Twentieth Century History
Seismology and Physics of the Earth's Interior at the University of Colorado in the 20th Century
Carl Kisslinger, University of Colorado at Boulder, USA, May 1999
Please note that this reflects the status of geophysics at CU as of 1999; we're leaving it here as a reminder of how the program has evolved.
Solid-earth geophysics was introduced into the Geology Department at the University of Colorado at Boulder (CU) in 1940 and a seismograph station was set up on campus soon after. A decision to create a program in geophysics in the Department of Geological Sciences was made in 1960. The first research efforts were in paleomagnetism and crustal deformation instrumentation and observations. In 1967, a major development was the creation of the Cooperative Institute for Research in the Environmental Sciences (CIRES), a joint institute of the University and the National Oceanic and Atmospheric Administration (NOAA) .The program in seismology was launched in 1972. The research plans have emphasized coordinated efforts in field observations, laboratory studies, computer simulations, and theoretical investigations, The principal subjects in the seismology program have been earthquake and explosion source physics, seismotectonics, earthquake prediction, and studies of structure and dynamics of the Earth's crust, mantle, and deep interior. Application to earthquake hazard assessment, prediction, and nuclear test monitoring has been the motivation for some research. Research and education in other topics within the scope of IASPEI are carried out through a number of other academic departments and research institutes on the Boulder campus.
**Manuscript submission date: May 18,1999
Professor Carl Kisslinger (deceased), University of Colorado, CIRES, Campus Box 216, Boulder, CO 80309-0216
As a result of historical developments driven by a few enterprising individuals, research programs and advanced study in seismology and the physics of the Earth's interior in the University of Colorado at Boulder are scattered among a number of academic departments and research institutes. An interdepartmental Ph.D. Program in Geophysics and the interdisciplinary emphasis in the Cooperative Institute for Research in Environmental Sciences (CIRES) have served to encourage integration of these components. In this report, this history will be traced briefly and the end-of-century activities in these programs summarized. The history begins quite recently, compared to other major geophysical organizations around the world.
In 1940, Warren W. Longley (Minnesota*) was appointed to the faculty of the Department of Geology (later renamed Geological Sciences). His background in mathematics and physics and his experience in exploration for mineral deposits provided a basis for bringing geophysics into the department offerings. The first effort in seismology came soon after with the installation of a seismographic station, consisting of three short-period instruments recording on film, in the basement of the Geology Building. Longeley accomplished this in cooperation with the U.S. Coast and Geodetic Survey. The station was operated by Longley through 1959. It was reactivated by J. C."Chris" Harrison (Cambridge ) in late 1965 and was in service through 1968 as part of the monitoring of the earthquakes associated with fluid injection at the Rocky Mountain Arsenal, near Denver.
The department made a decision in 1960 to initiate a distinct program in geophysics. David Strangway (Toronto) was recruited and brought his expertise in paleomagnetism to that program in 1961. He left in 1964, but the work in paleomagnetism was continued for the next 30 years by Edwin Larson (Colorado), Department of Geological Sciences and former Fellow of CIRES. Strangway's replacement on the faculty was Chris Harrison, whose broad interests in the geophysical sciences were centered on gravity and geodesy. He is recognized as the person most responsible for the birth and early nurturing of a prominent program in geophysics at CU, including the creation of CIRES and of the interdepartmental Ph.D. program.. The research on crustal deformation begun by Harrison has remained an important component of the global CU effort. He installed tiltmeters in the Poorman Mine, near Boulder, (1966), which led to his demonstration that, contrary to widespread practice of that time, mines are not good sites for tilt measurements because of cavity and topographic effects. In 1976 he proceeded to develop and deploy borehole tiltmeters. Concurrently, Judah Levine (NewYork U.)of the Joint Institute for Laboratory Astrophysics (JILA), was developing a laserstrainmeter, which he also deployed in the Poorman Mine.
The pivotal step in this history was the creation of CIRES in 1967, under the leadership of Harrison, who worked closely with the leadership of the Department of Commerce laboratories in Boulder. CIRES was formed as a joint enterprise of the University and the Environmental Sciences Service Administration, soon renamed the National Oceanic and Atmospheric Administration (NOAA), of the U.S. Department of Commerce. CIRES was conceived as, and remains, an interdisciplinary organization that bridges all aspects of the sciences of the solid, liquid, and gaseous components of the Earth. The Fellows and their research programs support and coordinate the interests of a number of academic departments of the University and a number of the research units of NOAA. A program in seismology as an area of instruction and research concentration in the University began with the development of CIRES.
In the late 1960's, one of the NOAA laboratories was the Earth Sciences Laboratory (ESL), which had absorbed many of the scientists from the U.S. Coast and Geodetic Survey when that organization, the oldest Federal scientific unit, was made a part of NOAA. The ESL was doing an excellent job of fulfilling its responsibilities for earthquake monitoring (it produced the Preliminary Determination of Epicenters reports, other data products, and had managed the installation and operation of the World Wide Standardized Seismograph Network (WWSSN)). NOAA was committed to strengthening the research capabilities of the ESL and saw the cooperative arrangement with the University as a mechanism for accomplishing this. Therefore, a decision was made to recruit a solid-earth geophysicist specializing in seismology as the first Director of CIRES. Meanwhile, Harrison was managing the launch of the new institute as its Acting Director.
Carl Kisslinger (St. Louis U. ) was appointed Director in 1972, a position he held until mid-1979 (he served a second term, 1993-1994). He promptly began the process of recruiting Max Wyss (Caltech) and Hartmut Spetzler (Caltech) as Fellows of CIRES and faculty members in Geological Sciences, and arranged for E. R. "Bob" Engdahl (St. Louis U.), then with NOAA, to be assigned fulltime to CIRES as a Fellow. Charles Archambeau (Caltech) joined the group soon after, providing strength in theoretical seismology to go with the laboratory expertise of Spetzler and the field observational interests of the others. Subhendu Datta (U. Jadhavpur), professor of Mechanical Engineering and later a Fellow of CIRES for several years, brought his expertise in theoretical wave propagation research to the group. Links were formed with other CU geophysicists whose work in non-seismological studies pre-dated, but complemented, the CIRES efforts: Jim Wait (Toronto), NOAA and a founding Fellow of CIRES, expert in electromagnetic wave propagation in the solid Earth and the atmosphere; Edward A. "Ned" Benton (Harvard), Astro-Geophysics and Fellow of CIRES, geodynamo theory and geophysical fluid dynamics; Jim Faller (Princeton), JILA, absolute gravity measurements; Peter Bender (Princeton), JILA, geodesy.
A significant change in circumstances occurred in January, 1973, when the Federal Office of Management and Budget, Executive Office of the President, decided to remove all research in solid-earth geophysics, except geodesy, from NOAA and consolidate it with the programs of the U.S.Geological Survey (U.S.G.S.). The ESL was terminated and many of its scientists transferred to the U.S.G.S.. In addition to the research functions, all of the seismological work, including the preparation of the PDE reports, except for the archiving and distribution of geophysical data bases, was transferred. Some key personnel became part of the National Geophysical Data Center of NOAA. The administration of NOAA and the Environmental Research Laboratories encouraged the CIRES leadership to continue the development of a broad research program in Earth-system science, but the funding for seismological and other solid-earth studies had to come from sources other than NOAA. During the following quarter century the principal thrusts of CIRES have shifted to the atmosphere, climate, and the oceans, with the solid-earth program rounding out a comprehensive Earth-system sciences effort.
From its beginning, CIRES has enjoyed excellent working relations with Federal Government agencies beyond its formal relation to NOAA. Collaboration with and research funding from many agencies engaged in solid-earth research has been fruitful. The National Science Foundation, U.S.G.S, NASA, and the Department of Energy have been especially supportive. The research efforts on topics within the scope of IASPEI that evolved from this beginning can be categorized broadly as seismic source physics and seismotectonics, investigation of potential techniques for earthquake prediction, and studies of the internal structure, composition, and processes of the Earth.
The research on seismic sources has been based on a combination of field and instrumental studies of earthquake generation, laboratory studies of fault formation in rock specimens using holographic techniques, the theory of fault rupture and wave generation, and computer simulations of earthquake processes. Engdahl's experience in designing and operating a seismic network at Amchitka Island before he came to CIRES provided a good basis for creating the Central Aleutians Seismic Network in 1974, when the U.S. Geological Survey began to support prediction-related research. This network, which operated from Adak Island until 1992 under the National Earthquake Hazards Reduction Program, was expanded and upgraded as time passed. It provided quality data from thousands of earthquakes in a part of the most prominent subduction zone in the territory of the United States. When Engdahl left CIRES to join the National Earthquake Information Center of the U.S.G.S., Kisslinger took over the direction of the network, with major operational management responsibilities going to Selena Billington (Cornell U.). S.T. Morrissey, on the staff at St. Louis University, provided the technical expertise for instrumentation installation and upgrades, and the maintenance of the network under very difficult field conditions. Several research assistants, including Sharon Kubichek, Robin Wright, Cyndi McDonald, Susanne Bell, Tom Toth, and Bruce Kindel provided the key staff support for the data analysis and catalog preparation, The data were the basis for many research papers and dissertations on the fundamentals of subduction zone seismology, as well as prediction research described below. The network was dismantled in 1992, when funding was ended.
The research on earthquake sources was not been confined to the Aleutians by any means. Studies spanned the globe, from east Asia, through central Asia and the eastern Mediterranean, to Hawaii, the mid-continent of the U.S. and, especially, California.
A series of aftershock studies was originally motivated by the thousands of aftershocks of the magnitude 8 earthquake of May 7, 1986, within the Central Aleutians Seismic Network. In addition to the Aleutians, the spatial and temporal distributions of aftershock sequences in California, Japan, and the Vrancea zone of Romania have been studied. One goal of these studies has been the clarification of fault-zone properties on the basis of aftershock behavior. Work on improved mathematical models of earthquake decay rate has been done by Kisslinger and, especially, Susanna Gross (U. Colorado).
Early laboratory investigations, under Spetzler and Ivan Getting, used laser holography to monitor the deformation of a rock mass as fault formation and failure proceeded. They were joined in this work by many co-workers over the years, including Randolph Martin (MIT), who became a Fellow of CIRES, and Carl Sondergeld (Cornell). The scope of this effort broadened and became thoroughly international, as specialists from a number of countries, listed at the end of this text, joined as Research Associates and visiting scientists. Processes associated with the preparation of a rock mass for fault slip were demonstrated and some effects could be correlated with field observations. More recent work in this laboratory, discussed below, has been concentrated on rock properties at high pressures, including wave attenuation, subjects more related to the interpretation of data on deep-earth processes than source studies.
Theoretical work on the production and propagation of earthquake- and explosion-generated seismic signals has been carried out by Archambeau, as a contribution to the national program of research on monitoring clandestine nuclear tests. He succeeded in developing useful criteria for discriminating between natural earthquakes and buried explosions. His work stimulated and was joined with observational studies of central Asian events under the Joint Seismic Program, led by Danny Harvey (U. Colorado) and Michael Ritzwoller (U. California, San Diego) of the Department of Physics. The University was the site of the IRIS Joint Seismic Program Center for about six years, 1991-1997. Anatoli Levshin (Inst. Earth Physics, Acad. Sci. USSR), former chief of the Laboratory of Wave Fields Interpretation, International Institute of Earthquake Prediction Theory and Mathematical Geophysics of the Russian Academy of Science, joined the group in 1992.
Work on seismic sources was led by John Rundle (U. California, Los Angeles). He concentrated on the development of computational and analytical techniques to model, simulate, interpret, and, ultimately, to understand the physics of earthquakes. In particular, he developed the application of the methods of statistical physics to the modeling and analysis of earthquake source mechanisms. Many of the techniques he developed may also have applications to other kinds of non-equilibrium driven systems, including complex non-linear earth systems. Rundle took the lead in the formulation and development of a new General Earthquake Model (GEM) simulation program. This collaboration of many scientists is based at the Southern California Earthquake Center.
Studies of crustal deformation on local and regional scales have continued as an important part of the tectonophysics program, much of which is directly linked to understanding earthquake source processes and crustal movements associated with volcanoes. Starting with the pioneering work of Harrison, mentioned above, this effort has employed point measurements using strain and tilt meters, creep meters, gravity measurements, electronic distance measuring devices (EDM) in fault zones, and now, Synthetic Aperture Radar (SAR) and Global Positioning System (GPS) surveys. The EDM work, concentrated on the San Andreas fault, was the primary contribution of Larry Slater, a former Fellow of CIRES. A major contribution to the interpretation of measurements of changes in surface elevation and ground tilt was provided by William Farrell (U.California, San Diego), who studied in detail the influence of ocean loading variations on crustal deformation observations. John Wahr (Colorado), Fellow of CIRES and Professor of Physics, contributed to theory and modeling of post-seismic deformation as one part of his analysis of surface elevation variations, described below.
In 1986, Roger Bilham (Cambridge U.) and Randolph Ware (Colorado) created the nucleus of UNAVCO, a consortium of universities (now more than 100) applying GPS methods to the study of global tectonics. CIRES currently owns 22 GPS receivers that are operating in permanent or mobile arrays in th e Himalayas, Tibet, China, Venezuela, Mexico, and Ethiopia.. The work continues to contribute to understanding regional or continental scale deformations, as well as local changes related to earthquake generation. The GPS work is currently led by Bilham and Kristine Larson (U. California, San Diego).Numerical modeling has been an important tool in the interpretation of the field data.
A recent innovative application of strain measurements is the monitoring of strains in the soft soil under Mexico City associated with strong motions following large subduction-zone earthquakes along Mexico's southern coast.
Much of the innovative research on prediction of earthquakes at CIRES was led by Max Wyss, who left CU in 1991. Variations in numerous geophysical parameters were examined for precursory patterns using global and regional (Hawaii, California, Turkey, Alaska) observations. In particular, Wyss and R. E. "Ted" Habermann (Colorado), (currently a NOAA Fellow of CIRES) focused attention on seismicity rate decreases (quiescence) prior to large earthquakes. As an integral part of these studies, they developed quantitative tools for recognizing and understanding apparent seismicity variations caused by changes in seismic networks and data processing procedures. They demonstrated that many seismicity rate variations proposed as precursors are artificial and can be explained by common network changes. Wyss also investigated crustal uplift and subsidence as revealed by tide-gauge data and systematic variations in b-value as plausible precursors.
Kisslinger and Engdahl carried out prediction studies based mostly on observations with the Central Aleutians Seismic Network. A number of phenomena that have been suggested as precursory to an imminent earthquake were investigated. These included systematic rotation of focal mechanisms of local events before a stronger event, the isolation of possible asperities on the subduction thrust surface on the basis of the distribution of background seismicity and stress drops, variations in attenuation based on measurements of coda-Q, investigations of localized changes in seismic body-wave velocities prior to a strong event, and seismic quiescence. Of these, quiescence as detected with the data from an adequate local network emerged as most promising, but no consistently successful precursor has been identified
Recent work by Rundle and others in the areas of non-linear continuum mechanics, chaotic behavior and the physics of complex systems may lead to advances in prediction technology, or, at least to a better understanding of approaches to overcoming the difficulties. Rundle is approaching earthquake forecasting through the analysis of neural networks. Projects done by his group have emphasized the development of techniques for understanding the space-time patterns and correlations that appear in many high-dimensional complex non-linear systems, including patterns of seismicity. The patterns are quantified in terms of eigenvectors of defined autocorrelation operators.
Measurements of tilt in the Long Valley Caldera, California, and of creep on the Hayward fault in the San Francisco Bay region, both directed by Bilham, are now part of early warning systems in these active zones. Data from these monitoring arrays are updated every 10 minutes on the following Web sites: http://quake.wr.usgs.gov/QUAKES/crustaldef/mal/sfb.creep.gif and
Studies of the structure, composition, and processes in the interior of the Earth have included investigations from the crust and upper mantle to the deep mantle and the core. The tools have included the deployment of portable broadband PASSCAL instruments in both passive and active field experiments, and studies of body waves, surface waves, and free oscillations using data from the global seismographic observing system.
Crust and Upper Mantle. The recent work on the structure and dynamics of the crust and upper mantle has been led by Anne Sheehan (MIT) and Craig Jones (MIT), both in the Department of Geological Sciences and Fellows of CIRES. Their geophysical studies are strongly supplemented by the geochemical research of G. Lang Farmer (U. California, Los Angeles), also affiliated with Geology and a Fellow of CIRES, who applies radiogenic isotope systematics to problems of the origin and evolution of continental crust and tectonic modification of continental mantle lithosphere.
Experiments in which Sheehan and Jones have participated include the Rocky Mountain Front experiment, Colorado Plateau/Great Basin seismic experiment, Sierran Paradox I and II, Snake River plain experiment, and the Continental Dynamics of the Rocky Mountains experiment. Sheehan and CIRES Research Associate Ken Dueker have developed high-resolution imaging techniques using phases converted at subsurface seismic discontinuities and have applied these to dense PASSCAL deployments. Other studies include seismic anisotropy and applications of seismic tomography.
The efforts of this group also extend to the sea floor, with participation in the MELT ocean bottom seismograph experiment on the East Pacific Rise, which explored the structure and dynamics of this mid-ocean ridge, and the Lau Basin experiment in the western Pacific.
The Deep Interior. The late Ned Benton carried out some of the earliest work at CU on the deep interior. His primary interest was the physics of the geodynamo and other problems in fluid dynamics related to core processes and the generation of the geomagnetic field. He was one of the creators of the IUGG program Study of the Earth's Deep Interior (SEDI).
Seismic array analysis by Bob Engdahl and co-workers permitted the recovery of clear observations of previously undetected or unexplained seismic phases. The source of these phases was convincingly associated with major boundaries in the Earth. Reflected core phases were used to fix accurately the radii of the inner and outer core and to describe the nature of these boundaries. These data are still used as a standard to test Earth models.
Some of the first CIRES work on the analysis of surface waves and normal modes was performed by Martin Smith (Princeton), a former Fellow of CIRES also associated with the Department of Physics.
John Wahr and his co-workers have made significant contributions to studies of the interior, with emphasis on the applications of geodetic information (Earth rotation, gravity), in addition to the analysis of seismic wave travel-times. He and graduate student Artie Rodgers tried to use ISC travel- times to infer core-mantle boundary (CMB) topography and concluded that the data were inadequate for providing accurate maps. He and Juergen Neuberg examined PcP reflections from a small portion of the CMB to derive topography and in the process noted a reflection that is presumably from the top of D".
This group also examined the possibility of laterally-varying structure in the core caused by density anomalies in the mantle. They were able to quantify the possible size of this variability and compute its effects on various observable quantities. With co-workers from Belgium, Wahr examined how internal density anomalies would perturb the geoid and the shapes of all boundaries, whether at the surface or internal.
Wahr's work on post-glacial rebound encompassed extension of the computational formalism and quantification of the sources of error in inferring mantle viscosity from rebound observations. His group carried out numerous studies of the use of new types of geodetic observations in rebound investigations and estimated the deformation of the outer surface and internal boundaries caused by the rebound. His work on earth tides and nutations led to an effective formalism for studying the shape of the CMB, as well as a method for using earth tides to infer mantle Q at tidal periods.
Work on the deep interior is also led by Michael Ritzwoller, Department of Physics and affiliate of CIRES. In addition to his Earth-oriented research, he contributes to efforts to understand how large-scale convection in the solar convection zone affects helioseismic oscillations. Some of his work with numerous colleagues, including J. Resovsky and E. Lavely, has focused on the theory and applications of normal mode analysis. They have developed three-dimensional models of mantle structure, including a recent (1999) model of mantle shear wave velocity structure. They are now working on the density structure of the mantle from normal modes. Ritzwoller contributed some of the earliest work leading to discovery of inner core anisotropy.
This group, with major contributions from A. Levshin, have completed group velocity tomography, in the period range 20-200 sec., to estimate continental crustal and uppermost mantle structure in Antarctica, Eurasia, South America and the Arctic. Levshin has contributed to the development of a new technique for measurement of surface wave characteristics (dispersion, polarization, spectral amplitudes) that is efficient and convenient for analyzing large volumes of data. Ritzwoller and co-authors were among the first to invert globally orbiting long-period surface waves for lateral attenuation models of the upper mantle.
The laboratory studies of mineral physics of the kind done by Spetzler and Getting, as well as Joseph Smyth of Geological Sciences, provide essential background for the interpretation of seismic observations in terms of internal composition and processes. Currently Spetzler is working on the chemical and physical effects of fluid flow in undersaturated porous rocks, with emphasis on seismic wave attenuation, and studies of mantle rheology through experimental determinations of shear anelasticity in candidate mantle minerals. He has also been carrying out geophysically important high-pressure, high-temperature equation-of-state measurements in a diamond anvil cell, using ultrasonic (GHz) interferometry and x-rays. Getting has developed a unique apparatus for measuring attenuation and dispersion in mantle minerals at seismic frequencies and small strains. The attenuation observed in these and other experiments is much more frequency-dependent than indicated by field observations and this raises some crucial questions for the science. Getting has also developed a number of devices for high-pressure experiments that have been installed in several of the premier laboratories in the United States.
* The place or university names indicate the institution from which the person received his or her highest degree. Additional information about many of the individuals mentioned here and the programs offered in the various institutes and academic departments mentioned here may be found at the following Internet addresses:
Fellows, Visiting Fellows, Research Associates, and other scientists who have contributed to the solid-earth geophysics program while in residence at CIRES who are not referred to in the above text.
Tomas L. Chelidze
F. A. Dahlen
Leonard E. Johnson
H. Paul Johnson
Charles M. Meertens
James H. Whitcomb
Representative sample of publications in IASPEI-related sciences by University of Colorado geophysicists and their co-authors (CIRES and others). This partial list is intended only to show the breadth and scope of research efforts.
Agnew, D., Nonlinearity in rocks: Evidence from earth tides, J. Geophys. Res. 86: 3969-3978, 1981.
Ando, M.,Y. Ishikawa, F. Yamazaki, Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan, J.Geophys. Res. 88: 5750-5864,1983.
Archambeau, C.B., J.B. Minster, Dynamics in prestressed media with moving phase boundaries: A continuum theory of failure in solids, Geophys. J. Roy. Astro. Soc. 52: 65-96, 1978.
Bilham, R., Surface slip subsequent to the 24 November 1987 Superstition Hills, California, earthquake monitored by digital creepmeters, Bull. Seism. Soc. Amer. 79: 424-450, 1989.
Bilham, R., The 1737 Calcutta earthquake and cyclone evaluated, Bull. Seism. Soc. Amer. 84: 1650-1657, 1994.
Bilham, R., K. Larson, J. Freymueller, Project Idyihim members, GPS measurements of present-day convergence across the Nepal Himalaya, Letter to Nature 386: 61-64, 1997.
Billington, S., E.R. Engdahl, S. Price, Changes in seismicity and focal mechanisms of small earthquakes prior to an Ms 6.7 earthquake in the central Aleutian island arc, Earthquake prediction-An Int'l Rev., Maurice Ewing Series 4:348-356, 1981.
Billington, S., C. Frohlich, E.R. Engdahl, A. Malahoff, Detection and location of earthquakes in the central Aleutian subduction zone using island and ocean bottom seismograph stations, J. Geophys. Res. 87: 6853-6864, 1982.
Bowman, J. R., C. Kisslinger, Seismicity associated with a cluster of earthquakes of mb>4.5 near Adak, Alaska: Evidence for an asperity?, Bull. Seism. Soc. Amer. 75: 223-236, 1985.
Brodsky, N., I. C. Getting, H. Spetzler, an experimental and theoretical approach to rock deformation at elevated temperature and pressure, Sp. Tech. Pub. 869, Amer. Soc. Testing and Materials: 37-54, 1985.
Burnley, P. C., The fate of olivine in subducting slabs: a reconnaissance study, Amer. Mineralog. 80: 1293-1301, 1995.
Chen, G., H. Spetzler, Complexities of rock fracture and rock friction from deformation of Westerly granite, Pure & Appl. Geophys. 140: 123-135, 1993.
Cormier, V. F., The synthesis of complete seismograms in an earth model specified by radially inhomogeneous layers, Bull. Seism. Soc. Amer. 70: 691-716, 1980.
Datta, S.K., A.H. Shah, W. Karunasena, Wave propagation in composite media and material characterization, in Elastic Waves and Ultrasonic Nondestructive Evaluation: 135-141, S. K. Datta, J.D. Achenbach, Y.S. Rajapakse,
Elsevier Sci. Publs, 1990.
Dahlen, F.H., M.L. Smith, The influence of rotation on the free oscillations of the Earth, Phil. Trans. Roy. Soc. London A 279: 583-629, 1975.
Douglas, B. J., S. L. Saul, C. R. Stern, Rheology of the upper mantle beneath southernmost South America inferred from peridotite xenoliths, J. Geol. 95: 241-253, 1987.
Dueker, K., A.F. Sheehan, Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track, J. Geophys. Res. 102 : 8313-8327, 1997.
Dueker, K., A.F. Sheehan, Mantle discontinuity structure beneath the Colorado Rocky Mountains and High Plains, J. Geophys. Res. 103: 7153-7169, 1998.
Engdahl, E.R., L.E. Johnson, Differential PcP travel times and the radius of the core, Geophysical J., 39: 435-456, 1974
Engdahl, E.R., E.A. Flinn, R.P. Mass, Differential PkiKP times and the radius of the inner core, Geophysical J. 30: 457-463, 1974
Engdahl, E. R., W. H. K. Lee, Relocation of local earthquakes by seismic ray tracing, J. Geophys. Res. 81: 4400-4066, 1976
Engdahl, E.R., S. Billington, Focal depth determination of central Aleutian earthquakes, Bull. Seis. Soc. Amer. 76: 77-93, 1986.
Engdahl, E.R., S. Billington, C. Kisslinger, Teleseismically recorded seismicity before and after the May 7, 1986, Andreanof Islands, Alaska, earthquake, J. Geophys. Res. 94: 15,481-15,498, 1989.
Evernden, J., C.B. Archambeau, E. Cranswick, An evaluation of seismic decoupling and underground nuclear test monitoring using high-frequency seismic data, Rev. Geophys. 24: 143-215, 1986.
Farmer, G.L., F.V. Perry, S. Semken, B. Crowe, D. Curtis, D. J. DePaolo, Isotopic evidence on the structure and origin of subcontinental lithospheric mantle in southern Nevada, J. Geophys. Res. 94: 7885-7898, 1989.
Farrell, W.E., Deformation of the Earth by surface loads, Rev. Geophys. Space Phys. 10: 761-797, 1972.
Farrell, W.E., Earth tides, ocean tides and tidal loading, Phil. Trans. Roy. Soc. London, A 274: 253-259, 1973.
Francis, T.J.G., The ratio of compressional to shear wave velocity and rock porosity on the axis of the mid-Atlantic ridge, J. Geophys. Res. 81: 4361-4364, 1976.
Getting, I.C., J. Paffenholz, H.A. Spetzler, Measuring attenuation in geologic materials at seismic frequencies and amplitudes, in The brittle-ductile transition in rocks, AGU Geophys. Monograph 56: 239-243, 1990.
Getting, I.C., S.J. Dutton, P.C. Burnley, S-I. Karato, H.A. Spetzler, Shear attenuation and dispersion in MgO, Phys. Earth and Planetary Interiors, 99: 249-257, 1997.
Gross, S.J., Magnitude distributions and slip scaling of heterogeneous seismic sources, Bull. Seism. Soc. Amer.86: 498- 504, 1996.
Gross, S.J. C. Kisslinger, Tests of Models of aftershock decay, Bull. Seism.Soc. Amer. 84: 1571-1579, 1994.
Gross, S.J., C. Kisslinger, Estimating tectonic stress rate and state with Landers aftershocks, J. Geophys. Res. 102: 7603-7612, 1997.
Gross, S.J., J.B. Rundle, A systematic test of time-to-failure analysis, Geophys. J. Int'l 133: 57-64, 1998.
Habermann, R.E., Consistency of teleseismic reporting since 1963, Bull. Seism. Soc. Amer. 72:93-111, 1982.
Habermann, R.E., M.S. Craig, Comparison of Berkeley and CALNET magnitude estimates as a means of evaluating temporal consistency of magnitudes in California, Bull. Seism. Soc. Amer. 78: 1255-1267, 1988.
Habermann, R.E., Seismicity rate variations and systematic changes in magnitudes in teleseismic catalogs, Tectonophysics 193:277-289, 1991.
Harrison, J.C., Cavity and topographic effects in tilt and strain measurement, J. Geophys. Res. 81: 319-328, 1976.
Harrison, J.C., Tilt observations in the Poorman mine, near Boulder, Colorado, J.Geophys. Res. 81: 329-336, 1976.
Harrison, J.C., J. Levine, C.M. Meertens, Design of a deep borehole tiltmeter, Proc. Ninth Intn'l Symp. on Earth Tides: 273-281, 1983.
Harvey, D.J., Seismogram synthesis using normal mode superposition: the locked mode approximation, Geophys. J. Roy. Astro Soc. 66: 37-69, 1981.
Harvey, D.J., M. Wyss, Comparison of a complex rupture model with the precursor asperities of the 1975 Hawaii Ms 7.2 earthquake, Pure & Appl. Geophys, 124: 957-973, 1986.
Hebenstreit, G.T., E.N. Bernard, Azimuthal variations in tsunami interaction with multiple-island systems, J. Geophys. Res. 90: 3353-3360, 1985.
Herrmann, R.B. The use of duration as a measure of seismic moment and magnitude, Bull. Seism. Soc. Amer. 65: 899- 913, 1975.
Johnson, L.E., A new datum for use in the body wave travel time inverse problem, Geophys. J. Roy. Astor. Soc.30: 1972.
Jones, C.H., H. Kanamori, S.W. Roecker, Missing roots and mantle "drips": regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California, J. Geophys. Res. 99: 4567-4601, 1994.
Jones, C.H., R. Phinney, Seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern Sierra Nevada and vicinity, California, J. Geophys. Res. 103: 10,065-10090, 1998.
Khattri, K., M. Wyss, Precursory variation in seismicity rate in the Assam area, India, Geology, 6: 685-688, 1978.
Kisslinger, C., Processes during the Matsushiro, Japan earthquake swarm as revealed by leveling, gravity, and spring- flow observations, Geology : 57-62, Feb. 1975.
Kisslinger, C., An experiment in earthquake prediction and the 7 May 1986 Andreanof Islands earthquake, Bull. Seism. Soc. Amer. 78: 218-2229, 1988.
Kisslinger, C., L.M. Jones, Properties of aftershock sequences in southern California, J. Geophys. Ree. 96: 11,947- 11,958, 1991.
Kisslinger, C., Aftershocks and fault-zone properties, Advances in Geophysics 30: 1-36, 1996.
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