Heterogeneity of Aftershock Productivity Along the Mainshock Ruptures and Its Advantage in Improving Short‐Term Aftershock Forecast

This study introduces an improved hypocentral version of the space‐time Epidemic‐Type Aftershock Sequence (ETAS) model, entitled as the 3D‐finite source (3D‐FS) ETAS model, and applies it to the analysis of the Southern California earthquake catalog. By stochastic reconstruction, we are able to reconstruct the patterns of aftershock productivity density along the mainshock ruptures. Detailed analysis of the productivity patterns reveals that: (1) Directly triggered aftershocks make up 21% to 41% of all earthquakes within the mainshock rupture areas, and show significant spatial heterogeneity and temporal migrations; (2) Major aftershocks tend to locate in low productivity areas, at the edges of clusters formed by small aftershocks; (3) Large slip areas are depleted of aftershocks, over 60% of all productivity distributes in areas with slip less than 0.3 times of the maximum slip, and the trajectory of the productivity pattern on the fault plane demonstrates apparent compensation to coseismic slip. We relate the difference in triggering abilities of four mainshocks to the heat flow in corresponding regions. Simulation results suggest that the 3D‐FS ETAS model has apparent advantages of improving the performance of short‐term aftershock forecast. Moreover, the later aftershocks are more correlated with the locations of subsequent events than earlier aftershocks, suggesting that the migration of aftershocks is important for mitigating aftershock hazard. Plain Language Summary How many aftershocks can be produced at each part along the rupture of a large earthquake is crucial for us to understand how aftershocks are generated and how the strain energy in the crust relaxes afterward. It is difficult to know whether an earthquake is an aftershock triggered by some previous earthquake or a background earthquake that has no ancestry since earthquake clusters not only overlap with each other but also with background seismicity in space and time. We proposed a new earthquake clustering model to disentangle such overlapping and quantify family trees among the earthquakes. Our study on the earthquake data in Southern California shows that aftershocks tend to occur at low slip areas in the mainshock rupture and that large aftershocks occur farther away near the boundary of such concentration of aftershocks. Moreover, this model improves the short‐term aftershock forecasting. Key Points Large aftershocks occur in the edge of areas with high aftershock productivity After