High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan

Abstract \nThe 1990-1995 eruption of Unzen Volcano in southwestern Japan was characterized by the extrusion of a dacite lava dome and frequent pyroclastic flows during the dome growth. Associated with the dome emergence on May 20, 1991, an intense swarm of high-frequency (HF) microearthquakes occurred just beneath the crater at very shallow depths. We used data from FG3, a seismic station located 500 m SSW of the crater, to identify 29,401 HF earthquakes between May 11 and 31, 1991. The rate of HF earthquakes increased starting on May 12 and peaked on May 17. The high seismicity continued until May 26, then dropped sharply, coinciding with a marked decline in the swelling on the southern upper flank of the volcano. The seismicity increased and decreased repeatedly within a 1- to 2-h period, which was correlated with tilt cycles observed 680 m west of the crater in such a way that the seismicity increased during uplifting on the craterward side. Defining an earthquake group as a series of earthquakes with waveforms that are similar or vary only slightly over time, we identified 10 such groups, each containing more than 300 events. The largest group comprised 3,214 events over 18 days. Seismicity rates of eight groups, including the largest, increased and decreased repeatedly, correlated with the tilt cycles. As the waveform data from station FG3 were considerably clipped for relatively large events, we analyzed data from two additional stations, KRA and CJA, located 3.5 and 8.7 km from the crater, respectively, and identified a total of five new groups. The activity of two groups recorded at KRA was correlated with the tilt cycles, but those of three groups recorded at CJA did not always show such a correlation. Hypocenters for the groups recorded at KRA were distributed to the east side of the conduit and a focal mechanism suggested that the events in this area occurred due to compressional stress produced by the inflation of the conduit. The groups recorded at CJA occurred to the north side of the dike trending westward from the conduit. The focal mechanisms have P-axes roughly trending to the dike, which can be explained by the compressional stress generated by the increasing thickness of the dike

Seismicity associated with the 1991-1995 dome growth at Unzen Volcano

From May 1991 until February 1995, seismicity in the crater area of Unzen Volcano, southwest Japan, intensified in conjunction with the growth of a dacite lava dome. We used data from seismic stations located near the crater to identify approximately 580,000 summit earthquakes with maximum amplitudes equal to or greater than 1 × 10- 3 cm/s. The temporal characteristics of the seismicity level were different for exogenous and endogenous periods of dome growth. Periods of solely exogenous growth were accompanied by several days or weeks of increased seismicity, and levels of seismicity were notably reduced between successive seismically active periods. In contrast, levels of seismicity were generally high during periods when the dome grew endogenously, with repeated cycles of increasing and decreasing seismicity of one to two months duration. We classified the waveforms of summit earthquakes into high-frequency (HF), medium-frequency (MF), and low-frequency (LF) types on the basis of spectral analysis. Dominant waveform types varied significantly over time: HF was dominant in May 1991, LF from June 1991 until August 1993, MF during September and October 1993, and HF and MF from November 1993. HF and MF events are mainly distributed at depths of 500-1100 m above sea level (ASL), just below the lava dome, while LF events are widely distributed at depths from 500 m ASL to the interior of the dome itself. To efficiently detect earthquake families, we conducted a cross-correlation analysis of waveforms. Using the events one-by-one as reference events, we calculated peak correlation coefficients between each reference event and events that occurred within 24 h either side of the reference event. The results show that many earthquake families of all waveform types occurred throughout the growth period of the dome. The durations of most families were less than two weeks and were not related to the intensity of seismic activity. The incidence rate of events within each family reached a peak in the middle of the respective activity period. The results also suggest that the waveforms in several analyzed families gradually evolved over time. During periods when HF events intensified, several families appeared contemporaneously. In contrast, during periods when LF events intensified, the occurrence pattern of earthquake families was relatively simple: a new family became active only once the activity of the previous family had declined or ceased completely. A possible source mechanism for families of HF events is stick-slip within the stiff rocks surrounding the conduit; however, several different types of source mechanisms should be considered for families of LF events

Broadband Seismic Observations of Artificial Sources in the Kirishima Volcanic Area, Japan

We conducted broadband seismic observations of artifical explosions on December 1, 1994, in the Kirishima volcanic area, southern Kyushu, Japan. We clarified the dominant characteristics of wave-propagating paths in this area and the potential of broadband seismic observations for artifical sources. We analyzed seismograms at three sites located around one of the shot points (near Karakuni Dake) at distances shorter than 1.3 km

Spine growth and seismogenic faulting at Mt. Unzen, Japan

The concluding episode of activity during the recent eruption of Mt. Unzen (October 1994 to February 1995) was characterized by incremental spine extrusion, accompanied by seismicity. Analysis of the seismic record reveals the occurrence of two dominant long‐period event families associated with a repeating, nondestructive source mechanism, which we attribute to magma failure and fault‐controlled ascent. We obtain constraints on the slip rate and distance of faulting events within these families. That analysis is complemented by an experimental thermomechanical investigation of fault friction in Mt. Unzen dacitic dome rock using a rotary‐shear apparatus at variable slip rates and normal stresses. A power density threshold is found at 0.3 MW m−2, above which frictional melt forms and controls the shear resistance to slip, inducing a deviation from Byerlee's frictional law. Homogenized experimentally generated pseudotachylytes have a similar final chemistry, thickness, and crystal content, facilitating the construction of a rheological model for particle suspensions. This is compared to the viscosity constrained from the experimental data, to assess the viscous control on fault dynamics. The onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event. This combination of experimental data, viscosity modeling, and seismic analysis offers a new description of material response during conduit plug flow and spine growth, showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock. This model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes

1997年鹿児島県北西部地震余震の広帯域強震観測

An earthquake with MJMA 6.5 occurred at the northwestern region of Kagoshima prefecture on March 26, 1997. We started observing the aftershocks at KKN (31.9794° N 130.4442° E) on March 28, 1997 using the broadband strong-motion seismometer (VSE 11 C/12 C). The observation was finished on September 30, 1997. Also another VSE 11 C/12 C was installed at the station SIBI (31.9677° N 130.3524° E). After all, we observed 100 events at KKN, including the second large earthquake（眈MA6.2) on May 13. The purpose of this paper is to describe the detail of observational conditions, and to report the result of the preliminary analysis of the observed seismograms. We transformed the horizontal components of seismograms observed at SIBI into the radial and transverse components, and analyzed the Fourier amplitude spectra for S-wave parts of the original records by using FFT. Most of the S-wave parts of the aftershocks observed at KKN have a sharp peek at the frequency of 4.0-6.0 Hz, which is not seen for the seismograms observed at SIBI. Also, we integrated the velocity seismograms to get the displacement waveforms. The observation in short epicentral distances made it possible to recognize the near-field term in all of the displacement waveforms.I. はじめに / II. 地震の概要 / III. 余震観測の詳細 / IV．観測結果 / V. おわり

Unzen Volcano : the 1900-1992 eruption

1.Chronological Table 2.Photographic Records of the 1990-1992 Eruptions at Unzen Volcano[Ohta] 3.Photographic Records of Eruption Products at Unzen Volcano during May 1991-May 1992[Nakada] 4.Photographic Records of People\u27s Life during the Eruption[The Nishinippon] 5.Geological and Tectonic Setting of Unzen Volcano[Okada] 6.The 1990-1992 Eruption of Unzen Volcano[Ohta, Matsuo, Yanagi] 7.Seismological Observations of Unzen Volcano before and during the 1990-1992 Eruption[Shimizu, Umakoshi, Matsuo, Ohta] 8.Surface Temperature Measurements of Lava Domes and Pyroclastic Flows by Infrared Thermal Video System[Umakoshi, Shimizu, Matsuo, Ohta] 9.Seismological Comparison in Eruptive Activity between Mt.Unzen and Mt.Usu[Suzuki] 10.Lava Domes and Pyroclastic Flows of the 1991-1992 Eruption at Unzen Volcano[Nakada] 11.Debris Flows in Mt.Fugen[Hirano, Hashimoto, Moriyama] 12.Geochemical Stuudy of Unzen Volcano by Noble Gas Measurement[Takaoka, Tokunaga, Nagao] 13.Temporal Variation in Specific Gravity of the Lava Extruded from the Jigokuato Crater[Yanagi, Nakada, Maeda] 14.Temporal Variation in Chemical Composition of the Lava Extruded from the Jigokuato Crater, Unzen Volcano[Yanagi, Nakada, Maeda] 15.Batch Franctionation Model for the Evolution of Unzen Volcanic Rocks[Yanagi, Nakada, Maeda] 16.Geological Examination of the Two Old Maps from the Tokugawa Era Concerning the “Shimabara Catastrophe”[Miyachi] 17.Monitoring Ground Movements of Chijiwa Foult and Mt. Mayuyama Using the Global Positioning System for Surevying[Esaki, Aikawa, Okubo, Shimizu, Ohta] 18.Slope Stability of Mt. Mayuyama under the Volcanic Activity of Unzen Volcano[Ochiai, Hayashi, Umemura, Iryo] 19.Earthquake Observations at Mt. Mayuyama[Tsutsumi, Aso, Kitagawa, Uno] 20.Decision Making Process of Both the Administration Bodies and the Inhabitations for Evacuation during the Eruption of Mt. Fugen in Unzen Volcano[Matsunaga, Takahashi] 21.A Documentary of People\u27s Life During the 1990-1992 Eruption of Unzen Volcano[Kawazoe, Okada] References本書は、1990年11月に噴火を開始した雲仙岳の火山活動について地球物理学的、地球科学的、地質学的観測、溶岩組成に基づく噴火モデルおよび眉山崩壊の土質工学的予測を含む総括的報告である

Unzen Volcano : the 1900-1992 eruption

本書は、1990年11月に噴火を開始した雲仙岳の火山活動について地球物理学的、地球科学的、地質学的観測、溶岩組成に基づく噴火モデルおよび眉山崩壊の土質工学的予測を含む総括的報告である。1.Chronological Table 2.Photographic Records of the 1990-1992 Eruptions at Unzen Volcano[Ohta] 3.Photographic Records of Eruption Products at Unzen Volcano during May 1991-May 1992[Nakada] 4.Photographic Records of People's Life during the Eruption[The Nishinippon] 5.Geological and Tectonic Setting of Unzen Volcano[Okada] 6.The 1990-1992 Eruption of Unzen Volcano[Ohta, Matsuo, Yanagi] 7.Seismological Observations of Unzen Volcano before and during the 1990-1992 Eruption[Shimizu, Umakoshi, Matsuo, Ohta] 8.Surface Temperature Measurements of Lava Domes and Pyroclastic Flows by Infrared Thermal Video System[Umakoshi, Shimizu, Matsuo, Ohta] 9.Seismological Comparison in Eruptive Activity between Mt.Unzen and Mt.Usu[Suzuki] 10.Lava Domes and Pyroclastic Flows of the 1991-1992 Eruption at Unzen Volcano[Nakada] 11.Debris Flows in Mt.Fugen[Hirano, Hashimoto, Moriyama] 12.Geochemical Stuudy of Unzen Volcano by Noble Gas Measurement[Takaoka, Tokunaga, Nagao] 13.Temporal Variation in Specific Gravity of the Lava Extruded from the Jigokuato Crater[Yanagi, Nakada, Maeda] 14.Temporal Variation in Chemical Composition of the Lava Extruded from the Jigokuato Crater, Unzen Volcano[Yanagi, Nakada, Maeda] 15.Batch Franctionation Model for the Evolution of Unzen Volcanic Rocks[Yanagi, Nakada, Maeda] 16.Geological Examination of the Two Old Maps from the Tokugawa Era Concerning the “Shimabara Catastrophe”[Miyachi] 17.Monitoring Ground Movements of Chijiwa Foult and Mt. Mayuyama Using the Global Positioning System for Surevying[Esaki, Aikawa, Okubo, Shimizu, Ohta] 18.Slope Stability of Mt. Mayuyama under the Volcanic Activity of Unzen Volcano[Ochiai, Hayashi, Umemura, Iryo] 19.Earthquake Observations at Mt. Mayuyama[Tsutsumi, Aso, Kitagawa, Uno] 20.Decision Making Process of Both the Administration Bodies and the Inhabitations for Evacuation during the Eruption of Mt. Fugen in Unzen Volcano[Matsunaga, Takahashi] 21.A Documentary of People's Life During the 1990-1992 Eruption of Unzen Volcano[Kawazoe, Okada] Reference