logo
ResearchBunny Logo
Tomography of the source zone of the great 2011 Tohoku earthquake

Earth Sciences

Tomography of the source zone of the great 2011 Tohoku earthquake

Y. Hua, D. Zhao, et al.

This groundbreaking research by Yuanyuan Hua, Dapeng Zhao, Genti Toyokuni, and Yixian Xu reveals critical insights into the megathrust zone beneath the Tohoku forearc, shedding light on the mechanisms behind the 2011 Tohoku-oki earthquake. Discover how coseismic slip initiated at the boundary of contrasting seismic velocities and the implications of a shallow low-velocity anomaly.

00:00
Playback language: English
Introduction
The 2011 Tohoku-oki earthquake (Mw 9.0) caused a devastating tsunami and significant damage. Its mechanism and rupture process remain poorly understood due to a lack of permanent near-field observations. Previous research proposed various models to explain the large coseismic slip near the Japan trench, including the abundance of weak clay (smectite) and fault weakening. However, inconsistencies persist. Understanding the structural variations along the plate boundary fault is crucial for comprehending the rupture process. The Japan Trench Fast Drilling Project (JFAST) suggested that weak clay might be responsible for the large shallow slip. Conversely, Bassett et al. (2016) proposed that the overriding Okhotsk plate's structure and frictional properties influence interplate coupling and seismogenic behavior. Nishikawa et al. (2019) suggested that variations in pore fluids and lithology affect slip behavior. Previous tomographic studies provided insights but lacked sufficient spatial resolution in the large-slip area near the Japan trench due to limited seafloor observations. The recent deployment of the Seafloor Observation Network for Earthquakes and Tsunamis (S-net) offers an opportunity to improve our understanding. This study utilizes high-quality arrival-time data from S-net and Hi-net to create a detailed 3-D seismic velocity structure model of the Tohoku forearc, aiming to shed light on the 2011 Tohoku-oki earthquake's mechanism and rupture process.
Literature Review
Several studies have attempted to explain the unusual characteristics of the 2011 Tohoku-oki earthquake. The large coseismic slip near the trench has been attributed to various factors, including the presence of weak clay minerals like smectite, which exhibit low friction and shear strength (Chester et al., 2013; Fulton et al., 2013; Ujiie et al., 2013). The role of dynamic weakening processes, such as thermal pressurization and powder lubrication, has also been investigated (Noda & Lapusta, 2013; Reches & Lockner, 2010). Depth-varying frictional models have been proposed to explain the observed distribution of high-frequency energy radiation (Lay et al., 2012; Satriano et al., 2014; Yao et al., 2013). However, the lack of near-field observations has made it challenging to definitively determine the precise mechanisms involved. Previous tomographic studies have contributed to our understanding of the structural variations in the megathrust zone, but they often lack the spatial resolution needed to fully characterize the near-trench area (Zhao et al., 2011; Huang & Zhao, 2013; Liu & Zhao, 2018). Bassett et al. (2016) suggested that the overriding plate's structure plays a significant role in controlling coseismic slip, while Nishikawa et al. (2019) highlighted the influence of along-strike variations in pore fluids and lithology on slip behavior.
Methodology
This study employs high-quality P-wave arrival-time data from 480 Hi-net stations and 120 S-net ocean bottom seismometer (OBS) stations, covering the period from June 2002 to December 2018. A total of 109,890 P-wave arrival times from 3757 well-located local earthquakes (uncertainty <3 km) were used in the tomographic inversion. The tomographic method of Zhao et al. (1992, 2012) was used, incorporating well-determined geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific plate (UBP). A 3-D grid with a lateral interval of 0.33° and a vertical interval of 5 km around the UBP was used to represent the 3-D P-wave velocity (Vp) structure. Vp perturbations at the grid nodes were treated as unknown parameters, with the Vp at any point calculated by linear interpolation. A 3-D starting Vp model was constructed using results from active-source seismic surveys of the Tohoku forearc region. An efficient 3-D ray tracing technique, considering station elevations and surface topography, was used to calculate theoretical travel times and ray paths. A least-squares method with damping and smoothing regularizations was applied to solve the system of observation equations. Local earthquakes were relocated during the inversion process. After obtaining the 3-D Vp model, a 2-D Vp model for the overriding Okhotsk plate was determined using a method similar to that of Liu and Zhao (2018). Residual Vp images were calculated using a spectral averaging method to compare the results with residual topography and gravity data. Resolution tests, including checkerboard resolution tests and synthetic tests, were conducted to assess the spatial resolution and robustness of the tomographic model. The tests indicated a spatial resolution of approximately 33 km laterally and 5 km in depth around the UBP.
Key Findings
The 3-D P-wave velocity tomography model reveals striking along-trench and along-dip Vp variations around the Tohoku megathrust. At depths greater than 20 km, the model is consistent with previous tomographic results and residual topography and gravity distributions. A high-velocity anomaly is observed from the forearc segment boundary (FSB) proposed by Bassett et al. (2016) to approximately 39°N, corresponding to high residual topography and positive gravity anomalies. North of 39°N, a low-velocity anomaly is present, corresponding to a low-seismicity region. At shallow depths (<20 km), a prominent low-velocity anomaly is observed from the 2011 mainshock epicenter to the Japan trench, coinciding with the area of large coseismic slip and lack of high-frequency radiation. The mainshock hypocenter is situated at the boundary between the down-dip high-velocity anomaly and the up-dip low-velocity anomaly. The low-velocity anomaly near the Japan trench correlates strongly with the largest coseismic slip area. In contrast, coseismic slip areas of larger megathrust earthquakes (M ≥ 7.0) that occurred before 2011 were mainly located in deeper high-velocity and high-Q zones. The study suggests that the low-velocity anomaly at shallow depths reflects low-rigidity materials, possibly subducted sediments and increased dynamic pore-fluid pressure, leading to weak high-frequency radiation during slip. The higher resolution of this tomographic model, enabled by the S-net data, allows for a more complete understanding of the along-dip velocity variations and their correlation with energy radiation and fault properties.
Discussion
The high-resolution tomography model provides new insights into the seismogenesis and seismotectonics of the Tohoku forearc, particularly the source zone structure of the 2011 Tohoku-oki earthquake. The correlation between the low-velocity anomaly and the large slip, weak high-frequency radiation area near the Japan trench is significant. The rupture initiation at the boundary between high and low-velocity anomalies suggests a role for the low-rigidity materials in facilitating the large slip. The strong high-frequency radiation from the deeper high-velocity anomaly supports previous findings. The along-dip Vp variation is consistent with depth-dependent frictional models, indicating a potential link between velocity variations and lithologic transitions in the upper plate. The postseismic displacement rates further corroborate the model, with landward movement in the low-velocity anomaly area and trench-ward movement in the high-velocity anomaly area. The differences may be attributed to variations in friction, material properties, and lithologic transitions in the upper plate. The study highlights the crucial role of near-field seafloor observations in improving our understanding of megathrust earthquake mechanisms and seismic hazard assessment.
Conclusion
This study's high-resolution tomography reveals a strong correlation between low-velocity anomalies at shallow depths and areas of large slip and weak high-frequency radiation during the 2011 Tohoku-oki earthquake. The rupture initiated at the boundary between high and low-velocity zones, highlighting the influence of weak materials on the earthquake's characteristics. The findings improve our understanding of megathrust earthquake seismogenesis and the importance of near-field observations for seismic hazard assessment. Future research could focus on integrating this tomographic model with detailed geological and geotechnical data to further refine our understanding of the fault zone's physical properties and rupture dynamics.
Limitations
While the high-resolution tomography significantly advances our understanding, several limitations should be considered. The model's spatial resolution is limited, particularly at shallower depths. The interpretation of low-velocity anomalies as low-rigidity materials requires further validation through independent methods. The effects of post-seismic processes on the velocity structure are also not fully accounted for, though the authors argue that the influence is minimal. Further research incorporating other geophysical datasets and detailed geological information could help to mitigate these limitations.
Listen, Learn & Level Up
Over 10,000 hours of research content in 25+ fields, available in 12+ languages.
No more digging through PDFs, just hit play and absorb the world's latest research in your language, on your time.
listen to research audio papers with researchbunny