Construction of Nominal Ionospheric Gradient Using Satellite Pair Method Based on GNSS CORS Observation in Indonesia

Ground-Based Augmentation System (GBAS) is a GNSS augmentation system that meets International Civil Aviation Organization (ICAO) requirements to support precision approach and landing. GBAS is based on the local differential GNSS technique with reference stations located around the airport to provide necessary integrity and accuracy. The performance of the GBAS system can be affected by the gradient in the ionospheric delay between the aircraft and the reference stations. A nominal ionospheric gradient, which is bounded by a conservative error bound, is represented by a parameter σvig. σvig was commonly determined using station pair to GNSS Continuous Operating Reference Station (CORS) data. The station pair method is susceptible to doubling of receiver bias error and is not suitable with the CORS conditions in Indonesia. We propose a satellite pair method that is found to be more suitable for the CORS network over Indonesia which is centered in Java and Sumatra islands. The value of σvig (4.48 mm/km) is obtained using this method along with the preliminary results of a comparison of σvig from Java and Sumatra islands.

Temporal Variation in Cultural Seismic Noise and Noise Correlation Functions during COVID-19 Lockdown in Canada

The COVID-19 pandemic of 2020 led to a widespread lockdown that restricted human activities, particularly land, air, and maritime traffic. The “quietness” on land and ocean that followed presents an opportunity to measure an unprecedented reduction in human-related seismic activities and study its effect on the short-period range of ambient noise cross-correlation functions (NCFs). We document the variations in seismic power levels and signal quality of short-period NCFs measured by four seismographs located near Canadian cities across the pandemic-defined timeline. Significant drops in seismic power levels are observed at all the locations around mid-March. These drops coincide with lockdown announcements by the various Canadian provinces where the stations are located. Mean seismic power reductions of ∼24% and ∼17% are observed near Montreal and Ottawa, respectively, in eastern Canada. Similar reductions of ∼27% and 17% are recorded in western Canada near Victoria and Sidney, respectively. None of the locations show full recovery in seismic power back to the pre-lockdown levels by the end of June, when the provinces moved into gradual reopening. The overall levels of seismic noise during lockdown are a factor of 5–10 lower at our study locations in western Canada, relative to the east. Signal quality of NCF measured in the secondary microseism frequency band for the station pair in western Canada is maximum before lockdown (late February–early March), minimum during lockdown (mid–late March), and increased to intermediate levels in the reopening phase (late May). A similar pattern is observed for the signal quality of the eastern Canada station pair, except for a jump in levels at similar periods during the lockdown phase. The signal quality of NCF within the secondary microseism band is further shown to be the lowest for the western Canada station pair during the 2020 lockdown phase, when compared with similar time windows in 2018 and 2019.

3-D crustal VS model of western France and the surrounding regions using Monte-Carlo inversion of seismic noise cross-correlation dispersion diagrams

Summary Due to a too sparse permanent seismic coverage during the last decades, the crustal structure of western France and the surrounding regions is poorly known. In this study, we present a 3-D seismic tomographic model of this area obtained from the analysis of 2-year continuous data recorded from 55 broad-band seismometers. An unconventional approach is used to convert Rayleigh wave dispersion diagrams obtained from ambient noise cross-correlations into posterior distributions of 1-D VS models integrated along each station pair. It allows to avoid the group velocity map construction step (which means dispersion curve extraction) while providing meaningful VS posterior uncertainties. VS models are described by a self-adapting and parsimonious parameterization using cubic Bézier splines. 1268 separately inverted 1-D VS profiles are combined together using a regionalization scheme, to build the 3-D VS model with a lateral resolution of 75 km over western France. The shallower part of the model (horizontal cross-section at 4 km depth) correlates well with the known main geological features. The crystalline Variscan basement is clearly associated with positive VS perturbations while negative heterogeneities match the Mesocenozoic sedimentary basins. At greater depths, the Bay of Biscay exhibits positive VS perturbations,which eastern and southern boundaries can be interpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale, and tends to support that both the South-Armorican Shear Zone and the Paris Basin Magnetic Anomaly are major crustal discontinuities that separate distinct domains.

Even moderate geomagnetic pulsations can cause fluctuations of foF2 frequency of the auroral ionosphere

Using data of the ionosonde in Sodankyla, (SOD, 67° N, 27° E, Finland), variations of the critical frequency of o-mode radiowave reflected from ionospheric F2 layer (foF2) in 1–5 mHz frequency range and their possible association with long period (Pc5/Pi3) geomagnetic pulsations are studied. For that, a technique of automatic detection of the foF2 critical frequency from an ionogram is developed and applied to daytime Pc5/Pi3 geomagnetic pulsations and foF2 fluctuations during several months of years 2014–2015 near the maximum of 24-th Solar cycle. The variations of foF2 are compared with the Pc5/Pi3 geomagnetic pulsations at SOD station, and the influence of pulsations' spatial scale is analyzed with the data of a station pair located at the same magnetic meridian but separated in latitude. The variations of foF2 are in the majority of cases decoupled from the geomagnetic pulsations on the ground. Meanwhile, the analysis of geomagnetic and foF2 variations show intervals with noticeable coherence for both horizontal components. These coherent pulsations are predominantly registered in the afternoon sector of the magnetic local time (MLT). Statistically, their spectral content, polarization and spatial distribution differ from averaged parameters of post-noon Pc5 pulsations. The pulsations, coherent to foF2 fluctuations, demonstrate features typical for Alfven field-line resonance. The analysis of space weather conditions favorable for the occurrence of coherent geomagnetic/foF2 pulsations show that these pulsations are registered mostly under moderately disturbed conditions. Comparison of space weather parameters for all the intervals analyzed and the intervals of high geomagnetic/foF2 coherence show that the latter correspond mostly to intermediate values of indexes of geomagnetic (Dst) and auroral (AE) activity, solar wind speed and dynamic pressure fluctuations.

Crustal scattering and intrinsic attenuation of S-waves in southern Aegean derived using envelope inversion

<p>Southern Aegean is the major part of the Eurasian plate overriding the subduction of African plate in eastern Mediterranean region. In this study, shallow depth (< 40 km) events recorded by temporary and permanent seismic networks in southern Aegean are used to study the crustal scattering attenuation (Q<sub>s</sub><sup>-1</sup>) and intrinsic attenuation (Q<sub>i</sub><sup>-1</sup>) of S-waves. The 3 component S-waveforms are filtered in 1-2, 2-4, 4-8, and 8-16 Hz bands and envelopes are calculated by smoothing the root mean square of individual components. The envelopes are modeled using the approximate analytical solution of 3D isotropic radiative transfer equation. The fitting is performed using grid search approach to obtain Q<sub>s</sub><sup>-1 </sup>and then linear inversion is used to calculate Q<sub>i</sub><sup>-1 </sup>for each source station pair. The results obtained from each source-station pair are assigned to an ellipsoid region and robust mean technique is used to map the results in each 0.20<sup>o </sup>x 0.20<sup>o</sup> bin. The final results indicate consistently high Q<sub>s</sub><sup>-1</sup> in western Crete in all 4 frequency bands. Also, high Q<sub>s</sub><sup>-1</sup> is observed in western Peloponnese in 1-2 and 2-4 Hz frequency bands. High Q<sub>i</sub><sup>-1</sup> is observed along the volcanic arc in all 4 frequency bands. Our results compare well with the recent S-wave scattering study in the region. They are also consistent with the geodynamics of southern Aegean subduction zone. Our study provides useful insight about the attenuation in the southern Aegean crust which has implications for ground motion and seismic hazard.</p>

Diet of Cataglyphis bicolor (Hymenoptera-Formicidae) in an insular condition in north-eastern Algeria

A study of the adaptation of the ant Cataglyphis bicolor in terms of feeding under insular conditions was conducted on the north-eastern coast of Algeria. For this, three pairs of stations (island-continent) were chosen. Analyzing fragments of prey species found in C. bicolor nests, the diet in each station pair was studied. The results obtained indicate that C. bicolor has an opportunistic diet characterised by insectivory. Indeed, more than 95% of the prey consumed in the six study stations were insects with a clear preference for other ants, with frequencies ranging from 52 to 87%. Among the latter, Messor barbarus, Camponotus sp., Camponotus laurenti, Pheidole pallidula and Tetramorium biskrense were the ants most predated by C. bicolor. The diversity of continental prey seemed greater than that of island environments. For the two Cap Sigli stations, the prey richness was 94 species for the continent against only 28 species for the island environment. For Boulimat, there were 27 prey species for the mainland and 20 prey species for the islet. Finally, for the Sahel region, C. bicolor was able to harvest 42 prey species on the mainland and 28 species on the island. The diversity of C. bicolor prey in the island environments seems to be a function of insect richness (prey availability) and floral richness.

Assessing Short‐Term Clock Errors and Drifts of Temporary Seismic Networks Using the Active Airgun Source in Binchuan, Yunnan

ABSTRACT We conducted a short‐term airgun experiment at the Binchuan Fixed Airgun Signal Transmission Station from 14 to 20 February 2017, and two different types of seismometers (Güralp CMG‐40T and QS05A) recorded 62 airgun shots triggered under the same conditions. However, we observed significant clock errors and drifts in seismic data recorded by four QS05A seismometers. To assess the short‐term clock errors and drifts for seismometers, we propose a new method that measures the P‐wave arrival‐time differences between airgun signals recorded at a station pair, using the matched filter method. We find ∼1.0 s absolute clock errors for two Güralp CMG‐40T stations (CKT2 and 53261) and one QS05A station (STA05), as well as ∼0.5 s timing leaps for four QS05A stations (STA19, STA21, STA31, and STA33) during the experiment. Furthermore, all the QS05A seismometers exhibit clock drifts with similar linear trends. Additionally, we use teleseismic waveforms to verify the absolute clock errors for stations CKT2, 53261, and STA05. After double‐checking several possible factors, we determine the hardware failure or malfunctioning that may cause clock errors for the two types of seismometers.