A Geologic Classification of Worldwide Seismic Sites

Broadband seismic data recorded by more than 200 stations in two permanent networks, GSN and GEOSCOPE, have been widely used to investigate a large array of problems related to the structure and dynamics of the earth's interior. Yet so far there still is a lacking of a standard database of the geological setting in the vicinity of each of the stations. We are being funded by the National Science Foundation to compile such a database based on information provided by many previous studies. The geological setting in the vicinity of the stations has been classified broadly into a number of categories: Mesozoic-Cenozoic or Precambrian plateau basalts, Phanerozoic or Precambrian platform sedimentary rocks, Mesozoic-Cenozoic or Paleozoic orogenic provinces, Precambrian orogenic provinces (0.8 to 1.6 Ga, 0.6 to 0.8 Ga, 1.6 to 2.5 Ga, 2.5 to 3.5 Ga, and older than 3.5 Ga years old), and oceanic islands. In addition, these groups have been further sub-classified as to the types of rocks or environments in which the rocks are present. For instance, the oceanic islands have been classified as oceanic rise tholeiites (e.g., Iceland), ocean floor hotspot islands (either mostly tholeiiitic sequences, or mostly alkalic), continental mid-ocean islands (e.g., Seychelles Islands-mostly composed of granitoids), oceanic-oceanic subduction (e.g., Adak Island, the Aleutian arc), or oceanic-continental subduction (e.g., Kodiak Island, the Aleutian arc). The resulting comprehensive database, once completed, will become an essential resource for the study of the crust and mantle beneath the stations using various geophysical techniques, such as shear-wave splitting analysis, crustal thickness and Vp/Vs measurements, and seismic tomography and seismic wave attenuation analyses.