タイトル:

Whispers of aftershocks in the seismically prolific San Jacinto Fault Zone, California

要旨:

The San Jacinto Fault Zone (SJFZ) in California is known for its large earthquakes and extensive seismicity below the locked-to-creeping transition depth. With a new 15-year earthquake catalog, we discover that this transition depth coincides with a marked difference in the seismicity dynamics. Near-surface seismicity occurs mainly as classical mainshock‒aftershock sequences. Beneath the locking depth, in the supposedly creeping region, seismic activity occurs virtually without triggering aftershocks, nor forming repeating earthquake sequences. Our numerical simulations of earthquake sequences and slow-slip on heterogeneous faults demonstrate that the observations at these depths place strong constraints on fault friction heterogeneity, supporting a change in frictional properties from more velocity-weakening shallower to more velocity-neutral at depth. Our conclusions have important implications for earthquake activity throughout the SJFZ and beyond.