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Li, YG, Aki K, Vernon FL.  1997.  San Jacinto fault zone guided waves: A discrimination for recently active fault strands near Anza, California. Journal of Geophysical Research-Solid Earth. 102:11689-11701.   10.1029/97jb01050   AbstractWebsite

We deployed three 350-m-long eight-element linear seismic arrays in the San Jacinto Fault Zone (SJFZ) near Anza, California, to record microearthquakes starting in August through December 1995. Two arrays were deployed 18 km northwest of Anza, across the Casa Loma fault (CLF) and the Hot Springs fault (HSF) strands of the SJFZ. The third array was deployed across the San Jacinto fault (SJF) in the Anza slip gap. We observed fault zone guided waves characterized by low-frequency, large amplitudes following S waves at the CLF array and the SJF array for earthquakes occurring within the fault zone. However, we did not observe guided waves at the HSF array for any events. The amplitude spectra of these guided waves showed peaks at 4 Hz at the CLF and 6 Hz at the SJF, which decreased sharply with the distance from the fault trace. In contrast, no spectral peaks at frequency lower than 6 Hz were registered at the HSF array. We used a finite difference method to simulate these guided modes as S waves trapped in a low-velocity waveguide sandwiched between high-velocity wall rocks. The guided mode data are adequately fit by a waveguide on the CLF with the average width of 120 m and S velocity of 2.5 km/s, about 25% reduced from the S velocity of the surrounding rock; this waveguide becomes 40 to 60 m wide with the S velocity of 2.8 km/s in the Anza slip gap. On the other hand, there is not a continuous waveguide on the HSF at depth. Locations of the events with guided modes suggest that the fault plane waveguide extends along the CLF between the towns of San Jacinto and Anza, dipping northeastward at 75 degrees-80 degrees to a depth of about 18 km; it becomes nearly vertical in the Anza gap. We speculate that the existence of a continuous low-velocity waveguide on the CLF can be caused by the rupture of the magnitude 6.9 earthquake on April 21, 1918,occurring near the towns of San Jacinto and Hemet. Further, the lack of a clear waveguide on the HSF suggests that it was not ruptured in this event.

Li, YG, Vernon FL, Aki K.  1997.  San Jacinto fault zone guided waves: A discrimination for recently active fault strands near Anza, California (vol 102, pg 11689, 1997). Journal of Geophysical Research-Solid Earth. 102:20437-20437.   10.1029/97jb02128   Website
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.

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.

Barker, B, Clark M, Davis P, Fisk M, Israelsson H, Khalturin V, Kim WY, McLaughin K, Meade C, Murphy J, North R, Orcutt J, Powell C, Richards PG, Stead R, Stevens R, Vernon F, Wallace T.  1998.  Seismology - Monitoring nuclear tests (vol 282, pg 1967, 1998). Science. 282:882-882.Website
Lewis, MA, Peng Z, Ben-Zion Y, Vernon FL.  2005.  Shallow seismic trapping structure in the San Jacinto fault zone near Anza, California. Geophysical Journal International. 162:867-881.   10.1111/j.1365-246X.2005.02684.x   AbstractWebsite

We analyse fault zone trapped waves, generated by similar to 500 small earthquakes, for high-resolution imaging of the subsurface structure of the Coyote Creek, Clark Valley and Buck Ridge branches of the San Jacinto fault zone near Anza, California. Based on a small number of selected trapped waves within this data set, a previous study concluded on the existence of a low-velocity waveguide that is continuous to a depth of 15-20 km. In contrast, our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3-5 km. This is based on the following lines of evidence. (1) Earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones. (2) A traveltime analysis of the difference between the direct S arrivals and trapped wave groups shows no systematic increase (moveout) with increasing hypocentral distance or event depth. Estimates based on the observed average moveout values indicate that the propagation distances within the low-velocity fault zone layers are 3-5 km. (3) Quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low-velocity fault zone layers. The results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones. The waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch. This may result from a preferred propagation direction of large earthquake ruptures on the San Jacinto fault.

Golos, EM, Fang H, Yao H, Zhang H, Burdick S, Vernon F, Schaeffer A, Lebedev S, van der Hilst RD.  2018.  Shear wave tomography beneath the United States using a joint inversion of surface and body waves. Journal of Geophysical Research-Solid Earth. 123:5169-5189.   10.1029/2017jb014894   AbstractWebsite

Resolving both crustal and shallow-mantle heterogeneity, which is needed to study processes in and fluxes between crust and mantle, is still a challenge for seismic tomography. Body wave data can constrain deep features but often produce vertical smearing in the crust and upper mantle; in contrast, surface wave data can provide good vertical resolution of lithospheric structure but may lack lateral resolution and are less sensitive to the deeper Earth. These two data types are usually treated and inverted separately, and tomographic models therefore do not, in general, benefit from the complementary nature of sampling by body and surface waves. As a pragmatic alternative to full waveform inversions, we formulate linear equations for teleseismic S wave traveltimes and surface wave phase velocities and solve them simultaneously for variations in shear wave speed anomalies in the crust and upper mantle. We apply this technique to data from USArray and permanent seismic networks and present a model of seismic shear wave speed anomalies beneath the continental United States. Our joint model fits the individual data sets almost as well as separate inversions but provides a better explanation of the combined data set. It is generally consistent with previous models but shows improvements over both body wave-only and surface wave-only tomography and can lead to refinements in interpretation of features on the scale of the lithosphere and mantle transition zone. Plain Language Summary Variations in the speed at which seismic waves travel through the Earth reveal information about the structure and history of the planet. In this study, we investigate seismic velocity variations using two common types of data from seismograms: body waves, which travel through the deep Earth, and surface waves, which provide information about the shallower layers. Commonly, these two waves are studied separately, but we adopt the method of Fang et al. (2016, ) to produce a model of the crust and mantle of the whole Earth by using both types of data. The goal of this paper is to validate the application of this technique on a large scale, using the continental United States as a test region. We perform qualitative and quantitative tests to show that this method improves upon models made with only body or surface waves while maintaining the best fits of the individual models. We conclude that this technique is a valuable and efficient tool to study the Earth's interior at multiple scales.

Wolfe, CJ, Vernon FL, Al-Amri A.  1999.  Shear-wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic shield. Geophysical Research Letters. 26:779-782.   10.1029/1999gl900056   AbstractWebsite

We constrain upper mantle anisotropy across the Arabian Shield from shear-wave splitting analyses of SKS phases at eight temporary broadband stations that operated in Saudi Arabia. The direction of fast polarization is consistently aligned north-south and the delay time between fast and slow shear waves is generally 1.0 to 1.5 s, indicating that the mantle anisotropy is relatively homogeneous and coherent. We cannot distinguish between two possible models for the origin of this signal. The observed splitting may reflect fossil upper mantle anisotropy associated with the dominantly east-west accretion of oceanic terranes and formation of the Proterozoic Arabian lithosphere. Our results may also be compatible with present-day asthenospheric anisotropy caused by the northward absolute plate motion of the Arabian plate or northward asthenospheric flow from an Ethiopian mantle plume.

Wolfe, CJ, Vernon FL.  1998.  Shear-wave splitting at central Tien Shan: Evidence for rapid variation of anisotropic patterns. Geophysical Research Letters. 25:1217-1220.   10.1029/98gl00838   AbstractWebsite

At active collisional belts, the fast polarization axis of shear-wave splitting is generally aligned parallel to the strike of the belt, which has been proposed to indicate mantle strain that is coherent with crustal deformation. A notable exception is central Tien Shan, where anomalous patterns of splitting have previously been observed. We here analyze shear-wave splitting of SKS phases across north central Tien Shan using digital data from the Kyrgyzstan Broadband Seismic Network (KNET). We find a pattern of short-wavelength anisotropic heterogeneity that supports complex mantle flow due to small-scale convection. The along-strike variations in mantle structure contrast with the coherent pattern of crustal shortening, and indicate that mantle flow is not directly coupled to crustal deformation in this region.

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.

Hartse, HE, Fehler MC, Aster RC, Scott JS, Vernon FL.  1994.  Small-Scale Stress Heterogeneity in the Anza Seismic Gap, Southern California. Journal of Geophysical Research-Solid Earth. 99:6801-6818.   10.1029/93jb02874   AbstractWebsite

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20-km aseismic (seismic gap) portion of the fault zone, where sigma1, the maximum compressive stress, is nearly horizontal and is oriented at 74-degrees +/- 13-degrees relative to the fault strike. This contrasts with orientations ranging from 62-degrees +/- 11-degrees to 49-degrees +/- 7-degrees along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that sigma1 is horizontal and trends north-south, while sigma3 is horizontal and trends east-west. Approximately, 15 km west of the seismic gap, in the off-fault Cahuilla swarm area, sigma1 and sigma3 solutions are rotated clockwise by about 25-degrees relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, sigma1 and sigma3 are rotated counterclockwise by about 10-degrees relative to the regional solution. Northwest of the seismic gap along the fault zone, sigma3 plunges about 30-degrees from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, sigma1 plunges about 45-degrees from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right-lateral strike-slip motion of the fault zone satisfies the first-order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.

Thomson, DJ, Lanzerotti LJ, Vernon FL, Lessard MR, Smith LTP.  2007.  Solar modal structure of the engineering environment. Proceedings of the Ieee. 95:1085-1132.   10.1109/jproc.2007.894712   AbstractWebsite

This paper describes some unanticipated effects of the normal modes of the sun on engineering and scientific systems. we begin with historical, scientific, and statistical background, then present evidence for the effects of solar modes on various systems. Engineering evidence for these modes was first noticed in an investigation of communications satellite failures and second in a study of excessive dropped calls in cellular phone systems. The paper also includes several sections on multitaper estimates of spectra, canonical coherences, robust, and cyclostationary variants of multitapering, and related statistical techniques used to separate the various components of this complex system. In our attempt to understand this unexpected source of problems, we have found that solar modes are detectable in the interplanetary magnetic fields and energetic particles at the Ulysses spacecraft, five astronomical units from the Earth. These modes couple into the magnetosphere, the ionosphere, the geomagnetic field, and atmospheric pressure. Estimates of the power spectrum of data from solar radio telescopes and induced voltages on ocean cables show what appear to be solar modes at both lower and higher frequencies than the optically measured solar p-modes. Most surprisingly, these modes are easily detected in seismic data, where they literally shake the Earth.

Thomson, DJ, Vernon FL.  2016.  Some comments on the analysis of "big" scientific time series. Proceedings of the Ieee. 104:2220-2249.   10.1109/jproc.2016.2598218   AbstractWebsite

Experience with long time series from space, climate, seismology, and engineering has demonstrated the need for even longer data series with better precision, timing, and larger instrument arrays. We find that almost all the data we have examined, including atmospheric, seismic data, and dropped calls in cellular phone networks contain evidence for solar mode oscillations that couple into Earth systems through magnetic fields, and that these are often the strongest signals present. We show two examples suggesting that robustness has been overused and that many of the extremes in geomagnetic and space physics data may be the result of a superposition of numerous modes. We also present initial evidence that the evolution of turbulence in interplanetary space may be controlled by modes. Returning to the theme of "big data," our experience has been that theoretical predictions that spectra would be asymptotically unbiased have turned out to be largely irrelevant with very long time series primarily showing that we simply did not understand the problems. Data that were considered to have excessively variable spectra appear to evolve into processes with dense sets of modes. In short data blocks, these modes are not resolved and as the relative phase of the modes within the estimator varies, so does the apparent power. Ideas that data series become uncorrelated at modest distances in either time or space do not seem to be true with the long duration continuous time series data we have examined.

Rebollar, CJ, Quintanar L, Castro RR, Day SM, Madrid J, Brune JN, Astiz L, Vernon F.  2001.  Source characteristics of a 5.5 magnitude earthquake that occurred in the transform fault system of the Delfin basin in the Gulf of California. Bulletin of the Seismological Society of America. 91:781-791.   10.1785/0120000077   AbstractWebsite

Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude M-w 5.5 event on 26 November 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754 degrees N and 113.708 degrees W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mechanism estimated from first motions (phi = 310 degrees, delta = 83 degrees, lambda = 97 degrees) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.050. 1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330 degrees +/- 15, dip 85 +/- 5, and slip of 165 degrees +/- 15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28 X 10(17) N m (1.28 X 10(24) dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.

Li, ZF, Peng ZG, Ben-Zion Y, Vernon FL.  2015.  Spatial variations of shear wave anisotropy near the San Jacinto Fault Zone in Southern California. Journal of Geophysical Research-Solid Earth. 120:8334-8347.   10.1002/2015jb012483   AbstractWebsite

We examine crustal anisotropy at several scales along and across the San Jacinto Fault Zone (SJFZ) by systematically measuring shear wave splitting (SWS) parameters. The analyzed data are recorded by 86 stations during 2012-2014, including five linear dense arrays crossing the SJFZ at different locations and other autonomous stations within 15 km from the main fault trace. Shear phase arrivals and SWS parameters (fast directions and delay times) are obtained with automated methods. The measurement quality is then assessed using multiple criteria, resulting in 23,000 high-quality measurements. We find clear contrast of fast directions between the SW and NE sides of the SJFZ. Stations on the SW side have fast directions consistent overall with the maximum horizontal compression direction (SHmax), while stations on the NE side show mixed patterns likely reflecting lithological/topographic variations combined with fault zone damage. The fast directions in the Anza gap section with relatively simple fault geometry agree with the inferred SHmax, and the delay times at an array within that section are smaller than those observed at other across-fault arrays. These indications of less pronounced damage zone in the Anza section compared to other segments of the SJFZ are correlated generally with geometrical properties of the surface traces. Significant variations of fast directions on several across-fault arrays, with station spacing on the orders of a few tens of meters, suggest that shallow fault structures and near-surface layers play an important role in controlling the SWS parameters.

Schulte-Pelkum, V, Earle PS, Vernon FL.  2004.  Strong directivity of ocean-generated seismic noise. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000520   AbstractWebsite

We measure direction and amplitude of ocean-generated continuous seismic noise in the western United States. Slowness direction of the noise is determined using array beamforming, and particle motion direction from individual three-component stations. We find two surprising results. First, the noise is highly monodirectional at all sites, regardless of coastal distance. A single narrow generation area dominates for most of the time period, interrupted by a second well defined direction during ocean swell events. Second, we find that a storm off the Labrador coast with not unusual wave heights generates coherent noise across the entire continent. We show the causal relationship between swells arriving at different North American coastal areas and the triggered microseisms in time-lapse movies (Animations 1 and 2) of ocean swells and concurrent microseisms. Our results have a number of implications for different fields of research. A useful by-product of our finding that microseisms are a strongly directional noise source is the possibility of using automated processing of the continuous noise as a near real-time check on station polarity and calibration problems, which would be a simply implemented indicator for the state of health of a seismic network. Consistent monodirectional noise may have an influence on seismic azimuthal measurements such as shear wave splitting. Most importantly, our findings should be taken into account in proposed studies which will use seismic noise as a proxy for ocean wave height in investigations of interdecadal climate change.

Share, PE, Allam AA, Ben-Zion Y, Lin FC, Vernon FL.  2019.  Structural properties of the San Jacinto Fault Zone at Blackburn Saddle from seismic data of a dense linear array. Pure and Applied Geophysics. 176:1169-1191.   10.1007/s00024-018-1988-5   AbstractWebsite

We image the San Jacinto fault zone at Blackburn Saddle using earthquake waveforms recorded by a similar to 2-km across-fault linear array with 108 three-component sensors separated by similar to 10-30m. The length and spatiotemporal sampling of the array allow us to derive high-resolution information on the internal fault zone structure with spatial extent that can be merged with regional tomography models. Across-fault variations in polarization, amplitude, and arrival time of teleseismic P waves indicate abrupt changes in subsurface structure near the surface trace of the fault (sensor BS55) and similar to 270m to the northeast (sensor BS34). Analysis of fault zone head waves from local events reveals the existence of a deep bimaterial interface that extends from the array to at least 50km southeast and has a section with>10% velocity contrast. This analysis also corroborates the teleseismic results and indicates a broad damage zone primarily northeast of the fault bounded by a shallow bimaterial interface near BS34 that merges with the deep interface. Detection and waveform inversions of Love-type fault zone trapped waves generated by local events indicate a trapping structure within the broader damage zone with width of similar to 150m, velocity reduction of similar to 55% from the surrounding rock and depth extent of similar to 2km. The performed analyses provide consistent results on the subsurface location of the main seismogenic fault and properties of a major bimaterial interface and damage structure. The imaged fault zone properties are consistent with preferred propagation direction of earthquake ruptures in the area to the northwest.

Berger, J, Eissler HK, Vernon FL, Nersesov IL, Gokhberg MB, Stolyrov OA, Tarasov NT.  1988.  Studies of High-Frequency Seismic Noise in Eastern Kazakhstan. Bulletin of the Seismological Society of America. 78:1744-1758.Website
Hedlin, MAH, Berger J, Vernon FL.  2002.  Surveying Infrasonic Noise on Oceanic Islands. Pure and Applied Geophysics. 159:1127-1152., Germany   10.1007/s00024-002-8675-1   AbstractWebsite

-An essential step in the establishment of an International Monitoring System (IMS) infrasound station is the site survey. The survey seeks a location with relatively low infrasonic noise and the necessary logistical support. This paper reports results from our surveys of two of the oceanic sites in the IMS - the Azores and Cape Verde. Each survey sampled infrasonic noise, wind velocity, air temperature and humidity for 3 weeks at 4 sites near the nominal IMS locations. The surveys were conducted on Sao Miguel (the main island in the Azores) and Maio (Cape Verde). Infrasonic noise was measured using the French MB2000 microbarometer.During our 3-week experiment in January the trade winds at Cape Verde varied little from an azimuth of 63 degree . Because of the unvarying wind azimuth, the experiment gave us an opportunity to examine the effectiveness of a forest at reducing both wind speed and infrasonic noise. We find that the thick Acacia forest on Maio reduces wind speeds at a 2m elevation by more than 50% but does not reduce infrasonic noise at frequencies below 0.25Hz. This forest serves as a high-frequency filter and clearly does not reduce long-period noise levels which are due to large-scale turbulence in the atmospheric boundary layer above the forest. This is consistent with our observations in the Azores where the relationship between infrasonic noise and wind speed is more complex due to frequent changes in wind azimuth.In Cape Verde, wind speed and infrasonic noise are relatively constant. The diurnal variations are clearly seen however the microbarom is only rarely sensed. In the Azores, during our 3-week experiment in November and December of 1998, wind speed and infrasonic noise change rapidly. At this location, daily noise level swings of 40 to 50dB at 0.1Hz are not uncommon in the early winter and are due to changes in wind speed and atmospheric turbulence. The effectiveness of an infrasound station in the Azores will be strongly dependent on time during the winter season.The two surveys illustrate some of the difficulties inherent in the selection of sites for 1 to 3km aperture arrays on oceanic islands. Due to elevated noise levels at these sites, 8 element, 2km aperture arrays are strongly preferred.