Revised seismotectonic model of NE Italy and W Slovenia based on focal mechanism inversion

The study is focusing on the stress and strain inversions from focal mechanisms in a revised seismotectonic zonation of northeastern Italy and western Slovenia. The recent increase of monitoring capability of the local seismic network, the updated geological-structural model of the area, and the novelties emerged from studies on the spatial organization of the seismicity allowed a redefinition of the seismotectonic zones. The stress and strain tensors inversion is inferred from 203 focal mechanisms, corresponding to earthquakes occurred between 1984 and 2016 with coda-duration magnitude range from 2.0 to 5.6. The inverted stress domains reveal an articulated picture of the interaction of the Adria microplate with the Eurasian plate. A dominant strike-slip stress field characterizes the eastern part of the area, while the seismotectonic zones of the central part are undergoing to thrusting regime. The stress pattern inferred in the western part of the study area outlines a complex picture with prevailing strike-slip regime and dominant compression only in a seismotectonic zone. The comparison of stress and strain tensor orientations evidences a relative uniformity of the crustal strength in the eastern and northwestern zones of the study area. The central and western zones appear to be characterized by planes of mechanical weakness not favorably oriented for failure with respect to the stress tensor.     

Stability of premonitory seismicity pattern and intermediate-term earthquake prediction in Central Italy

The algorithm CN makes use of normalized functions. Therefore the original algorithm, developed for the California-Nevada region, can be directly applied, without adjustment of the parameters, to the determination of the Time of Increased Probability (TIP) of strong earthquakes for Central Italy. The prediction is applied to the events with magnitudeMM ₀=5.6, which in Central Italy have a return period of about six years. The routinely available digital earthquake bulletins of the Istituto Nazionale di Geofisica (ING), Rome, permits continuous monitoring. Here we extend to November 1994 the first study made by Keilis-Boroket al. (1990b). On the basis of the combined analysis of seismicity and seismotectonic, we formulate a new regionalization, which reduces the total alarm time and the failures to predict, and narrows the spatial uncertainty of the prediction with respect to the results ofKeilis-Borok et al. (1990b).The premonitory pattern is stable when the key parameters of the CN algorithm and the duration of the learning period are changed, and when different earthquake catalogues are used.The anlysis of the period 1904–1940, for whichM ₀=6, allows us to identify self-similar properties between the two periods, in spite of the considerably higher seismicity level of the earlier time interval compared with the recent one.     

Spatially smoothed seismicity modeling of seismic hazard in Slovenia

A progress report on the mapping effort for construction of a peak ground acceleration (PGA) map of Slovenia for 475-year return period for rock and firm soil is presented. The methodology is similar to that recently applied in Central and Eastern United States. It is based on historical seismicity spatially smoothed to different length scales. The procedure is described by Frankel (1995). He uses the characteristic earthquake recurrence relationship and in his firt version four different seismicity models. We also use four seismicity models. But instead of characteristic earthquake recurrence, we use the doubly truncated exponential magnitude-frequency relationship; no evidence of characteristic earthquakes in Slovenia has been found yet. Three of our models are similar to Frankel's first three models. Model 1 uses spatially smoothed activity rate based on magnitude 3.7 and above events since 1880. Model 2 deals with spatially smoothed activity rate based on magnitude 5.0 abd above events since 1690. Model 3 smoothes the observed activity over the entire region; it represents a uniform seismicity zone. Frankel skipped this model in his latest version (Frankel et al. 1996). In model 4, we introduce a new approach of calculating seismic activity rate taking into account released seismic energy. The ground motion attenuation model of Pugliese and Sabetta (1989) is used for all models. PGA maps for models 1, 2, 3 and 4 have been calculated, and a weighted mean map derived from them. A map of model 1 has been compared with the corresponding source zone map; the two maps do not differ significantly. A worst-case map derived from all four models has also been produced.     

Spatial pattern analysis using hybrid models: an application to the Hellenic seismicity

Earthquakes are one of the most destructive natural disasters and the spatial distribution of their epicentres generally shows diverse interaction structures at different spatial scales. In this paper, we use a multi-scale point pattern model to describe the main seismicity in the Hellenic area over the last 10 years. We analyze the interaction between events and the relationship with geological information of the study area, using hybrid models as proposed by Baddeley et al. (2013). In our analysis, we find two competing suitable hybrid models, one with a full parametric structure and the other one based on nonparametric kernel estimators for the spatial inhomogeneity.     

Miocene to Quaternary deformation in NE Slovenia: complex paleomagnetic and structural study

Combined paleomagnetic and structural research was carried out in the Mura-Zala Basin including the western and southern surrounding hills in northeastern Slovenia. The Mura-Zala Basin was formed due to ENE–WSW trending crustal extension in the late Early Miocene (18.3–16.5 Ma). First, marine sedimentation took place in several more or less confined depressions, then in a unified basin. During thermal subsidence in the late Miocene deltaic to fluvial sediments were deposited. After sedimentation, the southernmost, deepest depression was inverted. Map-scale folds, reverse and strike-slip faults were originated by NNW–SSE compression. This deformation occurred in the latest Miocene–Pliocene and is reflected also in the magnetic fabric (low field susceptibility anisotropy). After this folding, the Karpatian sediments of the Haloze acquired magnetization, then suffered 30° counterclockwise rotation relative to the present north (40° counterclockwise with respect to stable Europe). This Pliocene (Quaternary?) rotation affected a wide area around the Mura-Zala Basin. The latest Miocene to Quaternary folding and subsequent rotation may be connected to the counterclockwise rotation of the Adriatic microplate.     

Lithosphere structure and statistical properties of seismicity in Italy and surrounding regions

A detailed statistical analysis applied to the seismicity of Italy and surrounding areas allows us to identify some correlations between several statistical parameters and the thickness and the elastic parameters of the lithospheric part of the mantle, the lid. In particular, the intensity of the flow of seismic events, deprived of aftershocks, shows concentrations in relation to large gradients in the lithospheric thickness, due to abrupt variations of the lid thickness. When this variation is not very large the intensity of flow of seismic events is smaller than on the average, but in these areas (e.g., the Calabrian Arc) the strongest shocks tend to occur. The average depth of crustal shocks tends to be very shallow in relation to the thin and soft (low shear-wave velocities) lid. The positive influence, probably reflecting the thermal state in the lithosphere, is very large in relation to the thin and soft lid and where strong gradients in lid thickness are observed, and is small in relation to thick and hard lid (high shear-wave velocities). From the observed correlation between seismicity properties and lid characteristics, it may be concluded that aseismic slip below the Moho might be a key seismogenetic process.     

Spatial distribution of aftershocks and background seismicity in central California

We examine the spatial distribution of earthquake hypocenters in four central California areas: the aftershock zones of the (1) 1984 Morgan Hill (2) 1979 Coyote Lake, and (3) 1983 Coalinga earthquakes, as well as (4) the aseismically creeping area around Hollister. The basic tool we use to analyze these data are frequency distributions of interevent distances between earthquakes. These distributions are evaluated on the basis of their deviation from what would be expected if earthquakes occurred randomly in the study areas. We find that both background seismic activity and aftershocks in the study areas exhibit nonrandom spatial distribution. Two major spatial patterns, clustering at small distances and anomalies at larger distances, are observed depending on tectonic setting. While both patterns are seen in the strike-slip environments along the Calaveras fault (Morgan Hill, Coyote Lake, and Hollister), aftershocks of the Coalinga event (a thrust earthquake) seem to be characterized by clustering only. The spatial distribution of earthquakes in areas gradually decreasing in size does not seem to support the hypothesis of a self-similar distribution over the range of scales studied here, regardless of tectonic setting. Spatial distributions are independent of magnitude for the Coalinga aftershocks, but events in strike-slip environments show increasing clustering with increasing magnitude. Finally, earthquake spatial distributions vary in time showing different patterns before, during, and following the end of aftershock sequences.     

Three-dimensional velocity structure and seismicity of Mt. Etna Volcano, Italy

A new set of three-dimensional velocity models beneath Mt. Etna volcano is derived in the present work. We have used P- and S-wave arrivals from local earthquakes recorded at permanent and temporary seismic networks installed since 1980. A set of 1249 earthquakes recorded at more than four seismic stations was selected for traveltime inversion. The velocity models obtained by using different data selection criteria and parametrization display similar basic features, showing a high P-wave velocity at shallow depth in the SE quadrant, in close connection with a high gravimetric Bouguer anomaly. This area shares a low V_p/V_s ratio. High P-wave velocities and high V_p/V_s ratios are obtained along the central conduits, suggesting the presence of dense, intrusive magmatic bodies extending to a depth of about 20 km. The central intrusive core is surrounded by lower P-wave velocities. The relocated earthquake hypocenters also display the presence of an outward dipping brittle region, away from the central conduits, surrounding a ductile zone spatially related to the high P-wave velocity anomalies located in proximity to the central craters.     

Analysis of seismicity of central Luzon by pattern recognition

The distribution of large shallow earthquakes (magnitude ?5.5) in central Luzon, Philippines, related to the underthrusting of the Eurasian plate, is examined by the technique of pattern recognition. The objective of the study is (1) to forecast restricted zones of seismic activity, (2) to determine geological features which, in combination, are associated with the earthquakes in the environment of subduction, and (3) to determine whether more insight into regional seismicity can be gained by pattern recognition studies than by conventional seismicity studies. It appears from this analysis that large earthquakes occur (a) in a zone between the shallower portion of Manila Trench on its landward side and the belt of volcanoes and (b) on large strike-slip faults in the region such as Verde Island Passage fault and the Philippine fault. In these zones, large earthquakes occur at locations identified by large topographic relief in the vicinity and a high degree of fractured crust. In contrast to characteristics in California and central Asia, the intersection of faults nearby is not a major characteristic for large earthquakes in and near central Luzon. Topography and degree of fractured crust are important characteristics in all areas studied so far. Several earthquakes which are known to have occurred were considered unknown, and control experiments were carried out to determine whether the locations of these earthquakes would be identified as having potential for large earthquakes. The results of control experiments indicate that the pattern recognition technique is fairly successful in spatial forecasting if the known historical record is assumed almost complete for various major zones (85% in this case). It fails to forecast if information is totally lacking about earthquake activity on a tectonic feature. In the present study, a control experiment was able to correctly identify the locations of two of the largest earthquakes known to have occurred in the area. It was better able to identify locations of large earthquakes than a simple contouring of known earthquake locations would suggest.     

Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy)

In this paper, we analyse the seismicity distribution in the CentralApennines (Italy) using the recordings of the Rete SismometricaMarchigiana (RSM). In particular, the selected events are relocated usinga 1-D model calculated by means of an inversion procedure. The robustnessof the 1-D model and the location accuracy are tested. The capability ofthe RSM to well constrain crustal and subcrustal events in the studied areais discussed. We find that in the inner side of the chain the seismicity liesin the upper crustal layers, following the structural trend of the Apenninicbelt. A W-deepening of the events is observed in the Apenninic foredeep,where the seismicity is mainly confined in the lower crust. This evidenceimplies the deepening of the brittle to ductile transition. Some well-locatedsubcrustal events are found. Their locations seem to confirm the W-dippingsubduction of the Adriatic lithosphere beneath the Apennines.     

Present-day seismicity of the south-eastern Elbe Fault System (NE Bohemian Massif)

The Variscan Bohemian Massif is disrupted by the NW-SE striking Elbe Fault System in its northern part. The increased tectonic activity associated with this structure is manifested by increased seismicity in the eastern part of the Sudetes. With the use of a temporary local seismic network, the total number of micro-earthquakes located in this region increased to 153 for the period 1996–2003. The local magnitudes vary between −0.6 and 1.8 and the seismic energy was often released in swarm-like sequences. Five seismic events with well-defined P-onset polarities at five or six stations enabled the estimation of focal mechanisms. The present-day activity of the WNW-ESE to NNW-SSE fault systems is discussed on the basis of source mechanisms, the alignment of the epicentres, as well as morphological and geological evidence. The majority of the recent seismic activity is concentrated in a 40–60 km wide zone of a generally NW-SE trend. This structure represents a regional zone of weakness within the SE termination of the Elbe Fault System, defined by a mesh of interconnected faults, of which many are deep-seated and highly permeable and some are associated with light to moderate historical earthquakes. Both in the areas due south and due north of this zone the present-day seismic activity is very low. The increased tectonic activity can be interpreted as a result of the abundance of suitably oriented faults and their interconnection into major fault systems, the proximity of the Outer Carpathian indentor and the Cainozoic volcanic and associated recent post-volcanic activity. The similar character of swarms and their coincidence with the post-volcanic activity in the southeastern part of the Elbe Fault System and in some focal zones of the western Bohemian seismically active area suggests that overpressurized fluids may represent a potential swarm-triggering mechanism.     

中国大陆强震前地震活动图像演化特征研究

将1966年以来中国大陆发生的75次6.5级以上地震分为52组, 按华北-东北、 西南、 青藏块体、 新疆不同构造区域, 分别对其6.5级以上强震前4级、 5级以上地震活动图像进行了全时空扫描, 选取震前特征图像, 按构造区域进行分析、 总结, 研究强震前中等地震活动图像演变的基本特征, 及其震前图像演化的共性和差异性。 研究结果表明: 强震前不同级别中等地震活动出现二个阶段异常图像转化具有普适性。 5级地震图像后期在时间和空间上配套出现的4级地震的异常图像, 可能是强震孕育过程中由中期向短期过渡的一种标志。     

Transfer function‐noise modelling of an aquifer system in NE Italy

In this article, we use a transfer function‐noise (TFN) modelling strategy with single output and multiple/single inputs to study the relationships among a large unconfined aquifer in the upper Venetian plain (Northeast Italy), its recharge components (rainfalls and losing river) and a multi‐layered confined system located in the middle Venetian plain. Model identification from the data raises a range of difficulties when seeking models with consistent physical behaviour, but no information related to the transfer function order and the lags with no zero weights is available. Therefore we use an automatic identification procedure for TFN models. The obtained results suggest that the rainfall component is more important than the river discharge in the unconfined aquifer, and the behaviours of the deep‐confined aquifers are synchronous with that observed in the Badoere area. Copyright © 2010 John Wiley & Sons, Ltd.     

Seismic source zonation for NE India on the basis of past EQs and spatial distribution of seismicity parameters

Journal of Seismology - The North East (NE) of India is an earthquake (EQ) prone zone and has experienced considerable damages in the past due to high intensity EQs. With rapidly growing...     

Characteristics of the low energy seismicity of central Apulia (southern Italy) and hazard implications

The central part of the Apulia region, in southern Italy, has been generally considered practically free from significant level of seismicity, but historical documentation, geological indicators and recent instrumental observations suggest that the activity of local minor tectonic structures could have been masked (and partly also induced) by that of major seismogenic structures located in the neighbouring regions. A revision of the central Apulia seismicity characteristics was conducted considering its space and time distribution, energy release rate and focal mechanisms, in view of possible hazard implications. To better constrain the seismicity rates inferable from the set of available historical data, special attention was paid to the declustering of a catalogue of low energy events (magnitude < 3.5) instrumentally detected in about 20 years: a new declustering procedure, useful for cases like to the one at hand, was purposely devised taking into account the peculiarity of local seismicity characteristics and the limitations of the available database. The results obtained by combining instrumental and historical data show that this area is affected by a rather sporadic seismicity, likely associated to a general tensional regime and possibly stimulated by the interaction with Apenninic and northern Apulia seismogenic activity. Even though less energetic, the local seismicity contributes to increase the moderately damaging shaking probability due to the activity of seismic sources located in the near areas, so to justify the adoption of at least a minimum level of caution in relation to the local definition of seismic protection measures.     

Spatial and temporal distribution of seismicity before the Umbria-Marche September 26, 1997 earthquakes

Spatial and temporal distribution of seismicityoccurring prior to the Umbria-Marche earthquake ofSeptember 26, 1997, are studied. By applying the RTLprognostic parameter, a quiescence stage followed bya period of foreshock activation is observed beforethe event. The main shock occurred soon after therecovery of the RTL parameter to its normal backgroundlevel. An investigation of the clustering process isperformed on the earthquakes with M 3.5,occurred since 1989 in the area of the epicenter ofthe September 26, 1997 event. In comparison to theaverage background of the previous period, theincrease of the area of rupture activated during thetwelve months leading up to the Umbria-Marcheconcentrates in the vicinity of the main shock. Someresults of application of the time-to-failure model toseismicity before the Umbria-Marche main shock, arealso discussed.     

Active tectonics and seismicity in the area of the 1997 earthquake sequence in central Italy: A short review

A first tentative comparison between the structural framework related to the active tectonics and the long-term seismicity of the Umbria–Marche Apennines (affected by the 1997 seismic sequence) has provided some insight for discussing the seismotectonic characteristics of the area. This Apennine sector is affected by 15 to 20-km-long active fault systems, consisting of minor fault-segments. Each of these fault-segments may be responsible for earthquakes characterised by magnitudes ranging between 5.5 and 6.0 (such as those occurred in 1599, 1730, 1838, 1859, 1979). However, the occurrence of one large-magnitude event (1703, Ms = 6.7) and of seismic sequences (1747–1751; 1997–1998) indicate that an entire fault system may be activated suddenly (at least in the southern part of the investigated area) or during seismic crises which may last many months. The comparison between the active faulting framework and the long-term seismicity also indicates that no significant earthquakes may be related to the Mt. Vettore Fault System since 1000 AD.     

The partitioning of radiated energy and the largest aftershock of seismic sequences occurred in the northeastern Italy and western Slovenia

We analyzed the most relevant seismic sequences that occurred from 1977 to 2007 in the Friuli-Venezia Giulia region (northeastern Italy) and western Slovenia. The eight aftershock sequences were triggered by low- to moderate-magnitude earthquakes with mainshock duration magnitude ranging from 3.7 to 5.6. The b-value of the Gutenberg–Richter law varies from 0.8 to 1.1. The modified Omori’s modeling of the sequences evidences values of the p exponent ranging from 0.8 to 1.0. Using the Reasenberg and Jones (Science 243:1173–1176, 1989; Science 265:1251–1252, 1994) approach, we computed the probabilistic estimate of the aftershock rates and the largest aftershock in given time intervals. The difference in magnitude between the mainshock and the largest aftershock is calculated according to the modified Båth law and using an approach that considers the partitioning of the radiated seismic energy between mainshock and aftershocks. The partitioning of the radiated seismic energy appears to play a significant role in the evolution of the sequences. We define the parameter R _ES as the ratio between the radiated seismic energy of the mainshock and the summation of the seismic energy radiated by the aftershocks. The difference in magnitude between the mainshock and the largest aftershock, calculated with the parameter R _ES, agrees well with the observed difference. In most sequences, the parameter R _ES decreases very quickly until the occurrence of the largest aftershock and then becomes constant. By analyzing the values of R _ES during the early hours following the mainshock, we found that the R _ES values after 24 h are well related to the final ones, calculated on the whole sequence, and to the differences in magnitude between the mainshock and the largest aftershock.