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Analysis of unusual earthquake and tremor seismicity at the Mendocino Triple Junction and Parkfield, California


The detection and the analysis of unusual seismic events such as slow/low-stress-drop earthquakes in the Mendocino Triple Junction (MTJ), and nonvolcanic tremors and repeating earthquakes near Parkfied-Cholame can be used to provide a clearer picture and a better understanding of fault rheology, mechanics, and active tectonics in California. Modeling of cataloged slow earthquakes, which show large high-frequency/low-frequency magnitude differentials, indicates that the differences come from the source rather than from strongly attenuated propagation paths. Because the restricted number of these events limits their study and a precise comprehension of the complex processes occurring in the MTJ, a new procedure is proposed using continuous scanning of long-period seismic waveforms and moment tensor inversions to automatically detect and characterize regular and slow events, with magnitude larger than 3.5 occurring in the region. A similar approach using quasi-finite-source Green's functions in a point-source inversion method is proposed for the fast detection and characterization of large and potentially tsunamigenic earthquakes along the Cascadia Subduction Zone as well as in other subduction zone regions such as offshore Japan. Furthermore, this approach has the potential to be included in future earthquake and tsunami early warning systems.

Nonvolcanic tremors on the other hand, near Parkfield, inform on the variations in the state of stress in the deep fault zone and as a consequence in possible loading of the San Andreas Fault (SAF) at the proximity of the 1857 Fort Tejon earthquake rupture. Tremors are indeed strongly modulated by small stress changes (i.e. in the order of kPa) transmitted by local and regional earthquakes into their source region, below the seismogenic zone. The effects on tremors can last from a few seconds to several years after local, regional, and/or teleseismic earthquakes. Understanding and quantifying the tremor activity is essential for understanding the physics underlying the earthquake and fault zone processes such as earthquake nucleation, stress dependences, and state of fault stress. Episodes of tremors are found for example to exhibit deep M~5 slow slip events along the SAF, where no other datasets have yet revealed them. Hence, the occurrence of large events might be better understood through modeling of atypical, and numerous micro-earthquakes, whose recurrence times are shorter.

Understanding the characteristics of the unusual seismicity in northern California gives insights on the processes responsible for the occurrence of regular small and large earthquakes in the region that might help the seismological community to develop better earthquake forecasting models in the future.

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