Fig. 3: Multi-stage activation of the fault hosting the ML 5.6 earthquake.
Seismicity patterns in the shallow (S) and deep (D) segments of P1 and P2 in 4 stages. Wells of H04, H39, and H37 were projected in section plots (A–D). H33 is outside the display range. A Stage I: with HF from H39. On-fault earthquakes occurred sequentially from (1) to (4) in the shallow segments of P1 and P2. Starting in 2019, the shallow segment of P1 was activated and seismicity extended deeper from (5) to (6). B Stage II: Numerous earthquakes occurred in P1. ML 3.7 and 3.8 events occurred in February 2019. C Stage III: with HF from H37. Earthquakes were mainly in the shallow segment during HF, first on P1 (labeled (1)), then on P2 (labeled (2)). Soon after H37 shut-in, some earthquakes happened near the ML 5.6 hypocenter (labeled (3)). D Stage IV: no HF. Low earthquake occurrence rate. During the last 5 days, events occurred in the deep segment of P1, including an ML 3.5 event 6 min before the mainshock. E Summary of HF operations and on-fault seismicity since 2015. Fault activation started in October 2018. Histograms represent the number of earthquakes on P1 every 2 days. Distance vs. time plot shows the seismicity front (red arrows) advancing toward the ML 5.6 hypocenter.
