Detection of microseismic compressional (P) body waves aided by numerical modeling of oceanic noise sources

Among the different types of waves embedded in seismic noise, body waves present appealing properties but are still challenging to extract. Here we first validate recent improvements in numerical modeling of microseismic compressional (P) body waves and then show how this tool allows fast detection and location of their sources. We compute sources at ~0.2 Hz within typical P teleseismic distances (30-90°) from the Southern California Seismic Network and analyze the most significant discrete sources. The locations and relative strengths of the computed sources are validated by the good agreement with beam-forming analysis. These 54 noise sources exhibit a highly heterogeneous distribution, and cluster along the usual storm tracks in the Pacific and Atlantic oceans. They are mostly induced in the open ocean, at or near water depths of 2800 and 5600 km, most likely within storms or where ocean waves propagating as swell meet another swell or wind sea. We then emphasize two particularly strong storms to describe how they generate noise sources in their wake. We also use these two specific noise bursts to illustrate the differences between microseismic body and surface waves in terms of source distribution and resulting recordable ground motion. The different patterns between body and surface waves result from distinctive amplification of ocean wave-induced pressure perturbation and different seismic attenuation. Our study demonstrates the potential of numerical modeling to provide fast and accurate constraints on where and when to expect microseismic body waves, with implications for seismic imaging and climate studies. © 2013. American Geophysical Union. All Rights Reserved.This work was supported by the European Research Council (IOWAGA project), the Program >Investment for the future” Labex Mer (grant ANR-10-LABX-19-01), and the Consolider-Ingeno (Topo-Iberia). M.O. performed the data analysis while visiting the Domaines Océanique laboratorPeer Reviewe

Annual cycle of benthic nutrient fluxes in Tomales Bay, California, and contribution of the benthos to total ecosystem metabolism

Benthic fluxes of dissolved nutrients, oxygen, dissolved inorganic carbon, and total alkalinity were measured over a 2 yr period in Tomales Bay, California, USA, using in situ incubation chambers. Release of dissolved nutrients from the sediment peaked in late summer and was lowest in winter. The difference between C:N: P flux ratios and composition of suspended particulates indicated the existence of a sink for regenerated N, relative to C and P. Total alkalinity flux revealed that carbon metabolism by net sulfate reduction represented ca one-third of total benthic metabohsm Partitioning net system fluxes into component fluxes suggested that the equivalent of ca 70 to 80 % of the available particulate C, N and P was respired within the water column, while about 20 to 30 O/O was respired by the benthos. During spring, increasing light resulted in higher water column productivity, followed closely by rising water column respiration. With low delivery of the new organic material to the benthos, and low residual organics in the sediment, benthic respiration remained low. Fallout of particulate material, coinciding with peak water temperature in late summer, resulted in a 'crossover' with benthic respiration temporarily exceeding water column respiration

Construction status and prospects of the Hyper-Kamiokande project

The Hyper-Kamiokande project is a 258-kton Water Cherenkov together with a 1.3-MW high-intensity neutrino beam from the Japan Proton Accelerator Research Complex (J-PARC). The inner detector with 186-kton fiducial volume is viewed by 20-inch photomultiplier tubes (PMTs) and multi-PMT modules, and thereby provides state-of-the-art of Cherenkov ring reconstruction with thresholds in the range of few MeVs. The project is expected to lead to precision neutrino oscillation studies, especially neutrino CP violation, nucleon decay searches, and low energy neutrino astronomy. In 2020, the project was officially approved and construction of the far detector was started at Kamioka. In 2021, the excavation of the access tunnel and initial mass production of the newly developed 20-inch PMTs was also started. In this paper, we present a basic overview of the project and the latest updates on the construction status of the project, which is expected to commence operation in 2027

Frequency-dependent noise sources in the North Atlantic Ocean

Secondary microseisms are the most energetic waves in the noise spectra between 3 and 10 s. They are generated by ocean wave interactions and are predominantly Rayleigh waves. We study the associated noise sources in the North Atlantic Ocean by coupling noise polarization analysis and source mapping using an ocean wave model that takes into account coastal reflections. From the Rayleigh wave polarization analysis, we retrieve the back azimuth to the noise sources in the time-frequency domain. Noise source modeling enables us to locate the associated generation areas at different times and frequencies. We analyze the distribution of secondary microseism sources in the North Atlantic Ocean using 20 broadband stations located in the Arctic and around the ocean. To model the noise sources we adjust empirically the ocean wave coastal reflection coefficient as a function of frequency. We find that coastal reflections must be taken into account for accurately modeling 7¿10 s noise sources. These reflections can be neglected in the noise modeling for periods shorter than 7 s. We find a strong variability of back azimuths and source locations as a function of frequency. This variability is largely related to the local bathymetry. One direct cause of the time-dependent and frequency-dependent noise sources is the presence of sea-ice that affects the amplitude and polarization of microseisms at stations in the Arctic only at periods shorter than 4 s. Components: 8,061M.S. acknowledges financial support by the projects Rifsis (CGL 2009–09727) and TopoIberia (CSD2006-00041). This is IPGP contribution 3449.Peer Reviewe

Anisotropic stratification beneath Africa from joint inversion of SKS and P receiver functions

International audienceThe analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress-strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3-D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth-dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere-asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle