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Gomberg, J, Wech A, Creager K, Obara K, Agnew D.  2016.  Reconsidering earthquake scaling. Geophysical Research Letters. 43:6243-6251.   10.1002/2016gl069967   AbstractWebsite

The relationship (scaling) between scalar moment, M-0, and duration, T, potentially provides key constraints on the physics governing fault slip. The prevailing interpretation of M-0-T observations proposes different scaling for fast (earthquakes) and slow (mostly aseismic) slip populations and thus fundamentally different driving mechanisms. We show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip M-0-T observations, and both slip populations have a change in scaling, where the slip area growth changes from 2-D when too small to sense the boundaries to 1-D when large enough to be bounded. We present new fast and slow slip M-0-T observations that sample the change in scaling in each population, which are consistent with our interpretation. We suggest that a continuous but bimodal distribution of slip modes exists and M-0-T observations alone may not imply a fundamental difference between fast and slow slip.

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.

Borsa, AA, Agnew DC, Cayan DR.  2014.  Ongoing drought-induced uplift in the western United States. Science.   10.1126/science.1260279   AbstractWebsite

The western United States has been experiencing severe drought since 2013. The solid earth response to the accompanying loss of surface and near-surface water mass should be a broad region of uplift. We use seasonally-adjusted time series from continuously operating GPS stations to measure this uplift, which we invert to estimate mass loss. The median uplift is 4 mm, with values up to 15 mm in California’s mountains. The associated pattern of mass loss, which ranges up to 50 cm of water equivalent, is consistent with observed decreases in precipitation and streamflow. We estimate the total deficit to be about 240 Gt, equivalent to a 10 cm layer of water over the entire region, or the annual mass loss from the Greenland Ice Sheet.

Agnew, DC.  2014.  Variable star symbols for seismicity plots. Seismological Research Letters. 85:775-780.   10.1785/0220130214   AbstractWebsite
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.  2013.  Realistic Simulations of Geodetic Network Data: The Fakenet Package. Seismological Research Letters. 84:426-432.   10.1785/0220120185   AbstractWebsite
Barbour, AJ, Agnew DC.  2012.  Noise Levels on Plate Boundary Observatory Borehole Strainmeters in Southern California. Bulletin of the Seismological Society of America. 101:2453-2466.   10.1785/0120110062   AbstractWebsite

To establish noise levels for the borehole strainmeters of the Plate Boundary Observatory (PBO), we have analyzed data recorded by eight of these instruments, all in the Anza region of southern California. We determine time-varying power spectra for frequencies from 10(-3) to 10 Hz, using a new method that combines multitaper spectrum estimation, smoothing by local regression, and computation of cumulative distribution functions. From about 2 Hz to the Nyquist frequency of 10 Hz, the noise floor is set by instrument resolution; for frequencies between 0.1 Hz and 1 Hz, it is set by microseisms. The lowest noise level is between 0.01 and 0.1 Hz, with a rapid increase at lower frequencies. However, in most instruments this low-noise range also contains narrowband noise that appears to be caused by power supply fluctuations. We compare these results with noise spectra from other types of strainmeters, which suggest two conclusions: (1) they are in agreement with results for surficial, long-baseline instruments; and (2) other subsurface strainmeters have lower noise in the seismic band than the PBO instruments do.

Barbour, AJ, Agnew DC.  2012.  Detection of Seismic Signals Using Seismometers and Strainmeters. Bulletin of the Seismological Society of America. 102:2484-2490.   10.1785/0120110298   AbstractWebsite

Using data from borehole and long-base strainmeters and from borehole and surface seismometers, we compare the seismic-wave detection capability of strainmeters and seismometers. We use noise spectra to determine the relative signal-to-noise ratios (SNRs) on different sensors, as a function of the phase velocity and frequency of a signal. For the instruments we analyze, signals with frequencies from 10(-3) to 10 Hz and phase velocities typical of (or higher than) surface and body waves will have lower SNRs on the strainmeters than on broadband seismometers. At frequencies from 0.1 to 10 Hz the borehole (short-period) seismometers have better SNRs than strainmeters for typical phase velocities; at lower frequencies strainmeter data signals would have higher SNRs.

Shen, ZK, King RW, Agnew DC, Wang M, Herring TA, Dong D, Fang P.  2011.  A unified analysis of crustal motion in Southern California, 1970-2004: The SCEC crustal motion map. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008549   AbstractWebsite

To determine crustal motions in and around southern California, we have processed and combined trilateration data collected from 1970 to 1992, VLBI data from 1979 to 1992, and GPS data from 1986 to 2004: a long temporal coverage required in part by the occurrence of several large earthquakes in this region. From a series of solutions for station positions, we have estimated interseismic velocities, coseismic displacements, and postseismic motions. Within the region from 31 N to 38 N. and east to 114 W, the final product includes estimated horizontal velocities for 1009 GPS, 190 trilateration, and 16 VLBI points, with ties between some of these used to stabilize the solution. All motions are relative to the Stable North American Reference Frame (SNARF) as realized through the velocities of 20 GPS stations. This provides a relatively dense set of horizontal velocity estimates, with well-tested errors, for the past quarter century over the plate boundary from 31 N to 36.5 N. These velocities agree well with those from the Plate Boundary Observatory, which apply to a later time period. We also estimated vertical velocities, 533 of which have errors below 2 mm/yr. Most of these velocities are less than 1 mm/yr, but they show 2-4 mm/yr subsidence in the Ventura and Los Angeles basins and in the Salton Trough. Our analysis also included estimates of coseismic and postseismic motions related to the 1992 Landers, 1994 Northridge, 1999 Hector Mine, and 2003 San Simeon earthquakes. Postseismic motions increase logarithmically over time with a time constant of about 10 days, and generally mimic the direction and relative amplitude of the coseismic offsets.

Agnew, DC.  2010.  Comment on "Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of M(L)" by Thessa Tormann, Stefan Wiemer, and Egill Hauksson. Bulletin of the Seismological Society of America. 100:3320-3324.   10.1785/0120100027   AbstractWebsite

Earthquake catalogs can be inhomogeneous because of changes in the definition of earthquake magnitude. Provided that a sufficient number of events have magnitudes defined in more than one system, it is possible to apply a Monte Carlo method to the observed joint distribution to convert sets of magnitudes from one system to another, improving any statistical analysis of the catalog. I demonstrate the method for the southern California catalog, in which the definition of local magnitude has recently been changed. Monte Carlo magnitude mapping appears to eliminate temporal changes that are otherwise present.

Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2009.  Reply to comment by J. C. Savage on "Aseismic slip and fault-normal strain along the creeping section of the San Andreas Fault''. Geophysical Research Letters. 36   10.1029/2009gl039167   AbstractWebsite
Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2008.  Aseismic slip and fault-normal strain along the central creeping section of the San Andreas fault. Geophysical Research Letters. 35   10.1029/2008gl034437   AbstractWebsite

We use GPS data to measure the aseismic slip along the central San Andreas fault (CSAF) and the deformation across adjacent faults. Comparison of EDM and GPS data sets implies that, except for small-scale transients, the fault motion has been steady over the last 40 years. We add 42 new GPS velocities along the CSAF to constrain the regional strain distribution. Shear strain rates are less than 0.083 +/- 0.010 mu strain/yr adjacent to the creeping SAF, with 1 - 4.5 mm/yr of contraction across the Coast Ranges. Dislocation modeling of the data gives a deep, long-term slip rate of 31 - 35 mm/yr and a shallow (0 - 12 km) creep rate of 28 mm/yr along the central portion of the CSAF, consistent with surface creep measurements. The lower shallow slip rate may be due to the effect of partial locking along the CSAF or reflect reduced creep rates late in the earthquake cycle of the adjoining SAF rupture zones.

King, NE, Argus D, Langbein J, Agnew DC, Bawden G, Dollar RS, Liu Z, Galloway D, Reichard E, Yong A, Webb FH, Bock Y, Stark K, Barseghian D.  2007.  Space geodetic observation of expansion of the San Gabriel Valley, California, aquifer system, during heavy rainfall in winter 2004-2005. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004448   AbstractWebsite

[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in 2004 - 2005. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.

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.

Agnew, DC, Hodgkinson K.  2007.  Designing compact causal digital filters for low-frequency strainmeter data. Bulletin of the Seismological Society of America. 97:91-99.   10.1785/0120060088   AbstractWebsite

For the strainmeter component of the Plate Boundary Observatory, filters are needed to produce low-frequency series (5-minute samples) from the higher-frequency (1 Hz) data generated by the instruments. We present design methods for finding filters that are efficient, causal, and compact. We use standard methods for generating symmetric finite impulse response filters, followed by root finding, selection of roots, and reconstruction of the weights, using procedures that make these processes numerically stable. The final filters show appropriate performance even in the presence of large teleseismic signals, but introduce unavoidable artifacts for strain data from large local earthquakes.

Agnew, DC.  2007.  Before PBO: an overview of continuous strain and tilt measurements in the United States. Journal of the geodetic Society of Japan. 53:157-182. Abstract
Agnew, DC.  2007.  Earth Tides. Treatise on Geophysics and Geodesy. ( Herring TA, Ed.).:163-195., New York: Elsevier Abstract
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).

Meltzner, AJ, Sieh K, Abrams M, Agnew DC, Hudnut KW, Avouac JP, Natawidjaja DH.  2006.  Uplift and subsidence associated with the great Aceh-Andaman earthquake of 2004. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003891   AbstractWebsite

Rupture of the Sunda megathrust on 26 December 2004 produced broad regions of uplift and subsidence. We define the pivot line separating these regions as a first step in defining the lateral extent and the downdip limit of rupture during that great M(w) approximate to 9.2 earthquake. In the region of the Andaman and Nicobar islands we rely exclusively on the interpretation of satellite imagery and a tidal model. At the southern limit of the great rupture we rely principally on field measurements of emerged coral microatolls. Uplift extends from the middle of Simeulue Island, Sumatra, at similar to 2.5 degrees N, to Preparis Island, Myanmar (Burma), at similar to 14.9 degrees N. Thus the rupture is similar to 1600 km long. The distance from the pivot line to the trench varies appreciably. The northern and western Andaman Islands rose, whereas the southern and eastern portion of the islands subsided. The Nicobar Islands and the west coast of Aceh province, Sumatra, subsided. Tilt at the southern end of the rupture is steep; the distance from 1.5 m of uplift to the pivot line is just 60 km. Our method of using satellite imagery to recognize changes in elevation relative to sea surface height and of using a tidal model to place quantitative bounds on coseismic uplift or subsidence is a novel approach that can be adapted to other forms of remote sensing and can be applied to other subduction zones in tropical regions.

Agnew, DC.  2005.  GHAM: A compact global geocode suitable for sorting. Computers & Geosciences. 31:1042-1047.   10.1016/j.cageo.2005.02.007   AbstractWebsite

The GHAM code is a technique for labeling geographic locations based on their positions. It defines addresses for equal-area cells bounded by constant latitude and longitude, with arbitrarily fine precision. The cell codes are defined by applying Morton ordering to a recursive division into a 16 by 16 grid, with the resulting numbers encoded into letter-number pairs. A lexical sort of lists of points so labeled will bring near neighbors (usually) close together; tests on a variety of global datasets show that in most cases the actual closest point is adjacent in the list 50% of the time, and within 5 entries 80% of the time. (C) 2005 Elsevier Ltd. All rights reserved.

Agnew, DC.  2004.  Robert Fitzroy and the myth of the 'Marsden Square': Transatlantic rivalries in early marine meteorology. Notes and Records of the Royal Society of London. 58:21-46.   10.1098/rsnr.2003.0223   AbstractWebsite

Marine data (especially in meteorology) are often grouped geographically using a set of numbered 10degrees latitude-longitude squares known as Marsden squares, which are usually attributed to William Marsden, Secretary of the Admiralty (and Vice-President of The Royal Society), who supposedly invented them early in the nineteenth century. Available records suggest that this system was in fact probably invented by Robert FitzRoy soon after his appointment as head of the British Meteorological Office in 1854. FitzRoy felt that early English work in marine meteorology was being ignored, notably by the American Matthew Fontaine Maury, who had pioneered the collecting of marine meteorological data from ship's logs. A desire to undo this wrong led FitzRoy to emphasize earlier (though abortive) British projects by A.B. Becher (in 1831) and by Marsden (probably in the 1780s), both of which involved grouping marine data geographically, though only over limited areas. FitzRoy's treatment of this earlier work seems to have created, much later, the belief that Marsden had invented the system of 10degrees squares. Given both Maury's and FitzRoy's desire to demonstrate priority in this field, it is ironic that the first clear proposal to collect and group data from ship's logs was made by the American (and British) natural philosopher Isaac Greenwood in 1728.

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.

Gonzalez-Garcia, JJ, Prawirodirdjo L, Bock Y, Agnew D.  2003.  Guadalupe Island, Mexico as a new constraint for Pacific plate motion. Geophysical Research Letters. 30   10.1029/2003gl017732   AbstractWebsite

[1] We use GPS data collected on Isla de Guadalupe and in northern Baja California, Mexico, to estimate site velocities relative to Pacific plate motion. The velocities of all three geodetic monuments on Guadalupe fit a rigid Pacific plate model with residuals of 1 mm/yr. Using the Guadalupe data and data from five IGS stations on the Pacific plate ( CHAT, KOKB, KWJ1, MKEA, and THTI) we estimate an angular velocity for this plate that is consistent with other recently-published estimates. Our results indicate that Isla de Guadalupe lies on the Pacific plate, and that GPS data collection on the island usefully constrains Pacific plate motion and rigidity.

Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.