Effects of topography and crustal heterogeneities on the source estimation of LP event at Kilauea volcano

The main goal of this study is to improve the modelling of the source mechanism associated with the generation of long period (LP) signals in volcanic areas. Our intent is to evaluate the effects that detailed structural features of the volcanic models play in the generation of LP signal and the consequent retrieval of LP source characteristics. In particular, effects associated with the presence of topography and crustal heterogeneities are here studied in detail. We focus our study on a LP event observed at Kilauea volcano, Hawaii, in 2001 May. A detailed analysis of this event and its source modelling is accompanied by a set of synthetic tests, which aim to evaluate the effects of topography and the presence of low velocity shallow layers in the source region. The forward problem of Green's function generation is solved numerically following a pseudo-spectral approach, assuming different 3-D models. The inversion is done in the frequency domain and the resulting source mechanism is represented by the sum of two time-dependent terms: a full moment tensor and a single force. Synthetic tests show how characteristic velocity structures, associated with shallow sources, may be partially responsible for the generation of the observed long-lasting ringing waveforms. When applying the inversion technique to Kilauea LP data set, inversions carried out for different crustal models led to very similar source geometries, indicating a subhorizontal cracks. On the other hand, the source time function and its duration are significantly different for different models. These results support the indication of a strong influence of crustal layering on the generation of the LP signal, while the assumption of homogeneous velocity model may bring to misleading results.     

Scattering of surface waves modelled by the integral equation method

The integral equation method is used to model the propagation of surface waves in 3-D structures. The wavefield is represented by the Fredholm integral equation, and the scattered surface waves are calculated by solving the integral equation numerically. The integration of the Green's function elements is given analytically by treating the singularity of the Hankel function at   R = 0  , based on the proper expression of the Green's function and the addition theorem of the Hankel function. No far-field and Born approximation is made. We investigate the scattering of surface waves propagating in layered reference models imbedding a heterogeneity with different density, as well as Lamé constant contrasts, both in frequency and time domains, for incident plane waves and point sources.     

An alternative method for calculation of Rayleigh and Love wave phase velocities by using three-component records on a single circular array without a central station

A method for the computation of phase velocities of surface waves from microtremor waveforms is shown. The technique starts from simultaneous three-component records obtained in a circular array without a central station. Then, Fourier spectra of vertical, radial and tangential components of motion are calculated for each station and considered as complex-valuated functions of the azimuthal coordinate. A couple of intermediate real physical quantities, B and C , can be defined from the 0- and ±1-order coefficients of the Fourier series expansion of such functions. Finally, phase velocities of Rayleigh and Love waves can be retrieved from B and C by solving respective one-unknown equations. The basic assumption is the possibility of expanding the wavefield as a sum of plane surface waves with Rayleigh and Love wavenumbers being univocal functions of the circular frequency. The method is tested in synthetic ambient noise wavefields confirming its reliability and robustness for passive seismic surveying.     

The mechanisms of shallow earthquakes and the monitoring of a comprehensive test ban

The ability of seismological criteria to identify earthquakes from underground explosions depends partly on the orientation of the earthquake source. Well-determined double-couple moment tensor solutions for a large number of earthquakes have been published in the Harvard centroid moment tensor (CMT) and United Slates Geological Survey (USGS) catalogues. Statistical analyses of these catalogues indicate that the distribution of the orientation of earthquake mechanisms is not random. The distribution of the T axes shows significant clustering around the downward vertical, indicating that a larger number of earthquake mechanisms radiate compressional P -wave energy to teleseismic distances from near the maximum of the radiation pattern than is predicted if earthquake sources are randomly oriented double couples. The clustered T axes correspond to compressional dip-slip mechanisms, and it is this type of mechanism which is believed to cause both the m _b: M _s (the ratio of body-wave to surface-wave magnitude) and first-motion criteria to misidentify an earthquake as an explosion.     

3-D linearized scattering of surface waves and a formalism for surface wave holography

Summary. Scattering of surface waves by lateral heterogeneities is analysed in the Born approximation. It is assumed that the background medium is either laterally homogeneous, or smoothly varying in the horizontal direction. A dyadic representation of the Green's function simplifies the theory tremendously. Several examples of the theory are presented. The scattering and mode conversion coefficients are shown for scattering of surface waves by the root of an Alpine-like crustal structure. Furthermore a 'great circle theorem'in a plane geometry is derived. A new proof of Snell's law is given for surface wave scattering by a quarter-space. It is shown how a stationary phase approximation can be used to simplify the Fourier synthesis of the scattered wave in the time domain. Finally a procedure is suggested to do 'surface wave holography'.     

Anomalous high-frequency wave propagation from the Tonga–Kermadec seismic zone to New Zealand

Summary. Bulletins of the International Seismological Centre (ISC) show very large residuals, up to 15 s early, for arrivals from events in the Tonga–Kermadec subduction zone to the New Zealand network of seismometers. The very early arrivals are confined to events south of about 22°S, and shallower than about 350 km. The waveforms show two distinct phases: an early, emergent, first phase with energy in the high-frequency band 2–10 Hz, and a distinct second phase, containing lower frequency energy, arriving at about the time predicted by JB tables.
The residuals are attributed to propagation through the cold, subducted lithosphere, which has a seismic velocity 5 per cent faster, on average, than normal. Ray tracing shows that the ray paths lie very close to the slab for events south of 22°S, but pass well beneath the slab for events further north, corresponding to the change in residual pattern. This characteristic of the ray paths is due to the curved shape of the seismic zone, and in particular to the bend in the zone where the Louisville ridge intersects the trench at 25°S.
The residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36°S. The very high-frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high Q , and to transmit 2–10 Hz energy with little attenuation.
The absence of low-frequency energy in the first phase is due to the narrowness of the high-velocity slab, which transmits only short-wavelength waves. The second phase, which contains low frequencies, is identified as a P -wave travelling beneath the subducted slab in normal mantle. There is no need to invoke any special structures, such as low-velocity waveguides or reflectors, to explain any of the observations. The S -wave arrivals show similar effects.     

Origin of the valley networks on Mars: a hydrological perspective

The geomorphology of the martian valley networks is examined from a hydrological perspective for the compatibility with an origin by rainfall, globally higher heat flow, and localized hydrothermal systems. Comparison of morphology and spatial distribution of valleys on geologic surfaces with terrestrial fluvial valleys suggests that most martian valleys are probably not indicative of a rainfall origin, nor are they indicative of formation by an early global uniformly higher heat flow. In general, valleys are not uniformly distributed within geologic surface materials as are terrestrial fluvial valleys. Valleys tend to form either as isolated systems or in clusters on a geologic surface unit leaving large expanses of the unit virtually untouched by erosion. With the exception of fluvial valleys on some volcanoes, most martian valleys exhibit a sapping morphology and do not appear to have formed along with those that exhibit runoff morphology. In contrast, terrestrial sapping valleys form from and along with runoff valleys. The isolated or clustered distribution of valleys suggests localized water sources were important in drainage development. Persistent groundwater outflow driven by localized, but vigorous hydrothermal circulation associated with magmatism, volcanism, impacts, or tectonism is, however, consistent with valley morphology and distribution. Snowfall from sublimating ice-covered lakes or seas may have provided an atmospheric source of water for the formation of some valleys in regions where the surface is easily eroded and where localized geothermal/hydrothermal activity is sufficient to melt accumulated snowpacks.     

Propagation of harmonic plane waves in a general anisotropic porous solid

Wave propagation is studied in a general anisotropic poroelastic solid. The presence of dissipation due to fluid-viscosity as well as hydraulic anisotropy of pore permeability are also considered. Biot's theory is used to derive a system of modified Christoffel equations for the propagation of plane harmonic waves in porous media. A non-trivial solution of this system is ensured by a determinantal equation. This equation is separated into two different polynomial equations. One is the quartic equation whose roots represent the complex velocities of four attenuating waves in the medium. The other is a eighth-degree polynomial whose roots represent the vertical slowness values for the four waves propagating upward and downward in a finite porous medium. Procedure is explained to associate the numerically obtained roots with the waves propagating in the medium. The slowness surfaces of waves reflected at the boundary of the medium are computed for a realistic numerical model. The behaviours of phase velocity surfaces are analysed with the help of numerical examples.