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We use a newly developed 2-D elastic reverse time migration (RTM) imaging algorithm based on the Helmholtz decomposition to test approaches for imaging the descending slab in subduction zone regions using local earthquake sources. Our elastic RTM method is designed to reconstruct incident and scattered wavefields at depth, isolate constituent P- and S- wave components via Helmholtz decomposition, and evaluate normalized imaging functions that leverage dominant P and S signals. This method allows us to target particular converted-wave scattering geometries, for example incident S to scattered P , which may be expected to have dominant signals in any given data set. The method is intended to be applied to dense seismic array observations that adequately capture both incident and converted wavefields. We draw a direct connection between our imaging functions and the first-order contrasts in shear wave material properties across seismic discontinuities. Through tests on synthetic data using either S → P or P → S conversions, we find that our technique can successfully recover the structure of a subducting slab using data from a dense wide-angle array of surface stations. We also calculate images with a small-aperture array to test the impact of array geometry on image resolution and interpretability. Our results show that our imaging technique is capable of imaging multiple seismic discontinuities at depth, even with a small number of earthquakes, but that limitations arise when a small aperture array is considered. In this case, the presence of artefacts makes it more difficult to determine the location of seismic discontinuities. [ABSTRACT FROM AUTHOR]

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]

The largest slip in great megathrust earthquakes often occurs in the 10–30 km depth range, yet seismic imaging of the material properties in this region has proven difficult. We utilize a dense onshore‐offshore passive seismic dataset from the southernmost Cascadia subduction zone where seismicity in the mantle of the subducted Gorda Plate produces S‐to‐P and P‐to‐S conversions generated within a few km of the plate interface. These conversions typically occur in the 10–20 km depth range at either the top or bottom of a ∼5 km thick layer with a high Vp/Vs that we infer to be primarily the subducted crust. We use their arrival times and amplitudes to infer the location of the top and bottom of the subducted crust as well as the velocity contrasts across these discontinuities. Comparing with both the Slab1.0 and the updated Slab2 interface models, the Slab2 model is generally consistent with the converted phases, while the Slab1.0 model is 1–2 km deeper in the 2–20 km depth range and ∼6–8 km too deep in the 10–20 km depth range between 40.25°N and 40.4°N. Comparing the amplitudes of the converted phases to synthetics for simplified velocity structures, the amplitude of the converted phases requires models containing a ∼5 km thick zone with at least a ∼10%–20% reduction in S wave velocity. Thus, the plate boundary is likely contained within or at the top of this low velocity zone, which potentially indicates a significant porosity and fluid content within the seismogenic zone. Key Points: We observe clear Ps and SP converted phases generated from the top and bottom of the oceanic crust of the subducted Gorda PlatePs and SP converted phases are used to constrain the geometry of the interface of the subducted slabAmplitude of the converted phases indicates that the subduction plate interface is a low shear‐wave velocity zone at seismogenic depth [ABSTRACT FROM AUTHOR]

We develop a semiautomated method for estimating with second seismic moments the directivity, rupture area, duration, and centroid velocity of earthquakes. The method is applied to 41 southern California earthquakes with magnitude in the range 3.5–5.2 and provides stable results for 28 events. Apparent source time functions (ASTFs) of P and S phases are derived using deconvolution with three stacked empirical Green's functions (seGf). The use of seGf suppresses nongeneric source effects, improves the focal mechanism correspondence to the analyzed earthquakes, and typically allows inclusion of 5 to 15 more ASTFs compared with analysis using a single eGf. Most analyzed earthquakes in the Trifurcation area of the San Jacinto Fault have directivities toward the northwest, while events around Cajon Pass and San Gabriel Mountain tend to propagate toward the southeast. These results are generally consistent with predictions for dynamic rupture on bimaterial interfaces associated with the imaged velocity contrasts in the area. The second moment inversions also provide constraints on the upper and lower bounds of rupture areas in our data set. Stress drops and uncertainties are estimated for elliptical ruptures using the derived characteristic rupture length and width. The semiautomated second moment method with seGfs can be used for routine application to moderate earthquakes in locations with good station coverage. Key Points: We develop a semiautomated method for estimating finite fault parameters of earthquakes with second seismic momentsResolved rupture directivities are generally consistent with expectations for dynamic ruptures on the imaged velocity contrastsStress drops and uncertainties are estimated for elliptical ruptures using the derived characteristic rupture length and width [ABSTRACT FROM AUTHOR]

The key kinematic earthquake source parameters: rupture velocity, duration and area, shed light on earthquake dynamics, provide direct constraints on stress drop, and have implications for seismic hazard. However, for moderate and small earthquakes, these parameters are usually poorly constrained due to limitations of the standard analysis methods. Numerical experiments by Kaneko and Shearer demonstrated that standard spectral fitting techniques can lead to roughly one order of magnitude variation in stress-drop estimates that do not reflect the actual rupture properties even for simple crack models. We utilize these models to explore an alternative approach where we estimate the rupture area directly. For the suite of models, the area averaged static stress drop is nearly constant for models with the same underlying friction law, yet corner-frequency-based stress-drop estimates vary by a factor of 5–10 even for noise-free data. Alternatively, we simulated inversions for the rupture area as parametrized by the second moments of the slip distribution. A natural estimate for the rupture area derived from the second moments is A = π L c W c, where L c and W c are the characteristic rupture length and width. This definition yields estimates of stress drop that vary by only 10 per cent between the models but are slightly larger than the true area averaged values. We simulate inversions for the second moments for the various models and find that the area can be estimated well when there are at least 15 available measurements of apparent duration at a variety of take-off angles. The improvement compared to azimuthally averaged corner-frequency-based approaches results from the second moments accounting for directivity and removing the assumption of a circular rupture area, both of which bias the standard approach. We also develop a new method that determines the minimum and maximum values of rupture area that are consistent with a particular data set at the 95 per cent confidence level. For the Kaneko and Shearer models with 20+ randomly distributed observations and $$\sim 10\hbox{ per cent}$$ noise levels, we find that the maximum and minimum bounds on rupture area typically vary by a factor of two and that the minimum stress drop is often more tightly constrained than the maximum. [ABSTRACT FROM AUTHOR]

An earthquake rupture process can be kinematically described by rupture velocity, duration and spatial extent. These key kinematic source parameters provide important constraints on earthquake physics and rupture dynamics. In particular, core questions in earthquake science can be addressed once these properties of small earthquakes are well resolved. However, these parameters of small earthquakes are poorly understood, often limited by available data sets and methodologies. The Incorporated Research Institutions for Seismology Community Wavefield Experiment in Oklahoma deployed ∼350 three-component nodal stations within 40 km2 for a month, offering an unprecedented opportunity to test new methodologies for resolving small earthquake finite source properties in high resolution. In this study, we demonstrate the power of the nodal data set to resolve the variations in the seismic wavefield over the focal sphere due to the finite source attributes of an M2 earthquake within the array. The dense coverage allows us to tightly constrain rupture area using the second moment method even for such a small earthquake. The M2 earthquake was a strike-slip event and unilaterally propagated towards the surface at 90 per cent local S -wave speed (2.93 km s−1). The earthquake lasted ∼0.019 s and ruptured L c ∼70 m and W c ∼45 m. With the resolved rupture area, the stress-drop of the earthquake is estimated as 7.3 MPa for M w 2.3. We demonstrate that the maximum and minimum bounds on rupture area are within a factor of two, much lower than typical stress-drop uncertainty, despite a suboptimal station distribution. The rupture properties suggest that there is little difference between the M2 Oklahoma earthquake and typical large earthquakes. The new three-component nodal systems have great potential for improving the resolution of studies of earthquake source properties. [ABSTRACT FROM AUTHOR]

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]

Subduction zones produce earthquakes on a set of faults that operate under a wide variety of conditions resulting from considerable variations in depth, temperature, rock type, and fluid pressure. These variations likely lead to variation in the stress levels that drives particular earthquakes and that in turn effects the magnitude of seismic shaking they produce. In the Mendocino Triple Junction (MTJ) region, intraplate faults within the mantle of the subducting plate fail regularly in energetic earthquakes while the adjacent thrust interface of the Cascadia subduction zone remains seismically quiet despite the likelihood that it operates at much lower levels of stress and strength. In 2012, as part of the Cascadia Initiative community experiment, an ocean bottom seismometer (OBS) array was deployed in the MTJ area, providing unusually dense data covering both the inter- and intra-plate earthquakes. Combining these data with onshore networks, we detect and relocate 1137 earthquakes with a three dimensional velocity model. We perform detailed spectral and time domain analysis to study variations in earthquake source properties between the different types of faults. We observe a wide variability of stress drops and systematic lateral and depth variations in the earthquake source spectra resulting from the different types of tectonic fault systems in this region: intraplate faults within the subducted oceanic mantle, the Mendocino transform plate boundary fault, and the thrust interface of the Cascadia subduction zone. Some of the depth variability of source spectra can be explained by the expected increase in rupture velocity with depth. However, the overall variation in stress drop estimates is consistent with the highest stress drop earthquakes occurring in the depth range predicted by strength envelopes. Moreover, the earthquakes in the vicinity of the thrust interface, likely including some within the subducted oceanic crust, show clearly lower stress drops and depletion of high-frequency radiation for their size compared to events in the oceanic mantle and along the transform fault at a given depth. Our results suggest that the variability in earthquake source spectra that can be expected in the Cascadia subduction zone is systematic and that the thrust interface operates at a lower stress level than the core of the subducted plate a few kilometers away. [ABSTRACT FROM AUTHOR]

Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology--shielded ocean bottom seismometers--is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data. [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]

The concept of a seismic cycle, where the stress on a fault repeatedly builds up over a long period of time and then is rapidly released in a large earthquake, influences studies of both the basic physics of faulting and applied research aimed at estimating earthquake hazards. This hypothesis suggests that large earthquakes might be quasi periodic and that the probability of a particular portion of a fault rupturing twice in quick succession should be low. However, this basic hypothesis has been difficult to verify owing to the long repeat times of the largest earthquakes on most faults. East Pacific Rise (EPR) transform faults are an advantageous location to evaluate the seismic cycle hypothesis owing to their fast slip rates and the moderate size (~Mw 6) of their largest earthquakes. Using surface-wave based determinations of the relative separations between earthquake centroids, I document 16 pairs of Mw≥5:5 events that had overlapping ruptures. The distribution of interevent times for these pairs is tightly clustered around 5 yr (with a coefficient of variation ~0:2) indicating that quasi periodicity may be prevalent for the largest events on these faults. Moreover, I find no pairs of overlapping Mw 5.5-6.2 earthquakes separated by less than 50 cm of elapsed plate motion, indicating that the two basic features of the seismic cycle hypothesis are evident in the timing of large EPR transform mainshocks. I have also confirmed earlier results demonstrating a high degree of short-term predictability of EPR mainshocks by combining teleseismic and hydroacoustic earthquake catalogs. Thus, there appears to be a high degree of both short and long-term predictability on EPR transforms. [ABSTRACT FROM AUTHOR]

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominately aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion. [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]

The large-scale continuous GPS networks that have emerged in the last decade have documented a wide-range of aseismic deformation transients that resulted from physical processes such as aseismic fault slip and magma intrusion. In particular, a new class of slow earthquakes with durations ranging from days to months have been observed with GPS arrays located above the downdip portion of subduction zone thrust interfaces. Interpretation of the displacement time-series resulting from these events is not straightforward owing to the contaminating effects of multiple contributing signals such as fault-slip, local benchmark motion, seasonal effects, and reference frame errors. We have developed a time-dependent inversion algorithm based on the extended Kalman filter which can separate the various signals and allow the space–time evolution of these slow-slip transients to be studied in detail. We applied the inversion algorithm to the 1999 Cascadia slow earthquake. This event had two primary episodes of moment-release separated by a two week period in which relatively little moment-release occurred. The Cascadia event and other slow earthquakes share numerous similarities with both ordinary earthquakes and afterslip transients suggesting that they may represent fault slip under a conditionally stable regime. [ABSTRACT FROM AUTHOR]

Develops a method that inverts measurements of the shifts in traveltime and amplitude of various seismic phases at global stations for the second-degree polynomial moments of an event's space-time distribution. General description of seismic sources; Spatial and temporal event of an earthquake's rupture; Affects of unmodelled lateral heterogeneity.

Reports on the first extensive deep-earthquake aftershock sequence observed after the Tonga earthquake in March 9, 1994. Review of related literature; Number of aftershocks recorded; Investigation of spatial relationships.

Great subduction zone earthquakes vary considerably in the updip extent of megathrust rupture. It is unclear if this diversity reflects variations in interseismic strain accumulation owing to the limited number of subduction zones with seafloor monitoring. We use a borehole seismic‐geodetic observatory installed at the updip end of the Cascadia fault offshore Vancouver Island to show that the megathrust there does not appear to slip in triggered tremor or slow‐slip events when subjected to moderate dynamic stress transients. Borehole tilt and seismic data from recent teleseismic M7.6–8.1 earthquakes demonstrate a lack of triggered slow slip above the Mw 4.0 level and an absence of triggered tremor despite shear‐stress transients of 1–10 kPa that were sufficient to trigger tremor on the downdip end of the interface. Our observations are most consistent with a model in which the Cascadia fault offshore Vancouver Island is locked all the way to the trench. Plain Language Summary: Subduction zone thrust faults are generally thought to contain three primary regions in terms of earthquake rupture. Both the shallowest region near the trench (depths <~5–10 km) and the deeper onshore region (depths >~35 km) are thought to fail primarily without earthquakes, while seismic slip is contained primarily within the intervening depth range (~5–35 km). In many subduction zones around the world, the deeper region has been observed to emit small amounts of seismic radiation when nonvolcanic tremor events are triggered by passing seismic waves. Similarly, the shallowest portions of a few subduction zones behave the same way. Here we study the Cascadia subduction zone with newly available subseafloor seismic and geodetic data that indicate a clear difference in behavior between the deep and shallow parts of the fault. This likely indicates that strain is accumulated across most of the breadth of the accretionary prism and that the shallow part of the fault may rupture during great earthquakes. Key Points: New borehole seismic and geodetic data from the updip end of the Cascadia subduction zone show no evidence of triggered tremor or slow slip during the passage of teleseismic surface wavesThere is a clear contrast in the behavior of triggered tremor between the updip and downdip ends of the Cascadia subduction zone at Vancouver IslandBorehole tilt can rule out slow slip events at the Mw 4.0 level [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]