Systematic Search of Remotely Triggered Tremor in Northern and Southern California

Non-volcanic tremor is a seismic signal observed away from volcanoes, and is characterized with long durations and no clear body wave arrivals. Recent studies have found that non-volcanic tremor can be triggered instantaneously during the surface waves of large teleseismic events in Central California, Japan, Cascadia, and Taiwan. However, it is still not clear how widespread the triggering phenomenon is, and what are the necessary conditions for tremor to occur. Here we conduct a systematic search of remotely triggered tremor in Northern and Southern California, focusing on the following regions with dense instrumentations: the Central Calaveras fault, the Northern Hayward fault, the San Jacinto fault near Hemet and the Simi Valley, and the San Gabriel Mountains. Out of the 30 large teleseismic events with Mw ≥ 7.5 since 2001, our visual inspection shows that only the 2002 Mw7.8 Denali Fault earthquake has triggered tremor in these regions, including the San Gabriel Mountains where neither triggered nor ambient tremor has been observed before. The tremor observed near the Calaveras fault and San Jacinto fault appears to be initiated by Love waves, and becomes intensified during the large amplitude Rayleigh waves. The tremor triggered in Simi Valley and San Gabriel Mountains only shows weak correlations with the Rayleigh waves. Our results suggest a lack of widespread triggering in Northern and Southern California, which is in contrast with the finding of multiple events that triggered tremor in Central California, Japan, Cascadia, and Taiwan. However, such observation is consistent with a general lack of ambient tremor activities in these regions. Possible reasons for a lack of widespread triggering in Northern and Southern California include: elevated background noises for surface stations that may hide weak triggered tremor signals, different ambient tremor rate, or different tremor triggering threshold in different regions. Updated results will be presented at the meeting.