Local Seismicity Research Articles

Moment Capacity Ratio at Column – Beam Joint in a RC Framed Structure

Beam-column joint is the gap in the modern ductile design of building. Especially under the earthquake loading this is more susceptible to damage. Due to brittle nature of failure this type of failure cannot be afford. Since 1970’s this areas is under the light of research, but with the paper of Park and Paul, It got momentum. But still due to versatile nature of the joints core behaviour, the problem is still persisting. Though many international codes recommend the moment capacity ratio at beam column joint to be more than one, There are discrepancies among the major international codes with regard to MCR. Indian standard codes for design of RC framed buildings are silent on this aspect. Draft 13920 (2014) code suggests a value of MCR similar to other international codes without proper theoretical basis. Hence a rational study is required on the values of MCR. A computationally attractive procedure for calculating flexural capacity of column developed for determining MCR at a beam-column joint. To reach at an appropriate and acceptable MCR for capacity design of RC framed building reliability based approach is done. This research deals with the fragility and reliability analysis of five, seven and ten storey RC frames designed using various values of MCR ranging from 1.0 to 3.2. The RC frames are designed as per IS 1893 (2002) for all seismic zones. Hazard curves required of various seismic location in India (like zone II, III, IV and V) has been selected from National Disaster Management Authority, Government of India. Seismic risk assessment of all the designed buildings is conducted and based on the achieved Reliability Index and the Target Reliability Index minimum value of Moment Capacity Ratio (MCR) is suggested.

Seasonal patterns of 3D crustal motions across the seismically active southeastern Tibetan Plateau

We study the three-dimensional (3D) seasonal crustal deformation and its relationship with local seismicity based on continuous GPS (cGPS), mass loading models and seismicity data covering the period from 2010 to 2018 in the southeastern Tibetan Plateau (SETP). Of the 3D seasonal motions, vertical motion is the most significant, followed by the northward and eastward directions, with average annual amplitudes from cGPS of 9.28 mm, 1.62 mm and 0.72 mm, respectively. The annual motions predicted by the hydrological mass loading (HYDL) model in all directions are about 3–4 times larger than that of the atmospheric mass loading (ATML) model, but smaller than the cGPS measurements. The initial phase difference between the cGPS and loading models can be nearly eliminated by combining the ATML and HYDL models which helps to fit the cGPS results better. The average weighted root-mean-square (WRMS) reduction ratios of the ATML, HYDL model and their combination are 0.30, 0.77 and 0.88, respectively. This indicates that the seasonal vertical crustal motions observed by cGPS in the SETP are dominated by seasonal loading motions from the ATML and HYDL. In four subregions where no M > 4 earthquakes had occurred in the past decade, we observe an anti-correlation between equivalent water height (EWH) from the GRACE (Gravity Recovery and Climate Experiment) and the vertical displacement derived by cGPS. It shows an anti-correlation relationship between the EWH and seismicity in three subregions and positive correlation in one subregion, which underscores the importance of hydrological loading on seismicity in the SETP.

Earthquake detection in the Jiangsu region, China using graphics-processing-unit-based Match & Locate and rapid earthquake association and location*

Earthquake detection and location are essential in earthquake studies, which generally consists of two main classes: waveform-based and pick-based methods. To evaluate the ability of two different methods, a graphics-processing-unit-based Match & Locate (GPU-M&L) method and a rapid earthquake association and location (REAL) method are applied to continuous seismic data recorded by 24 digital seismic stations from Jiangsu Seismic Network during 2013 for comparison. GPU-M&L is one of waveform-based methods by waveform cross-correlations while REAL is one of pick-based method to associate arrivals of different seismic phases and locate events through counting the number of P and S picks and travel time residuals. Twenty-six templates are selected from the Jiangsu Seismic Network local catalog by using the GPU-M&L. The number of newly detected and located events is about 2.8 times more than those listed in the local catalog. We both utilize a deep-neural-network-based arrival-time picking method called PhaseNet and a short-term/long-term average (STA/LTA) trigger algorithm for seismic phase detection and picking by applying the REAL. We then refine seismic locations using a least-squares location method (VELEST) and a high-precision relative location method (hypoDD). By applying STA/LTA and PhaseNet, 1006 and 1893 events are associated and located, respectively. The newly detected events are mainly clustered and show steeply dipping fault planes. By analyzing the performance of these methods based on long-term continuous seismic data, the detected catalogs by the GPU-M&L and REAL show that the magnitudes of completeness are 1.4 and 0.8, respectively, which are smaller than 2.6 given by the local catalog. Although REAL provides improvement compared with GPU-M&L, REAL is highly dependent on phase detection and picking which is strongly affected by signal-noise ratio (SNR). Stations at southeast of the study region with low SNR may lead to few detections in the same area.

Earthquake-induced loss assessment of steel buildings designed to Eurocode 8

The main objective of this paper is to understand and characterize the level of expected earthquake-induced economic losses of steel buildings designed to Eurocode 8. This is attained through the use of a comprehensive population of case-study buildings that considers different lateral load resisting systems, plan configurations and building heights. Different locations of low and high seismicity, reflecting the majority of the European seismic reality, are considered. The results indicate that EC8-compliant steel moment-resisting frame (MRF) buildings and concentrically-braced frame (CBF) buildings generally exhibit good seismic performance in terms of mean annual frequency of reaching collapse. Although all buildings showed good performance against collapse, when compared to the requirements present in literature, clear indications on a lower performance of chevron-braced CBFs are observed. Furthermore, low expected annual losses (EALs) are recorded across the population of steel buildings considered, with a predominant contribution of losses coming from structural and non-structural damage. Based on the findings reported in this paper, different insights towards the incorporation of loss performance objectives in the European seismic design code are provided.

Recent Advances and Challenges of Waveform‐Based Seismic Location Methods at Multiple Scales

AbstractSource locations provide fundamental information on earthquakes and lay the foundation for seismic monitoring at all scales. Seismic source location as a classical inverse problem has experienced significant methodological progress during the past century. Unlike the conventional traveltime‐based location methods that mainly utilize kinematic information, a new category of waveform‐based methods, including partial waveform stacking, time reverse imaging, wavefront tomography, and full waveform inversion, adapted from migration or stacking techniques in exploration seismology has emerged. Waveform‐based methods have shown promising results in characterizing weak seismic events at multiple scales, especially for abundant microearthquakes induced by hydraulic fracturing in unconventional and geothermal reservoirs or foreshock and aftershock activity potentially preceding tectonic earthquakes. This review presents a comprehensive summary of the current status of waveform‐based location methods, through elaboration of the methodological principles, categorization, and connections, as well as illustration of the applications to natural and induced/triggered seismicity, ranging from laboratory acoustic emission to field hydraulic fracturing‐induced seismicity, regional tectonic, and volcanic earthquakes. Taking into account recent developments in instrumentation and the increasing availability of more powerful computational resources, we highlight recent accomplishments and prevailing challenges of different waveform‐based location methods and what they promise to offer in the near future.

Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismically active region of northern Italy (MwMAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH4–dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of 4He in the crust and a long-lasting diffusion through the crust are not the main processes that rule the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.

S-Wave Attenuation Field and Seismotectonics of Eastern Anatolia

This work studies the characteristics of seismic coda waves attenuation in Eastern Anatolia. The short-period coda waves from about 400 earthquakes that occurred in Eastern Anatolia between 1989 and 2018 are analyzed. The three-component digital records of seismic events with M ≥ 3.5, recorded at epicentral distances of 10–600 kilometers, are used. S-wave attenuation was estimated from short-period coda envelopes (including those of Lg wave coda) at a frequency of ~1 Hz. The structural features of the S-wave attenuation field in the upper mantle of the Lake Van area are similar to those for other seismoactive zones. The attenuation field is represented by blocks, which have weak attenuation and are subisometric in plan view, and strong attenuation zones; in the case of blocks, attenuation decreases in the direction from the boundaries to the central of these blocks (Q-factor is up to 300–700). Among the zones of strong attenuation, which are usually linearly elongated, the most noticeable are the en echelon zone (north of 39° N), associated with the North Anatolian Fault; here, QS ~ 80–110 (up to QS ~ 60 in the north); and the Bitlis-Zagros fold and thrust belt (QS ~ 60–70). Sources of the strongest earthquakes with M > 6.0, including the 1976 Caldiran–Muradiye and 2011 Van earthquakes, are confined to the boundaries of the blocks. The attenuation field structure corresponds to the velocity field structure: low-velocity anomalies correspond to low-Q zones. Vertically elongated volumes, with which intensive localized seismicity (earthquake swarms) is associated and along which deep fluids can migrate, are revealed. Seismic activation in such volumes precedes large seismic events (for example, the 2011 Van earthquake). It is assumed that such volumes can be interpreted as local seismogenic sources.

К оценке протяженности по латерали участков наличия углеводородов по данным сейсмической локации нефти и газа (СЛОНГ)

In the method of seismic location of oil and gas (SLONG), intended for predicting the oil and gas prospects of objects based on the data of seismic exploration method CDP-2D, the length of the laterally allocated sections of the presence of hydrocarbons is determined by statistical processing of all graphs of the energy distribution by depth of micro tremors. However, in this case, the results of processing depend on the repetition factor of impulse accumulations of fluctuations used. Therefore it is proposed to use a different approach to determine the length of the lateral sections of the presence of HC based on the processing of micro tremors recorded during CDP-2D, according to the method proposed by G.V. Vedernikov, E.A. Khagoev and T.I. Chichinina. As a result of applying this technique, it was found that all oil and gas fields are accompanied by anomalies in the micro tremors field in terms of energy and spectral characteristics of vibrations. Not stopping on the possibility of solving the inverse problem, we only note that according to this provision, the presence of a deposit should lead to anomalies in the micro tremors field and, in particular, to an increase in the energy oscillation. Therefore, if the presence of a deposit is established in any way, then we should expect the appearance of a section with an increased energy oscillation, which can be used to estimate the length of the deposit laterally. The fact of the presence of a deposit can be established by the results of processing the same CDP-2D profile using SLONG technology. If a positive SLONG result is obtained, a section with an increased energy of micro tremors should appear on the corresponding section of the profile, which will lead to the solution of the problem. Key words: Location, microtremors, length, anomaly.

Exploring for the Future—a new oil and gas frontier in northern Australia

Exploring for the Future (EFTF) is a four-year, AU$100.5 million initiative from the Australian Government conducted by Geoscience Australia in partnership with state and Northern Territory government agencies, CSIRO and universities to provide new geoscientific datasets for frontier regions. As part of this program, Geoscience Australia acquired two new seismic surveys that collectively extend across the South Nicholson Basin (L120 South Nicholson seismic line) and into the Beetaloo Sub-basin of the McArthur Basin (L212 Barkly seismic line). Interpretation of the seismic has resulted in the discovery of new basins that both contain a significant section of presumed Proterozoic strata. Integration of the seismic results with petroleum systems geochemistry, structural analyses, geochronology, rock properties and a petroleum systems model has expanded the knowledge of the region for energy exploration. These datasets are available through Geoscience Australia’s newly developed Data Discovery Portal: an online platform delivering digital geoscientific information, including seismic locations and cross-section images, and field site and well based sample data. Specifically for the EFTF energy project, a petroleum systems framework with supporting organic geochemical data has been built to access source rock, crude oil and natural gas datasets via interactive maps, graphs and analytical tools that enable the user to gain a better and faster understanding of a basin’s petroleum prospectivity.

Link Slab Capacity In Bridges Supported By Lead Rubber Bearing And Elastomer

Debris accumulation in bridge slab gaps which use expansion joints can restrain deck expansion, causing undesirable forces on floor deck and damage to the structure. In order to avoid the worst possibility that can occur, an alternative using link slab is utilized. The use of link slab at high level seismic force location, requires the Seismic Isolation System on bridge to reduce the seismic force. The application of Seismic Isolation System can be conducted by Lead Rubber Bearing (LRB) type of seismic isolator. This study compares the use of Lead Rubber Bearing (LRB) and elastomer on bridge link slabs against the dimension of the link slab. In this study structural modeling used 2 models: bridges supported by elastomer and bridges supported by LRB with software-made. The link slab analysis approach used were analytical methods or classical methods. Based on results of the analysis, the width of the crack that occured on bridge supported by LRB is 0.218 mm while on the bridge supported by elastomer is 0.269 mm. The use of Lead Rubber Bearing (LRB) type of support will give more advantages to the design of the link slab since it results in smaller crack design criteria.

Сейсмические воздействия от землетрясений разной удалённости на территорию Беларуси

The control of the geological environment is one of the most important tasks of the seismological monitoring in the territory of Belarus. The seismological monitoring in Belarus is carried out with a system of continuous round-the-clock computer-aided observations of the seismic events of natural and artificial origin in a wide range of distances and energies. The major task of the seismic environment observation network in the territory of Belarus is recording of the distant, regional, and local seismic events. Since 1966 till the present, the environmental monitoring network of the seismic stations located within the territory of Belarus recorded 60,876 seismic events in various regions of the Earth. The data obtained from strong distant, regional and local seismic events recorded by the seismic stations were analyzed, and the intensity of the seismic impact of the recorded earthquakes upon the studied territory was calculated using the N.V. Shebalin’s formula. The results of investigations performed were used to assess the seismic impact upon the territory of Belarus. A catastrophic earthquake that occurred in the Northern Sumatra western coasts in December 26, 2004, should be mentioned first among the largest earthquakes of the Earth that exerted the strongest impact upon the studied territory. An earthquake in southern Greece on January 8, 2006, and the second one in Turkey on October 23, 2011, are the events that should be mentioned among the strongest earthquakes of Europe and its adjacent areas which impact was important for the territory under study. An earthquake in Rumania on September 23, 2016, was one of the regional earthquakes that had a strong influence on the territory of Belarus. The results of the quantitative assessment of the ground shaking from earthquakes differently distant from the territory of Belarus are used for upgrading maps of the general and local seismicity, as well as for solving several other scientific and applied problems.

Сейсмологические наблюдения в Антарктиде

The history of seismological observations development in Antarctica is shown. Maps of the existing seismic stations and earthquake epicenters location on the mainland territory for the instrumental monitoring period (1956–2018) are presented according to data from International centers. Russian seismic stations monitor major earthquakes around the globe, earthquakes in the seismic zone around Antarctica, and local seismic phenomena in Antarctica, including local earthquakes and ice sheet ruptures. Since 1999, the Novolazarevskaya seismic station has been equipped with digital equipment. An analysis of the sixth continent seismicity was made; the records of the Antarctic strongest earthquakes (2007, 2008, and 2012) by the GS RAS stations Mirny and Novolazarevskaya were shown.

A new 3-D P-wave velocity model for the Gulf of Cadiz and adjacent areas derived from controlled-source seismic data: application to nonlinear probabilistic relocation of moderate earthquakes

SUMMARYThe SW Iberian margin is well known for its complex tectonic setting and crustal structure and by the occurrence of moderate magnitude earthquakes and some great tsunamigenic earthquakes. Fortunately, many seismic reflection and refraction profiles have been carried out, providing detailed information about the crustal structure of the main geologic domains in this region. These studies show a first-order variation due to the transition from oceanic to continental domain, large-scale heterogeneities within the crust and an irregular Moho topography. Routine earthquake locations in this area have been usually computed using a general 1-D velocity model which is clear that cannot account for such a heterogeneous structure. In addition, regional seismic stations used to locate the Gulf of Cadiz seismicity are on land and far away to the east, implying large azimuthal gaps and distances. In this context, a 3-D approach seems necessary to properly solve the crustal velocity field and improve earthquake location in this area. With this purpose, we present a new digital 3-D P-wave velocity distribution for the crust and uppermost mantle derived from previously published controlled-source seismic experiments carried out in SW Iberia and the Gulf of Cadiz over the last 40 yr. We have reviewed more than 50 wide-angle and multichannel seismic reflection and refraction profiles and digitized the most significant published 2-D seismic velocity models, performing an updated compilation of crustal parameters (P-wave velocities and geometry and depth of the main crustal interfaces). These velocities as a function of position and depth have been interpolated using ordinary kriging algorithm to obtain, in the form of a regular georeferenced 20 × 20 × 1 km grid spacing, a high-resolution 3-D P-wave velocity distribution for the crust and uppermost mantle and a continuous Moho depth map of the whole area of this study (33°N–41°N latitude and 15°W–5°W longitude). Since current seismic location tools allow the implementation of 3-D grid structures, we have applied our 3-D model to relocate a selection of moderate earthquakes occurred in the studied region using a probabilistic nonlinear method. In the Gulf of Cadiz area the probabilistic approximation provides maximum likelihood hypocentres located within the uppermost mantle with the majority of depths ranging between 20 and 45 km. This model would subsequently be implemented at the Spanish Seismic Network for the routine relocation of the seismicity of the area.

Fast MW estimation of microearthquakes recorded around the underground gas storage in the Montello-Collalto area (Southeastern Alps, Italy)

Underground fluid injection and extraction is able to change pore fluid pressure at depth and make faults unstable, due to friction-force reduction, with an increased possibility of triggering earthquakes. Studying the local seismicity, down to microearthquakes, and stress field in areas where such activities are developed are essential steps to discriminate between natural and induced events. In this context, the moment magnitude (MW) is a key-parameter to both evaluate the energy balance and the stress involved in earthquake rupture process and assess seismic hazard accurately. Here, we focus on the fast MW estimation of microearthquakes recorded around the underground gas storage of Collalto (Northeastern Italy) by a dedicated seismic monitoring network. The area of Montello-Collalto, where this industrial activity is carried out, is densely populated and characterized by relevant seismic hazard. We compute MW from the response spectra (SA) calculated at fixed periods (i.e., 1.0 and 0.3 s); we show that log (SA) and MW scale as 2/3 and extend our method to microseismicity by using response spectra at 0.1 s. We eventually estimate MW for 1659 events (0.4 ≤ MW ≤ 3.5) and find that ML and MW scale as 2/3 too. The discrepancy between these two magnitude scales affects both the Gutenberg-Richter parameters and completeness magnitude estimations; therefore, it has consequences when those quantities are used for physical interpretation. Our procedure shows to be efficient and suitable to be implemented within standard routine analyses of real-time monitoring and feed decision-making processes about plant management, such as the traffic light protocols.

Lateral Variations in Lithospheric Mantle Structure Control the Location of Intracontinental Seismicity in Australia

AbstractDespite decades of study, the mechanisms that lead to the localization of intracontinental seismicity remain vigorously debated. We find a very strong correlation between the attenuation of teleseismic P waves and the occurrence of intraplate seismicity in Australia. The regions with the highest attenuation host ~2 orders of magnitude more earthquakes per unit of area than the least attenuating regions. We argue that the attenuation we observe is produced by lateral variations in the thickness and/or viscosity of the lithospheric mantle and further suggest that the correlation we document implies that lithospheric mantle structure exerts first‐order controls on the localization of intraplate seismicity.

Earthquakes Felt in the Juan Fernandez Islands: Where Are They Coming from?

Abstract The Juan Fernandez Islands (JFI) are located in the Pacific Ocean 675 km west of the Chilean coast. This archipelago has historically been affected by large tsunamis. Robinson Crusoe Island (RCI), the main island of the JFI, was first inhabited in 1749. Since then, several tsunamis have destroyed RCI port structures and sometimes caused deaths. Ground shaking perceived by the inhabitants has preceded some tsunami arrivals. Seismological instrumentation was temporarily deployed on RCI in 1999, and a permanent station has been operating since 2014. Here, we use these data to characterize the seismic waves that arrive at the JFI and to determine whether shaking perception could be used as a tsunami early warning system. We compute peak ground accelerations (PGAs) from P, S, and T waves generated by Peruvian and Chilean earthquakes and find that the largest ground shakings are mostly related to T‐wave arrivals, which correlate with macroseismic modified Mercalli intensities lower than III. From the analysis of PGAs and macroseismic intensities, we conclude that shaking perception can be associated with large megathrust earthquakes, subduction events generated in the deep zone of seismogenic contact, and local seismicity. Unfortunately, potential tsunami earthquakes that occur on the Chilean coast will not be felt on RCI. Consequently, ground shaking in the JFI would not be a good proxy for tsunami warning, and a robust tsunami early warning system is necessary for RCI.

Along‐Arc Heterogeneity in Local Seismicity across the Lesser Antilles Subduction Zone from a Dense Ocean‐Bottom Seismometer Network

AbstractThe Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

An Empirical Formula to Classify the Quality of Earthquake Locations

Abstract Providing a quantitative estimate of earthquake location quality is not a simple task. Traditional methods, used in literature, are not exhaustive and depend on a subjective point of view, because they consist of empirical choice of quality thresholds of different estimators of location uncertainty. However, we notice that these estimators are correlated with one another, implying the need to combine them to obtain a numerical and impartial estimate of the quality of a seismic location. Therefore, we provide a formula that associates a quality factor (qf) value with a seismic location, which is based on the combination of a set of uncertainty estimators, suitably normalized. We apply the criterion to two different‐type and different‐scale earthquake catalogs, located by two different methods, obtaining encouraging results. The qf parameter definition is a fast, simple, and objective instrument to provide a user‐friendly classification of location quality. Thus, the qf could represent a powerful tool for routine monitoring location computation.

Seismic location and tracking of snow avalanches and slush flows on Mt. Fuji, Japan

Abstract. Avalanches are often released at the dormant stratovolcano Mt. Fuji, which is the highest mountain of Japan (3776 m a.s.l.). These avalanches exhibit different flow types from dry-snow avalanches in winter to slush flows triggered by heavy rainfall in late winter to early spring. Avalanches from different flanks represent a major natural hazard as they can reach large dimensions with run-out distances up to 4 km, destroy parts of the forest, and sometimes damage infrastructure. To monitor the volcanic activity of Mt. Fuji, a permanent and dense seismic network is installed around the volcano. The small distance between the seismic sensors and the volcano flank (<10 km) allowed us to detect numerous avalanche events from the seismic recordings and locate them in time and space. We present the detailed analysis of three avalanche or slush flow periods in the winters of 2014, 2016, and 2018. The largest events (size class 4–5) are detected by the seismic network at maximum distances of about 15 km, and medium-size events (size class 3–4) within a radius of 9 km. To localize the seismic events, we used the automated approach of amplitude source location (ASL) based on the decay of the seismic amplitudes with distance from the moving flow. The recorded amplitudes at each station have to be corrected by the site amplification factors, which are estimated by the coda method using data from local earthquakes. Our results show the feasibility of tracking the flow path of avalanches and slush flows with considerable precision (on the order of magnitude of 100 m) and thus estimating information such as the approximate run-out distance and the average front speed of the flows, which are usually poorly known. To estimate the precision of the seismic tracking, we analyzed aerial photos of the release area and determined the flow path and run-out distance, estimated the release volume from the meteorological records, and conducted numerical simulations with Titan2D to reconstruct the dynamics of the flow. The precision as a function of time is deduced from the comparison with the numerical simulations, showing mean location errors ranging between 85 and 271 m. The average front speeds estimated seismically, which ranged from 27 to 51 m s−1, are consistent with the numerically predicted speeds. In addition, we deduced two scaling relationships based on seismic parameters to quantify the size of the mass flow events. Our results are indispensable for assessing avalanche risk in the Mt. Fuji region as seismic records are often the only available dataset for this natural hazard. The approach presented here could be applied in the development of an early-detection and location system for avalanches based on seismic sensors.

Microseismicity analysis in the geothermal area of Torre Alfina, Central Italy

The geothermal field of Torre Alfina is located in central Italy at the northern edge of the Vulsini Volcanic District, the northernmost area of the so-called Quaternary Roman Co-Magmatic Province. In the framework of a medium-enthalpy geothermal exploitation project, INGV installed a local seismic network close to the future geothermal production site for monitoring natural local seismicity. In this paper, we show the results of a study of the microseismicity recorded from June 2014 to May 2016 in a small area of about 10 km2 around Torre Alfina. Analyzing seismic signals recorded by a local temporary network of ten short-period stations and by four permanent stations of the INGV national seismic network, we detected 846 local earthquakes. Then, we accurately relocated 799 events using HypoDD code. Our results show that the region of Torre Alfina is characterized by intense microseismicity, with hypocentral depths between 3 and 7 km and with moderate magnitudes between Md = 0.1 and ML = 2.8. Moreover, more than half of the earthquakes are grouped into six main swarm-like clusters each lasting few days. Furthermore, we computed 36 well-constrained fault plane solutions, which show a clear transtensional deformation regime in the whole study area. Three main tectonic directions have been evidenced from the focal mechanisms analysis: E-W, WSW-ENE, and NW-SE. The understanding of the seismogenic structural setting of the Torre Alfina geothermal field, and the study of its background natural seismicity can be of great importance in recognizing any possible future seismicity induced by the exploitation of the field.