EarthRef.org Reference Database (ERR) -- Morinaga et al. 2000

Paleomagnetic investigation was performed on 26 Potassium-Argon dated Pliocene to Pleistocene lava flows, collected at 34 sites from northern Hyogo, Southwest Japan. Most magnetic polarities were found to be coincident with the geomagnetic polarity timescale (GPTS), although two lava flows do not agree with the timescale. One of 2 lava flows incompatible with the GPTS, the Olivine Andesite lava flow from the Oginocene volcano group dated at 0.92+ or -/-0.05 Ma, shows normal polarity. This finding is a possible evidence confirming the existence of the Santa Rosa Event proposed recently. Twenty-two paleomagnetic data sets were used to investigate paleosecular variation (PSV) having passed the following criteria: 1) characteristic components are obtained from four specimens or more for a lava flow, 2) alpha (sub 95) of the lava flow mean direction is <10 degrees and 3) the virtual geomagnetic pole (VGP) latitude calculated is >50 degrees or < -50 degrees. The mean VGP was calculated to be latitude = 85.5 degrees N and longitude = 285.3 degrees E (k = 34.8 and alpha (sub 95) = 5.3 degrees). The resultant angular standard deviation (ASD) with respect to the geographic pole was calculated to be 14.5 degrees with the upper limit = 18.3 degrees and the lower limit = 12.1 degrees. The ASD value is almost equal to most existing results from the Japan Islands. Brunhes Chron PSV in Japan was estimated using the identical criteria for selection; 66 of 87 available data sets passed. The mean VGP was calculated to be latitude = 89.3 degrees N and longitude = 14.4 degrees E (k = 33.1 and alpha (sub 95) = 3.1 degrees). The resultant ASD with respect to the geographic pole was calculated to be 14.2 degrees with the upper limit = 16.1 degrees and the lower limit = 12.7 degrees. The ASD values for the past 3 million years in northern Hyogo and for the Brunhes Chron in Japan are compatible with the global trend for the past 5 million years fitted by physical PSV Model F and phenomenological PSV Model G. This suggest that the PSV in Japan is not low compared with anomalously low PSV area in the Pacific Region.