Hot spot at Hawaii? Not so fast

– AUGUST 18, 2017

Hot spot at Hawaii? Not so fast

Rice University scientists’ model shows global mantle plumes don’t move as quickly as thought

HOUSTON – (Aug. 18, 2017) – Through analysis of volcanic tracks, Rice University geophysicists have concluded that hot spots like those that formed the Hawaiian Islands aren’t moving as fast as recently thought.

Hot spots are areas where magma pushes up from deep Earth to form volcanoes. New results from geophysicist Richard Gordon and his team confirm that groups of hot spots around the globe can be used to determine how fast tectonic plates move.

Rice University geophysicists have developed a method that uses the average motion of hot-spot groups by plate to determine that the spots aren't moving as fast as geologists thought. For example, the Juan Fernandez Chain (outlined by the white rectangle) on the Nazca Plate west of Chile was formed by a hot spot now at the western end of the chain as the Nazca moved east-northeast relative to the hotspot forming the chain that includes Alejandro Selkirk and Robinson Crusoe islands. The white arrow shows the direction of motion of the Nazca Plate relative to the hot spot, and it is nearly indistinguishable from the direction predicted from global plate motions relative to all the hot spots on the planet (green arrow). The similarity in direction indicates that very little motion of the Juan Fernandez hot spot relative to other hot spots is needed to explain its trend. Illustration by Chengzu Wang

Gordon, lead author Chengzu Wang and co-author Tuo Zhang developed a method to analyze the relative motion of 56 hot spots grouped by tectonic plates. They concluded that the hot-spot groups move slowly enough to be used as a global reference frame for how plates move relative to the deep mantle. This confirmed the method is useful for viewing not only current plate motion but also plate motion in the geologic past.

The study appears in Geophysical Research Letters.

Hot spots offer a window into the depths of Earth, as they mark the tops of mantle plumes that carry hot, buoyant rock from deep Earth to near the surface and produce volcanoes. These mantle plumes were once thought to be straight and stationary, but recent results suggested they can also shift laterally in the convective mantle over geological time.

The primary evidence of plate movement relative to the deep mantle comes from volcanic activity that forms mountains on land, islands in the ocean or seamounts, mountain-like features on the ocean floor. A volcano forms on a tectonic plate above a mantle plume. As the plate moves, the plume gives birth to a series of volcanoes. One such series is the Hawaiian Islands and the Emperor Seamount Chain; the youngest volcanoes become islands while the older ones submerge. The series stretches for thousands of miles and was formed as the Pacific Plate moved over a mantle plume for 80 million years.

Rice University geophysicists have developed a method that uses the average motion of hot-spot groups by plate to determine that the spots aren’t moving as fast as geologists thought. From left, Chengzu Wang, Richard Gordon and Tuo Zhang. Photo by Jeff Fitlow

The Rice researchers compared the observed hot-spot tracks with their calculated global hot-spot trends and determined the motions of hot spots that would account for the differences they saw. Their method demonstrated that most hot-spot groups appear to be fixed and the remainder appear to move slower than expected.

“Averaging the motions of hot-spot groups for individual plates avoids misfits in data due to noise,” Gordon said. “The results allowed us to say that these hot-spot groups, relative to other hot-spot groups, are moving at about 4 millimeters or less a year.

“We used a method of analysis that’s new for hot-spot tracks,” he said. “Fortunately, we now have a data set of hot-spot tracks that is large enough for us to apply it.”

For seven of the 10 plates they analyzed with the new method, average hot-spot motion measured was essentially zero, which countered findings from other studies that spots move as much as 33 millimeters a year. Top speed for the remaining hot-spot groups — those beneath the Eurasia, Nubia and North America plates — was between 4 and 6 millimeters a year but could be as small as 1 millimeter per year. That’s much slower than most plates move relative to the hot spots. For example, the Pacific Plate moves relative to the hot spots at about 100 millimeters per year.

Gordon said those interested in paleogeography should be able to make use of the model. “If hot spots don’t move much, they can use them to study prehistorical geography. People who are interested in circum-Pacific tectonics, like how western North America was assembled, need to know that history of plate motion.

“Others who will be interested are geodynamicists,” he said. “The motions of hot spots reflect the behavior of mantle. If the hot spots move slowly, it may indicate that the viscosity of mantle is higher than models that predict fast movement.”

“Modelers, especially those who study mantle convection, need to have something on the surface of Earth to constrain their models, or to check if their models are correct,” Wang said. “Then they can use their models to predict something. Hot-spot motion is one of the things that can be used to test their models.”

Gordon is the W.M. Keck Professor of Earth Science. Wang and Zhang are Rice graduate students. The National Science Foundation supported the research.

 

Read the paper at http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

Data mining finds more than expected beneath Andean Plateau

Seismic data suggests means of producing massive volumes of continental crust

Seismologists investigating how Earth forms new continental crust have compiled more than 20 years of seismic data from a wide swath of South America’s Andean Plateau and determined that processes there have produced far more continental rock than previously believed.

“When crust from an oceanic tectonic plate plunges beneath a continental tectonic plate, as it does beneath the Andean Plateau, it brings water with it and partially melts the mantle, the layer below Earth’s crust,” said Rice University’s Jonathan Delph, co-author of the new study published online this week in Scientific Reports. “The less dense melt rises, and one of two things happens: It either stalls in the crust to crystallize in formations called plutons or reaches the surface through volcanic eruptions.”

Jonathan Delph

Delph, a Wiess Postdoctoral Research Associate in Rice’s Department of Earth, Environmental and Planetary Science, said the findings suggest that mountain-forming regions like the Andean Plateau, which geologists refer to as “orogenic plateaus,” could produce much larger volumes of continental rock in less time than previously believed.

Study lead author Kevin Ward, a postdoctoral researcher at the University of Utah, said, “When we compared the amount of trapped plutonic rock beneath the plateau with the amount of erupted volcanic rock at the surface, we found the ratio was almost 30:1. That means 30 times more melt gets stuck in the crust than is erupted, which is about six times higher than what’s generally believed to be the average. That’s a tremendous amount of new material that has been added to the crust over a relatively short time period.”

A true-color image of the Central Andes and surrounding landscape acquired by the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard NASA’s Terra spacecraft. (Image courtesy of NASA)

The Andean Plateau covers much of Bolivia and parts of Peru, Chile and Argentina. Its average height is more than 12,000 feet, and though it is smaller than Asia’s Tibetan Plateau, different geologic processes created the Andean Plateau. The mountain-building forces at work in the Andean plateau are believed to be similar to those that worked along the western coast of the U.S. some 50 million years ago, and Delph said it’s possible that similar forces were at work along the coastlines of continents throughout Earth’s history.

Most of the rocks that form Earth’s crust initially came from partial melts of the mantle. If the melt erupts quickly, it forms basalt, which makes up the crust beneath the oceans on Earth; but there are still questions about how continental crust, which is more buoyant than oceanic crust, is formed. Delph said he and Ward began their research in 2016 as they were completing their Ph.D.s at the University of Arizona. The pair spent several months combining public datasets from seismic experiments by several U.S. and German institutions. Seismic energy travels through different types of rock at different speeds, and by combining datasets that covered a 500-mile-wide swath of the Andean Plateau, Ward and Delph were able to resolve large plutonic volumes that had previously been seen only in pieces.

West-east cross sections from north (top) to south (bottom) of a 500-mile-wide portion of the Andean Plateau show subsurface features to a depth of 80 kilometers. Colors represent the speed at which seismic waves pass through the Earth; arrows point to plutonic regions of continent-building material in each section. (Image courtesy of J. Delph/Rice University)

Over the past 11 million years, volcanoes have erupted thousands of cubic miles’ worth of material over much of the Andean Plateau. Ward and Delph calculated their plutonic-to-volcanic ratio by comparing the volume of regions where seismic waves travel extremely slowly beneath volcanically active regions, indicating some melt is present, with the volume of rock deposited on the surface by volcanoes.

“Orogenic oceanic-continental subduction zones have been common as long as modern plate tectonics have been active,” Delph said. “Our findings suggest that processes similar to those we observe in the Andes, along with the formation of supercontinents, could have been a significant contributor to the episodic formation of buoyant continental crust.”

Additional co-authors include George Zandt and Susan Beck of the University of Arizona and Mihai Ducea of the University of Arizona and University of Bucharest.

The research was supported by the National Science Foundation and Rice University, and data was obtained by request from the Incorporated Research Institutions for Seismology and the German Research Centre for Geosciences, Potsdam.

 

Hidden river once flowed beneath Antarctic ice

Antarctic researchers from Rice University have discovered one of nature’s supreme ironies: On Earth’s driest, coldest continent, where surface water rarely exists, flowing liquid water below the ice appears to play a pivotal role in determining the fate of Antarctic ice streams.

Rice University researchers (from left) Lindsay Prothro, Lauren Simkins and John Anderson and colleagues discovered a long-dead river system that once flowed beneath Antarctica’s ice. (Photo by Jeff Fitlow/Rice University)

The finding, which appears online this week in Nature Geoscience, follows a two-year analysis of sediment cores and precise seafloor maps covering 2,700 square miles of the western Ross Sea. As recently as 15,000 years ago, the area was covered by thick ice that later retreated hundreds of miles inland to its current location. The maps, which were created from state-of-the-art sonar data collected by the National Science Foundation research vessel Nathaniel B. Palmer, revealed how the ice retreated during a period of global warming after Earth’s last ice age. In several places, the maps show ancient water courses — not just a river system, but also the subglacial lakes that fed it.

Today, Antarctica is covered by ice that is in some places more than 2 miles thick. Though deep, the ice is not static. Gravity compresses the ice, and it moves under its own weight, creating rivers of ice that flow to the sea. Even with the best modern instruments, the undersides of these massive ice streams are rarely accessible to direct observation.

This schematic depicts a subglacial Antarctic river and overlying ice sheet. Black lines t1, t2 and t3 show where the ice sheet was grounded to the seafloor during pauses in ice retreat. Rice University researchers used such lines from precise maps of the Ross Sea floor to study how liquid water influenced the ice sheet during a period of its retreat starting about 15,000 years ago. (Image courtesy of L. Prothro/Rice University)

“One thing we know from surface observations is that some of these ice streams move at velocities of hundreds of meters per year,” said Rice postdoctoral researcher Lauren Simkins, lead author of the new study. “We also know that ice, by itself, is only capable of flowing at velocities of no more than tens of meters per year. That means the ice is being helped along. It’s sliding on water or mud or both.”

Because of the paucity of information about how water presently flows beneath Antarctic ice, Simkins said the fossilized river system offers a unique picture of how Antarctic water drains from subglacial lakes via rivers to the point where ice meets sea.

“The contemporary observations we have of Antarctic hydrology are recent, spanning maybe a couple decades at best,” Simkins said. “This is the first observation of an extensive, uncovered, water-carved channel that is connected to both subglacial lakes on the upstream end and the ice margin on the downstream end. This gives a novel perspective on channelized drainage beneath Antarctic ice. We can track the drainage system all the way back to its source, these subglacial lakes, and then to its ultimate fate at the grounding line, where freshwater mixed with ocean water.”

An example of seafloor bathymetry data that Rice University oceanographers used to identify a paleo-subglacial channel, grounding line landforms, volcanic seamounts and other features used in their study. (Image courtesy of L. Simkins/Rice University)

Simkins said meltwater builds up in subglacial lakes. First, intense pressures from the weight of ice causes some melting. In addition, Antarctica is home to dozens of volcanoes, which can heat ice from below. Simkins found at least 20 lakes in the fossil river system, along with evidence that water built up and drained from the lakes in episodic bursts rather than a steady stream. She worked with Rice co-author and volcanologist Helge Gonnermann to confirm that nearby volcanoes could have provided the necessary heat to feed the lakes.

Study co-author John Anderson, a Rice oceanographer and veteran of nearly 30 Antarctic research expeditions, said the size and scope of the fossilized river system could be an eye-opener for ice-sheet modelers who seek to simulate Antarctic water flow. For example, the maps show exactly how ice retreated across the channel-lake system. The retreating ice stream in the western Ross Sea made a U-turn to follow the course of an under-ice river. Simkins said that’s notable because “it’s the only documented example on the Antarctic seafloor where a single ice stream completely reversed retreat direction, in this case to the south and then to the west and finally to the north, to follow a subglacial hydrological system.”

Location of study area in the western Ross Sea. (Image courtesy of L. Simkins/Rice University)

Simkins and Anderson said the study may ultimately help hydrologists and modelers better predict how today’s ice streams will behave and how much they’ll contribute to rising sea levels.

“It’s clear from the fossil record that these drainage systems can be large and long-lived,” Anderson said. “They play a very important role in the behavior of the ice sheet, and most numerical models today are not at a state where they can deal with that kind of complexity.”

He said another key finding is that drainage through the river system took place on a time scale measured in tens to several hundreds of years.

“We’re kind of in this complacent mode of thinking right now,” Anderson said. “Some people say, ‘Well, the ice margin seems to be stable.’ Some people may take comfort in that, but I don’t because what this new research is telling us is that there are processes that operate on decadal time scales that influence ice behavior. The probability of us having observed a truly stable condition in the contemporary system, given our limited observation time, is pretty low.”

Additional co-authors are Lindsay Prothro of Rice, Sarah Greenwood of Stockholm University, Anna Ruth Halberstadt and Robert DeConto of the University of Massachusetts-Amherst, Leigh Stearns of the University of Kansas and David Pollard of Pennsylvania State University.

The research was supported by the National Science Foundation and the Swedish Research Council.

Glaciers may have helped warm Earth

Glaciers may have helped warm Earth

Rice professor’s study details effect of glacial versus nonglacial weathering on carbon cycle

It seems counterintuitive, but over the eons, glaciers may have made Earth warmer, according to a Rice University professor.

Weathering of Earth by glaciers may have warmed Earth over eons by aiding the release of carbon dioxide into the atmosphere.
A new study shows the cumulative effect may have created negative feedback that prevented runaway glaciation.
Photo by Paul Quackenbush

Mark Torres, an assistant professor of Earth, environmental and planetary sciences, took a data-driven dive into the mechanics of weathering by glaciation over millions of years to see how glacial cycles affected the oceans and atmosphere and continue to do so.


Mark Torres

Torres, who joined the Rice faculty in July, is lead author of a paper in the Proceedings of the National Academy of Sciences. He wanted to know how and when chemicals released by weathering of the land reached the atmosphere and ocean, and what effect they have had.

The study shows that glaciation, through enhanced erosion, probably increased the rate of carbon dioxide released to the environment. The researchers determined enhanced oxidation of pyrite, an iron sulfide also known as fool’s gold, most likely generated acidity that fed carbon dioxide into the oceans and altered the carbon cycle. The oscillation of glaciers over 10,000 years could have changed atmospheric carbon dioxide by 25 parts per million or more. While this is a significant percentage of the 400 parts per million measured in recent months, present anthropogenic carbon dioxide release is occurring at a much faster rate than it is naturally released by glaciation.

Over long timescales, they found, glaciers’ contribution to the release of carbon dioxide could have acted as a negative feedback loop that may have inhibited runaway glaciation.“The ocean stores a lot of carbon,” Torres said. “If you change the chemistry of the ocean, you can release some of that stored carbon into the atmosphere as carbon dioxide. This release of carbon dioxide affects Earth’s climate, due to the greenhouse effect.”

Glacial runoff appeared to have an outsize effect on carbon dioxide levels compared with that of rivers in warmer climes. Torres, until recently a postdoctoral researcher at the California Institute of Technology, studied glacier-fed rivers and used existing databases to compare their chemical contents with that of thousands of rivers around the world.

The goal was to evaluate the dominant chemical reactions associated with glacial weathering and explore the long-term implications. “Mainly, we’re thinking about the effect of glaciers and glaciation on the way our planet works,” he said. “In particular, we’re looking at rivers that drain areas of land surface that are covered by glaciers, and whether or not there are any differences in the chemical composition of those rivers.”

Mark Torres, an assistant professor of Earth, environmental and planetary sciences,
looked into the mechanics of weathering by glaciation over millions of years to see
how glacial cycles affected the oceans and atmosphere.

The researchers acknowledged that glaciers are equal-opportunity weathering agents, as they also break down silicates in rocks. Silicates release alkalinity that removes carbon from the atmosphere. Still, they believe the net effect of glaciation could be to supply carbon dioxide to the atmosphere rather than to remove it.

The results support a couple of interesting additional theories. One is that billions of years ago in the Archeaneon and Paleoproterozoic era, when the atmosphere contained little oxygen, Earth may indeed have been a “snowball” as oxidative weathering in glaciated regions and the subsequent release of carbon would have been less active. Another is that the growth of a sulfide reservoir in Earth’s crust over time may have helped to stabilize the climate, which is important for maintaining Earth’s habitability over geologic timescales.

The paper’s authors include Nils Moosdorf of the Center for Tropical Marine Ecology in Bremen, Germany, Jens Hartmann of the University of Hamburg, Jess Adkins of the California Institute of Technology and A. Joshua West of the University of Southern California.

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks PNAS 2017 published ahead of print July 31, 2017doi:10.1073/pnas.1702953114

 

Zealandia should hold answers about tectonics and past climate

Zealandia should hold answers about tectonics, past climate

JADE BOYD – JULY 18, 2017

Scientific expedition will explore Tasman Sea for clues about submerged continent

Thirty scientists will sail from Australia July 27 on a two-month ocean drilling expedition to the submerged continent of Zealandia in search of clues about its history, which relates to key questions about plate tectonic processes and Earth’s past greenhouse climate.

Jerry Dickens standing on a map of Zealandia in Rice’s Keith-Wiess Geological Laboratories (Photo by Jeff Fitlow/Rice University)

“We’re really looking at the best place in the world to understand how plate subduction initiates,” said expedition co-chief scientist Gerald Dickens, professor of Earth, environmental and planetary science at Rice University. “This expedition will answer a lot of lingering questions about Zealandia.”

Expedition 371, a cruise sponsored by the National Science Foundation and its international partners in the International Ocean Discovery Program (IODP), will sail from Townsville, Australia, aboard JOIDES Resolution, one of the world’s most sophisticated scientific drill ships. Expedition scientists will join more than 20 scientific crew members in drilling at six Tasman Sea sites at water depths ranging from 1,000 to 5,000 meters. At each site, the crew will drill from 300 to 800 meters into the seafloor to collect cores — complete samples of sediments deposited over millions of years. The cores hold fossil evidence the scientists can use to assemble a detailed record of Zealandia’s past.

“Some 50 million years ago a massive shift in plate movement happened in the Pacific Ocean,” said Jamie Allan, program director in the National Science Foundation’s Division of Ocean Sciences, which supports IODP. “It resulted in the diving of the Pacific Plate under New Zealand, the uplift of New Zealand above the waterline and the development of a new arc of volcanoes. This IODP expedition will look at the timing and causes of these changes, as well as related changes in ocean circulation patterns and ultimately Earth’s climate.”

IODP Expedition 371 map (Image courtesy IODP)

Zealandia is a mass of continental crust about half the size of Australia that surrounds New Zealand. Increasingly detailed seafloor maps have brought Zealandia into focus in recent decades. Unlike other continents, though, more than 90 percent of Zealandia is submerged.

“If you go way back, about 100 million years ago, Antarctica, Australia and Zealandia were all one continent,” Dickens said. “Around 85 million years ago, Zealandia split off on its own, and for a time, the seafloor between it and Australia was spreading on either side of an ocean ridge that separated the two.”

The relative movements of Zealandia and Australia are due to plate tectonics, the constant movement of interlocking sections of Earth’s surface. There are some 25 tectonic plates that fit like puzzle pieces to form Earth’s crust. Plates are in constant motion. They can crash together to form mountain ranges and slide past one another in earthquake zones. Oceanic plates form on either side of ocean ridges and also can slide beneath lighter, more buoyant continental plates in a process known as subduction.

JOIDES Resolution (Photo courtesy IODP)

Expedition 371 will examine a shift that occurred about 50 million years ago in the direction of movement of the enormous Pacific Plate northeast of Zealandia. In its scientific prospectus, the expedition refers to this shift as “the most profound subduction initiation event and global plate-motion change” in the past 80 million years. Prior to the shift, Australia and New Zealand were spreading apart, and after the shift, the area that separated them was under compression for millions of years. Then, in the final stage of the tectonic shift, the Pacific Plate dove beneath Zealandia, forming a new subduction zone. This relieved the compressive forces across the region.

“What we want to understand is why and when the various stages from extension to relaxation occurred,” Dickens said. “The cores will help tell us that. They’ll be analyzed for sediment composition, microfossil components, mineral and water chemistry and physical properties.”

He said the research also could answer many questions about the way Earth’s climate has evolved in the last 60 million years. For example, Zealandia is left out of many climate models, and Dickens said this could be one reason that this region has been among the most difficult parts of Earth to accurately model in greenhouse climates around 50 million years ago.

“When the community does climate modeling for the Eocene (Epoch), this is the area that causes consternation, and we’re not sure why,” he said. “It may be because we had continents that were much shallower than we had thought. Or we could have the continents right but at the wrong latitude. Either way, the cores will help us figure that out.”

IODP is an international research collaboration that coordinates seagoing expeditions to study the history of Earth recorded in sediments and rocks beneath the ocean floor. Scientists apply to participate in IODP expeditions, and participation is open to all scientists from IODP’s member countries. Opportunities exist for researchers, including graduate students, in all shipboard specialties, which include but are not limited to sedimentology, micropaleontology, paleomagnetism, inorganic/organic geochemistry, petrology, petrophysics, microbiology and borehole geophysics. For more information, visit http://iodp.tamu.edu/index.html.

JR in a Minute the Whole Ship!

 

Bounds on Geologically Current Rates of Motion of Groups of Hotspots

 Chengzu Wang, Richard G. Gordon*, and Tuo Zhang

It is widely believed that groups of hotspots in different regions of the world are in relative motion at rates of 10 to 30 mm a–1 or more. Here we present a new method for analyzing geologically current motion between groups of hotspots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hotspot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hotspot motion perpendicular to the direction of absolute plate motion, vperp, differs significantly from zero for only three of ten plates and then by merely 0.3 to 1.4 mm a–1. The global mean upper bound on |vperp| is 3.2 ±2.7 mm a–1. Therefore, hotspots move slowly and can be used to define a global reference frame for plate motions.

Link: http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

DOI: 10.1002/2017GL073430

The rigid-plate and shrinking-plate hypotheses: Implications for the azimuths of transform faults

Jay Kumar Mishra and Richard G. Gordon*

The rigid-plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools (hereinafter “rigid plate”). An alternative hypothesis, that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction (hereinafter “shrinking plate”), predicts subtly different azimuths for transform faults. The size of the predicted difference is as large as 2.44° with a mean and median of 0.46° and 0.31°, respectively, and changes sign between right-lateral (RL)-slipping and left-lateral (LL)-slipping faults. For the MORVEL transform-fault data set, all six plate pairs with both RL- and LL-slipping faults differ in the predicted sense, with the observed difference averaging 1.4° ± 0.9° (95% confidence limits), which is consistent with the predicted difference of 0.9°. The sum-squared normalized misfit, r, to global transform-fault azimuths is minimized for γ = 0.8 ± 0.4 (95% confidence limits), where γ is the fractional multiple of the predicted difference in azimuth between the shrinking-plate (γ = 1) and rigid-plate (γ = 0) hypotheses. Thus, observed transform azimuths differ significantly between RL-slipping and LL-slipping faults, which is inconsistent with the rigid-plate hypothesis but consistent with the shrinking-plate hypothesis, which indicates horizontal shrinking rates of 2% Ma−1 for newly created lithosphere, 1% Ma−1 for 0.1 Ma old lithosphere, 0.2% Ma−1 for 1 Ma old lithosphere, and 0.02% Ma−1 for 10 Ma old lithosphere, which are orders of magnitude higher than the mean intraplate seismic strain rate of ~10−6 Ma−1 (5 × 10−19 s−1).

 

Link: http://onlinelibrary.wiley.com/doi/10.1002/2015TC003968/full

DOI: 10.1002/2015TC003968

Comparison of full wavefield synthetics with frequency-dependent traveltimes calculated using wavelength-dependent velocity smoothing

Comparison of full wavefield synthetics with frequency-dependent traveltimes calculated using wavelength-dependent velocity smoothing

Jianxiong Chen and Colin A. Zelt

Journal of Environmental and Engineering Geophysics, 22, 133-141, 2017.

Ray theory-based traveltime calculation that assumes infinitely high frequency wave propagation is likely to be invalid in the near-surface (upper tens of meters) due to the relatively large seismic wavelength compared with the total travel path lengths and the scale of the near -surface velocity heterogeneities. The wavelength-dependent velocity smoothing (WDVS) algorithm calculates a frequency-dependent, first-arrival traveltime by assuming that using a wavelength-smoothed velocity model and conventional ray theory is equivalent to using the original unsmoothed model and a frequency-dependent calculation. This paper presents comparisons of WDVS-calculated traveltimes with band-limited full wave field synthetics including the results from 1) different velocity models, 2) different frequency spectra, 3) different values of a free parameter in the WDVS algorithm, and 4) different levels of added noise to the synthetics. The results show that WDVS calculates frequency-dependent travel times that are generally consistent with the first arrivals from band-limited full wavefield synthetics. Compared to infinite-frequency traveltimes calculated using conventional ray theory, the WDVS frequency-dependent traveltimes are more consistent with the first arrivals picked from full wavefield synthetics in terms of absolute time and trace-to-trace variation. The results support the use of WDVS as the forward modeling component of a tomographic inversion method, or any seismic method that involves modeling first-arrival traveltimes.

 

Rice wins 1st Place Poster at AAPG Student Expo 2016

expo_2016-20

Pankaj Khanna, PhD Candidate, won the 1st Place Poster award at the AAPG Student Expo, Houston 2016.

Poster title – ‘ Uppermost Pleistocene coralgal Reefs and Upper Cambrian microbial bioherms: Morphologies and sea-level induced evolution’

 

 

Earth Science Students Participate in Rice University’s 90 Second Thesis Competition

The Earth Science Ph.D. program fielded its first team of student participants in the university-wide 90 second thesis competition this spring.  Students Tamunoisoala LongJohn, Harsha Vora, Tuo Zhang, Zuolin Liu, and Lacey Pyle had 90 seconds to describe their Ph.D. thesis topics to a diverse panel of judges.  Follow this link here to see their performances.