Decay and expansion of the early aftershock activity following the 2011 Mw9.0 Tohoku earthquake

journal articl

Extending Earthquakes' Reach Through Cascading.

International audienceEarthquakes, whatever their size, can trigger other earthquakes. Mainshocks cause aftershocks to occur, which in turn activate their own local aftershock sequences, resulting in a cascade of triggering that extends the reach of the initial mainshock. A long-lasting difficulty is to determine which earthquakes are connected, either directly or indirectly. Here we show that this causal structure can be found probabilistically, with no a priori model nor parameterization. Large regional earthquakes are found to have a short direct influence in comparison to the overall aftershock sequence duration. Relative to these large mainshocks, small earthquakes collectively have a greater effect on triggering. Hence, cascade triggering is a key component in earthquake interactions

Seismicity related to the hydraulic stimulation of GRT1, Rittershoffen, France

International audienc

Constraining Spatiotemporal Characteristics of Magma Migration at Piton De La Fournaise Volcano From Pre-eruptive Seismicity

International audienceVolcano-tectonic seismicity has been recorded for decades on various volcanoes and is linked with the magma transport and reservoir pressurization. Yet earthquakes often appear broadly distributed such that magma movement is difficult to infer from its analysis. We explore the seismicity that occurred before eruptions at Piton de la Fournaise in the last 5 years. Using template matching and relocation techniques, we produce a refined image of the summit seismicity, demonstrating that most earthquakes are located on a ring structure. However, only a portion of this structure is activated before each eruption, which provides an indication as to the direction of the future eruptive site. Furthermore, we show that the delay between the pre-eruptive swarm and the eruption onset is proportional to the distance of the eruptive fissures relative to the summit cone. This reveals that the beginning of the intrusion already bears information regarding the future eruption location. Plain Language Summary Earthquakes on volcanoes are usually related to the ascent of magma in the edifice and can therefore provide information concerning an impending eruption. Yet on many occasions, it is not easy to relate the recorded seismicity to the propagation of magma in the volcano. In this study, we detect and locate earthquakes that occurred before 13 eruptions at Piton de la Fournaise, a hawaiian-type volcano located on La Réunion island in the Indian Ocean. We demonstrate that a detailed analysis of the seismicity occurring when the magma starts its ascent can bring important information regarding the future eruption location

Carte de l'édifice volcanique du Piton de la Fournaise

Les triangles orange indiquent les stations sismiques utilisés pour l'étude. Les fissures éruptives associées aux plus récentes éruptions sont indiquées par des lignes rouges. Les points bleus et verts sur la carte en bas correspondent respectivement aux séismes superficiels (au dessus du niveau de la mer) et profonds (sous le niveau de la mer).Avec plus d’une éruption par an en moyenne, le Piton de la Fournaise est l’un des volcans les plus actifs au monde. Comme sur la plupart des volcans, les éruptions y sont généralement précédées par une abondante activité sismique. On observe ainsi de véritables essaims de micro-séismes : des milliers de petits tremblements de terre correspondant à autant de petites ruptures déclenchées au sein de l’édifice volcanique. Au Piton de la Fournaise, cette activité est tellement importante qu’il devient même difficile de détecter et de localiser précisément tous ces signaux en utilisant des procédures classiques.Pour identifier un maximum de tremblements de terre, les chercheurs ont donc essayé de resserrer les mailles du filet en appliquant de nouvelles méthodes de détection automatique. Les résultats furent sans appels avec un grand nombre de séismes détectés automatiquement. En étudiant finement le délai temporel entre ces signaux, tous ces séismes nouvellement détectés ont également pu être localisés précisément. A l'occasion de cette étude, les chercheurs ont découvert une remontée profonde de magma qui correspond au remplissage d’une chambre magmatique superficielle suivie quelques jours plus tard par une éruption volcanique

Seismicity distribution and locking depth along the Main Marmara Fault, Turkey

International audienc

Thermal Cracking in Westerly Granite Monitored Using Direct Wave Velocity, Coda Wave Interferometry, and Acoustic Emissions

International audienceTo monitor both the permanent (thermal microcracking) and the nonpermanent (thermo‐elastic) effects of temperature on Westerly Granite, we combine acoustic emission monitoring and ultrasonic velocity measurements at ambient pressure during three heating and cooling cycles to a maximum temperature of 450°C. For the velocity measurements we use both P wave direct traveltime and coda wave interferometry techniques, the latter being more sensitive to changes in S wave velocity. During the first cycle, we observe a high acoustic emission rate and large—and mostly permanent—apparent reductions in velocity with temperature (P wave velocity is reduced by 50% of the initial value at 450°C, and 40% upon cooling). Our measurements are indicative of extensive thermal microcracking during the first cycle, predominantly during the heating phase. During the second cycle we observe further—but reduced—microcracking, and less still during the third cycle, where the apparent decrease in velocity with temperature is near reversible (at 450°C, the P wave velocity is decreased by roughly 10% of the initial velocity). Our results, relevant for thermally dynamic environments such as geothermal reservoirs, highlight the value of performing measurements of rock properties under in situ temperature conditions

Short term forecasting of explosions at Ubinas volcano, Peru

Most seismic eruption forerunners are described using Volcano-Tectonic earthquakes, seismic energy release, deformation rates or seismic noise analyses. Using the seismic data recorded at Ubinas volcano (Peru) between 2006 and 2008, we explore the time evolution of the Long Period (LP) seismicity rate prior to 143 explosions. We resolve an average acceleration of the LP rate above the background level during the 2-3 hours preceding the explosion onset. Such an average pattern, which emerges when stacking over LP time series, is robust and stable over all the 2006-2008 period, for which data is available. This accelerating pattern is also recovered when conditioning the LP rate on the occurrence of an other LP event, rather than on the explosion time. It supports a common mechanism for the generation of explosions and LP events, the magma conduit pressure increase being the most probable candidate. The average LP rate acceleration toward an explosion is highly significant prior to the higher energy explosions, supposedly the ones associated with the larger pressure increases. The dramatic decay of the LP activity following explosions, still reinforce the strong relationship between these two processes. We test and we quantify the retrospective forecasting power of these LP rate patterns to predict Ubinas explosions. The prediction quality of the forecasts (e.g. for 17% of alarm time, we predict 63% of Ubinas explosions, with 58% of false alarms) is evaluated using error diagrams. The prediction results are stable and the prediction algorithm validated, i.e. its performance is better than the random guess

Seismicity and deformation induced by magma accumulation at three basaltic volcanoes

International audienceWe analyzed the evolution of volcano-tectonic (VT) seismicity and deformation at three basaltic volcanoes (Kilauea, Mauna Loa, Piton de la Fournaise) during phases of magma accumulation. We observed that the VT earthquake activity displays an accelerating evolution at the three studied volcanoes during the time of magma accumulation. At the same times, deformation rates recorded at the summit of Kilauea and Mauna Loa volcanoes were not accelerating but rather tend to decay. To interpret these observations, we propose a physical model describing the evolution of pressure produced by the accumulation of magma into a reservoir. This variation of pressure is then used to force a simple model of damage, where damage episodes are equivalent to earthquakes. This model leads to an exponential increase of the VT activity and to an exponential decay of the deformation rate during accumulation phases. Seismicity and deformation data are well fitted by such an exponential model. The time constant, deduced from the exponential increase of the seismicity, is in agreement with the time constant predicted by the model of magma accumulation. This VT activity can thus be a direct indication of the accumulation of magma at depth, and therefore can be seen as a long-term precursory phenomenon, at least for the three studied basaltic volcanoes. Unfortunately, it does not allow the prediction of the onset of future eruptions, as no diverging point (i.e., critical time) is present in the model