Alessia Maggi

Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere

Almost all earthquakes on the continents are confined within a crustal layer that varies in thickness (Ts) from about 10 to 40 km, and are not in the mantle. Variations in Ts correlate with variations in the effective elastic thickness ( Te), both of them having similar values, although Te is usually the smaller of the two. These observations suggest that the lower crust, at least in some places, is stronger than the mantle beneath the Moho, contrary to most models of continental rheology. Thus the strength of the continental lithosphere is likely to be contained within the seismogenic layer, variations in the thickness of this strong layer determining the heights of the mountain ranges it can support. The aseismic nature of the continental mantle and the lower crustal seismicity beneath some shields are probably related to their water contents.

Adjoint Tomography of the Southern California Crust

Crustal Details Revealed In seismic tomography, a large collection of data representing paths through Earth are inverted to provide an analysis of variation of density in which errors are minimized. Typically, the inversion starts with a simple layered model of the tomographic region. Tape et al. (p. 988) show how, starting with a three-dimensional model, based on synthetic seismograms, an improved iterative inversion approach can lead to a much more detailed view of a region. Using the rich data for Southern California, the model reveals details of the geologic history of the crust in this region. Analysis of seismic data using a more realistic crustal model reveals detailed variations in density beneath southern California. Using an inversion strategy based on adjoint methods, we developed a three-dimensional seismological model of the southern California crust. The resulting model involved 16 tomographic iterations, which required 6800 wavefield simulations and a total of 0.8 million central processing unit hours. The new crustal model reveals strong heterogeneity, including local changes of ±30% with respect to the initial three-dimensional model provided by the Southern California Earthquake Center. The model illuminates shallow features such as sedimentary basins and compositional contrasts across faults. It also reveals crustal features at depth that aid in the tectonic reconstruction of southern California, such as subduction-captured oceanic crustal fragments. The new model enables more realistic and accurate assessments of seismic hazard.

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.

S‐velocity model and inferred Moho topography beneath the Antarctic Plate from Rayleigh waves

Since 2007/2008, seismographs were deployed in many new locations across much of Antarctica. Using the records from 122 broadband seismic stations, over 10,000 Rayleigh wave fundamental-mode dispersion curves have been retrieved from earthquake waveforms and from ambient noise. Using the processed data set, a 3-D S-velocity model for the Antarctic lithosphere was constructed using a single-step surface wave tomographic method, and a Moho depth map was estimated from the model. Using the derived crustal thicknesses, the average ratio of lithospheric mantle and crustal densities of Antarctica was calculated. The calculated density ratio indicates that the average crustal density for Antarctica is much higher than the average values for continental crust or the average density of lithospheric mantle is so low as to be equal to low-density bound of Archean lithosphere. The latter implies that the lithospheric mantle in much of Antarctica should be old and of Archean age. The East Antarctic Mountain Ranges (EAMOR) represent a thick crustal belt, with the thickest crust (~60 km) located close to Dome A. Very high velocities can be found at depths greater than 200 km beneath parts of East Antarctica, demonstrating that the continental lithosphere extends deeper than 200 km. The very thick crust beneath the EAMOR may represent the collision suture of East Gondwana with Indo-Antarctica and West Gondwana during the Pan-African orogeny.

Automated identification, location, and volume estimation of rockfalls at Piton de la Fournaise volcano

Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Reunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate, and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 m. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a data set of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit.

Continuous Kurtosis‐Based Migration for Seismic Event Detection and Location, with Application to Piton de la Fournaise Volcano, La Réunion

Abstract We present an automatic earthquake detection and location technique based on migration of continuous waveform data. Data are preprocessed using a kurtosis estimator in order to enhance the first arrival information, then migrated onto a predefined search grid using precalculated P ‐wave travel times, and finally stacked. Local maxima in the resulting 4D space–time grid indicate the locations and origin times of seismic events. We applied our technique to earthquake swarms occurring on Piton de la Fournaise volcano, La Reunion, France. We located 5000 events from 12 different swarms that occurred between 2009 and 2011. Our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level. Multiplet analysis of the detected events and subsequent double‐difference relocation produce sharper images of the earthquake swarms. Our code, Waveloc, is released in open source. Online Material: Figures of seismicity distributions from Waveloc, synthetic test, and stack amplitude values versus magnitudes.

Temperature, lithosphere‐asthenosphere boundary, and heat flux beneath the Antarctic Plate inferred from seismic velocities

We estimate the upper-mantle temperature of the Antarctic Plate based on the thermoelastic properties of mantle minerals and S velocities using a new 3-D shear velocity model, AN1-S [An et al., 2015, JGR]. Crustal temperatures and surface heat fluxes are then calculated from the upper-mantle temperature assuming steady-state thermal conduction. The temperature at the top of the asthenosphere beneath the oceanic region and West Antarctica is higher than the dry mantle solidus, indicating the presence of melt. From the temperature values, we generate depth maps of the lithosphere–asthenosphere boundary and the Curie-temperature isotherm. The maps show that East Antarctica has a thick lithosphere similar to that of other stable cratons, with the thickest lithosphere (~250 km) between Domes A and C. The thin crust and lithosphere beneath West Antarctica are similar to those of modern subduction-related rift systems in East Asia. A cold region beneath the Antarctic Peninsula is similar in spatial extent to that of a flat-subducted slab beneath the southern Andes, indicating a possible remnant of the Phoenix Plate, which was subducted prior to 10 Ma. The oceanic lithosphere generally thickens with increasing age, and the age–thickness correlation depends on the spreading rate of the ridge that formed the lithosphere. Significant flattening of the age–thickness curves is not observed for the mature oceanic lithosphere of the Antarctic Plate.

Frequency‐dependent noise sources in the North Atlantic Ocean

M.S. acknowledges financial support by the projects Rifsis (CGL 2009–09727) and TopoIberia (CSD2006-00041). This is IPGP contribution 3449.

Seismic Wave Interactions Between the Atmosphere - Ocean - Cryosphere System and the Geosphere in Polar Regions

At the time of the International Geophysical Year (IGY; 1957-1958), it was generally understood by a majority of seismologists that no extreme earthquakes occurred in polar regions, particularly around Antarctica. Despite the Antarctic being classified as an aseismic region, several significant earthquakes do occur both on the continent and in the surrounding oceans. Since IGY, an increasing number of seismic stations have been installed in the polar regions, and operate as part of the global network. The density of both permanent stations and temporary deployments has improved over time, and has recently permitted detailed studies of local seismicity (Kaminuma, 2000; Reading, 2002; 2006; Kanao et al., 2006).

Seismological constraints on ice properties at Dome C, Antarctica, from horizontal to vertical spectral ratios

Abstract The French-Italian Concordia (CCD) seismological station at Dome C is one of two observatories setup on the ice cap in the interior of the Antarctic continent. We analysed the seismic signal due to ambient noise at this station and at three temporary stations 5 km away from Concordia, in order to specify the ice properties beneath them. A method based on the horizontal to vertical (H/V) spectral ratio, commonly used to analyse soil response in seismic regions, was applied to the Antarctic stations. The main peak in the spectral ratios is observed at frequencies 6.7–8 Hz at the Dome C stations, but it is not observed at another station on the ice cap, QSPA, where the sensor is buried at 275 m depth. This peak can be explained by a 23 m thick unconsolidated snow or firn layer with a low S-wave velocity of 0.7 km s-1, overlying a consolidated layer with S-wave velocity 1.8 km s-1. Despite the non-uniqueness of the solutions obtained by fitting the H/V spectra, this model is preferred because the depth of the velocity contrast coincides with the density at which ice particles arrange themselves in a continuous, dense lattice. A small variability of this structure is observed around Dome C.