Knowledge of crustal stress fields is essential for understanding tectonics and earthquake generation. One approach for estimating the crustal stress field is based on the focal mechanisms of earthquakes. This study investigated the focal mechanisms of approximately 110,000 microearthquakes in the area of the Japanese islands that occurred at a depth shallower than 20 km, based on the first-motion polarities picked by a simple neural network model. The model was first trained using a data set of mainly moderate to large earthquakes throughout Japan. Following on, the model was re-trained using a data set of microearthquakes in two regions of Japan. The threshold of the confidence score from the neural network model was chosen to maximize the overall quality of the focal mechanism solutions. The P- and T-axes of the numerous focal mechanism solutions provided more detailed distributions of the crustal stress field. For example, in the Chugoku region, small differences were observed in the trend of P-axes azimuths between the northern and southern areas, spatially corresponding to geodetic observations. The results of this study are useful for revealing the crustal stress field, and, as such, for assessing past and current tectonic activities and potential future earthquake generation.

To better quantify how injection, prior seismicity and fault properties control rupture growth and propagation of induced earthquakes, we perform finite-fault slip inversion on a $M_{w}$ 4.1 earthquake that occurred in April 2015, which is the largest earthquake of an induced sequence near Guthrie, Oklahoma. The slip inversion reveals a complex rupture with multiple slip patches that are anti-correlated to the cumulative slip distributions of prior seismicity. This indicates that the $M_{w}$ 4.1 earthquake likely ruptured relatively strong asperities, while earlier seismicity driven by pore pressure occurred in weaker area. Compared to similar magnitude events in swarms from other regions, intraplate earthquakes in Oklahoma have higher number of well separated slip patches, indicating a difference in fault characteristics between regions. These observations suggest that both pore pressure perturbations, earthquake interactions, and fault characteristics control rupture propagation in moderate size earthquakes in Oklahoma, with the latter likely the dominant factor.