Global seismology

Three complete data sets of strong earthquakes (M -> 5.5), which occurred in the seismic regions of Chile, Mexico and Kamchatka during the time period 1899-1985, have been used to test the existence of a time-lag in the seismic energy release between these regions. These data sets were cross-correlated in order to determine whether any pair of the sets are correlated. For this purpose statistical tests, such as the T-test, the Fisher's transformation and probability distribution have been applied to determine the significance of the obtained correlation coefficients. The results show that the time-lag between Chile and Kamchatka is -2, which means that Kamchatka precedes Chile by 2 years, with a correlation coefficient significant at 99.80% level, a weak correlation between Kamchatka-Mexico and noncorrelation for Mexico-Chile.

We analyze the sensitivity of PP precursor traveltimes that are often used to infer lateral variation in the depths of the 410-and 660-km discontinuities in the mantle. Previous results were inconclusive due to complex wave phenomena, such as multiple energy conversions and focusing/defocusing, that hamper their interpretation. Using spectral-element synthetics and Fréchet derivatives calculated with adjoint methods, we compute sensitivity kernels for volumetric and boundary parameters in a 1-D model for representative epicentral distances of past studies, and a dominant period of 11-25 s. Our results indicate that the boundary sensitivity of PP precursors is low and that these phases are not coherently seen in exact synthetics. Our most important finding is the strong sensitivity to both shear and compressional wave speeds, indicating that wave interference and wave conversions are dominant. The PP precursor traveltimes appear more sensitive to structural parameters, that is, compressional and shear wave speed, than to the boundaries; therefore, they are unlikely sources for valuable insight into discontinuity topography. Plain Language Summary Seismologists frequently use traveltimes of seismic waves to understand how the Earth's internal structure changes at various depths. This study presents pictorial examples of how traveltimes of seismic waves sense the depth variations inside the Earth. We focus on waves that reflect from specific depths, and they are often used to explain topography along interfaces at 410-and 660-km depths. Along these depths, the ambient mantle conditions change significantly, indicating areas of major changes in material distribution, which enable vigorous movements within the Earth and cause geological phenomena observed at the surface. We aim at explaining why resulting models using these traveltimes are not in agreement. To achieve that, we use exact methods that give an accurate picture of the sensitivity of the traveltimes under investigation. Our contribution is important because we visualize many effects relating to how the waves travel, convert, and interact with each other. These effects, if not taken into account in an exact manner, could lead to inaccurate mapping of topography. Our explanations can help seismologists understand the reasons of disagreement between models produced using these traveltime data. We conclude that further interpretations relating to temperature and compositional variations should be considered with great care.

A complete data set of globally distributed shallow (h ~< 60 Km) earthquakes have been used for first time to test the possible existence of periodicities in the seismic energy release. Only main shocks of magnitude, M >/7.0 were considered, which occurred in the whole Earth during 1898-1985. These magnitudes are converted in seismic energy, which is released during the occurrence of earthquakes, through Bath's formula. The detection of such kind of periodicities is important in seismology, because these patterns may lead to the prediction of large earthquakes. Statistical techniques, such as Maximum Entropy (ME), and two Fourier approaches, the Fast Fourier Transform (FFT) and Power Spectrum (PS) of truncated subrecords of the whole time series have been applied to examine the possible existence of such periodicities in seismic energy release. Furthermore, the even-spacing technique is used to validate our results and a type-curve has been constructed for the data set.

Focusing effects on body waves are influenced by the second derivative of the velocity field perpendicular to the ray path, and contain potentially valuable information on the Earth's heterogeneity. A new generation of well calibrated, broadband, digital stations reduces the effect of instrumental errors in amplitude measurements. We report on an extensive study of P wave amplitudes from 217 intermediate to deep earthquakes recorded at Global Seismic Network (GSN) broadband stations