R E P O R T S rial. Futtire planeta~y accretion models will ha! e to accolmt for this obse~~,ation. T11e real-izatio~l of a variation in Fe Si ratio among the terrestrial planets \I ill also alter the details of our ~nodels of maltia~~ lna~ltle chemistly and core fomlation sce~larios. Models of core formation in the renestrial planets cannot assume bulk C 1 sitlerophile eleinent abu~ldances.Despite the many complexities concerning their initiation and propagation, forest fires exhibit power-law frequency-area statistics over many orders of magnitude. A simple forest fire model, which is an example of self-organized criticslity, exhibits similar behavior. One practical implication of this result is that the frequency-area distribution of small and medium fires can be used to quantify the risk of large fires, as is routinely done for earthquakes.Frequency-size distributions of natural hazards provide i~nportant information 011 calculating risk and are used ill hazard mitigation ( 1 ). R o b~~s t pon.er-la\\ frequent) -size distributions are associated nit11 self-organized critical behavior. Examples of this beha1 ior are fou~ld in a il~ulnber of conlputer models: the sandpile model ( 2 ) . the slider-block model (31, and the forest fir? nod el ( I ) . The slider-block model is considered to be an analog for eal-tilquakes. Earthqualtes exhibit a power-la~v dependence of occurrence fre-~L I U I C~ on ruphlre area and are co~lsidered to be the type esalllple of self-organized critical behavior in nature ( 5 ) . We found that. under a \vide ~a r i e t y of circ~umsta~lces. forest fires exhibit a pon-er-lan dependence of occurrence fi-equencq on burn area over many orders of magnitude and that actual forest fires call be directly associated n-it11 the forest fire nod el. The only previous major application of the forest fire lnodel lvas to epidemics of measles in isolated populations ( 6 ) . The forest fire model consists of randomly planting trees on a square grid at successi\.e

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S U M M A R YI n order to understand the underlying physics of distributed seismicity better we have considered a 2-D array of slider blocks connected by springs and interacting via static friction with a surface. There is no driving plate in this model. The time evolution of the system is found from numerical simulations in a cellular automata formulation. Energy is conserved and is the single control parameter. The distribution of energies in the springs is found to obey a modified Maxwell-Boltzmann statistics. It is found that the number-size statistics of clusters of unstable sliding blocks is identical to those in percolation clusters in the site-to-site percolation model. There is a well-defined critical point when unstable blocks become connected across the array. It has been previously suggested that distributed seismicity in a seismic zone is the percolation backbone of a 3-D percolation cluster. The fact that low-level seismicity satisfies the GutenbergRichter frequency-magnitude relation and is nearly constant in time also suggests that this background seismicity is similar to thermally induced noise.