Study on Low-Frequency Abnormal Signal and Structural Characteristics of 2015 Azuoqi Ms5.8 Earthquake

In this article, the phenomenon of low-frequency abnormal signals before earthquakes, which reflects the three elements of earthquakes and the beneath structure change information, is discussed. Based on the data recorded at the Shizuishan (SZS), Wuhai (WUH) and Dongshenmiao seismic stations around the epicenter of the Ms5.8 earthquake in Azuoqi, Inner Mongolia, in 2015, the low-frequency abnormal signal from the seismic waves before this earthquake is extracted. At the same time, the autocorrelation method is used to extract the reflected waves of the main interface from teleseismic events recorded by the seismic array in the epicenter area, and then the change information from the beneath structure is obtained. It is explained in time and space that the low-frequency abnormal signal before the main earthquake, extracted from the continuous waveform, is directly related to the change in the underground structure near the epicenter, and it can be determined that the wave propagation direction f the crustal stress before the earthquake is from south to north, and it continues to accumulate near the epicenter until the main earthquake occurs.

3D crustal structure of the Ligurian Basin revealed by surface wave tomography using ocean bottom seismometer data

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-process the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. In onshore France, we observe a high-velocity area beneath the Argentera Massif, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The maps also indicate that the south-western and north-eastern Ligurian Basin are structurally separate. The lack of high crustal vP/vS ratios beneath the south-western part of the Ligurian Basin preclude mantle serpentinisation there.

Noise Levels and Signals Observed on Submarine Fibers in the Canary Islands Using DAS

In this study, we investigate 70 days of distributed acoustic sensing (DAS) recordings in the Canary Islands using an undersea fiber-optic telecommunication cable that links the islands of Tenerife and Gran Canaria. Two DAS interrogators connected to both ends of the cable turned the fiber into an array of 11,968 strain sensors covering a total length of ∼120 km. We present the details of the experiment, noise analysis, and examples of recorded signals. Seismic ambient noise levels assessment indicates poor local coupling of the cable due to the irregular bathymetry that results in high-amplitude acoustic oscillations in some channels. The DAS array recorded several types of nonseismic (vehicles, surface gravity waves, ships) and seismic signals. Local and regional earthquakes were detected with magnitudes mbLg≥2. Surface waves from teleseismic events at a distance of ∼3000 km were also identified in the strain recordings. Here, we report the first observations with DAS of hydroacoustic T waves generated by oceanic earthquakes located at the Central Mid-Atlantic Ridge and the Cape St. Vincent region. Events had magnitudes from Mw 4.2 to 6.9, and the hydroacoustic waves were recorded at epicentral distances from 780 to 3400 km. Our findings show that submarine fiber-optic cables can effectively be used to assess the seismic activity in remote oceanic areas.

Seismic Phase Association Based on the Maximum Likelihood Method

Phase association is a process that links seismic phases triggered at the stations of a seismic network to declare the occurrence of earthquakes. During phase association, a set of phases from different stations is examined to determine the common origin of phases within a specific region, predominantly on the basis of a grid search and the sum of observations. The association of seismic phases in local earthquake monitoring systems or earthquake early warning systems is often disturbed not only by transient noises, but also by large regional or teleseismic events. To mitigate this disturbance, we developed a seismic phase association method, binder_max, which uses the maximum likelihood method to associate seismic phases. The method is based on the framework of binder_ew, the phase associator of Earthworm, but it uses a likelihood distribution of the arrival information instead of stacking arrival information. Applying binder_max to data from seismic networks of South Korea and Ohio, United States, we found a significant improvement in the robustness of the method against large regional or teleseismic events compared to binder_ew. Our results indicate that binder_max can associate seismic phases of local earthquakes to the same degree as binder_ew as well as can avoid many of the false associations that have limited binder_ew.

Moho topography beneath the European Eastern Alps by global-phase seismic interferometry

In this work we present the application of the global-phase seismic interferometry (GloPSI) technique to a dataset recorded across the Eastern Alps with the EASI (Eastern Alpine Seismic Investigation) temporary seismic network. GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium and is used here to retrieve information about the crust–mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions where it shows a nearly planar behaviour and corresponds to a strong velocity contrast (i.e. in the northern part of the profile, from the Bohemian Massif to the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence of the boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards and confirm the observations of previous studies suggesting a structurally complex and faulted internal Alpine crustal structure.

Use of a vibrating beam MEMS accelerometer for surface microgravimetry

<p>A vibrating beam MEMS gravimeter with an Allan deviation of 9 μGal for a 1000 s integration time, a noise floor of 10 μGal/√Hz, and measurement over the full ±1 g dynamic range (1 g = 9.81 ms<sup>−2</sup>) is presented. In addition to a direct digital signal output, the sensor system possesses built-in tilt compensation capabilities and a 2-stage temperature control that is stable to 500 µK.</p><p>Instances of Earth tidal tracking and ground motion records corresponding to several teleseismic events are demonstrated. The output response from tracking of the Earth tides is compared to the data obtained from the software TSoft and a statistical correlation R of 0.92 is obtained between the conditioned MEMS dataset over a period of ~4 days and the predicted Earth tides model from TSoft following correction for ocean loading effects.</p><p>The device also recorded the ground motion from several teleseismic events during the testing period, a prominent event among them is the 6.2 M<sub>L</sub> earthquake near to Petrinja, Croatia, which occurred on December 29<sup>th</sup>, 2020. The MEMS sensor has demonstrated excellent performance as a long-period seismometer and the response is compared to the seismograms recorded by two nearby BGS broadband seismic stations. </p><p>Advances in microgravity sensor detection capability will be shown to match feasibility modelling for void detection. Results demonstrate that a vibrating beam MEMS accelerometer can be used for measurements requiring high levels of stability and resolution with wider implications for precision measurement. Gravimetry use to warn of imminent failures due to a range of shallow hazards include assessing damage in the built environment, transmission losses in utilities, territory breach and storage containment loss.</p>

Integrating Data under the European Plate Observing System from the Regional and Selected Local Seismic Networks in Poland

High-quality and open-access seismic data are of great importance for both research and increasing public awareness of actual seismic hazards and risks. We present four seismic networks that currently operate in Poland: the backbone Polish Seismological Network (PLSN), which monitors natural teleseismic events as well as regional events from Poland, and three networks that mainly serve the monitoring of anthropogenic seismicity. The acquired data from all four networks are openly available through the European Plate Observing System (EPOS) Information Technology (IT) facilities: the PLSN data within the Observatories and Research Facilities for European Seismology–European Integrated Data Archive and the anthropogenic seismicity data episodes through the induced seismicity-EPOS platform of EPOS Thematic Core Service Anthropogenic Hazards. For each network, we describe briefly the recorded seismic activity, the equipment and composition of the network, the acquisition system, and the data availability. Information from recent studies is used to demonstrate the scientific potential of the acquired anthropogenic seismicity data.

Teleseismic P-Wave Coda Autocorrelation Imaging of Crustal and Basin Structure, Bighorn Mountains Region, Wyoming, U.S.A.

ABSTRACT We demonstrate successful crustal imaging via teleseismic P-wave coda autocorrelation, using data recorded on a 261 station array of vertical-component high-frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P-wave coda of 30 teleseismic events and use phase-weighted stacking to yield seismic profiles comparable to low-passed versions of those produced via controlled-source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid-crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P-wave coda energy using modern large-array seismic data, and they corroborate previous interpretations of crustal structures in the study area.

Moho topography beneath the Eastern European Alps by global phase seismic interferometry

In this work we present the application of the Global-Phase Seismic Interferometry (GloPSI) technique to a data-set recorded across the Eastern Alps with the EASI temporary seismic network (Eastern Alpine Seismic Investigation). GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium, and is used here to retrieve information about the crust-mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions, where it shows a nearly planar behaviour (i.e. in the northern part of the profile, from the Bohemian massif to beneath the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence for a typical boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards. On the contrary, our results confirm the observations of previous studies suggesting a structurally complex Moho topography and faulted internal Alpine crustal structure.

2D velocity profiles along south- and northwestern Norway: An approach using receiver function analysis and Markov chains

<p>A receiver function analysis was carried out along two profiles located in north- and southwestern Norway. We selected and processed 801 teleseismic events registered by twelve seismic stations belonging to the 2002-2005 Geofon/Aarhus temporary network. The HK (depth vs Vp/Vs) stacking procedure and a Reversible jump Markov chain Monte Carlo (Rj-McMC) inversion were applied independently with the objective to reveal new crustal and crust-mantle transitional contrasts gaining a better understanding of the geology. In the southern profile, the most noticeable feature corresponds to a Moho offset of about ~5 km ca. 85 km to the east of the Norwegian coast: That feature was previously observed in several occasions and is also well-supported from this research. Furthermore, a very deep Moho discontinuity – at between 45 – 50 km depth - was found beneath the northern profile, approximately 70 km inland from the coast, and dipping about 30° to the northwest. Even when this deep structure was previously inferred through other methods, its presence was not certainly confirmed and so far, the origin of this feature is still disputed. We discuss two hypotheses, which are valid to explain the occurrence of the noticeable anomaly. First, a gradual and wide crust-mantle transition zone, which is also reflected in the velocity model or second, the presence of a paleo-slab of Fennoscandian basement subducted and deformed during the Caledonian Orogen (490-390 Ma).</p>