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Al-Amri, AM, Mellors R, Vernon FL.  1999.  Broadband seismic noise characteristics of the Arabian Shield. Arabian Journal for Science and Engineering. 24:99-113. AbstractWebsite

A total of nine portable broadband stations were deployed across the Arabian Shield from November 1995 to March 1997. The stations consisted of STS-2 seismometers recorded continuously at 40 samples per second on RefTek dataloggers. Noise studies showed that most stations were exceptionally quiet with noise levels near the USGS low noise model for frequencies higher than 0.1 Hz. At lower frequencies, the horizontal components showed high noise levels, possibly due to instrumental characteristics. High frequency (>1 Hz) noise varied as much as 10 dB between day and night for some stations (RAYN and TAIF) while for more isolated stations (HALM) was constant. Seasonal noise levels also varied, with April to June being the quietest months. Slight changes in peak microseism frequency also occurred seasonally. The quietest stations were HALM, RAYN, AFIF, and UQSK, ail of which were located in central Saudi Arabia and show noise levels near the low noise model for frequencies between 0.1 and 4 Hz. The optimal site for a new quiet station would be near HALM which showed very little diurnal variations of cultural noise. These stations appear to be among the best sites in the world for the properties of detection thresholds and ground noise levels. Events with mb >3.5 could be detected at distances from 10 to 100 degrees.

Allam, AA, Ben-Zion Y, Kurzon I, Vernon F.  2014.  Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography. Geophysical Journal International. 198:978-999.   10.1093/gji/ggu176   AbstractWebsite

We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V-P, V-S and hypocentre location within 50 x 50 x 20 km(3) volumes, using 266 969 P and 148 249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S-wave arrival times, we also obtain stable maps of V-P/V-S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V-P/V-S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V-S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V-P/V-S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V-P/V-S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.

AlShukri, HJ, Pavlis GL, Vernon FL.  1995.  Site effect observations from broadband arrays. Bulletin of the Seismological Society of America. 85:1758-1769. AbstractWebsite

We present evidence of significant variations in time-domain amplitude and spectral estimates of local earthquake recorded by small-aperture arrays. We examine data from three arrays: two arrays of different scales deployed at Pinon mat, California, in 1990 and 1991 and an array deployed in the summer of 1993 in the former Soviet republic of Turkmenistan. We find consistent evidence in all cases for significant variations in measured amplitudes over scale lengths comparable to feasible wavelengths of incident wave fields. This phenomenon, however, is strongly frequency dependent. At the Pinon Flat site, variations in power spectral estimates exceed a factor of 100 at frequencies over 4 Hz. Analysis of teleseismic signals, however, demonstrates that these variations diminish rapidly at lower frequencies and falls to negligible levels below 0.6 Hz. The Turkmenistan site shows similar overall characteristics; high-frequency variations are less dramatic. Variations comparable to the Pinon mat site do not occur below 20 Hz. Analysis of teleseismic signals yield results similar to Pinon Flat, although the transition to negligible variability seems to occur at a slightly lower frequency of 0.3 Hz. The 1990 Pinon Flat experiment utilized simultaneous recording in two boreholes directly beneath the array. Comparison of spectral estimates from these boreholes to the surface sensors strongly suggests that the deviations in high-frequency spectral estimates we observe across the array are due to interaction of the wave field with the near-surface, weathered layer. We suggest the differences in high-frequency variations of signals recorded at Pinon Flat compared with that of the Turkmenistan site can be explained by differences in near-surface conditions at the two sites. The low-frequency transition to negligible signal variation seen at both sites occurs when the array aperture becomes small compared with the wavelength of surface waves, suggesting that signal variations in intermediate frequencies may be influenced by body-wave to surface-wave conversions caused by crustal velocity variations and topography.

Anchieta, MC, Wolfe CJ, Pavlis GL, Vernon FL, Eakins JA, Solomon SC, Laske G, Collins JA.  2011.  Seismicity around the Hawaiian Islands Recorded by the PLUME Seismometer Networks: Insight into Faulting near Maui, Molokai, and Oahu. Bulletin of the Seismological Society of America. 101:1742-1758.   10.1785/0120100271   AbstractWebsite

Instrumental limitations have long prevented the detailed characterization of offshore earthquakes around the Hawaiian Islands, and little is known about the spatial distribution of earthquakes in regions outside the vicinity of the well-monitored island of Hawaii. Here, we analyze data from the deployment of two successive networks of ocean-bottom seismometers (OBSs) as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME) to better determine seismicity patterns along the Hawaiian Islands and their offshore regions. We find that earthquake detection rates are improved when seismograms are high-pass filtered above similar to 5 Hz to reduce the background seismic noise. Hypocentral solutions have been determined for 1147 previously undetected microearthquakes, and an additional 2880 events correspond to earthquakes already in the catalog of the United States Geological Survey (USGS) Hawaiian Volcano Observatory (HVO). The spatial patterns of earthquakes identified solely on the PLUME network provide complementary information to patterns identified by the HVO network. A diffuse pattern of seismicity is found to the southeast of the island of Hawaii, and clusters of earthquakes are located west of the island. Many microearthquakes are observed in the vicinity of Maui and Molokai, including some located at mantle depths. A small number of microearthquakes are found to occur near Oahu. There is no evidence from our analyses that the Molokai fracture zone (MFZ) is seismically active at this time, and no evidence was found of a previously hypothesized Diamond Head fault (DHF) near Oahu. However, on the basis of both the PLUME and HVO locations, there is a northeast-southwest-trending swath of epicenters extending northeastward of Oahu that may indicate the locus of moderate-sized historic earthquakes attributed to the Oahu region.