Constituents of Vertical-component Coda Waves at Long Periods

Abstract

AKI (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude 7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than 35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f-k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh waves for synthesizing the envelope. At lapse times later than 20,000 s-35,000 s, higher modes with phase velocities around 20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus earthquake. The small coda attenuation factor from (1.90±0.23) · 10 )3 to (2.38±0.32) · 10 )3 estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very large lapse times.

The lack of equipartitioning in global body wave coda

“…At later lapse times amplitudes at high slownesses decay faster than those of small slownesses thereby narrowing the slowness range of late coda energy. This is in accordance with observations by Maeda et al [2006] at a period of 128 s. The wavefield at late lapse times is dominated by waves propagating in the deep Earth because of the stronger attenuation of seismic waves in the shallow Earth and high Q values in the mantle and core. The superposition of waves propagating in the high Q deep Earth and these shallow propagating waves leaves an imprint on the envelope that has a significant curvature in a logarithmic scale, indicating a combination of different attenuation processes (green curve of Figure 1).…”

Circularly Polarized Luminescence Switching in Small Organic Molecules

“…[13] At the same time, the CPL switching (i.e.,i nversion of handedness,t he on/off switching, and the shifto fe mission wavelength), which can be triggered by the external stimuli such as light, solvent, pressure and ions, is becoming an important topic. [14] The CPL-switchable SOMs are expected to find promising applicationsi ns ecurity technologiesa nd sensing devices, which promoted us to review the recent progress in this field. CPL has been frequently observed for supramolecular assemblies, the structure of which has great influence on the CPL properties.…”

“…[18] Most of the relatedr eports have been well summarized by Maeda in 2013. [14] In addition to the assembly structure, other factors, such as excimer formation, [19][20][21][22][23][24] conformation of monomeric fluorophores, [25][26][27][28][29][30][31] electronic nature of p-conjugated skeleton or peripherals ubstituent, [32][33][34][35] and intramolecular charge-transfer (CT) dynamics, [36] maya lso exhibits significant influences on the CPL properties. Our discussion will focus on the strategies to tune the factors that influence CPL properties, which is expected to complementw ell the recent review about polymers and SOMs displayings timuli-responsive CPL.…”

Interference of long-period seismic wavefield observed by the dense Hi-net array in Japan

“…[36] Our interpretation of the phase splitting as a Moiré pattern explains well the observed features of the shortperiod wavefield of T = 20 to 50 s. Since Moiré patterns appear as a result of the interference of two plane waves, this feature is expected to be caused by medium heterogeneity at the far-field rather than by medium heterogeneity just beneath Japan. Scattering by mountains [Ji et al, 2005;Stich and Morelli, 2007] or seamounts [Obara and Matsumura, 2010], by continental margin [Capon, 1970;Meier and Malischewsky, 2000], or by medium heterogeneity [Maeda et al, 2006;Sato and Nohechi, 2001] between the epicenter and Japan could be the cause of these multipath effects.…”

“…Although a true broadband seismograph network (F-net) is also deployed in Japan, the station interval of this network, which is roughly 100 km, is sparser than that of the Hi-net. Maeda et al [2006] used F-net stations as a large-aperture broadband array to detect scattered waves of long-period Rayleigh waves of the period band of from 90-180 s. The use of F-net as an array for shorter-period waves is quite difficult due to the sparse station separation. Therefore, the Hi-net high-density and high-gain shortperiod network in Japan with the shorter station intervals may potentially be a powerful tool for understanding propagation of short-period seismic wave with a compliment of the F-net broadband network.…”