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Retrospective and Near-fault Constraints for High-resolution Earthquake Relocation

Description: 

Accurate earthquake locations are fundamental for resolving fault geometry, rupture processes, and earthquake physics. Although improved seismic network coverage and double-difference relocation techniques have substantially enhanced location precision, two challenges remain: improving earthquake locations during periods of sparse network coverage, and robustly constraining mainshock hypocenters as they were in short of S picks and cross-correlation constrained differential times.

We present a set of practical strategies for earthquake location refinement, including a retrospective relocation framework, grid-search-based relative relocation for mainshocks, and hypocenter depth refinement using near-epicentral observations. These methods are applied to the Weiyuan shale gas field, where seismic monitoring evolved from a sparse network with ~27 km station spacing during 2015–2018 to a dense near-fault network with ~5 km spacing since 2019. By first relocating earthquakes using the dense network and then retrospectively constraining earlier events, location uncertainties in the sparse-network stage are reduced from kilometer-scale to <100 m, representing an improvement of approximately one order. We further implement a grid-search relative location approach to refine the largest two M5 earthquakes, which significantly improved earthquake locations that were biased during double-difference relocation.

We further examine how station distance affects mainshock depth determination using the 2008 M_w 7.9 Wenchuan earthquake as a case study. While previous studies reported hypocenter depths of ~15 km based on mixed near and far observations, our analysis demonstrates that inclusion of distant stations introduces systematic depth bias. Using near constraints that are sensitive to depths, we obtain a robust hypocenter depth of ~8 km. Combined with first-motion polarity analysis, these results indicate rupture initiation on a low-dip fault (<30°), rather than a high-angle reverse fault.

Session: New Frontiers in Seismic Observations and Modeling with Innovative Methods and Emerging Data on Earth and Other Planets [Poster]

Type: Poster

Date: 4/17/2026

Presentation Time: 08:00 AM (local time)

Presenting Author: Jinping Zi

Student Presenter: No

Invited Presentation: 

Poster Number: 104

Authors