Future Cascadia megathrust rupture delineated by episodic tremor and slip

A suite of 15 episodic tremor and slip events imaged between 1997 and 2008 along the northern Cascadia subduction zone suggests future coseismic rupture will extend to 25 km depth, or approximately 60 km inland of the Pacific coast, rather than stopping offshore at 15 km depth. An ETS-derived coupling profile accurately predicts GPS measured interseismic deformation of the overlying North American plate, as measured by approximately 50 continuous GPS stations across western Washington State. When extrapolated over the 550-year average recurrence interval of Cascadia megathrust events, the coupling model also replicates the pattern and amplitude of coseismic coastal subsidence inferred from previous megathrust earthquakes here. For only the Washington State segment of the Cascadia margin, this translates into an Mw = 8.9 earthquake, with significant moment release close to its metropolitan centers

Episodic Tremor and Slip in the Pacific Northwest

Every 14 months the Pacific Northwest experiences slow slip on a fault that is the equivalent of about a magnitude 6.5 earthquake. While a typical earthquake of this magnitude happens in less than 10 seconds, the duration of these slip events is two to several weeks. The most recent event occurred from January 14 through February 1, 2007

Precursory transient slip during the 2001 M\u3csub\u3ew\u3c/sub\u3e = 8.4 Peru earthquake sequence from continuous GPS

Two-hour position estimates from a continuous GPS station located at Arequipa, Peru, document precursory deformation beginning 18 hours prior to an Mw = 7.6 aftershock of the June 23rd 2001 Mw = 8.4 earthquake. This preseismic signal appears on the north and east components as a slow displacement with an amplitude twice that of the subsequent coseismic. Analysis of three years of 18-hour rate measurement shows this signal to be unprecedented and beyond four standard deviations from the mean rate. The best fitting centroid is directionally consistent with slow slip along the plate interface and suggests the preseismic deformation arises from creep near the aftershock rupture. This implies the Nazca-South American plate interface slipped slowly prior to seismogenic faulting. These observations indicate the Mw = 7.6 earthquake grew out of slow slip along the plate interface and clearly demonstrate the breadth of slip rates accommodated by subduction zone plate interfaces

Detection of arbitrarily large dynamic ground motions with a dense high-rate GPS network

We describe the detection of teleseismic surface waves from the 3 November 2002 Mw 7.9 Denali fault earthquake in Alaska with a dense network of 1 Hz GPS stations in southern California, about 3900 km from the event. Relative horizontal displacements with amplitudes in excess of 15 mm and duration of 700 seconds agree with integrated velocities recorded by nearby broadband seismometers with an rms difference of 2–3 mm. The displacements are derived from independent 1 Hz instantaneous positions demonstrating that a GPS network can provide direct measurements of arbitrarily large dynamic and static ground horizontal displacements at periods longer than 1 s and amplitudes above 2 mm, with an inherent precision (signal to noise) that improves indefinitely with amplitude without clipping and in real time. High‐rate, real‐time GPS networks can enhance earthquake detection and seismic risk mitigation and support other applications such as intelligent transportation and civil infrastructure monitoring

GPS Vertical Land Motion Corrections to Sea-Level Rise Estimates in the Pacific Northwest

We construct coastal Pacific Northwest profiles of vertical land motion (VLM) known to bias long-term tide-gauge measurements of sea-level rise (SLR) and use them to estimate absolute sea-level rise with respect to Earth’s center of mass. Multidecade GPS measurements at 47 coastal stations along the Cascadia subduction zone show VLM varies regionally but smoothly along the Pacific coast and inland Puget Sound with rates ranging from +4.9 to –1.2 mm/yr. Puget Sound VLM is characterized by uniform subsidence at relatively slow rates of +0.1 to –0.3 mm/yr. Uplift rates of 4.5 mm/yr persist along the western Olympic Peninsula of northwestern Washington State and decrease southward becoming nearly 0 mm/yr south of central coastal Washington through Cape Blanco, Oregon. South of Cape Blanco, uplift increases to 1–2 mm/yr, peaks at 4 mm/yr near Crescent City, California, and returns to zero at Cape Mendocino, California. Using various stochastic noise models, we estimate long-term (~50–100 yr) relative sea-level rise rates at 18 coastal Cascadia tide gauges and correct them for VLM. Uncorrected SLR rates are scattered, ranging between –2 mm/yr and + 5 mm/yr with mean 0:52±1:59 mm/yr, whereas correcting for VLM increases the mean value to 1.99 mm/yr and reduces the uncertainty to ±1:18 mm/yr, commensurate with, but approximately 17% higher than, twentieth century global mean

Moment release rate of Cascadia tremor constrained by GPS

A comparison of GPS and seismic analyses of 23 distinct episodic tremor and slip events, located throughout the Cascadia subduction zone over an 11-year period, yields a highly linear relationship between moment release, as estimated from GPS, and total duration of nonvolcanic tremor, as summed from regional seismic arrays. The events last 1–5 weeks, typically produce ~5 mm of static forearc deformation, and show cumulative totals of tremor that range from 40 to 280 h. Moment released by each event is estimated by inverting GPS-measured deformation, which is sensitive to all rates of tremor-synchronous faulting, including aseismic creep, for total slip along the North American-Juan de Fuca plate interface. Tremor, which is shown to be largely invariant in amplitude and frequency content both between events and with respect to its duration, is quantified using several different parameterizations that agree to within 10%. All known Cascadia events detected since 1997, which collectively span the Cascadia arc from northern California to Vancouver Island, Canada, release moment during tremor at a rate of 5.2 ± 0.4 X 1016 N m per hour of recorded tremor. This relationship enables estimation of moment dissipation, via seismic monitoring of tremor, along the deeper Cascadia subduction zone that poses the greatest threat to its major metropolitan centers

Southern Cascadia Episodic Slow Earthquakes

Continuous GPS and seismic data from northern California show that slow earthquakes periodically rupture the Gorda-North America plate interface within southern Cascadia. On average, these creep events have occurred every 10.9±1.2 months since at least 1998. Appearing as week-long GPS extensional transients that reverse secular forearc contraction, the data show a recurrence interval 22% shorter than slow events recognized to the north. Seismic tremor here accompanies the GPS reversals, correlated across as many as 5 northern California seismometers. Tremor occurs sporadically throughout the year, but increases in duration and intensity by a factor of about 10 simultaneous with the GPS reversals. Beneath west-central Oregon, three reversals are also apparent, but more stations are needed to confirm sporadic slip on the plate interface here. Together, these measurements suggest that slow earthquakes likely occur throughout the Cascadia subduction zone and add further evidence for the role of fault-fluid migration in controlling transient slow-slip events here

Southern Cascadia episodic slow earthquakes

Continuous GPS and seismic data from northern California show that slow earthquakes periodically rupture the Gorda‐North America plate interface within southern Cascadia. On average, these creep events have occurred every 10.9 ± 1.2 months since at least 1998. Appearing as week‐long GPS extensional transients that reverse secular forearc contraction, the data show a recurrence interval 22% shorter than slow events recognized to the north. Seismic tremor here accompanies the GPS reversals, correlated across as many as 5 northern California seismometers. Tremor occurs sporadically throughout the year, but increases in duration and intensity by a factor of about 10 simultaneous with the GPS reversals. Beneath west‐central Oregon, three reversals are also apparent, but more stations are needed to confirm sporadic slip on the plate interface here. Together, these measurements suggest that slow earthquakes likely occur throughout the Cascadia subduction zone and add further evidence for the role of fault‐fluid migration in controlling transient slow‐slip events here

Real-time Monitoring of Tectonic Displacements in the Pacific Northwest through an Array of GPS Receivers

The Pacific Northwest Geodesic Array at Central Washington University collects telemetered streaming data from 450 GPS stations. These real-time data are used to monitor and mitigate natural hazards arising from earthquakes, volcanic eruptions, landslides, and coastal sea-level hazards in the Pacific Northwest. The displacement measurements are performed at millimeter-scale, and require stringent analysis and parameter estimation techniques. Recent improvements in both accuracy of positioning measurements and latency of terrestrial data communication have led to the ability to collect data with higher sampling rates, of up to 1 Hz. For seismic monitoring applications, this means 1350 separate position streams from stations located across 1200 km along the West Coast of North America must be able to be both visually observed and analyzed automatically. We aim to make the real-time information from GPS sensors easily available, including public access via interfaces for all intelligent devices with a connection to the Internet. Our contribution is a dashboard application that monitors the real-time status of the network of GPS sensors. We are able to visualize individual and multiple sensors using similar time series scales. We are also able to visualize groups of sensors based on time-dependent statistical similarity, such as sensors with the the highest variance, in real-time. In addition to raw positioning data, users can also display derived quantities, such as the Allan variance or the second derivative of a data stream

Noise Characteristics of Operational Real‐Time High‐Rate GNSS Positions in a Large Aperture Network

Large earthquakes are difficult to model in real‐time with traditional inertial seismic measurements. Several algorithms that leverage high‐rate real‐time Global Navigation Satellite Systems (HR‐GNSS) positions have been proposed, and it has been shown that they can supplement the earthquake monitoring effort. However, analyses of the long‐term noise behavior of high‐rate real‐time GNSS positions, which are important to understand how the data can be used operationally by monitoring agencies, have been limited to just a few sites and to short time spans. Here, we show results from an analysis of the noise characteristics of 1 year of positions at 213 GNSS sites spanning a large geographic region from Southern California to Alaska. We characterize the behavior of noise and propose several references noise models which can be used as baselines to compare against as technological improvements allow for higher precision solutions. We also show how to use the reference noise models to generate realistic synthetic noise that can be used in simulations of HR‐GNSS waveforms. We discuss spatiotemporal variations in the noise and their potential sources and significance. We also detail how noise analysis can be used in a dynamic quality control to determine which sites should or should not contribute positions to an earthquake modeling algorithm at a particular moment in time. We posit that while there remain important improvements yet to be made, such as reducing the number of outliers in the time series, the present quality of real‐time HR‐GNSS waveforms is more than sufficient for monitoring large earthquakes