Fluids Triggered the 2021 Mw 6.1 Yangbi Earthquake at an Unmapped Fault: Implications for the Tectonics at the Northern End of the Red River Fault

On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

A Numerical Method for the Design of the U-Shaped Segmental Tunnel Lining under the Impact of Earthquakes: A Case Study of a Tunnel in the Hanoi Metro System

Circular tunnels are usually encountered when excavation tunnel. However, the U-shapedtunnel lining is used a lot in practice because of it’s advantages. However, there are not many studies inthe world to calculate and design for underground structures with U-shaped tunnel lining, especially inthe case of tunnels being affected by earthquakes. This paper proposes a new numerical-HRM methodapproach for the analysis of U-shaped segmental tunnel lining under the impact of earthquakes. Hanoi isthe capital of Vietnam, this is a big city with more than 8 million people. Hanoi is located between twomajor fault systems, the Red River fault system and the Son La-Dien Bien-Lai Chau fault system.Therefore, the Hanoi area is assessed as likely to be affected by earthquakes of magnitude Mw = 6.1 up to6.5 Richter. The Hanoi metro system is constructed by TBM and the U-shaped segmental tunnel lining isalso one of the types of tunnel lining considered for use in the construction of metro tunnels in Hanoi. Theimproved HRM method has been used to investigate the effect of joints in the tunnel lining from the Hanoisystem metro under the impact of earthquakes is conducted considering from the results of the tunnellining behavior in terms of bending moment (M), normal forces (N) and tunnel lining displacements (δn)in both cases: the U-shaped continuous tunnel lining and the U-shaped segmental tunnel lining.

Massive lithospheric delamination in southeastern Tibet facilitating continental extrusion

Significant left-lateral movement along the Ailao Shan-Red River fault accommodated a substantial amount of the late Eocene to early Miocene India-Asia convergence. However, the activation of this critical strike-slip fault remains poorly understood. Here, we show key seismic evidence for the occurrence of massive lithospheric delamination in southeastern Tibet. Our novel observation of reflected body waves (e.g. P410P and P660P) retrieved from ambient noise interferometry sheds new light on the massive foundered lithosphere currently near the bottom of the mantle transition zone beneath southeastern Tibet. By integrating the novel seismic and pre-existing geochemical observations, we highlight a linkage between massive lithospheric delamination shortly after the onset of hard collision and activation of continental extrusion along the Ailao Shan-Red River fault. This information provides critical insights into the early-stage evolution of the India-Asia collision in southeastern Tibet, which has significant implications for continental collision and its intracontinental response.

Initiation of Clockwise Rotation and Eastward Transport of Southeastern Tibet Inferred from Deflected Fault Traces and GPS Observations

Eastward transport and clockwise rotation of crust around the southeastern margin of the Tibetan Plateau dominates active deformation east of the Eastern Himalayan Syntaxis. Current crustal movement inferred from GPS measurements indicates ongoing distortion of the traces of the active Red River fault and the Mesozoic Yalong-Yulong-Longmen Shan thrust belt. By extrapolating current rates back in time, we infer that this pattern of deformation developed since 10.1 ± 1.5 Ma. This date of initiation is approximately synchronous with a suite of tectonic phenomena, both near and far, within the wide Eurasia/Indian collision zone, including the initiation of slip on the Ganzi-Yushu-Xianshuihe fault and crustal thinning and E-W extension by normal faulting on N-S−trending rifts in the plateau interior. Accordingly, the eastward movement of eastern Tibet and the clockwise rotation of that material seem to be local manifestations of a larger geodynamic event at ca. 10−15 Ma that changed the kinematic style and reorganized deformation not only on the plateau-wide scale, but across the entire region affected by the India/Eurasia collision. Convective removal of some or all of Tibet’s mantle lithosphere seems to offer the simplest mechanism for these approximately simultaneous changes.

Identification of Deep Tectonic Structures of the Pho Lu area, northwestern Vietnam using Digital Elevation Model and Earth focal mechanism

The digital elevation model and the earthquake focal mechanism are utilized to define the geological structure of the Pho Lu area, northwestern Vietnam. The results allow the identification of lineaments and recognition of the correlation between the lineaments and geological structures directed in the study area. The digital elevation model (DEM) was used in the methodology of interpretation trends of lineaments derived from various enhancing techniques to show that the most lineament trend in the NW‒ SE direction. Further more, the interpreted lineament map demonstrates the NW‒SE system is correlated with the Red River fault zone, which is interpreted as a positive flower structure combined with the focal mechanism of earthquake. The results also demonstrate the capacity to used the digital elevation model and focal mechanism of the earthquake to identify deep geological structures.

A local lithospheric structure model for Vietnam derived from a high-resolution gravimetric geoid

High-resolution Moho and lithosphere–asthenosphere boundary depth models for Vietnam and its surrounding areas are determined based on a recently released geoid model constructed from surface and satellite gravity data (GEOID_LSC_C model) and on 3ʹʹ resolution topography data (mixed SRTM model). A linear density gradient for the crust and a temperature-dependent density for the lithospheric mantle were used to determine the lithospheric structure under the assumption of local isostasy. In a first step, the impact of correcting elevation data from sedimentary basins to estimate Moho depth has been evaluated using CRUST1.0 model. Results obtained from a test area where seismic data are available, which demonstrated that the sedimentary effect should be considered before the inversion process. The geoid height and elevation-corrected sedimentary layer were filtered to remove signals originating below the lithosphere. The resulting Moho and lithosphere–asthenosphere boundary depth models computed at 1ʹ resolution were evaluated against seismic data as well as global and local lithospheric models available in the study region. These comparisons indicate a consistency of our Moho depth estimation with the seismic data within 1.5 km in standard deviation for the whole Vietnam. This new Moho depth model for the study region represents a significant improvement over the global models CRUST1.0 and GEMMA, which have standard deviations of 3.2 and 3.3 km, respectively, when compared to the seismic data. Even if a detailed geological interpretation of the results is out of scope of this paper, a joint analysis of the obtained models with the high-resolution Bouguer gravity anomaly is finally discussed in terms of the main geological patterns of the study region. The high resolution of our Moho and lithosphere–asthenosphere boundary depth models contribute to better constrain the lithospheric structure as well as tectonic and geodynamic processes of this region. The differences in Moho depth visible in the northeast and southwest sides of the Red River Fault Zone confirmed that the Red River Fault Zone may be considered the boundary between two continental blocks: South China and Indochina blocks. However, no remarkable differences in lithosphere–asthenosphere boundary depth were obtained from our results. This suggests that the Red River Fault Zone developed within the crust and remained a crustal fault.