Tectonic stress fields inferred from long-term CMT data ranging over different periods

We have developed an inversion method for estimating 3-D tectonic stress fields from centroid moment tensor (CMT) data of seismic events. To obtain reliable inversion analysis results, we need to use as much data as possible, ranging over different periods. In this study, we improved the previously developed CMT data inversion method to incorporate preceding inversion analysis results into the present analysis as direct prior information about the stress field. The spatial distribution of seismic events and/or their focal mechanisms in a region would change from one period to another, reflecting temporal stress changes there. In the improved inversion method, in addition to the hyperparameter controlling the roughness of stress fields, we introduced another hyperparameter that controls the relative weight of direct prior information to the observed data to be analysed. The relative weights of direct and indirect prior information to the observed data are appropriately determined by using Akaike's Bayesian Information Criterion. We applied the improved inversion method to the observed CMT data of seismic events of 24 yr (1997−2020) in northeast Japan, where the Tohoku-oki megathrust event (Mw 9.0) occurred on 11 March 2011 at the North American–Pacific Plate boundary. To investigate temporal changes in tectonic stress orientation, we divided the target period into the inter-, pre- and post-seismic periods of the megathrust event: P1 (January 1997−January 2007), P2 (February 2007−February 2011), and P3 (March 2011−December 2020). According to successive inversion analysis results for P1 and P2, we confirmed that the tectonic stress orientation was stable for 14 yr before the megathrust event. On the other hand, successive inversion analysis results for P2 and P3 indicated a possibility that the 2011 megathrust event caused significant changes in tectonic stress orientation in three regions. However, two of them were negative in the quantitative evaluation of the reliability of detected temporal changes. In conclusion, the stress orientation change in the region off Fukushima and Ibaraki, the southern margin of the main rupture area of the megathrust event, seems to be real.