タイトル：

Constraining Focal Mechanisms of Microearthquakes (M ≤ 3): Methods and Case Studies

要旨：

Earthquake focal mechanisms represent a key tool for both the characterization of seismogenic structures and the investigation of earthquake source properties and rupture processes, as they constrain fault geometry, source kinematics, and the regional stress field.

Fault-plane solutions can be determined using several techniques applicable over different magnitude ranges, both in the time and frequency domains and using different types of seismological data. For low-magnitude earthquakes, however, the limited number of available observations, their uncertainties, and the geometry of the seismic network can significantly affect the stability and reliability of focal mechanism solutions. For this reason, a robust assessment of solution uncertainties is essential to distinguish well-constrained mechanisms from poorly resolved ones and to evaluate their contribution to seismotectonic interpretations.

This seminar will focus on the main challenges involved in constraining focal mechanisms of microearthquakes (M ≤ 3), discussing different methodological approaches and the factors controlling the quality and reliability of the solutions. Particular attention will be devoted to methods combining complementary observations, including P-wave polarities and P-to-S spectral amplitude ratios, and to Bayesian approaches aimed at quantifying uncertainties and assessing the robustness of the inferred fault-plane solutions. Recent developments based on machine learning and artificial intelligence will also be discussed, with emphasis on their potential for improving data analysis and supporting focal mechanism determination for large microseismic datasets.

Finally, selected case studies from recent seismicity will be presented to illustrate the application, performance, and limitations of these approaches, and to evaluate their contribution to the characterization of active fault systems and seismic source processes.