Passive seismic depth sounding by statistical array analysis

We can determine efficiently the Rayleigh wave dispersion curve for a frequency range between 2 Hz and 80 Hz by analyzing the seismic noise recorded on mini-arrays. Due to the short recording time of 15 min we are able to map an area up to 6 ha per day. The dispersion curve gives us a first qualitative image of the subsurface

Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.Comment: 15 pages, 5 figure

Crustal structure from teleseismic P-wave receiver function analysis in the Maule Region, Central Chile

EGU2011-12780 A temporary passive seismic network of 31 broad-band stations was deployed in the region around Talca and Constitución between 35°S to 36°S latitude and 71°W to 72.5°W longitude. The network was operated between March and October 2008. Thus, we recorded data prior the magnitude Mw=8.8 earthquake of 27 February 2010 at a latitude of the major slip and surface uplift. The experiment was conducted to address fundamental questions on deformation processes, crustal and mantle structures, and fluid flow. We present results of a teleseismic P receiver function study that covers the coastal region and reaches to the Andes. The aim is to determine the structure and thickness of the continental crust and constrain the state of hydration of the mantle wedge. The P-wave receiver function technique requires large teleseismic earthquakes from different distances and backazimuths. A few percent of the incident P-wave energy from a teleseismic event will be converted into S-wave (Ps) at significant and relatively sharp discontinuities beneath the station. A small converted S phase is produced that arrives at the station within the P wave coda directly after the direct P-wave. The converted Ps phase and their crustal multiples contain information about crustal properties, such as Moho depth and the crustal vp/vs ratio. We use teleseismic events with magnitudes mb > 5.5 at epicentral distances between 30° and 95° to examine P-to-S converted seismic phases. Our preliminary results provide new information about the thickness of the continental crust beneath the coastal region in Central Chile. At most of the stations we observed significant energy from P to S converted waves between 4 and 5 s after the direct P-wave within a positive phase interpreted as the Moho, occurring at 35 to 40 km. The great Maule earthquake of 27 February 2010 nucleated up-dip of the continental Moho. The rupture of this earthquake seems to have propagated down-dip of the Moho. The Moho reflection show a positive polarity, indicating that the mantle is either dry or only moderately hydrated. We observed converted energy from an intracrustal boundary at around 2 s that disappears near the coast. Further, positive polarity peaks occur that are possibly caused by the down going plate

Determination of the subsurface structure of the Chora Plain (Samos) by seismic noise analysis

Seismic noise was passively recorded with mini-arrays, allowing shear wave velocities of individual layers to be estimated. A statistical analysis by beamforming was performed. A 1D underground model of the Chora plain was derived. Three dominant structures were found: coastal barrier, siltated lagoon and the interior of the Chora plain

application to the Tyrrhenian sea

In this study different full waveform techniques was developed for the investigation of seismic ocean bottom single station data. These techniques were applied to data obtained in a pilot ocean bottom experiment in the Tyrrhenian Sea/Italy (TySea experiment) from December 2000 to May 2001. A network of broadband seven ocean bottom seismometers and seven ocean bottom hydrophones was installed above the subducting Ionian plate which descends from Southeast to Northwest. Local and teleseismic earthquakes were recorded by the stations. The newly developed techniques produce very promising results in reconstructing the sea floor structure beneath the stations and in attenuating waveform effects generated by water layer multiples. Additionally the techniques offer a possibility to determine the orientation of free fall ocean bottom seismometers. The main results are: 1. The waveform recorded at the seafloor differs from waveforms recorded at land stations. This is primarily due to multiple reflections in the water layer. These multiple reflections show different patterns on seismometer and hydrophone recordings depending on the seafloor structure. This opens the possibility to constrain the P-wave velocity structure beneath the station by means of a full waveform inversion...thesi

Seismic evidence of a link between subducted oceanic faults and volcanism: A case study from South Central Chile

The south-central Chilean subduction zone was investigated at 39-40°S by a passive seismic experiment. The investigation area comprises the maximum slip of the great 1960 Mw 9.5 Valdivia earthquake. The incoming Nazca plate is permeated by a number of major fault zones including the Valdivia fault zone and the Mocha fault zone which seem to have behaved as a barriers for the rupture propagation of large earthquakes in the past. The investigated sector is also home to the Villarrica volcano - one of South America’s most active volcanoes. In the extension of the Valdiva fault zone we observed a cluster of increased seismicity in the subducting plate at depths between 80 km and 120 km, where dehydration of the subducting plate occurs. The focal plane solutions of this cluster show predominantly strike-slip motion. Tomographic images show decreased P- and S-velocity and increased ratio between the seismic cluster and the volcanic center of Villarrica, Quetrupillán and Lanin, corresponding to an increased content of fluids or melt. Additional geochemical investigations show that the magma of Villarrica volcano has an enhanced fluid signal compared to the other volcanoes of the Southern Volcanic Zone of Chile. It can be assumed that the Valdivia fault zone serves as the source for the fluids. Before the plate subducts, water can penetrate the plate through faults within the Valdivia fault zone. Serpentinization would build the water into minerals. Inside the subduction zone the Valdiva fault zone is reactivated by dehydration reactions at a depth of about 100 km. The released fluids rise towards the volcanic center causing the tomographic anomalies. At the end this leads to an increased degree of melting and a higher activity of Villarrica volcano