Fluid migrations and volcanic earthquakes from depolarized ambient noise

Ambient noise polarizes inside fault zones, yet the spatial and temporal resolution of polarized noise on gas-bearing fluids migrating through stressed volcanic systems is unknown. Here we show that high polarization marks a transfer structure connecting the deforming centre of the caldera to open hydrothermal vents and extensional caldera-bounding faults during periods of low seismic release at Campi Flegrei caldera (Southern Italy). Fluids pressurize the Campi Flegrei hydrothermal system, migrate, and increase stress before earthquakes. The loss of polarization (depolarization) of the transfer and extensional structures maps pressurized fluids, detecting fluid migrations after seismic sequences. After recent intense seismicity (December 2019-April 2020), the transfer structure appears sealed while fluids stored in the east caldera have moved further east. Our findings show that depolarized noise has the potential to monitor fluid migrations and earthquakes at stressed volcanoes quasi-instantaneously and with minimum processing.

Seismic Assessment and Rehabilitation of a RC Structure Under Mainshock-Aftershock Seismic Sequences Using Beam-Column Bonded CFRP Strategy

A mainshock is usually accompanied by a group of ground motions. In many design codes, the effects of the seismic sequences have been neglected or underestimated. Aftershocks can increase structural damage or even cause failure. The current study evaluated the seismic behavior of a rehabilitated and as-built RC structure under real scaled mainshock-aftershocks using nonlinear analysis. Verification was done in two modes. The inter-story drift ratio, maximum residual, and relative displacements were studied. The seismic study of the as-built structure showed that the residual displacement grew, on average, more than 90% under the mainshock-aftershock sequence compared to the mainshock-only record. A beam-column bonded CFRP rehabilitation strategy using six layers of T-700 CFRP was chosen based on the specific performance level. The strategy showed the ability to transfer the plastic strain from the columns to the beams, which could be considered as a change from a weak column-strong beam concept to a strong column-weak beam concept. Compared to the as-built structure under the seismic sequences, the rehabilitated structure showed an average growth of 78% in the first-story drift ratio, which was significant. Unlike the as-built structure, seismic sequences caused no growth drift in the rehabilitated structure. It also was observed that the ratio of aftershock PGA to mainshock PGA could have an intensive effect on the seismic behavior of both rehabilitated and as-built structures.

Seismic Behavior of RC Moment Resisting Structures with Concrete Shear Wall under Mainshock-aftershock Seismic Sequences

A structure may subject to several aftershocks after a mainshock. In many seismic design provisions, the effect of the seismic sequences is not directly considered or underestimated. This paper studies the seismic behavior of RC moment-resisting structures with concrete shear wall under seismic sequences. Two three-dimensional structures of short and medium height were designed and analyzed. The former models were studied under a group of real mainshock-aftershock seismic sequences. The models were loaded and designed according to the fourth edition of the Iranian seismic code of standard no. 2800 and ACI-318 respectively. Furthermore, the non-linear dynamic time-history finite element analysis of models was performed via the explicit method. The parameters of maximum displacement, inter-story drift ratio, residual displacement, and finally the effect of the ratio of aftershock acceleration to mainshock acceleration were investigated and assessed. Due to the high lateral stiffness of shear walls, parallel with the complete elastic behavior, aftershocks cause no growth in inter-story drift ratio and relative displacement in the short structure model. In contrast, compared to the structure under the solely mainshock, the medium height structure model under seismic sequences showed significant growth in the amount of relative displacement (even more than 50% growth), inter-story drift ratio, plastic strain, and residual displacement (almost 30% growth). Furthermore, unlike the moment-resisting frame structures, models showed no significant growth in the drift ratio with the height. Assessments indicated that the ratio of aftershock to mainshock acceleration is a determinative parameter in structural behavior under seismic sequences.

Geomorphological Geometries and High-Resolution Seismic Sequence Stratigraphy of Malay Basin’s Fluvial Succession

This study identified the Pleistocene depositional succession of the group (A) (marine, estuarine, and fluvial depositional systems) of the Melor and Inas fields in the central Malay Basin from the seafloor to approximately −507 ms (522 m). During the last few years, hydrocarbon exploration in Malay Basin has moved to focus on stratigraphic traps, specifically those that existed with channel sands. These traps motivate carrying out this research to image and locate these kinds of traps. It can be difficult to determine if closely spaced-out channels and channel belts exist within several seismic sequences in map-view with proper seismic sequence geomorphic elements and stratigraphic surfaces seismic cross lines, or probably reinforce the auto-cyclic aggregational stacking of the avulsing rivers precisely. This analysis overcomes this challenge by combining well-log with three-dimensional (3D) seismic data to resolve the deposition stratigraphic discontinuities’ considerable resolution. Three-dimensional (3D) seismic volume and high-resolution two-dimensional (2D) seismic sections with several wells were utilized. A high-resolution seismic sequence stratigraphy framework of three main seismic sequences (3rd order), four Parasequences sets (4th order), and seven Parasequences (5th order) have been established. The time slice images at consecutive two-way times display single meandering channels ranging in width from 170 to 900 m. Moreover, other geomorphological elements have been perfectly imaged, elements such as interfluves, incised valleys, chute cutoff, point bars, and extinction surfaces, providing proof of rapid growth and transformation of deposits. The high-resolution 2D sections with Cosine of Phase seismic attributes have facilitated identifying the reflection terminations against the stratigraphic amplitude. Several continuous and discontinuous channels, fluvial point bars, and marine sediments through the sequence stratigraphic framework have been addressed. The whole series reveals that almost all fluvial systems lay in the valleys at each depositional sequence’s bottom bars. The degradational stacking patterns are characterized by the fluvial channels with no evidence of fluvial aggradation. Moreover, the aggradation stage is restricted to marine sedimentation incursions. The 3D description of these deposits permits distinguishing seismic facies of the abandoned mud channel and the sand point bar deposits. The continuous meandering channel, which is filled by muddy deposits, may function as horizontal muddy barriers or baffles that might isolate the reservoir body into separate storage containers. The 3rd, 4th, and 5th orders of the seismic sequences were established for the studied succession. The essential geomorphological elements have been imaged utilizing several seismic attributes.

Near-fault seismic monitoring reveals the long-lasting activation of a local fault in the Marmara region controlled by slow slip

<p>Recent laboratory and field observations show that fault seismic and aseismic slip may occur concurrently. Here, we combine microseismicity recordings from a temporary near-fault seismic network (SMARTnet) and borehole strainmeter data from the eastern Marmara region in NW Turkey to track seismic and aseismic deformation around the hypocentral region of a M<sub>W</sub> 4.5 earthquake that occurred in 2018. The strainmeter data show a clear strain signal transient starting at the time of the M<sub>W</sub> 4.5 event and lasting for about 150 days. We study about 1,200 microseismic events following the mainshock within and beyond the mainshock fault rupture. The temporal distribution of the seismicity reveals a strong temporal clustering, including four semi-periodic seismic sequences each containing more than 50 events in two days. Two seismic sequences occurred during the strain transient showing different characteristics compared to two sequences occurring afterwards. Seismicity occurring during the transient displayed typical characteristics driven by aseismic slip, such as the activation of a broader region from the mainshock, and the absence of a clear mainshock in each sequence. Seismic sequences occurring after the transient correspond to typical mainshock-aftershock sequences and activated a region closer to the original M<sub>W</sub> 4.5 mainshock. We suggest post-strain transient seismicity originate from stress redistribution and breaking of remaining asperities. Our observations from a newly installed combined dense seismic and strainmeter network in the eastern Sea of Marmara region allows identifying repeated triggering of aseismic transients within an observation period of three years suggesting these may occur more often than previously thought.</p>

Variations of the earthquake diffusion rates in the Western Gulf of Corinth (Greece)

<p>Earthquake diffusion is frequently observed in the spatiotemporal evolution of seismic clusters and regional seismicity, a characteristic that is attributed to a triggering mechanism, such as fluid flow, aseismic creep and/or stress transfer effects. In this work, we study the earthquake diffusion properties in the Western Gulf of Corinth (central Greece), an area that presents high extension rates, moderate to large magnitude earthquakes, intense microseismicity and frequent seismic swarms. We focus on the period 2013–2014 that is characterized by intense background microseismic activity along with significant seismic sequences. More specifically, the latter include the 2013 Helike swarm, the 2014 seismic sequence between Nafpaktos and Psathopyrgos, which culminated with an Mw 4.9 event on 21 September 2014, as well as moderate magnitude events that were followed by aftershock sequences. In the herein analysis, we employ a relocated earthquake catalogue of ~9000 events which delineates the activated areas during the study period in high-resolution. We consider the most significant seismic sequences and calculate their respective spatial correlation histograms and the evolution of the mean squared distance of the hypocenters with time, in order to study the earthquake diffusion rates and possible variations that might be related to the triggering mechanisms of seismicity. Our results demonstrate a weak earthquake diffusion process, analogous to subdiffusion within a stochastic framework, for the seismic sequences under consideration, providing further evidence for slow earthquake diffusion in regional and global seismicity. In addition, the earthquake diffusion rates exhibit variations that can be associated with the triggering mechanism. In particular, seismic sequences which are related with pore-fluid pressure diffusion present considerably higher diffusion rates than mainshock/aftershock sequences or the background activity. Such results may provide novel constraints on the triggering mechanisms of clustered seismic activity based on the study of the earthquake diffusion rates. </p><p><strong>Acknowledgements</strong></p><p>We would like to thank the personnel of the Hellenic Unified Seismological Network (http://eida.gein.noa.gr/) and the Corinth Rift Laboratory Network (https://doi.org/10.15778/RESIF.CL) for the installation and operation of the stations used in the current article. The present research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning 2014-2020» in the context of the project “The role of fluids in the seismicity of the Western Gulf of Corinth (Greece)” (MIS 5048127).</p>

Investigating the relationship between seismic sequences and hydrological loading in the Azores

<p>Hydrological loads can be either surface loads induced by precipitation, changes in water levels at crater volcanic lakes, or subsurface loads created by seasonal changes in groundwater levels. These may contribute to strain and stress transients that trigger small earthquake swarms at faults that are already near failure. This work focusses on how annual and multi-annual stress changes of hydrological origin may affect the generation of seismic sequences on several tectonic settings, such as the New Madrid Seismic Zone and the Azores. The New Madrid seismic Zone is used as a benchmark test study region, while the Azores has been chosen for its intense seismic activity of both tectonic and volcanic origin. The magnitude of the hydrologically derived variations in stress is small compared with the long-term tectonic stresses, so we look for seasonal and inter-annual modulations of the earthquake occurrence rate. This requires the manipulation of seismic catalogues and the use of statistical methods to check if the seasonal and inter-annual variations are statistically significant, and not the result of extreme climatic events. The impact of hydrologic loads on faults is addressed using high-quality time series of seismic sequences, rainfall and other loads produced by variations in water levels, methods of decomposition and reconstruction of geophysical time series (SSA and wavelet transform) to identify modes of oscillation, and correlation analysis to recognize common patterns in seismicity and water loads. The results provide the first assessment of cyclic variations in seismicity and its relationship with atmospheric disturbances and hydrologically-driven load in the Azores region, and contributes to improve our understanding of the physics of earthquake triggering processes. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL. This is a contribution to the RESTLESS project PTDC/CTA-GEF/6674/2020.</p>