Large Slip Rate Research Articles

Asymmetric Bilateral Rupture of the 2022 Ms 6.8 Luding Earthquake on a Continental Transform Fault, Tibetan Border, China

Abstract The Xianshuihe (XSH) fault on the eastern boundary of the Tibetan plateau is one of the most active faults in the world. However, the Moxi fault, the southernmost segment of the XSH fault, remained seismically quiescent for over 230 yr, with a large slip rate and high locking degree. Thus, this region is considered to be a seismic gap capable of hosting earthquakes of Mw 7.0 or greater. In September 2022, the Ms 6.8 Luding earthquake occurred on the Moxi fault, resulting in destructive landslide damage, with 93 people dead and 25 missing. Using regional and global seismic recordings, we integrate the multiple-point-source analysis, finite-fault inversions, and backprojection imaging to investigate in detail the rupture process associated with the 2022 Luding, China, earthquake. Our results show that this event is characterized by an asymmetric bilateral rupture with three episodes: (1) an initial bilateral rupture propagation mainly toward north-northwest; (2) major rupture propagating toward south-southeast and up-dip direction; (3) south-southeast-rupture propagation speed accelerates to ∼2 km/s. Over 70% of the seismic moment is released at shallow depths <12 km thus explaining significant damaging effects. Furthermore, the seismic potential of the remaining locked portions of the XSH and Anninghe faults is still high and positive increases of the Coulomb stress on these faults due to the Luding earthquake might bring them closer to future failure.

High‐Resolution Fault‐Rupture Imaging by Combining a Backprojection Method With Binarized MUSIC Spectral Image Calculation

AbstractBackprojection (BP) methods are widely used for estimating the fault rupture processes; however, they are inherently susceptible to noise. Hence, noise suppression is an important research target. In this paper, we develop a fault rupture imaging method by combining beamforming‐based BP and MUltiple Signal Classification (MUSIC), which realizes artificial noise suppression at a high spatial resolution. The stations are grouped into arrays according to the SH wave radiation coefficients, and MUSIC analysis is performed on each array. The MUSIC spectral images of these arrays are binarized and then multiplied by the BP images. Spatial filtering is also applied to the images based on the possible range of the rupture velocity and rise time. When tested using synthetic test data, the proposed method worked as expected. We then applied this method to the 2016 Kumamoto earthquake by interpolating the travel times from the observed travel times of relocated hypocenters using a 3‐D velocity structure model. In the area of large slip and slip rate approximately 30–50 km from the hypocenter on the Futagawa fault, the spatiotemporal evolution of the fault ruptures and waveform inversion results were generally in harmony. The distributions of the low‐ and high‐frequency seismic radiations are complementary, as is understood in the context of fault rupture physics. This method can aid in understanding and modeling the details of seismic radiation sources, enabling the accurate prediction of strong ground motion even in near‐fault areas.

Nucleation mechanism of the 2021 Mw 7.4 Maduo earthquake, NE Tibetan Plateau: Insights from seismic tomography and numerical modeling

The 2021 Mw 7.4 Maduo earthquake ruptured the Jiangcuo fault, a secondary fault within the Bayan Har block, NE Tibetan Plateau, which attests that large earthquakes can occur on faults with very low slip rates. In this study, we use data from local seismic stations to explore the 3D velocity structure beneath the source region of the Maduo earthquake. Our results indicate that there is a distinct low-velocity anomaly in the mid-lower crust, and the thickness of this low-velocity zone decreases sharply to the east. The Maduo earthquake occurred in the transition zone between the high- and low-velocity regions. Based on our local velocity model and our knowledge of the regional crustal structures, we constructed numerical models to investigate the stress and strain energy distributions in our study area. As indicated by the stress concentration, strain energy accumulation, and low strain rate in the shallow upper crust around the source region, we conclude that the seismic velocity heterogeneity and the weak ductile mid-lower crust provide a unique tectonic environment that facilitated the nucleation of the Maduo earthquake. This study helps to understand the mechanisms of the Maduo earthquake, and more importantly reconciles the seeming contradiction between large earthquakes and low slip and strain rates.

Fault locking of the Qilian–Haiyuan fault zone before the 2022 Menyuan Ms6.9 earthquake and its seismic hazards in the future

By using GPS-derived velocities of 2015–2021 and a negative dislocation program, we inverted the locking degree and slip rate deficit in the Qilian–Haiyuan fault zone, and combined with the distribution of small earthquakes in the fault, we studied the characteristics before the 2022 Menyuan MS6.9 earthquake and analyzed the future seismic hazards of each segment within this fault zone. The regional crustal deformation pattern is discussed with regard to the fault slip rate and regional strain rate field. The preliminary results show that before the earthquake, the seismogenic fault was strong locked, with a high locking depth, the slip rate deficit was large, and the distribution of small earthquakes was relatively few, these characteristics are closely related to the occurrence of strong earthquakes, according to the aftershock relocation results, further, it is believed that the earthquake may link the Lenglongling and Tuolaishan faults into a large strike-slip fault. The Jinqianghe fault, the Lenglongling fault, and the eastern segment of the Tuolaishan fault are strongly locked, with high locking depth and large slip rate deficit, combined with the occurrence of small earthquakes and the locking degree before the 2022 Menyuan MS6.9 earthquake, indicate that the eastern segment of the Tuolaishan fault is highly likely to have strong earthquakes in the future, which requires further attention. In addition, the strike-slip rate of the Qilian–Haiyuan fault zone is mainly between 3.9 and 4.3 mm/yr, the overall movement of the fault is consistent, and the compressional rate gradually decreases from 2.9 mm/yr in the western segment to 1 mm/yr in the eastern segment; the fault compressional rate may be related to the crustal shortening (formation basin and uplift mountain). Therefore, the present-day crustal deformation in the northeastern margin of the Tibetan Plateau is mainly distributed in the shortened region of the crust on the Qilian Shan area and left-lateral strike-slip localized on the Qilian–Haiyuan fault zone.

Deep Electrical Structure of the Langshan Mountain-Linhe Basin Area on the Northwest Edge of the Ordos Block, China

A series of fault depression structures have developed around the Ordos Block. The Langshan Mountain-Linhe Basin area (LLA), located on the northwest edge of the Ordos Block, is a typical, normal tension fault system. A geological survey shows that the Langshan Piedmont fault (LPF) in this area has a large slip rate and indicates risk of earthquake preparation. Broadband magnetotelluric (MT) exploration research was recently carried out across the LLA in the NW–SE direction, and the three-dimensional deep electrical structure thus obtained revealed that the LPF in the LLA is an evident electrical boundary zone on the whole crustal scale and is the main boundary fault of the primary structural block of the Alxa and Ordos Blocks. The MT results also show that the Linhe Basin and Ordos Block belong to the same tectonic basement. The Linhe and Dengkou faults belong to the internal faults of the Ordos Block. The upper crust of the Langshan Mountain on the west side of LPF is characterized by high-resistivity, the middle and lower crust have a low-resistivity layer, and the Linhe Basin on the east side has a Cenozoic low-resistivity sedimentary layer of approximately 10 km thick, which reveals that the Linhe Basin is a faulted basin with sedimentary thickness around the Ordos Block. This indicates that the LLA has experienced continuous and strong tension, normal fault depression sedimentary activities since the Cenozoic era. The current Global Positioning System velocity field shows that there is an apparent NW–SE acceleration zone in the LLA. The leveling data indicate that Linhe Basin shows a subsidence trend relative to the Ordos Block, indicating that the area is undergoing continuous NW–SE tension and faulting. It is speculated that there is a risk of earthquake preparation in the LPF.

Working Performance of the Low-Adhesion and Anti-Slip Bionic Press Roller in the Rice–Wheat Rotation Area

During the working process of a conventional press roller in the rice–wheat rotation area, the phenomenon of soil adhesion and a large slip rate, which are directly related to the growth and the output of the wheat, occurred widely. It was found that the geometric structure of the back surface was one of the key reasons for Carabus formosus nili exhibiting low adhesion against the soil. In this paper, lessons are drawn from the non-smooth morphology theory and bionics principle. The bionic press rollers with bionically convex hull structures were designed after learning from the geometric structure of Carabus formosus nili, which modifies the geometric structure of the conventional press roller. The three-dimensional simulation model of a bionic press roller and soil was constructed in the ANSYS/LS-DYNA software(ANSYS, Canonsburg, PA, USA) to simulate the dynamic process of the interaction between the bionic roller and soil and to verify the rationality of the design of the bionic press roller. Based on the simulation of the interaction between the bionic press roller and the soil, the main factors affecting the bionic roller were determined: forward speed, load, and structural parameters. The field tests were carried out, and the orthogonal test was used to investigate the effects of forward speed, load, and structural parameters on the operation performance of the bionic press roller. Through the orthogonal test, the primary and secondary order of forward speed, load, and axial spacing were obtained. The optimal combination was: forward speed 7 km/h, load 400 N, and axial spacing 40 mm. This study provides a new idea and reference for the design of a press roller for wheat sowing in the rice–wheat rotation area.

Coupled dynamic behaviours of the brake system considering wheel–rail interactions

ABSTRACT During vehicle operation, the brake system is influenced by wheel–rail interactions. To investigate the dynamic behaviours of the brake system, we propose a novel dynamic model of the brake system by considering the coupling effects between the disc–pad and the wheel–rail subsystems. Using the established model, the nonlinear coupled dynamic behaviours including wheel–rail adhesion characteristics, wheel–disc torsional vibrations and disc–pad stick–slip vibrations were comprehensively explored. The results showed that braking conditions remarkably influenced the longitudinal slip rate. The wheelset and brake units affect each other, and the wheels on one wheelset influence each other when they are asynchronous. The brake system is stable in the wheel–rail stick state, but becomes unstable in the wheel–rail stick–slip state or slip state. The limit cycle analysis showed that the brake pad stick–slip vibrations were easier to occur at a large slip rate.

The Influence of Depth‐Varying Elastic Properties of the Upper Plate on Megathrust Earthquake Rupture Dynamics and Tsunamigenesis

AbstractMegathrust earthquakes are strongly influenced by the elastic properties of rocks surrounding the fault. In contrast to friction, these properties can be derived in situ from geophysical measurements along the seismogenic zone. However, they are often overestimated in numerical simulations, particularly in the shallow megathrust. Here we explore the influence that realistic depth‐varying upper‐plate elastic properties along the megathrust have on earthquake rupture dynamics and tsunamigenesis using 3D dynamic rupture and tsunami simulations. We compare results from three subduction zone scenarios with homogeneous and heterogeneous elastic media, and bimaterial fault. Elastic properties in the heterogeneous model follow a realistic depth‐distribution derived from controlled‐source tomography models of subduction zones. We assume the same friction properties for all scenarios. Simulations in the heterogeneous and homogeneous models show that rigidity depth variations explain the depth‐varying behavior of slip, slip rate, frequency content, and rupture time. The depth‐varying behavior of slip, frequency content, and rupture duration quantitatively agree with previous predictions based on worldwide data compilations, explaining the main depth‐dependent traits of tsunami earthquakes and large shallow megathrust earthquakes. Large slip, slow rupture and slip rate amplification in bimaterial simulations are largely controlled by elastic rock properties of the most compliant side of the fault, which in subduction zones is the upper plate. Large shallow slip and trenchward increasing upper‐plate compliance of the heterogeneous model lead to the largest co‐seismic seafloor deformation and tsunami amplitude. This highlights the importance of considering realistic upper‐plate rigidity variations to properly assess the tsunamigenic potential of megathrust earthquakes.

Friction during earthquakes: 25 years of experimental studies

Dynamic fault strength τ (rock friction in the broad sense) and its evolution with seismic slip and slip rate are among the most relevant parameters in earthquake mechanics. Given the large slip rate (1 m s−1 on average), displacement (up to tens of meters), effective stress (tens of MPa), typical of seismic faulting at depth, thermo-mechanical effects become outstanding: dynamic fault strength is severely affected by fluid and rock phase changes, extreme grain size reduction, and the production of amorphous and unstable materials in the slipping zone. Here, first we will summarize the most relevant findings about dynamic fault strength during seismic slip mainly obtained thanks to the exploitation of dedicated experimental machines (i.e., rotary shear apparatus). However, the interpretation of this experimental dataset remains debated because of technical limitations which impede us to measure fundamental parameters such as temperature, strain rate, pore fluid pressure and grain size in the slipping zone. Without a sound estimate of these physical parameters, any constitutive law proposed to describe the evolution of dynamic fault strength during simulated seismic slip remains speculative. Then, we will discuss the results of some recent experiments which exploit new technical approaches to overcome the main limitations of the previous studies. The experimental approach, together with field studies of the geometry and architecture of exhumed faults and modelling, remains our most powerful tool to investigate seismic-related deformation mechanisms in both natural and human-induced earthquakes.

The relationship between synsedimentary fault activity and reservoir quality — A case study of the Ek1 formation in the Wang Guantun area, China

The activity of synsedimentary faults plays an important role in controlling the distribution of sand bodies in basins and furthermore the porosity and permeability of reservoirs. We have used fault interpretation, the method of image and granularity size analysis, and the seismic pumping effect to investigate the control of the activity of the Kongdong fault on the development degree of the dissolution pores and grain size, further studying the controlling mechanism of the activity of synsedimentary faults on reservoir quality (porosity and permeability). The results showed that the slip rate of synsedimentary faults is one of the main factors in controlling reservoir quality. The slip rate controls the accommodation space and hydrodynamic conditions and it furthermore controls the grain size. The higher the slip rate, the bigger the grain size in the downthrow wall of synsedimentary faults; the seismic pump produced by synsedimentary faults activity also controls the development degree of dissolution pores. The development degree of dissolution pores in the downthrown wall of synsedimentary faults is greater than that in the upthrown wall. Dissolution pores are more developed in areas with a large slip rate of synsedimentary faults. Porosity increases gradually with the increase of plane porosity of dissolution pores, whereas the changes of permeability are not obvious.

Robust Variable Structure Anti-Slip Control Method of a Distributed Drive Electric Vehicle

Slip rate control is important in improving vehicle stability and driving efficiency. In this paper, a robust slip rate control system is designed for distributed drive electric vehicles that consists of two slip rate estimators for multi-driving conditions, a vehicle speed estimator, and an anti-windup robust variable structure slip rate tracking controller. Because there is no driven wheel in a four-in-wheel-motor distributed drive electric vehicle, the estimators for small and large slip rates are designed based on dynamic and kinematic methods, respectively, which can switch according to the slip conditions. The convergence of the estimation error is discussed with the Lyapunov stability law and is less than 2% under the condition of acceleration on a low-friction road. The slip rate tracking controller is designed based on the sliding mode control law and the proportional-integral (PI) control method to handle model nonlinearity, modelling and estimation errors, and disturbances and to control the input chattering and saturation. The asymptotic stability of the tracking error is proven by Lyapunov theory. A joint control variable composed of the wheel angular acceleration and slip rate is designed to improve the robustness of the controller against the slip rate estimation error. Simulations and experiments under various conditions are performed to verify the proposed anti-slip control method. The results show that compared with a distributed drive vehicle without a slip rate controller, the controlled vehicle can prevent serious wheel skid on low-adhesion roads and improve the driving performance.

Geological and geomorphic evidence for activity of the Mengzi fault along the southeastern margin of the Tibetan Plateau

Few studies have been conducted on the activity of the Mengzi fault system, which is located to the southeast of the Sichuan-Yunnan block. The Mengzi fault is distributed along the eastern margin of the Mengzi basin. However, little is known about the location, length, and activity of the fault or the occurrence of paleo-earthquakes during the late Quaternary. This paper studies the geometric distribution and activity of the fault via remote sensing imagery interpretation, fault outcrops in the area, faulted landforms and chronological analyses. The results show that the Mengzi fault extends from Nandong (Kaiyuan basin) in the north through the eastern margin of the Mengzi basin to Pingbian in the southeast, eventually intersecting with the Honghe fault. The Mengzi fault is approximately 250 km long and is characterized as a reverse strike-slip fault. We survey and calculate the offset gullies and terraces dislocated by the fault; combined with sample dating, we calculate the horizontal slip rate of the fault since 30 ka BP to be 0.91–1.56 mm/a. Three paleo-events since 30–40 ka BP are observed with a 10-ka major earthquake recurrence period. We analyze the relationships among the Mengzi fault, southern Xiaojiang fault, and Qujiang-Jianshui fault and note a slip rate loss of ≥8 mm/a from the southern Xiaojiang fault that was absorbed by reverse dextral strike-slip faulting and lateral shortening of the Qujiang-Jianshui fault and the Mengzi fault. The Mengzi fault plays an important role in the adjustment of the large slip rate reduction along the southern Xiaojiang fault. Moreover, the Mengzi fault has a similar tectonic position to the seismogenic faults of the 2008 Wenchuan earthquake, 2013 Lushan earthquake and 2014 Ludian earthquake, and the Mengzi fault and southern Xiaojiang fault both accumulate the energy generated by the southeastward movement of the Sichuan-Yunnan block. Studies on the Mengzi fault are helpful for renewing our understanding of the tectonic patterns of the southeastern Sichuan-Yunnan block and delimiting the boundary of the southeastern block margin. Furthermore, because the cities of Mengzi and Kaiyuan are located along the Mengzi fault, this study has practical significance for earthquake disaster mitigation.

Development of autonomous navigation system for rice transplanter

Rice transplanting requires the operator to manipulate the rice transplanter in straight trajectories. Various markers are proposed to help experienced drivers in keeping straightforward and parallel to the previous path, which are extremely boring in terms of large-scale fields. The objective of this research was to develop an autonomous navigation system that automatically guided a rice transplanter working along predetermined paths in the field. The rice transplanter used in this research was commercially available and originally manually-operated. An automatic manipulating system was developed instead of manual functions including steering, stop, going forward and reverse. A sensor fusion algorithm was adopted to integrate measurements of the Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS) and Inertial Measurement Unit (IMU), and calculate the absolute moving direction under the UTM coordinate system. A headland turning control method was proposed to ensure a robust turning process considering that the rice transplanter featured a small turning radius and a relatively large slip rate at extreme steering angles. Experiments were designed and conducted to verify the performance of the newly developed autonomous navigation system. Results showed that both lateral and heading errors were less than 8 cm and 3 degrees, respectively, in terms of following straight paths. And headland turns were robustly executed according to the required pattern. Keywords: autonomous navigation, rice transplanter, sensor fusion, headland turning DOI: 10.25165/j.ijabe.20181106.3023 Citation: Yin X, Du J, Noguchi N, Yang T X, Jin C Q. Development of autonomous navigation system for rice transplanter. Int J Agric & Biol Eng, 2018; 11(6): 89–94.

Development of an automatically guided rice transplanter using RTK-GNSS and IMU

The objective of this research was to develop an automatically guided rice transplanter by using RTK-GNSS and IMU as navigation sensors. The rice transplanter used in this research was commercially available and manually-operated. Automatic operation mechanisms were developed instead of manual functions including turning, stop, going forward and reverse. Sensor fusion was used to integrate measurements by sensors and calculate the absolute heading direction of the rice transplanter. A headland turning method was proposed to ensure the turning process in considering that a relatively large slip rate at extreme steering angles. Experiments were conducted to verify the performance of the automatically guided rice transplanter. Results showed that lateral and heading errors were less than 10 cm and 5 degrees, respectively, in terms of straight paths. And headland turns were automatically executed according to the desired pattern.

Transient switching control strategy from regenerative braking to anti-lock braking with a semi-brake-by-wire system

Regenerative braking is an important technology in improving fuel economy of an electric vehicle (EV). However, additional motor braking will change the dynamic characteristics of the vehicle, leading to braking instability, especially when the anti-lock braking system (ABS) is triggered. In this paper, a novel semi-brake-by-wire system, without the use of a pedal simulator and fail-safe device, is proposed. In order to compensate for the hysteretic characteristics of the designed brake system while ensure braking reliability and fuel economy when the ABS is triggered, a novel switching compensation control strategy using sliding mode control is brought forward. The proposed strategy converts the complex coupling braking process into independent control of hydraulic braking and regenerative braking, through which a balance between braking performance, braking reliability, braking safety and fuel economy is achieved. Simulation results show that the proposed strategy is effective and adaptable in different road conditions while the large wheel slip rate is triggered during a regenerative braking course. The research provides a new possibility of low-cost equipment and better control performance for the regenerative braking in the EV and the hybrid EV.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.

Slow Slip Transients Before the 2011 Tohoku-Oki Earthquake

I review a spatiotemporal evolution of slow-slip transients on the plate interface of the subducting Pacific plate that happened in and around the mainshock rupture area prior to the 2011 Tohoku-Oki earthquake. Based on foreshock activity before the mainshock, two sequences of slow-slip transients were identified by earthquake migrations toward the initiation point of the mainshock rupture. These two slow-slip transients were also detected by geodetic measurement. The second sequence of slow-slip transients, which involved large slip rates, may have caused significant stress loading onto the hypocenter of the mainshock and prompted the initiation of unstable dynamic rupture. In addition, decadal slip-behavior on the plate interface revealed by geodetic measurement and small repeating earthquakes show that slow-slip transients occurred in the down-dip and up-dip portions of the mainshock rupture area. These slow-slip transients imply the reduction of coupling between the subducting and overlying plates, that could be interpreted as the late stage of mega-thrust earthquake cycle, although this notion remains conjectural.

An episodic creep-slip event detected by precise levelling surveys in the central part of the Longitudinal Valley Fault, eastern Taiwan, in 2011–2012

Abstract Precise levelling surveys were conducted across the central Longitudinal Valley Fault, eastern Taiwan, to understand the deformation of the transition zone between the stable fault creep area and the asperity area. In order to investigate the surface relationship between the fault creep area and the geological condition of the transition zone, we established levelling routes in the Yuli, Chike-san, and Reishuei areas. The Yuli area forms the geological boundary of the Lichi Melange Formation, which is composed of chaotic mudstones containing numerous exotic blocks of various sizes and lithologies. Along the Yuli route, located on the Lichi Melange, an uplift rate of 30 mm/yr was detected during the period 2010–2012, suggesting that aseismic fault creep might be continuing with long-term stability. Along the Chike-san route, a vertical deformation rate of 8 mm/yr was detected in the period 2010–2011. However, a large deformation with an uplift rate of 40 mm/yr was detected in the period 2011–2012. Along the Reisuei route, we detected a deformation of 8 mm/yr in the period 2011–2012. A two-dimensional single-fault model was developed to discuss the slip distributions in the periods 2010–2011 and 2011–2012. Relatively large slip rates were estimated at two parts of the fault plane—one at a depth of ~ 1.5 km and another at a depth of ~ 4 km—in both periods. Because both parts of the fault plane show approximately the same slip distribution, we believe that the detected deformation resulted from an episodic acceleration event of creeping slip. The northern limit of the stable creep area may be the Yuli area. The episodic creep event occurred in the transition zone between the stable fault creep area and the asperity area. The boundary between the stable creep area and the episodic creep area is consistent with the geological boundary of the Lichi Melange Formation.

Propagation of Slow Slip Leading Up to the 2011 M w 9.0 Tohoku-Oki Earthquake

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 (M(w)) 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.

東海地域下の三次元地震波減衰構造

Long term slow slip (LTSS) and non-volcanic low frequency earthquakes (LFEs) were reported in the central part of the Tokai district, central Japan. Such LTSS and LFE events are considered to take place at a transition zone of frictional property from stick-slip to stable sliding on the top of subducting Philippine Sea plate. To clarify the spatial variation of physical properties in this region, we estimated a three dimensional seismic attenuation structure using a joint inversion method. In the shallow depths from the surface to 5km, we found a lower Q zone located along the Median Tectonic Line which divides the southwestern Japan into two parts; an old geologic belt and a new accretionary belt. In the lower crust of the land plate at the depths of 17 to 25km, a very high Q zone (about 2000) exists just above the region where large slip rates were observed during the LTSS between 2001 and 2005. Since very few earthquakes occur in this high Q zone, that portion might consist of harder rocks than surroundings. On the contrary, the region just beneath the large slip zone has lower Q values than those of surrounding area. Comparing our results with a seismic velocity structure derived from travel time tomography, we found the high Q zone approximately coincides with a zone of relatively high velocities and the lower Q zone corresponds to a zone of relatively low velocities and high VP/VS values. A low Q zone with low velocities and high VP/VS can be interpreted as the zone which involves high-pressure fluid. Probably the high Q zone above the large slip zone works as a cap rock and prevents the fluid from moving toward the shallow part, and then the fluid pressure becomes high and it affects the occurrence of slow slip in this region.