The largest deep-focus Sea of Okhotsk earthquake May 24, 2013, Mw=8.3

This is the second part of an report of Chebrov et al. on notable events in Kamchatka in 2013 for the Summary of the Bulletin of the International Seismological Centre including the largest deep-focus Sea of Okhotsk earthquake from 24 May 2013 with a magnitude of Mw=8.3

Deep and shallow long-period volcanic seismicity linked by fluid-pressure transfer

International audienceVolcanic long-period earthquakes are attributed to pressure fluctuations that result from unsteady mass transport in the plumbing system of volcanoes. Whereas most of the long-period seismicity is located close to the surface, the volcanic deep long-period earthquakes that occur in the lower crust and uppermost mantle reflect the activity in the deep parts of magmatic systems. Here, we present observations of long-period earthquakes that occurred in 2011-2012 within the Klyuchevskoy volcano group in Kamchatka, Russia. We show two distinct groups of long-period sources: events that occurred just below the active volcanoes, and deep long-period events at depths of ~30 km in the vicinity of a deep magmatic reservoir. We report systematic increases of the long-period seismicity levels prior to volcanic eruptions with the initial activation of the deep long-period sources that reflects pressurization of the deep reservoir and consequent transfer of the activity towards the surface. The relatively fast migration of the long-period activity suggests that a hydraulic connection is maintained between deep and shallow magmatic reservoirs. The reported observations provide evidence for the pre-eruptive reload of the shallow magmatic reservoirs from depth, and suggest that the deep long-period earthquakes could be used as a reliable early precursor of eruptions

New features in the subsurface structure model of El Hierro island (Canaries) from low-frequency microseismic sounding: an insight into the 2011 seismo-volcanic crisis

To study the deep structure of El Hierro Island, Canarian Archipelago, we have used a microseismic sounding method (MSM) based on the fact that heterogeneities of the Earth's crust disturb the spectrum of the low-frequency microseismic field in their vicinity. So, at the Earth's surface, the spectral amplitudes of definite frequency f above the high-velocity heterogeneities are decreasing, and above the low-velocity ones they are increasing. Moreover, the frequency f is connected with the depth of a heterogeneity H and the velocity of the fundamental mode of Rayleigh waves V (R)(f) through the relation H a parts per thousand 0.4V (R)(f)/f. From these relations, the MSM lets us model the subsurface structure in a 3D context by inverting the amplitude-frequency spatial distribution of the microseismic field of low frequency. The validity of the method is shown through of numerical simulations and previous applications with known or verified solutions. This MSM is now used to invert the microseismic data registered in El Hierro Island. The obtained subsurface model reveals two large intrusive bodies beneath the island. Joint interpretation of microseismic and gravimetric data and their comparison with the available geological studies relate the central-eastern intrusive body to the early stage of the island formation. With respect to the western intrusive body, at the depths of 15-25 km, an area with lowest seismic velocities is identified, where we suggest that a modern magmatic reservoir is located. This reservoir could be associated with the recent submarine eruption in October 2011 and the accompanying seismic swarm, which started in July 2011. Several correlations between the shallowest structures identified by the gravity and MSM approaches are also found. Besides the numerical simulation and previous studies of this method, the correlation between gravity results, the MSM model, the geological information and the possible explanation of the features of the seismic swarm through the model obtained offer us a valid proof about the plausibility of the subsurface structures identified from MSM