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The Earth's upper inner core has a quasi-hemispherical velocity structure. One of the two quasi-hemispheres has a high-velocity region centered beneath Southeast Asia and is referred to as “East”. The remaining low-velocity region is referred to as “West”. The boundary positions between the quasi-hemispheres are rather scattered but well estimated at low- and mid-latitudes, whereas discrepancies between seismological hemisphere models increase at high latitudes. However, the boundary position at high latitudes is important when considering the origin of the quasi hemisphericity. Recent deployment of a temporary broadband seismic array in Thailand (Thai Seismic Array; TSAR) provides a means to analyze the inner core structure in the north polar region because rays from Central American earthquakes to the network are transmitted through the inner core and have their turning points approximately 150–400 km beneath the north polar region of the inner core boundary. We measured the differential traveltimes of earthquake waveforms between a seismic phase transmitted through the inner core (PKIKP, DF) and a phase that turns in the lower outer core (PKPbc, BC). The observed differential times are approximately 0.2–0.3 s smaller than theoretical estimates predicted by the Preliminary Reference Earth Model (PREM) over the observed distance range and 0.3–0.5 s smaller than those of AK135 at larger distances, which suggests lower velocities in the analyzed region of the upper inner core. Our analyses show that models with velocities ~0.03 km/s lower than those of PREM in the upper 300 km of the inner core, with a velocity about midway between PREM and AK135 in the F layer, sufficiently fit the observed differential times. This low-velocity feature in the upper inner core is characteristic of the “West”. The observations are also largely explained by the model obtained for the structure beneath the northeast Pacific in mid-latitudes classified as “West”. Our previous study also shows that the south polar region of the upper inner core has “West” characteristics. Our results thus suggest that the north and south polar regions of the upper inner core have a “West” structure and that the Earth's upper inner core has an eyeball-shaped high-velocity anomaly called the “East”. The shape of the hemispheric boundary is possibly an ellipse shortened in the north-south direction rather than a circle.

Three months of acoustic emission (AE) monitoring in a South African gold mine down to Mw −5 revealed a newly emergent planar cluster of 7557 events −3.9 ≤ Mw ≤ −1.8 (typical rupture radius of 6–70 cm) that expanded with time to reach a size of 20 m on a preexisting geological fault near an active mining front 1 km beneath the ground. It had a sharply defined, planar configuration, with hypocenters aggregated within a thickness of only several decimeters. We infer that the zone defines an aseismic slip patch on the fault, wherein the individual AEs represent failures of very small asperities being loaded by the aseismic slip. Additional support for the interpretation was obtained by analyzing composite focal mechanisms and repeating events. The patch expansion over 2 months was likely quasistatic because all individual AEs ruptured much smaller areas than the cluster size at the corresponding time. The b values dropped gradually from 2.6 to 1.4, consistent with a significant increase in shear stress expected of the mining style. Another cluster with similar characteristics emerged later on a neighboring part of the same fault and grew to a 10 m extent in the last weeks of the study period. The quasi-static expansion of inferred localized slow-slip patches to sizes of 10–20 m suggests that the critical crack length on natural faults can be at least as large, much exceeding the decimeter range derived from laboratory stick-slip experiments on saw-cut rocks.

Broadband magnetotelluric (MT) measurements were conducted in 2010 and 2011 in the vicinity of Shinmoe-dake Volcano in the Kirishima volcano group, Japan, where sub-Plinian eruptions took place 3 times during 26–27 January 2011. By combining the new observations with previous MT data, it is found that an anomalous phase in excess of 90° is commonly observed in the northern sector of the Kirishima volcano group. Because the anomalous phase is not explained by 1-D or 2-D structure with isotropic resistivity media, 3-D inversions were performed. By applying small errors to the anomalous phase, we successfully estimated a 3-D resistivity structure that explains not only the normal data but also the anomalous phase data. The final model shows a vertical conductor that is located between a deep-seated conductive body (at a depth greater than 10 km) and a shallow conductive layer. By applying the findings of geophysical and petrological studies of the 2011 sub-Plinian eruptions, we infer that the subvertical conductor represents a zone of hydrothermal aqueous fluids at temperatures over 400°C, in which a magma pathway (interconnected melt) is partially and occasionally formed before magmatic eruptions. To the north of the deep conductor, earthquake swarms occurred from 1968 to 1969, suggesting that these earthquakes were caused by volcanic fluids.

We investigated frequency-magnitude distribution (FMD) of acoustic emissions (AE) occurring near an active mining front in a South African gold mine, using a catalog developed from an AE network, which is capable of detecting AEs down to M W −5. When records of blasts were removed, FMDs of AEs obeyed a Gutenberg−Richter law with similar b values, not depending on post-blasting time from the initial 1-min interval through more than 30 h. This result denies a suggestion in a previous study (Richardson and Jordan Bull Seismol Soc Am, 92:1766–1782, 2002) that new fractures generated by blasting disturb the size distribution of background events, which they interpreted as slip events on existing weak planes. Our AE catalog showed that the GR law with b ∼ 1.2 was valid between M W −3.7 and 0 for AEs around the mining front. Further, using the mine’s seismic catalog, which covers a longer time period of the same area, we could extend the validity range of the GR law with the same b value up to M W 1.

The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass’ position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 μs and less than 0.1% in velocity over 1000 m between the source and the receiver.

[1] We have conducted a temporal broadband seismic observation in Kii Peninsula, southwest Japan, and detected a swarm of 110 very low frequency earthquakes (VLFs) and deep low frequency tremors. During three days of the swarm activity, VLFs and tremors occur concurrently in two localized regions separated by ∼10 km. Stacking analyses are also employed to detect VLF signals of a period longer than 50 s, whose focal mechanisms are determined for the first time from data and shown to be consistent with the subducting plate motion. Evaluation of the VLF seismic moment implies that a substantial portion of the SSE seismic moment is released as a VLF swarm.

Very-long-period (VLP) signals showing simple decaying harmonic oscillations with periods near 10 seconds and lasting for about 300 seconds were observed in association with an earthquake swarm that occurred beneath Hachijo Island, Japan. Results from the source-mechanism analysis and waveform simulation based on a fluid-filled crack model consistently point to the resonance of a dike filled with a basaltic magma as the source of the VLP signals. Thus, VLP signals can be used to probe the state of the fluid and dynamic processes within a volcanic system.

[1] Temporal variations in the P- and S-wave travel times were continuously monitored using ACROSS, which is a newly developed system for utilizing accurately controlled continuous sinusoids for seismic exploration. We made an experiment lasting for 15 months at a site near the Nojima fault which ruptured during the 1995 Kobe earthquake (M7.2). The elastic waves emitted from the ACROSS vibrators located at the surface were observed with seismometers at 800 m and 1700 m depth. The long-term variations of about 2 ms for both the P and S waves were observed during the 15-month experiment. The major cause of these variations is identified to be that of a near-surface property which changes the near-source vibration. The variations in the deeper structures estimated after the reduction of the near-source variation were about 0.5 ms. An S-wave delay of about 1 ms was detected due to the 2000 Western Tottori earthquake (M6.6).

In the early morning of 27 May 2006, local time, central Java was jolted by strong seismic ground motion. In spite of the moderate size of the earthquake (Mw = 6.4), it caused severe damage nearYogyakarta city. According to a report from the Social Affairs Ministry of Indonesia, more than 5700 people were killed, 38,000 injured, and 423,000 evacuated. As a result of the shaking, more than 126,000 buildings collapsed, and more than 392,000—including those of the famous Prambanan temple complex, located about 17 kilometers east of Yogyakarta—were severely damaged (Figure 1).

The bond energies of ennea- and octa-aqua-coordinated polyhedra of lanthanoid ions have been calculated on the electrostatic model. Sums of coordination bond energies and Born hydration enthalpies of the aqua complexes predict an exclusive stabilization of the enneahydrate regardless of whether the central metal ion is of the earlier or later elements of the series. However, the difference between the hydration free energies evaluated for octa- and ennea-hydrate ions shows that there is a preference to form the enneahydrate for the earlier elements, but the octahydrate is formed for the later elements. A small difference in hydration free energy results in the transitional coordination number for the elements near to gadolinium. As an application of the present model, the equilibrium constant for the 5d� 4f excited states of [Ce(OH2)8]3+ and [Ce(OH2)9]3+ in aqueous media have been calculated.