EARTHQUAKE 3D. Enhanced Edition Features: Six Preset Views that you can use to save display setting configurations Custom Colors that you can use to customize the display colors Lunar and Solar Rings that that show the positions of the Sun and Moon Custom Captions let. Treating shots as earthquakes with a fixed location in LET has been. New additional LET data significantly enhances the internal crustal structure. 3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece). New, improved version of the Generic Mapping Tools released.
. 191 Downloads. Abstract Reliable analysis of low-energy earthquakes (microseismic) depends on how accurately one can detect and pick the arrival times, which are strongly influenced by the noise content. The study of microseismic events becomes even more challenging when the sensors are located on the surface because of the poor signal-to-noise ratio (SNR). Consequently, efficient and robust techniques for denoising microseismic data are necessary. In this study, we propose a method based on an infinite impulse response (IIR) Wiener filter. The proposed method uses statistics based on signal observations (noisy data) and the underlying noise, both recorded by various sensors.
The method presented here precludes the need for statistics or prior knowledge of the signal of interest. The second-order statistics of the noise and the noisy data are extracted from the recorded data only. As an advantage, in deriving the filter’s impulse response, no underlying structure of noise is assumed. Therefore, our method works for various types of noise, e.g., uncorrelated, spatially correlated, temporally correlated, Gaussian and non-Gaussian noise. Hence, the proposed method can be suitable as well for microseismic data recorded in diverse seismic noise environments.
The criteria used to optimize the filter impulse response is the minimization of the mean square error. The proposed method is tested on synthetic and field data sets and found to be effective in denoising microseismic data with very low SNR ( (-12 ) dB).
The 2011 Tohoku earthquake (M w = 9.1) highlighted previously unobserved features for megathrust events, such as the large slip in a relatively limited area and the shallow rupture propagation. We use a Finite Element Model (FEM), taking into account the 3D geometrical and structural complexities up to the trench zone, and perform a joint inversion of tsunami and geodetic data to retrieve the earthquake slip distribution. We obtain a close spatial correlation between the main deep slip patch and the local seismic velocity anomalies, and large shallow slip extending also to the North coherently with a seismically observed low-frequency radiation. These observations suggest that the friction controlled the rupture, initially confining the deeper rupture and then driving its propagation up to the trench, where it spreads laterally. These findings are relevant to earthquake and tsunami hazard assessment because they may help to detect regions likely prone to rupture along the megathrust, and to constrain the probability of high slip near the trench. Our estimate of 40 m slip value around the JFAST (Japan Trench Fast Drilling Project) drilling zone contributes to constrain the dynamic shear stress and friction coefficient of the fault obtained by temperature measurements to 0.68 MPa and 0.10, respectively. On March 11 th 2011 one of the largest earthquakes ever recorded occurred at the subduction interface between the Pacific and the Okhotsk plates and struck the Tohoku region in Japan.
This M w9.1 earthquake, located at 142.68°E 38.19°N, generated a tsunami that devastated the Japanese coasts, including towns and important infrastructures such as the Sendai airport and the Fukushima nuclear power plant causing more than 16,000 fatalities. The Tohoku earthquake is also the best observed ever megathrust event and consequently it has been investigated by modelling the unprecedented high-quality data set recorded by the Japanese dense seismological, geodetic and marine observational networks. The numerous studies published in recent literature are based on different kind of data and methodologies, including teleseismic, strong motion, geodetic, tsunami waveforms, and joint inversions, which were performed to investigate the earthquake rupture process. (a) Red star indicates the epicentre position. Red and white “beach ball” represents the focal mechanism of 2011 Tohoku earthquake.
Yellow triangles indicate the DART stations used in the inversion; (b) Cyan circles indicate GPS stations onshore, magenta circles indicate the geodetic seafloor observation sites, yellow triangles indicate the bottom pressure sensors and GPS-buoys. White arrow indicates the approximate convergence direction of the Pacific plate (estimated velocity of 9.2 cm/yr). Maps are created using GMT (Generic Mapping Tools, ) software.
The resulting source models share two common features of the coseismic rupture, stimulating further investigations to explore the physical processes controlling the genesis and the impact of megathrust events. The first feature is that the overall Tohoku rupture area is mainly concentrated in a relatively small portion of the plate interface and the retrieved peak slip values range between 30 and 60 m. This long-wavelength feature is common to most of the slip models obtained using different kind of data. At the same time, differences in terms of maximum slip value or number of slip patches can be observed among models, due to the data resolution and fault parameterization,.
A rather small rupture area characterized by very large slip is quite unusual for great earthquakes such as the 2004, M w = 9.2, Sumatra, and the 2010, M w = 8.8, Maule megathrust events. Some authors observed also a first order correlation between the coseismic slip patch and the positive seismic velocity anomaly at the subduction interface. The relatively concentrated Tohoku rupture area may be related to the lithosphere structure and the consequent heterogeneous pattern of pre-stress as well as to the fault frictional properties of the plate interface that could promote or inhibit the rupture propagation.
The relative variations in shear-wave and bulk-sound speed detected in the coseismic slip area may reflect mechanical heterogeneities of the subduction interface, which may have acted as asperities allowing this event to build up large slip in the near-trench zone. The second intriguing feature of the 2011 Tohoku earthquake is the shallow rupture propagation up to the trench.
The large (15 m) and shallow coseismic slip is at odds with the quite common view on the coseismic behaviour of the shallow portion of megathrusts, where aseismic slip, lack of strain accumulation and low coupling are expected to be dominant. Recent results from the Japan Trench Fast Drilling Project (JFAST,) pointed out the presence of smectite-rich weak clay, up-dip from the hypocentre in the very shallow portion of the subduction plate boundary. Thus, this distinctive feature of the Tohoku earthquake might have been controlled by very low friction on a relatively thin fault zone rich of clay sediments. Both these features, contributing to confine a vertical seafloor displacement larger than 10 m in a relatively small area, may have increased the tsunamigenic potential of the 2011 earthquake, generating tsunami waves higher than 10 m and runups larger than 30 m along the Iwate prefecture coasts. Accounting for detailed information on the geometry and structure of the subducting plate is necessary in order to reduce the epistemic uncertainties related to the modelling, to get a robust image of the coseismic slip distribution, to further constrain its spatial extension with respect to previous studies, and to focus on the shallow near-trench portion of the megathrust. We achieve this by carrying out, for the first time to our knowledge, a joint inversion of tsunami and both inland and seafloor geodetic data, constraining the slip distribution by means of a 3D Finite Element Model (FEM) of the subduction zone. We characterize each single element of the FEM grid by the 3D elastic structure inferred from seismic tomography, whereas the shallow near-trench portion of the megathrust, rich of sediments, is modelled by assuming a much more compliant material with respect to the surrounding medium (see section Methods).
Our modelling approach enables to account for the effects of the elastic contrasts both within the crust and between the crust and the mantle and, furthermore, for mimicking the contrast between the crust and the sediments at the trench (i.e. Clay-rich sediments), which may play a key role in controlling the coseismic slip distribution. Aim of this work is to image the slip distribution adopting a more realistic representation of the Green's functions through the 3D FEM model. In particular, we discuss whether and how the rupture extent and its propagation near the trench have been controlled by the regional and fault-zone structural heterogeneities. We also relate the shallow slip propagation to the recent JFAST results, drawing some possible relationships with the observed seismic radiation. The 3D geometry of the FEM (cf.
Section Methods for details) is built taking into account the main features of the subduction zone including the Tohoku region, such as the slab geometry and the topography/bathymetry. A FEM mesh of about 280,000 8-nodes brick elements is then created.
The 3D elastic structure of each element is constrained by the 3D P-wave and S-wave velocities (Vp and Vs, respectively) of a regional tomographic model; the resulting crust, slab and mantle are highly heterogeneous, with rigidity values ranging from 20 GPa in the crust, to 50–60 GPa in the slab. The elastic structure of the shallow part of the megathrust (i.e. The prism above the subduction fault) is characterized by independent data as imaged by seismic reflection surveys. The prism volume is limited by the trench to the East and by a steep listric plane to the West and is considered as made of unconsolidated clay-rich sediments and volcanic materials, for which we assume uniform elastic constants (rigidity = 5GPa and Poisson's ratio = 0.33). The chosen elastic values are compatible with those attributable to weak-clay (smectite) observed by the JFAST drilling project.
(a) Total domain of the 3D FEM model (2900 × 2500 km 2). Yellow dots represent the free surface grid nodes. (b) Central part of the 3D FEM model, including the active fault, viewed from SW.
The light blue line is the trench and the red line is the section of the active fault. (c) The inset shows a zoom of the interface between trench and the uppermost part of the fault. The shaded element edges lines on the top identify the prism extent. Details about the elastic layering of the model can be found in the. Panel (a) is created using GMT software, panels (b,c) using AMIRA (, Date of access: ) software. The FEM model is then used to compute a set of Green's Functions which are combined with a robust joint inversion scheme adopted in several previous papers to retrieve the slip distribution,. The inverse problem is solved by using the method of Green's functions superposition and the Heat Bath algorithm, a particular implementation of the Simulated Annealing technique.
The slip distribution is obtained through a global search technique. Since the solution of this inverse problem is intrinsically non-unique, and to account for possible modelling uncertainties, we compute and show the average slip model that we consider more representative than the best fitting solution. The average slip model is calculated as a weighted mean of a subset of the explored models. We assess the dispersion of the model parameters around their average values by performing an a-posteriori analysis of the explored models ensemble so that the error associated to each parameter is its weighted standard deviation,. In this analysis the coefficient of variation (i.e. The standard deviation divided by the average slip value) is also computed for each subfault parameter (further details in Methods section). The 2011 Tohoku earthquake slip distribution The slip distribution of the 2011 Tohoku earthquake extends mainly along strike from 36.5°N to 39.5°N and up-dip from the hypocentre in the SE direction (from 142°E to 144°E).
The portion of megathrust with slip values greater than 10 m (i.e. 20% of the maximum peak of slip) corresponds to an area of about 350 × 200 km 2. Amplitudes ranging between 25 m and 30 m characterize the slip distribution in the earthquake nucleation zone. Slip amplitudes increase moving away from the hypocentre towards the trench; slip direction is consistent with both the relative convergence of the Pacific and the Okhotsk plates and the centroid moment tensor solution (GCMT). Rake angle averaged over the whole slipping area is 88°.
The rupture reaches the shallow and less locked part of the megathrust with slip values greater than 30 m, up to 45 m around 143.5°E, 38°N. The inferred slip distribution shown in displays a narrow patch of slip located to the NE (from 38.5°N to 39.5°N) in the shallow part of the subduction interface roughly corresponding to the area where the 1896 Meiji-Sanriku earthquake occurred. In addition, at around 35 km depth a relatively low slip feature extends from the hypocentre zone to the latitudes of Fukushima and Ibaraki prefectures. This feature, generally imaged by using strong-motion and seismic data, approximately corresponds to a source of high frequency radiation detected by means of back-projection analysis and amplitude source location method. Even though our inverted distribution represents a long-wavelength image of the coseismic slip pattern, the complex model adopted and the inversion technique allow sensing this peculiarity of the Tohoku earthquake even using only tsunami and geodetic data. We recognize, however, that the pattern of the variation coefficient indicates that the slip distribution is particularly well resolved (variation coefficient lower than 0.4) in the portion of megathrust with slip values greater than 10 m. Slip distribution for the 2011 Tohoku-oki earthquake obtained from the joint inversion of tsunami and geodetic data.
(a) Orange arrows represent the slip direction (rake, ). Thin dashed black contours above the fault plane indicate the interseismic coupling (from 10% to 100%, at 10% intervals) along the megathrust. Black arrow indicates the approximate convergence direction of the Pacific plate (estimated velocity of 9.2 cm/yr). Red star as of. (b) Green contour lines (intervals are at 0.25, 0.50, 1.00, and 1.50 m) and green star indicate the slip distribution and the epicenter position of the foreshock occurred on 9 March 2011, respectively; magenta dashed rectangle represents approximately the rupture area of the 1896 Meiji-Sanriku earthquake; yellow coloured region approximately indicates the zone of coseismic high frequency radiation; black dot indicates the JFAST ocean drilling site C0019; (c) coefficients of variation associated to the average slip model of the 2011 Tohoku-oki earthquake resulting from the joint inversion. Black contour lines (10meters interval) indicate the slip distribution of Tohoku earthquake.
Maps are created using GMT software. The estimated total seismic moment, computed by taking into account the elastic parameters used into the FEM model (see Methods section) is M 0 = 5.72 × 10 22 Nm, corresponding to a magnitude M w = 9.1.
The inferred slip model yields a general good fit to tsunami (RMS = 0.50 m) and geodetic (RMS = 0.38 m, 0.19 m, and 0.14 m for East-West, North-South, and vertical components, respectively) data. It is noteworthy that the tsunami signals measured by the GPS-buoys off Iwate (G802, G804, G807) and by the bottom pressure sensors located very close to the source are extremely well reproduced, in particular their short-period wavelengths. This feature is probably tied up to the narrow patch of slip located in the shallow part of the megathrust from 38.5°N to 39.5°N.
However, a slight phase shifting is observed at few tsunami stations (e.g. G801) and it may be due to our assumption of a simplified circular rupture front (propagating from the hypocenter with a velocity of 1 km/s,) while the actual rupture history of Tohoku event, is likely more complex. We also notice, as already observed in previous studies, that the tsunami amplitude is amplified by the contribution of the horizontal seafloor deformation near the trench axis (due to the presence of steep bathymetric slopes, further details in section Methods).
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This effect is particularly evident for the stations located near or above the source region (TM1, TM2, P02, P06, G802, G804, G807) where the wave amplitudes are smaller by 20–40% if the horizontal deformation contribution is not included. The present slip model confirms at the first order the rupture pattern retrieved in our previous study as well as in other works, i.e. The large slip in a relatively concentrated rupture area and the shallow rupture propagation. In particular, a shallow patch of slip in the northern portion of the megathrust is clearly distinguishable, a feature also observed using seismic data, and that will be discussed in a later section. In addition, the present slip model highlights a new interesting feature, thanks to the employment here of an enlarged geodetic and tsunami data set (particularly above the source, e.g.
P02 and P06 sensors), and of a FEM model honouring the structural heterogeneities. Indeed, we observe a relative minimum of slip ( 10 m) and the PVA present in the Vp model; the rupture area almost completely overlaps the PVA zone (70% of the slipping area) and borders the NVA zones. Such a correlation holds for the Vs anomalies as well. This close correspondence suggests that the Tohoku rupture might have been efficiently controlled by the observed velocity anomalies, perhaps associated to the variation of the frictional properties of the Tohoku megathrust zone.
(a) Vp and (b) Vs anomaly distributions on the subducting plate interface. Black contour lines (10 m interval) indicate the slip distribution of Tohoku earthquake. Open green circles indicate the large (M 7) earthquakes occurred in the Tohoku earthquake region since 1900. Orange dashed contour lines (18 km interval) indicate the depth of plate interface.
Red star as of. (c) Slip distribution for the 2011 Tohoku-oki earthquake.
Red star and thin dashed black contours above the fault plane as of; (d) Vp/Vs ratio anomaly distribution on the subducting plate interface. Black contour lines (10 m interval) indicate the slip distribution of Tohoku earthquake. Red star as of. Maps are created using GMT software.