Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones

International audienceDouble Wadati-Benioff seismic zones (DSZ) with two parallel planes of seismicity separated by 15-30 km are a global feature of subduction zones in the 50-200 km depth range. Upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated. Serpentine or hydrous-phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration. High-resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization. Earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs. The contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2-3 kilometers beneath oceanic crust

Moment tensors for rapid characterization of megathrust earthquakes: the example of the 2011 M9 Tohoku-oki, Japan earthquake

The rapid detection and characterization of megathrust earthquakes is a difficult task given their large rupture zone and duration. These events produce very strong ground vibrations in the near field that can cause weak motion instruments to clip, and they are also capable of generating large-scale tsunamis. The 2011 M9 Tohoku-oki earthquake that occurred offshore Japan is one member of a series of great earthquakes for which extended geophysical observations are available. Here, we test an automated scanning algorithm for great earthquakes using continuous very long-period (100-200 s) seismic records from K-NET strong-motion seismograms of the earthquake. By continuously performing the cross-correlation of data and Green's functions (GFs) in a moment tensor analysis, we show that the algorithm automatically detects, locates and determines source parameters including the moment magnitude and mechanism of the great Tohoku-oki earthquake within 8 min of its origin time. The method does not saturate. We also show that quasi-finite-source GFs, which take into account the effects of a finite-source, in a single-point source moment tensor algorithm better fit the data, especially in the near-field. We show that this technique allows the correct characterization of the earthquake using a limited number of stations. This can yield information usable for tsunami early warnin

Observation of Microtremors in the Tsukuba Area, Japan, using a Portable Broadband Seismometer

Studies of microtremors have been advanced by different approaches, that is, a variety of observational studies and analyses, for both the short-period and the long-period ranges since the microtremors for each range have their own source and site characteristics in time and space. We, therefore, conducted microtremor observation around Tsukuba City, Ibaraki, Japan, in July and August, 1991, in order to clarify site characteristics of six locations around the Tsukuba Mountain, deploying a portable broadband seismometer (Streckeisen STS-2) in the field of subsurface structure and studying ground motion in both frequency ranges, simultaneously. By a comparison with an STS-1 seismometer, the STS-2 gives reliable frequency ranges higher than 0.09 Hz and 0.05 Hz in horizontal and vertical components, respectively. The correlation of the reference site and the other sites implies that the source of microtremors shares common characteristics for the lower range ( 1 Hz), particularly 1.2 ～ 2.5 Hz, in this area. Three types of dominant peak frequencies for the range of 0.1 ～ 1 Hz are revealed: (1) the peak frequencies ranging from 0.2 to 0.3 Hz observed near Mt.Tsukuba can be explained by the topographic high model of Bard (1982). Two frequency peaks ranging (2) from 0.2 to 0.4 Hz and (3) from 0.5 to 0.8 Hz, observed at the stations on alluvials, are related to any vertical resonance in soft layers, consistent with other geological information. Amplitude ratios at sedimentary sites with respect to TSK (Mt.Tsukuba), a rock site, are greater than unity over 5 Hz where the ratios are reported to be smaller than unity in many areas, which implies relatively hard sedimentary layers in the Tsukuba area

On the Realtime Monitoring of the Long-period Seismic Wavefield

A possibility of monitoring the long-period seismic wavefield in realtime is suggested. The seismic wavefield below 0.1 Hz may be consistently modeled by the earthquake activity field defined by a point source moment tensor on lOkm-mesh grid points. With the current level of personal computers, it should be possible to perform moment tensor inversions for all the mesh points to find the best moment tensor every second. A sparse regional broadband seismometer network appears suffice to perform such realtime monitoring, which may eventually enable us to predict the short-period ground motions in realtime as well

長周期波動場のリアルタイムモニタリングについて

A possibility of monitoring the long-period seismic wavefield in realtime is suggested. The seismic wavefield below 0.1 Hz may be consistently modeled by the earthquake activity field defined by a point source moment tensor on lOkm-mesh grid points. With the current level of personal computers, it should be possible to perform moment tensor inversions for all the mesh points to find the best moment tensor every second. A sparse regional broadband seismometer network appears suffice to perform such realtime monitoring, which may eventually enable us to predict the short-period ground motions in realtime as well

Global Surface Wave Tomography Using Seismic Hum

International audienceThe development of global surface wave tomography using earthquakes has been crucial to exploration of the dynamic status of Earth’s deep. It is naturally believed that only large earthquakes can generate long-period seismic waves that penetrate deep enough into Earth for such exploration. The discovery of seismic hum, Earth’s background free oscillations, which are randomly generated by oceanic and/or atmospheric disturbances, now provides an alternative approach. We present results of global upper-mantle seismic tomography using seismic hum and without referring to earthquakes. At periods of 100 to 400 seconds, the phase-velocity anomalies of Rayleigh waves are measured by modeling the observed cross-correlation functions between every pair of stations from among 54 globally distributed seismic stations. The anomalies are then inverted to obtain the three-dimensional S-wave velocity structure in the upper mantle. Our technique provides a new means for exploring the three-dimensional structure of the interior of terrestrial planets with an atmosphere and/or oceans, particularly Mars