Constraints on the location of the 2008, MW 6.4 Achaia-Ilia earthquake fault from strong motion data.

The June 2008, Mw6.4 Achaia-Ilia earthquake was the first recorded dextral strikeslip event of considerable magnitude in western Peloponnese, which, nevertheless, could not be related to any of the known/mapped structures at the ground surface. Published locations of the mainshock focus by various agencies/researchers differ by as much as 6 km and 16 km in the horizontal and vertical dimensions, respectively, making even more difficult the accurate siting of the seismogenic fault. However, the 2008 earthquake provided a valuable set of near-fault strong motion data, which could shed some light on the problem of accurately locating the earthquake source. To this end, we use the discrete wavenumber method to forward model the strongground motion records at three stations, located close to the prolongation of the 2008 strike. We test different locations and lengths for the ruptured plane and compare synthetic polarities and amplitudes of the first strong S-wave pulse to actual data. We conclude that the line of maximum moment release (our fault models are vertical planes) during the 2008 earthquake is located to the east of the imaginary line connecting stations PAT2 and AMAA and to the west of station PYR1.

Effects of an off-centered tool on dipole and quadrupole logging

An acoustic logging tool in inclined or horizontal boreholes may be placed apart from the center and produce additional complicated wavefields. We investigate the effects of an off-centered tool on monopole, dipole, and quadrupole logs due to off-centering of the tool. In recent logging tools, monopole, dipole, and quadrupole logs can be obtained by adding or subtracting responses at four monopole (pressure) receiver arrays at right angles. We examine the responses of the four monopole receiver array system for three directions of eccentricity: inline direction of the dipole source (Off [Formula: see text] case), crossline direction (Off [Formula: see text] case), and [Formula: see text] to the dipole source (Off [Formula: see text] case). To simulate responses at each receiver array, we use the discrete wavenumber method. In a dipole tool positioned on the borehole axis, the inline receiver arrays have only the flexural wave while the cross receiver arrays have zero response. However, an off-centered dipole source produces nondipole modes as well as the dipole mode. For all cases with a dipole source, the dipole mode was enhanced by subtracting responses at one of the inline receiver arrays from those of the other receiver array, and we could extract the formation shear velocity. Similar to the dipole source, an off-centered quadrupole source introduces additional nonquadrupole modes. The quadrupole mode was enhanced by subtracting the sum of responses at receiver arrays below negative poles of a quadrupole source from the sum of responses at receiver arrays below positive poles for all cases. In addition, the formation shear velocity was obtained from the time semblance calculation.