The best documented observations of earthquake lights are from Japanese earthquakes in the early 1930's and mid-1960's. In the latter case, color and black and white photographs were taken of bright, hemispherical, white luminescences based at ground level, about 20 to 200 m in diameter, of duration 10 sec to 2 min, restricted to mountain summits in a quartz-diorite faulted rock. Great difficulties and uncertainties accompany any attempt to explain the phenomenon. Recent calculations include attempts to show that earthquake lights may be associated with auroras through a solar magnetic triggering mechanism. Other more probable explanations include ultrashort-period air oscillations and generation of a large potential difference in quartz-bearing rock by the piezoelectric effect. Considering the existence of well-documented pictures, reproduced here from the work of Yasui, the existence of the phenomenon is considered well-established, although no completely satisfactory explanation has been advanced to date.

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