Anomalous Crustal Activity in the Izu Peninsula, Central Honshu

Abstract

Earthquake prediction research on the recent anomalous crustal activity in the Izu Peninsula is summarized. Swarm activity of shallow microearthquakes in the eastern Izu Peninsula began in August 1975 and became more active in October. The epicenters clustered near Togasa-yama at first but spread over other places in the eastern part of and east off the peninsula in early 1976. Releveling carried out in January-April 1976 has disclosed crustal uplift of 15cm centered at Hiekawatoge, several kilometers north of Togasa-yama. The uplift area, more than 30km in diameter, covered the eastern half of the peninsula. Leveling, gravimetric, and tidal data showed that this uplift had developed only during the preceding 1-1.5 year period. Rapid changes in length of base-lines were also detected by repeated geodimeter measurements.These phenomena were noted by the Coordinating Committee for Earthquake Prediction and observations were intensified. The information about the crustal activity was made public by the Committee in May 1976.On August 18, the Kawazu earthquake (M=5.4) occurred in the southern part of the uplift area. Short-term precursors except for foreshocks were not so clear in spite of the intensified observations.In the eastern part of the Izu Peninsula, Central Honshu, remarkable crustal uplift has developed since around the end of 1974, with accompanying microearthquake swarms. An indication of anomalous crustal activity was firstly noticed in increasing microseismicity in August 1975, and the crustal uplift of 15cm was found by releveling in early 1976. Rapid changes in sea level, gravity and base-line length were also found. These facts were noted by the Coordinating Committee for Earthquake Prediction (CCEP) and various observations were intensified by its cooperating institutions. The information about this anomalous activity was made public by the CCEP in May 1976. On August 18, 1976, the Kawazu earthquake (M=5.4) occurred in the southernmost part of the uplift area, and caused some property damage. Short-term precursors were not clear in spite of the intensified observations, except for the foreshock sequence during about 1.5hr prior to the main shock and slight change in groundwater temperature. The process of the research and the response of the CCEP for this anomalous activity are reviewed chronologically as an actual example of the Japanese efforts to predict earthquakes.S 51

Muographic mapping of the subsurface density structures in Miura, Boso and Izu peninsulas, Japan

“…5 , 6 and 7 show muon-deduced maps of above-tunnels density distributions in Miura, South-Boso and South-Izu peninsulas, respectively. These plots are overlaid with indications of active faults 21 22 26 , geological boundary lines 20 ( Fig. 8 ), and old fault lines (considered nearly or completely inactive) drawn for reference.…”

Quaternary and geodetically measured crustal movements in the Tokai District, central Honshu, Japan

“…The Izu peninsula itself contains several active volcanic centers and is being broken up by a dense pattern of north-south and east-west striking conjugate strike-slip faults [Somerville, 1978]. Minor seismicity is quite high, and there have been two recorded episodes of aseismic uplift, one around 1930 and the other during 1974-1979 [Tsumura, 1977]. Since 1930 there have been three earthquakes of about M ---7.…”