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Records of pressure variations on seismographs were historically considered unwanted noise; however, increased deployments of collocated seismic and acoustic instrumentation have driven recent efforts to use this effect induced by both wind and anthropogenic explosions to invert for near-surface Earth structure. These studies have been limited to shallow structure because the pressure signals have relatively short wavelengths (0.8) correlations between pressure and vertical component ground motion at 83% of the stations, but only 30% of stations show this on the radial component, likely due to complex tilt effects. We use average elastic properties in the upper 5.2 km from the CRUST1.0 model to estimate vertical seismic/acoustic coupling coefficients (SV/A) across the GSN network and compare these to recorded observations. We exclude many island stations from these comparisons because the 1° resolution of the CRUST1.0 model places a water layer below these stations. Our simple modeling can predict observed SV/A within a factor of 2 for 94% of the 51 non-island GSN stations with high correlations between pressure and ground motion. These results indicate that analysis of acoustic-to-seismic coupling from the eruption could be used to place additional constraints on crustal structure models at stations with collocated seismic and pressure sensors. Ultimately, this could improve tomographic imaging models, which rely on methods that are sensitive to local structure.

We have developed a method to determine shear-modulus (rigidity) structure for the upper 20-50m of the Earth. The method is based on the analysis of co-located pressure and seismic instruments. We applied this method to about 200 (co-located) stations in Alaska and examined seasonal variation in shallow shear-modulus structure at each site; in this report we quantify this seasonal change by taking the ratio (R) of the highest shear-modulus to the lowest throughout a year and use it as a characteristic feature for each station.R is smaller than 2 at many stations but there are some stations in and near the Arctic zone that have R larger than 10. Such a large seasonal change implies that there occurs massive melting of shallow permafrost and a significant development of the active layer every summer. As a side product, because of such a huge reduction in near-surface shear-modulus, horizontal amplitudes in seismic noise become 30 times larger in summer than amplitudes in winter.These seasonal changes may not be surprising because thawing of ice is common every summer in the permafrost region. But regions with large R show a systematic geographic pattern on a large-scale map; large-R stations are typically found near the coast (ocean) and tend to decrease toward the interior of the continent (Alaska and NW Canada). Large R stations are found in the NW Territories in Canada, the North Slope region northern side of the Brooks Range, near the Seaward Peninsula (west), and the Yukon-Kuskokwim Delta (west). These locations suggest a strong influence by the nearby ocean on the climate at each station. Proximity to the ocean (coast) seems to be an important factor in evaluating periglacial hazards.There are a few exceptions in the northernmost coastal stations as they show small R despite the fact that they are at the coast. But the ICEsat-2 (satellite) data show that sea ice seems to remain thick near the peninsula (near Barrow, Alaska) much longer than other coastal areas in this study; temperature is colder because of thicker sea ice and the amount of melting at these exception sites remains low. This would strengthen the hypothesis that near-coastal ocean has strong influence on the climate of continental interior.

Seismic noise with frequencies above 1 Hz is often called “cultural noise” and is generally correlated quite well with human activities. Recently, cities in mainland China and Italy imposed restrictions on travel and day-to-day activity in response to COVID-19, which gave us an unprecedented opportunity to study the relationship between seismic noise above 1 Hz and human activities. Using seismic records from stations in China and Italy, we show that seismic noise above 1 Hz was primarily generated by the local transportation systems. The lockdown of the cities and the imposition of travel restrictions led to an ∼4–12 dB decrease in seismic noise power in mainland China. Data also show that different Chinese cities experienced distinct periods of diminished cultural noise, related to differences in local response to the epidemic. In contrast, there was only ∼1–6 dB decrease of seismic noise power in Italy, after the country was put under a lockdown. The noise data indicate that traffic flow did not decrease as much in Italy and show how different cities reacted distinctly to the lockdown conditions.

We propose a single-station approach to estimate near-surface elastic structure using collocated pressure and seismic instruments. Our main result in this study is near-surface rigidity (shear modulus) structure at 784 EarthScope Transportable Array (TA) stations in operation from mid-2011 to the end of 2018 using coherent horizontal seismic and pressure signals at 0.02 Hz. We isolate time periods for which surface pressure change is the dominant excitation source for seismic signals by searching for data windows with large pressure variations and high-seismic-pressure coherence. We emphasize the importance of using horizontal seismic components for two reasons: first, horizontal seismic signals are significantly higher than vertical signals at 0.02 Hz due to ground tilt, and second, we can analytically compute the predicted horizontal signals without an assumption of atmospheric pressure wavespeed (which is required for predicting the vertical excitation). Sensitivity kernels from 0.01 to 0.05 Hz show that this pressure–seismic coupling is mostly dependent on rigidity shallower than 50 or 100 m. Our estimates of shallow elastic structure show good spatial agreement with large-scale surface geological features. For instance, stations in the Appalachian Mountains mostly have high rigidity, whereas low-rigidity sites dominate the Mississippi Alluvial Plain. Because of the lack of measured velocity profiles, we quantitatively validate our approach by comparing with VS30 models that are based on proxies such as topographic slopes and large-scale surface geology. We estimate near-surface rigidity at 784 TA stations, where these locations have no prior structure information. Our method provides independent information for seismic hazard studies.

SUMMARY Single-component ring lasers have provided high-resolution observations of Earth’s rotation rate as well as local earthquake- or otherwise-induced rotational ground motions. Here, we present the design, construction and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 m side length provide rotational motions that can be combined to construct the complete vector of Earth’s rotation from a point measurement with very high resolution. Combined with a classic broad-band seismometer, we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific design and construction details are discussed as are the resulting consequences for permanent observations. We present seismic observations of local, regional and global earthquakes as well as seasonal variations of ocean-generated rotation noise. The current resolution of polar motion is discussed and strategies how to further improve long-term stability of the multicomponent ring laser system are presented.

Author(s): Tanimoto, Toshiro; Wang, Jiong | Abstract: SUMMARY An algorithm developed in Tanimoto a Wang for deriving shallow elasticity structure from colocated pressure and seismic instruments is applied to data at nine stations in the Pinon Flat Observatory (PFO) in Southern California. Depth resolution kernels indicate that this approach can recover near-surface shear-modulus structure in the upper 50–100 m of the Earth. Our estimate for Vs30 at the closest station (BPH01) agrees with an independent result by Yong et al. which was based on an active source survey with a geophone array. Vs30 derived from the borehole S-wave speed model by Fletcher et al. also agrees with estimates at nearby locations, considering the size of uncertainties. Derived structures are much smoother than their models, however. This is because our method analyses slow quasi-static deformation of the solid Earth in the frequency range 0.005–0.05 Hz that inherently limits depth resolution. Lack of detailed variations in structures does not seem to be the problem for estimating Vs30 as Vs30 is an averaged quantity from the upper 30 m of the Earth. Also our estimates are not affected by different choices of frequency range in the inversion. However, our method may have some difficulty at stiff, hard-rock sites because ground deformation caused by surface pressure can become small and the key observables in our method, the ratios between seismic amplitude and surface pressure change, become difficult to measure accurately.

Stochastic excitation of seismic waves by a hurricane Toshiro Tanimoto* and Anne Valovcin Department of Earth Science and Earth Research Institute, University of California, Santa Barbara, California 93106, USA. *Corresponding author: Email toshiro@geol.ucsb.edu Abstract We investigate how a tropical cyclone (Hurricane Isaac in 2012) generated seismic ground motions using seismic and barometric data from the Earthscope network. In the frequency band 0.01-0.02 Hz, seismic and surface-pressure amplitudes show a systematic decreasing trend with distance from the center of the hurricane. However, the decreasing rate is much higher for seismic waves than for pressure. We develop a stochastic theory of seismic-wave excitation by surface pressure that connects these two observed data sets; surface pressure is the excitation source and seismic data are the resulting seismic- wave field. This theory contains two parameters: (i) the pressure power spectral density (pressure PSD, S p ) and (ii) the correlation length in the pressure field ( L ). Using the formula, we solve for the spatial variation of correlation lengths. The solution shows that longer correlation lengths in pressure are near the hurricane center. Because seismic- wave excitation is proportional to L 2 S p , the excitation for seismic waves becomes effectively more localized closer to the center. Also the scaling relation between L and S p leads to an excitation source which is approximately proportional to the third power of surface pressure. This centralized source for seismic-wave excitation explains why the

Pressure power spectral density (Pressure PSD), Seismic PSD (three components), Wind PSD, and Coherence between vertical data and pressure data. They constitute all the data that were used for figures in the paper by Tanimoto and Wang.

©2015. American Geophysical Union. All Rights Reserved. Using a colocated ring laser and an STS-2 seismograph, we estimate the ratio of Rayleigh-to-Love waves in the secondary microseism at Wettzell, Germany, for frequencies between 0.13 and 0.30 Hz. Rayleigh wave surface acceleration was derived from the vertical component of STS-2, and Love wave surface acceleration was derived from the ring laser. Surface wave amplitudes are comparable; near the spectral peak about 0.22 Hz, Rayleigh wave amplitudes are about 20% higher than Love wave amplitudes, but outside this range, Love wave amplitudes become higher. In terms of the kinetic energy, Rayleigh wave energy is about 20-35% smaller on average than Love wave energy. The observed secondary microseism at Wettzell thus consists of comparable Rayleigh and Love waves but contributions from Love waves are larger. This is surprising as the only known excitation mechanism for the secondary microseism, described by Longuet-Higgins (1950), is equivalent to a vertical force and should mostly excite Rayleigh waves.

©2014. American Geophysical Union. All Rights Reserved. A dense seismic array can provide new perspectives for a decaying hurricane after its landfall. The case of Hurricane Isaac in 2012 is presented using a seismic array from EarthScope (USArray). The amplitude-distance plots from the center of the hurricane showed a sharp peak at a distance of 75 km at the time of landfall. This peak decayed and moved outward from the center over the next 1.5 days. The sharp peak can be explained by strong surface pressure fluctuations under the eyewall in which a focused ascending flow is known to exist. We reconstructed the time evolution of surface pressure that explains seismic data. Pressure solutions indicate that the eyewall stayed at 75 km in the first 10 h after the landfall, while the ascending flow weakened significantly. In the following 24 h, the eyewall diffused and moved to distances about 200-300 km, suggesting its collapse during this period. Key Points Hurricane eyewall can be studied from seismic amplitude versus distance plotsSeismically derived surface pressure shows the location and strength of eyewallTime evolution of this decaying process can be tracked

Using closely located seismographs at Pinon Flat (PFO), California, for 1 year long record (2015), we estimated the Rayleigh-to-Love wave energy ratio in the secondary microseism (0.1-0.35 Hz) in four seasons. Rayleigh wave energy was estimated from a vertical component seismograph. Love wave energy was estimated from rotation seismograms that were derived from a small array at PFO. Derived ratios are 2-2.5, meaning that there is 2-2.5 times more Rayleigh wave energy than Love wave energy at PFO. In our previous study at Wettzell, Germany, this ratio was 0.9-1.0, indicating comparable energy between Rayleigh waves and Love waves. This difference suggests that the Rayleigh-to-Love wave ratios in the secondary microseism may differ greatly from region to region. It also implies that an assumption of the diffuse wavefield is not likely to be valid for this low frequency range as the equipartition of energy should make this ratio much closer.

Excitation of seismic waves by atmospheric pressure changes is examined from data for two tropical cyclones, Tropical Storm Lee (2011) and Hurricane Isaac (2012). They moved through the Earthscope Transportable Array (TA) and generated variations in pressure and ground motions that spanned 4-5 orders of magnitude in power spectral density (PSD). For vertical seismic ground velocity PSD (SV) for frequencies between 0.01 and 0.02 Hz, there is a threshold pressure at about pressure PSD (SP) of 10 (Pa2s), below which vertical motion is not affected by local atmospheric pressure. Above this threshold pressure, vertical ground motion increases with surface pressure as SV ~ SP1.5. In order to understand the land-atmosphere interaction, pressure above this threshold is the only useful range. Horizontal-component PSDs are about two orders of magnitude larger than vertical-component PSDs and change with pressure for its entire range. This overall trend is most likely caused by ground tilt.

SUMMARY Non-linear interaction of ocean waves is a widely accepted mechanism of microseism excitation for frequencies approximately between 0.05 and 0.5Hz. Longuet-Higgins published the most influential paper on this subject in 1950 and his main contributions can be summarized in two points; the first is on the double-frequency mechanism and the second is on the dependency of excitation on ocean depth. The two results were derived for different media, the first point for an incompressible liquid (ocean) and the second point for a compressible liquid layer over an elastic half-space. The two features naturally come out in the normal-mode formulae. We note, however, that the use of the Longuet-Higgins formula, that showed efficiency of excitation based on ocean depth, is not suitable for interpreting land-based observations because his formula was for displacement at ocean bottom. Reevaluation of this dependency for a land-based observation shows similar results for the depth of maximum excitation, but the sharpness of this peak is reduced significantly. Some recent observational studies tried to identify the source area of excitation by using this ocean-depth dependency, but such a result needs to be revised to a larger source region according to our results. We confirm that the maximum excitation of microseisms exists and occurs when the ocean depth is about 2.7km. It is thus not surprising to find an efficient pelagic source for microseisms, although existence of the wave‐wave interaction in the source region is essential.

S U M M A R Y We present an approach for seismic noise which improves Rayleigh-wave ellipticity estimates by reducing the influences from body waves and Love waves. The method requires three-component seismographs and uses the phase-shift information between the vertical and horizontal components. We select data that show the phase-shift of about 90◦ in order to separate Rayleigh waves from body waves and Love waves. In comparison to the usual H/V approach, the estimates for Rayleigh-wave ellipticity are systematically different by about 20 per cent because the existence of S waves and Love waves systematically increases horizontal amplitudes. The differences in the inverted S-wave velocity structure reach up to about 20 per cent in the upper 20 km. The method is limited to a relatively low-frequency range, below 0.3 Hz, as the 90◦ phase-shift peaks become difficult to identify above 0.3 Hz. Despite this limitation in the frequency range, this approach can improve the S-wave velocity structure in the upper 5–10 km, which should lead to a better prediction of long-period ground motions for periods of about 5–10 s. An improved prediction of such long-period ground motion is important for hazard mitigation in the world’s metropolitan areas with high-rise buildings. This approach is applied to the structure in the broader Los Angeles basin in Southern California and improvement of the reference velocity structure in the upper 5–10 km is demonstrated.

Monthly variations in the ratio of Rayleigh‐to‐Love waves in the secondary microseism are obtained from a colocated ring laser and an STS‐2 seismograph at Wettzell, Germany. Two main conclusions are derived for the Rayleigh‐to‐Love wave kinetic energy ratios in the secondary microseism; first, the energy ratio is in the range 0.8–0.9 (

SUMMARY An approach to detect afterslips in the source process of giant earthquakes is presented in the normal-mode frequency band (0.3–2.0 mHz). The method is designed to avoid a potential systematic bias problem in the determination of earthquake moment by a typical normal-mode approach. The source of bias is the uncertainties in Q (modal attenuation parameter) which varies by up to about ±10 per cent among published studies. A choice of Q values within this range affects amplitudes in synthetic seismograms significantly if a long time-series of about 5–7 d is used for analysis. We present an alternative time-domain approach that can reduce this problem by focusing on a shorter time span with a length of about 1 d. Application of this technique to four recent giant earthquakes is presented: (1) the Tohoku, Japan, earthquake of 2011 March 11, (2) the 2010 Maule, Chile earthquake, (3) the 2004 Sumatra-Andaman earthquake and (4) the Solomon earthquake of 2007 April 1. The Global Centroid Moment Tensor (GCMT) solution for the Tohoku earthquake explains the normal-mode frequency band quite well. The analysis for the 2010 Chile earthquake indicates that the moment is about 7–10 per cent higher than the moment determined by its GCMT solution but further analysis shows that there is little evidence of afterslip; the deviation in moment can be explained by an increase of the dip angle from 18° in the GCMT solution to 19°. This may be a simple trade-off problem between the moment and dip angle but it may also be due to a deeper centroid in the normal-mode frequency band data, as a deeper source could have steeper dip angle due to changes in geometry of the Benioff zone. For the 2004 Sumatra-Andaman earthquake, the five point-source solution by Tsai et al. explains most of the signals but a sixth point-source with long duration improves the fit to the normal-mode frequency band data. The 2007 Solomon earthquake shows that the high-frequency part of our analysis (above 1 mHz) is compatible with the GCMT solution but the low-frequency part requires afterslip to explain the increasing amplitude ratios towards lower frequency. The required slip has the moment about 19 per cent of the GCMT solution and the rise time of 260 s. The total moment of these earthquakes are 5.31 × 1022 N m (Tohoku), (1.86–1.96) × 1022 N m (Chile), 1.33 × 1023 N m (Sumatra) and 1.86 × 1021 N m (Solomon). The moment magnitudes are 9.08, 8.78–8.79, 9.35 and 8.11, respectively, using Kanamori’s original formula between the moment and the moment magnitude. However, the trade-off problem between the moment and dip angle can modify these estimates for moment up to about 40–50 per cent and the corresponding magnitude ±0.1.

The authors investigate the structural change of a ZnO–P 2 O 5 sintered matrix for the preparation of SnO-doped zinc phosphate (SZP) glass using a platinum crucible. Heat treatment of zinc oxide and ammonium phosphate at 800 °C causes the formation of a Zn 2 P 2 O 7 -like species, which is effective in preventing damage to the platinum crucible. Although the thermal property of the glass was affected by added Al 2 O 3 , Al 2 O 3 hardly affects the quantum efficiency of the SZP glass phosphor. This process is important for the fabrication of contamination-free phosphate glass using ammonium phosphate as a starting material.

Tornadoes generate seismic signals when they contact the ground. Here we examine the signals excited by the Joplin tornado, which passed within 2 km of a station in the Earthscope Transportable Array. We model the tornado-generated vertical seismic signal at low frequencies (0.01–0.03 Hz) and solve for the strength of the seismic source. The resulting source amplitude is largest when the tornado was reported to be strongest (EF 4–5), and the amplitude is smallest when the tornado was weak (EF 0–2). A further understanding of the relationship between source amplitude and tornado intensity could open up new ways to study tornadoes from the ground.

SUMMARY Direct detection of Rayleigh-wave azimuthal anisotropy is reported by applying an array analysis to broad-band seismic data in Southern California, USA. Our approach has excellent resolution for frequencies between 30 and 60 mHz and good resolution between 10 and 30 mHz. Limitation from array size limits accuracy below 10 mHz and complicated wave propagation effects lead to difficulty above 60 mHz. Between 30 and 60 mHz, azimuthal anisotropy of Rayleigh-wave phase velocity is detected cleanly. Phase velocity results show that the 2θ components dominate the 4θ components. Strength of anisotropy hovers around 1 per cent for the 2θ components with standard error (1 σ) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110° clockwise from north and its opposite direction 290°. Depth sensitivity kernels for P-wave and S-wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths. We speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the Pacific Plate and the North America Plate in the region.

SUMMARY Colliding ocean waves is considered to be the major source of seismic noise. In this paper, we examine the question, what would be the equivalent forces for such colliding ocean waves? In addition to the vertical pressure force term pointed out by Longuet-Higgins, we find that a lateral pressure gradient term is included in this system. This leads to a lateral dipole force and the excitation of noise that is different from vertical forcing. Numerical evaluation with PREM-like models shows that this term may become more important than vertical pressure term for frequencies below 5 millihertz (mHz). Importance of this horizontal term at low frequencies is related to the fact that horizontal motions of ocean waves become dominant in the low frequency band and its square is proportional to the excitation.

SUMMARY Rayleigh wave ellipticity has high sensitivity to shallow depth and contains complementary information to phase and group velocity data whose sensitivity extends to deeper region. The inversion of this parameter has often been conducted by using numerically derived depth sensitivity kernels. We present compact formulas based on a variational principle which includes both volumetric seismic velocity perturbations and boundary perturbations. Use of this formula can speed up the inversion process considerably.

SUMMARY The particle motion of surface waves, in addition to phase and group velocities, can provide useful information forS-wave velocity structure in the crust and upper mantle. In this study, we applied a new method to retrieve velocity structure using the ZH ratio, the ratio between vertical and horizontal surface amplitudes of Rayleigh waves. Analysing data from the GEOSCOPE network, we measured the ZH ratios for frequencies between 0.004 and 0.05 Hz (period between 20 and 250 s) and inverted them for S-wave velocity structure beneath each station. Our analysis showed that the resolving power of the ZH ratio is limited and final solutions display dependence on starting models; in particular, the depth of the Moho in the starting model is important in order to get reliable results. Thus, initial models for the inversion need to be carefully constructed. We chose PREM and CRUST2.0 in this study as a starting model for all but one station (ECH). The eigenvalue analysis of the least-squares problem that arises for each step of the iterative process shows a few dominant eigenvalues which explains the cause of the inversion’s initial-model dependence. However, the ZH ratio is unique in having high sensitivity to near-surface structure and thus provides complementary information to phase and group velocities. Application of this method to GEOSCOPE data suggest that low velocity zones may exist beneath some stations near hotspots. Our tests with different starting models show that the models with low-velocity anomalies fit better to the ZH ratio data. Such low velocity zones are seen near Hawaii (station KIP), Crozet Island (CRZF) and Djibuti (ATD) but not near Reunion Island (RER). It is also found near Echery (ECH) which is in a geothermal area. However, this method has a tendency to produce spurious low velocity zones and resolution of the low velocity zones requires further careful study. We also performed simultaneous inversions for volumetric perturbation and discontinuitydepth perturbation. While its formulation and inversion were straightforward, there seemed to be a difficult trade-off problem between volumetric perturbation and discontinuity-depth perturbation.

SUMMARY Exact normal-mode solutions for the seismic noise-correlation method are derived by assuming that excitation sources of seismic noise are distributed over the globe uniformly. While a previous derivation by Snieder was for propagating waves, using the stationary-phase approach, our results show that the method could work for normal modes. Comparison of the formula for the cross-correlated seismogram (correlogram) against the Green's function shows differences in four aspects: (1) the correlogram is proportional to the number of sources (density of sources), (2) depends on excitation-source spectra, (3) shows a larger weight on high-frequency band (inversely proportional to the square of modal mass) and (4) has time dependence which requires one extra integration over time. Waveform modelling of the correlograms seems difficult in practice, particularly because of the lack of information on the excitation sources, often the temporarily changing ocean waves. On the other hand, phase and group velocity measurements should be possible as long as the correlograms contain good surface wave signals.

It is well known that off-great-circle path propagation causes a technical difficulty for surface wave analysis in higher frequency ranges. We propose a new approach that combines a beamforming technique and two-station phase velocity measurement to resolve this problem. Beamforming allows us to determine the correct azimuth of incoming surface waves which can be taken into account in phase velocity measurement. Beamforming results also support that a plane-wave approximation is mostly acceptable for frequencies up to about 50–60 mHz (millihertz), although evidence of multipathing is occasionally recognized in beamforming results as multiple peaks. Application of this correction scheme for Rayleigh-wave data in Southern California seems to make the largest impact on the results of azimuthal anisotropy. Effects are not large for frequencies up to 30 mHz but fast velocity axes in azimuthal anisotropy maps change significantly for higher frequencies.

SUMMARY Recent development in seismology has shown that seismic noise is a useful source of Earth structure study. Non-linear interaction of ocean waves is considered to be one of the important causes for seismic noise, although source locations and detailed mechanisms are not clear yet. In order to improve our understanding of this mechanism quantitatively, we derive a normal-mode excitation formula due to this effect. Longuet-Higgins' pressure formula naturally comes out from the vertical forcing term in the excitation coefficient. A novel aspect of our formula is the horizontal forcing term that becomes comparable to the vertical forcing term for frequencies below about 5 mHz (millihertz), especially in shallow oceans. This term is not likely to be important for microseisms because the main frequency range of microseisms is about 0.05–0.4 Hz but it may make significant contributions to the excitation of the continuous background oscillations (the hum) whose frequency range is 2–7 mHz.

© 2015 Elsevier B.V. We present a new, 3D description of crust and upper mantle velocity structure in southern California implemented as a Unified Structural Representation (USR). The USR is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity (Vp, Vs) measurements and incorporates the locations and displacement of major fault zones that influence basin structure. These basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure, which were subsequently iterated and improved using 3D waveform adjoint tomography. A geotechnical layer (GTL) based on Vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component. The resulting model provides a detailed description of the structure of the southern California crust and upper mantle that reflects the complex tectonic history of the region. The crust thickens eastward as Moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust. Deep sedimentary basins and underlying areas of thin crust reflect Neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late Pliocene. To illustrate the impact of this complex structure on strong ground motion forecasting, we simulate rupture of a proposed M 7.9 earthquake source in the Western Transverse Ranges. The results show distinct basin amplification and focusing of energy that reflects crustal structure described by the USR that is not captured by simpler velocity descriptions. We anticipate that the USR will be useful for a broad range of simulation and modeling efforts, including strong ground motion forecasting, dynamic rupture simulations, and fault system modeling. The USR is available through the Southern California Earthquake Center (SCEC) website (http://www.scec.org).

The largest mechanical components of the Earth system are the solid body and two fluid envelopes: ocean and atmosphere. Each component in this system – the atmosphere, the oceans, and the solid Earth – interacts with the others in complex ways on various timescales. The fluid envelopes generate mechanical noise in the solid body from which structural information can be retrieved because Green's functions for the solid Earth can be constructed from such ocean- or atmosphere-generated noise. Likewise, various sources in the solid Earth generate waves that propagate in the atmosphere from which quantification of earthquakes and volcanic sources is possible. Broadening the study of the solid Earth to include interactions with the fluid spheres will allow new data to inform and enrich traditional solid Earth science.

SUMMARY Teleseismic surface wave data from the dense seismographic network in Southern California (CISN, California Integrated Seismic Network) are used to construct S-wave velocity maps for the region. Surface wave phase velocity measurements are achieved by way of a waveform-matching spectral-domain technique that was developed for use within the CISN array. A two-station phase velocity measurement technique was developed, which takes advantage of the fact that, for surface wave arrivals from any azimuth, there are multiple pairs of CISN stations, which are almost exactly on the same great-circle paths. To ensure the validity of this assumption, particle motions were analysed for measured pairs within the network, showing that the deviation between expected backazimuth from ray theory and observed backazimuths are small enough to be analysed by a simple distance correction. This method was applied to 114 large earthquakes for time periods between 1999 and 2002 and led to the retrieval of phase velocity maps for Rayleigh waves between 20 and 55 mHz and for Love waves between 25 and 45 mHz. Finite frequency effect for propagating surface waves was approximately taken into consideration using a geometrical technique. An ellipse, determined by wavelength and great-circle distance between the source and receiver can describe an influence zone for the finite frequency effect. Phase velocity map reconstructions, using a method which incorporates this effect, show minimal changes in phase velocity maps for our region, but do extend solvable velocity regions. Inversion for S-wave velocity distribution with depth shows that deep structural contrast across the San Andreas extends into the upper mantle. A slow velocity anomaly is evident under the Southern Sierras and the Salton Sea, the latter region indicating the northward extension of tectonic activity in the Gulf of California. Fast velocity roots that reach 60 km are found under the western part of Transverse Ranges and the Northern Peninsular Ranges. Reconstruction of past location of the Western Transverse Range microplate implies that these two ranges may have been adjacent to one another historically. Both the Western Transverse range fast velocity root and the Peninsular range fast velocity root may be a remnant of old oceanic plate.

SUMMARY Microseisms are a continuous source of seismic signal which mainly consist of Rayleigh waves but are known to contain some other types of seismic waves. We developed a simple processing procedure for single-station three-component seismic data which allows us to select Rayleigh-wave dominated portions. Application of this procedure to data from Southern California led us to confirm that excitation of the secondary-peak microseisms (the predominant energy at about 0.15 Hz) occurs in coastal regions; the source directions, viewed at each station, do not change very much throughout the year. We also discovered that the ratio of the horizontal to vertical amplitudes in Rayleigh waves, termed H/Z in this paper, is shown to have seasonal variations. The H/Z estimates typically reach their maximum in winter and their minimum in summer. Seasonal variations are observed at most stations but peak-to-peak amplitudes of seasonal variations vary greatly from station to station, ranging between 0 and 40 per cent. Two hypotheses were examined to explain this phenomenon; the first hypothesis is that it is caused by seasonal changes in seismic velocities in a layer between the surface and the groundwater level. The second hypothesis is that it is caused by seasonal changes in relative excitation of higher modes compared to dominant fundamental-mode Rayleigh waves. The first hypothesis is not likely, because predicted amplitudes of seasonal variations in H/Z are too small to explain observed variations. The second hypothesis seems quite plausible if source regions move seasonally among regions with different ocean depths. The technique developed in this paper, however, is not sufficient to answer this question conclusively.

SUMMARY S-wave velocity in the shallow crust is an important controlling parameter for ground motion amplification. It is a key parameter for prediction of ground motion and thus for earthquake hazard mitigation in general. Shallow S-wave velocity structure has often been obtained from phase velocity of microtremors and microseisms. We present a new approach which directly inverts the ratio of horizontal to vertical amplitudes of microseisms which is referred to as the HZ ratio in this paper. Our approach consists of (i) isolation of Rayleigh-wave dominant portions in seismograms by using the 90-degree phase shift between horizontal and vertical component, (ii) measurement of the HZ ratios as a function of frequency and (iii) iterative inversion for S-wave velocity structure in the crust. Depth sensitivity kernels of the HZ ratios for density, P-wave velocity and S-wave velocity are derived by a numerical method. Examples of the application of this method to data in southern California show that the HZ ratio data require modification of the standard seismic velocity model in this region (Southern California Earthquake Center Community Velocity Model 3.0 or SCEC CVM) in the upper 10–15 km. The derived models show amplification of incident SH waves by factors of 2–8, relative to the prediction by the SCEC CVM. The proposed approach has promise for applications to other regions in the world because microseisms are observed everywhere in the world. The method may become even more powerful if it is combined with phase velocity data.

We report the emission and optical properties of RO-substituted (R = Mg, Ca, Sr, and Ba) 2.5SnO–57.5ZnO–40P2O5 glasses. Although the optical basicity of the host glass is changed by substitution of...

Using liquid-phase synthesis, SnO–ZnO–P2O5 amorphous coating films exhibiting broad emission of Sn2+ center were prepared. It is found that the P–O–Zn bonds of the film were generated during the heat-treatment. The emission property due to Sn2+ depended on the heat-treatment temperature, which correlated with the residual chloride ion content. Although the peak position of the photoluminescence (PL) excitation band was almost constant, that of PL red-shifted with increasing amount of SnO, which was quite different from the result observed for SnO–ZnO–P2O5 bulk glass containing Sn2+ center.

Continuous oscillations of the Earth are observed for frequencies between 2 and 7 mHz at almost every seismically quiet site in the world. These oscillations ride on a broad noise peak which spans the frequency band from 3 to 15 mHz and reaches its maximum at 7-9 mHz. We propose an oceanic excitation hypothesis which explains both the modal oscillations and the broad noise peak, specifically the action of oceanic infragravity waves on the solid Earth. Using the estimated amplitudes of oceanic infragravity waves from observation, although they are limited in number at the moment, we show that there is sufficient energy in these waves to excite observed seismic signals; for a given time window, the contribution from a small area (which may be as small as 100 km x 100 km) is all that is required to explain the seismic observations. The advantage of this oceanic mechanism over the previously proposed atmospheric mechanism is in the simultaneous explanation of the above two features in seismograms, whereas the atmospheric hypothesis has only explained the modal oscillations. The oceanic mechanism naturally explains the predominant 6-month periodicity as a result of semi-hemispheric ocean-wave activities in the Northern and Southern hemispheres, showing a good match between seismic data and satellite ocean-wave data both in the amplitude and phase of seasonal variation. Our Earth seems to be filled with ubiquitous propagating Rayleigh waves, generated directly by oceanic infragravity waves, for the frequency band 3-15 mHz.

SUMMARY Compact analytical formulae are derived for particle motions of surface waves in a weakly anisotropic, flat-layered medium. Theory only incorporates coupling between fundamental mode Rayleigh and Love waves but comparison against numerical results show that a good match is achieved for anisotropy up to approximately 10 per cent. Among various types of particle motions for Rayleigh and Love waves, it is proposed that Love wave vertical polarization, defined by the ratio of the vertical to the transverse displacement, may be a good, cleanly measurable quantity. Derived formulae show that vertical polarization contains 2θ and 4θ azimuthal dependence, similar to the well-known phase velocity variations. Formulae show depth kernels explicitly and can be used for the inversion of anisotropic parameters. The measurement of polarization as a function of azimuth then provides constraints on anisotropy under a seismic station. The derived formulae are a natural extension of Smith & Dahlen's results on phase velocity to particle motions.

We study the cumulative change of the coseismic, crustal gravitational energy caused by earthquakes occurring in two regions with active tectonics: the western USA during 1992 to 1999, and Japan during January 1997 to August 2000. We use regional earthquake source mechanism catalogues available for these regions in order to increase spatial coverage and resolution. The catalogues, however, contain very shallow earthquakes for which the conventional normal mode approach has difficulties in calculating the gravitational energy change under slow convergence in the mode summations. To overcome this problem, we employ a ‘direct’ solution approach which allows to generate very accurate kernels for the integration of the coseismic gravitational energy change. In the western USA, the Coast Ranges and the Transverse Ranges show systematically positive gravitational energy changes while the margin of the Basin and Range shows negative changes. In Japan, the northeastern part shows predominantly positive changes while the southwestern part shows negative to weak positive changes. These systematic patterns well correspond to the regional tectonic regime: gravitational energy gain for compressional tectonics and energy loss for extensional tectonics. The total (global), cumulative crustal gravitational energy change caused by the earthquakes occurring in the western USA is +6.1×10 17 J, and it is +2.7×10 17 J by the earthquakes in Japan. These energy changes are positive in both regions which indicates the predominance of the compressional stress state with respect to the earthquake occurrence.

In order to study high-frequency seismic wave propagation, seismic wavefields generated by resonant-shaking experiments of the Millikan Library, on the campus of California Institute Technology (Pasadena, California, USA), were analysed. Because the resonant shaking frequencies are 1.12 Hz (the east-west direction) and 1.64 Hz (the north-south direction), this active-source experiment can provide opportunities for studying high-frequency seismic wave propagation in Southern California. Because they are very narrow frequency band data, the analyses must be quite different from ordinary time-domain analyses. We show, theoretically, that the signals must be dominated by surface waves. Adopting this surface wave assumption, we proceed to make two separate analyses, one on spectral amplitude and the other on phase. We present a new method to derive group velocity from phase based on the cross correlations between the station in the Millikan Library (MIK) and stations in the regional network. Our results support that an active-source experiment by resonant shaking of a building is a feasible approach for high-frequency seismic wave studies. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.

▪ Abstract In 1997, after almost forty years since the initial attempt by Benioff et al (1959) , continuous free oscillations of the Earth were discovered. Spheroidal fundamental modes between 2 and 7 millihertz are excited continuously with acceleration amplitudes of about 0.3–0.5 nanogals. The signal is now commonly found in virtually all data recorded by STS-1 type broadband seismometers at quiet sites. Seasonal variation in amplitude and the existence of two coupled modes between the atmosphere and the solid Earth support that these oscillations are excited by the atmosphere. Stochastic excitation due to atmospheric turbulence is a favored mechanism, providing a good match between theory and data. The atmosphere has ample energy to support this theory because excitation of these modes require only 500–10000 W whereas the atmosphere contains about 1017 W of kinetic energy. An application of this phenomenon includes planetary seismology, because other planets may be oscillating owing to atmospheric excitation. The interior structure of planets could be learned by determining the eigenfrequencies in the continuous free oscillations. It is especially attractive to pursue this idea for tectonically quiet planets, since quakes may be too infrequent to be recorded by seismic instruments.

Earthquakes cause a potential energy change in Earth's interior because they generate permanent vertical deformation in the gravitational field. The cumulative change of this potential energy in the crust, computed for the period between 1977 and 1998, shows systematic patterns in various tectonic provinces. Regions of extensional tectonics show systematic potential energy loss and regions of compressional tectonics typically show energy gain. Therefore, we claim that the crustal potential energy change can serve as a good indicator of compressional and extentional tectonics.

Maxwell's multiple pole theory provides the basis for a convenient means of determining the spherical harmonics of geomagnetic fields from directional paleomagnetic data. The relationship between the Maxwell poles and axes and the corresponding spherical harmonics was given by Maxwell. We show that the distribution of virtual geomagnetic poles (VGPs) is symmetrical about the Maxwell axes and converge to their poles. Utilizing this property of VGPs and a complimentary property of the distribution of equatorial virtual poles (EVPs), which are defined as points 90° from the VGP on the site meridian, leads to a means of obtaining the Gaussian spherical harmonic coefficients from paleomagnetic data. This VGP method involves the minimization of the horizontal components of the magnetic fields at VGPs and the radial components of the magnetic field at EVPs to yield the best fitting spherical harmonic coefficients. A hybrid variant of the VGP method involving this minimization and fitting the available mean vectors measured at sites has also been developed. Tests of the VGP and hybrid methods on model fields derived from the International Geomagnetic Reference Field (IGRF) 1995 demonstrate that they are effective means of determining the Gauss coefficients for any field requiring no a priori assumptions about the field. The hybrid method has been applied in a preliminary analysis of the time-averaged paleomagnetic data for the past 5 Myr and for the Bruhnes chron. The resulting mean fields were dominantly dipolar, but there were also persistent second order features, suggesting some long-term smaller-scale control over the dynamo process. The hybrid method was also applied to data from the Brunhes-Matuyama reversal. Four models were obtained: (1) a mean field model for reversed polarity, (2) a mean field model for normal polarity, (3) a mean field model for the entire reversal, and (4) a time sequence field model. The results were consistent with the Americas being a persistent site of inward field lines in the nondipole field and suggested that the reversal is initiated by decreases in strength of dipole, quadrupole, and octupole terms. The true dipole path of the transitional field tracks across eastern Asia, but a strong radially inward flux bundle moves over Africa. The paucity of demonstrably reliable data in these reversal records, however, requires that these results be interpreted cautiously.