The current topography of our planet represents just the latest frame of a long film that began 4.6 billion years ago; in fact, this frame is gradually morphing into the next, transitioning at a rate that is too slow to appreciate during our brief human life span. For man, earthquakes and volcanism are the only clues that Earth's evolution is still underway.
The modern Continents, sitting on their large tectonic plates, have changed size, shape and location throughout the history of our planet. They are the products of past land masses that have merged, rifted apart or collected small, exotic terrains along their margins. Some suture lines, from past mergers or terrain accretions, and valleys, from ongoing rifts, are evident today; the former are often represented by mountain ranges, where continents (or sub-continents) have collided, while rift valleys, and their associated volcanism, are currently represented by the Rio Grande Rift of North America and the East African Rift Valley, among others. However, suture lines and rift zones from the distant past may now be obscure (if evident at all), having eroded to flat plains or having been filled in and covered over by glacial, volcanic, oceanic and erosional debris.
While most earthquakes occur along tectonic plate margins, where collision, subduction or friction is occurring, some are produced by pressure release along old, buried faults, many of which represent aborted rifts or past suture lines. Since seismic mapping of our Continents is incomplete and since many of these faults are very deep (and thus difficult to detect) earthquakes may occur at sites with no expectation of tectonic activity. This week's earthquake in northern Illinois is a prime example, having occurred at a location with no previous recording of seismic events; of course, man's record of these events is extremely limited, covering but a nanosecond of geologic time. The lesson: there is no reason to assume that any site on our planet is immune to earthquakes.
