Anomaly Detection in Borehole Strain Data with CNN and Frequency-Aware VAE

Earthquakes pose significant threats to human life and property, often triggering secondary disasters such as landslides, mudslides, and collapses. Although earthquake prediction remains a challenging global scientific problem, the study of seismic precursors plays a critical role. Earthquakes occur due to the instability and fracturing of underground rock layers when concentrated stress exceeds their strength limit. Borehole strainmeters provide direct observations of crustal strain, making pre-earthquake strain anomaly detection essential for precursor studies. In recent decades, variational autoencoders (VAEs) have been widely adopted for anomaly detection due to their powerful denoising capabilities. However, traditional VAE-based methods face difficulties in capturing both long-term heterogeneous patterns and fine-grained short-term trends simultaneously. To address this, we propose a new approach combining convolutional neural networks (CNN) with frequency-aware conditional VAE frameworks. The CNN extracts spatial dependencies among strain observation components, while frequency analysis improves temporal feature capture. By incorporating a target attention mechanism, our model selects the most relevant frequency-domain information to enhance reconstruction of both long-term and short-term trends. Experimental results on borehole strain data show that our model outperforms state-of-the-art methods. These findings confirm the practical value of our approach in overcoming current VAE-based detection limitations and emphasize the importance of integrating spatial and frequency representations in seismic precursor studies.