Timothy Ian Melbourne

Rapid postseismic transients in subduction zones from continuous GPS

[1] Continuous GPS time series from three of four recently measured, large subduction earthquakes document triggered rapid postseismic fault creep, representing an additional moment release upward of 25% over the weeks following their main shocks. Data from two Mw = 8.0 and Mw = 8.4 events constrain the postseismic centroids to lie down dip from the lower limit of coseismic faulting, and show that afterslip along the primary coseismic asperities is significantly less important than triggered deep creep. Time series for another Mw = 7.7 event show 30% postseismic energy release, but here we cannot differentiate between afterslip and triggered deeper creep. A fourth Mw = 8.1 event, which occurred in the broad Chilean seismogenic zone, shows no postseismic deformation, despite coseismic offsets in excess of 1 m. For the three events which are followed by postseismic deformation, stress transferred to the inferred centroids (at 34, 60, and 36 km depths) by their respective main shock asperities increased reverse shear stress by 0.5, 0.8, and 0.2 bar with a comparatively small decrease in normal stress (0.01 bar), constraining the Coulomb stress increase required to force slip along the metastable plate interface. Deep triggered slip of this nature is invisible without continuous geodesy but on the basis of these earthquakes would appear to constitute an important mode of strain release from beneath the seismogenic zones of convergent margins. These events, captured by some of the first permanent GPS networks, show that deep moment release is often modulated by seismogenic rupture updip and underscore the need for continuous geodesy to fully quantify the spectrum of moment release in great earthquakes. INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8150 Tectonophysics: Plate boundary—general (3040); 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: subduction zone, earthquake, postseismic, moment release

The geodetic signature of the M8.0 Oct. 9, 1995, Jalisco subduction earthquake

The October, 1995 Mw 8.0 Jalisco subduction earthquake has provided a thorough geodetic observation of the coseismic subduction process. An 11 station regional GPS network located directly onshore of the rupture demonstrates consistent vertical subsidence verified by tide gauge data and southwest-directed extension, with measured displacements reaching 1 meter. Unusually shallow and non-uniform faulting is required to explain the displacements. We determine that up to 5 meters of slip occurred within the upper 15 km of the thrust fault zone and 2 meters possibly as shallow as 8 km, and that slip was likely distributed in two main patches. The paucity of continental sediments in this subduction zone could be responsible for the anomalously shallow faulting.

Mantle Control of Plate Boundary Deformation

The seismic wavefield propagating along the recently instrumented Pacific-North American plate boundary (California) displays remarkable variation, with regional shear waves arriving at coastal stations up to 20 seconds earlier than equidistant stations in eastern California. Broadband modeling of this data reveals that coastal paths sample fast upper mantle typical of Miocene-aged ocean plate (>50 Km thickness). Inland paths sample slower uppermost mantle, with the seismic lithosphere, or lid, measuring less than 5 Km thick, characteristic of the Basin and Range extensional province. The boundary in the uppermost mantle between these provinces is sharp, expressing the juxtaposition of the stronger Pacific plate with weaker continental North America. The lid step coincides with regionally maximum dextral strain rates measured with GPS, suggesting the uppermost mantle modulates long term, regional-scale continental margin deformation and evolution.

Fine structure of the 410-km discontinuity

The April 14, 1995, earthquake in western Texas (M_w 5.7) produced a strong topside reflection off the 410-km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern United States. Data from 394 vertical short-period and 24 broadband instruments provide dense coverage of this event from distances of 11° to 19° and provide a detailed look at the subcontinental 410-km structure. The salient features of this data set are (1) the strong dependence on wavelength of the 410-km triplication range, (2) the uniform amplitude ratio of the direct P and reflected P_(410) phases on both short-period and broadband recordings throughout the triplication, and (3) the abrupt termination of the short-period P_(410) phase at 13.3°. These features are best modeled by a composite discontinuity in which a sharp velocity jump of 3% is overlain by a linear velocity jump of 3.5% spread over 14 km. The interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long- and short-period triplication range and the step-like behavior of the P_(410) short-period amplitude, which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity. This composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency. Additionally, this is the structure expected from mineralogical arguments for the α to β olivine phase transition.

Plate Boundary Observatory and related networks: GPS data analysis methods and geodetic products

The Geodesy Advancing Geosciences and EarthScope (GAGE) Facility Global Positioning System (GPS) Data Analysis Centers produce position times series, velocities and other parameters for approximately two thousand continuously operating GPS receivers spanning a quadrant of Earths surface encompassing the high Arctic, North America and Caribbean. The purpose of this review is to document the methodology for generating station positions and their evolution over time, and to describe the requisite tradeoffs involved with combination of results. GAGE GPS analysis involves formal merging within a Kalman filter of two independent, loosely-constrained solutions: one is based on precise point positioning produced with the GPS Inferred Positioning System, Orbit Analysis Simulation (GIPSY/OASIS) software at Central Washington University (CWU) and the other is a network solution based on phase and range double-differencing produced with the GPS at MIT (GAMIT) software at New Mexico Institute of Mining and Technology (NMT). The primary products generated are the position time series that show motions relative to a North America reference frame and secular motions of the stations represented in the velocity field. The position time series themselves contain a multitude of signals in addition to the secular motions. Co-seismic and post-seismic signals, seasonal signals from hydrology, and transient events, some understood and other not yet fully explained, are all evident in the time series and ready for further analysis and interpretation. We explore the impact of analysis assumptions on the reference frame realization and on the final solutions, and we compare within the GAGE solutions and with others.

Whole mantle shear structure beneath the East Pacific Rise

We model broadband seismograms containing triplicated S, S^2, and S^3 along with ScS to produce a pure path one-dimensional model extending from the crust to the core-mantle boundary beneath the East Pacific Rise. We simultaneously model all body wave shapes and amplitudes, thereby eliminating depth-velocity ambiguities. The data consist of western North American broadband recordings of East Pacific Rise (EPR) affiliate transform events that form a continuous record section out to 82° and sample nearly the entire East Pacific Rise. The best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages. The 660-km discontinuity is particularly well resolved and requires a steep gradient (4%), extending down to 745 km. We find no discernible variation in apparent depths of the 405- and 660-km discontinuities over ridge-orthogonal distances on the order of 1000 km (or 20 Ma lithosphere). Body waveform comparisons indicate that we can resolve discontinuity depths to less than ±10 km, providing an upper limit to transition zone topography. These depth estimates, in conjunction with the fan shot nature of the ray paths, lower the detection limit from S^2 precursor analysis of the lateral length scale over which short-wavelength topographic variation could occur and indicate the sub-EPR Transition Zone and upper mantle are remarkably homogeneous. The lower mantle beneath the East Pacific Rise is well modeled by PREM, with the greatest variation occurring in ScS, reflecting strong heterogeneity along the core-mantle boundary. Together, these observations require that the East Pacific Rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients. Dynamically, the transition zone therefore appears vertically decoupled from the overlying East Pacific Rise spreading system.

Anticipating the successor to Mexico's largest historical earthquake

Note in proof: On October 9, as this article was being prepared for publication, a magnitude 7.6 earthquake occurred beneath the Jalisco region and caused significant loss of life and property. This earthquake highlights the societal need for accurate measurements of crustal strain rates in earthquake-prone zones. In the coming months, we plan to measure the amount of displacement that occurred within the GPS network during and after this earthquake.

Critical Infrastructure Monitoring with Global Navigation Satellite Systems

AbstractThis paper documents measured deformation within four global positioning system (GPS) networks deployed on critical, heavy-engineered infrastructure in real time over a combined 5-year period. The first is an ∼2-km, four-lane floating freeway that deforms daily in response to temperature and traffic loads and seasonal lake-level variation. The second is a 6-lane elevated freeway ∼1 km in length that has subsided unevenly and discontinuously since it was damaged by the 2001 (MW 6.8) Nisqually earthquake. Two additional structures comprise ∼300-m-long, earth-filled dams forming major reservoirs (Howard A. Hanson and Tolt dams). Real-time kinematic processing of high rate (1s or 5-s epochs) GPS observations over short baselines (0.1 to ∼1 km) permits continuous deformation monitoring at centimeter-level accuracy, whereas long-term deformation was measured at subcentimeter accuracy through postprocessing of 24-h observations. The floating freeway showed 60 cm of annual vertical displacement and a 1.4-...

Clustering and visualization of geodetic array data streams using self-organizing maps

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. 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. 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 automatically analyzed at a sampling rate of up to 1 Hz. Our goal is to efficiently extract and visualize useful information from these data streams. We propose a method to visualize the geodetic data by clustering the signal types with a Self-Organizing Map (SOM). The similarity measure in the SOM is determined by the similarity of signals received from GPS stations. Signals are transformed to symbol strings, and the distance measure in the SOM is defined by an edit distance. The symbol strings represent data streams and the SOM is dynamic. We overlap the resulted dynamic SOM on the Google Maps representation.

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.