Earthquakes Around and Mountains the Pacific

Even after a century of intensive study, the problem of the origin of mountain systems remains as possibly the greatest enigma of the earth sciences. Specifically, what is the origin of mountain-building forces, and how are these forces manifested in the crustal rocks of the earth's surface? Field geologists who have studied such classical mountain systems as the Alps and the Appalachians have always been impressed by the abundant evidence of uplift and compression shown by the highly deformed rocks in the cores of these ranges; consequently, most theories of mountain building have been intimately concerned with the mechanics of compression and vertical uplift. Contraction of the earth's crust (possibly due to cooling), and thermal convection currents in the earth's interior, are two of the theories most often discussed at present, although they are by no means the only ones. Geological field studies of ancient mountain systems have told us much about the nature of the associated deformation, and have emphasized some of the conditions and constraints that must be satisfied by any valid theory. If such mountainbuilding processes are taking place anywhere in the world today, however, one might expect to observe them best in the regions bordering the Pacific Ocean, inasmuch as this circum-Pacific "ring of fire" can claim the great majority of the world's current earthquakes, most of its active volcanoes, and most of its very youthful mountain systems. Great earthquakes certainly represent one of the most obvious manifestations of present-day mountain building, and the 1906 earthquake in San Francisco was one of the earliest circumPacific shocks studied in detail. The results were surprising to most geologists, because instead of the associated vertical uplift and evidence of great compression that might have been expected, the ground displacement along the San Andreas fault that caused this earthquake was almost entirely horizontal. During the earthquake, points