In this paper, we present numerical modeling aimed to explain Deep Long Period (DLP) events occurring in middle-to-lower crust beneath volcanoes and often observed in association with volcanic eruptions or their precursors. We consider a DLP generating mechanism caused by the rapid growth of gas bubbles in response to the slow decompression of H\textsubscript{2}O–CO\textsubscript{2} over-saturated magma. The nucleation and rapid growth of gas bubbles lead to rapid pressure change in the magma and elastic rebound of the host rocks, radiating seismic waves recorded as DLP events. The magma and host rocks are modeled as Maxwell bodies with different relaxation times and elastic moduli. Simulations of a single sill-shaped intrusion with different parameters demonstrate that realistic amplitudes and frequencies of P and S seismic waves can be obtained when considering intrusions with linear sizes of the order of 100 m. We then consider a case of two closely located sills and model their interaction. We speculate on conditions that can result in consecutive triggering of the bubble growth in multiple closely located batches of magma, leading to the generation of earthquake swarms or seismic tremors.

In this paper, we present numerical modeling aimed to explain Deep Long Period (DLP) events occurring in middle-to-lower crust beneath volcanoes and often observed in association with volcanic eruptions or their precursors. We consider a DLP generating mechanism caused by the rapid growth of gas bubbles in response to the slow decompression of H\textsubscript{2}O–CO\textsubscript{2} over-saturated magma. The nucleation and rapid growth of gas bubbles lead to rapid pressure change in the magma and elastic rebound of the host rocks, radiating seismic waves recorded as DLP events. The magma and host rocks are modeled as Maxwell bodies with different relaxation times and elastic moduli. Simulations of a single sill-shaped intrusion with different parameters demonstrate that realistic amplitudes and frequencies of P and S seismic waves can be obtained when considering intrusions with linear sizes of the order of 100 m. We then consider a case of two closely located sills and model their interaction. We speculate on conditions that can result in consecutive triggering of the bubble growth in multiple closely located batches of magma, leading to the generation of earthquake swarms or seismic tremors.

The overwhelming amount of seismic, geodesic and in-situ observations accumulated over the last 30 years clearly indicate that, from a mechanical point of view, faults should be considered as both damageable elastic solids in which highly localized features emerge as a result of very short-term brittle processes and materials experiencing ductile strains distributed in large volumes and over long time scales. The interplay of both deformation mechanisms, brittle and ductile, give rise to transient phenomena associating slow slip and tremors, known as slow earthquakes, which dissipate a significant amount of stress in the fault system. The physically-based numerical models developed to improve our comprehension of the mechanical and dynamical behaviour of faults must therefore have the capacity to treat simultaneously both deformation mechanisms and to cover a wide range of time scales in a numerically efficient manner. This capability is essential, both for simulating accurately their deformation cycles and for improving our interpretation of the available observations. In this paper, we present a numerically efficient visco-elasto-brittle numerical framework that can simulate transient deformations akin to that observed in the context of subduction zones, over the wide range of time scales relevant for slow earthquakes. We implement the model in idealized simple shear simulations and explore the sensitivity of its behavior to the value of its main mechanical parameters.

A document by Rene Steinmann. Click on the document to view its contents.

We develop a method to estimate relative seismic moments M0 and corner frequencies fc of acoustic emission events recorded in laboratory experiments from amplitude spectra of signal’s coda composed of reverberated and scattered waves. This approach has several advantages with respect to estimations from direct waves that are often clipped and also are difficult to separate in experiments performed on small samples. Also, inversion of the coda spectra does not require information about the source locations ans mechanisms. We use the developed method to analyze the data of two experiments: (1) on granite from the Voronezh crystal massif and (2) on Berea sandstone. The range of absolute corner frequencies estimated in both experiments is around 70-700 kHz. The range of relative seismic moments covers 103.5. The relation between fc and M0 observed on the first stages of both experiments, consisted of increasing isotropic confining pressure, approximately follow M0 ~ fc-3 scaling and the b-value of the Gutenberg-Richter distribution was found close to 1. This can be interpreted as rupturing of preexisting material defects with a nearly constant stress-drop and has a similarity with observations of ‘natural’ earthquakes. Deviations from this ‘earthquake-like’ behavior observed after applying axial loading and initiation of sample damaging can be interpreted as changes in stress-drop. Lower stress-drops prevail for sandstone and higher for granite sample respectively that can be related to the strength of corresponding material.

Along the strike of subduction zones, tectonic tremor activity is segmented on a geologic scale, indicating local variations of the tremor-generating process. Here, we study how strong temporal clustering and long-term recurrence of activity can emerge from the synchronization of elementary tremor sources, as they interact through fluid pressure transients. We model tremor sources as rapid openings of low-permeability valves in the permeable fault zone channeling the upward flow of deep metamorphic fluids. Valve openings trigger fast pressure transients that generate seismic waves. In such a system, tremor activity is thus shaped by unsteady fluid circulation. Using numerical simulations of fluid flow for a large number of different valve populations, we show that the synchronized, collective activity of sources generates episodic activity, and that along-strike variations of fluid flux and fluid transport properties can lead to the segmentation of tremor activity. Strong tremor bursts that coherently activate wide parts of the fault and recur with a long period are associated with patches densely populated with valves and characterized by below-average permeability. Long-term tremor episodicity emerges from the synchronous activity of valves in such patches and is responsible for fluid-pressure cycling at the subduction scale. In the tremor zone of the Shikoku, Japan, subduction interface, the most temporally clustered segment coincides with a downgoing seamount chain, suggesting that the segmentation of the fault zone permeability, and hence of tremor activity, could be inherited from the topography of the subducting oceanic plate.

Low-frequency earthquakes are peculiar energy-release events mostly occurring at the transition between the seismogenic and the freely creeping zones of a subducting slab. The source characterization of these events is of fundamental importance to understand physical processes that govern the slow out of equilibrium evolution of the subduction interface that may lead to the generation of large, destructive earthquakes. Nevertheless, their source mechanisms still remain unclear. Here, we estimate the source parameters of ~23,000 low-frequency earthquakes continuously detected from 2013 to 2015 in Shikoku, Japan. We show that a cubic moment-duration scaling characterizes these events, suggesting a self-similar process as for regular earthquakes. However, their high-frequency fall-off suggests an omega-cube decay in contrast to the omega-squared model of earthquakes. Source characteristics do not change when low-frequency earthquake bursts occur during the analyzed three years. On the other hand, we observe a coherent along-strike variation of the product of stress drop and the cube of rupture velocity, possibly related to a weaker behavior of tremor patches in central Shikoku. Secondary microseismic noise and network-dependent completeness magnitude lead to missing event detections that do not allow discriminating between Gutenberg-Richter event size distribution and any deviation from it. Our findings suggest that the same observational limits might affect worldwide detection of low-frequency earthquakes.

Most of the largest volcanic activity in the world occurs in remote places as deep oceans or poorly monitored oceanic islands. Thus, our capacity of monitoring volcanoes is limited to remote sensing and global geophysical observations. However, the rapid estimation of volcanic eruption parameters is needed for scientific understanding of the eruptive process and rapid hazard estimation. We present a method to rapidly identify large volcanic explosions, based on analysis of seismic data. With this methodology, we promptly detect the January 15, 2022 Hunga Tonga eruption. We then analyze the seismic waves generate by the volcanic explosion and estimate important first-order parameters of the eruption. We further relate the parameters with the volcanic explosivity index (VEI). Our estimate of VEI~6, indicate how the Hunga Tonga eruption is among the largest volcanic activity ever recorded with modern geophysical instrumentation, and can provide new insights about the physics of large volcanoes.