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The interactions between the North American, Pacific, and Gorda plates at the Mendocino Triple Junction (MTJ) create one of the most seismically active regions in North America. The earthquakes rupture all three plate boundaries but also include considerable intraplate seismicity reflecting the strong internal deformation of the Gorda plate. Understanding the stress levels that drive these ruptures and estimating the locking state of the subduction interface are especially important topics for regional earthquake hazard assessment. However owing to the lack of offshore seismic and geodetic instruments, the rupture process of only a few large earthquakes near the MTJ have been studied in detail and the locking state of the subduction interface is not well constrained. In this paper, first, we use the second moments inversion method to study the rupture process of the January 28, 2015 M w 5.7 earthquake on the Mendocino transform fault that was unusually well recorded by both onshore and offshore strong motion instruments. We estimate the rupture dimension to be approximately 6 km by 3 km corresponding to a stress drop of ∼4 MPa for a crack model. Next we investigate the frictional state of the subduction interface by simulating the afterslip that would be expected there as a result of the stress changes from the 2015 earthquake and a 2010 M w 6.5 intraplate earthquake within the subducted Gorda plate. We simulate afterslip scenarios for a range of depths of the downdip end of the locked zone defined as the transition to velocity strengthening friction and calculate the corresponding surface deformation expected at onshore GPS monuments. We can rule out a very shallow downdip limit owing to the lack of a detectable signal at onshore GPS stations following the 2010 earthquake. Our simulations indicate that the locking depth on the slab surface is at least 14 km, which suggests that the next M8 earthquake rupture will likely reach the coastline and strong shaking should be expected there. [ABSTRACT FROM AUTHOR]

Many of the most fundamental questions in earthquake science are currently limited by a lack of knowledge about small earthquake ruptures. Small earthquakes are difficult to study owing to the poor constraints placed on many of the interesting physical parameters by bandlimited, far-field, seismic data. Traditionally, dynamic models, such as an expanding circular crack, have been utilized to bridge the gap between the easily measurable quantities for small earthquakes and more interesting physical parameters such as stress drop and rupture velocity. Here I present a method for estimating the basic finite source properties of a rupture that is independent of any a priori model and utilizes the description of a finite source, the second moments, that far-field waves are inherently sensitive to. Application to two magnitude 5 events in southern California demonstrates the ability of an empirical Green's function approach to estimating the second moments to resolve the fault-plane ambiguity, rupture length, and overall directivity. Additional results are presented for two example M 2.7 events from the creeping section of the San Andreas fault to examine the likely lower bound on event size that can be studied with surface seismometers. The creeping section earthquakes have very similar rupture areas but would be improperly interpreted as significantly different using the traditional methodology. One of these events presents a relatively clear interpretation of the velocity of rupture front propagation, which is about 0.8 of the Rayleigh speed, suggesting little difference in rupture velocity between it and typical large earthquakes. [ABSTRACT FROM AUTHOR]