[1] A detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern Mid-Atlantic Ridge between 15°and 35°N is presented. Two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones. The seismically active zone extends $20 km to either side of the ridge axis, consistent with earlier results from studies of fault morphology. Along the axis, hydrophone-recorded activity shows important variations. Areas with intense and persistent seismic activity (stripes) stand in sharp contrast to areas that lack seismicity (gaps). The regions of persistent activity are a new observation at mid-ocean ridges. In general, the patterns of seismically active/inactive regions are also recognized in the 28-year teleseismic record, implying that these patterns are maintained at timescales between a few years and a few decades. We find no simple relationship between individual segment variables (e.g., length or trend of the segment, maximum offset of discontinuities, or along-axis change in mantle Bouguer anomaly (MBA) and water depths) and number of hydrophone-recorded events. There does appear to be a correlation between axial thermal structure and seismicity. Regions of low and high numbers of events would thus correspond to thinner (hotter) and thicker (colder) lithosphere, respectively. Seismicity may reflect thermal structure at short timescales (decadal or longer), while relief and inferred crustal thickness may integrate this structure over longer periods of time (order of 1 Myr). INDEX TERMS: 3025 Marine
Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling. The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.
The Çınarcık Basin is a transtensional basin located along the northern branch of the northern North Anatolian Fault (NAF) in the Sea of Marmara, the eastern half of which has been identified as a seismic gap. During the SEISMARMARA (2001) experiment, a dense grid of multichannel seismic reflection profiles was shot, covering the whole Çınarcık Basin and its margins. The new seismic images provide a nearly three‐dimensional view of the architecture of the basin (fault system at depth and sedimentary infill) and provide insight into its tectonic evolution. Along both northern and southern margins of the basin, seismic reflection data show deep‐penetrating faults, hence long‐lived features, which have accommodated a large amount of extension. There is no indication in the data for a single throughgoing strike‐slip fault, neither a cross‐basin fault nor a pure strike‐slip fault running along the northern margin. Faster opening is presently observed in the eastern part of the basin. The Çınarcık Basin seems to have developed as a transtensional basin across strike‐slip segments of the northern NAF for the last few million years.
The determination of melt distribution in the crust and the nature of the crust-mantle boundary (the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.
Landslide early warning systems (EWSs) have to be implemented in areas with large risk for populations or infrastructures when classical structural remediation measures cannot be set up. This paper aims to gather experiences of existing landslide EWSs, with a special focus on practical requirements (e.g., alarm threshold values have to take into account the smallest detectable signal levels of deployed sensors before being established) and specific issues when dealing with system implementations. Within the framework of the SafeLand European project, a questionnaire was sent to about one-hundred institutions in charge of landslide management. Finally, we interpreted answers from experts belonging to 14 operational units related to 23 monitored landslides. Although no standard requirements exist for designing and operating EWSs, this review highlights some key elements, such as the importance of pre-investigation work, the redundancy and robustness of monitoring systems, the establishment of different scenarios adapted to gradual increasing of alert levels, and the necessity of confidence and trust between local populations and scientists. Moreover, it also confirms the need to improve our capabilities for failure forecasting, monitoring techniques and integration of water processes into landslide conceptual models
[1] Most tsunami models apply dislocation models that assume uniform slip over the entire fault plane, followed by standard analytical models based on Volterra's theory of elastic dislocations for the seabed deformation. In contrast, we quantify tsunami runup variability for an earthquake with fixed magnitude but with heterogeneous rupture distribution assuming plane wave propagation (i.e., an infinitely long rupture). A simple stochastic analysis of 500 slip realizations illustrates the expected variability in coseismic slip along a fault plane and the subsequent runup that occurs along a coastline in the near field. Because of the need for systematically analyzing different fault geometries, grid resolutions, and hydrodynamic models, several hundred thousand model runs are required. Thus, simple but efficient linear models for the tsunami generation, propagation, and runup estimation are used. The mean value and variability of the maximum runup is identified for a given coastal slope configuration and is analyzed for different dip angles. On the basis of the ensemble runs, nonhydrostatic effects are discussed with respect to their impact on generation, nearshore propagation, and runup. We conclude that for the geometry and magnitude investigated, nonhydrostatic effects reduce the variability of the runup; that is, hydrostatic models will produce an artificially high variability.Citation: Løvholt, F., G. Pedersen, S. Bazin, D. Kühn, R. E. Bredesen, and C. Harbitz (2012), Stochastic analysis of tsunami runup due to heterogeneous coseismic slip and dispersion,
[1] On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (M w = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.
[Bazin, 2000]. In this study, we investigate the regional geometry of layer 2A using velocity models obtained by the refraction/tomography experiment. The air gun/ocean bottom receiver technique provides detailed information on upper crustal velocities owing to the combination of high source repetition rate and broad coverage.Earlier investigations identified regions of high magnetization associated with the propagating limb of OSCs, which were 16,101
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