Earth Sciences Commons^™

Relative Motion Between The Caribbean And North American Plates And Related Boundary Zone Deformation From A Decade Of Gps Observations, Timothy H. Dixon, Frederic Farina, Charles Demets, Pamela Jansma, Paul Mann, Eric Calais

School of Geosciences Faculty and Staff Publications

Global Positioning System (GPS) measurements in 1986, 1994, and 1995 at sites in Dominican Republic, Puerto Rico, Cuba, and Grand Turk define the velocity of the Caribbean plate relative to North America. The data show eastward motion of the Caribbean plate at a rate of 21 ± 1 mm/yr (1 standard error ) in the vicinity of southern Dominican Republic, a factor of 2 higher than the NUVEL-1A plate motion model prediction of 11 ± 3 mm/yr. Independent measurements on San Andres Island, and an Euler vector derived from these data, also suggest a rate that is much higher than …

Continuous Monitoring Of Surface Deformation At Long Valley Caldera, California, With Gps, Timothy H. Dixon, Ailin Mao, Marcus Bursik, Michael Heflin, John Langbein, Ross Stein, Frank Webb

School of Geosciences Faculty and Staff Publications

Continuous Global Positioning System (GPS) measurements at Long Valley Caldera, an active volcanic region in east central California, have been made on the south side of the resurgent dome since early 1993. A site on the north side of the dome was added in late 1994. Special adaptations for autonomous operation in remote regions and enhanced vertical precision were made. The data record ongoing volcanic deformation consistent with uplift and expansion of the surface above a shallow magma chamber. Measurement precisions (1 standard error) for “absolute” position coordinates, i.e., relative to a global reference frame, are 3–4 mm (north), 5–6 …

Constraints On Present-Day Shortening Rate Across The Central Eastern Andes From Gps Data, Lisa Leffler, Seth Stein, Ailin Mao, Timothy H. Dixon, Michael A. Ellis, Leonidas Ocola, I. Selwyn Sacks

School of Geosciences Faculty and Staff Publications

Two years of continuous GPS data from several sites in South America indicate that Arequipa in the southern Peruvian Andes has a velocity of 13±3 mm/yr (two standard errors) to the northeast with respect to stable South America. We interpret these data as reflecting a combination of elastic strain accumulation associated with a locked Nazca-South America subduction zone and a small amount of crustal shortening across the fold and thrust belt on the eastern margin of the Andes. Models of elastic strain accumulation for fully locked and partly locked subduction zones constrain shortening in the eastern Andes to 0–3 mm/yr …

Kilauea Volcano Provides Hot Seismic Data For Joint Japanese-U.S. Experiment, Stephen R. Mcnutt, Y. Ida, B. A. Chouet, P. Okubo, J. Oikawa, G. Saccorotti

School of Geosciences Faculty and Staff Publications

A team of 25 researchers from the United States, Japan, and Italy joined the staff of the Hawaiian Volcano Observatory (HVO) from January 8 through February 9, 1996, to make the most detailed seismic recordings on Kilauea Volcano ever. One-hundred-sixteen portable seismographs were installed in and near Kilauea Crater in Hawaii Volcanoes National Park as a joint Japanese-U.S. research project to record volcanic earthquakes and tremor. The importance of these events has long been recognized, but their origin remains poorly understood due to inadequate network coverage and limitations of the analog instrumentation used in the past.

On February 1, a …

A Gps Estimate Of Relative Motion Between North And South America, Timothy H. Dixon, Ailin Mao

School of Geosciences Faculty and Staff Publications

GPS velocity data are used to estimate the Euler vector describing rigid body motion of North America relative to South America. Assuming the boundary between the North and South American plates is located near the Fifteen Twenty fracture zone in the equatorial Atlantic, the Euler vector predicts extension across the Royal Trough up to 1 mm/yr, and convergence across the Barracuda Ridge at about 2 mm/yr, in agreement with geological estimates averaged over tens of millions of years. Further west, convergence between North and South America at rates up to 8 mm/yr may contribute to deformation of the Caribbean plate …

Magnetic Surveys Help Reassess Volcanic Hazards At Yucca Mountain, Nevada, Charles B. Connor, Sammantha Lane-Magsino, John A. Stamatakos, Ronald H. Martin, Peter C. Lafemina, Brittain E. Hill, Steve Lieber

School of Geosciences Faculty and Staff Publications

Natural disasters like volcanic eruptions occur infrequently, but if they occur near nuclear power plants or high‐level radioactive waste repositories, local and global communities can be threatened. Ideally, such facilities should be constructed only where geologic risk is very low.

Estimating the probabilities of such events requires a comprehensive understanding of site geology and the geologic processes operating in the site region on timescales of 104 to 107years. In light of these requirements, geologists and geophysicists must continually improve techniques for site characterization.

New Constraints On Source Processes Of Volcanic Tremor At Arenal Volcano, Costa Rica, Using Broadband Seismic Data, John P. Benoit, Stephen R. Mcnutt

School of Geosciences Faculty and Staff Publications

Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenal's tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2–3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200–660 m-long, vertically oriented 1-D resonator with matched boundary …

Variations In The Frequency-Magnitude Distribution With Depth In Two Volcanic Areas: Mount St. Helens, Washington, And Mt. Spurr, Alaska, Stefan Wiemer, Stephen R. Mcnutt

School of Geosciences Faculty and Staff Publications

The frequency-magnitude distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b-value (b ≥ 1.3) at a depth of about 2.3–4.5 …

Observations At Costa Rican Volcano Offer Clues To Causes Of Eruptions, M. Hagerty, S. Y. Schwartz, M. Protti, Timothy H. Dixon, M. Garcés

School of Geosciences Faculty and Staff Publications

Recent measurements of ground deformation at an active volcano show great promise for elucidating the processes that lead to volcanic eruptions. A long-term effort to continuously monitor ground deformation over a very wide bandwidth using state-of-the-art geodetic, seismic, and acoustic instruments at Arenal Volcano in Costa Rica is producing high-quality recordings of harmonic tremor and explosive events and offers an exciting opportunity to explore possible lunar periodicities in volcanic eruptivity.

Timing Of Basaltic Volcanism Along The Mesa Butte Fault In The San Francisco Volcanic Field, Arizona, From 40Ar/39Ar Dates: Implications For Longevity Of Cinder Cone Alignments, F. M. Conway, David A. Ferrill, Chris M. Hall, Alan P. Morris, John A. Stamatakos, Charles B. Connor, Alex N. Halliday, Christopher Condit

School of Geosciences Faculty and Staff Publications

Using high‐precision stepwise‐heating 40Ar/39Ar methods, we dated volcanic products of at least 12 vents cropping out along the northern segment of the Mesa Butte Fault in the San Francisco Volcanic Field, Arizona, where the relationship between fault and vent alignment is unambiguous. The 40Ar/39Ar isochron dates indicate that the Mesa Butte Fault hosted five episodes of volcanism over a 1 million year interval, from 1400 to 300 ka. The first episode occurred 1 km northwest of the trace of the Mesa Butte Fault, whereas the next two episodes were centered near the intersection of the Mesa Butte and Cedar Ranch …

Geometry Of Continental Normal Faults: Seismological Constraints, Jochen Braunmiller, John L. Nabalek

School of Geosciences Faculty and Staff Publications

Teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the Aegean. Waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low-angle detachment faulting at the …

How Rigid Is The Stable Interior Of The North American Plate?, Timothy H. Dixon, Ailin Mao, Seth Stein

School of Geosciences Faculty and Staff Publications

We analyze data from eight permanent GPS stations broadly distributed through the interior of the North American plate, and use the resulting velocities to estimate an Euler vector describing motion of “stable” North America as a single rigid plate. The site velocities fit the single plate model with a mean residual of 1.3 mm/yr. The residuals do not appear to reflect post-glacial rebound, and tests for differential motion between eastern and western North America at the New Madrid seismic zone show no resolvable motion within uncertainties. The residuals likely reflect observational error, and thus our estimate of the stability of …

Three Nonhomogeneous Poisson Models For The Probability Of Basaltic Volcanism: Application To The Yucca Mountain Region, Nevada, Charles B. Connor, Brittain E. Hill

School of Geosciences Faculty and Staff Publications

The distribution and timing of areal basaltic volcanism are modeled using three nonhomogeneous methods: spatio‐temporal nearest neighbor, kernel, and nearest‐neighbor kernel. These models give nonparametric estimates of spatial or spatio‐temporal recurrence rate based on the positions and ages of cinder cones and related vent structures and can account for migration and shifts in locus, volcano clustering, and development of regional vent alignments. The three methods are advantageous because (1) recurrence rate and probability maps can be made, facilitating comparison with other geological information; (2) the need to define areas or zones of volcanic activity, required in homogeneous approaches, is eliminated; …

Constraints On Present-Day Basin And Range Deformation From Space Geodesy, Timothy H. Dixon, Stefano Robaudo, Jeffrey Lee, Marith C. Reheis

School of Geosciences Faculty and Staff Publications

We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1±1.2 mm/yr oriented N38°W±5° (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east-west extension concentrated in the eastern Basin and Range and north-northwest directed right lateral shear concentrated in the western Basin and Range. …

Gps Monitoring Data For Active Volcanos Available On Internet, Timothy H. Dixon, Frederic Farina, Ailin Mao, Frank Webb, Marcus Bursik, Ross Stein, Grant Marshall

School of Geosciences Faculty and Staff Publications

Recent technological advances have now made it possible for volcanologists to continuously monitor active volcanoes with the Global Positioning System (GPS). These observations allow detection of large changes in surface strain near active volcanoes that may precede major volcanic eruptions by days, weeks or even months. They are therefore a critical component of programs designed to assess hazard and predict eruptions of active volcanoes.

Researchers who are unfamiliar with GPS technology or who do not have facilities to process large amounts of GPS data may have difficulty obtaining GPS results, however. To address this problem, we recently initiated a “proof …

Inflation Of Long Valley Caldera From One Year Of Continuous Gps Observations, Frank H. Webb, Marcus Bursik, Timothy H. Dixon, Frederic Farina, Grant Marshall, Ross S. Stein

School of Geosciences Faculty and Staff Publications

A permanent Global Positioning System receiver at Casa Diablo Hot Springs, Long Valley Caldera, California was installed in January, 1993, and has operated almost continuously since then. The data have been transmitted daily to the Jet Propulsion Laboratory for routine analysis with data from the Fiducial Laboratories for an International Natural sciences Network (FLINN) by the JPL FLINN analysis center. Results from these analyses have been used to interpret the on going deformation at Long Valley, with data excluded from periods when the antenna was covered under 2.5 meters of snow and from some periods when Anti Spoofing was enforced …

The 1993 Klamath Falls, Oregon, Earthquake Sequence: Source Mechanisms From Regional Data, Jochen Braunmiller, John Nábělek, Beate Leitner, Anthony Qamar

School of Geosciences Faculty and Staff Publications

We use regional broadband seismograms to obtain seismic moment-tensor solutions of the two September 20, 1993, M_w=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (M_D>3.5). Several sub-groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment-tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east-dipping normal fault, possibly related to the Lake of the Woods fault system. Rotation of T-axes between the two main shocks is consistent with the two dominant trends of the aftershocks …

Sar Interferometry And Surface Change Detection, Timothy H. Dixon

School of Geosciences Faculty and Staff Publications

Recent events have brought synthetic aperture radar (SAR) interferometry into vogue—at least in some sectors of the geophysical science community. For one thing, the image of coseismic displacement associated with the Landers earthquake, recently published in Nature [Massonnet et al., 1993], coupled with the recent disastrous Northridge earthquake of January 17,1994, has created interest in the Earth science community about SAR interferometry. In particular, there is widespread appreciation that this technique can make a significant contribution to earthquake studies in California and other technically active regions. Similarly, a recent demonstration that ice stream velocity could be measured directly …

Aftershocks Of The 13 May 1993 Shumagin Alaska Earthquake, Zhong Ludwig, Max Wyss, Guy Tytgat, Stephen R. Mcnutt, Scott Stihler

School of Geosciences Faculty and Staff Publications

The 13 May 1993 M_s 6.9 Shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1.5 by 1 June 1993. Of these aftershocks, 79 were located by using S-P travel times at the only two stations within 570 km of the mainshock epicenter. The rupture area inferred from the aftershocks is about 600 km² and we estimate for the mainshock a mean fault displacement of 1.0 m and a 28 bar stress drop. The magnitude-frequency plots give a b-value for the aftershock sequence of about 0.4, which is low compared to …

Continuous Monitoring Of High‐Temperature Fumaroles On An Active Lava Dome, Volcán Colima, Mexico: Evidence Of Mass Flow Variation In Response To Atmospheric Forcing, Charles B. Connor, Bradford M. Clement, Xiaodan Song, Sammantha B. Lane, Jennifer West-Thomas

School of Geosciences Faculty and Staff Publications

Rapid, multichannel monitoring of fumaroles on Volcán Colima, Mexico, provides new insight into the time‐scales and magnitudes of fumarole temperature variation. Temperatures in five fumaroles, all located along a single fracture cutting the summit lava dome of the volcano, were monitored at 20‐min intervals between May 1991 and May 1992. Measurements were made using a programmable data logger deployed near the fumarole field, and data were radiotelemetered to a nearby volcano observatory at regular intervals. Mean fumarole temperatures varied between 350°C and 550°C. Statistical analysis of these time series shows that significant diurnal variation occurs in each fumarole. Magnitudes of …

Gps Measurement Of Relative Motion Of The Cocos And Caribbean Plates And Strain Accumulation Across The Middle America Trench, Timothy H. Dixon

School of Geosciences Faculty and Staff Publications

Global Positioning System (GPS) measurements in 1988 and 1991 on Cocos Island (Cocos plate), San Andres Island (Caribbean plate), and Liberia (Caribbean plate, mainland Costa Rica) provide an estimate of relative motion between the Cocos and Caribbean plates. The data for Cocos and San Andres Islands, both located more than 400 km from the Middle America Trench, define a velocity that is equivalent within two standard errors (7 mm/yr rate, 5 degrees azimuth) to the NUVEL-1 plate motion model. The data for Liberia, 120 km from the trench, define a velocity that is similar in azimuth but substantially different in …

Assessment Of Watershed-Lake Interactions Influencing The Cultural Eutrophication Of Little Crooked & Crooked Lakes, Indiana, Thomas Crisman, William R. Demott, Paul M. Stewart

School of Geosciences Faculty and Staff Publications

No abstract provided.

Evidence Of Regional Structural Controls On Vent Distribution: Springerville Volcanic Field, Arizona, Charles B. Connor, Christopher Condit, Larry S. Crumpler, Jayne C. Aubele

School of Geosciences Faculty and Staff Publications

Quantitative analysis of the geographic distribution of vents and comparison with regional structural, petrologic, and vent age data provide insight into the processes governing the emplacement of vents in the Springerville volcanic field, Arizona. A total of 409 vents in the Springerville volcanic field (SVF) have a mean distance to nearest neighbor vents of 955 m, a much closer spacing than is common in some platform‐type volcanic fields. Based upon a cluster analysis search radius parameter of 4500 m, these vents comprise seven geographic clusters, with only five outlying vents occurring in the entire field. Cinder cone clusters in the …

Tri-Lakes Watershed Feasibility Study Whitley County June 1992, Thomas Crisman, Gensic & Associates

School of Geosciences Faculty and Staff Publications

No abstract provided.

Paleolimnological Assessment Of The Effectiveness Of A Constructed Sedimentation Basin At Reducing Sediment And Nutrient Loading To Skinner Lake, Indiana, Thomas Crisman

School of Geosciences Faculty and Staff Publications

No abstract provided.

Reply [To “Comment On ‘Topographic And Volcanic Asymmetry Around The Red Sea: Constraints On Rift Models’”], Timothy H. Dixon, Erik R. Ivins, Brenda J. Franklin

School of Geosciences Faculty and Staff Publications

No abstract provided.

Preliminary Determination Of Pacfic-North America Relative Motion In The Southern Gulf Of Calfornia Using The Global Positioning System, Timothy H. Dixon, G. Gonzalez, S. M. Lichten, D. M. Tralli, G. E. Ness, J. P. Dauphin

School of Geosciences Faculty and Staff Publications

Global Positioning System (GPS) data from experiments conducted in 1985 and 1989 in the southern Gulf of California, Mexico, allow a determination of relative motion between the Pacific and North American plates. The data indicate motion of Cabo San Lucas on the Pacific plate relative to North America at a rate of 47±7 mrn/yr and azimuth of 57±6° west of north (1σ errors), equivalent within uncertainties to the NUVEL-1 global plate motion model.

An Introduction To The Global Positioning System And Some Geological Applications, Timothy H. Dixon

School of Geosciences Faculty and Staff Publications

Receivers equipped to measure dual frequency carrier phase signals from satellites of the Global Positioning System (GPS) have been capable, under special conditions, of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s. The major obstacles to making this capability routine, extending it to all parts of the globe, and extending it to longer station separations, have been equipment cost, limitations in the GPS satellite constellation, arduous data analysis, uncertainties in satellite orbits, uncertainties in propagation delays associated with variable tropospheric water vapor, …

First Epoch Geodetic Measurements With The Global Positioning System Across The Northern Caribbean Plate Boundary Zone, Timothy H. Dixon, G. Gonzalez, S. M. Lichten, E. Katsigris

School of Geosciences Faculty and Staff Publications

The first geodetic survey across the northern Caribbean plate boundary zone with the Global Positioning System (GPS) was conducted in June 1986. Baseline vectors defined by the six station regional GPS network ranged from 170 to 1260 km in length. Repeatability of independent daily baseline estimates was better than 8 mm plus 1.3 parts in 10⁸ of baseline length for horizontal components. The wet tropospheric path delay during the experiment was both high, sometimes exceeding 30 cm at zenith, and variable, sometimes exceeding 5 cm variation over several hours. Successful carrier phase cycle ambiguity resolution (“bias fixing”) could not …

Cinder Cone Clustering In The Transmexican Volcanic Belt: Implications For Structural And Petrologic Models, Charles B. Connor

School of Geosciences Faculty and Staff Publications

Cinder cone distributions have most often been characterized using univariate statistics. Here a new technique to volcano distribution studies, cluster analysis, is applied to cinder cone distribution in the central TransMexican Volcanic Belt (TMVB). A total of 1016 cinder cones are identified over an area of approximately 60,000 km2. Application of cluster analysis reveals structure in cinder cone distribution. Using a search radius parameter of 16 km, 75% of the cinder cones within the central TMVB are found within eight clusters of 45–159 cones each. These clusters are each 2000 to 5000 km2 in area. Only 22 cones are found …