A 2024 Geophysical Journal International study analysed seismicity in Southwestern China (2008-2021, China Earthquake Networks Center catalogue) with four space-time Epidemic-Type Aftershock Sequence (ETAS) variants, finding that background rates can jump by up to about 50 per cent after major earthquakes and that reservoir-induced earthquakes substantially raise the regional seismicity level; a principled joint model separating tectonic background-rate changes from anthropogenic forcing is not yet implemented. The open problem is to formulate, fit, and validate such a joint model for this region: an ETAS model whose background intensity is non-stationary and explicitly carries the reservoir signal, estimated together with the tectonic background and the triggered (aftershock) component, with uncertainties and adequate handling of catalogue incompleteness, and to use it to re-evaluate the 2024 study's conclusions about background-rate changes and the reservoir contribution to regional seismic hazard.

Background

Statistical seismology describes earthquake catalogues as spatiotemporal point processes. The Epidemic-Type Aftershock Sequence (ETAS) model, introduced by Ogata (1988, 1998), specifies the conditional intensity of earthquake occurrence, that is, the instantaneous expected event rate given the observed history, as the sum of two parts: a spatially varying background rate reflecting spontaneous, mainly tectonic loading, and a triggering term in which each earthquake of magnitude MM stochastically generates offspring (aftershocks) at a rate growing exponentially with MM. Stochastic declustering (Zhuang, Ogata and Vere-Jones, 2002) uses the fitted ETAS model to compute, for every event, the probability that it is a background event rather than an offspring, which decomposes a catalogue into background and clustered parts. Liu, Zhuang, Guo, Jiang, Tian, Zhang and Long (2024) applied four ETAS variants (two-dimensional and three-dimensional, with point-source and finite-source triggering kernels) to the earthquake catalogue of the China Earthquake Networks Center (CENC) for Southwestern China from 1 January 2008 to 30 June 2021, a period beginning with the MSM_S (surface-wave magnitude) 8.0 Wenchuan earthquake and including seven MS≥6.0M_S\geq 6.0 sequences. They found that the natural background level is usually stable but can change suddenly by up to about 50 per cent after events such as the 2014 Ludian and Jinggu earthquakes, and that reservoir-induced earthquakes substantially increase the regional seismicity level, so that they cannot be ignored in hazard evaluation. Reservoir-triggered seismicity (RTS) is earthquake activity caused by the impoundment and operation of large water reservoirs, classically distinguished into a rapid initial response to water-level change and a protracted response driven by pore-pressure diffusion, the slow spreading of fluid pressure through connected pores and cracks in the rock, down to earthquake depths (Simpson, Leith and Scholz, 1988). Southwestern China is an extreme case: it is one of the world's most seismically active regions and hosts the planet's densest cluster of giant hydropower reservoirs (including Xiluodu, Baihetan, Wudongde and Xiaowan), whose impoundment-associated hydro-mechanical effects and regional seismicity have been documented geophysically (Zhang, Ge, Yang and Ma, 2021; Duan, Zhou, Zhao and Zhang, 2024); the Sichuan Basin portion additionally hosts shale-gas hydraulic-fracturing sequences (Lei et al., 2017). The methodological difficulty is that the ETAS variants used in the 2024 study assume a time-constant background rate during fitting, so any slowly varying anthropogenic or transient signal is absorbed into either the background estimate or the declustering probabilities; the authors handled reservoir sequences only by removing geographically proximate events and comparing background histories with and without them. Non-stationary extensions of ETAS exist (time-dependent parameters estimated by empirical Bayes, a technique that treats the time-varying parameters as random effects whose smoothness is tuned by maximizing an integrated likelihood — Kumazawa and Ogata, 2014; detection of fluid-driven transients — Hainzl and Ogata, 2005), ETAS has been extended with external covariates and with background components driven by fluid-injection rates or by slow slip events (episodes of slow, aseismic fault creep observed mainly on subduction-zone interfaces) in other settings, but none of these has been combined into a regional joint model that attributes Southwestern China seismicity between tectonic and reservoir-induced sources.

Problem Statement

Formulate, fit, and validate a joint space-time ETAS model for the Southwestern China regional earthquake catalogue (CENC data, 2008-2021 and any available extension) in which the background intensity is non-stationary and explicitly represents anthropogenic forcing. The model must be estimated jointly with the triggered (aftershock) component while adequately handling catalogue incompleteness shortly after large mainshocks and near reservoir sequences, so that missing early aftershocks do not bias the background and productivity estimates. The required results are: (i) a fully specified model with parameter estimates and uncertainty quantification; (ii) an estimated time-varying partition of regional seismicity into tectonic background, reservoir-induced (anthropogenic) background, and triggered components, with uncertainties; and (iii) a re-evaluation of the 2024 study's conclusions, namely whether the natural background rate genuinely changes suddenly by up to about 50 per cent after major earthquakes such as Ludian and Jinggu, and how large the reservoir-induced contribution to regional seismicity and seismic hazard really is. Data acquisition is part of the task: the solver must assemble the CENC 2008-2021 catalogue (the catalogue underlying the 2024 study) together with the reservoir water-level or impoundment histories for the region's major reservoirs from their documented sources; each of the three required results depends on these inputs.

References

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