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Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.

Thomson, DJ, Vernon FL.  2015.  Unexpected, high-Q, low-frequency peaks in seismic spectra. Geophysical Journal International. 202:1690-1710.   10.1093/gji/ggv175   AbstractWebsite

It was established over a decade ago that the normal modes of the Earth are continuously excited at times without large earthquakes, but the sources of the 'seismic hum' have remained unresolved. In addition to the normal modes of the Earth, we show spectral lines in seismic data with frequencies which correspond closely to normal modes of the Sun. Moreover, the widths of the low-frequency lines in the seismic spectra are similar to those of solar modes and much narrower than those of the Earth's normal mode peaks. These seismic lines are highly coherent with magnetic fields measured on both the Geostationary Operations Environmental Satellite (GOES)-10 satellite and the Advanced Composition Explorer (ACE) spacecraft located at L1, 1.5 million km sunward of Earth suggesting that the solar modes are transmitted to the Earth by the interplanetary magnetic field and solar wind. The solar modes are split by multiples of a cycle/day and, surprisingly, by the 'quasi two-day' mode and other frequencies. Both the phase of the coherences and slight frequency offsets between seismic and geomagnetic data at observatories exclude the possibility that these effects are simply spurious responses of the seismometers to the geomagnetic field. We emphasize data from low-noise seismic observatories: Black Forest (BFO), Pion Flat (PFO), Eskdalemuir (ESK) and Obninsk (OBN). Horizontal components of seismic velocity show higher coherences with the external (ACE) magnetic field than do the vertical components. This effect appears to be larger near the seismic torsional, or T-mode, frequencies.

Pavlis, GL, Sigloch K, Burdick S, Fouch MJ, Vernon FL.  2012.  Unraveling the geometry of the Farallon plate: Synthesis of three-dimensional imaging results from USArray. Tectonophysics. 532:82-102.   10.1016/j.tecto.2012.02.008   AbstractWebsite

We compare 12 recent three-dimensional (3D) seismic imaging results that made extensive use of data from the Earthscope Transportable Array (TA). Our goal is to sort out what can be said about the geometry of the Farallon plate. Our main approach is 3D visualization using a kinematic plate motion model as a framework. Comparison of results from all 12 image volumes indicates that the results are most consistent with a single, coherent Farallon slab overridden by North American. The Farallon can be tracked from the trench in the Pacific Northwest to its remnants in the lower mantle under eastern North America. From the trench the lithosphere has a low dip to the volcanic arc. Immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity. The gently dipping section varies along strike. Under Washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern Idaho. There is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern Oregon. Scattered wave imaging results, however, suggest the top of the anomaly is continuous. These can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the Yellowstone system. We find that all results are consistent with a 4D kinematic model of the Mendocino slab window under Nevada and Utah. In the eastern US the larger scale models all show a lower mantle anomaly related to the older history of Farallon subduction. The link between the lower mantle and new results in the U.S. Cordillera lies under the High Plains where the required USArray coverage is not yet complete. Image volumes in a unified format are supplied in an electronic supplement. (C) 2012 Elsevier B.V. All rights reserved.

Burdick, S, Li C, Martynov V, Cox T, Eakins J, Mulder T, Astiz L, Vernon FL, Pavlis GL, van der Hilst RD.  2008.  Upper mantle heterogeneity beneath north America from travel time tomography with global and USArray transportable array data. Seismological Research Letters. 79:384-392.   10.1785/gssrt.79.3.384   Website
Collins, JA, Vernon FL, Orcutt JA, Stephen RA.  2002.  Upper mantle structure beneath the Hawaiian swell: Constraints from the ocean seismic network pilot experiment. Geophysical Research Letters. 29   10.1029/2001gl013302   AbstractWebsite

[1] Data from two broadband, ocean-bottom seismographic stations deployed similar to 225 km southwest of Oahu, Hawaii during the Ocean Seismic Network Pilot Experiment provide constraints on upper mantle structure beneath the Hawaiian swell. Receiver functions show that the mantle transition zone is thinned by >50 km relative to reference model PA5, which, in the absence of compositional changes, implies excess temperatures of >350 K in the transition zone. The combination of the measurements reported here and the thickness variations reported by Li et al. [2000] imply that the transition zone is thinned by 30 +/- 15 km over an along-swell dimension of at least 700 km. At similar to80 km depth, P-to-S converted phases are identified from the Gutenberg discontinuity marking the lid of the oceanic low-velocity zone and the base of the lithosphere. Shear-wave splitting measurements imply that fast-polarization azimuths are intermediate between the absolute plate-motion vector and the fossil spreading direction; multi-event stacked values of o and deltat are -80degrees and 1.5 s, respectively.

Tytell, J, Vernon F, Hedlin M, Hedlin CD, Reyes J, Busby B, Hafner K, Eakins J.  2016.  The USARRAY transportable array as a platform for weather observation and research. Bulletin of the American Meteorological Society. 97:603-619.   10.1175/bams-d-14-00204.1   AbstractWebsite