By David Schaff
Cross correlation is applied to a variety of useful scientific applications. We have created a relocated double-difference earthquake catalog for northern California comprising 456,041 events and 48 billion correlation measurements. The average improvement in location errors is one to two orders of magnitudes (three orders of magnitude for repeats). For correlation detection, we achieve an order of magnitude improvement (one magnitude unit reduction in detection threshold). It is possible to detect aftershocks buried in the coda of a mainshock. Also a difference of 3.3 magnitude units between the master template and slave event can produce statistically significant detections. It has been discovered there is an abundance of repeating events in China (13%) and northern California (12%). They are clustered in time as well as space and able to assess location errors of routine catalogs (~15 km). About half are foreshocks. We have analyzed foreshock sequences for 612 M ≥ 4 mainshocks in northern California (ten times the number of mainshocks as in previous studies). A negative correlation of foreshock occurrence with depth is observed. There is a positive correlation of foreshock magnitude with mainshock magnitude (first time observed). One prediction of a pre-slip model is that foreshock magnitude may scale with nucleation region. We investigate the presence of preseismic velocity changes before large earthquakes. For an event in Korea using the doublet method on a rare foreshock sequence produces biases up to 1% velocity change. Fitting with a plane wave to correct delays due to slight position differences improves the measurements by an order of magnitude. The upper bound on preseismic changes ranges from 0.02% to 0.08%. Applying an ambient noise technique at Parkfield, CA, using all possible pairs (702) measures an upper bound on preseismic changes to be 0.026% for one day stacks or 17% of the -0.15% coseismic signal.
By James P. M. Syvitski
Exec. Director of the Community Surface Dynamic Modeling System (CSDMS)
Fellow of the Institute of Arctic and Alpine Research (INSTAAR)
Professor of Geological Sciences, University of Colorado at Boulder
Abstract: Data and computer simulations are reviewed to help better define the timing and magnitude of human influence on sediment flux-the Anthropocene epoch. Impacts on the Earth surface processes are not spatially or temporally homogeneous. Human influences on this sediment flux have a secondary effect on floodplain and delta-plain functions and sediment dispersal into the coastal ocean. Human impact on sediment production began 3000 years ago but accelerated more widely 1000 years ago. By the sixteenth century, societies were already engineering their environment. Early twentieth century mechanization has led to global signals of increased sediment flux in most large rivers. By the 1950s, this sediment disturbance signal reversed for many rivers owing to the proliferation of dams, and sediment load reduction below pristine conditions is the dominant signal today. A delta subsidence signal began in the 1930s and is now a dominant signal in terms of sea level for many coastal environments, overwhelming even the global warming imprint on sea level. Humans have engineered how most water and sediment are discharged into the coastal ocean. Hyperpycnal flow events have become more common for some rivers, and less common for other rivers. Bottom trawling is now widespread, suggesting that even continental shelves have received a significant but as yet quantified Anthropocene impact. The Anthropocene attains the level of a geological climate event, such as that seen in the transition between the Pleistocene and the Holocene.
This lecture is part of the The Earth Science Colloquium video series. The Earth Science Colloquium Series is sponsored by the Lamont-Doherty Earth Observatory and the Columbia University Department of Earth and Environmental Sciences (DEES). This series provides a lively forum for discussing a wide variety of topics within the earth sciences and related fields. Colloquia are attended by the full range of scientific and technical staff at LDEO.
By: Yair Rosenthal (Rutgers University)
Featuring: Michelle Mack, Associate Professor, University of Florida, Department of Biology
A predicted consequence of climate warming in the Arctic is an increase in the frequency, intensity and size of wildfires. Because Arctic tundra and boreal forests store globally important stocks of carbon in plants and soil, there has been considerable interest in understanding the effects of changing fire regimes on the carbon balance of these ecosystems. Fire rapidly releases carbon stored in plants and soil to the atmosphere, particularly if it burns deeply into organic soils characteristic of Arctic ecosystems. Over longer timescales, changes in organic soils can alter controls over ecosystem carbon dynamics by influencing plant species composition, nutrient availability, and the integrity of permafrost—permanently frozen soil. In this talk, I will explore two aspects of changing fire regimes in Arctic ecosystems: increasing fire severity in the boreal forests of Interior Alaska, where fire has been part of the historic disturbance regime, and unprecedented fire in the Arctic tundra of Alaska’s North Slope, where fire has been largely absent since the early Holocene. For these two biomes, I will compare patterns of burning and discuss the consequences of intensifying fire regimes for carbon cycling feedbacks to climate.
Featuring: Matthew J. Fouch, Assistant Professor
School of Earth and Space Exploration, Arizona State University
The geological evolution of the western United States is an extensively studied problem with important consequences for understanding the nature of a host of Earth processes, including earthquake patterns and volcanic activity. The region is home to several famous case studies of Earth system processes occurring away from tectonic plate boundaries, including the substantial and ongoing extension of the Great Basin, and widespread hotspot volcanism across the Snake River Plain and Yellowstone regions. Other components of this complex system, such as the massive magmatic events expressed by the High Lava Plains of Oregon, and subduction of the Juan de Fuca plate system beneath North America that is responsible for the Cascade Volcanic Chain, are more poorly understood. Until recently, the only available seismic images of regional crust and mantle structure have been of limited resolution, with the exception of a few localized high-resolution studies. The lack of broad-scale, high-resolution seismic images across the entire region has limited our ability connect deep processes with their surface manifestations.
In this talk, I will show results from several recent studies that place new constraints on the structure and evolution of the western United States. I will focus primarily on high-resolution seismic investigations in which we have utilized data collected from the ongoing EarthScope Project (earthscope.org) and the High Lava Plains Project (dtm.ciw.edu/research/HLP). I will present images of the structure and dynamics of the northwestern United States, and will concentrate on detailed studies of both the central Great Basin, where we find clear evidence for a newly-discovered zone of mantle downwelling, and the Snake River Plains / Yellowstone system, where we find no evidence for an upwelling deep mantle plume source. These new results demonstrate the need for revisions to models of the tectonic evolution of the western United States.
Earth Science Colloquium Series
The Earth Science Colloquium Series is sponsored by the Lamont-Doherty Earth Observatory and the Columbia University Department of Earth and Environmental Sciences (DEES). This series provides a lively forum for discussing a wide variety of topics within the earth sciences and related fields.