PRECURSORS AND DEEP EARTH GASES IN RELATION TO EARTHQUAKE PREDICTIONS
V. T. Jones
The fact that earthquakes may sometimes be preceded by geochemical anomalies was first discovered at Tashkent in the USSR (Fursov, 1968), although earlier studies were conducted in Japan (Okabe, 1956). Extensive earthquake prediction studies have been carried out since that time in Russia, China, and Japan. Results from Chinese geochemical data are reported to have, at least partly, contributed to the successful prediction of several strong earthquakes (F. Press, et al. Eos Trans, AGU 56(11), 838-881, 1975). In contrast, the Earthquake Hazards Reduction Program in the United States mainly emphasizes geophysical methods.
In the late 1970's, a few scientific institutions in the U.S. began to make geochemical measurements for predicting earthquakes. The most common geochemical parameter studied was Radon, concentrated primarily in water wells placed on or near faults, with no concern for the level of seepage occurring on the fault at the location of the stations. Areas of particularly high activity are related to zones of deep tectonic fracturing, and the accompanying jointing in which mineralization is sometimes located. However, examination of the published literature, coupled with the experience of Gulf Oil Company scientists, have demonstrated that the placement of earthquake sampling locations can be significantly improved by soil gas surveys designed to locate active seeps (Jones and Drozd, 1983).
In an attempt to improve the basic understanding of deep earth gases and their relationship to earth tectonics, Gulf Oil Company entered into a joint research project with Caltech scientists who had developed and were operating a network of computed controlled monitoring stations. These stations were capable of handling multiple geochemical variables and the real-time monitoring requirements necessary for improvement of this science.
The first active seep monitored by this Gulf/Caltech joint venture was the Arrowhead Hot Spring located within the San Andreas Fault system near San Bernardino, California. This hot spring releases significant quantities of free gases (40 cc/minute), including helium, hydrogen, radon, and the light hydrocarbons; methane, ethane, propane, iso-butane, and normal butane. Monitoring was conducted on a semi-continuous basis from this natural 80 degree Centigrade spring for approximately one year (12/8/81 to 12/1/82). Although sample collection intervals were variable over this time period, systematic magnitude and compositional changes were recorded for the measured gases. Hydrocarbon magnitudes nearly doubled from December 1981 to July 1982 and remained fairly stable until sample collection termination on 12/1/82. These measured changes in gas flux, which could indicate a precursory signal related to potential earthquake activity on the locked southern section of the San Andreas fault, demonstrated the rapid flux changes that were expected to occur in response to tectonic activity along such a major basement fault.
This Gulf/Caltech program included only the study of a single hot spring site in an attempt to relate gas emissions to earthquakes. Although no clearly earthquake related events were documented, significant gas magnitude and compositional changes were recorded in these very sensitive and volatile gases. Regional stress variations were interpreted as the most probable cause for the recorded changes in gas flux at this site. Plans for additional sites and longer term monitoring were planned by the Gulf/Caltech scientists and are highly recommended for future research. Industry downsizings and takeovers prevented the final implementation of these research plans. No comment needs to be made regarding the value this data might have had to California and the nation if this program had continued until today.
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