Sébastien Chevrot

The Poisson ratio of the Australian crust: geological and geophysical implications

The Poisson ratio, which depends on the VP/VS ratio, provides much tighter constraints on the crustal composition than either the compressional or the shear velocity alone. The crustal Poisson ratio can be determined from the joint analysis of the travel times of waves converted at the Moho and of crustal multiples reflected at the top of the Moho. We have analyzed the records of the permanent stations installed on the Australian continent, complemented by the data of the SKIPPY experiment. The results reveal substantial variations in the Poisson ratio in the different tectonic units. For the Proterozoic crust, an increase of the Poisson ratio with increasing crustal thickness is systematically observed while for the Phanerozoic crust, the Poisson ratio tends to decrease for increasing crustal thicknesses. These observations are in remarkable agreement with the results of the deep seismic soundings that were performed in the former Soviet Union. The variations observed in the Proterozoic provinces can perhaps be explained by underplating of mafic materials at the base of the crust. fl 2000 Elsevier Science B.V. All rights reserved.

Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversions

The technique of receiver function analysis is applied to the study of crustal and upper mantle structures beneath the Kaapvaal craton in southern Africa and its surroundings. Seismic data were recorded by the seismic array of 82 sites deployed from April 1997 to April 1999 across southern Africa, as well as a dense array of 32 sites near Kimberley, in operation from December 1998 to June 1999. Arrival times for phases converted at the Moho are used to determine crustal thickness. The Moho depth in the south–western section of the craton was found to vary between 37 and 40 km, except for one station that recorded a depth of 43 km (SA23). Farther north along the western block of the craton (into Botswana) the depth increases up to 43 km. The depth increases even further in the north–eastern section of the craton, where results vary from 40 to 52 km. Just north of the Kaapvaal craton, in the neighbouring Zimbabwe craton, the crustal thickness drops significantly. The results obtained there varied from 36 to 40 km. For the Kimberley area, using the dense array, the Moho depth was found to be 37.3 km. Arrivals of the Ps and Ppps phases were used to determine the Poisson’s ratio in the region. This was found to be 0.26 ± 0.01. Arrivals of phases from the 410 and 660 km mantle discontinuities are used to interpret the relative positions of these discontinuities, as well as for comparison of mantle temperatures and seismic velocities in the region with global averages. In the Kimberley area the 410 and 660 km discontinuities were found at their expected depth, implying that mantle temperatures in the region are close to the global average. The seismic velocities above the ‘410’ were found up to 5% faster than the averages from the global iasp91 model, which is fast even by Precambrian standards. In other sections of the Kaapvaal craton, the velocities are also faster than global averages, but not as fast as beneath Kimberley. In these sections, the ‘410’ is also slightly elevated, while the ‘660’ is depressed, which implies a slightly lower mantle temperature relative to the global average. Beneath the Kaapvaal craton we find evidence suggesting the presence of a zone with a reduced wavespeed gradient at an upper bound of approximately 300 km, which may mark the lower chemical boundary of the craton.

Simultaneous Inversion of Source Spectra, Attenuation Parameters, and Site Responses: Application to the Data of the French Accelerometric Network

Displacement spectra of earthquakes recorded by the French accelero- metric network at regional scale are modeled as the product of source, propagation (including geometric and anelastic attenuation), and site effects. We use an iterative Gauss-Newton inversion to solve the nonlinear problem and retrieve these different terms. This method is easy to implement because the partial derivatives of the am- plitude spectrum with respect to the different parameters have simple analytic forms. After convergence, we linearize the problem around the solution to compute the cor- relation matrix, which allows us to identify the parameters which are poorly resolved. We analyze data from two tectonically active regions: the Alps and the Pyrenees. Eighty-three earthquakes with local magnitudes between 3.0 and 5.3 are analyzed, with epicentral distances in the range 15-200 km. S-wave displacement spectra are computed using a fast Fourier transform and integration in the 0.5-15-Hz fre- quency domain. We assume a Brune-type source, with a geometric attenuation of the form R � γ , γ being constant, and a frequency-dependent quality factor of the form QQ0 × f α . The results reveal that the attenuation parameters are correlated to each other and to the seismic moments. The two regions have different attenuation patterns. The geometrical spreading factor is equal to 1 for the Alps and 1.2 for the Pyrenees. The anelastic attenuation exhibits low Q0 values (322 and 376 for the Alps and the Pyrenees, respectively) with regional variations for α (0.21 in the Alps and 0.46 in the Pyrenees). Computed moment magnitudes are generally 0.5 unit smaller than local magnitudes, and the logarithms of the corner frequencies decrease linearly with mag- nitude according to log10� fc �� 1:72 � 0:32 × Mw. Stress drops range from 10 5 to 10 7 Pa (i.e., 1-100 bars), with a slight dependence to magnitude (large stress drops for large magnitudes). Finally, robust site responses relative to an average rock-site response are derived, allowing us to identify good reference rock sites.

Correlation between the shear-speed structure and thickness of the mantle transition zone

The 410 and 660 km seismic discontinuities that bound the mantle transition zone (TZ) are attributed to phase transformations in olivine structure. This implies that variations in TZ thickness ( HTZ) should correlate with those in TZ temperature. Pertinent seismic evidence has so far been ambiguous, however. We measure converted-wave (P ds) differential times tdiff = tP 660s − tP 410s in SE Asia and Australia and compare them with S-velocity (βTZ) estimates from regional tomographic models. Both tdiff and βTZ vary on a scale of a few hundred kilometers. Inferred variations in HTZ are up to ±30 km over length scales larger than 500 km, implying ±200 K thermal heterogeneity if the effect of composition can be neglected. tdiff and βTZ correlate strongly; the linear dependence of HTZ on the average temperature within the TZ is consistent with olivine Clapeyron slopes. We also show that this relationship holds on a global-scale as well, provided that the scalelengths and uncertainties of the variations in tdiff and βTZ are taken into account. These results confirm that the transformations in olivine structure give rise to the 410 and 660 km discontinuities globally.

The 660-km discontinuity within the subducting NW-Pacific lithospheric slab

Abstract The 660-km seismic discontinuity ( 660 ) in Earth’s mantle is generally attributed to the breakdown of the ringwoodite phase of olivine, but other mineral reactions are also thought to occur near 660-km depth. Recently, complex arrivals of P 660 s waves (converted from P to s at the 660 ) in active and recently active subduction zones have been interpreted as evidence for additional seismic discontinuities caused by the garnet–perovskite and garnet–ilmenite–perovskite phase transformations ( gt → pv , gt → il → pv ) at relatively low temperatures. Here we show that the P 660 s phases converting at the 660 within the subducting NW-Pacific slab beneath the station MDJ in Northeast China are clear and coherent, with no additional arrivals in the vicinity. P 660 s waves that convert near the boundaries of the area where the 660 occurs within the slab produce distinctly more complex, multiple arrivals, but they are more likely to be caused by small-scale topography rather than ‘multiplicity’ of the 660 . Our observations suggest that the gt → pv transformation and the gt → il → pv , if it occurs in the mantle, are spread over tens of kilometers and do not have sharp onsets visible to short-period seismic waves.

Teleseismic travel time residuals in North America and anelasticity of the asthenosphere

Abstract The slope a of the relation δ t S = a δ t P + b for the teleseismic S and P travel time residuals δ t s and δ t p is practically the only robust observational value that is equally sensitive to the lateral S and P velocity variations in the upper mantle. Published estimates of a for North America are about twice higher than those at ultrasonic frequencies in olivine at comparable pressures and temperatures. In our study, lateral variations of the S residuals in North America are found not from the S wave readings, which can be easily biased, but from accurately determined delays relative to P of the Ps phases converted from `410 km and `660 km discontinuities. Nevertheless, the estimates of a thus derived are close to the values obtained with the standard technique. The discrepancy between the ultrasonic and seismic estimates of a can be explained by physical velocity dispersion. On the assumption of a single absorption band, our value of a is consistent with the published values of Q in the seismic frequency band and velocities in the ultrasonic range, if the high-frequency cutoff of the absorption band in the upper mantle beneath western North America is on the order of several hundred Hertz. However, in the literature there are indications of a cutoff at around 1 Hz. Then our data imply the presence of another absorption band at high frequencies outside the seismic frequency band. This band may correspond to the viscous dissipation in the Walsh [Walsh, J.B., 1969. A new analysis of attenuation in partially melted rock. J. Geophys. Res., 74, 4333–4337] model of penny-shaped inclusions of melt.

A hybrid technique for 3-D modeling of high frequency teleseismic body waves in the Earth

In the last decade, the deployment of dense regional arrays such as the USArray transportable array has considerably improved our capacity to image the interior of the Earth.