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Ben-Zion, Y, Vernon FL, Ozakin Y, Zigone D, Ross ZE, Meng HR, White M, Reyes J, Hollis D, Barklage M.  2015.  Basic data features and results from a spatially dense seismic array on the San Jacinto fault zone. Geophysical Journal International. 202:370-380.   10.1093/gji/ggv142   AbstractWebsite

We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1-10 Hz and likely ongoing earthquake sources dominant in the frequency range 20-40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. The median noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions. Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust.

Tolstoy, M, Vernon FL, Orcutt JA, Wyatt FK.  2002.  Breathing of the seafloor: Tidal correlations of seismicity at Axial volcano. Geology. 30:503-506.   10.1130/0091-7613(2002)030<0503:botstc>2.0.co;2   AbstractWebsite

Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.

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.

Collins, JA, Vernon FL, Orcutt JA, Stephen RA, Peal KR, Wooding FB, Spiess FN, Hildebrand JA.  2001.  Broadband seismology in the oceans: Lessons from the Ocean Seismic Network Pilot Experiment. Geophysical Research Letters. 28:49-52.   10.1029/2000gl011638   AbstractWebsite

The fundamental objective of the Ocean Seismic Network Pilot Experiment (OSNPE) - which was carried out over a period of about 4 months at a site 225 km southwest of Oahu, Hawaii - was to learn how to make high-quality, broadband seismic measurements in the deep oceans. The OSNPE results demonstrate that broadband data of quality similar to that of quiet land stations can be acquired with seafloor seismographs, but that the location of the seismometer - whether it be on the seafloor, surficially buried within the seabed, or in a deep borehole - has a profound effect on data quality. At long-periods (< 0.1 Hz), data quality was highest for a seismometer buried just beneath the seafloor, while at short-periods (> 0.1 Hz), data quality was best for a seismometer deployed 242 m below the seafloor in a borehole.