relative motion between two tectonic plates is accommodated by earthquakes and how much is taken up by slow creep, either steady or episodic. Understanding the ratio of fast, seismic (earthquake-producing) slip to slow aseismic slip is fundamentally important in the quest to assess the danger of active geologic faults.

The use of GPS arrays capable of continuously monitoring a large region provides the resolution needed to monitor short- and long-term displacements that occur during and after earthquakes. In addition to making estimates of the component of aseismic creep between major earthquakes, we also can now estimate the relative amounts of seismic and aseismic slip associated with a particular earthquake. Cases have been documented in which the aseismic slip after an earthquake has accommodated as much slip or more slip than the quake itself. If this is a common occurrence, more than half of plate-boundary slip may be aseismic.

Several GPS arrays are being deployed across plate boundaries in an effort to monitor slip events. For example, an array being installed across the Cascadia subduction zone offshore of Oregon and Washington State is capable of detecting purely creep events. These events result from slip on a fault that does not radiate seismic energy detectable by seismometers and hence do not produce traditional earthquakes. These creep events may explain the paradox that many fault zones currently have high strain rates, although their histories are largely devoid of earthquakes, or the quakes that did happen were too small to account for the long-term rate of slip.

Extrasolar Planets

The discovery of extrasolar planets has brought with it a number of surprises. To put matters in context, say Najita et al., the planet Jupiter has been a benchmark in planet searches because it is the most massive planet in the solar system and is the object that we are most likely to detect in other systems.22 Even so, this is a challenging task. All the known extrasolar planets have been discovered through high-resolution stellar spectroscopy, which measures the line-of-sight reflex motion of the star in response to the gravitational pull of the planet. In our solar system, Jupiter induces in the Sun a reflex motion of only about 12 meters per second, which is challenging to measure given that the typical spectral resolution employed is approximately several kilometers per second. Fully aware of this difficulty, planet-searching groups have worked hard to achieve this velocity resolution by reducing the systematic effects in their experimental method.

22  

Frontiers of Science/1999. Joan Najita, Willy Benz, and Artie Hatzes, at <http://www.pnas.org/cgi/content/full/96/25/14197>.



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