A hybrid method is proposed to calculate complete synthetic seismograms based on a spherically symmetric and self-gravitating Earth with a multilayered structure of atmosphere, ocean, mantle, liquid core and solid core. For large wavelengths, a numerical scheme is used to solve the geodynamic boundary-value problem without any approximation on the deformation and gravity coupling. With decreasing wavelength, the gravity effect on the deformation becomes negligible and the analytical propagator scheme can be used. Many useful approaches are used to overcome the numerical problems that may arise in both analytical and numerical schemes. Some of these approaches have been established in the seismological community and the others are developed for the first time. Based on the stable and efficient hybrid algorithm, an all-in-one code QSSP is implemented to cover the complete spectrum of seismological interests. The performance of the code is demonstrated by various tests including the curvature effect on teleseismic body and surface waves, the appearance of multiple reflected, teleseismic core phases, the gravity effect on long period surface waves and free oscillations, the simulation of near-field displacement seismograms with the static offset, the coupling of tsunami and infrasound waves, and free oscillations of the solid Earth, the atmosphere and the ocean. QSSP is open source software that can be used as a stand-alone FORTRAN code or may be applied in combination with a Python toolbox to calculate and handle Green's function databases for efficient coding of source inversion problems.
<p>A period of intense seismicity started more than a year prior to the 2021 Fagradalsfjall eruption in Iceland. During the same period, repeated cycles of surface uplift and subsidence were observed in the Svartsengi and Kr&#253;suv&#237;k high-temperature (HT) fields, about 8-10 km west and east of the eruption site in Fagradalsfjall, respectively. Such an uplift has never been observed during 40 years of surface deformation monitoring of the exploited Svartsengi HT field. However, cycles of uplift followed by subsidence have been observed earlier at the unexploited Kr&#253;suv&#237;k HT field.</p><p>Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and &#205;SOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.</p><p>The data was used for multidisciplinary modelling of the pre-eruption processes (see Fl&#243;venz et al, 2022. Cyclical geothermal unrest as a precursor to Iceland's 2021 Fagradalsfjall eruption. Nature Geoscience (in revision)). It included a poroelastic model that explains the repeated uplift and subsidence cycles at the Svartsengi HT field, by cyclic fluid intrusions into a permeable aquifer at 4 km depth at the observed brittle-ductile transition (BDT). The model gives a total injected volume of 0.11&#177;0.05 km<sup>3</sup>. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850&#177;350 kg/m<sup>3</sup>. A high-resolution seismic catalogue of 39,500 events using the optical cable recordings was created, and the poroelastic model explains very well the observed spatiotemporal seismicity.</p><p>The geodetic, gravity, and seismic data are explained by ingression of magmatic CO<sub>2</sub> into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.</p><p>The poroelastic model and the feeder-channel model are combined into a conceptual model that is consistent with the geochemical signature of the erupted magma. It explains the pre-eruption processes and gives estimates of the amount of magma involved.</p><p>The conceptual model incorporates a magmatic reservoir at 15-20 km depth, fed by slowly upwelling currents of mantle derived magma. Volatiles released from inflowing enriched magma into the sub-Moho reservoir migrated upwards. The volatiles were possibly trapped for weeks or months at the BDT at ~7 km depth beneath Fagradalsfjall, generating overpressure, but not high enough to lift the overburden (~220 MPa) and cause surface deformation. After reaching a certain limiting overpressure, or when a certain volume had accumulated, the magmatic volatiles were diverted upwards, just below the BDT towards the hydrostatic pressurized aquifer (~ 40 MPa) at 4 km depth at the bottom of the convective HT fields. They passed through the BDT and increased the pressure sufficiently (>110 MPa) to cause the uplift.</p><p>The lessons learned enlighten the most important factors to help detect precursory volcanic processes on the Reykjanes Peninsula; including detailed monitoring of seismicity, surface deformation, gravity changes and gas content in geothermal fluids. Furthermore, geophysical exploration of the deeper crust by seismic and resistivity measurements are crucial to map possible melt and possible pathways towards the surface.</p>
Abstract. Geothermal systems in the Hengill volcanic area, SW Iceland, started to be exploited for electrical power and heat production since the late 1960s. Today the two largest operating geothermal power plants are located at Nesjavellir and Hellisheiði. This area is a complex tectonic and geothermal site, located at the triple junction between the Reykjanes Peninsula (RP), the Western Volcanic Zone (WVZ), and the South Iceland Seismic Zone (SISZ). The region is seismically highly active with several thousand earthquakes located yearly. The origin of such earthquakes may be either natural or anthropogenic. The analysis of microseismicity can provide useful information on natural active processes in tectonic, geothermal and volcanic environments as well as on physical mechanisms governing induced events. Here, we investigate the microseismicity occurring in Hengill area, using a very dense broadband seismic monitoring network deployed in Hellisheiði since November 2018, and apply sophisticated full-waveform based method for detection and location. Improved locations and first characterization indicate that it is possible to identify different types of microseismic clusters, which are associated with either production/injection or the tectonic setting of the geothermal area.
The design of an array configuration is an important task in array seismology during experiment planning. Often the array response function (ARF), which depends on the relative position of array stations and frequency content of the incoming signals, is used as the array design criterion. In practice, additional constraints and parameters have to be taken into account, for example, land ownership, site-specific noise levels or characteristics of the seismic sources under investigation. In this study, a flexible array design framework is introduced that implements a customizable scenario modelling and optimization scheme by making use of synthetic seismograms. Using synthetic seismograms to evaluate array performance makes it possible to consider additional constraints. We suggest to use synthetic array beamforming as an array design criterion instead of the ARF. The objective function of the optimization scheme is defined according to the monitoring goals, and may consist of a number of subfunctions. The array design framework is exemplified by designing a seven-station small-scale array to monitor earthquake swarm activity in Northwest Bohemia/Vogtland in central Europe. Two subfunctions are introduced to verify the accuracy of horizontal slowness estimation; one to suppress aliasing effects due to possible secondary lobes of synthetic array beamforming calculated in horizontal slowness space and the other to reduce the event’s mislocation caused by miscalculation of the horizontal slowness vector. Subsequently, a weighting technique is applied to combine the subfunctions into one single scalar objective function to use in the optimization process.
Abstract Flank instability and sector collapses, which pose major threats, are common on volcanic islands. On 22 Dec 2018, a sector collapse event occurred at Anak Krakatau volcano in the Sunda Strait, triggering a deadly tsunami. Here we use multiparametric ground-based and space-borne data to show that prior to its collapse, the volcano exhibited an elevated state of activity, including precursory thermal anomalies, an increase in the island’s surface area, and a gradual seaward motion of its southwestern flank on a dipping décollement. Two minutes after a small earthquake, seismic signals characterize the collapse of the volcano’s flank at 13:55 UTC. This sector collapse decapitated the cone-shaped edifice and triggered a tsunami that caused 430 fatalities. We discuss the nature of the precursor processes underpinning the collapse that culminated in a complex hazard cascade with important implications for the early detection of potential flank instability at other volcanoes.
Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and eruption. We use multiparameter geophysical and geochemical data to show that the 110-square-kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014-2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, near-exponential decline of both collapse rate and the intensity of the 180-day-long eruption.
