The motivation and objective of the EarthScope Transportable Array (TA) is to record earthquake signals and image the structure of the North American plate, however the observations collected by this National Science Foundation funded project have enabled unanticipated discoveries, innovative data analysis techniques, and ongoing investigations across many disciplines in the Earth and space sciences. The Transportable Array utilized a survey approach to collect data in which high-quality stations were systematically installed in a dense geospatial grid. From the very beginning of the deployment, this strategy allowed for data-driven discovery, such as using seismic data to map out extensive travel time curves for acoustic waves in the atmosphere (Hedlin et al., 2010). While the emplacement of the seismic sensors was kept uniform along with the core components for power and communications, the Transportable Array station design evolved over time to include additional barometric pressure and infrasound sensors and, eventually, meteorological sensors measuring external temperature, wind, and precipitation. As the array rolled across the Lower 48 and the TA became more recognized outside of seismology, collaborations were forged and strengthened with researchers in the infrasound and meteorological communities. Along with standard approaches using direct measurements, inventive techniques were used to apply environmental data for observing tectonic phenomena as well as applying seismic data for observing environmental phenomena. The value of integrated scientific infrastructure became even more apparent with the Transportable Array deployment in Alaska and western Canada, with autonomous and telemetered stations occupying sites within large swaths of previously unmonitored and inaccessible terrain. The majority of Alaska TA stations collect weather data and a subset also include a detached soil temperature probe. As a result, data collected by the Alaska Transportable Array have been used to observe throughout the ‘spheres: the lithosphere (earthquakes, volcanoes, landslides), the cryosphere (sea ice), the hydrosphere (precipitation, fire preparation), the atmosphere and biosphere (weather forecasting, storm systems, bolides), and even into the magnetosphere (space weather).

Over the past third of a century the Incorporated Research Institutions for Seismology (IRIS) has facilitated observational seismology in many ways. At the beginning of IRIS in 1984, and with the support of the National Science Foundation and in partnership with the US Geological Survey, IRIS embarked on deploying the Global Seismographic Network (GSN). Key characteristics of the GSN are its use of high-performance digitizers, very broad band seismometers, strong motion accelerometers, and high frequency sensors to provide multi-decadal observations across a wide frequency band and dynamic range. The IRIS Portable Array Seismic Studies of the Continental Lithosphere (PASSCAL) program has also operated since 1984. PASSCAL’s extensive inventory of seismic equipment has been used by scientists to make observations on every part of the globe. The number and breadth of observations made with this equipment has fueled thousands of research papers and contributed to the education of hundreds, if not thousands, of students. More recently, the IRIS-operated EarthScope Transportable Array (TA) provided a breakthrough in the systematic collection of data using an array of unprecedented size. The success of the TA has ushered in a new era of “Large N” seismology, focused on dense spatial coverage of sensors to reduce aliasing and provide more complete recording of the full wavefield. The TA highlighted the power of survey mode data collection, where systematic, spatially-dense, and high-quality data fuel data-driven discovery, as opposed to deployments made to test a specific hypothesis. Key future directions in observational seismology include an increasing emphasis on wavefield measurements. Deploying instruments in large numbers requires reductions in the size, weight, and power of units, as well as a focus on dirt-to-desktop data management strategies that merge data and metadata while minimizing human intervention with the data flow from the sensor in the dirt to the scientist’s desktop. Other critical frontiers include pervasive seafloor observations to enable studies of key structures like subduction zones, more accessible satellite telemetry to enable ubiquitous sensing of the environment, and new sensing technologies such as MEMS and Distributed Acoustic Sensing.