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Inbal, A, Cristea-Platon T, Ampuero JP, Hillers G, Agnew D, Hough SE.  2018.  Sources of long-range anthropogenic noise in Southern California and implications for tectonic tremor detection. Bulletin of the Seismological Society of America. 108:3511-3527.   10.1785/0120180130   AbstractWebsite

We study anthropogenic noise sources seen on seismic recordings along the central section of the San Jacinto fault near Anza, southern California. The strongest signals are caused by freight trains passing through the Coachella Valley north of Anza. Train-induced transients are observed at distances of up to 50 km from the railway, with durations of up to 20 min, and spectra that are peaked between 3 and 5 Hz. Additionally, truck traffic through the Coachella Valley generates a sustained hum with a similar spectral signature as the train transients but with lower amplitude. We also find that wind turbine activity in northern Baja California introduces a seasonal modulation of 1- to 5-Hz energy across the Anza network. We show that the observed train-generated transients can be used to constrain shallow attenuation structure at Anza. Using the results from this study as well as available borehole data, we further evaluate the performance of approaches that have been used to detect nonvolcanic tremor at Anza. We conclude that signals previously identified as spontaneous tremor (Hutchison and Ghosh, 2017) were probably generated by other nontectonic sources such as trains.

Donner, S, Lin CJ, Hadziioannou C, Gebauer A, Vernon F, Agnew DC, Igel H, Schreiber U, Wassermann J.  2017.  Comparing direct observation of strain, rotation, and displacement with array estimates at Pinon Flat Observatory, California. Seismological Research Letters. 88:1107-1116.   10.1785/0220160216   AbstractWebsite

The unique instrument setting at the Pinon Flat Observatory in California is used to simultaneously measure 10 out of the 12 components, completely describing the seismic-wave field. We compare the direct measurements of rotation and strain for the 13 September 2015 M-w 6.7 Gulf of California earthquake with array-derived observations using this configuration for the first time. In general, we find a very good fit between the observations of the two measurements with cross-correlation coefficients up to 0.99. These promising results indicate that the direct and array-derived measurements of rotation and strain are consistent. For the array-based measurement, we derived a relation to estimate the frequency range within which the array-derived observations provide reliable results. This relation depends on the phase velocity of the study area and the calibration error, as well as on the size of the array.

Barbour, AJ, Agnew DC, Wyatt FK.  2015.  Coseismic strains on plate boundary observatory borehole strainmeters in Southern California. Bulletin of the Seismological Society of America. 105:431-444.   10.1785/0120140199   AbstractWebsite

Strainmeters can record offsets coincident with earthquakes, but how much these represent strain changes from elastic rebound, and how much they are contaminated by local effects, remains an open question. To study this, we use a probabilistic detection method to estimate coseismic offsets on nine borehole strainmeters (BSMs) operated by the Plate Boundary Observatory (PBO) in southern California, from 34 earthquakes with a wide range of magnitudes and distances. In general, the offsets estimated for the BSM data differ substantially from the static strain predicted by elastic dislocation theory, which is well supported by other techniques, though 10% of the observed offsets agree well with theory. For one earthquake, the BSM offsets significantly disagree with collocated long-base laser strainmeter data. Comparisons with collocated seismic data provide strong evidence that the absolute errors between the observed and predicted strains scale with the level of seismic energy density but also that relative errors (normalized by the model size) do not. We conclude that apparent strain offsets are induced by seismic waves, occurring presumably because of irreversible deformation, whether in the rock or cementing materials close to the BSMs, or in the instruments themselves. Coseismic offsets seen in PBO BSM data should therefore be viewed with caution before being used as a measure of large-scale coseismic deformation.

Agnew, DC, Wyatt FK.  2014.  Dynamic strains at regional and teleseismic distances. Bulletin of the Seismological Society of America. 104:1846-1859.   10.1785/0120140007   AbstractWebsite

We develop formulas for the size of dynamic strains caused by seismic waves from an earthquake of given magnitude and distance. These formulas include peak strain, peak dissipated power, and total dissipated energy, and they are applicable at regional and teleseismic distances. The formulas are fits to data from 89 large (6.5 <= M-w <= 9.0) shallow earthquakes, with source distances between 500 and 16,000 km, recorded between 1977 and 2013 by three long-base laser strainmeters at Pinon Flat Observatory in southern California; these strainmeters provide uniquely well-calibrated measurements of tensor strain. The residuals to the fits suggest that strain values can usually be predicted to within a factor of 2. These data also show that the strain tensor can be substantially different from that expected for plane waves: in particular, the extension perpendicular to the back azimuth, which should be zero, is always 20% or more of the extension along that azimuth. How much the strains resemble those for plane waves depends on their path, perhaps because inhomogeneities along different paths produce different amounts of multipathing. The observed strains are systematically 10%-30% larger at nearby laser strainmeter sites in the Salton trough, suggesting local amplification from inhomogeneous crustal structure.

Wang, TH, Cochran ES, Agnew D, Oglesby DD.  2013.  Infrequent triggering of tremor along the San Jacinto Fault near Anza, California. Bulletin of the Seismological Society of America. 103:2482-2497.   10.1785/0120120284   AbstractWebsite

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 M-w >= 7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller-magnitude, regional event, the 2009 M-w 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 M-w 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love-wave arrival, when surface-wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (similar to 35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20-40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 M-w 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient-shear stress (17-35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well-studied San Andreas fault.

Agnew, DC, Larson KM.  2007.  Finding the repeat times of the GPS constellation. Gps Solutions. 11:71-76.   10.1007/s10291-006-0038-4   AbstractWebsite

Single-epoch estimates of position using GPS are improved by removing multipath signals, which repeat when the GPS constellation does. We present two programs for finding this repeat time, one using the orbital period and the other the topocentric positions of the satellites. Both methods show that the repeat time is variable across the constellation, at the few-second level for most satellites, but with a few showing much different values. The repeat time for topocentric positions, which we term the aspect repeat time, averages 247 s less than a day, with fluctuations through the day that may be as much as 2.5 s at high latitudes.

Elkhoury, JE, Brodsky EE, Agnew DC.  2006.  Seismic waves increase permeability. Nature. 441:1135-1138.   10.1038/nature04798   AbstractWebsite

Earthquakes have been observed to affect hydrological systems in a variety of ways-water well levels can change dramatically, streams can become fuller and spring discharges can increase at the time of earthquakes(1-7). Distant earthquakes may even increase the permeability in faults(8). Most of these hydrological observations can be explained by some form of permeability increase(1,5). Here we use the response of water well levels to solid Earth tides to measure permeability over a 20-year period. At the time of each of seven earthquakes in Southern California, we observe transient changes of up to 24 degrees in the phase of the water level response to the dilatational volumetric strain of the semidiurnal tidal components of wells at the Pinon Flat Observatory in Southern California. After the earthquakes, the phase gradually returns to the background value at a rate of less than 0.1 degrees per day. We use a model of axisymmetric flow driven by an imposed head oscillation through a single, laterally extensive, confined, homogeneous and isotropic aquifer to relate the phase response to aquifer properties(9). We interpret the changes in phase response as due to changes in permeability. At the time of the earthquakes, the permeability at the site increases by a factor as high as three. The permeability increase depends roughly linearly on the amplitude of seismic-wave peak ground velocity in the range of 0.21-2.1 cm s(-1). Such permeability increases are of interest to hydrologists and oil reservoir engineers as they affect fluid flow and might determine long-term evolution of hydrological and oil-bearing systems. They may also be interesting to seismologists, as the resulting pore pressure changes can affect earthquakes by changing normal stresses on faults(10).

Anderson, G, Agnew DC, Johnson HO.  2003.  Salton trough regional deformation estimated from combined trilateration and survey-mode GPS data. Bulletin of the Seismological Society of America. 93:2402-2414.   10.1785/0120030014   AbstractWebsite

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We remeasured 37 of the USGS baselines using survey-mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 mustrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 +/- 0.03 mustrain/yr total engineering shear strain along an axis oriented 311.6degrees +/- 23degrees east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11.

Gomberg, J, Agnew D.  1996.  The accuracy of seismic estimates of dynamic strains: An evaluation using strainmeter and seismometer data from Pinon Flat Observatory, California. Bulletin of the Seismological Society of America. 86:212-220. AbstractWebsite

The dynamic strains associated with seismic waves may play a significant role in earthquake triggering, hydrological and magmatic changes, earthquake damage, and ground failure. We determine how accurately dynamic strains may be estimated from seismometer data and elastic-wave theory by comparing such estimated strains with strains measured on a three-component long-base strainmeter system at Pinon Flat, California. We quantify the uncertainties and errors through cross-spectral analysis of data from three regional earthquakes (the M(0) = 4 x 10(17) N-m St. George, Utah; M(0) = 4 X 10(17) N-m Little Skull Mountain, Nevada; and M(0) 1 x 10(19) N-m Northridge, California, events at distances of 470, 345, and 206 km, respectively). Our analysis indicates that in most cases the phase of the estimated strain matches that of the observed strain quite well (to within the uncertainties, which are about +/-0.1 to +/-0.2 cycles). However, the amplitudes are often systematically off, at levels exceeding the uncertainties (about 20%); in one case, the predicted strain amplitudes are nearly twice those observed. We also observe significant epsilon(phi phi) strains (phi = tangential direction), which should be zero theoretically; in the worst case, the rms epsilon(phi phi) Strain exceeds the other nonzero components. These nonzero epsilon(phi phi) strains cannot be caused by deviations of the surface-wave propagation paths from the expected azimuth or by departures from the plane-wave approximation. We believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities.

Larsen, SC, Agnew DC, Hager BH.  1993.  Strain Accumulation in the Santa-Barbara Channel - 1970-1988. Journal of Geophysical Research-Solid Earth. 98:2119-2133.   10.1029/92jb02043   AbstractWebsite

Geodetic observations between 1970 and 1988 indicate appreciable strain accumulation in the Santa Barbara Channel, California. Eleven line-length changes from a six-station geodetic network spanning the eastern two-thirds of the channel were determined from electronic distance measurements in 1970/1971 and Global Positioning System observations in 1987/1988. Within this network the strains observed are spatially nonuniform. In the easternmost channel the strain is nearly uniaxial, with convergence of 6.4 +/- 0.9 mm/yr oriented N25-degrees-E +/-5-degrees; this direction is consistent with the seismicity, which is dominated by thrust mechanisms with P axes directed to the northeast. In the central channel the strain is less well determined, but appears to include a significant component of shear that is left-lateral when resolved on an east-west plane.