Paleointensities of the geomagnetic field determined from recent four lava flows of Sakurajima Volcano, West Japan.

Abstract

Geomagnetic paleointensities were determined by making use of the Thellier's double heating method from the recent four lava flows of Sakurajima Volcano, West Japan. Bunmei (1471) and An-ei (1779) lavas were used for inferring the geomagnetic intensities at the respective ages of eruption, and Taisho (1914) and Showa (1946) lavas were used for checking the validity of paleointesity determination by this method. The NRM's of these lavas are highly stable and the paleointensity experiment was successfully applied to all the four lavas. The fact that the intensity determined from Showa lava, i. e. 0.46±0.02Oe, agrees very well with the present-day intensity of geomagnetic field shows that the present measuring device works well. The usual paleointensity experiment was a little modified by making heating and cooling time shorter with the aid of a movable heater. Amounting to 0.42±0.03Oe, the measurement of Taisho lava led to a value a little smaller than the value expected from the actual observation. This could be attributed to the local geomagnetic anomaly on the volcano. As for the historical lava flows, the results indicate that the total intensity of geomagnetic field was 0.53 and 0.48Oe at 1471 and 1779, respectively, and had a tendency to decrease at Sakurajima since the 15th century. This tendency agrees with the geomagnetic secular changes observed at other places in Japan.