Fully probabilistic seismic source inversion - Part 1: Efficient parameterisation

Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Seismic source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of > 1000 source time functions (STFs) by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits propagating these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible

Triplicated P-wave measurements for waveform tomography of the mantle transition zone

Triplicated body waves sample the mantle transition zone more extensively than any other wave type, and interact strongly with the discontinuities at 410 km and 660 km. Since the seismograms bear a strong imprint of these geodynamically interesting features, it is highly desirable to invert them for structure of the transition zone. This has rarely been attempted, due to a mismatch between the complex and band-limited data and the (ray-theoretical) modelling methods. Here we present a data processing and modelling strategy to harness such broadband seismograms for finite-frequency tomography. We include triplicated P-waves (epicentral distance range between 14 and 30°) across their entire broadband frequency range, for both deep and shallow sources. We show that is it possible to predict the complex sequence of arrivals in these seismograms, but only after a careful effort to estimate source time functions and other source parameters from data, variables that strongly influence the waveforms. Modelled and observed waveforms then yield decent cross-correlation fits, from which we measure finite-frequency traveltime anomalies. We discuss two such data sets, for North America and Europe, and conclude that their signal quality and azimuthal coverage should be adequate for tomographic inversion. In order to compute sensitivity kernels at the pertinent high body wave frequencies, we use fully numerical forward modelling of the seismic wavefield through a spherically symmetric Earth

The future of passive seismic acquisition

It is an exciting time to be a seismologist. In November 2018, the InSight lander touched down on Mars and the first seismometer was deployed on another planet. This incredible feat means planetary seismologists are currently searching for marsquakes and will hopefully soon be providing images of its interior and helping us to understand how rocky planets form. However, we have been doing this for a long time in more familiar territory back home on Earth, where the field of terrestrial seismology has reached a turning point with significant developments in instrumentation and the manner of their deployment in recent years. However, equipment available to the UK community has not kept pace and needs urgent regeneration if the UK is to lead in the field of passive seismology in the future. To begin the process of redesigning the UK’s equipment for the next few decades, the British Geophysical Association sponsored a meeting in Edinburgh in late 2018 to discuss the future of passive seismic acquisition. What follows is a historical account of how and why we arrived at the present day UK seismological research and resource base, a summary of the Edinburgh meeting, and a vision for the passive seismic facilities required to support the next 20 years of seismological research

How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot

Increasingly, spatial geochemical zonation, present as geographically distinct, subparallel trends, is observed along hotspot tracks, such as Hawaii and the Galapagos. The origin of this zonation is currently unclear. Recently zonation was found along the last B70 Myr of the Tristan-Gough hotspot track. Here we present new Sr–Nd–Pb–Hf isotope data from the older parts of this hotspot track (Walvis Ridge and Rio Grande Rise) and re-evaluate published data from the Etendeka and Parana flood basalts erupted at the initiation of the hotspot track. We show that only the enriched Gough, but not the less-enriched Tristan, component is present in the earlier (70–132 Ma) history of the hotspot. Here we present a model that can explain the temporal evolution and origin of plume zonation for both the Tristan-Gough and Hawaiian hotspots, two end member types of zoned plumes, through processes taking place in the plume sources at the base of the lower mantle

Mantle provinces under North America from multifrequency P wave tomography

This is a survey of mantle provinces (large-scale seismic anomalies) under North America, from the surface down to 1500-1800 km depth. The underlying P velocity model was obtained by multifrequency tomography, a waveform-based method that systematically measures and models the frequency-dependence of teleseismic body waves. A novel kind of three-dimensional rendering technique is used to make the considerable structural complexities under North America accessible. In the transition zone and below, the North American mantle is dominated by seismically fast provinces, which represent distinct subduction episodes of the Farallon plate. I attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes. Differences in vertical sinking velocity have led to large vertical offsets across adjacent, coeval slabs. Some of the mantle provinces have not been discussed much previously, including (1) a seismically slow blanket overlying the oldest Farallon subduction along the eastern continental margin, (2) a transition zone slab coeval with the Laramide orogeny (ca. 80-60 Myr), which I discuss in analogy to the "stagnant slab" subduction style commonly found in the western Pacific today, (3) the lower mantle root of present-day Cascadia subduction, which may have started out as intraoceanic subduction,(4) a lower mantle slab under Arizona and New Mexico, the last material to subduct before strike-slip motion developed along the San Andreas boundary, and (5) two narrow plate tears thousands of kilometers long, one of which is the subducted conjugate of the Mendocino Fracture Zone. Copyright 2011 by the American Geophysical Union

Circum-Arctic mantle structure from global tomography – consistent with plate reconstructions?

I present a preliminary, global-scale tomographic P-velocity model, and discuss it with a focus on the Arctic hemisphere. The model was obtained from waveform inversion of teleseismic P-waves, specifically using the method of multi-frequency inversion, an extension of finite-frequency tomography that systematically exploits the entire usable body-wave spectrum. The transition zone and mid-mantle are decently sampled, since the Arctic is surrounded by well-instrumented continents (Eurasia, North America, Japan). In addition, the past decade has seen the addition of a significant number of stations on Greenland and surrounding islands. I use a rigorously quality-controlled data set of broadband seismograms from IRIS, which is rather complete for the years 1999-2009, together with a smaller data set from the European data center ORFEUS. Global tomography models have rarely been discussed with a focus on the circum-Arctic region. Accordingly, this integrated investigation of tomography and plate tectonics is still in a reconnaissance stage. I compare my own model and a few other body-wave tomographies to a plate reconstruction model, in an attempt match up seismically fast anomalies (subducted slabs), with predicted paleo-trench locations. Shallow anomalies should correspond to recent subduction, deeper slabs to older subduction episodes. Slabs that are not overlain by a modeled trench at any time, or paleo-trenches without fast anomaly observed underneath, can point to gaps in our current understanding of the Arctic’s plate-tectonic evolution

Circum-Arctic mantle structure from global tomography – consistent with plate reconstructions?

I present a preliminary, global-scale tomographic P-velocity model, and discuss it with a focus on the Arctic hemisphere. The model was obtained from waveform inversion of teleseismic P-waves, specifically using the method of multi-frequency inversion, an extension of finite-frequency tomography that systematically exploits the entire usable body-wave spectrum. The transition zone and mid-mantle are decently sampled, since the Arctic is surrounded by well-instrumented continents (Eurasia, North America, Japan). In addition, the past decade has seen the addition of a significant number of stations on Greenland and surrounding islands. I use a rigorously quality-controlled data set of broadband seismograms from IRIS, which is rather complete for the years 1999-2009, together with a smaller data set from the European data center ORFEUS. Global tomography models have rarely been discussed with a focus on the circum-Arctic region. Accordingly, this integrated investigation of tomography and plate tectonics is still in a reconnaissance stage. I compare my own model and a few other body-wave tomographies to a plate reconstruction model, in an attempt match up seismically fast anomalies (subducted slabs), with predicted paleo-trench locations. Shallow anomalies should correspond to recent subduction, deeper slabs to older subduction episodes. Slabs that are not overlain by a modeled trench at any time, or paleo-trenches without fast anomaly observed underneath, can point to gaps in our current understanding of the Arctic’s plate-tectonic evolution

obspyDMT: A Python toolbox for retrieving and processing of large seismological datasets

We present obspyDMT, a free, open source software toolbox for the query, retrieval, processing and management of seismological data sets, including very large, heterogeneous, and/or dynamically growing ones. obspyDMT simplifies and speeds up user-interaction with data centers, in more versatile ways than existing tools. The user is shielded from the complexities of interacting with different data centers and data exchange protocols, and is provided with powerful diagnostic and plotting tools to check the retrieved data and meta-data. While primarily a productivity tool for research seismologists and observatories, easy-to-use syntax and plotting functionality also make obspyDMT an effective teaching aid. Written in the Python programming language, it can be used as a stand-alone command line tool (requiring no knowledge of Python) or can be integrated as a module with other Python codes. It facilitates data archival, pre-processing, instrument correction, and quality control – routine but non-trivial tasks that can consume much user time.We describe obspyDMT’s functionality, design and technical implementation, accompanied by an overview of its use cases. As an example of a typical problem encountered in seismogram preprocessing, we show how to check for inconsistencies in response files of two example stations. We also demonstrate the fully automated request, remote computation, and retrieval of synthetic seismograms from IRIS DMC’s Syngine webservice

Yellowstone debate erupts again

The heat driving Yellowstone’s volcanism originates from a depth of at least 700 km, according to images of the mantle created using novel seismic methods