Toshihiro Igarashi

Propagation of slow slip leading up to the 2011 mw 9.0 tohoku-oki earthquake

Before Tohoku-Oki Recordings by Japans dense seismic network in the days and weeks before the 2011 Mw 9.0 Tohoku-Oki earthquake provide an opportunity to interrogate what caused the dynamic rupture of one of the largest earthquakes on record. Using a method to extract small earthquakes that are often obscured by overlapping seismic waves, Kato et al. (p. 705, published online 19 January) identified over a thousand small repeating earthquakes that migrated slowly toward the hypocenter of the main rupture. Based on the properties of these foreshocks, the plate interface experienced two sequences of slow slip, the second of which probably contributed a substantial amount of stress and may have initiated the nucleation of the main shock. Two sequences of slow slip preceded and migrated toward the main rupture. Many large earthquakes are preceded by one or more foreshocks, but it is unclear how these foreshocks relate to the nucleation process of the mainshock. On the basis of an earthquake catalog created using a waveform correlation technique, we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki earthquake in Japan. The time history of quasi-static slip along the plate interface, based on small repeating earthquakes that were part of the migrating seismicity, suggests that two sequences involved slow-slip transients propagating toward the initial rupture point. The second sequence, which involved large slip rates, may have caused substantial stress loading, prompting the unstable dynamic rupture of the mainshock.

Repeating earthquakes and interplate aseismic slip in the northeastern Japan subduction zone

[1] On the basis of a waveform similarity analysis, we detected 321 earthquake clusters with very similar (cross-correlation coefficient >0.95) waveforms on the plate boundary in the northeastern Japan subduction zone. Most of them were not found within the subducting Pacific plate with a few exceptions. Moreover, even on the plate boundary, they were not located in the large moment release areas of large interplate earthquakes that occurred recently or in the areas where the plates are inferred to be strongly coupled from GPS data analyses. These observations suggest that these similar earthquakes are caused by repeating slips of small asperities with a dimension of around 0.1 to 1 km surrounded by stable sliding areas on the plate boundary. If the aseismic slip portion in these small asperities is negligible, we can estimate the cumulative amount of aseismic slip in the area surrounding each asperity. In other words, repeating earthquake data potentially can be used to estimate the spatiotemporal aseismic slip distribution on the plate boundary. We estimated the spatial distribution of slip rate on the plate boundary from repeating earthquake data. The scaling relation between seismic moment and seismic slip by Nadeau and Johnson [1998] is used for the estimation of the slip amount by each repeating earthquake cluster. Obtained spatial distribution is consistent with that estimated from GPS data on land.

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low-angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT-type events. Almost all the earthquake clusters are composed of LT-type events. The western limit of the region where LT-type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double-planed deep seismic zone, which is characterized mainly by DC-type event. These NF-type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double-planed but triple-planed, even beneath the land area, which cannot be explained by any simple models.

Repeating earthquakes and quasi-static slip on the plate boundary east off northern Honshu, Japan

We have investigated spatio-temporal variation in small repeating earthquake activity in the 1989 earthquake swarm and used them to infer quasi-static slip distribution on the plate boundary off Sanriku, northern Honshu, Japan. Seismicity and inferred quasi-static slip accelerations propagated to the west and to the south during the swarm activity to trigger the occurrence of the largest earthquake (M7.1). To explain the migration of the seismicity and inferred quasi-static slip acceleration, we propose a conceptual model named the ‘chain reaction model’ in which large earthquakes generate large afterslips and then the afterslips accelerate the ruptures of the nearby asperities to generate the next earthquakes which are also followed by large afterslips, and so on. The model is applicable to a plate boundary where asperities are located sparsely but the afterslip associated with the rupture of an asperity can reach nearby asperities. Aftershock area expansion, which is conspicuous off Sanriku, is also explained by the model. If we can evaluate the slip deficit of a large asperity correctly, we will be able to issue a warning of large earthquake occurrence in some areas when we detect the acceleration of quasi-static slip near the asperity although the prediction is inevitably probabilistic.

Spatial changes of inter‐plate coupling inferred from sequences of small repeating earthquakes in Japan

[1] We extract sequences of small repeating earthquakes to clarify inter-plate coupling of subducting plates over a large area of the Japanese Islands. As a result, many sequences arc detected at the Philippine Sea plate subducting from the Ryukyu trench and Pacific plate subducting from the Kuril-Japan trench. The average slip-rates and standard deviations estimated from the sequences show substantial spatial changes of inter-plate coupling. The large deviations of slip-rates correspond to the occurrence of episodic slips such as after-slips following large earthquakes. Constant slip-rates approaching the relative plate motion indicate weak coupling areas. Slip deficits and sparse distributions of repeating groups suggest locked areas. In the Nankai trough, deep low-frequency earthquakes in the transition zone and burst-type repeating sequences within plates have not been located in the downdip direction of groups with slow slip-rates. This suggests that the space-time characteristics of inter-plate coupling affected these seismic events.

Imaging heterogeneous velocity structures and complex aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake by a dense seismic observation

The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-Vp body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high-Vp body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.

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.

Detection of a hidden Boso slow slip event immediately after the 2011 Mw 9.0 Tohoku‐Oki earthquake, Japan

Utilizing a cross-correlation detector technique, we discovered an increase in swarm-like seismicity within the source area of the Boso slow slip events (SSEs) immediately after the 2011 Tohoku-Oki earthquake. The epicentral distribution of the detected seismicity was similar to that of previously recognized Boso SSEs. In addition, small repeating earthquakes were identified within this seismic swarm sequence. These seismic observations indicate that a hidden SSE occurred along the top surface of the subducting Philippine Sea plate immediately after the Tohoku-Oki earthquake. We propose that external stress transfer by the coseismic slip of the Tohoku-Oki earthquake and the following afterslip could have led to the occurrence of the newly detected Boso SSE. In contrast to previous work, we demonstrate that the recurrence interval of the Boso SSEs has not shortened over time but has shown a more complex evolution as a result of external stress perturbations imposed by the Tohoku-Oki earthquake.

Three-dimensional velocity structure in the source region of the Noto Hanto Earthquake in 2007 imaged by a dense seismic observation

The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P-wave velocity (Vp) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-Vp body of the hanging wall has a relatively high Vp/Vs ratio. Conversely, the low-Vp body in the footwall appears to have a low Vp/Vs ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low Vp and high Vp/Vs are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low-Vp and low-Vp/Vs zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.