Lateral structure of the subducting pacific plate beneath the Hokkaido corner from intermediate and deep earthquakes

We present a study of the lateral structure and mode of deformation in the transition between the Kuril and Honshu subduction zones. We begin by examining the source characteristics of the January 19, 1969, intermediate depth earthquake north of Hokkaido in the framework of slab-tearing, which for the December 6, 1978 event has been well documented by previous studies. We use a least-squares body wave inversion technique, and find that its focal mechanism is comparable to the 1978 event. To understand the cause of these earthquakes, which in the case of the 1978 event occurred on a vertical tear fault but does not represent hinge faulting, we examine the available International Seismological Centre [ISC] hypocenters and Harvard centroid-moment tensor [CMT] solutions to determine the state of stress, and lateral structure and segmentation in the Kuril and northern Honshu slabs. These data are evaluated in the framework of two models. Model (A) requires the subducting slab at the Hokkaido corner to maintain surface area. Model (B) requires slab subduction to be dominated by gravity, with material subducting in the down-dip direction. The distribution of ICS hypocenters shows a gap in deep seismicity down-dip of the Hokkaido corner, supporting model (B). From the CMT data set we find that three types of earthquake focal mechanisms occur. The first (type A) represents dip-slip mechanisms consistent with down-dip tension or compression in the slab in a direction normal to the strike of the trench. These events occur throughout the Honshu and Kuril slabs with focal mechanisms beneath Hokkaido showing NNW plungingP andT axes consistent with the local slab geometry. The second (type B) occurs primarily at depths over 300 km in the southern part of the Kuril slab with a few events in the northern end of the Honshu deep seismicity. These earthquakes have focal mechanisms with P axes oriented roughly E-W, highly oblique to the direction of compression found in the type A events, with which they are spatially interspersed. The third (type C) group of earthquakes are those events which do not fit in either of the first two groups and consist of either strike-slip focal mechanisms, such as the tearing events, or oddly oriented focal mechanisms. Examination of the stress axes orientations for these three types reveals that the compressional axes of the type C events are consistent with those of type B. The slab tearing events are just differential motion reflecting the E-W compressive states of stress which is responsible for the type B family of events. There is no need to invoke down-dip extension which does not fit the slab geometry. We conclude that these two states of stress can be explained as follows: 1) The type A events and the seismicity distribution support model (B). 2) The type B and C events upport model (A). The solution is that the slab subducts according to model (B), but the flow in the mantle maintains a different trajectory, possibly induced by the plate motions, which produces the second state of E-W compressive stress.     

Evidence of a partial melt zone beneath Stromboli volcano (Italy) from inversion of teleseismic receiver functions

Teleseismic body waves from broadband seismic stations are used to investigate the crustal and uppermost mantle structure of Stromboli volcano through inversion of the receiver functions (RFs). First, we computed RFs in the frequency domain using a multiple-taper spectral correlation technique. Then, the non-linear neighbourhood algorithm was applied to estimate the seismic shear wave velocity of the crust and uppermost mantle and to define the main seismic velocity discontinuities. The stability of the inversion solution was tested using a range of initial random seeds and model parameterizations. A shallow Moho, present at depth of 14.8 km, is evidence of a thinned crust beneath Stromboli volcano. However, the most intriguing and innovative result is a low S velocity layer in the uppermost mantle, below 32 km. The low S velocity layer suggests a possible partial melt region associated with the volcanism, as also recently supported by tomographic studies and petrological estimations.     

Event Scenario Analysis for the Design of Rockslide Countermeasures

The Torgiovannetto quarry（Assisi municipality,central Italy） is an example of a site where the natural equilibrium was altered by human activity,causing current slope instability phenomena which threaten two roadways important for the local transportation.The quarry front,having a height of about 140 m,is affected by a 182,000 m3 rockslide developed in intensely fractured limestone and is too large to be stabilized.In 2003 some tension cracks were detected in the vegetated area above the quarry upper sector.From then on,several monitoring campaigns were carried out by means of different instrumentations（topographic total station,extensometers,inclinometers,ground-based interferometric radar,laser scanner and infrared thermal camera）,allowing researchers to accurately define the landslide area and volume.The latter’s major displacements are localized in the eastern sector.The deformational field appears to be related to the seasonal rainfall.The landslide hazard associated with the worst case scenario was evaluated in terms of magnitude,intensity and triggering mechanism.For the definition of the possible runout process the DAN 3D code was employed.The simulation results were used in order to design and construct a retaining embankment.Furthermore,in order to preserve both the safety of the personnelinvolved in its realization and of the roadways users,an early warning system was implemented.The early warning system is based on daily-averaged displacement velocity thresholds.The alarm level is reached if the prediction based on the methods of Saito（1969） and Fukuzono（1985） forecasts an imminent rupture.     

Estimates of surface drifter trajectories in the equatorial Atlantic: a multi-model ensemble approach

We compared the estimates of surface drifter trajectories from 1 to 7?days in the equatorial Atlantic over an 18-month period with five eddying ocean general circulation model (OGCM) reanalyses and one observational product. The cumulative distribution of trajectory error was estimated using over 7,000?days of drifter trajectories. The observational product had smaller errors than any of the individual OGCM reanalyses. Three strategies for improving trajectory estimates using the ensemble of five operational ocean analysis and forecasting products were explored: two methods using a multi-model ensemble estimate and also spatial low-pass filtering. The results were insensitive to the method used to create the ensemble estimates, and by most measures, the results were better than the observational product. Comparison of relative skill of the various OGCM reanalyses suggested promising avenues for exploration for further improvements: forcing with higher frequency wind stress and quality control of input data. One of the lowest horizontal resolution OGCMs, with 1/4° longitude horizontal resolution, made the best trajectory estimates. The individual OGCMs were dominated by errors at spatial scales smaller than about 100 to 200?km, i.e., less than the local deformation radius. But buried in those errors were valuable signals that could be retrieved by combining all the OGCM velocity fields to produce a multi-model ensemble-based estimate. This estimate had skill down to spatial scales about 75?km. Results from this study are consistent with previous work showing that ensemble-mean forecast skill is superior to individual forecasts.     

Damage field and site effects: multidisciplinary studies of the 1964 earthquake series in Central Switzerland

Central Switzerland shows comparatively high seismic activity by Swiss standards. Many historical earthquakes are known and several of them caused damage. The last major event dates back to 1964 and has the characteristics of an earthquake swarm. Among dozens of felt shocks were two main shocks (Mw = 5 and 5.7) that moderately damaged a limited area with hundreds of buildings suffering loss. Our aim here was to reconstruct the damage field and to analyze whether it was influenced by site effects. Given the existence of a contemporary damage assessment and other historical sources, we could describe the damage field in detail. For about 95% of the affected buildings, we could reconstruct the location and extent of loss, using assessments from the European Macroseismic Scale (EMS 98). Spatial analysis of the resulting data showed that most losses were concentrated in the villages of Sarnen and Kerns. Damage to residential houses and barns was by far most frequent (90%), but expensive losses to the relatively few sacral buildings were responsible for almost 50% of the repair costs. We compared the damage data with deposit thickness and soil composition and carried out field experiments using H/V spectral ratios to measure the fundamental frequency of ground resonance at 75 sites to estimate the frequency band in which amplification occurs. Our results show that locations on both thick fluviatile sediments and large alluvial cones showed higher intensities than did other ground types. Moreover, at some sites, intensity was probably increased by a layer of weathered rock below thin deposits.     

The Precambrian geology of the “Borborema”-Belt (States of Paraíba and Rio Grande do Norte; northeastern Brazil) and the origin of its mineral provinces

In the crystalline area of the “Borborema” in NE Brazil the final geologic event was a strong static recrystallization, which is manifest in regional granitization, hybride granite, or pegmatite veins. The manifestation varies within the mapped area, it depends on lithology, thickness and structure of the Precambrian metasediments. The age of this phase is between 550 and 450 M. years. Recrystallization is accompanied by weak folding of pre-existing anticlines as indicated by the structural control of the Borborema pegmatites. The principal phase of deformation and the accompanying regional metamorphism are considered to be older, approximately 850 M. years or more. This older regional metamorphism produced rocks of the almandine-staurolite subfacies of the amphibolite facies; locally higher (cordierite and sillimanite subfacies) or lower (greenschist facies) degree of metamorphism occurs. The pyroxene-hornfels facies, which is found in the contact aureoles of hybride granites, occurs exclusively in the tactites (metamorphic dolomitic marls). In clastic metasediments, high grade metamorphic facies is restricted to the contact aureole of a small hypersthene diorite stock. The structure is simple, and consists of synclines and anticlines the axial planes of which are vertical or, rarely, inclined. Faults are rare. The degree of lateral compression varies: low in the NW part of the area mapped, intermediate in die NE, and high in the southern part. The southern part belongs to the “Paraíba Transversal Zone”, an E-W striking fault zone along which the northern parts have been dislocated horizontally to the East. Stratigraphically the sequence is subdivided into three major groups by unconformities, which, however, are masked intensely by the principal tectonic events. These groups are: The oldest member occurs but locally in the cores of some anticlines; the youngest constitutes the cores of most synclines. The Caicó can be subdivided, in the most part, as follows: Florânia and Parelhas are essentially composed by unsorted clastic sediments (graywackes and arcoses). The Equador is a local lithofacies of the latter restricted to one anticline and a small part of another, and composed of well sorted clastics (muscovite quartzite). Another quartzite, locally developed at the top of the Florânia in the western part of the area, has been named “Member S. José do Seridó”. The Quixaba contains essentially carbonate rocks, but with intercalations of clastic sediments. The scheelite province of the area is generally connected with the Quixaba-Formation, the marbles of which served as transportation channels, whereas tactites were the receptors of the ore-bringing solutions, derived from some palingenetic granites and thus, indirectly, from the metasediments of the Parelhas. The deposition does not belong to the principal phase of contact metamorphism, but to a later-phase with hydrothermal paragenesis. Scheelite occurs also in common quartz-albite-veins. The famous pegmatite-province (Ta, Nb, Li, Be) cannot be considered as a product of magmatic differentiation, either. It must be the product of the statical recrystallization of the muscovite-rich Equador quartzite, by addition of thermal energy and K-rich solutions from the depth. Thus this province is only a special case of the general “granitization” with K-metasomatism: $$Quartz + Muscovite + K^ + \to Microclin + H^ + + Water.$$ The resulting fluids formed pegmatites in the rocks overlying the quartzite, i. e., generally in the micaschists of the Seridó-Group. Productive dykes always fill longitudinal or transversal tension cracks. The Borborema province of zoned pegmatites is thus an excellent example for the recent genetic hypothesis ofGresens, all requirements of it being fulfilled.