Crustal Deformation Detection Capability Associated With a Fault Slip on the Interplate Boundary in the Gnss-a Seafloor Geodetic Observation Array (SGO-A), Provided By Japan Coast Guard

The GNSS-A technique is an observation method that can detect seafloor crustal deformations with centimeter level accuracy. The GNSS-A seafloor geodetic observation array operated by the Japan Coast Guard, called SGO-A, has been constructed near the Japan Islands along the Nankai Trough and the Japan Trench. This observation array has detected several earthquakes’ displacements and episodic slow crustal deformation. To compare the detection results of SGO-A with other observation networks and expand the SGO-A distribution, it is necessary to correctly understand its detection capability. In this paper, the capabilities of current GNSS-A (frequency: f = 4–6 times/year, position accuracy: σ (standard deviation) = 1.5 cm) to detect a crustal deformation rate only, an event only, and crustal deformation rate and event together were arranged by numerical simulations. Results suggested the following features: when it is known that there is no event, the 95% confidence level (CL) for the estimation of crustal deformation rate with 4-year observation is about 0.5–0.8 cm/year; when the deformation rate is known, a signal of about 3.0 cm can be detected by observations of about 4 times before and after the event. When the deformation rate and the event are detected together, to keep the false positive low (about 0.05), the false negative becomes high (about 0.2–0.7 for detecting a signal of 4.5–6.0 cm). The determined rate and event variations are approximately 1.8 cm/year (95%CL) and 1.5 cm (standard deviation), respectively. We also examined the detection capability for higher frequency and accuracy, to examine how the detection capability improves by technological advancements in the future. Additionally, we calculated the spatial range of event detectability using the determined values of detection sensitivity. Each seafloor site can detect a slip event larger than 0.1 m scale within about 50 km radius. A subseafloor slip event smaller than about 1 m at the distance of 100 km or more from the land can often be detected only on the seafloor observation array.

GNSS-A seafloor geodetic observation capability in 2021 and its applicability to global geodesy

<p>Recently, the GNSS-A (Global Navigation Satellite System – Acoustic combination technique) seafloor geodetic observation technology, developed in the Hydrographic and Oceanographic department in Japan Coast Guard (JCG), was upgraded to be able to monitor a subseafloor interplate coupling condition of about 1 cm/year and an interplate shallow slow slip event of about 5 cm (e.g., Yokota et al., 2018, Scientific Data; Ishikawa et al., 2020, Front. Ear. Sci.). By observing such small-scale seafloor crustal movements, GNSS-A technology makes a decisive contribution to subduction seismology and disaster prevention sciences (e.g., Yokota et al., 2016, Nature; Yokota and Ishikawa, 2020, Sci. Adv.). This technology was achieved by connecting high-precision underwater acoustic ranging technology and high-rate GNSS on a vessel at sea surface.</p><p>The GNSS-A, which is carried out all over the world, especially in the Pacific Rim, has been constructed for observation of plate boundary subduction processes and fault movement processes. Unlike the GNSS network, GNSS-A has never contributed to global geodesy within the framework of the Global Geodetic Observing System (GGOS). However, it can be a unique observation method for the construction of the International Terrestrial Reference Frame (ITRF). It can make an important contribution in determining the movement and Euler pole on an oceanic plate that have few land area.</p><p>In the future, if an extensive seafloor geodetic observation network as shown by Kato et al. (2018, JDR) will be established, there is a possibility of constructing a next-generation reference frame that completely explains the plate motion on the earth's surface. This presentation will present the current state of the GNSS-A ability and cost and future prospects for the contribution to global geodesy.</p>

Testing normality of chosen R-estimates used in deformation analysis

The normal distribution is one of the most important distribution in statistics. In the context of geodetic observation analyses, such importance follows Hagen’s hypothesis of elementary errors; however, some papers point to some leptokurtic tendencies in geodetic observation sets. In the case of linear estimators, the normality is guaranteed by normality of the independent observations. The situation is more complex if estimates and/or the functional model are not linear. Then the normality of such estimates can be tested theoretically or empirically by applying one of goodness-of-fit tests.This paper focuses on testing normality of selected variants of the Hodges-Lehmann estimators (HLE). Under some general assumptions the simplest HLEs have asymptotical normality. However, this does not apply to the Hodges-Lehmann weighted estimators (HLWE), which are more applicable in deformation analysis. Thus, the paper presents tests for normality of HLEs and HLWEs. The analyses, which are based on Monte Carlo method and the Jarque–Bera test, prove normality of HLEs. HLWEs do not follow the normal distribution when the functional model is not linear, and the accuracy of observation is relatively low. However, this fact seems not important from the practical point of view.