S43A-82 INVITED 1330h
Imaging the 3D Volcanic Structure of Hawaiian Volcanoes: Onshore-Offshore Seismic Investigations of the Island of Hawaii
Frequent, and ongoing seismicity at Kilauea and Mauna Loa volcanoes in Hawaii has enabled good quality seismic tomographic imaging of these volcanic edifices. Due to the limited distribution of earthquakes and receivers, however, these images have been largely restricted to the subaerial portions of the volcanoes. A 1998 marine seismic survey carried out on board the R/V {\it Maurice Ewing} generated ~2,000 km of multichannel reflection lines over the submarine flanks of the Island of Hawaii. The offshore shots were also recorded by the permanent onland HVO-USGS Seismic Network. This unique onshore-offshore data set has provided an unprecedented opportunity to image the combined subaerial and submarine portions of Kilauea volcano and surroundings. As part of this study, we have conducted 3D tomographic inversions of first arrival traveltimes to determine seismic velocity distributions within the volcanoes and underlying lithosphere. The results reveal distinct high velocity anomalies beneath the onland summits and rift zones of Mauna Loa and Kilauea volcanoes, as well as the offshore Loihi seamount. As noted by others, these high velocity regions are associated with dense dike and cumulate complexes resulting from magmatic intrusion and volcanic spreading. The new tomographic images provide a more complete view of these intrusive complexes, allowing us to better constrain their geometries and volumes. The submarine flanks of both Mauna Loa and Kilauea, in contrast, exhibit anomalously low velocities suggestive of thick packages of volcaniclastic sediments or landslide debris, further constrained by the reflection data. In addition, 2D forward and inverse modeling of later reflected and refracted arrivals, parallel to the offshore survey lines, should help to constrain the depths and geometries of internal crustal structures, such as the base of the volcanic edifice and the Moho discontinuity. In combination, these imaging tools are offering an improved view of the complex internal structure of Hawaiian volcanoes, leading to better understanding of the dynamic processes active in this setting.
S43A-83 1330h
The Spread of the Dike and the Migration of the Magma around the Miyake-jima Island in 2000.
From June 26, 2000, an intensive earthquake swarm started under the Miyake-jima Island, 180km south off Japan. This swarm was closely related to the eruption of the Miyake-jima Island, probably dominated by the underground magmatic activity. The swarm spread toward northwestern ocean region from Miyake-jima Island, in which a huge number of earthquakes (over about 100,000) including five large events with M>6.0 were detected for about 2 months. This earthquake swarm was the most active one since we started the seismic observation in 1970's. Although some telemetered observation stations exist on the Izu volcanic islands, no offshore instruments were operated in the area of this earthquake swarm. To understand both the spatial and temporal changes of this activity, we conducted a series of ocean bottom seismometer observations. According to the variation in the seismic activity with time, we changed the array configuration of OBSs six times. Furthermore, a real time seismic observation was undertaken using a buoy-telemetering OBS system. Combining the OBS data with those of the island stations, very precise earthquake locations were determined. The epicenter distribution obtained strongly indicates a northwest-southeastern lineament. The dike intrusion was a vertical thin plane by the distribution of the earthquake. But the distribution was not uniform and a gathering of some long tablets or columns. They consist of some burst activities, of which the hypocenters migrated upward. In view of the migrated hypocenter, we investigate the spread of the dike and the migration of the magma.
S43A-84 1330h
Imagin of the scatterer distribution beneath Unzen Volcano
Unzen Volcano is one of the active volcanos in Japan, located in Southwest Japan behind the Ryukyu-Arc. Latest eruption of Unzen began in November 1990, leaving 198 years dormancy. It continued until early 1995, effusion of dacite lava resulted in the formation of lava dome at the summit and frequent generation of Merapi-type pyroclastic flows. To detect the volcanic vent used in latest eruption, a seismic reflection exploration was conducted with vibratory energy sources in December 2001, as a program of Unzen Scientific Drilling Project. A profile of the exploration was performed along about 12 km line crossing Unzen graben and passed through 2 km west from the lava dome. The survey line was composed of 201 sweep points along the profile. In this study, we try to detect scatterer distribution beneath Unzen Volcano using vibration signals observed at seismic stations of Kyushu University in Unzen area. A seismograph with natural frequency of 1 Hz is installed in the seismic stations, and recorded with sampling frequency of 100 Hz. According to the reciprocity theorem, the common receiver gather from the vibratory sources on the survey line obtained at the station is equivalent to common shot gather obtained at the profile for a source locating at the station. Therefore, we assumed that seismic data observed at the stations are data observed in survey line with vibratory sources at the stations. We obtained the image of scatterer distribution beneath Unzen Volcano by calculating semblance coefficient with slant stacked waveform, and we found strong scattering part at 8 km deep and 5 km west of lava dome. From geodetic data, four pressure sources (A-D) is detected in Unzen Volcano, and pressure sources C, which is considered a major magma reservoir, is locating very cross to the strong scattering part. Thus we consider the strong scattering part indicates volcanic pressure source.
http://www.sevo.kyushu-u.ac.jp/index-e.html
S43A-85 1330h
The visualization of velocity structure and magma distribution from upper mantle to upper crust in Hakone volcano.
Three-dimensional seismic reflection and refraction survey was carried out in Hakone volcanic area, northern part of Izu peninsula. The region is one of the most famous hot spring areas in Japan. Hakone volcano morphologically resembles one big caldera. However, the depression of the volcano consists of several small calderas which has been formed by multiple eruptions. Although sprouts of fumarolic gas and steam are identified in a few areas of the volcano, there is no historical record of volcanic eruption. Main purpose of our study is to determine the 3-dimensional deep velocity structure around the volcano using the seismic tomography processing. We deployed 44 sets of temporal offline seismic stations and a line of multi-channels seismic reflection survey cable. The seismic waves generated by some natural earthquakes and 14 dynamite explosions were observed, and their data were processed for tomography. The observation coverage was 20 km in diameter. Our result demonstrates the usefulness of high dense seismic observation in identifying and locating low velocity zones beneath the particular area. According to our tomography, low velocity zone was identified only in surface layer of the old caldera part of the volcano. We could not identify any remarkable reflector in deeper crust, as the result of wide-angle reflection survey using explosive shots. Moreover, we could not identify any other low velocity zone as far as 32 km depth by incorporating the results of other study. In other words, we think that magma is no longer supplied to Hakone volcanic area.
S43A-86 1330h
Three-Dimensional Velocity Strutures in the Tatun Volcanic Area of Taiwan
The Tatun volcanic group, composed of more than 20 volcanoes, is situated at the northern tip of Taiwan. Although the volcanic activity has been extinct for a long time, hydrothermal activity in the form of hot springs, high heat flow and clustering earthquakes is still going on. In this study three-dimensional velocity structures have been inverted from P- and S-wave arrivals generated by 304 local earthquakes recorded by a local temporary network during the period of 1996-2001. The results of the tomographic study indicate strong lateral velocity variations, with low velocity bodies probably representing zones of hydrothermal fluid circulation and high velocity areas representing solidified magmatic intrusions.
S43A-87 1330h
Low Q Zone Beneath the Onikobe Caldera as Inferred From Coda Amplitude Decay of Local Earthquakes
Coda waves of local earthquakes are interpreted as scattered waves from inhomogeneities in the Earth. As the shape of the coda envelope is affected by spatial variations of attenuation and scattering strength, the distribution of the attenuation and scattering strength can be estimated by analyzing coda wave envelopes. In the present study, we show an example of how the strong attenuation zone in the upper crust beneath a caldera was detected by investigating characteristics of coda wave envelopes observed by a local seismographic network. The Onikobe area is an active geothermal area situated in the Ou backbone range in northeastern Japan. There are Tertiary to Quaternary calderas and active volcanoes in this area. Earthquakes with magnitudes 5.9 and 5.7 occurred near the Onikobe caldera in August 1996. After these earthquakes, a dense temporary seismographic network was set up in about a 30 km$\times$30 km area for aftershock observation. Seismograms recorded at stations located within 10km of the epicentral distance for earthquakes which occurred at depths shallower than 10km were used in the present study. Q of coda waves (Qc) was calculated by applying a single scattering model for the time window starting from twice the S-wave travel time to a lapse time of about 18 s. We found a systematic variation of Qc with station location: Qc values were low (200-350 for 8-16Hz band) at stations in and around calderas, and high (400-600) at other stations. The amplitude of the coda wave characterized by low Qc values decreased smoothly with the lapse time, while the decay rate of the coda wave with high Qc values changed at around 6 s after the origin time. By taking these observational facts into account, we proposed a model in which the scattering coefficient was very large in the lower crust, and a small and strong attenuation zone ( low Qs zone, LQZ ) was embedded in the upper crust beneath a caldera. In order to estimate the location, size and Qs of the LQZ to explain observed characteristics of Qc and decay curves, we synthesized coda envelopes and compared them with observations. We homogeneously distributed scatterers in the medium and synthesized a coda envelope by summing up coda wave energy scattered by one of the scatterers The relative scattering coefficient was assumed to increase with depth. Qs outside of the LQZ was given as Qs=250. By calculating synthetic envelopes for different locations, sizes and Qs of the LQZ, we found that the model containing 10km$\times$10km$\times$5km LQZ with Qs$\sim$15 at a depth of 5--10km beneath the Onikobe caldera explained the difference in the gross feature of observed Qc and decay curves.