Eleonore Stutzmann

Understanding seismic heterogeneities in the lower mantle beneath the Americas from seismic tomography and plate tectonic history

A process for catalytically cracking a hydrocarbon-containing oil employs a cracking catalyst comprising aluminum borate and zirconium borate.

Global climate imprint on seismic noise

In the absence of earthquakes, oceanic microseisms are the strongest signals recorded by seismic stations. Using the GEOSCOPE global seismic network, we show that the secondary microseism spectra have global characteristics that depend on the station latitude and on the season. In both hemispheres, noise amplitude is larger during local winter, and close to the equator, noise amplitude is stable over the year. There is an excellent correlation between microseism amplitude variations over the year and changes in the highest wave areas. Considering the polarization of the secondary microseisms, we show that stations in the Northern Hemisphere and close to the equator record significant changes of the secondary microseism source azimuth over the year. During Northern Hemisphere summer, part or all of the sources are systematically located farther toward the south than during winter. Stations in French Guyana (MPG) and in Algeria (TAM) record microseisms generated several thousand kilometers away in the South Pacific Ocean and in the Indian Ocean, respectively. Thus, secondary microseism sources generated by ocean waves which originate in the Southern Hemisphere can be recorded by Northern Hemisphere stations when local sources are weak. We also show, considering a station close to Antarctica, that primary and secondary microseism noise amplitudes are strongly affected by changes of the sea ice floe and that sources of these microseisms are in different areas. Microseism recording can therefore be used to monitor climate changes.

GEOSCOPE Station Noise Levels

The noise level at GEOSCOPE seismograph stations operating in 1995 has been studied in order to quantify the quality of stations for periods ranging from 0.2 to 8000 sec. The power spectral density curves presented in this article are a useful tool for selecting stations as a function of signal-to-noise ratio in the frequency band of interest. Seismic-noise level is the lowest for continental stations in the entire frequency band. It is similarly low at most coastal stations (stations located less than 150 km away from the coast). Finally, the noise level is low for island stations at long periods but increases significantly for periods smaller than 20 seconds, and in particular in the period range of the microseismic peak. The noise level on horizontal components varies, in most stations, as a function of local time for periods greater than 20 sec, being higher during the day than during the night. Only stations located in cold areas with little daily temperature variations and stations installed in a long tunnel do not display these daily variations. There is no seasonal variations of short-period noise (periods less than 5 sec). For some continental stations, we observe variations in the amplitude of the 7-sec microseismic peak during the year. For all three components, the peak amplitude is higher and shifted toward longer periods in fall and winter than in spring and summer. This phenomenon can be explained by the increase of the number and the size of oceanic storms in fall and winter. Long-period seismic noise (periods greater than 30 sec) also varies for some stations as a function of the season; however, no systematic characteristics have been observed.

How ocean waves rock the Earth: Two mechanisms explain microseisms with periods 3 to 300 s

Microseismic activity, recorded everywhere on Earth, is largely due to ocean waves. Recent progress has clearly identified sources of microseisms in the most energetic band, with periods from 3 to 10 s. In contrast, the generation of longer-period microseisms has been strongly debated. Two mechanisms have been proposed to explain seismic wave generation: a primary mechanism, by which ocean waves propagating over bottom slopes generate seismic waves, and a secondary mechanism which relies on the nonlinear interaction of ocean waves. Here we show that the primary mechanism explains the average power, frequency distribution, and most of the variability in signals recorded by vertical seismometers, for seismic periods ranging from 13 to 300 s. The secondary mechanism only explains seismic motions with periods shorter than 13 s. Our results build on a quantitative numerical model that gives access to time-varying maps of seismic noise sources.

Numerical modeling of the Mount Steller landslide flow history and of the generated long period seismic waves

The rock-ice avalanche that occurred in 2005 on Mount Steller, Alaska and the resulting long period seismic waves have been simulated for different avalanche scenarios (i.e., flow histories), with and without erosion processes taken into account. This 40-60 Mm3 avalanche traveled about 10 km down the slope, mainly on top of a glacier, eroding a significant amount of ice. It was recorded by 7 broadband seismic stations. The simulations were compared with the recorded long period seismic signal and with the inverted flow history. The results show that, when erosion processes are taken into account, the simulations reproduce the observed signal at all the stations over a wide range of azimuths and source-station distances (37-623 km). This comparison makes it possible to constrain the rheological parameters involved which should help constrain the volume of eroded material. Because landslides are continuously recorded by seismic networks, this method could significantly broaden quantitative insights into natural flow dynamics.

Polarized Earth's ambient microseismic noise

We quantify, analyze, and characterize the frequency-dependent microseismic noise recorded by worldwide distributed seismic stations. Microseismic noise is generated through the interaction of ocean waves. It is the strongest ambient noise, and it is observed everywhere on Earth. We introduce a new approach which permits us to detect polarized signals in the time-frequency domain and which we use to characterize the microseismic noise. We analyze 7 years of continuous seismograms from the global GEOSCOPE network. Microseisms are dominated by Rayleigh waves, and we therefore focus on elliptically polarized signals. The polarized signals are detected in the time-frequency domain through a degree of polarization measure. We design polarization spectra and show that microseismic noise is more strongly polarized than noise in other frequency bands. This property is used to measure the directions of the polarized noise at individual stations as a function of time and frequency. Seasonal variations are found for the back azimuths and for the number of polarized signals at many stations. We show that the back azimuth directions are robust measurements that point toward the source areas computed from ocean wave models.

MOISE: A pilot experiment towards long term sea-floor geophysical observatories

We describe the scientific purposes and experimental set-up of an international deployment of a 3 component broadband seismometer package on the ocean floor in Monterey Bay which took place during the summer of 1997. Highlights of this experiment were the installation, performed using a remotely operated vehicle (ROV), the underwater connection of the different components of the package, and the successful retrieval of 3 months of broadband seismic and auxiliary data. Examples of recordings of teleseisms and regional earthquakes are presented and the background noise characteristics are discussed, in comparison with those of near-by broadband land sites, current-meter data from the vicinity of the ocean bottom package, as well as pressure data from deeper ocean sites.

Anisotropic tomography of the Atlantic Ocean from Rayleigh surface waves

Abstract The depth extent of the Mid Atlantic Ridge and the role of hotspots in the Atlantic opening are still a matter of debate. In order to constrain the structure and the geodynamic processes below the Atlantic Ocean, we provide the first anisotropic phase velocity maps of this area, obtained at a regional scale. We have determined Rayleigh wave phase velocities along 1311 direct epicentre to station paths. For each path, phase velocities are calculated by a technique of cross-correlation with a synthetic seismogram. These phase velocities are corrected for the effect of shallow layers. They are then inverted, without a priori constraints, to obtain maps of the lateral variations of the anisotropic phase velocities in the period range 50–250 s. The ridge axis corresponds to a low velocity anomaly, mainly visible at short periods. A good correlation between hotspot locations and low velocity anomalies is obtained for the whole period range. Furthermore, a low velocity anomaly elongated along a North–South direction is visible for every period and seems to be correlated with hotspot positions. On average, the North Atlantic is associated with higher velocities than the South Atlantic. The shields below Canada, Brazil and Africa display high velocity anomaly at short periods and only the Brazilian and African shields are still visible for a period of 200 s, thus suggesting that the Canadian shield is a shallower structure. The maps of phase velocity anisotropy under the Atlantic Ocean are interpreted in the Mid-Atlantic area, where we have the best resolution. Close to the ridge, the fast axis of Rayleigh wave phase velocity is found perpendicular to the ridge axis. A comparison of anisotropy directions and plate motion shows that seismic anisotropy integrates also deeper phenomena such as mantle convection.

On the shaping factors of the secondary microseismic wavefield

Seismic noise in the period band 3-10 s is known as secondary microseism and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. In this paper, we investigate the seismic content of the wavefield generated by a source at the ocean surface and three of the major wavefield shaping factors using the 2D spectral-element method: the ocean-continent boundary, the source site effect and the thickness of seafloor sediments. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed of the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean-continent boundary. The presence of a continental shelf at the ocean-continent boundary produces a negligible effect on land-recorded seismograms, whereas the source site effect, i.e. the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land-recorded long period signals (T > 6 s) are mostly due to deep water sources, whereas land-recorded short period signals (T > 6 s) are due to sources in relatively shallow water, located close to the shelf break. Seafloor sediments around the source region trap seismic waves reducing the amplitude of land-recorded signals, especially at long periods (T > 6 s).

Tracking major storms from microseismic and hydroacoustic observations on the seafloor

Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseismic noise. Here we use a network of 57 ocean bottom seismometers (OBS) deployed around La Reunion Island in the southwest Indian Ocean to investigate the noise generated in the secondary microseismic band as a tropical cyclone moved over the network. Spectral and polarization analyses show that microseisms strongly increase in the 0.1–0.35 Hz frequency band as the cyclone approaches and that this noise is composed of both compressional and surface waves, confirming theoretical predictions. We infer the location of maximum noise amplitude in space and time and show that it roughly coincides with the location of maximum ocean wave interactions. Although this analysis was retrospectively performed, microseisms recorded on the seafloor can be considered a novel source of information for future real-time tracking and monitoring of major storms, complementing atmospheric, oceanographic, and satellite observations.