Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling

The 25 October 2010 Mentawai tsunami earthquake (M_w 7.8) and the tsunami hazard presented by shallow megathrust ruptures

The 25 October 2010 Mentawai, Indonesia earthquake (M_w 7.8) ruptured the shallow portion of the subduction zone seaward of the Mentawai islands, off-shore of Sumatra, generating 3 to 9 m tsunami run-up along southwestern coasts of the Pagai Islands that took at least 431 lives. Analyses of teleseismic P, SH and Rayleigh waves for finite-fault source rupture characteristics indicate ∼90 s rupture duration with a low rupture velocity of ∼1.5 km/s on the 10° dipping megathrust, with total slip of 2–4 m over an ∼100 km long source region. The seismic moment-scaled energy release is 1.4 × 10^(−6), lower than 2.4 × 10^(−6) found for the 17 July 2006 Java tsunami earthquake (M_w 7.8). The Mentawai event ruptured up-dip of the slip region of the 12 September 2007 Kepulauan earthquake (M_w 7.9), and together with the 4 January 1907 (M 7.6) tsunami earthquake located seaward of Simeulue Island to the northwest along the arc, demonstrates the significant tsunami generation potential for shallow megathrust ruptures in regions up-dip of great underthrusting events in Indonesia and elsewhere

Depth-varying rupture properties of subduction zone megathrust faults

Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra-Andaman (M_w 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (M_w 9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones – coherent teleseismic short-period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short-period radiation. We represent these and other depth-varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ∼35 km deep, large earthquake displacements occur over large-scale regions with only modest coherent short-period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ∼35 to 55 km deep. These isolated patches produce bursts of coherent short-period energy both in great ruptures and in smaller, sometimes repeating, moderate-size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short-period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30–45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low-frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone

Effects of Kinematic Constraints on Teleseismic Finite-Source Rupture Inversions: Great Peruvian Earthquakes of 23 June 2001 and 15 August 2007

Two great underthrusting earthquakes that occurred along the coast of Peru in 2001 and 2007 involve spatiotemporal slip distributions that differ from the predominantly unilateral or bilateral rupture expansion of many great events. Commonly used finite-source rupture model parameterizations, with specified rupture velocity and/or short duration of slip at each grid point applied to the seismic data for these two events, lead to incorrect slip-distributions or inaccurate estimation of rupture velocities as a result of intrinsic kinematic constraints imposed on the model slip distributions. Guided by large aperture array back projections of teleseismic broadband P-wave signals that image slip locations without imposing a priori kinematic constraints on the rupture process, we exploit the availability of large global broadband body and surface wave data sets to consider the effects of varying the kinematic constraints in teleseismic finite-source waveform inversions. By allowing longer than usual rupture durations at each point on the fault using a flexible subfault source-time function parameterization, we find that the anomalous attributes of the 2001 and 2007 Peru earthquake ruptures are readily recognized and accounted for by compound rupture models. The great 23 June 2001 (M_w 8.4 8.4) earthquake involved an initial modest-size event that appears to have triggered a much larger secondary event about 120 km away that developed an overall slip distribution with significant slip located back along the megathrust in the vicinity of the initial rupture. The great 15 August 2007 (M_w 8.0 8.0) earthquake was also a composite event, with a modest size initial rupture followed by a 60-sec delayed larger rupture that initiated 50–60 km away and spread up-dip and bilaterally. When back projections indicate greater rupture complexity than captured in a simple slip-pulse-type rupture model, one should allow for possible long-subfault slip-duration or composite triggered sequences, and not overly constrain the earthquake slip distribution

Curated Pacific Northwest AI-ready Seismic Dataset

The curation of seismic datasets is the cornerstone of seismological research and the starting point of machine-learning applications in seismology. We present a 21-year-long AI-ready dataset of diverse seismic event parameters, instrumentation metadata, and waveforms, as curated by the Pacific Northwest Seismic Network and ourselves. The dataset contains about 190,000 three-component (3C) waveform traces from more than 65,000 earthquake and explosion events, and about 9,200 waveforms from 5,600 exotic events. The magnitude of the events ranges from 0 to 6.4, while the biggest one is 20 December 2022 M6.4 Ferndale Earthquake. We include waveforms from high-gain (EH, BH, and HH channels) and strong-motion (EN channels) seismometers and resample to 100 Hz. We describe the earthquake catalog and the temporal evolution of the data attributes (e.g., event magnitude type, channel type, waveform polarity, and signal-tonoise ratio, phase picks) as the network earthquake monitoring system evolved through time. We propose this AI-ready dataset as a new open-source benchmark dataset

Метод калибровки и коррекции выходных сигналов трехосного акселерометра

Проблематика. Калібрування є важливим етапом роботи при введенні в експлуатацію таких навігаційних датчиків, як акселерометри. Мета дослідження. Метою роботи є дослідження можливості використання методу калібрування та корекції вихідних сигналів тривісного акселерометра. Методика реалізації. Для роботи з моделями вихідних сигналів застосовуються матричні методи лінійної алгебри. Зокрема, для визначення невідомих параметрів моделі використано метод найменших квадратів. Алгоритм корекції вихідних сигналів представлено у вигляді матричного запису розв’язку системи лінійних рівнянь. Для оцінки ефективності корекції вихідних сигналів використовувались такі методи математичної статистики, як середньоквадратичне відхилення та математичне сподівання. Результати дослідження. Отримано розрахункові формули для визначення параметрів калібрування, розроблено коректувальну ланку для вихідних сигналів тривісного акселерометра. Організовано експериментальні калібрування та опрацьовано отримані дані за допомогою алгоритму калібрування та коректувальної ланки. Приведено результати досліджень для кількох дослідних зразків тривісних акселерометрів. Висновки. Використання калібрувальних коефіцієнтів, що поєднують в собі похибки масштабних коефіцієнтів та неортогональності осей дає змогу значно зменшити обчислювальне навантаження на електронні блоки обробки сигналів та отримувати вихідні сигнали із задовільною точністю.Background. Calibration is one of the most important stages of work for putting into operation such navigation sensors as accelerometers. Objective. The aim of this study is to investigate the possibility of using the calibration and correction method of the output signals of the triaxial accelerometer. Methods. To work with the model outputs matrix methods of linear algebra are used. In particular, determining the unknown parameters of the model is based on the method of least squares. Correction algorithm is given in the form of a matrix notation for solving system of linear equations. Such methods of mathematical statistics as the standard deviation and the mathematical expectation were used for the output signals correction estimation. Results. Formulas for calculations of calibration parameters were obtained, and correction link for output accelerometer signals was developed. Experimental calibration was organized and the data obtained by the algorithm of calibration and correction link was processed. The results of studies for several test samples of triaxial accelerometers are presented. Conclusions. Using the calibration coefficients, which combine error scale factors and non-orthogonal axes errors can significantly reduce the computational load on the electronic signals processing unit and make output signals with a satisfactory accuracy.Проблематика. Калибровка является важным этапом роботы при введении в эксплуатацию таких навигационных датчиков, как акселерометры. Цель исследования. Целью работы является исследование возможности использования метода калибровки и коррекции выходных сигналов трехосного акселерометра. Методика реализации. Для работы с моделями выходных сигналов применяются матричные методы линейной алгебры. В частности, для определения неизвестных параметров модели использован метод наименьших квадратов. Алгоритм коррекции выходных сигналов приведен в виде матричной записи решения систем линейных уравнений. Для оценки эффективности коррекции выходных сигналов использовались такие методы математической статистики, как среднеквадратическое отклонение и математическое ожидание. Результаты исследования. Получены расчетные формулы для определения параметров калибровки, разработано корректирующее звено для выходных сигналов акселерометра. Организованы экспериментальные калибровки и обработаны полученные данные с помощью алгоритма калибровки и корректирующего звена. Приведены результаты исследований для нескольких исследуемых образцов трехосных акселерометров. Выводы. Использование калибровочных коэффициентов, которые объединяют в себе погрешности масштабных коэффициентов и неортогональности осей, позволяет значительно уменьшить вычислительную нагрузку на электронные блоки обработки сигналов и получать выходные сигналы с удовлетворительной точностью

Modeling the ratios of SKKS and SKS amplitudes with ultra‐low velocity zones at the core‐mantle boundary

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94737/1/grl26377.pd

Tomographic errors from wavefront healing: more than just a fast bias

Wave front healing, in which diffractions interfere with directly travelling waves causing a reduction in recorded traveltime delays, has been postulated to cause a bias towards faster estimated earth models. This paper reviews the theory from the mathematical physics community that explains the properties of diffractions and applies it to a suite of increasingly complicated numerical examples. We focus in particular on the elastic case and on the differences between P and S healing. We find that rather than introducing a systemic fast bias, wave front healing gives a more complicated bias in the results of traveltime tomography, with fast anomalies even manifesting themselves as slow anomalies in some situations. Of particular interest, we find that a negative correlation between the bulk and shear or compressional velocities may result to a large extend from healing.Netherlands Organization for Scientific Research (NWO:VICI865.03.007

Evaluation of 1‐D and 3‐D seismic models of the Pacific lower mantle with S, SKS, and SKKS traveltimes and amplitudes

In this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep‐focus earthquakes in the Tonga‐Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one‐dimensional (1‐D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S‐SKS differential traveltime of up to 10 s. SKKS‐SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1‐D and 3‐D) waveforms for three 1‐D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great‐circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short‐wavelength features from the 1‐D profiles. The large‐scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core‐mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear‐velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other. Key Points Waveform modeling and tomographic imaging is requiredPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97536/1/jgrb50054.pd