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A note on Northern Marche seismicity: new focal mechanisms and seismological evidence  [cached]
S. Santini
Annals of Geophysics , 2003, DOI: 10.4401/ag-4374
Abstract: A geodynamic interpretation of the Northern Marche region is diffi cult, the zone being characterized by complex structures which cannot be defi ned in the form of a simple, standard model. It is unquestionable that the geodynamic setting, whatever it is, bears a strong infl uence on the seismic hazard assessment of a region, and this is the background reason for the present note. In order to obtain a more detailed picture of seismological evidence in this zone, 11 new fault plane solutions of crustal events with 2.9 < M < 4.3 were calculated, using data recorded by the national seismic network of the Istituto Nazionale di Geofi sica e Vulcanologia in the period 1990-2000. The aim is to add local information to the previous studies by Frepoli and Amato (1997, 2000). A possible result of this new study is the division of the Northern Marche region into three areas with different focal mechanism categories: the inner area of the Apenninic belt, the Adriatic on-shore and the Adriatic off-shore. This note is intended to be a contribution to update seismological evidence in the Northern Marche region.
Seismicity, focal mechanisms, and stress distribution in the Tres Vírgenes volcanic and geothermal region, Baja California Sur, Mexico
Wong, Victor;Munguía, Luis;
Geofísica internacional , 2006,
Abstract: in october 1993 we carried out a seismic monitoring in the tres vírgenes volcanic region in order to record the background seismicity associated with the volcanic structures, the geothermal field and the tectonic features of the area. hypo-centers for 257 microearthquakes were located in the volcanic edifices and along the northwest right-lateral, strike-slip la virgen fault. focal depths range from close to the earth surface to about 8 km. shallow depths occur mainly in the volcanic edifices. deeper seismic events occurred outside the volcanic area. the duration magnitudes of the located microearthquakes range between 1 and 3. the vp/vs ratio and the low-q values estimated suggest heterogeneous material properties in the volcanic structures mainly toward the el azufre fault and the el aguajito caldera, where hydrothermal activity has been reported. the p- and t-axes of focal mechanisms for 90 microearthquakes suggest that the region is under n-s compression and e-w extension, in agreement with the regional tectonic stress field of the nw-se right-lateral strike-slip transform fault system of the gulf of california.
Seismicity, focal mechanisms, and stress distribution in the Tres Vírgenes volcanic and geothermal region, Baja California Sur, Mexico  [PDF]
Victor Wong,Luis Munguía
Geofísica internacional , 2006,
Abstract: In October 1993 we carried out a seismic monitoring in the Tres Vírgenes volcanic region in order to record the background seismicity associated with the volcanic structures, the geothermal field and the tectonic features of the area. Hypocenters for 257 microearthquakes were located in the volcanic edifices and along the northwest right-lateral, strike-slip La Virgen fault. Focal depths range from close to the Earth surface to about 8 km. Shallow depths occur mainly in the volcanic edifices. Deeper seismic events occurred outside the volcanic area. The duration magnitudes of the located microearthquakes range between 1 and 3. The Vp/Vs ratio and the low-Q values estimated suggest heterogeneous material properties in the volcanic structures mainly toward the El Azufre fault and the El Aguajito Caldera, where hydrothermal activity has been reported. The P- and T-axes of focal mechanisms for 90 microearthquakes suggest that the region is under N-S compression and E-W extension, in agreement with the regional tectonic stress field of the NW-SE right-lateral strike-slip transform fault system of the Gulf of California.
Decomposing spatio-temporal seismicity patterns
C. Goltz
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2001,
Abstract: Seismicity is a distributed process of great spatial and temporal variability and complexity. Efforts to characterise and describe the evolution of seismicity patterns have a long history. Today, the detection of changes in the spatial distribution of seismicity is still regarded as one of the most important approaches in monitoring and understanding seismicity. The problem of how to best describe these spatio-temporal changes remains, also in view of the detection of possible precursors for large earthquakes. In particular, it is difficult to separate the superimposed effects of different origin and to unveil the subtle (precursory) effects in the presence of stronger but irrelevant constituents. I present an approach to the latter two problems which relies on the Principal Components Analysis (PCA), a method based on eigen-structure analysis, by taking a time series approach and separating the seismicity rate patterns into a background component and components of change. I show a sample application to the Southern California area and discuss the promising results in view of their implications, potential applications and with respect to their possible precursory qualities.
Analysis of the seismicity of Southeastern Sicily: a proposed tectonic interpretation
R. Azzaro,M. S. Barbano
Annals of Geophysics , 2000, DOI: 10.4401/ag-3628
Abstract: Southeastern Sicily is one of the Italian regions with high seismic risk and is characterised by the occurrence in the past of large destructive events (MS = 6.4-7.3) over a territory which is densely urbanised today. The main earthquakes were analysed and some minor damaging shocks reviewed to investigate the main seismogenic features of the region. The comparison between the pattern of seismicity and evidence of Quaternary tectonics allowed us to propose a first tentative, tectonic interpretation of the earthquakes. On the whole, the seismicity of SE Sicily seems distributed along regional fault systems which have had a role in the recent geodynamic evolution of the area. The Malta escarpment, the only structure whose late Quaternary-recent activity is currently known, appears the most probable source for earthquakes with about 7 magnitude. Although no evidence of tectonics subsequent to the middle Pleistocene is available for them, the Scicli line and the NE-SW fault system delimiting the northern sector of the Hyblean plateau seem seismically active with events with maximum magnitude of 5.2 and 6.4, respectively.
Detecting premonitory seismicity patterns based on critical point dynamics  [PDF]
G. Z?ller,S. Hainzl
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2001,
Abstract: We test the hypothesis that critical point dynamics precedes strong earthquakes in a region surrounding the future hypocenter. Therefore, we search systematically for regions obeying critical point dynamics in terms of a growing spatial correlation length (GCL). The question of whether or not these spatial patterns are correlated with future seismicity is crucial for the problem of predictability. The analysis is conducted for earthquakes with M > 6.5 in California. As a result, we observe that GCL patterns are correlated with the distribution of future seismicity. In particular, there are clear correlations in some cases, e.g. the 1989 Loma Prieta earthquake and the 1999 Hector Mine earthquake. We claim that the critical point concept can improve the seismic hazard assessment.
Patterns of distribution and abundance of larval phosichthyidae (actinopterygii, stomiiformes) in southeastern Brazilian waters
Go?alo, Cássia G?ngora;Katsuragawa, Mario;Silveira, Ilson Carlos Almeida da;
Brazilian Journal of Oceanography , 2011, DOI: 10.1590/S1679-87592011000300002
Abstract: horizontal and vertical distribution patterns and abundance of larval phosichthyids were investigated from oblique and depth-stratified towns off southeastern brazilian waters, from s?o tomé cape (41ow.; 22os.) to s?o sebasti?o island (45ow.; 24os.). the sampling was performed during two cruises (january/2002 -summer; august/2002 -winter). overall 538 larvae of phosichthyidae were collected during summer and 158 in the winter. three species, pollichthys mauli, vinciguerria nimbaria and ichthyioccoccus sp. occurred in the area, but ichthyioccoccus sp. was extremely rare represented by only one specimen, caught in the oceanic region during the summer. geographically, larval were concentrated in the oceanic region, and vertically distributed mainly between the surface and 80 m depth in the summer and winter. larvae were more abundant during the night, performing a diel vertical migration in the water column. the results suggest that the meandering and eddies of brazil current play important role on the transport and distribution patterns of larval phosichthyids over the oceanic and neritic area in the southeastern brazil.
Seismicity and focal mechanism of some recent earthquakes in North-east India and neighbourhood  [cached]
H. M. CHAUDHTJRY,H. N. SRIVASTAVA
Annals of Geophysics , 1976, DOI: 10.4401/ag-4792
Abstract: The seismicity of northeast India has been studied in the light of the known geotectonic features based on the data from 1881 to 1972. Now focal mechanism solutions of earthquakes using double couple hypothesis have been discussed with reference to the plate theory of global tectonics and the geological faults. It was found that the constant b in Gutenberg-Ricliter's frequency magnitude relationship was nearly I for both the periods of observations namely 1920-1972 and 1963-1972 which was in agreement with the other highly seismic regions of the world. Most of the new fault plane solutions show significant strike slip movements with thrust faulting in this region. Near the main boundary fault in northeast Himalaya, the pressures are oriented almost along the strike of the fault while along the Manipur-Burina border region, the pressures are acting at right angles or at acute angles to the mountains and are shallow dipping.
Stress distribution and seismicity patterns of the 2011 seismic swarm in the Messinia basin, (South-Western Peloponnesus), Greece  [PDF]
G. Chouliaras,G. Drakatos,K. Pavlou,K. Makropoulos
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2013, DOI: 10.5194/nhess-13-45-2013
Abstract: In this investigation we examine the local stress field and the seismicity patterns associated with the 2011–2012 seismicity swarm in the Messinia basin, south-western Peloponnesus, Greece, using the seismological data of the National Observatory of Athens (NOA). During this swarm more than 2000 events were recorded in a 12 month period by the Hellenic Unified Seismological Network (HUSN) and also by the additional local installation of four portable broadband seismographic stations by NOA. The results indicate a Gaussian distribution of swarm activity and the development of a seismicity cluster in a pre-existing seismic gap within the Messinia basin. Centroid Moment Tensor solutions demonstrate a normal fault trending northwest–southeast and dipping to the southwest primarily due to an extensional stress field. During this seismicity swarm an epicentre migration of the three largest shocks is observed, from one end of the rupture zone in the north-western part of the cluster, towards the other edge of the rupture in the south-eastern part of the cluster. This migration is found to follow the Coulomb failure criterion that predicts the advancement and retardation of the stress field and the patterns of increases and decreases of the seismicity rate (b-value) of the frequency–magnitude relation.
Comparison of the diel and spatial distribution patterns of ichthyoplankton and ichthyoneuston in the Southeastern Brazilian Bight
Katsuragawa, Mario;Matsuura, Yasunobu;
Boletim do Instituto Oceanográfico , 1990, DOI: 10.1590/S0373-55241990000200003
Abstract: fish larvae were collected by neuston and bongo nets, in the southeastern brazil, during four océanographie cruises (january 1980; january 1981; october 1981; and march 1982). the results are compared and analyzed in order to study the patterns of diel vertical distributions of fish larvae and to detect sampling problems. larvae collected with neuston net were quite different in types and size composition, when compared with those collected with bongo net in oblique tows. clear tendency of stratification in size composition at the uppermost layer can be observed on juveniles ( > 19 mm) of sardinella brasiliensis which also showed day time avoidance. most of the neustonic taxa were also caught in the deeper layers, although some groups, e.g. mugilidae, mullidae, and gerreidae, were more abundant at the surface. density of ichthyoneuston is much higher than those observed in the northwest atlantic and number of fish larvae collected during the day by neuston net exceeded that of night samples.
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