Seismic Amplitude Ratio Analysis of the 2014–2015 Bár ∂arbunga-Holuhraun Dike Propagation and Eruption

Magma is transported in brittle rock through dikes and sills. This movement may be accompanied by the release of seismic energy that can be tracked from the Earth’s surface. Locating dikes and deciphering their dynamics is therefore of prime importance in understanding and potentially forecasting volcanic eruptions. The Seismic Amplitude Ratio Analysis (SARA) method aims to track melt propagation using the amplitudes recorded across a seismic network without picking the arrival times of individual earthquake phases. This study validates this methodology by comparing SARA locations (filtered between 2- 16 Hz) with the earthquake locations (same frequency band) recorded during the 2014-15 Bárðarbunga-Holuhraun dike intrusion and eruption in Iceland. Integrating both approaches also provides the opportunity to investigate the spatio-temporal characteristics of magma migration during the dike intrusion and ensuing eruption. During the intrusion SARA locations correspond remarkably well to the locations of earthquakes. Several exceptions are however observed. [1] A low-frequency signal was possibly associated with a subglacial eruption on 23 August. [2] A systematic retreat of the seismicity was also observed to the back of each active segment during stalled phases and was associated with a larger spatial extent of the seismic energy source. This behavior may be controlled by the dike’s shape and/or by dike inflation. [3] During the eruption SARA locations consistently focused at the eruptive site. [4] Tremor-rich signal close to ice cauldrons occurred on 3 September. This study demonstrates the power of the SARA methodology, provided robust site amplification, Quality Factors and seismic velocities are available

Experimental Study on the Effect of Bubble Concentration on the Effective Viscosity of Liquids

It is widely accepted that the volatile gas in magma plays an important role in volcanic eruptions. With the ascent of magma, pressure gradually decreases and increases the degree of supersaturation of volatile components and consequently, nucleation of the gas phase is initiated. Though the dynamical process of magma in a volcanic conduit is very complicated, the viscosity of magma is known to be a fundamentally important factor. As for the effective viscosity of a liquid containing small concentration of bubbles, Taylor, Batchelor and Shimozuru derived theoretically the following formula, ηe/η0 = (1 + φ), where φ is the volume concentration of bubbles. But if violent nucleation happens, the concentration of bubbles will increase rapidly and the interaction of bubbles will not be ignored. The author tried to examine experimentally how the effective viscosity varies when bubble concentration is large. The viscosity was measured using a specially deviced rotation viscometer in which air bubbles were injected from the bottom into silicon oil. The torque of the inner cylinder was measured for various bubble concentration. The results of experiments show that the change of the effective viscosity coincides with the theoretical predicted curve within the bubble concentration below 30%, above which, the experimental value tends to increase. The empirical formula which is written below is the best fit curve of the change of the effective viscosity of liquid which contains bubbles with volume fraction less than 40%. ηe/η0=(1-1.5φ)-0.55 (O.O<φ<O.4)

Deep seismic imaging of the eastern Nankai trough, Japan, from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration

Journal of Geophysical Research, v. 109, p. B02111, 2004. http://dx.doi.org/10.1029/2003JB002689International audienc

Heterogeneous structure around the rupture area of the 2003 Tokachi-oki earthquake (Mw=8.0), Japan, as revealed by aftershock observations using Ocean Bottom Seismometers

Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw = 8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~ 7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock

Very long period seismic signals observed before the caldera formation with the 2000 Miyake-jima volcanic activity, Japan

Very long period (VLP) seismic signals, whose waveform consists of an initial impulsive signal and a later oscillatory wave with 0.2 or 0.4 Hz in dominant frequency, were observed before the caldera formation in the 2000 activity of Miyake-jima volcano, Japan. The results of waveform inversion show that the initial and later parts can be explained by a northward single-force of 1.5×10^8 N working at a depth of 2 km beneath the summit and a moment tensor solution at a depth of 5 km below and 2 km southwest of the summit with ～10^12 Nm, respectively. A clear positive correlation of the amplitudes between the two sources strongly suggests that the shallow single-force triggers the deeper moment source in spite of the several km distance between the two sources. To analyze the source time functions of the moment tensor that do not always oscillate in phase, we introduce a new method of moment tensor diagonalization which is performed in the frequency domain. According to the analysis, the two principal components have similar amplitudes and are greater than the third principal component, suggesting an axially symmetric oscillation. One of the possible systems is a combination of two cracks intersecting perpendicularly. Our interpretation is that the single-force was generated when magma containing rock blocks suddenly began to move in a choked subsurface magma path, and the resultant pressure waves propagated and excited a resonance oscillation of the two cracks