Science Panel: Activities in 2022-2023

Short report of the Global Geodetic Observing System (GGOS) Science Panel during the period 2022-2023

Space geodetic observations of repeating slow slip events beneath the Bonin Islands

The Pacific Plate subducts beneath the Philippine Sea Plate along the Izu-Bonin Trench. We investigated crustal movements at the Bonin Islands, using Global Navigation Satellite System and geodetic Very Long Baseline Interferometry data to reveal how the two plates converge in this subduction zone. These islands are located similar to 100 km from the trench, just at the middle between the volcanic arc and the trench, making these islands suitable for detecting signatures of episodic deformation such as slow slip events (SSEs). During 2007-2016, we found five SSEs repeating quasi-periodically with similar displacement patterns. In estimating their fault parameters, we assumed that the fault lies on the prescribed plate boundary, and optimized the size, shape and position of the fault and dislocation vectors. Average fault slip was similar to 5 cm, and the average moment magnitude was similar to 6.9. We also found one SSE occurred in 2008 updip of the repeating SSE in response to an M6 class interplate earthquake. In spite of the frequent occurrence of SSEs, there is no evidence for long-term strain accumulation in the Bonin Islands that may lead to future megathrust earthquakes. Plate convergence in Mariana-type subduction zones may occur, to a large extent, episodically as repeating SSEs

Apparent ionospheric total electron content variations prior to major earthquakes due to electric fields created by tectonic stresses

Growing evidence for ionospheric signatures of impending earthquakes comes from electron content measurements along slanted paths from GPS satellites to multiple ground stations located up to 500 km away from the epicenters. These slant total electron content (STEC) measurements deviate from the classic U-shape pattern, starting about 40 min to over an hour before major earthquakes. Unlike other naturally occurring STEC fluctuations at midlatitudes, we show here that these earthquake-induced deviations are simultaneous over a wide geographical area and do not propagate, thereby indicating a ground-based origin. Prior to the 11 March 2011 Tohoku-Oki earthquake (Mw 9.0), the deviations were as much as 10% of the undisturbed STEC. We argue that such deviations must be due to an electric field-forced rise or fall of the main ionosphere with little change in the vertical electron density profile. Hence, “apparent” is used in the title. We show how stress-related underground electric fields penetrate to 80 km altitude (above which penetration to the main ionosphere easily occurs) with magnitudes high enough to create STEC variations comparable to those observed. Since many thousands of GPS receivers exist worldwide, our theory suggests the possibility of early warning systems that could provide 10 to 20 min notice prior to large earthquakes, after allowing time for signal processing. This theory for prequake-induced STEC fluctuations also explains the ground-based ULF magnetic field data acquired by Fraser-Smith et al. 40 min prior to the Loma Prieta earthquake

Preface to the Special Issue on “Geophysical and Climate Change Studies in Tibet, Xinjiang, and Siberia (TibXS) from Satellite Geodesy”

This special issue publishes papers on recent results in geophysical and climate change studies over Tibet, Xinjiang and Siberia (TibXS) based upon some of the key sensors used in satellite geodesy, including satellite gravimetric sensors (GRACE and GOCE), satellite altimeters (TOPEX, Jason-1 and -2, and ENVISAT), and Global Positioning System satellites. Results from ground- and airborne-based geodetic observations, notably those based on airborne gravimeter, superconducting gravimeter (SG) and seismometers are also included in the special issue. In all, 22 papers were submitted for this special issue; 17 papers were accepted

The GGOS Bureau of Products and Standards: Description and Promotion of Geodetic Products

The Bureau of Products and Standards (BPS) is a key component of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG). It supports GGOS in its goal to provide consistent geodetic products needed to monitor, map, and understand changes in the Earth’s shape, rotation, and gravity field. In addition to the operational structure, the Committees “Earth System Modeling” and “Essential Geodetic Variables” as well as the Working Group “Towards a consistent set of parameters for the definition of a new Geodetic Reference System (GRS)” are associated to the BPS. This contribution presents the structure and role of the BPS. It highlights some of the recent activities, which are focused on the classification of geodetic products and on the generation of user-friendly product descriptions to support the establishment of a comprehensive Internet portal for Geodesy under the responsibility of GGOS. The GGOS website www.ggos.org serves as an “entrance door” and information platform to geodetic data and products, and should become an essential tool to make these data and products easier findable and accessible. With this, GGOS is contributing to address different user needs (e.g., geodesists, geophysicists, other geoscientists and further customers) and to make other disciplines and society aware of Geodesy and the importance of its products

GGOS Bureau of Products and Standards: Description and Promotion of Geodetic Products

The Bureau of Products and Standards (BPS) is a key component of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG). It supports GGOS in its goal to provide consistent geodetic products needed to monitor, map, and understand changes in the Earth’s shape, rotation, and gravity field. In its present structure, the two Committees "Earth System Modeling" and "Essential Geodetic Variables" as well as the Working Group "Towards a consistent set of parameters for the definition of a new Geodetic Reference System (GRS)" are associated to the BPS. This paper presents the structure and role of the BPS and it highlights some of the recent activities. A major focus is on the classification and description of geodetic products and their representation at the renewed GGOS website (www.ggos.org). This website serves as an "entrance door" to geodetic products to satisfy different user needs and communities (e.g., geodesists, geophysicists, other geosciences and further customers) in order to make geodesy more visible to other disciplines and to society

Ionospheric electron enhancement preceding the 2011 Tohoku-Oki earthquake

he 2011 March 11 Tohoku‐Oki earthquake (Mw9.0) caused vast damages to the country. Large events beneath dense observation networks could bring breakthroughs to seismology and geodynamics, and here I report one such finding. The Japanese dense network of Global Positioning System (GPS) detected clear precursory positive anomaly of ionospheric total electron content (TEC) around the focal region. It started ∼40 minutes before the earthquake and reached nearly ten percent of the background TEC. It lasted until atmospheric waves arrived at the ionosphere. Similar preseismic TEC anomalies, with amplitudes dependent on magnitudes, were seen in the 2010 Chile earthquake (Mw8.8), and possibly in the 2004 Sumatra‐Andaman (Mw9.2) and the 1994 Hokkaido‐Toho‐Oki (Mw8.3) earthquakes, but not in smaller earthquakes

Explosion energy of the 2004 eruption of the Asama Volcano, central Japan, inferred from ionospheric disturbances

The Japanese dense array of Global Positioning System recorded ionospheric disturbances as changes in Total Electron Content ∼12 minutes after the September 1 2004 eruption of the Asama Volcano, Central Japan. The disturbance had a period of one and a quarter minutes and propagated as fast as ∼1.1 km/s, suggesting its origin as the acoustic wave generated by the explosion. By comparing the disturbance amplitudes with those by a surface mine blast with a known energy, the overall Asama explosion energy is inferred to be about 2 × 1014 J