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The ShakeAlert earthquake early warning (EEW) system issues public alerts in California and will soon extend to Oregon and Washington. The Cascadia subduction zone presents significant new challenges and opportunities for EEW. Initial publications suggested that EEW algorithms based on Global Navigation Satellite System (GNSS) data could provide improved warning for intraslab events and dramatically improved warning for offshore megathrust events, both of which contribute significantly to hazard in Cascadia. We find that some expectations in these publications were unrealistic, and we demonstrate that in general geodetic algorithms would not produce timely warnings for intraslab events nor warning times of two minutes or more for severe shaking from megathrust earthquakes. Nonetheless, lessons from recent earthquakes in Japan and California, for which alerts from seismic algorithms suffered from magnitude saturation and high data latencies, demonstrate the urgent need for rigorous testing of geodetic EEW as a potential complement to seismic EEW. Plain Language Summary: ShakeAlert was initially implemented and optimized in California where most earthquakes occur on shallow faults in the Earth's crust. These earthquakes typically start just a few miles underground and can begin directly underneath population centers. Thus, earthquake early warning (EEW) needs to rely on the very first detections of seismic waves to be able to issue warnings before stronger shaking arrives. In a subduction zone, many damaging earthquakes are located offshore on the plate boundary megathrust, and many inland large earthquakes occur in the subducted plate at depths of 30+ miles. Because some seismic EEW systems have underestimated large earthquakes in the past, approaches are being tested that use GNSS data as the primary input for quantifying the magnitude and length of a large, growing rupture. We discuss realistic expectations from this approach and identify key topics for future research. Key Points: Geodetic earthquake early warning (EEW) algorithms require rigorous testingRegularization operators are a key feature to prevent over alertingGeodetic EEW will likely not help with most intraslab earthquakes [ABSTRACT FROM AUTHOR]

In the uppermost ∼ 3-5 km of continental strike-slip faults, a significant fraction of the total slip is typically accommodated by stable sliding, or fault creep. Creep can be continuous or episodic, lasting only a few hours, and varies throughout the earthquake cycle and from one fault to another. The most commonly used mechanical model attributes episodic creep events to the transition from unconsolidated sediments to lithified rocks at depth. However, this model cannot explain the wide variability in observed shallow creep characteristics on strike-slip faults in California. Here, we use numerical simulations to examine a range of alternative mechanical models that can reproduce the variability of shallow creep behaviour in both postseismic and interseismic periods. We find that geodetic observations of creep behaviour on a number of significant fault segments in California are matched when an additional unstable layer is embedded within the shallow, stable zone. This layer may result from fine-scale lithological heterogeneities within the stable zone-frictional behaviour varies with lithology, generating the instability. Our model suggests that the displacement of and interval between creep events are dependent on the thickness, stress and frictional properties of the shallow, unstable layer. We also suggest that such frictional heterogeneity may be the mechanism responsible for slow slip events in many subduction zones. [ABSTRACT FROM AUTHOR]

Swarm-like earthquake sequences are commonly observed in a diverse range of geological settings including volcanic and geothermal regions as well as along transform plate boundaries. They typically lack a clear mainshock, cover an unusually large spatial area relative to their total seismic moment release, and fail to decay in time according to standard aftershock scaling laws. Swarms often result from a clear driving phenomenon, such as a magma intrusion, but most lack the necessary geophysical data to constrain their driving process. To identify the mechanisms that cause swarms on strike-slip faults, we use relative earthquake locations to quantify the spatial and temporal characteristics of swarms along Southern California and East Pacific Rise transform faults. Swarms in these regions exhibit distinctive characteristics, including a relatively narrow range of hypocentral migration velocities, on the order of a kilometre per hour. This rate corresponds to the rupture propagation velocity of shallow creep transients that are sometimes observed geodetically in conjunction with swarms, and is significantly faster than the earthquake migration rates typically associated with fluid diffusion. The uniformity of migration rates and low effective stress drops observed here suggest that shallow aseismic creep transients are the primary process driving swarms on strike-slip faults. Moreover, the migration rates are consistent with laboratory values of the rate-state friction parameter b (0.01) as long as the Salton Trough faults fail under hydrostatic conditions. [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]

Since a regional earthquake in 1951, shallow creep events on strike-slip faults within the Salton Trough, Southern California have been triggered at least 10 times by M ≥ 5.4 earthquakes within 200 km. The high earthquake and creep activity and the long history of digital recording within the Salton Trough region provide a unique opportunity to study the mechanism of creep event triggering by nearby earthquakes. Here, we document the history of fault creep events on the Superstition Hills Fault based on data from creepmeters, InSAR, and field surveys since 1988. We focus on a subset of these creep events that were triggered by significant nearby earthquakes. We model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate- and state-dependent friction. We find that the static stress changes from the causal earthquakes are less than 0.1 MPa and too small to instantaneously trigger creep events. In contrast, we can reproduce the characteristics of triggered slip with dynamic perturbations alone. The instantaneous triggering of creep events depends on the peak and the time-integrated amplitudes of the dynamic Coulomb stress change. Based on observations and simulations, the stress change amplitude required to trigger a creep event of a 0.01-mm surface slip is about 0.6 MPa. This threshold is at least an order of magnitude larger than the reported triggering threshold of non-volcanic tremors (2–60 kPa) and earthquakes in geothermal fields (5 kPa) and near shale gas production sites (0.2–0.4 kPa), which may result from differences in effective normal stress, fault friction, the density of nucleation sites in these systems, or triggering mechanisms. We conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the Superstition Hills Fault. [ABSTRACT FROM AUTHOR]