Yasuo Yabe

Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

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.

Twenty Thousand Aftershocks of a Very Small (M 2) Earthquake and Their Relation to the Mainshock Rupture and Geological Structures

We have determined the locations of more than 20,000 aftershocks (as small as moment magnitude Mw 4:4 or even smaller) following an M 2 event in a South African gold mine, using manually picked arrival times. Spatial clustering into fivegroupswasclearlydiscerned.Amajorityoftheaftershocksformedaplanarcluster (∼4 m in apparent thickness, ∼100 × 80 m in areal extent). This cluster is thought to delineate the rupture area of the mainshock because its orientation and spatial extent wereconsistent with thenodalplaneofthecentroid moment tensor (CMT)solution and withthecornerfrequencyofthemainshock,respectively.Theclustersattitudesuggests that the mainshock was a Mohr-Coulomb failure (or formation of a shear rupture sur- faceinintactrockatananglethatobeystheCoulombfailurecriterion)thattookplacein a vertical compression stress field that is indicated by borehole breakout patterns. The aftershock distribution also shows that the mainshock rupture was largely confined to the interior of a 25-m-thick vertical dike, although there are indications of interactions taking place between the rupture and the dikes material boundary with the host rock.

Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

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.

In-situ stress at the northern portion of the Chelungpu fault, Taiwan, estimated on boring cores recovered from a 2-km-deep hole of TCDP

We have investigated the depth variation in the stress state at the northern part of the Chelungpu fault, Taiwan, which slipped during the 1999 Chi-Chi earthquake (Mw 7.6). In-situ stress around the fault was estimated based on the stress memory of rocks recovered from five depths (739–1316 m) of a 2-km-deep-hole. The borehole intersects a fault zone at a depth of 1111 m (FZ1111), which is the best candidate for the Chelungpu fault. Our main results are: (1) the SHmax direction is parallel to the slip for the Chi-Chi earthquake and agrees with those of the local, regional, or tectonic scales estimated from various stress indicators; (2) significant aspects of the stress field are identical to those of paleostress; (3) the horizontal differential stress was significantly reduced just above FZ1111, which may be attributable to the existence of a significantly deformable zone at this depth.

Elastic property of damaged zone inferred from in-situ stresses and its role on the shear strength of faults

The Nojima fault in Hyogo prefecture, Japan, ruptured during the 1995 Hyogo-ken Nanbu earthquake (MJMA = 7.3). The stress measurements at sites close to this fault have revealed that the direction of the largest horizontal stress is almost perpendicular to the strike of this sub-vertical fault and that, in the zone within about 100 m from the fault core axis, the ratio of the largest shear stress to the normal stress is significantly small compared with that of the outside. It is thus the logical consequence that the principal stress outside the zone tends to direct perpendicularly to the fault plane. A model called the fracture process model is introduced for the relationship between fracture strength and elastic property of rocks. Making use of this model on the assumption that the observed shear stress equilibrates to the shear strength of damaged zone, it is found that the elastic wave velocities estimated from the stress well explain the observed velocities of damaged zone. This model suggests further that the friction coefficient of fault can be smaller than 0.15 due to the characteristic deformation of damaged zone and that the pressurized fluid is not essential for the formation of weak faults.

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.

Evolution of source characteristics of AE events during frictional sliding

The evolution of source characteristics of acoustic emission (AE) events and the surface topography of the fault surface during frictional sliding were investigated with the aim of improving current understanding of the microscopic process of friction. Both the AE source dimensions and the grain-scale topographies were well conserved against abrasion during the sliding, but the magnitude of stress drop was significantly reduced. The fractal-domain topographies, the wavelength of which is shorter than the fractal limit wavelength (λc), were significantly worn during the sliding. These results suggest that the grain-scale topography determined the AE source dimension, while the fractal-domain asperities controlled the magnitude of the stress drop. Since elastic wave radiation is one of the major energy consumption processes, the grain-scale and the fractal-domain topographies may play different roles, not only in the AE source process but also in the friction of rocks.

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.