Characterization of Damage in Sandstones along the Mojave Section of the San Andreas Fault: Implications for the Shallow Extent of Damage Generation

Following theoretical calculations that suggest shallow generation of rock damage during an earthquake rupture, we measure the degree of fracture damage in young sedimentary rocks from the Juniper Hills Formation (JHF) that were displaced 21 km along the Mojave section of San Andreas Fault (SAF) and were not exhumed significantly during their displacement. In exposures adjacent to the fault, the JHF typically displays original sedimentary fabrics and little evidence of bulk shear strain at the mesoscopic scale. The formation is, however, pervasively fractured at the microscopic scale over a zone that is about a 100 m wide on the southwest side of the SAF near Little Rock. The abundance of open fractures, the poor consolidation, and the shallow inferred burial depth imply that the damage was generated close to the surface of the Earth. The spatial correlation of this damage with a seismically active trace of the SAF suggests that it was generated by SAF slip events that by assumption were of a seismic nature throughout the displacement history of the JHF. Thus the JHF provides a very shallow upper bound for the generation of brittle damage in a seismic fault zone. The fracture fabric is characterized by preferred orientations of fractures that split grains between contact points and is consistent with overall deformation under directed compression. However, the available results cannot be used to distinguish between proposed off-fault damage mechanisms. Fracture orientations are compatible with a maximum compressive stress oriented at a high angle to the fault at about 10 m, and at a lower, more variable angle farther away from the fault. The fracture distribution and fabric are consistent with observations made of the microscale damage characteristics of the Hungry Valley Formation in the northwestern section of the SAF in the Mojave, and with previous observations of exhumed, ancestral strands of the SAF.     

Effect of an exotic prey on the feeding pattern of a predatory snail

The mussel Brachidontes pharaonis, which invaded the Mediterranean from the Red Sea about 120 years ago, has recently become abundant in many midlittoral and some infralittoral rocky habitats along the Israeli rocky shore. We investigated the influence of B. pharaonis, as a novel prey, on the foraging patterns of the large whelk Stramonita haemastoma in the field, and examined food preferences in the laboratory. S. haemastoma has shifted from indigenous species to feeding on the novel mussel when abundant. The whelk prefers to prey upon the invasive mussel over all indigenous species offered (e.g. barnacles and mussels), probably due to its larger size. In the midlittoral zone, the foraging activity of S. haemastoma is considerably low even where refuges are readily available (incisioned-rocks) and food density is high (mainly B. pharaonis). Higher proportions of whelks are actively foraging in the infralittoral zone but usually on smaller prey, mostly barnacles. We suggest that this differential foraging activity in the two zones is related to the degree of exposure to wave action. The midlittoral is inherently more exposed to wave action than the infralittoral, where sea conditions are more benign and the whelks may enjoy longer activity periods.     

Epimenia arabica spec, nov., a Solenogaster (Mollusca) Feeding on the Alcyonacean Scleronephthya corymbosa (Cnidaria) from Shallow Waters of the Red Sea

Abstract. The body organization and some biological data of Epimenia arabica , a new species of Sotenogastres of the order Cavibelonia , are presented. The species is described based on four large-sized specimens (13–21 cm × 0.7-1 cm) from three different localities off the coast of the Sinai Peninsula (Red Sea). It feeds on Scleronephthya corymbosa V erseveldt & C ohen ( Octocorallia: Alcyonacea ), generally at depths of 2–5 meters.     

Theoretical amplitudes of body waves from a dislocation source in the Earth II. Core phases

Expressions are obtained for the ray-theoretical spectral amplitudes of body waves induced by a shear dislocation of arbitrary orientation and depth situated in a radially heterogeneous model of the earth. Account is taken of the azimuthal and colatitudinal radiation patterns of the source, the geometrical spreading, and the reflections and refractions at the free surface and at the mantle-core boundary.In this work spectral amplitudes are calculated for PKP, PKS, SKP and SKS. The results are presented in the form of tables for a source of strength U₀dS = 10¹⁵ cm³, where U₀ is the amount of the dislocation and dS is the fault area. Given the slip and dip angles of the source, the amplitudes of the four core phases can be obtained from these tables for all azimuths, for most of the epicentral distances at which a particular phase is observable, and for all the fourteen focal depths included in the Jeffreys-Bullen tables. It is found that the depth of the source has a strong effect on the amplitudes of the body-wave signals.     

Seismic observations at Lake Emosson,Switzerland

Since 1973 several seismic stations have been brought into operation at Lake Emosson, the last large conventional hydrodam in the Swiss Alps. The reservoir is situated in a seismically active zone which stretches from the Rhone valley in southwestern Switzerland into Haute Savoie, France. The monitoring program was started before the first impounding and continued during four full load cycles. A certain correlation is indicated between water level changes and the occurrence near the reservoir of weak seismic events (M_L<0) with high frequency content. A transient series of 30 tremors occurred in August 1974, during a period of rising water level. Local events of a different type frequently occur during the periods of decreasing water level. Their daily distribution indicates a mechanism which is caused by temperature variations.Contribution No. 131 of the Geophysical Institute, Karlsruhe University. Sonderforschungsbereich Felsmechanik of Karlsruhe University. Contribution No. 299 of the Institute for Geophysics, ETH Zürich.     

A simple failure criterion for rough joints and compound shear surfaces

The subject under investigation is the strength of a single shear plane which exhibits a regular, asymmetric roughness pattern. In the shear direction the asperities are so steeply inclined that the joint becomes mechanically non-effective with the result that the asperities are sheared off. Against the shear direction the asperities are only gently inclined. It is shown that this particular roughness pattern is of some general importance in geomechanics (examples: unconfined compression test; shear plane with secondary fractures).

Simple analytical considerations allow the formulation of a shear criterion, which is dependent on friction angle ø_m and cohesionc_m of the intact rock and on the inclination γ of the gently inclined parts of the asperities which are dipping against the shear direction. In the Mohr-diagram the criterion results in envelopes which converge at high normal stresses against the envelope of intact rock. Furthermore, the criterion expresses that both the slope of the envelopes and the dilation angle continuously decrease with increasing normal stress. Therefore the criterion adequately describes features which are regarded as most important when shearing rough joints or compound shear surfaces.     

Theoretical amplitudes of body waves from a dislocation source in the earth III. P,S and surface reflections

Expressions are obtained for the ray-theoretical spectral amplitudes of body waves induced by a shear dislocation of arbitrary orientation and depth situated in a radially heterogeneous model of the earth. Account is taken of the azimuthal and colatitudinal radiation patterns of the source, the geometrical spreading, and the reflections and refractions at the free surface and at the mantle-core boundary.In this work spectral amplitudes are calculated for P, SH, SV, PP, PPP, PS, PSS, SP, SPP, SSH, SSSH, SSV, and SSSV. The results are presented in the form of tables for a source of strength U₀dS = 10¹⁵cm³, where U₀ is the amount of the dislocation and dS is the fault area. Given the slip and dip angles of the source, the amplitudes of these surface reflections and direct phases can be obtained from these tables for all azimuths, for most of the epicentral distances at which a particular phase is observable, and for all the fourteen focal depths included in the Jeffreys-Bullen tables. It is found that the depth of the source has a strong effect on the amplitudes of the body wave signals.