19 Clustered Foreshock Sequences Along the San Jacinto Fault Zone: Possible Role of Frictional Heterogeneity in Extended Earthquake Nucleation

Journal article

Plain Language Summary: Earthquake initiation in nature is still a mystery, although lab experiments or theoretical models suggest that there is a nucleation phase of slow but accelerating slip before the earthquake rupture begins. Foreshocks are widely used to probe the nucleation process, but the connection between foreshocks and the mainshock nucleation is still uncertain. Here we study earthquake nucleation along the SJFZ, where abundant seismicity and good station coverage make it an ideal site. Specifically, we detect extended and clustered foreshock sequences around the mainshock hypocenter which may indicate extended earthquake nucleation. We track the temporal evolution of foreshocks' source power by using a new coherence based power metric. Our results show that 19 clustered foreshock sequences occur near the base of the seismogenic zone. These observations are rare, but might indicate that prolonged earthquake nucleations occur under some fault conditions. We discuss possible rupture scenarios under conditions of frictional heterogeneity that lead to our observations. We observe growth of source power during foreshocks, which might reflect accelerating aseismic slip or growing cascades of ruptures. Our results provide new constraints on the fault condition control of earthquake nucleation behaviors along the SJFZ.

Authors: Huang, H; Hawthorne, JC

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Geophysical Research Letters
• Volume: 53
• Issue: 8
• Article number: e2025GL120398
• Publication date: 2026-04-14
• Acceptance date: 2026-03-31
• DOI: 10.1029/2025gl120398
• EISSN: 1944-8007
• ISSN: 0094-8276
• Language: English
• Keywords: earthquake detection; earthquake nucleation; frictional heterogeneity; foreshock sequences; earthquake precursor