EarthScope
The EarthScope project was a National Science Foundation funded Earth science program using 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, Stanford University, the United States Geological Survey and National Aeronautics and Space Administration. Several international organizations also contributed to the initiative. EarthScope data are publicly accessible.
Observatories
There were three EarthScope project observatories:- The San Andreas Fault Observatory at Depth
- The Plate Boundary Observatory
- The Seismic and Magnetotelluric Observatory
Seismic and Magnetotelluric Observatory (USArray)
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.
Transportable Array
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 had been occupied. The Array Network Facility was responsible for data collection from the Transportable Array stations.Flexible Array
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.Magnetotelluric Facility
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.Image:EarthScope-geosensor.jpg|thumb|left|An EarthScope GPS Geosensor, a component of the Plate Boundary Observatory
Plate Boundary Observatory (PBO)
The Plate Boundary Observatory PBO consisted of a series of geodetic instruments, Global Positioning System 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 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 and LiDAR 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.Image:San Andreas Fault SAFOD Project.jpg|thumb|right|Schematic representation of the SAFOD main borehole and pilot hole
San Andreas Fault Observatory at Depth (SAFOD)
The San Andreas Fault Observatory at Depth 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 Products
Data collected from the various observatories were used to create different types of data products. Each data product addressed a different scientific problem.P-Wave Tomography
is a method of producing a three-dimensional image of the internal structures of a solid object 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 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.Ambient Seismic Noise
The main function of the Advanced National Seismic System 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, 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 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. With the use of this new method it became much easier to compare seismic noise characteristics between different networks in different regions.