Koji Miyakawa

Three‐dimensional resistivity structure and magma plumbing system of the Kirishima Volcanoes as inferred from broadband magnetotelluric data

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.

Enhancing volcano‐monitoring capabilities in Ecuador

Ecuador has 55 active volcanoes in the northern half of the Ecuadorian Andes. There, consequences of active volcanism include ashfalls, pyroclastic flows (fast moving fluidized material of hot gas, ash, and rock), and lahars (mudflows), which result in serious damage locally and regionally and thus are of major concern to Ecuadorians. In particular, Tungurahua (elevation, 5023 meters) and Cotopaxi (elevation, 5876 meters) are high-risk volcanoes. Since 1999, eruption activity at Tungurahua has continued and has produced ashfalls and lahars that damage towns and villages on the flanks of the volcano. More than 20,000 people live on these flanks.

Source estimates of the May 2006 Java earthquake

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).

Quasi‐static slip patch growth to 20 m on a geological fault inferred from acoustic emissions in a South African gold mine

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.