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>
<p>A moderate seismicity accompanied the dike intrusion which preceded the 2021 volcanic eruption at La Palma, Canary Islands, Spain. Nevertheless, the largest magnitudes were recorded during the eruption, from September 19th to December 13th, 2021. This volcanotectonic activity accompanied the upward magma transfer to feed the eruption and provides important clues to the understand the feeding system geometry, as we are dealing with the first fully monitored eruption in the island. Seismicity during the eruption displayed a stable bimodal spatial distribution, with hypocenters clustering at two, well separated depth intervals. A shallower seismic cluster was active beneath the central area of Cumbre Vieja &#160;~10-14 km depth, starting by September 27, just after a short quiescence of about 3 hours in the tremor signal and with peaks of intensification rates in mid and late November. A deeper and larger cluster (~33-39 km) extended further to the Northeast. Here, the activity started with some delay on October 5th and the cluster was mostly active over October and November 2021, reaching a peak magnitude mbLg 5.1 November 19th, 2021, the largest earthquake of the whole seismic sequence. In this study, we use a variety of seismological methods to resolve hypocentral and centroid location at the two clusters, as well as full moment tensors for 156 earthquakes, including largest ones at each cluster. The hypocentral relocation of 7150 earthquakes reconstructs the geometry of the active seismogenic structures, resolving small-scale details within each of the two clusters. The centroid moment tensor inversion resolves different families of moment tensors in each cluster including earthquakes with almost reversed focal mechanism that respond to local stress perturbations introduced by the magma rise through a complex path and multiple magmatic reservoirs. The source studies are complemented by a temporal analysis of the families based on waveform characterization, which allows to reconstruct the timeline of the magma transfer and seismogenic processes. Our seismological analysis provides details of seismicity accompanying the volcanic unrest at La Palma and documents the evolution of seismogenic processes in response to the rise of magma batches through the complex plumbing system.</p><p>&#160;</p>
