Characteristics of the Seismic Sequence and Determination of Post-Earthquake Trends for the MS 7.3 Earthquake in the Sea Area Off Hualien County, Taiwan, April 3, 2024

1 Research Objectives and Methods

To clarify the sequence characteristics and post-earthquake trend of the Ms 7.3 earthquake in the sea area off Hualien County, Taiwan, on April 3, 2024 (focal depth 12 km, epicenter at 23.81°N latitude and 121.74°E longitude), this study is based on observational data from the China Earthquake Networks Center (CENC), combined with the regional geological and tectonic background. Selecting seismic catalog data from November 2023 to December 2024, systematic analysis was conducted on the spatiotemporal distribution, intensity variation, frequency characteristics and dynamic evolution law of the earthquake using analytical tools such as M-T diagrams, H-T diagrams, h-value curves, b-value curves and creep curves.

2 Study Area Characteristics and Seismic Sequence Analysis

2.1 Tectonic setting and spatial distribution of seismic activity: The study area is located at the subduction boundary between the Eurasian Plate and the Philippine Sea Plate, controlled by the tectonic background of the northern segment of the Huadong Valley Fault Zone. Seismic activity features a spatial pattern of "stronger and denser in the east, weaker and sparser in the west". Epicenters are concentrated within the range of 121-123°E longitude and 23-25°N latitude, showing an overall northeast-southwest trend.

2.2 Seismic sequence type and source characteristics: The earthquake sequence is a typical mainshock-aftershock type, with the mainshock releasing 98.2% of the total energy of the sequence. Aftershocks are active after the mainshock, and their attenuation follows the modified Omori formula. Shallow-focus earthquakes (0-50 km) dominate, which are highly destructive; a small number of deep-focus earthquakes also occur, reflecting stress adjustment processes at different crustal levels.

2.3 Seismic sequence parameter analysis: Analysis of the seismic sequence parameters reveals that the post-earthquake h-value is 1.1 (faster than the conventional attenuation rate), and the b-value is 1.0166 (with a higher proportion of small and medium-sized earthquakes). There is a significant linear correlation between magnitude and logarithmic frequency, consistent with the Gutenberg-Richter law. The creep curve clearly shows a three-stage evolutionary characteristic: "strain accumulation — mainshock release — post-earthquake adjustment".

3 Post-Earthquake Trend Determination and Research Significance

3.1 Post-earthquake trend judgment: The intensity and frequency of aftershock activity will continue to attenuate, and the probability of a magnitude 7.0 or above strong earthquake occurring in the short term (within several months) is extremely low. However, deep-seated stress adjustment in the region is not yet complete; special attention should be paid to stress transfer in the unruptured area of the northern segment of the Huadong Valley Fault Zone to prevent the occurrence of delayed strong aftershocks.

3.2 Research significance: The conclusions of this study provide scientific support for the research on earthquake mechanisms and disaster prevention and control work in eastern Taiwan.