Исследование структуры Североамериканского континента в рамках проекта EarthScope.

Earth science program exploring the structure of the North American continent

The EarthScope project was an National Science Foundation (NSF) funded earth science program that, from 2003 2018, used geological and geophysical techniques to explore the structure and evolution of the North American continent and to understand the processes controlling earthquakes and volcanoes. The project had three components: USArray, the Plate Boundary Observatory, and the San Andreas Fault Observatory at Depth. Organizations associated with the project included UNAVCO, the Incorporated Research Institutions for Seismology (IRIS), Stanford University, the United States Geological Survey (USGS) and National Aeronautics and Space Administration (NASA). Several international organizations also contributed to the initiative. EarthScope data are publicly accessible.

There were three EarthScope observatories: the San Andreas Fault Observatory at Depth (SAFOD), the Plate Boundary Observatory (PBO), and the Seismic and Magnetotelluric Observatory (USArray). These observatories consist of boreholes into an active fault zone, global positioning system (GPS) receivers, tiltmeters, long baseline laser strainmeters, borehole strainmeters, permanent and portable seismographs, and magnetotelluric stations. The various EarthScope components will provide integrated and highly accessible data on geochronology and thermochronology, petrology and geochemistry, structure and tectonics, surficial processes and geomorphology, geodynamic modeling, rock physics, and hydrogeology.

USArray, managed by IRIS, was a 15 year program to place a dense network of permanent and portable seismographs across the continental United States. These seismographs recorded the seismic waves released by earthquakes that occur around the world. Seismic waves are indicators of energy disbursement within the earth. By analyzing the records of earthquakes obtained from this dense grid of seismometers, scientists could learn about Earth structure and dynamics and the physical processes controlling earthquakes and volcanoes. The goal of USArray was primarily to gain a better understanding of the structure and evolution of the continental crust, lithosphere, and mantle underneath North America. The USArray was composed of four facilities: a Transportable Array, a Flexible Array, a Reference Network, and a Magnetotelluric Facility. The Transportable Array was composed of 400 seismometers that were deployed in a rolling grid across the United States over a period of 10 years. The stations were placed 70 km apart, and could map the upper 70 km of the Earth. After approximately two years, stations were moved east to the next site on the grid – unless adopted by an organization and made a permanent installation. Once the sweep across the United States was completed, over 2000 locations will have been occupied. The Array Network Facility was responsible for data collection from the Transportable Array stations. The Flexible Array was composed of 291 broadband stations, 120 short period stations, and 1700 active source stations. The Flexible Array allowed sites to be targeted in a more focused manner than the broad Transportable Array. Natural or artificially created seismic waves could be used to map structures in the Earth. The Reference Network was composed of permanent seismic stations spaced about 300 km apart. The Reference Network provided a baseline for the Transportable Array and Flexible Array. EarthScope added and upgraded 39 stations to the already existing Advanced National Seismic System, which was part of the Reference Network. The Magnetotelluric Facility was composed of seven permanent and 20 portable sensors that recorded electromagnetic fields. It is the electromagnetic equivalent of the seismic arrays. The portable sensors were moved in a rolling grid similar to the Transportable Array grid, but were only in place about a month before they were moved to the next location. A magnetotelluric station consists of a magnetometer, four electrodes, and a data recording unit that are buried in shallow holes. The electrodes are oriented north south and east west and are saturated in a salt solution to improve conductivity with the ground.

The Plate Boundary Observatory PBO consisted of a series of geodetic instruments, Global Positioning System (GPS) receivers and borehole strainmeters, that were installed to help understand the boundary between the North American Plate and Pacific Plate. The PBO network included several major observatory components: a network of 1100 permanent, continuously operating Global Positioning System (GPS) stations, many of which provide data at high rate and in real time, 78 borehole seismometers, 74 borehole strainmeters, 26 shallow borehole tiltmeters, and six long baseline laser strainmeters. These instruments were complemented by InSAR (interferometric synthetic aperture radar) and LiDAR (light detection and ranging) imagery and geochronology acquired as part of the GeoEarthScope initiative. PBO also included comprehensive data products, data management and education and outreach efforts. These permanent networks were supplemented by a pool of portable GPS receivers that could be deployed for temporary networks to researchers, to measure the crustal motion at a specific target or in response to a geologic event. The Plate Boundary Observatory portion of EarthScope was operated by UNAVCO, Inc. UNAVCO was a non profit, university governed consortium that facilitated research and education using geodesy.

The San Andreas Fault Observatory at Depth (SAFOD) consisted of a main borehole that cut across the active San Andreas Fault at a depth of approximately 3 km and a pilot hole about 2 km southwest of San Andreas Fault. Data from the instruments installed in the holes, which consisted of geophone sensors, data acquisition systems, and GPS clocks, as well as samples collected during drilling, helped to better understand the processes that control the behavior of the San Andreas Fault.

Data collected from the various observatories were used to create different types of data products. Each data product addressed a different scientific problem.

Tomography is a method of producing a three dimensional image of the internal structures of a solid object (such as the human body or the earth) by the observation and recording of differences in the effects on the passage of energy waves impinging on those structures. The waves of energy are P waves generated by earthquakes and are recording the wave velocities. The high quality data that was collected by the permanent seismic stations of USArray and the Advanced National Seismic System (ANSS) allowed the creation of high resolution seismic imaging of the Earth's interior below the United States. Seismic tomography helps constrain mantle velocity structure and aids in the understanding of chemical and geodynamic processes that are at work. With the use of the data collected by USArray and global travel time data, a global tomography model of P wave velocity heterogeneity in the mantle could be created. The range and resolution of this technique allowed investigation into the suite of problems that are of concern in the North American mantle lithosphere, including the nature of the major tectonic features. This method gives evidence for differences in thickness and the velocity anomaly of the mantle lithosphere between the stable center of the continent and the more active western North America. These data are vital for the understanding of local lithosphere evolution, and when combined with additional global data, allow the mantle to be imaged beyond the current extent of USArray.

EarthScope Automated Receiver Survey (EARS), created a prototype of a system that was used to address several key elements of the production of EarthScope products. One of the prototype systems was the receiver reference model. It provided crustal thickness and average crustal Vp/Vs ratios beneath USArray transportable array stations.

The main function of the Advanced National Seismic System (ANSS) and USArray, was to provide high quality data for earthquake monitoring, source studies and Earth structure research. The utility of seismic data is greatly increased when noise levels, unwanted vibrations, are reduced; however broadband seismograms will always contain a certain level of noise. The dominant sources of noise are either from the instrumentation itself or from ambient Earth vibrations. Normally, seismometer self noise will be well below the seismic noise level, and every station will have a characteristic noise pattern that can be calculated or observed. Sources of seismic noise within the Earth are caused by any of the following: the actions of human beings at or near the surface of the Earth, objects moved by wind with the movement being transferred to the ground, running water (river flow), surf, volcanic activity, or long period tilt due to thermal instabilities from poor station design. A new approach to seismic noise studies was introduced with the EarthScope project, in that there were no attempts to screen the continuous waveforms to eliminate body and surface waves from the naturally occurring earthquakes. Earthquake signals are not generally included in the processing of noise data, because they are generally low probability occurrences, even at low power levels. The two objectives behind the collection of the seismic noise data were to provide and document a standard method to calculate ambient seismic background noise, and to characterize the variation of ambient background seismic noise levels across the United States as a function of geography, season, and time of day. The new statistical approach provided the ability to compute probability density functions (PDFs) to evaluate the full range of noise at a given seismic station, allowing the estimation of noise levels over a broad range of frequencies from 0.01–16 Hz (100 0.0625s period). With the use of this new method it became much easier to compare seismic noise characteristics between different networks in different regions.

Seismometers of USArray transportable array recorded the passage of numerous seismic waves through a given point near the Earth's surface, and classically these seismograms are analyzed to deduce properties of the Earth's structure and the seismic source. Given a spatially dense set of seismic recordings, these signals could also be used to visualize the actual continuous seismic waves, providing new insights and interpretation techniques into complex wave propagation effects. Using signals recorded by the array of seismometers, the EarthScope project animated seismic waves as they sweep across the USArray transportable array for selected larger earthquakes. This illustrated the regional and teleseismic wave propagation phenomena. The seismic data collected from both permanent and transportable seismic stations was used to provide these computer generated animations.

The seismic moment tensor is one of the fundamental parameters of earthquakes that can be determined from seismic observations. It is directly related to earthquake fault orientation and rupture direction. The moment magnitude, Mw derived from the moment tensor magnitude, is the most reliable quantity for comparing and measuring the size of an earthquake with other earthquake magnitudes. Moment tensors are used in a wide range of seismological research fields, such as earthquake statistics, earthquake scaling relationships, and stress inversion. The creation of regional moment tensor solutions, with the appropriate software, for moderate to large earthquakes in the U. S. came from USArray transportable array and Advance National Seismic System broadband seismic stations. Results were obtained in the time and the frequency domain. Waveform fit and amplitude phase match figures were provided to allow users to evaluate moment tensor quality.

Global Positioning System (GPS) equipment and techniques provide a unique opportunity for earth scientists to study regional and local tectonic plate motions and conduct natural hazards monitoring. Cleaned network solutions from several GPS arrays merged into regional clusters in conjunction with the EarthScope project. The arrays included the Pacific Northwest Geodetic Array, EarthScope's Plate Boundary Observatory, the Western Canadian Deformation Array, and networks run by the US Geological Survey. The daily GPS measurements from ~1500 stations along the Pacific/North American plate boundary provided millimeter scale accuracy and could be used monitor the displacements of the earths crust. With the use of data modeling software and the recorded GPS data, the opportunity to quantify crustal deformation caused by plate tectonics, earthquakes, landslides and volcanic eruptions was possible.

The goal was to provide models of time dependent strain associated with a number of recent earthquakes and other geologic events as constrained by GPS data. With the use of InSAR (Interferometric Synthetic Aperture Radar), a remote sensing technique, and PBO (Plate Boundary Observatory), a fixed array of GPS receivers and strainmeters, the EarthScope project provided spatially continuous strain measurements over wide geographic areas with decimeter to centimeter resolution.

The Global Strain Rate Map (GSRM) is a project of the International Lithosphere Program whose mission is to determine a globally self consistent strain rate and velocity field model, consistent with geodetic and geologic field observations collected by GPS, seismometers, and strainometers. GSRM is a digital model of the global velocity gradient tensor field associated with the accommodation of present day crustal motions. The overall mission also includes: (1) contributions of global, regional, and local models by individual researchers; (2) archive existing data sets of geologic, geodetic, and seismic information that can contribute toward a greater understanding of strain phenomena; and (3) archive existing methods for modeling strain rates and strain transients. A completed global strain rate map provided a large amount of information which will contribute to the understanding of continental dynamics and for the quantification of seismic hazards.

There were seven topics the EarthScope program addressed with the use of the observatories.

The Education and Outreach Program was designed to integrate EarthScope into both the classroom and the community. The program reached out to scientific educators and students as well as industry professionals (engineers, land/resource managers, technical application/data users), partners of the project (UNAVCO, IRIS, USGS, NASA, etc. ), and the general public. To accomplish this, the EOP offered a wide array of educational workshops and seminars, directed at various audiences, to offer support on data interpretation and implementation of data products into the classroom. Their job was to make sure that everyone understood what EarthScope was, what it was doing in the community, and how to use the data it was producing. By generating new research opportunities for students in the scientific community, the program also hoped to expand recruitment for future generations of earth scientists.

"To use EarthScope data, products, and results to create a measurable and lasting change on the way that Earth science is taught and perceived in the United States."

Create a high profile public identity for EarthScope that emphasizes the integrated nature of the scientific discoveries and the importance of EarthScope research initiatives. Establish a sense of ownership among scientific, professional, and educational communities and the public so that a diverse group of individuals and organizations can and will make contributions to EarthScope. Promote science literacy and understanding of EarthScope among all audiences through informal education venues. Advance formal Earth science education by promoting inquiry based classroom investigations that focus on understanding Earth and the interdisciplinary nature of EarthScope. Encourage use of EarthScope data, discoveries, and new technology in resolving challenging problems and improving our quality of life.

Education and outreach developed tools for educators and students across the United States to interpret and apply this information for solving a wide range of scientific issues within the earth sciences. The project tailored its products to the specified needs and requests of educators.

The EarthScope Education and Outreach Bulletin was a bulletin targeted for grades 5 8 that summarized a volcanic or tectonic event documented by EarthScope and put it into an easily interpretable format, complete with diagrams and 3D models. They followed specific content standards based on what a child should be learning at those grade levels. The EarthScope Voyager, Jr. allowed students to explore and visualize the various types of data that were collected. In this interactive map, the user could add various types of base maps, features, and plate velocities. Educators could access to real time GPS data of plate movement and influences through the UNAVCO website.

EarthScope promised to produce a large amount of geological and geophysical data to the door for numerous research opportunities in the scientific community. As the USArray Big Foot project moved across the country, universities adopted seismic stations near their areas. These stations were then monitored and maintained by not only the professors, but their students as well. Scouting for future seismic station locations created field work opportunities for students. The influx of data helped creaate projects for undergraduate research, master's thesis, and doctoral dissertations. A list of funded proposals can be found on the NSF website.

Many applications for EarthScope data currently exist, as mentioned above. The EarthScope program was dedicated to determining the three dimensional structure of the North American continent. Future uses of the data that it produced might include hydrocarbon exploration, aquifer boundary establishment, remote sensing technique development, and earthquake risk assessment. Due to the open and free to the public data portals that EarthScope and its partners maintain, the applications are limited only by the creativity of those who wish to sort through the gigabytes of data. Also, because of its scale, the program will undoubtedly be the topic of casual conversation for many people outside of the geologic community. EarthScope chatter will be made by people in political, educational, social, and scientific arenas.

The multidisciplinary character of EarthScope helped create stronger network connections between geologists of all types and from around the country. Building an Earth model of this scale required a complex community effort, and this model is largely the first EarthScope legacy. Researchers analyzing the data left us with a greater scientific understanding of geologic resources in the Great Basin and of the evolution of the plate boundary on the North American west coast. Another geologic legacy desired by the initiative, was to invigorate the Earth sciences community. Invigoration is self perpetuating as evidenced by participation from thousands of organizations from around the world and from all levels of students and researchers. This leads to a significantly heightened awareness within the general public, including the next cohort of prospective Earth scientists. With further evolution of the EarthScope project, there were opportunities to create new observatories with greater capabilities, including extending the USArray over the Gulf of Mexico and the Gulf of California. There is much promise for EarthScope tools and observatories, even after retirement, to be used by universities and professional geologists. These tools include the physical equipment, software invented to analyze the data, and other data and educational products initiated or inspired by EarthScope.

The science produced by EarthScope and the researchers using its data products help guide lawmakers in environmental policy, hazard identification, and ultimately, federal funding of more large scale projects like this one. Besides the three physical dimensions of North America's structure, a fourth dimension of the continent is being described through geochronology using EarthScope data. Improving understanding of the continent's geologic history allow future generations to more efficiently manage and use geologic resources and live with geologic hazards. Environmental policy laws have been the subject of some controversy since the European settlement of North America. Specifically, water and mineral rights issues have been the focus of dispute. Representatives in Washington D. C. and the state capitals require guidance from authoritative science in drafting the soundest environmental laws for our country. The EarthScope research community was in a position to provide the most reliable course for government to take concerning environmental policy. Hazard identification with EarthScope is an application already in use. In fact, the Federal Emergency Management Agency (FEMA) has awarded the Arizona Geological Survey and its partner universities funding to adopt and maintain eight Transportable Array stations. The stations will be used to update Arizona's earthquake risk assessment.

For EarthScope to live up to its potential in the Earth sciences, the connections between the research and the education and outreach communities must continue to be cultivated. Enhanced public outreach to museums, the National Park System, and public schools will ensure that these forward thinking connections are fostered. National media collaboration with high profile outlets such as Discovery Channel, Science Channel, and National Geographic may secure a lasting legacy within the social consciousness of the world. Earth science has already been promoted as a vital modern discipline, especially in today's “green” culture, to which EarthScope is contributing. The size of the EarthScope project augments the growing public awareness of the broad structure of the planet on which we live.

Given that IRIS and UNAVCO operated the seismology and geodesy components of the instrumentation that the project relied on, when these two organizations merged in 2023 they adopted the name EarthScope Consortium to represent the shared vision of the new organization.