Rotational sensor on a volcano: New insights from Etna, Italy

<p>Rotational seismology is an emerging field of seismology with rotational sensors such as blueSeis-3A as portable devices. We deployed one of these rotational sensors on Etna volcano from August to September 2019 in the middle of a 26 stations broadband seismic array and a fibre-optic cable deployed for Distributed Acoustic Sensing (DAS). We, therefore, recorded continuously the full seismic wavefield using a 6C station (rotational sensor co-located with a broadband seismometer) for 30 days.</p><p>We will present an overview of our work on the rotational data in combination with a broadband seismometer. We will (i) compare the translational with rotational data and show how they complement each other, (ii) calculate back azimuths using only a 6C station or using merely the horizontal components of the rotational sensor, (iii) determine Love and Rayleigh wave velocities from the rotation rate and (iv) perform a simple inversion for the shallow velocity structure below the station, and finally (v) discuss the usefulness of such a sensor in a volcanic environment and (vi) highlight what new it would bring to volcano-related research.</p>

Automatic monitoring of crustal seismic activity in Galati region of southeastern Romania using full waveform-based approach

<p>In September–November 2013 a seismic swarm occurred in Galati region of southeastern Romania. The area was previously known as characterized by low seismic activity along the major crustal faults. During the period of swarm, between September 23rd and November 5<sup>th</sup>, over 1000 events with the magnitudes (Ml) of 0.2–4.0, located at the depth of 5–10 km, have been detected. Despite the relatively small magnitude, events generated ground motions that were well felt by local people, leading to panic in the area. The proximity of active oil fields caused additional annoyance.</p><p>Advanced seismic monitoring in the region started in 2013 with deployment of mobile seismic stations immediately after the beginning of the swarm. Additionally, active seismic measurements were performed in order to characterize the shallow velocity structure at specific sites. Starting from July 2015 new permanents stations were installed in the area marking the beginning of Galati local network development. The routine seismic catalog derived using the acquired data and applying the standard detection and location techniques pointed that area continues to be seismically active, however with low rate of activity and magnitude of events. These made it a perfect study case for development of new advanced schemes for seismic monitoring of the regions with low and complex seismicity aiming on an understanding of the phenomenon underlying the 2013 seismic swarm as well as the current seismic activity in the area.</p><p>We developed and automatic monitoring scheme based on the network-based full waveform detection and location method BackTrackBB (Poiata et al. 2016) that exploits the coherency of signals’ statistical features recorded across the seismic network. Once extracted from the flux of continuous data, seismic events are compared against the database of previously detected events using coherency and allowing to identify potential repeaters or multiplets. The earthquake catalog provided by the system starting from 2017 was compared to the routine ROMPLUS catalog of NIEP showing an increase in the number of detected events by the order of 3. We present the details of the implementation and discuss its advantages and drawbacks.</p>

Using a Dense Seismic Array to Determine Structure and Site Effects of the Two Towers Earthflow in Northern California

We deployed a network of 68 three-component geophones on the slow-moving Two Towers earthflow in northern California. We compute horizontal-to-vertical spectral ratios (HVSRs) from the ambient seismic field. The HVSRs have two prominent peaks, one near 1.23 Hz and another between 4 and 8 Hz at most stations. The 1.23 Hz resonance is a property of the background noise field and may be due to a velocity contrast at a few hundred meters depth. We interpret the higher frequency peaks as being related to slide deposits and invert the spectral ratios for shallow velocity structure using in situ thickness measurements as a priori constraints on the inversion. The thickness of the shallowest, low-velocity layer is systematically larger than landslide thicknesses inferred from inclinometer data acquired since 2013. Given constraints from field observations and boreholes, the inversion may reflect the thickness of deposits of an older slide that is larger in spatial extent and depth than the currently active slide. Because the HVSR peaks measured at Two Towers are caused by shallow slide deposits and represent frequencies that will experience amplification during earthquakes, the depth of the actively sliding mass may be less relevant for assessing potential slide volume and associated hazard than the thicknesses determined by our inversions. More generally, our results underscore the utility of combining both geotechnical measurements and subsurface imaging for landslide characterization and hazard assessment.