Near-Fault Monitoring Reveals Combined Seismic and Slow Activation of a Fault Branch within the Istanbul–Marmara Seismic Gap in Northwest Turkey

Various geophysical observations show that seismic and aseismic slip on a fault may occur concurrently. We analyze microseismicity recordings from a temporary near-fault seismic network and borehole strainmeter data from the eastern Marmara region in northwest Turkey to track seismic and aseismic deformation around the hypocentral region of an Mw 4.5 earthquake in 2018. A slow transient is observed that lasted about 30 days starting at the time of the Mw 4.5 event. We study about 1200 microseismic events that occurred during 417 days after the Mw 4.5 event around the mainshock fault rupture. The seismicity reveals a strong temporal clustering, including four episodic seismic sequences, each containing more than 30 events per day. Seismicity from the first two sequences displayed typical characteristics driven by aseismic slip and/or fluids, such as the activation of a broader region around the mainshock and swarm-like topology. The third and fourth sequences correspond to typical mainshock–aftershock sequences. These observations suggest that slow slip and potentially fluid diffusion along the fault plane could have controlled the seismicity during the initial 150 days following the Mw 4.5 event. In contrast, stress redistribution and breaking of remaining asperities may have caused the activity after the initial 150 days. Our observation from a newly installed combined dense seismic and borehole strainmeter network follows an earlier observation of a slow transient occurring in conjunction with enhanced local seismic moment release in the same region. This suggests a frequent interaction of seismic and aseismic slip in the Istanbul–Marmara seismic gap.

Anatomy of a Paroxysmal Lava Fountain at Etna Volcano: The Case of the 12 March 2021, Episode

On 13 December 2020, Etna volcano entered a new eruptive phase, giving rise to a number of paroxysmal episodes involving increased Strombolian activity from the summit craters, lava fountains feeding several-km high eruptive columns and ash plumes, as well as lava flows. As of 2 August 2021, 57 such episodes have occurred in 2021, all of them from the New Southeast Crater (NSEC). Each paroxysmal episode lasted a few hours and was sometimes preceded (but more often followed) by lava flow output from the crater rim lasting a few hours. In this paper, we use remote sensing data from the ground and satellite, integrated with ground deformation data recorded by a high precision borehole strainmeter to characterize the 12 March 2021 eruptive episode, which was one of the most powerful (and best recorded) among that occurred since 13 December 2020. We describe the formation and growth of the lava fountains, and the way they feed the eruptive column and the ash plume, using data gathered from the INGV visible and thermal camera monitoring network, compared with satellite images. We show the growth of the lava flow field associated with the explosive phase obtained from a fixed thermal monitoring camera. We estimate the erupted volume of pyroclasts from the heights of the lava fountains measured by the cameras, and the erupted lava flow volume from the satellite-derived radiant heat flux. We compare all erupted volumes (pyroclasts plus lava flows) with the total erupted volume inferred from the volcano deflation recorded by the borehole strainmeter, obtaining a total erupted volume of ~3 × 106 m3 of magma constrained by the strainmeter. This volume comprises ~1.6 × 106 m3 of pyroclasts erupted during the lava fountain and 2.4 × 106 m3 of lava flow, with ~30% of the erupted pyroclasts being remobilized as rootless lava to feed the lava flows. The episode lasted 130 min and resulted in an eruption rate of ~385 m3 s−1 and caused the formation of an ash plume rising from the margins of the lava fountain that rose up to 12.6 km a.s.l. in ~1 h. The maximum elevation of the ash plume was well constrained by an empirical formula that can be used for prompt hazard assessment.

Fifteen Years of Continuous High-Resolution Borehole Strainmeter Measurements in Eastern Taiwan: An Overview and Perspectives

As one of the most sensitive instruments for deformation monitoring in geophysics, borehole strainmeter has the capability to record a large spectrum of tectonic and environmental signals. Sensors are usually deployed near active faults and volcanoes and provide high-resolution continuous recordings of seismic and aseismic signals, hydrological variations (rainfall, groundwater level) and natural hazards (tropical cyclones, landslides, tsunamis). On the occasion of the 50th anniversary of the installation of the first Sacks–Evertson borehole strainmeter, in central Japan, we present an overview of the major scientific contributions and advances enabled by borehole strainmeter measurements in Taiwan since their installation in the mid 2000s. We also propose a set of future research directions that address recent challenges in seismology, hydrology and crustal strain modeling.

The 2019 eruptive phase of Stromboli volcano through multiparametric geophysical observations.

<p>In summer 2019, two paroxysmal explosions occurred in Stromboli. The first one occurred on July 3, when the Strombolian ordinary eruptive activity did not show a significant intensification. The explosion formed an eruptive column more than 3 km high. A pyroclastic flow ran down the “Sciara del Fuoco” slope causing a victim and some injuries. Moreover, the pyroclastic flow spread over the sea surface for about one kilometer. On August 28, a second paroxysmal explosion occurred, similar to the previous one. Also in this case the eruption formed an eruptive column of more than 3 km and a pyroclastic flow that expanded along the “Sciara del Fuoco” slope and traveled about 1 km on the sea surface. In the period between the two paroxysms, effusive activity occurred from the summit crater area. The eruptive phase of summer 2019, which began with the paroxysm of 3 July, was not preceded by significant changes in the routinely monitored parameters, such as the hourly frequency (daily average) of the VLP events (typical of Stromboli) and the amplitude of the seismic signal (RSAM). For this reason, we have analyzed the seismic and dilatometric data, which were recorded by the INGV geophysical network in the period November 2018 - September 2019, focusing our attention on other parameters that can give indications on the activity state of the volcano. In particular, we analyzed the data of the broadband seismic stations, equipped with the Guralp CMG40T sensors, and the data of one Sacks-Evertson borehole strainmeter. We defined the "VLP size", which takes into account the waveform of the VLP events, in terms of both amplitude and duration. We also applied time varying Fractal Dimension (FD) analysis to the seismograms of a seismic station close to the crater area and we analyzed the polarization of the same signal. We carried out the polarization analysis both without applying a filter and by filtering the seismic signal in the typical frequency bands of the Stromboli volcanic tremor (1-3 Hz) and of the VLPs (0.5-0.05 Hz). We found that the "VLP size", the FD and the polarization parameters showed significant changes about one month before the paroxysm of July 3. In the short term, we applied an appropriately tuned STA/LTA algorithm to the data of the borehole strainmeter, which is installed on the island at about 2km from the craters, and we obtained an automatic detection of the paroxysmal events 10 and 7.5 minutes before the explosion of July 3 and August 28, respectively.</p>

Visualization of 4-component borehole strainmeter data in China

<p>With the accumulation of 4-component borehole strainmeter data and the improvement of observation reliability, it is a primary of current research to improve the efficiency of processing, analyzing and visualizing of these data. The visualization of borehole strain observation data is a key means to convey the information behind the data, display the data research results and extract the shallow surface stress state revealed by borehole strain.</p><p>Borehole strainmeter data are of great significance for earthquake prediction research due to its’ high resolution in short-medium term time scale of earthquake prediction. With the progress of observation technology, many four-component borehole strain gauges in China had experienced the data stabilization period, the early years of establishing the instrument, and the borehole strain station began to obtain a batch of high-quality observation data.</p><p>By using the normal stress petal diagram to show the change of the ground stress, it can not only qualitatively analyze the change of the relative ground stress of the station, but also quantitatively read  the observed normal stress in any direction at a certain time. In this paper, the method of normal stress petals diagram is combined with map visualization technology to process and analyze the strain observation data of four-component borehole across the country. The main works are as follows: first of all, The construction of the stress petal visualization platform can display the dynamic stress effectively in all directions of 30 stations across the country; secondly, Variable sliding window length and sliding spacing added according to specific needs can not only directly display the change of the stress petal over the years, but also show the stress petal map of the solid tide strain all over the country; thirdly, The platform can display the co-seismic stress petal variation image observed at the national borehole strain stations and visually show the stress changes observed by the local borehole strain gauge during the seismic wave propagation. Finally, The borehole strainmeter data can monitor the relative geostress state of the fault near the borehole. Then the magnitude and direction of the maximum principal stress at the borehole strain station reflected by the stress petal can further calculate the corresponding changes of dynamic coulomb stress and static coulomb stress which can help to analyze seismic dynamic triggering problems.</p>

Installation of a High Sensitivity Ocean Borehole Strainmeter in the Nankai Trough Under Severe Sea Current Conditions

A high-sensitivity volumetric strainmeter has been installed into the C0010 borehole observatory using the drilling vessel (D/V) Chikyu during the Expedition 365 cruise in the Nankai Trough, Japan. At this location, crustal deformation occurs in association with large interplate earthquakes. However, strong Kuroshio ocean currents cause vortex-induced vibrations (VIVs) in the region, which can cause fatal damage to the strainmeter. Therefore, laboratory vibration tests were performed prior to installation to confirm that the antivibration mechanism inside the strainmeter was functional against the severe vibrations during installation. VIV was measured prior to installing the strainmeter into the C0010A borehole using accelerometers at the installation site. The results indicated that the VIV were within the specification of the antivibration mechanism. This meant that installation of the strainmeter into the borehole was possible. To maximize sensor sensitivity, it is extremely important to ensure mechanical coupling of the strainmeter with the borehole wall by cementing operation after installation. The cementing process was confirmed using a pressure recording device incorporated within the strainmeter. Pressure data clearly showed that seawater had been displaced with cement slurry. Data from the strainmeter clearly showed tidal waveforms, which are comparable to those of pressure data recorded by a borehole pressure sensor installed at approximately the same depth. Accuracies of the strain data were validated through the procedure. They suggest that the first installation of the ocean borehole strainmeter in the Nankai Trough was successful, and therefore, highly sensitive strain measurement is now possible in a seismically active area.