A new method for estimating the correlation of seismic waveforms based on the NTFT

Summary We propose a new correlation function called the similarity coefficient (SC) based on the normal time-frequency transform (NTFT) to evaluate the similarity between two nonstationary seismic signals as a function of the delay time. The SC is defined in the time-frequency spectrum of the NTFT, and the instantaneous phase and amplitude of each frequency component in a signal are employed to calculate the SC. Our simulation experiments demonstrate that the SC method can effectively recognize similar signals compared to the conventional normalized cross-correlation coefficient (NCC) under high background noise conditions. The SC presents good robustness in identifying similar signals and performs well in the case of an extremely low signal-to-noise ratio (SNR), which makes it suitable for detecting weak seismic signals concealed by noise. As a real application case, we use the SC method to detect quasi-Love (QL) surface waves. QL waves are scattered Love waves and are important indicators for lateral anisotropic gradients in the upper mantle. We detect the QL waves at 21 stations deployed across Japan after the December 23 2004 Mw 8.1 Macquarie earthquake by using the SC method. Obvious QL waves are observed at 19 stations, and we locate the Love-to-Rayleigh scatterers by applying the delay times between the QL and main Love waves. Our results show that the QL wave scatterers were mostly generated in two areas: Mariana subduction and Papua New Guinea. The observations of QL waves suggest the presence of lateral gradients in anisotropy beneath those two areas. The spatial distribution of the 13 scatterers in the Mariana subduction zone agrees well with the Mariana Island Arc, and we infer that the Mariana slab may have melted and coupled with the surrounding mantle at depth.

Comparative geodynamics of Aleutian and Izu-Bonin-Mariana island-arc systems

Fulfilled comparative analysis of the Aleutian and Izu-Bonin-Marian island-arc systems structure and geodynamic development. Izu-Bonin-Maian island-arc systems situated along сontinental margin of Eurasia in the West of Pacific Ocean. The Aleutian island-arc system is situated between the North American and Eurasian continents. Aleutian and Izu-Bonin-Marian island-arc systems appeared to be of the same age. Both island-arc systems form autonomous Philippine and Beringia small lithospheric plates. Izu-Bonin-Marianas island-arc system formed on exclusively geodynamic interaction of oceanic plate and back-arc basins, with the main role of the Pacific subduction. Aleutian system at the initial stage was formed as a result from separation of the part of Pacific Cretaceous crust by Aleutian subduction zone. The subsequent process of Aleutian system development was caused by geodynamics of movement of North American and Eurasian lithospheric plates. Pacific plate constant oblique subduction led to expansion of Aleutian island-arc system in the Western direction.

Effects of Remote Generation Sites on Model Estimates of M2 Internal Tides in the Philippine Sea*

This study investigates the impact of remotely generated internal tides on model estimates of barotropic to baroclinic tidal conversion for two generation sites bounding the Philippine Sea: the Luzon Strait and the Mariana Island Arc. A primitive equation model is used to characterize the internal tides generated by the principal semidiurnal tide (M2) over a domain encompassing the two generation sites. Energetic internal tides are generated at the Luzon Strait where nearly 17 GW of barotropic tide energy is converted to baroclinic energy, of which 44% (4.78 GW) is radiated eastward into the Philippine Sea. From the Mariana Arc, baroclinic energy propagates westward into the Philippine Sea as a result of 3.82 GW of barotropic to baroclinic energy conversion. Simulations that focus on each generation site without influence of the other are performed, and comparisons show that remotely generated internal tides have a significant effect on local conversion at the two sites. Total conversion is greater in the absence of remotely generated internal tides at both sites: 11% greater at the Luzon Strait and 65% greater at the Mariana Arc. The first three modes of the remotely generated internal tides traverse the basin and alter the amplitude and phase of bottom pressure. The arrival of the remote internal tides varies significantly with changing stratification and mesoscale circulation. The results suggest that an important source of variability in local conversion around the globe is due to remotely generated internal tides.