EarthScope

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.^[1] 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.

Observatories

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.

Seismic and Magnetotelluric Observatory (USArray)

USArray, managed by IRIS, is a 15-year program to place a dense network of permanent and portable seismographs across the continental United States. These seismographs record 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 can learn about Earth structure and dynamics and the physical processes controlling earthquakes and volcanoes. The goal of USArray is primarily to gain a better understanding of the structure and evolution of the continental crust, lithosphere, and mantle underneath North America.

The USArray is composed of four facilities: a Transportable Array, a Flexible Array, a Reference Network, and a Magnetotelluric Facility.

The Transportable Array is composed of 400 seismometers that are being deployed in a rolling grid across the United States over a period of 10 years. The stations are placed 70 km apart, and can map the upper 70 km of the Earth. After approximately two years, stations are 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 is completed, over 2000 locations will have been occupied. The Array Network Facility is responsible for data collection from the Transportable Array stations.

The Flexible Array is composed of 291 broadband stations, 120 short period stations, and 1700 active source stations. The Flexible Array allows sites to be targeted in a more focused manner than the broad Transportable Array. Natural or artificially created seismic waves can be used to map structures in the Earth.

The Reference Network is composed of permanent seismic stations spaced about 300 km apart. The Reference Network provides a baseline for the Transportable Array and Flexible Array. EarthScope added and upgraded 39 stations to the already existing Advanced National Seismic System, which is part of the Reference Network.

The Magnetotelluric Facility is composed of seven permanent and 20 portable sensors that record electromagnetic fields. It is the electromagnetic equivalent of the seismic arrays. The portable sensors are moved in a rolling grid similar to the Transportable Array grid, but are only in place about a month before they are 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.

Plate Boundary Observatory (PBO)

The Plate Boundary Observatory PBO consists of a series of geodetic instruments, Global Positioning System (GPS) receivers and borehole strainmeters, that have been installed to help understand the boundary between the North American Plate and Pacific Plate. The PBO network includes 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 are 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 includes comprehensive data products, data management and education and outreach efforts. These permanent networks are supplemented by a pool of portable GPS receivers that can 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 is operated by UNAVCO, Inc. UNAVCO is a non-profit, university-governed consortium that facilitates research and education using geodesy.

San Andreas Fault Observatory at Depth (SAFOD)

The San Andreas Fault Observatory at Depth (SAFOD) consists of a main borehole that cuts 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 consist of geophone sensors, data acquisition systems, and GPS clocks, as well as samples collected during drilling, will help to better understand the processes that control the behavior of the San Andreas Fault.

Data Products

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

P-Wave Tomography

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 is being collected by the permanent seismic stations of USArray and the Advanced National Seismic System (ANSS) will allow 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 can be created. The range and resolution of this technique will allow 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. This data is vital for the understanding of local lithosphere evolution, and when combined with additional global data, will allow the mantle to be imaged beyond the current extent of USArray.

Receiver Reference Models

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

Ambient Seismic Noise

The main function of the Advanced National Seismic System (ANSS) and USArray, is 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 will be introduced with the EarthScope project, in that there are 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 are 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 will provide 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 will be much easier to compare seismic noise characteristics between different networks in different regions.

Earthquake Ground Motion Animations

Seismometers of USArray transportable array record 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 can 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 will be able to animate seismic waves as they sweep across the USArray transportable array for selected larger earthquakes. This will be able to illustrate the regional and teleseismic wave propagation phenomena. The seismic data collected from both permanent and transportable seismic stations will be used to provide these computer generated animations.

Regional Moment Tensors

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. will be from USArray transportable array and Advance National Seismic System broadband seismic stations. Results are obtained in the time and the frequency domain. Waveform fit and amplitude-phase match figures are provided to allow users to evaluate moment tensor quality.

Geodetic Monitoring of the Western US and Hawaii

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 have merged into regional clusters in conjunction with the EarthScope project. The arrays include 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 provide millimeter-scale accuracy and can 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 will be possible.

Time-dependent Strain

The goal is 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 will provide spatially continuous strain measurements over wide geographic areas with decimeter to centimeter resolution.

Global Strain Rate Map

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 will provide a large amount of information which will contribute to the understanding of continental dynamics and for the quantification of seismic hazards.

Science

There are seven topics that EarthScope will address with the use of the observatories.

Convergent Margin Processes

Convergent margins, also known as convergent boundaries, are active regions of deformation between two or more tectonic plates colliding with one another. Convergent margins create areas of tectonic uplift, such as mountain ranges or volcanoes. EarthScope is focusing on the boundary between the Pacific Plate and the North American Plate in the western United States. EarthScope will provide GPS geodetic data, seismic images, detailed seismicity, magnetotelluric data, InSAR, stress field maps, digital elevation models, baseline geology, and paleoseismology for a better understanding of convergent margin processes.