关键词: 青藏高原, 正断层地震, 地壳减薄

Abstract:

The Qinghai-Xizang Plateau, known as the “Roof of the World,” is characterized by complex deformation mechanisms driven by the ongoing India-Eurasia collision. The deformation characteristics and mechanisms of the Qinghai-Xizang Plateau have long been a focus of attention in the field of geophysics, and many studies have been conducted both domestically and internationally to deeply explore its internal deformation structural features. Additionally, the frequent occurrence of normal-faulting earthquakes within the plateau has a significant impact on crustal deformation. However, previous research has not extensively explored the relationship between these earthquakes and the overall crustal deformation of the Tibetan plateau, particularly in terms of their quantitative contribution to regional crustal thinning. Previous studies have primarily focused on horizontal strain rates, leaving a gap in understanding how seismic activity vertically impacts crustal evolution. In this study, we aim to quantify the role of normal-faulting earthquakes in crustal thinning by integrating seismic catalogs, geodetic observations, and theoretical models. Our objectives include: 1)evaluating the spatiotemporal distribution of normal-faulting earthquakes and their cumulative seismic moment release; 2)developing refined methodologies to estimate crustal volume loss caused by these earthquakes; and 3)reconciling theoretical calculations with geodetic measurements to clarify their relative contributions to observed crustal thinning.

We combined a comprehensive earthquake catalog(2000-2024, M_W>5.0) from the Global Centroid Moment Tensor(GCMT) project with GPS-derived horizontal strain rate fields. Earthquakes were filtered using T-axis and P-axis dip angles (<50°) and manually verified to ensure normal-faulting mechanisms. Geodetic data from CMONOC and InSAR were utilized to derive horizontal strain rates, thus linking horizontal extension to vertical thinning. Two theoretical approaches were employed: 1)Seismic Moment Tensor Analysis: We calculated vertical strain rates using the cumulative seismic moment tensor's M_zz component, accounting for slip angles and excluding strike-slip contributions. 2)Coseismic Vertical Deformation Integration: We integrated vertical surface displacements over infinite and finite domains to estimate crustal volume loss.

Both methodologies yielded consistent results. Normal-faulting earthquakes contributed approximately 2.5×10^-9/a to the crustal thinning rate, accounting for ~30% of the GPS observation result(~7.5×10^-9/a). Including smaller-magnitude earthquakes(M_W<5.0) could increase this contribution to ~35%. The cumulative volume loss from 2000-2024 totaled 1.9×10⁹m³, driven predominantly by high-elevation(>4 200m) events. Earthquake depth showed no influence on crustal thinning due to compensating far-field uplift and near-field subsidence in an infinite domain. At the same time, the error is permissible in a finite domain. Spatial heterogeneity in strain rates highlighted stronger extension in southern Tibet, aligning with eastward crustal extrusion.

This study quantitatively demonstrates that normal-faulting earthquakes are a significant driver of crustal thinning in the Qinghai-Xizang Plateau, contributing ~30% to observed rates. The consistency between seismic moment and deformation integral methods underscores the robustness of our approach. Our findings bridge seismic activity with long-term tectonic processes, offering insights into plateau evolution and earthquake hazard assessment. Future work should incorporate smaller earthquakes, post-seismic relaxation, and erosional effects to refine these estimates.

Key words: Qinghai-Xizang Plateau, normal-faulting earthquakes, crustal thinning