TECTONISM AND VOLCANISM OF THE LAST ONE MILLION YEARS: A REALISTIC MAP OF PRESENT-DAY GEOLOGIC ACTIVITY

The NASA Crustal Dynamics Progam of the 1970s applied space geodesy techniques (satellite laser ranging and very long baseline interferometry) to the measurement of plate motions and rigidity. To plan baselines and station locations, a global map of present tectonic activity was required. The National Geographic “Physical World” map of 1975 was the base for such a map in 1977, which was used for several years by NASA for space geodesy research. Since then, the map has undergone several revisions and has appeared in 16 different textbooks and many technical papers. The most recent version, published in 2003, is a Robinson projection digital map based on sea surface altimetry for the ocean basins. Spreading rates are those of NUVEL-1. Terrestrial features were drawn from published literature, orbital photographs, and seismic epicenters. The one million year figure was chosen as a time long enough to be representative, but short enough that physiographic features such as fault scarps and volcanoes were still recognizable. The map has been useful in teaching geology courses at all levels. It shows recognized plates, but also shows that there are many major features not on distinct plate boundaries. Some of these have produced catastrophic earthquakes; examples include the New Madrid events of 1811, the Charleston earthquake of 1886, and most recently the Sechuan earthquake of 2008. The tectonic activity map may help increase public awareness of the need for seismic mitigation measures in non-plate boundary areas. The map is available on line as “Digital Tectonic Activity Map,” accompanied by complementary seismicity and space geodesy baseline maps, and as a public domain publication can be freely reproduced if properly credited.