Brendan W. Crowell

Earthquake magnitude calculation without saturation from the scaling of peak ground displacement

GPS instruments are noninertial and directly measure displacements with respect to a global reference frame, while inertial sensors are affected by systematic offsets—primarily tilting—that adversely impact integration to displacement. We study the magnitude scaling properties of peak ground displacement (PGD) from high-rate GPS networks at near-source to regional distances (~10–1000 km), from earthquakes between Mw6 and 9. We conclude that real-time GPS seismic waveforms can be used to rapidly determine magnitude, typically within the first minute of rupture initiation and in many cases before the rupture is complete. While slower than earthquake early warning methods that rely on the first few seconds of P wave arrival, our approach does not suffer from the saturation effects experienced with seismic sensors at large magnitudes. Rapid magnitude estimation is useful for generating rapid earthquake source models, tsunami prediction, and ground motion studies that require accurate information on long-period displacements.

Seismogeodesy of the 2014 Mw6.1 napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike‐slip fault

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 Mw6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

A model for the production of sulfur floc and “snowblower” events at mid‐ocean ridges

At mid-ocean ridges following magmatic eruptions, biogenic floc emerges from the seafloor and blankets regions of the seafloor in what have been called “snowblower” events. The floc often consists of filaments of elemental sulfur, and similar byproducts have been produced by hydrogen sulfide oxidizing bacteria in the laboratory. In this paper we estimate the rate of sulfur floc production in two ways. First, we compare the flux of H2S from high temperature vents and adjacent diffuse flow near 9°50′N on the East Pacific Rise to estimate the rate at which subsurface microbes use H2S to produce elemental sulfur in the shallow crust since the 1991 eruption. Second, we use the data from laboratory experiments to provide an upper estimate of sulfur floc production during a volcanic eruption. The results suggest that that the floc observed during “snowblower” events is most likely a combination of a bloom event and floc that has been stored in the crust between eruption cycles. The calculations also suggest that a ∼1% reduction in crustal porosity would result from the microbial production and storage of sulfur floc during the 1991 to 2000 time interval.

Single‐station automated detection of transient deformation in GPS time series with the relative strength index: A case study of Cascadian slow‐slip

The discovery of slow-slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of slow-slip events is necessary for studies of regional interseismic, coseismic and postseismic deformation, and miscalculations can affect our understanding of the regional stress field and tectonic hazard. Because of the proliferation of GPS data over the last two decades, an automated algorithm is required to analyze the signals and model the deformation on a station by station basis. Using the relative strength index (RSI), a financial momentum oscillator, we test the ability to detect events of various sizes and durations. We first determine the statistics of the RSI under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals. We then apply the technique to daily GPS displacement time series from 213 stations along the Cascadia subduction zone to form a record of transient deformation between 2005 and 2016. Our estimates of the spatial extent, duration, and propagation of major episodic tremor and slip (ETS) events are consistent with previous studies. We use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications. Finally, we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ETS events.

Modeling and On-the-Fly Solutions for Solid Earth Sciences: Web Services and Data Portal for Earthquake Early Warning System

We report on a unified on-the-fly, Web services-based observation/analysis/modeling environment for crustal deformation and natural hazards research, intended as a plug-in service for early warning systems, transfer of rapid information to civilian decision makers and the media, and educational purposes. We demonstrate an early warning system for a large earthquake in southern California using the components developed under several NASA-funded projects, including real-time GPS network infrastructure, Geophysical Resources Web Services, and GPS Explorer data portal with its on-the-fly earthquake modeling software. We generate an on-the-fly earthquake fault model based on simulated real-time (dynamic and coseismic) displacements computed by the GPS network, and viewable through GPS Explorer. Our objective is to demonstrate an operational service that could be transitioned to appropriate civilian agencies in southern California.

Earthquake Early Warning: ShakeAlert in the Pacific Northwest

We evaluate the performance of earthquake early warning algorithm ElarmS‐2 (earthquake alarm system v. 2) in the Pacific Northwest. Real‐time and prerecorded seismic data from Oregon, California, and Washington in the United States and British Columbia in Canada are used. The earthquakes tested range up to moment magnitude 7.2, the limit for which the ElarmS‐2 magnitude method is accurate. ElarmS‐2 reliably detects catalog magnitude 3 and larger earthquakes within the network, but the cut‐off magnitude for accurate event recognition is higher for events offshore and on the edges of the network. We have made several adjustments that make the ElarmS‐2 algorithm less likely to falsely report multiple alerts for earthquakes. Replaying of past earthquakes shows that the new settings improve the algorithm’s behavior for edge cases while not degrading previously well‐constrained solutions within the dense part of the network. We expect few false alerts, none that would predict significant shaking anywhere in our region. Even though ElarmS‐2 assumes a fixed earthquake depth, the epicenter and magnitude estimates are accurate enough to provide good predictions of shaking intensities. Online Material: Tables listing parameters of 31 calibration earthquakes, ElarmS‐2 real‐time detections, and comparison of ElarmS‐2 performance for calibration events before and after adjusting configuration.

Development of a Geodetic Component for the U.S. West Coast Earthquake Early Warning System

An earthquake early warning (EEW) system, ShakeAlert, is under development for the West Coast of the United States. This system currently uses the first few seconds of waveforms recorded by seismic instrumentation to rapidly characterize earthquake magnitude, location, and origin time; ShakeAlert recently added a seismic line source algorithm. For large to great earthquakes, magnitudes estimated from the earliest seismic data alone generally saturate. Real-time Global Navigation Satellite System (GNSS) data can directly measure large displacements, enabling accurate magnitude estimates for Mw7 events, possibly before rupture termination. GNSS-measured displacements also track evolving slip and, alone or in combination with seismic data, constrain finite-fault models. Particularly for large-magnitude, long-rupture events, GNSS-based magnitude and rupture extent estimates can improve updates to predicted shaking and thus alert accuracy. GNSS data processing centers at ShakeAlert partner institutions provide real-time streams to the EEWsystem, and three geodetic EEW algorithms have been developed through the ShakeAlert collaboration. These algorithms will undergo initial testing within ShakeAlert’s computational architecture using a suite of input data that includes simulated real-time displacements from synthetic earthquakes and GNSS recordings from recent earthquakes worldwide. Performance will be evaluated using metrics and standards consistent with those adopted for ShakeAlert overall. This initial assessment will guide method refinement and synthesis of the most successful features into a candidate geodetic algorithm for the ShakeAlert production system. In parallel, improvements to geodetic networks and streamlining approaches to data processing and exchange will ensure robust geodetic data availability in the event of an earthquake. Electronic Supplement: Table listing recent earthquakes for which high sample-rate (≥ 1 Hz) processed Global Positioning System data and seismic data have been gathered for use in testing geodetic earthquake early warning algorithms and a summary of ground-motion metrics adopted by ShakeAlert, the U.S. West Coast EEW system, for evaluating new or updated components before adoption in the production system, and a schematic diagram of the real-time Global Navigation Satellite Systems data flow for ShakeAlert.