A digital tectonic activity map of the earth
Paul Lowman, Jacob Yates,Penny Masuoka, Brian Montgomery,
Jay O'Leary, and Demetra Salisbury
Abstract
This paper presents two Robinson Projection versions of a map showing tectonic and volcanic activity of the geologic present, defined as the last one million years. One version is a shaded relief base map computer-generated from a National Geophysical Data Center digital elevation model, the other a schematic version showing in addition the distribution of continental and oceanic crust. Volcanic features active within the last one million years were generalized from Smithsonian Institution Holocene volcanism compilations, supplemented by geologic maps and orbital photography. Global seismicity (1963-1998) was digitally compiled from National Geophysical Data Center and U.S. Geological Survey databases.Horizontal motions of Very Long Baseline Interferometry (VLBI) space geodesy stations were compiled from Goddard Space Flight Center data. Sources for all data are given, including the Internet URLs. The maps are all in the public domain and may be freely reproduced if the source is credited. Tectonic implications are briefly discussed. The maps show that the Earth's crust can not be realistically described in terms of a small number of rigid plates; that the Appalachians and Urals may still be active; that some of the most intense seismic activity (western Europe) is between two of the most slowly-moving plates (African and Eurasian); and that space geodesy does not necessarily measure plate motion relative to the mantle.
Introduction
The subject of neotectonics, covering the structures and structural activity of the last 5 million years (i.e., post-Miocene) is a well-recognized field, including "active tectonics," focussed on the last 500,000 years in a 1986 National Research Council report of that title. However, there is a cartographic gap between tectonic maps, generally showing all features regardless of age, and maps of current seismic or volcanic activity. We have compiled a map intended to bridge this gap, using modern data bases and computer-aided cartographic techniques.
The maps presented here are conceptually descended from an earlier map showing tectonic and volcanic activity of the last one million years (Lowman, 1981,1982). Drawn by hand with the National Geographic Society's 1975 "The Physical World" map as a base, the 1981 map in various revisions has been widely reproduced in textbooks (e.g.,Siegal and Gillespie,1980) and various technical publications (e.g., McKelvey, 1986). However, two decades of progress call for a completely new map that can take advantage of new knowledge and cartographic techniques. The digital tectonic activity map (DTAM), presented in shaded relief
(Fig. 1) and schematic
(Fig. 2) versions, is the result.
The DTAM is intended to show tectonism and volcanism of the last one million years, a period long enough to be representative of present global activity, but short enough that features such as fault scarps and volcanos are still geomorphically recognizable.
Data Sources and Cartographic Methods
The DTAM is based on a wide range of digital data sources, summarized in Table 1. The DTAM itself can be found at
http://dtam.gsfc.nasa.gov or
http://denali.gsfc.nasa.gov/dtam/. The most important data source is the digital elevation model, used to construct a shaded relief map. The bathymetry is largely from satellite altimetry, specifically the marine gravity compilations by Smith and Sandwell (1996). The shaded relief map was designed to match the new National Geographic Society world physical map (1992), although drawn independently, from the digital elevation model. The Robinson Projection is used instead of the earlier Van der Grinten one. Although neither conformal nor equal-area, it provides a reasonable compromise and retains useful detail at high latitudes. Like the National Geographic Society map, the DTAM is centered on the prime meridian to avoid splitting major land masses.
The DTAM shaded relief map is an objective compilation, unlike the 1975 National Geographic Van der Grinten map, an artist's rendition drawing heavily on the classic Tharp-Heezen ocean floor maps. The Global Tectonic Activity Map of the Earth (GTA)
(Fig. 2) was drawn by hand with the shaded relief map as a base. Delineation of active tectonic features, such as the mid-ocean ridges, follows the bathymetry or topography as closely as possible, taking into account the distribution of seismic activity. Landsat imagery was extensively used for tectonic activity mapping, especially in central Asia.
A series of seismic activity maps was compiled, equatorial and polar (
Fig.3,
Fig.4 &
Fig.5), the latter also showing volcanism of the last one million years. Since development in the 1960s of the World Wide Standardized Seismographic Network to monitor nuclear tests (Bolt, 1976), our knowledge of global seismicity has expanded enormously. A Robinson Projection map of 358,214 epicenters of all magnitudes, between 1963 and 1998, was compiled. Although the map brought out many little-known active areas, regions such as western Europe were saturated with non-tectonic events such as quarry blasts. This problem has long challenged those involved in nuclear test monitoring (Hennet et al., 1996). Stump et al. (1994) cite a figure of 10,000 man-made seismic events per day in the United States. We therefore compiled equatorial and polar seismicity maps showing only events with magnitudes (chiefly mb) over 3.5. According to Hennet et al., a magnitude 4 corresponds roughly to a 1 kiloton explosion. Most industrial blasts are much less powerful than this, generally below 50 tons (Stump et al., 1994).
Many interesting zones of seismic activity are in the oceans, as off the coast of Norway. To see if these events might be offshore petroleum prospecting blasts, an epicenter map showing only events less than magnitude 3.5 was compiled. Most of the events in this particular area were thus eliminated. Furthermore, there were virtually no events in areas of intense petroleum exploration activity, such as the Grand Banks or the Gulf of Mexico. We conclude, therefore, that most of the events shown on
(Fig. 3) are natural, outside areas of known nuclear testing.
No effort has been made to screen nuclear explosions from the epicenter map. Even relatively small explosions, in the 30 - 50 kiloton range, can produce mb events with values well over 3.5, and some of those on the map are undoubtedly nuclear tests. The isolated event in southern Algeria, and the cluster on Novaya Zemlya, are obvious examples.
Areas of volcanic activity include both the mid-ocean ridges, presumably the site of recent fissure eruptions even though few such have been observed (e.g., in Iceland), and features of the central eruptive type. The latter are highly generalized, with any one symbol on the DTAM generally representing several volcanos. The primary data source was the Smithsonian Institution's compilation, and Simkin and Siebert (1994), supplemented by geologic maps used with the aid of orbital photography (Lowman, 1982). It was assumed that in most areas, volcanos remain recognizable for roughly one million years after their last activity. Any feature that still looks like a volcano has probably been active within this period. Similarly, a fault scarp (as distinguished from a fault-line scarp, formed by differential erosion) that is still distinct has also been active in the last million years.
Sea-floor spreading rates were taken from the NUVEL-1 model of DeMets et al. (1990), based on the spacing of dated magnetic anomalies. This approach, used by Minster et al. (1974) and Minster and Jordan (1978), has been supported by other evidence, in particular space geodesy. A Robinson Projection map of Very Long Baseline Interferometry (VLBI) site motion vectors (Fig. 6) has been compiled, showing directions and approximate magnitudes. Methods and results of this and other space geodesy programs have been summarized in the volume edited by Smith and Turcotte (1993).
Major plates have been labeled using accepted nomenclature. However, as will be discussed, the DTAM is not a "plate map" in that plate boundaries in, for example, Asia are extremely broad and diffuse. Oceanic plate boundaries, in contrast, are easily drawn on the basis of topography and seismic activity. The extent of continental crust, on the Global Tectonic Activity Map, was drawn on the basis of bathymetry supplemented by other geophysical, geological, or geochemical data. The Rockall Plateau, for example, has been known for decades to be continental crust. However, the Rockall Trough, despite its abyssal depth, is now considered continental in composition on the basis of reflection profiling (Hauser et al., 1995). The Faeroe Plateau, though surfaced with basalt, has been found by geochemical studies to be underlain at depth by sialic crust (Gariepy et al., 1983).
The DTAM and GTA were created with commercially available PCI image-processing software. After the GTA was drawn by hand, it was digitized, converting each feature to a point or line. ArcView, a geographic information system, was used to integrate the newly created raster and vector files. The integrated format was then imported into Adobe Illustrator. This software permits manipulation of all image formats, and was used for combining and manipulating text, line features, raster files, and the geologic interpretation (the GTA) for the final cartographic output.
