Takao Ohminato

Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment‐tensor inversions of very‐long‐period data

[1] Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and � 160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip � 60� to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are � 200 m 3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7280 Seismology: Volcano seismology (8419); 8414 Volcanology: Eruption mechanisms; KEYWORDS: very-long-period seismicity, moment tensor inversions, eruption mehanics

Waveform inversion of very long period impulsive signals associated with magmatic injection beneath Kilauea volcano, Hawaii

We use data from broadband seismometers deployed around the summit of Kilauea Volcano to quantify the mechanism associated with a transient in the flow of magma feeding the east rift eruption of the volcano. The transient is marked by rapid inflation of the Kilauea summit peaking at 22 μrad 4.5 hours after the event onset, followed by slow deflation over a period of 3 days. Superimposed on the summit inflation is a series of sawtooth displacement pulses, each characterized by a sudden drop in amplitude lasting 5–10 s followed by an exponential recovery lasting 1–3 min. The sawtooth waveforms display almost identical shapes, suggesting a process involving the repeated activation of a fixed source. The particle motion associated with each sawtooth is almost linear, and its major swing shows compressional motion at all stations. Analyses of semblance and particle motion are consistent with a point source located 1 km beneath the northeast edge of the Halemaumau pit crater. To estimate the source mechanism, we apply a moment tensor inversion to the waveform data, assuming a point source embedded in a homogeneous half-space with compressional and shear wave velocities representative of the average medium properties at shallow depth under Kilauea. Synthetic waveforms are constructed by a superposition of impulse responses for six moment tensor components and three single force components. The origin times of individual impulses are distributed along the time axis at appropriately small, equal intervals, and their amplitudes are determined by least squares. In this inversion, the source time functions of the six tensor and three force components are determined simultaneously. We confirm the accuracy of the inversion method through a series of numerical tests. The results from the inversion show that the waveform data are well explained by a pulsating transport mechanism operating on a subhorizontal crack linking the summit reservoir to the east rift of Kilauea. The crack acts like a buffer in which a batch of fluid (magma and/or gas) accumulates over a period of 1–3 min before being rapidly injected into a larger reservoir (possibly the east rift) over a timescale of 5–10 s. The seismic moment and volume change associated with a typical batch of fluid are approximately 1014 N m and 3000 m3, respectively. Our results also point to the existence of a single force component with amplitude of 109 N, which may be explained as the drag force generated by the flow of viscous magma through a narrow constriction in the flow path. The total volume of magma associated with the 4.5-hour-long activation of the pulsating source is roughly 500,000 m3 in good agreement with the integrated volume flow rate of magma estimated near the eruptive site.

Mechanism of Phreatic Eruptions at Aso Volcano Inferred from Near-Field Broadband Seismic Observations

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.

10s‐period volcanic tremors observed over a wide area in southwestern Japan

Unusual seismic oscillations with a period of 10 seconds were observed over a wide area of southwestern Japan. Several lines of evidence indicate that the origin of these 10s waves must be around the Aso volcano, located in the central part of Kyushu Island, Japan. Although a 10s period is unusually long for a volcanic tremor and no corresponding surface activity at the crater was reported by the local observatory, the volcanic origin of the tremor is indisputable. A further astonishing aspects of the observation is that there exists a strong correlation between the onset of 10s waves observed remotely and cessation of high-frequency (>1Hz) volcanic tremors at the Aso volcano. The Aso volcano appears to be trembling with a 10s period without this having been noticed. Since seismic monitoring of volcanic activity is limited to high frequencies (above 1 Hz) at the most of the volcanoes in the world, long-period tremors of the type observed here might be a widespread phenomenon that has been overlooked.

Broadband seismic observation at the Sakurajima Volcano, Japan

We installed a portable broadband seismometer (Streckeisen STS-2) at the Sakurajima volcano, which has been very active in the recent years. The recorded seismograms show a wide variety (both in temporal and spectral contents) of seismic events, from explosions to tremors, and exhibit the importance of such broadband seismometry at volcanos. We present examples of seismograms to show the potential of broadband seismic observation in monitoring volcanic activities.

Vulcanian eruptions with dominant single force components observed during the Asama 2004 volcanic activity in Japan

On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

Broadband seismic observations at Satsuma‐Iwojima Volcano, Japan

This paper reports the results of preliminary analysis of broadband seismic observations conducted at Satsuma-Iwojima volcano, Japan. The recorded broadband data show a wide variety of seismic features. Small (M ∼ 0), short-period volcanic earthquakes characterized by a long (∼5 s) emergent phase and by nearly identical waveforms are located at the very shallow part of the summit crater. Long-period seismic pulses synchronized with a regular amplitude modulation of volcanic tremor have an interval of approximately 46 to 50 min. Each of the long-period pulses has almost identical waveform. Polarization of the long-period events suggests a very shallow source which can be interpreted as a result of a volume expansion mechanism. The correlation between the long-period seismic pulses and volcanic tremor seems to be associated with magma convection and the degassing process inside the conduit of the volcano.

Radiographic imaging below a volcanic crater floor with cosmic-ray muons

We present a novel application of cosmic-ray muon radiography to image the shallow density structure beneath Asama Volcano, Japan. We use a single detector (emulsion cloud chamber) set up in an underground vault at an elevation of 2250 m on the eastern flank of Asama, 310 m below the summit of the edifice and 1 km away from the crater. The results point to two high-density anomalies located between the original pre-2004 eruption crater floor and post-2004 eruption crater profile. A third low-density anomaly is imaged immediately below the pre-2004 eruption crater floor. The spatial extent of each density anomaly is about 100 to 200 m. To know if this method, applied to other volcanoes, would produce contrasting results, we performed the measurement in 1944 Usu lava dome. We confirmed a bulbous shape measuring approximately 300 m in diameter and narrowing downwards. The diameter of the uppermost part of the conduit is estimated at 100 ± 15 m at an elevation of 260 m a.s.l. and 50 ± 15 m at an elevation of 217 m a.s.l., demonstrating a resolution that is significantly better than that typically achieved with seismic tomography based on picks of first arrival times from earthquakes or artificial sources.

Slow initial phase generated by microearthquakes occurring in the western Nagano Prefecture, Japan ‐The source effect‐

Near-source observations of microearthquakes occurring in the Western Nagano prefecture show that durations of recorded slows initial phases were almost constant independently of focal distance. Although a closer inspection revealed that the durations of the slow initial phases slightly increased with focal distance, they were not proportional to focal distance. Thus, it is concluded that the slow initial phases do not mainly reflect the whole path Q. If the slow initial phases were attributed to the strong anelastic attenuation in the surface layer combined with that of the whole path Q, the rise time of the P-wave velocity pulse at the source for a M2.4 event would be estimated as about 3 ms and the source radius of the M2.4 event would be estimated as about 10 m. This value of the source radius is too small for M2.4. This suggests that the slow initial phase does not arise from the strong anelastic attenuation in the surface layer. It is likely that the slow initial phase does not mainly reflect a path effect but a source effect.

Correction to “Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data”

[1] Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and 160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip 60 to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are 200 m for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit.