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ABSTRACT: Linear scattering of ocean wave energy at the ocean–continent transition structure causes the primary microseism at a period of 14 s. Subsequent nonlinear wave–wave interactions produce the secondary microseism signal at half the primary microseism period (Longuet-Higgins, 1950; Haubrich et al., 1963). We use three years (2018–2022) of seismic data from an ongoing microarray deployment in the UC Santa Barbara Sedgwick Reserve, situated in the Santa Ynez Valley, to constrain seasonal and long-term microseismic noise characteristics for this portion of California’s central coast. Ancillary buoy data (spectral data, wave height, wind speed and direction) from the National Oceanic and Atmospheric Administration are used to explore the causal relationship between ocean swell and the generation of microseisms. This region is found to exhibit strong seasonality in the primary and secondary microseism bands (0.05–0.1 and 0.1–0.3 Hz, respectively), with much higher noise levels in the winter compared with the summer, especially for the secondary microseism (15.4 dB). We also observe a systematic shift in the peak frequency of the secondary microseism between the winter (∼0.14 Hz) and summer (∼0.20 Hz) months, which may reflect a difference in sources of secondary microseisms between the two seasons. Local buoy wave height and spectral data are well correlated with seismic power spectra during times of incoming storm swell in winter, indicating locally generated microseisms along the central coast during this season.

SUMMARY An algorithm developed in Tanimoto & Wang for deriving shallow elasticity structure from colocated pressure and seismic instruments is applied to data at nine stations in the Piñon 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.

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.

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).

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

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.

High-resolution seismic tomographic models of the upper mantle provide powerful new constraints on theories of plate tectonics and hotspots. Midocean ridges have extremely low seismic velocities to a depth of 100 kilometers. These low velocities imply partial melting. At greater depths, low-velocity and high-velocity anomalies record, respectively, previous positions of migrating ridges and trenches. Extensional, rifting, and hotspot regions have deep (>200 kilometers) low-velocity anomalies. The upper mantle is characterized by vast domains of high temperature rather than small regions surrounding hotspots; the asthenosphere is not homogeneous or isothermal. Extensive magmatism requires a combination of hot upper mantle and suitable lithospheric conditions. High-velocity regions of the upper 200 kilometers of the mantle correlate with Archean cratons., Seismic tomography is a new but rapidly developing method for imaging Earth's interior [1-4]. Early results had limited resolution, but important problems regarding the geoid and the large-scale structure of [...]

Earth breathes in and out, murmuring gently to itself as it does so. The habit has been ascribed to the tickling effects of ocean waves--but a new-found twisting oscillation might [...]