the volca nic area. The type of seismicity observed for the strong earthquakes in the subI duction zone has been, majo quake and aftershocks. The mean value of the felt earthquakes (I= III) in the metropolitan aren of San José mas 14.7 , Per Yeat, that is, about a monthly earthquake. Pee highest seismic activity (VI - VII) mas felt during April 1983. d.le td violent activity around Golfito (Ms = 7.3). The year 1983 has been the most ac,we of tha wbole period as measured by the energy release and material damages, mainly che arcas of Golfito-Osa ami Pérez Zele-dón. However, landslides due o Tilarán's earthquake (1973) yie1ded the largest number of casualties (23), during the whole period.

Accelerograms For Building Design For Hard SoilIn Mexico City

A method of disaggregation probabilistic seismic hazard assessment is applied to determine the most probable magnitude, M, and distance, R, of certain seismic events that can generate a fixed seismic intensity (spectral ordinate) associated to an exceedance rate for hard soil in Mexico City. The main seismic sources which affect the firm land structures are determined. In addition, the following criteria are used: i) to establish in which structural periods the spectral ordinates should be used so that the response of the expected motion does not exceed the uniform hazard spectra and ii) to find the minimum number of design strong ground motions that must be considered to include spectral ordinates of uniform hazard spectra.

Seismic hazard reappraisal from combined structural geology, geomorphology and cosmic ray exposure dating analyses: the Eastern Precordillera thrust system (NW Argentina)

Because earthquakes on large active thrust or reverse faults are not always accompanied with surface rupture, paleoseismological estimation of their associated seismic hazard is a difficult task. To improve the seismic hazard assessments in the Andean foreland of western Argentina (San Juan Province), this paper proposes a novel approach that combines structural geology, geomorphology and exposure age dating. The Eastern Precordillera of San Juan is probably one of the most active zones of thrust tectonics in the world. We concentrated on one major regional active reverse structure, the 145 km long Villic´um-Pedernal thrust, where this methodology allows one to: (1) constrain the Quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes; (2) analyse the geometry and the geomorphic characteristics of the Villic´um-Pedernal thrust; and (3) estimate uplift and shortening rates through determination of in situ-produced 1 0 Be cosmic ray exposure (CRE) ages of abandoned and uplifted alluvial terraces. From a structural point of view, the Villic´um-Pedernal thrust can be subdivided into three thrust portions constituting major structural segments separated by oblique N40°E-trending fault branches. Along the three segments, inversion of fault slip data shows that the development of the Eastern Precordillera between 31°S and 32°S latitude is dominated by a pure compressive reverse faulting stress regime characterized by a N110° ′ 10°E-trending compressional stress axis (σ 1 ). A geomorphic study realized along the 18 km long Las Tapias fault segment combined with CRE ages shows that the minimum shortening rate calculated over the previous ∼20 kyr is at least of the order of 1 mm yr - 1 . An earthquake moment tensor sum has also been used to calculate a regional shortening rate caused by seismic deformation. This analysis of the focal solutions available for the last 23 yr shows that the seismic contribution may be three times greater than the shortening rate we determined for the Las Tapias fault (i.e. ∼3 mm yr - 1 ), suggesting that the San Juan region may have experienced a seismic crisis during the 20th century. Moreover, the ramp that controls the development of the Eastern Precordillera appears to be one of the main seismic sources in the San Juan area, particularly the 65 km long Villic´um-Las Tapias segment. A first-order evaluation of the seismic hazard parameters shows that this thrust segment can produce a maximum earthquake characterized by a moment magnitude of ∼7.3 (′0.1) and a recurrence interval of 2.4 (′1.5) kyr. This part of the Villic´um-Pedernal ramp may have ruptured during the Ms =7.4, 1944 San Juan earthquake producing very few surface ruptures and only distributed flexural slip deformation on to the Neogene foreland bedding planes between the Eastern Precordillera and Pie de Palo.

Faulting, shallow seismicity and seismic hazard analysis for the Costa Rican Central Valley

Faulting, shallow seismicity (0–30km), and seismic hazard of the Costa Rican Central Valley were analyzed. Faults in the study area are oriented northwest or northeast. There is an active fault system in the south flank of the Central Volcanic Ridge and another in the north flank of the Talamanca Ridge. Faults of these systems have generated 15 destructive earthquakes in the area during the last 228 years all of them shallow and their locations show one cluster near the Poas Volcano and another southward the Central Valley. These earthquakes have damaged cities of the Central Valley, two of them destroyed Cartago city and almost 1000 people were killed. Regarding recent seismicity, there are three main seismic sources at the Central Volcanic Ridge: Irazu, Bajo de la Hondura and Poas and other three in the Talamanca Ridge: Puriscal, Los Santos and Pejibaye.A seismic hazard map for the Metropolitan Area of San José has been elaborated, based on local tectonic and seismic information. The area for the hazard computation covers an area of 20×15km2 and includes the zone where the most population and socioeconomic activities are concentrated. The computation analysis are based on areas zones and faults, each one characterized by recurrence parameters, geometry, minimum and maximum magnitude and source depth. A recent local spectral attenuation model, which includes relations for shallow crustal sources and subduction zone earthquakes, has been applied in this study. The seismic hazard results are presented in terms of contour plots of estimated peak ground acceleration (PGA) for bedrock conditions for return period of 50, 100 and 500 years. In the Central Park of San Jose City the following PGA values were found: 0.29g for 50 years, 0.36g for 100 years, and 0.53g for 500 years.