IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1986
Luminescence Associated With Uni-Axial Rock Fracture Experiments
Dr. Mamoru Kato
Graduate School of Human and Environmental Studies Kyoto University IASPEI
Yuta Mitsui, Takashi Yanagidani
The sky being illuminated before and during an earthquake is one of the phenomena that are often
referred as macroscopic anomaly. Such phenomena are termed as earthquake lightening, and reports of
observations can be found in ancient documents in , and . The first unambiguous photographs of the
illuminated sky were taken during swarm activities in Matsushiro, , in mid-1960s, and reports of similar
observations, some of which are less credible than others, follow large earthquakes in , such as 1995
Kobe Earthquake, as high interest in this phenomenon, in accord with high interest in short-term
earthquake prediction, is shared in . Brady and Rowell [1986] was first to experimentally investigate
luminescence during rock fracture, and postulated from spectrographic observations that luminescence
is caused by exoelectrons emitted from fresh rock surfaces which are created during fracture. In other
experimental studies on electromagnetic behavior of rocks subjected to compression [e.g., Yoshida,
2001], piezoelectric effect of quartz is asserted to play an important role, and it appears that
luminescence and emission of other electromagnetic waves do not share the same origin. We have
experimentally studied luminescence of rock in uniaxial compression. Rock samples such as granite,
sandstone, and basalt are prepared in both dry and wet conditions, and are compressed with high strain
rates in ambient atmosphere. Luminescence of rocks is weak but visible to naked eye when conditions
are met. Photographic records of faint luminescence with digital cameras are also possible. Emission of
light is instantaneous, and appears to be dominant at the final explosive failure. Its intensity
significantly decreases when samples are in wet condition. Mineralogy controls luminescence, as, for
example, quartz-rich samples usually emit stronger light than quartz-poor samples. Grain size is another
controlling parameter, as we observe stronger light from granite with coarser grains. These
observations are not successfully explained by the exoelectron hypothesis of Brady and Rowell, but
rather suggest that discharge of piezoelectric field is primarily responsible for luminescence.
Keywords: earthquake lightening, rock experiment