Dyke intrusion model for the 1989 seismovolcanic activity off Ito, central Japan

In the last 10 years, extensive earthquake swarms have occurred intermittently in the region east of Izu Peninsula, central Japan. In July 1989 an active swarm again took place off Ito City, and among all past activities, it was nearest to land and shallowest. This activity eventually became a weak submarine eruption, the first of its kind in this region. Associated with this event, a borehole tiltmeter at 92 m depth, installed several kilometers from the source area, detected distinct continuous tilt change with the following features: (1) Tilt of 20 μrad was observed within 1 week preceding the eruption. (2) Tilt change showed a good correlation to swarm activity. (3) No obvious coeruption tilt change was observed. On the basis of the tilt record and swarm distribution, we constructed a dislocation model to interpret the seismovolcanic activity. The model consists of two tensile faults, each of which corresponds to magma intrusions in May and July, and a right-lateral reverse fault representing the largest earthquake (M5.5) at 1109 LT on July 9. This model can simulate not only the above tilt data but also the record of a nearby dilatometer, the results from daily laser distance measurements and daily Global Positioning System monitoring, as well as results from other conventional distance surveys and leveling surveys in the surrounding region. From this model, we conclude that the intersection of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom, creating a lava dome. Later an instability brought about a magma-phreatic explosion which did not generate any substantial crustal deformations at deeper levels.

[1]  D. Hill.  A model for earthquake swarms , 1977 .

[2]  K. Shimazaki,et al.  Subduction, bending and education; present and Quaternary tectonics of the northern border of the Philippine Sea Plate , 1984 .

[3]  Takao Eguchi,et al.  Detection of a volcanic fracture opening in Japan using Global Positioning System measurements , 1990, Nature.

[4]  Y. Okada.  Surface deformation due to shear and tensile faults in a half-space , 1985 .

[5]  Y. Sasai,et al.  Tectonomagnetic Signals Related to the Seismo-Volcanic Activity in the Izu Peninsula , 1991 .

[6]  Keiiti Aki,et al.  Generation and Propagation of G Waves from the Niigata Earthquake of June 16, 1964. : Part 2. Estimation of earthquake moment, released energy, and stress-strain drop from the G wave spectrum. , 1966 .

[7]  M. Ishida.  Spatial-temporal variation of seismicity and spectrum of the 1980 earthquake swarm near the Izu Peninsula, Japan , 1984 .

[8]  N. Nasu.  Recent Seismic Activities in the Idu Peninsula. (Part 2.) , 1931 .

[9]  Ryosuke Sato,et al.  THEORETICAL BASIS ON RELATIONSHIPS BETWEEN FOCAL PARAMETERS AND EARTHQUAKE MAGNITUDE , 1979 .

[10]  Y. Tsuneishi.  Continuous Monitoring of Crustal Activity in East Izu Peninsula by Automatic Electronic Distance Measurement , 1991 .

[11]  Y. Fujinawa,et al.  Preliminary results of the observation by fixed-point GPS simultaneous baseline determination network in Kanto-Tokai district , 1989 .

[12]  Y. Fujinawa,et al.  Fixed-Point GPS Observation of Crustal Movement Associated with the 1989 Seismic Swarm and Submarine Volcanic Activities off Ito, Central Japan. , 1991 .

[13]  K. Ishibashi.  Possibility of a Large Earthquake near Odawara, Central Japan, Preceding the Tokai Earthquake , 1985 .

[14]  F. Yamamizu,et al.  Reflection Profiles of the Active Seismic and Volcanic Region off the East Coast of Izu Peninsula , 1991 .

[15]  T. Seno.  THE INSTANTANEOUS ROTATION VECTOR OF THE PHILIPPINE SEA PLATE RELATIVE TO THE EURASIAN PLATE , 1977 .

[16]  S. Kato,et al.  Birth of a Submarine Volcano "Teisi Knoll". , 1991 .

[17]  M. Ohtake,et al.  A High Quality Digital Network for Microearthquake and Ground Tilt Observations in the Kanto-Tokai Area, Japan , 1985 .

[18]  Y. Okada,et al.  Ground Tilt Changes Preceding the 1989 Submarine Eruption off Ito, Izu Peninsula , 1991 .