Rotational Seismology with a Quartz Rotation Sensor

We describe the construction and performance of a new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS). The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. The noise floor is measured to be ∼45 pico-radians/root (Hz) near 1 Hz, and the resonant period of the sensor is about 10 s, making it a broadband sensor. Among similarly sized broadband rotation sensors, this represents more than two orders of magnitude improvement in noise floor near 0.1 Hz. We present measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

Resonant Periods of Seiches in Semi-Closed Basins with Complex Bottom Topography

Seiches and resonances are two closely related phenomena that can cause damage to coastal areas. Seiches that occur in a basin at a distinct period named the resonant period may generate resonance when a wave induced by external forces enters the basin and has the same period as the seiches. Studying this period has become essential if we want to understand the resonance better. Thus, in this paper, we derive the resonant period in various shapes of semi-closed basin using the shallow water equations. The equations are then solved analytically using the separation of variables method and numerically using the finite volume method on staggered grid to discover the resonant period for each basin. To validate the numerical scheme, we compare its results against the analytical resonant periods, resulting in a very small error for each basin, suggesting that the numerical model is quite reliable in the estimation of the analytical resonant period. Further, resonant wave profiles are also observed. It is revealed that, in the coupled rectangular basin, the maximum wave elevation is disproportionate to the ratio of the length of the basin, while, in the trapezoidal basin, the ratio of the depth of the basin has no significant impact on the maximum wave elevation.

Testing the Depths to 1.0 and 2.5 km/s Velocity Isosurfaces in a Velocity Model for Japan and Implications for Ground‐Motion Modeling

In the Next Generation Attenuation West2 (NGA‐West2) project, a 3D subsurface structure model (Japan Seismic Hazard Information Station [J‐SHIS]) was queried to establish depths to 1.0 and 2.5 km/s velocity isosurfaces for sites without depth measurement in Japan. In this article, we evaluate the depth parameters in the J‐SHIS velocity model by comparing them with their corresponding site‐specific depth measurements derived from selected KiK‐net velocity profiles. The comparison indicates that the J‐SHIS model underestimates site depths at shallow sites and overestimates depths at deep sites. Similar issues were also identified in the southern California basin model. Our results also show that these underestimations and overestimations have a potentially significant impact on ground‐motion prediction using NGA‐West2 ground‐motion models (GMMs). Site resonant period may be considered as an alternative to depth parameter in the site term of a GMM.

Tidal resonance in the Gulf of Thailand

The Gulf of Thailand is dominated by diurnal tides, which indicates that the resonant period of the gulf is potentially close to one day. However, when applied to the gulf, the classic quarter wavelength resonant theory fails to give a diurnal resonant period. In this study, we first perform a series of numerical experiments showing that the resonant period of the gulf is approximately one day and that the resonance of the South China Sea body has a critical impact on the resonance of the gulf. In contrast, the resonance of the Gulf of Thailand has little influence on the resonance of the South China Sea body. An idealised two-channel model that can reasonably explain the dynamics of the tidal resonance in the Gulf of Thailand is then established in this study.

Systematic Accuracy Assessment of Two Frequently Used QTF Approximations in the Case of a Rectangular Barge

This paper presents numerical results of the monodirectional second order forces acting on a rudimentory FPSO hull shape. The exact calculation of second order loads through potential theory is computationally challenging and requires the evaluation of a slowly-convergent free surface integral. In practice, this integral is often neglected as it is widely assumed that it does not contribute significantly to the overall load and few commercial seakeeping software propose this option anyway. An even rougher and widely used approximation consists in expressing the second order loads occuring at a non-zero difference frequency from the mean drift loads. It is called the Newman’s approximation; it enables serious CPU time gains, but at the cost of a worse accuracy. The object of this paper is to investigate on a simple case the accuracy of each of these two approximations (without the free surface integral and Newman’s approximation) and the influence of parameters such as the water depth and the sea state wave length. Depending on the resonant period of the considered mooring system, it may give some insight on a good compromise between CPU time and accuracy when choosing the way of determing the second order loads.

Evidence of Supersolidity in Rotating Solid Helium

Supersolidity, the appearance of zero-viscosity flow in solids, was first indicated in helium-4 torsional oscillator (TO) experiments. In this apparatus, the irrotationality of the superfluid component causes it to decouple from the underlying normal solid, leading to a reduction in the resonant period of the TO. However, the resonant period may be altered for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Superimposing rotation onto oscillatory measurements may distinguish between supersolidity and classical effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity.

Tidal Resonance in Juan de Fuca Strait and the Strait of Georgia

The resonant period and quality factor Q are determined for the semienclosed sea comprising Juan de Fuca Strait, Puget Sound, and the Strait of Georgia. The observed tidal elevation gain and phase change, from the Pacific Ocean to this inland sea, are fitted to the predictions of simple analytic models, which give a resonant period of 17–21 h and a Q of about 2. The low Q value, indicative of a highly dissipative system, is consistent with the need for numerical models for the area to employ large bottom friction coefficients. These include the effects of form drag.