Abstract: When the ground moves, it does not only shake in translation, but it twists too! Indeed, any motion has 6 degrees of freedom. But all the seismometers provide only translation measurement. Rotational seismology is a hot topic in seismology since instruments have been built to bring first ground rotation measurement. It started as laboratory instrument with ultimate performances thanks to Giant Ring Laser Gyroscope. It is now portable on field with optimized performances thanks to Fiber Optic Gyroscope. And now Giant Fiber Optic Gyroscope arises to make extreme performances deployable. Based on experiences developing very low noise fiber-optic gyroscopes (FOG), recent performance results on deployable large fiber-optic coils of up to 1m diameter are presented, with a focus on comparison with Array Derivated Method (ADR) and Giant Ring Laser Gyroscopes (Giant-RLG). The goal for constructing large FOGs is to evaluate experimentally the physical limits of this kind of technology and to get closer of most innovative seismologic application. While these experiments are probing the fundamental limits of the FOG technology, they also serves as a first step for a cost effective very low noise laboratory rotational seismometer and would contribute in a second step to performance improvements on the portable rotational seismometer ‘blueSeis-3A’. To demonstrate a very low self-noise and characterize the quality of the data provided by a new instrument, the ideal way is a direct co-location measurement with an already qualified and much more precise instrument. The Giant-RLG, as the one located in Wettzell (Germany) is the ideal instrument to do so. Results of different prototypes on the road map will be presented to underline the applicability of each technological response to the Large-FOG requirements. Finally we conclude with the achieved results with a 1m scale diameter FOG having 6000m of fiber length upgraded with patented optical intensity noise subtraction.
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Abstract: One of the most common tasks in seismology is preparing data for analysis, which commonly involves parsing, transforming, and cleaning text data. When data are in tidy formats, like tabular files (e.g. CSV) or other structured files (e.g. JSON or XML), parsing and transforming data can leverage existing tools. When data are semistructured or otherwise ad hoc, such as legacy “in-house” formats or data collected directly from instruments, existing tools for reading and parsing data may not exist. Writing ad hoc parsing scripts is a common solution, but these are generally fragile, inflexible, and require re-writing for each new format. Parser generators, originally developed in the fields of linguistics and computers science, were developed to solve these problems. Here, we present the application of parser generators towards the parsing and transforming of two non-standard seismic bulletin formats. Parsing is done by declaring a “grammar”, or syntax, for the format, and then an existing parser generator tool is used to parse and optionally transform the data. A command-line utility written in Python, called Bulletproof, further facilitates parsing and conversion of new textual seismic data through a plugin infrastructure.
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Abstract: Oblique convergence between the Caribbean (CAR) and North American (NOAM) plates near Hispaniola and the island’s observed clockwise rotation have led to competing hypotheses to explain its tectonics. Clockwise rotation of Hispaniola could be the result of slab edge push generated by (a) subducting NOAM plate, (b) two subducted lithospheric slabs, one southward-dipping from the NOAM plate and one northward-dipping from the CAR Plate at the Muertos Trough interacting at depth, or (c) collision with the carbonate Bahamas Platform. Any of these scenarios or a combination of them can cause intermediate-depth earthquakes. Examining the location of intermediate depth earthquakes along with regional tomography, recorded via a dense temporary network of seismic stations, has the potential to resolve differences between competing hypotheses. A new 1D P velocity model was determined and then used as a starting model for iterative, nonlinear inversion for 3D structure. A catalog of over 2000 teleseismic events and 200 moderate-sized local events recorded by a total of 30 stations during the interval 2013-2017 is used. The catalog averages 25 P and S wave arrival picks per event with the largest local event magnitude being 5.8. While some events are located as deep as 185 km, over 80% are shallower than 35 km. The intermediate-depth earthquakes appear to be associated with a slow P velocity anomaly. The shape of the slow anomaly suggests a southward-dipping NOAM subducting slab with a change in dip from 30° to 50° at ~67 km depth. Deeper intermediate-depth earthquakes mark the leading edge of the feature we interpret to be the NOAM slab at ~100 km depth. A slow velocity zone extends to the surface beneath central Hispaniola and may be associated with the uplifted Cordillera Central, which includes Pico Duarte, the highest point in the Caribbean. There is no indication in the tomography of a northward-dipping CAR slab subducted beneath eastern Hispaniola.
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Abstract: The diapirism process in Colombia has generated the mud volcanoes in the Sinú Belt, which extends from the Urabá Gulf to the Barranquilla region, including land and marine areas. Most of these events have been associated with the presence of faults that have allowed the mud to rise to the surface. The implementation of an inversion scheme of the S-coda and P-coda waves was proposed in order to estimate the non-homogeneous spatial distribution of the scattering and attenuation coefficients and their relationship with the structures that originate the mud volcanism and thus quantify and characterize the terrestrial medium, allowing to make a structural analysis in the Sinú belt and establish the recurrence of these volcanic events associated with seismicity. The approach proposed in this work shows promising results in the characterization of temporal variations of volcanic structures due to its greater sensitivity to small changes than the direct waves-based analysis of velocity or attenuation.
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Abstract: Although it is generally thought that the Earth’s outer core is well mixed due to vigorous convection, and is therefore more or less compositionally homogeneous, there are, however, increasing evidence that suggests stratification may exist within the outermost outer core due to the presence of light elements. Recent seismic studies show that top a few hundreds kilometers of outer core possess a P-wave velocity slightly lower than the PREM model, which cannot be explained by self-compression of a chemically homogeneous outer core. Most studies utilize the SmKS arrival, which is a core phase being reflected m-1 times from the lower side of the core-mantle boundary (CMB) and can be observed at epicentral distances of 120°-180°. Differential arrival times between SmKS pairs, such as S3KS and SKKS, S4KS and S3KS, are usually employed in determine the P-wave velocity structure in the top part of the outer core since these pairs have very similar traveling ray paths in the mantle. Since there is a π/2 phase shift between two consecutive SmKS arrivals due to the internal caustic surface for underside reflection, measuring the differential times between the two arrivals using waveform cross-correlation requires an operation of Hilbert transform of the first arrival before the regular cross-correlating. We investigated the SmKS waveforms of deep earthquakes occurring in South America recorded by several large and dense seismic arrays in China, and measured the differential arrival times of the SmKS pairs. We found significant waveform distortion of the SmKS caused by postcritical refraction and reflection at the CMB. For example, the π/2 phase between SKKS and S3KS cannot be confirmed from some major arc records at ~170°. This waveform distortion can introduce significant bias to the measured differential times, leading to incorrect estimate of P-wave velocity of the outer core.
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Abstract: The September 19th, 2017 Puebla earthquake caused life loss and severe property damage in Mexico City, even though the epicenter was located ~100 km away from the city. Mexico City is built on a thick clay-rich sedimentary sequence and, hence, is susceptible to land subsidence (at rates as high as 350 mm/yr), surface faulting, and seismic acceleration during earthquakes. The earthquake damage in the eastern part of the city, characterized by the collapse of several buildings, can be explained by seismic amplification. However, the damage in the southern part of the city, characterized by the collapse of small houses and surface faulting, requires a different explanation. We present here geodetic observations suggesting that the surface faulting in Mexico City triggered by the Puebla earthquake occurred in areas already experiencing differential displacements. Our study is based on Sentinel-1A satellite data from before and after the earthquake. We process the data using Interferometric Synthetic Aperture Radar (InSAR) to produce a coseismic interferogram. The results of our analysis reveal the locations and patterns of coseismic phase discontinuities, mainly in the piedmont of the Sierra de Santa Catarina, which agree with the location of earthquake’s damage reported by official and unofficial sources (GCDMX, 2017; OSM, 2017), the location of preexisting, subsidence-related faults (GCDMX, 2017), and differential displacements identified using a Fast Fourier Transform residual technique on high-resolution InSAR results from 2012 (Solano-Rojas et. al, 2017). We propose that the seismic energy released by the 2017 Puebla earthquake induced fast soil consolidation, which remotely triggered slip on the preexisting subsidence-related faults. The slip observed during this earthquake represents a hazard that needs to be considered in future urban development plans of Mexico City.
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