Hiroshi Ogasawara

Observation of numerous aftershocks of an Mw 1.9 earthquake with an AE network installed in a deep gold mine in South Africa

This is the first report from the JAGUARS (JApanese-German Underground Acoustic Emission Research in South Africa) project, the overall aim of which is to observe ultra-small fracturing in a more or less natural environment. We installed a local (∼40-m span) network of eight acoustic emission (AE) sensors, which have the capability to observe up to 200 kHz at a depth of 3.3 km in a South African gold mine. Our specific objective was to monitor a 30-m thick dyke that remains as a dip pillar against active mining ∼90 m above our network. An Mw 1.9 earthquake whose hypocenter was ∼30 m above the network occurred in the dyke. Although the mineowned geophone (4.5 Hz) network detected only five earthquakes in the surrounding 200×200×150-m3 volume within the first 150 h following the main shock, our AE network detected more than 20,000 earthquakes in the same period. More than 13,000 of these formed a distinct planar cluster (∼100×80 m2) on which the main shock hypocenter lay, suggesting that this cluster delineates the main shock rupture plane. Most of the aftershocks were presumably very small, probably as low as M ∼ −4. The aftershock cluster dipped ∼60°. This is consistent with normal faulting under a nearly vertical compression field, as indicated by nearly horizontal breakouts found in a borehole crossing the rupture plane.

Nucleation process of an M2 earthquake in a deep gold mine in South Africa inferred from on-fault foreshock activity

Using a network of sensitive high-frequency acoustic emission sensors, we observed foreshock activity of an Mw 2.2 earthquake (main shock) in a deep gold mine in South Africa. Foreshock activity, which selectively occurred on a part of the rupture plane of the forthcoming main shock, lasted for at least 6 months until the main shock. Rock samples recovered from the main shock source region showed evidence of ancient hydrothermal alteration on the main shock rupture plane, suggesting that the foreshock activity occurred on a preexisting weakness. The foreshocks during 3 months before the main shock were concentrated in three clusters (F1–F3), which we interpret as representing localized preslip at multiple sites. While the location of mining area, the source of stress perturbations, changed with time, the locations of foreshock clusters did not change, suggesting that the preslip patches were controlled by strength heterogeneity rather than stress distribution. Activity over the entire foreshock area was generally constant, but the largest cluster (F2) showed accelerated activity starting at least 7 days before the main shock, while mining stress did not increase in this period. The main shock initiated at a point close to F1, away from F2. All the six foreshocks during the final 41 h occurred in F1 and F2 and in-between. These suggest that in the last stage of the preparation process of the main shock, preslip patches interacted with each other through the stress concentration ahead of the expanding preslip patch (F2), which should be the only driving force of the preparation process under the constant external load.

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.

Stress change prior to the major events in the 1989 earthquake swarm off the eastern Izu Peninsula, Japan

We investigate a temporal change in a stress parameter of earthquakes that occurred in the 1989 swarms off the eastern Izu Peninsula in Japan. We use the energy index (EI), which have been monitored in deep South African gold mines for predicting major events, as an estimate of apparent stress that is proportional to the ratio of seismic energy (E) to seismic moment (Mo). EI measures an excess or shortage in E with respect to the empirical relationship between E and Mo. We check that EI is almost proportional to the apparent stress for the ranges of Mo and frequency for our analysis, although E is underestimated due to the artifact of limited frequency band of monitoring. The largest events (M = 5.2 and 5.5) took place off the tip of a vertical crack, which opened associated with magma intrusion. While the opening continued to load the source areas of the M 5.2 and 5.5, we find significant decrease in EI prior to the events. Based on the experimental result that the stress decreases when yielding takes place prior to final failure, we interpret this observation as yielding in the vicinity of the large earthquake hypocenters, following a rapid increase in stress caused by magma intrusion.

Unexpectedly frequent occurrence of very small repeating earthquakes (–5.1 ≤ MW ≤ –3.6) in a South African gold mine: implications for monitoring intraplate faults

We observed very small repeating earthquakes with −5.1 ≤ Mw ≤ −3.6 on a geological fault at 1 km depth in a gold mine in South Africa. Of the 851 acoustic emissions that occurred on the fault during the 2 month analysis period, 45% were identified as repeaters on the basis of waveform similarity and relative locations. They occurred steadily at the same location with similar magnitudes, analogous to repeaters at plate boundaries, suggesting that they are repeat ruptures of the same asperity loaded by the surrounding aseismic slip (background creep). Application of the Nadeau and Johnson (1998) empirical formula (NJ formula), which relates the amount of background creep and repeater activity and is well established for plate boundary faults, to the present case yielded an impossibly large estimate of the background creep. This means that the presently studied repeaters were produced more efficiently, for a given amount of background creep, than expected from the NJ formula. When combined with an independently estimated average stress drop of 16 MPa, which is not particularly high, it suggests that the small asperities of the presently studied repeaters had a high seismic coupling (almost unity), in contrast to one physical interpretation of the plate boundary repeaters. The productivity of such repeaters, per unit background creep, is expected to increase strongly as smaller repeaters are considered (∝ Mo −1/3 as opposed to Mo −1/6 of the NJ formula), which may be usable to estimate very slow creep that may occur on intraplate faults.

Dynamic Strain in a South African Gold Mine Produced by the 2011 Tohoku Earthquake

The 2011 Tohoku earthquake (2011/03/11 05:46:18 UT, M W 9.0) produced huge permanent displacements of up to 50 m and dynamic strain of up to 10−5 in the near-field. We observed dynamic strain (10−7) produced by this earthquake at a depth of 1 km in a gold mine in the Republic of South Africa at a distance of more than 14,000 km (125.25∘) from epicenter. The dynamic strain was observed by two Ishii-type borehole strainmeters about 30 m apart on opposite sides of a fault filled with gouge of several decimeters thickness, allowing the response of the fault to be investigated. The Tohoku earthquake seismic waves passed through the gold mine outside blasting hours, allowing us to analyze the tele-seismic body waves and the surface waves that circled the globe several times (R3 − R5 and G3 − G5, \(\sim 10^{-8}\)). We discuss the fault deformation associated with the dynamic strains by the wave packets. The seismograms of some sub-parallel components of the two strainmeters installed on the opposite sides of the fault appeared similar, although with differences on the order of \(\sim 10^{-8}\). These differences may imply a complex response of the fault (fault rheology; i.e. not only elastic, but perhaps plastic or even indicating the effects of fluid).

Flexure of the crust estimated from in-situ stress measurements and its relation to tectonics

Abstract In a previous study, the author proposed a numerical method to estimate the static flexure of the uppermost crust and the force causing the flexure. He applied the method to in-situ stress data measured in the Kanto-Tokai area, where the active margin of three tectonic plates meet, and estimated the static flexure and the force causing the flexure. He found that the estimated flexure is consistent with the tectonics of the area and that it is caused by the subduction of the Philippine Sea plate. The present paper presents the solution to the problem of the reference state of stress which remained unsolved in the previous study. The previous results are reviewed in the light of additional information and the following conclusions are reached: 1. (1) The vertical cross-section of the estimated flexure shows a good resemblance to that of the vertical deformation of the Earths surface numerically estimated from a two-dimensional finite-element model which calculate the influence on the Japanese Islands of the drag along the interface between the islands and the descending oceanic plate. From a rough comparison, it is inferred that the accumulated drag on the interface is 1–2 km. 2. (2) The vertical movement in recent years around the Kanto-Tokai area shows a positive correlation with the estimated static flexure, suggesting that stress is presently accumulating.

Observations of Natural Ultramicro-Fracturings at Ikuno Geophysical Observation Station (1)

Since February 1985, we have observed natural ultramicro-fracturings occurring in an area of several tens square meters at Ikuno Geophysical Observation Station in Hyogo Prefecture. From the present observation, we have attempted to obtain an adequate idea of the mechanism of the anomalous crustal deformation: it was recorded only by the extensometer No. 4 at this station before and after the Yamasaki fault earthquake (115. 6; 4=30 km) on May 30, 1984. The Observation system was designed to detect an event occurring near the extensometer No. 4 with frequencies of 100 Hz-30 kHz and an amplitude larger than 30 gals. Another anomalous crustal deformation was recorded only by the same extensometer No. 4 in October-November 1985. The events that could be regarded as ultramicro-fracturings were observed in this period. The larger one had a predominant frequency of about 3 kHz and a maximum amplitude greater than 50 gals. This event was observed only by the sensor near the interface between rhyolite and tuff across which the extensometer No. 4 was installed. The other was observed by two sensors also near the interface. It can, therefore, be thought that the sources of these events are near this interface and that the anomalous crustal deformation and the natural ultramicro-fracturings are probably caused by local slips on this interface. This view, however, requires more datato be investigated and confirmed.