Naofumi Aso

Volcanic‐like low‐frequency earthquakes beneath Osaka Bay in the absence of a volcano

[1]xa0Among the many deep low-frequency earthquakes (LFEs) recently discovered worldwide, LFEs beneath Osaka Bay, western Japan, are especially unusual. Their waveforms are monochromatic, resembling those of some volcanic LFEs, but there are no active volcanoes around. The area is close to but clearly distinct from a belt of tectonic LFEs, and is near the site of a large inland earthquake (the 1995 Kobe earthquake). To characterize the activity of these LFEs, we present an extensive catalog constructed using a matched filter analysis on continuous seismic records with template LFEs determined by the Japan Meteorological Agency. The relocated catalog of 1378 events over a period of 5 years shows spatially concentrated activity in two volumetric zones, with several active periods including successive tremor-like events. The magnitude–frequency statistics satisfy the Gutenberg–Richter law with a b-value of 2. Unlike tectonic LFEs, which are highly sensitive to tidal stress, the LFEs beneath Osaka Bay show no spectral peak in activity at tidal periods, and the overall pattern of the spectrum is similar to that of volcanic LFEs beneath Sakurajima, Japan. These findings suggest that the Osaka Bay LFEs are almost same as volcanic LFEs in origin, or at least related to fluid upwelling from the mantle.

Cooling magma model for deep volcanic long‐period earthquakes

Deep long-period events (DLP events) or deep low-frequency earthquakes (deep LFEs) are deep earthquakes that radiate low-frequency seismic waves. While tectonic deep LFEs on plate boundaries are thought to be slip events, there have only been a limited number of studies on the physical mechanism of volcanic DLP events around the Moho (crust-mantle boundary) beneath volcanoes. One reasonable mechanism capable of producing their initial fractures is the effect of thermal stresses. Since ascending magma diapirs tend to stagnate near the Moho, where the vertical gradient of density is high, we suggest that cooling magma may play an important role in volcanic DLP event occurrence. Assuming an initial thermal perturbation of 400°C within a tabular magma of half width 41u2009m or a cylindrical magma of 74u2009m radius, thermal strain rates within the intruded magma are higher than tectonic strain rates of ~u200910^(−14)u2009s^(−1) and produce a total strain of 2u2009×u200910^(−4). Shear brittle fractures generated by the thermal strains can produce a compensated linear vector dipole mechanism as observed and potentially also explain the harmonic seismic waveforms from an excited resonance. In our model, we predict correlation between the particular shape of the cluster and the orientation of focal mechanisms, which is partly supported by observations of Aso and Ide (2014). To assess the generality of our cooling magma model as a cause for volcanic DLP events, additional work on relocations and focal mechanisms is essential and would be important to understanding the physical processes causing volcanic DLP events.

Focal mechanisms of deep low‐frequency earthquakes in Eastern Shimane in Western Japan

[1]xa0Deep low-frequency earthquakes (LFEs) are small earthquakes (Mu2009<u20092) that occur at depths of ~10–45 km with seismic wave radiation mainly in the frequency range of 2–8 Hz. LFEs in western Japan are categorized into tectonic LFEs in the Nankai subduction zone, volcanic LFEs beneath active volcanoes, and semi-volcanic LFEs that occur far from active volcanoes but are otherwise similar to volcanic LFEs. While tectonic LFEs are considered to reflect shear slip on a plate boundary, the mechanisms of volcanic and semi-volcanic LFEs remain unclear. We have determined the mechanisms of 38 semi-volcanic LFEs in eastern Shimane, which is the site of the most frequent semi-volcanic LFE activity in Japan. For each event, velocity seismograms at five stations were inverted into a focal mechanism and moment rate function by a combined grid search and linear inversion analysis. Synthetic waveforms were calculated for a one-dimensional structure and the local site amplification effects were corrected using body waves from deep earthquakes. The estimated moment rate function was found to often oscillate between positive and negative values, which are unlike those of regular earthquakes. The focal mechanisms for many LFEs are dominated by a compensated linear vector dipole with symmetry axes parallel to the lineation formed by the hypocenter distribution and the direction of the minimum principal axis of regional stress.

Mathematical review on source-type diagrams

A source-type diagram is a visualization tool used to display earthquake sources, including double-couples, compensated linear vector dipoles, and isotropic deformation. Together with recent observations of non-double-couple events in a variety of tectonic settings, it is important to be able to recognize the source type intuitively from a representative diagram. Since previous works have proposed diagrams created using a range of projections, we review these diagrams in the framework of the moment tensor eigenvalue space. For further applications, we also provide complete formulas for conversion between moment tensor representation and the coordinate system of each diagram style. Using both a global catalog and synthetic data, we discuss differences between types of diagrams and the relative effectiveness of each.

Seismologically Observed Spatiotemporal Drainage Activity at Moulins

Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by back-projecting the amplitude of the cross-correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2-km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation back-projection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor.