Deep Aseismic Slip Identified as a Key Precursor to the 2024 Hualien Earthquake
24 Years of Seismic Data Reveal a Multiyear Preparation Process and a Critical Slow-Slip Trigger
A research team led by Professor Hui-Hsuan Chen of the Department of Earth Sciences at National Taiwan Normal University (NTNU) has found evidence that accelerated aseismic slip on a deep fault began as early as three years before the magnitude-7.3 Hualien earthquake on April 3, 2024. This behavior suggests features of a potential earthquake precursor. The team’s findings, published in Nature Communications, offer important insight into how major earthquakes may develop over long timescales and highlight the role of slow slip along the Central Range Fault in eastern Taiwan.
Aseismic Slip and Swarm Activity Before the 2024 Hualien Earthquake
The study was conducted by Professor Hui-Hsuan Chen, postdoctoral researcher Wei Peng, Professor Roland Bürgmann of the University of California, Berkeley, and Research Fellow Ya-Ju Hsu with research assistant Yen-Hung Chen of Academia Sinica’s Institute of Earth Sciences. Their analysis shows that deep aseismic slip played a central role in the preparation of the 2024 Hualien earthquake. The research was published on October 13, 2025.
Using seismic data from 2000 to 2024, the team examined the spatial and temporal behavior of repeating earthquakes (magnitude ≥ 2) and earthquake swarms. These phenomena often reflect aseismic slip or elevated pore-fluid pressure within deeper portions of the fault.
Eastern Taiwan lies within the convergence zone where the Philippine Sea Plate subducts beneath the Eurasian Plate. This tectonic setting has formed the west-dipping Central Range Fault and the east-dipping Longitudinal Valley Fault, two major, roughly parallel structures responsible for many magnitude-6-and-above earthquakes and extensive aseismic slip.
The analysis shows that in the northern Longitudinal Valley, seismicity clusters primarily along the Central Range Fault, while in the southern segment it concentrates along the Chishang Fault. Both repeating earthquakes and swarms are tightly linked to deep aseismic slip or high pore-fluid pressure, demonstrating that faults do not always release energy through sudden rupture.
Evidence of Slow Slip in the 2021 Shoufeng Swarms
In 2021, four months of earthquake swarms in the Shoufeng region revealed a gradual upward migration of events. The observed diffusion rate of 5 to 6 meters per second, which far exceeded typical pore-fluid diffusion rates, indicating that slow slip at depth accompanied this activity. GNSS (Global Navigation Satellite System) measurements simultaneously recorded 1 to 2 centimeters of surface displacement, supporting a combined effect of deep aseismic slip and increasing pore-fluid pressure.
A Four-Stage Preparation Process
The buildup to the 2024 Hualien earthquake can be described in four stages:
Stage 1 (April–August 2021): Acceleration of aseismic slip and swarm activity, accompanied by a sharp rise in shallow seismicity.
Stage 2 (September 2021–late 2022): A series of magnitude-6-plus earthquakes occurred south of the valley, intensifying regional seismicity, while activity in the northern segment remained low.
Stage 3 (2023–early 2024): Deep repeating earthquakes increased, seismic moment rose gradually, and fault-slip rates also increased slightly. Meanwhile, seismicity at depths shallower than 15 kilometers climbed steadily, with both a- and b-values rising. These trends indicate concurrent deep slow slip and shallow brittle failure before the mainshock.
Stage 4 (after April 3, 2024): The magnitude-7.3 mainshock occurred, followed by two magnitude-6.0-plus aftershocks.
How Slow Slip Led to a Major Earthquake
To understand how accelerated deep slip in 2021 could connect to a major earthquake three years later, the team applied static Coulomb stress modeling. They found that the 2021 slow-slip event significantly influenced the triggering of Stage 2 earthquakes. When slow-slip episodes from 2021 to 2023 were combined with three magnitude-6-plus earthquakes in 2022, including the Guanshan–Chishang sequence, the total stress increase reached approximately 30 kilopascals, equivalent to the pressure exerted by a three-meter column of water. This amount of stress aligns with conditions that could trigger the 2024 Hualien earthquake.
The study reveals a cascading process linking deep slow slip, fluid-pressure changes, and earthquake initiation. The 2021 swarms, deeper aseismic slip, and rapid fluid migration likely weakened the fault zone. Subsequent large earthquakes further raised Coulomb stress on the eventual rupture plane, ultimately accumulating about 30 kilopascals, which is enough to significantly advance the timing of the 2024 mainshock.
Lead author Wei Peng, Ph.D., stated: “We observed precursory phenomena driven by aseismic slip, which is extremely rare in an orogenic suture zone. The Hualien mainshock developed over a three-year period shaped by interactions between deep slip and swarm activity.”
Corresponding author Professor Hui-Hsuan Chen added: “If we can continue to monitor interactions between aseismic slip and moderate-to-large earthquakes, along with the resulting stress accumulation, we will improve our ability to identify earthquake precursors and enhance short-term forecasting.”