Izumi Yokoyama

Seismological significance of the 1977–1978 eruptions and the magma intrusion process of usu volcano, Hokkaido

Abstract An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations.

Geophysical characteristics of dacite volcanism — the 1977–1978 eruption of Usu volcano

Abstract The 1977–1978 eruption of Usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism. The activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas; the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes. The hypocenters of the earthquakes occurring beneath Usu volcano have been located precisely. Their distribution defines an earthquake-free zone which underlies the area of doming within the summit crater. This zone is regarded as occupied by viscous magma. The domings within the summit crater forming the cryptodomes have amounted to about 160 m. In addition to uplift they showed thrusting towards the northeast. As a result, the northeastern foot of the volcano has contracted by about 150 m. The relation between crustal deformation and earthquake occurrence is examined, and it is found that the abrupt domings are accompanied by the larger earthquakes ( M = 3–4.3). Both the seismic activity and the ground deformation are shown to have a unique and common energy source. The energy of activities of Usu volcano consists of the explosive type, the deformation type and the seismic type; the second and the third are in parallel with each other in discharges, and both energies are complementary to the explosive energy. The explosive energy and the seismic energy have been calculated for an explosion sequence, and it is concluded that the deformation energy is about 10 times greater than the seismic energy. The discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption, except for a small increase at the end of January 1978. Eruptions are governed not only by the supply of the energies but also by the depth of the magma, which has gradually approached the surface. The last eruption occurred in October 1978; however, the crustal deformations and the earthquake swarms are still proceeding as of January 1980, albeit at a lower rate of activity.

A scenario of the 1883 Krakatau tsunami

Abstract The 1883 Krakatau tsunami provides an important clue for solving the mechanisms of the eruption of Krakatau. First, sea waves of volcanic origin are basically classified. The travel times of the crest of the largest tsunami in the 1883 Krakatau eruption are recalculated taking the wave heights and the sea depths along the path into consideration. As a special case of sea waves, air waves coupled with those of the sea are examined and the necessary conditions of their generation are discussed: the sea depths must be around 5 km, the sea wave paths require long stretches of sea, and the magnitudes of volcanic explosions must be large to produce the air-sea waves. The information collected suggests that the tsunamis registered at Batavia (Jakarta) were not air sea-waves. Some results of the recent reflection survey of the subbottom materials and a supplemental gravity survey in the Krakatau area are introduced to provide further evidence for explosive mechanisms. The origin times of the onset and the crest of the tsunami and the air wave, both registered at Batavia, are calculated from their travel times. Adopting these origin times, the author reconstructs a scenario of the tsunami. In order to generate a tsunami, as long as about 60 km wave length, successive developments of surges and water domes seem to be necessary.

The subsurface structure of the AIRA caldera and its vicinity in Southern Kyushu, Japan

Abstract Southern Kyushu, Japan, includes a chain of large and small calderas and active volcanoes, and the greatest part of it is covered with thick pyroclastic ejecta. The regional and local structures of this area are discussed from the standpoint of physical volcanology, with consideration of all available data. The regional structure of this area is examined in the light of gravity and geomagnetic anomalies. Two layers of the earths uppermost crust are defined by spectrum analysis of the gravity anomalies. These two layers are identical with the two identified by seismicwave velocities. The Bouguer gravity anomalies are relatively high and rather monotonous over outcrops of the Mesozoic basement and the granite, but are relatively low and perturbed over calderas and caldera-like structures. Two low-gravity anomalies in Kagoshima Bay are remarkable. One is circular, with its center on the Aira caldera. The other is elongated between the Satsuma and Oosumi peninsulas. The southern end of the latter anomaly is occupied by the Ata caldera. Discussion of the gravity anomalies of the Aira caldera suggests that the subsurface basement has a funnel shape and is overlain by ‘fallback’. The sub bottom geology of the caldera suggests that it is formed by a few smaller depressions, though the distribution of the overall gravity anomalies is parallel with its shape. The southern part of Kagoshima Bay is characterized by a graben-like topography and low-gravity anomalies and, moreover, by several calderas. The middle part, between the Aira and Ata calderas, may have a graben-like structure. A profile crossing the bay through Sakurajima volcano is modeled on the basis of results from drilling and gravity surveys. The basement has a graben-like structure and is filled with coarse and low-density deposits, and the structure continues northwards to the Aira caldera with a funnel shape. A comparison of this area with the Taupo-Rotorua depression in New Zealand and Lake Toba in Indonesia, leads the authors to the conclusion that such major volcanic depressions may have been formed by amalgamation of a series of caldera-like structures which were formed by multiple violent explosions accompanied by ejection of a tremendous amount of pyroclastic material.

Seismic energy releases from volcanoes

Seismic energy release during the precursory, eruptive and declining stages of volcanic activities provides various information about the mechanisms of volcanic eruptions and the temporary developments of their activities. Hitherto the energy release patterns from precursory earthquake swarms were used to predict the eruption times, especially of andesitic or dacitic volcanoes. In this paper the discussion is expanded to quantify the total amount of seismic energy released at the threshold of volcanic eruptions, with reference to the results observed at several volcanoes. The results generally indicate that the cumulative seismic energy release from the precursory earthquake swarms exceed 1017∼18ergs before eruptions at any andesitic or dacitic volvanoes. This allows the seismic efficiency, or the ratio of energy radiated seismically, and the energy required for the volumetric expansion to be estimated by incorporating available deformation data with the seismic data. The dependency of seismic efficiency on the type of volcanic activity, i. e. non-explosive outbreaks, phreatic and magmatic eruptions, dome formation, etc., was evaluated from observations at a few volcanoes that provided a variety of examples.

Microgravity and height changes caused by volcanic activity: four Japanese examples

Microgravity measurements and levelling surveys on volcanoes are not always easy to make, but are useful for studying volcanic processes quantitatively. Gravity changes associated with volcanic activity are not always significant. Precision of microgravity measurements depend critically on the procedures adopted, and those applied in the present paper are described. Levelling technique is now orthodox, and some empirical laws relating ground deformation to volcanic activity are deduced from the accumulated data. Gravity changes occur at the same time and places as ground deformations. The relationship between microgravity and height changes are discussed from the standpoint of analyzing the data obtained on volcanoes. The observational results obtained on four volcanoes in Japan are separately analyzed because each volcano exhibits different patterns of gravity changes and deformations. During the 1977–1982 activity of Usu volcano, deformation was accompanied by microgravity changes frequently observed at a particular benchmark at the base of the volcano for about five years. The gravity changes prove to be not a direct effect of magma movements but to be caused by the deformations of ground strata and aquifers around the benchmark. The 1983 eruption of Miyakejima volcano was associated with local gravity changes around the eruptive fissures due to magma intrusion which was approximately modelled. Similarly the 1986 eruption of Ooshima volcano caused gravity changes on the volcano, but these were poorly correlated with elevation changes and their origins were not uniquely interpreted. To detect gravity changes associated with the activity of Sakurajima volcano, an equigravity point was selected at the north of the volcano besides the gravity points on and around the volcano itself. The probable gradual accumulation of magmas beneath the volcano for eight years is substantiated by observed microgravity and elevation changes.

Crustal deformation caused by the 1914 eruption of Sakurajima volcano, Japan and its secular changes

Abstract Analysis of ground-deformation data obtained at and around Sakurajima volcano during the 1914 eruption indicates that the deformation may be interpreted by assuming a model with two pressure sources, one shallow (about 2 km deep) and vertically directive and the other deep (about 8 km deep) and obliquely, directly beneath the volcano. This model is reasonable from the viewpoint of the volcanic processes. The local upheaval near the centers of eruption has scarcely recovered because it surpassed the elastic limit. The recovery of the regional depression after the eruption can be interpreted as pressure accumulation beneath the volcano. It may be concluded that the center of pressure would remain at the deeper source beneath the volcano, but that the pressure would change, resulting in surface deformation. The depression and its recovery suggest the presence of a pressure focus or a ‘magma reservoir’ beneath the volcano.

Crustal deformations associated with volcanic activities

Abstract Two models of pressure source to interpret the crustal deformations associated with volcanic activities are discussed by example. It is pointed that the magma pressure causing the crust to be deformed may sometimes trigger remarkable tectonic earthquakes around volcanoes.

A gravity survey on Volcano Huzi, Japan by means of a Worden gravimeter

SummaryObservations and reductions of the gravimeter survey on Volcano Huzi are fully described. The survey was accompanied by various inevitable difficulties but the accuracy of calculating the Bouguer anomalies is kept high enough to bear volcanological discussion. The results are rather suggestive to the theory of caldera-formation. Comparison with the gravity anomaly on the volcanic islands in the middle of the ocean is also made.

Gravimetric studies of Taal Volcano, philippines

Taal Volcano is peculiar in its violent explosivity in spite of its low altitude. The surrounding topographies suggest that the origin of Taal Volcano is either a caldera or a graben structure. To confirm the caldera origin one must search for a vast quantity of caldera ejecta balancing with the depression. As a first step, a gravity survey was carried out on and around Taal Volcano, and high gravity anomalies were lound on Volcano Island. The distribution of the gravities may suggest a graben structure overlying a denser layer of igneous material.