Array Science for the IRIS Community Wavefields Experiment

University Of Memphis, Memphis TN

Investigators

Abstract

Seismic waves that move through the Earth give critical information about the earthquake sources that create them and about the physical nature of the crust and mantle that they move through. Seismic waves are generally recorded by a seismic instrument situated at a single spot on Earth's surface so that wave motion is recorded can be seen as time changes. A seismic array is a collection of seismic instruments arranged in a pattern such that waves can be imaged over space as well as time. This additional information is important to the solution of many practical problems in hydrocarbon exploration, earthquake source physics, explosion detection, earthquake hazards assessments, and scientific studies of the structure of Earth's crust and mantle. Research in this project is centered on the study of new array geometries and data analysis methods that have been made possible by the deployment of a scientific-community experiment in Northern Oklahoma by the Incorporated Research Institutions in Seismology (IRIS) consortium. Several experimental arrays were deployed over an area of active earthquake activity to collect seismic data in order to validate the new array methods. Expected results will be important in the design of future arrays and to the understanding of the relatively new Oklahoma seismic activity. In addition, the results will be incorporated into the IRIS Data Analysis and Processing short course held during the summer. The IRIS consortium deployed a community-planned experiment in northern Oklahoma in an area of presumed active induced seismicity from June 22, 2016 until the end of November 2016. Scientific goals for this experiment included: densely recording small to moderate induced earthquakes to understand their source processes, deploying experimental arrays to examine the utility of certain array geometries and methods in determining wave attributes of local and regional seismicity (e.g., horizontal phase velocity, azimuth of propagation), dense recording over two 5- and one 12-km-long profiles to probe for crustal structure, and use of local infrasound arrays in discriminating wave sources seen in the seismic signals. The present project is concerned with the PI-'s contribution to the experiment in testing two experimental array designs. These include a phased, frequency-wavenumber array in the defined by the 'Golay 3x6' spatial geometry 3x6 geometry consisting of 18 broadband stations with a 6 km aperture (3x6 geometry) and a separate 7-level, nested gradiometer consisting of 112 three-component industry nodal seismometers deployed over a square 800m on a side. Deployment of these two arrays offers an unprecedented opportunity to rigorously test the performance of a gradiometer against a phased array using the same earthquake sources that generate the wavefield. A positive result for the much smaller gradiometer would allow array deployments in areas with difficult field conditions such as inaccessibility or having poor security. Objectives for testing the performance of the "Golay 3x6' array are to examine its stability as a function of frequency and degradation of the array response due to field siting requirements. The great redundancy in sampling the wavefield will allow testing of methods to compute the spatial wave gradients and dealing with amplitude statics problems due to site, installation, and instrument response factors. In addition to evaluating the arrays, the seismic data and wave attributes will be used to study local and regional wave propagation and will serve to train young scientists on array processing techniques. The project will support a graduate student.