Nonlinear Joint Inversion Research Articles

Global variations of temperature and water content in the mantle transition zone from higher mode surface waves

We performed a fully nonlinear joint inversion of surface wave overtone phase velocities for velocity structure down to a depth of 1500 km and topography of transition zone discontinuities using neural networks. The resulting models are compared with existing mantle models from linearized inversions techniques and the agreement between vertically averaged S-wave velocity models and various 660-km topography models is very good, whereas the agreement between all the 400-km topography models is very poor. The latter might be due to a broader than previously reported 400-km discontinuity where the minerals co-exist with water induced melt. From our models of transition zone thickness and S-wave velocity we estimated lateral variations of temperature and water content in Earth's mantle transition zone, linearly relating variations in S-wave velocity and transition zone thickness to variations in temperature and water content. Within the transition zone we observe relatively cold subduction zones as well as relatively warm regions beneath continents and hot spots. We find that the transition zone is wettest away from subduction zones.

Rupture process of the 2007 Niigata‐ken Chuetsu‐oki earthquake by non‐linear joint inversion of strong motion and GPS data

We image the rupture history of the 2007 Niigata‐ken Chuestu‐oki (Japan) earthquake by a nonlinear joint inversion of strong motion and GPS data, retrieving peak slip velocity, rupture time, rise time and slip direction. The inferred rupture model contains two asperities; a small patch near the nucleation and a larger one located 10 ÷ 15 km to the south‐west. The maximum slip ranges between 2.0 and 2.5 m and the total seismic moment is 1.6 × 1019 Nm. The inferred rupture history is characterized by rupture acceleration and directivity effects, which are stable features of the inverted models. These features as well as the source‐to‐receiver geometry are discussed to interpret the high peak ground motions observed (PGA is 1200 gals) at the Kashiwazaki‐Kariwa nuclear power plant (KKNPP), situated on the hanging‐wall of the causative fault. Despite the evident source effects, predicted PGV underestimates the observed values at KKNPP by nearly a factor of 10.

Source parameters of the 1908 Messina Straits, Italy, earthquake from geodetic and seismic data

We carry out a nonlinear joint inversion of P wave first‐motion polarities and coseismic surface displacement data of the 1908 Messina earthquake. We model the earthquake using a single planar fault: Slip is at first assumed to be uniform across the whole fault, then independent in a small set of coplanar subfaults, and finally smoothly variable across the fault. The first two steps are accomplished using a global minimization technique. The main features of the retrieved model are very robust and independent of the seismic velocity profile, of the presence of questionable bench marks, and of the assumed residual distribution. The along‐strike component of slip is about half the along‐dip component, in agreement with the direction of the extensional stress axis retrieved from geological observations. Surface vertical displacement is consistent with tsunami data and with the morphology of the Messina Straits.

2-D nonlinear joint inversion of VLF and VLF-R data using simulated annealing

Two-dimensional (2-D) individual and joint inversions of very low frequency (VLF) data (the real and imaginary anomalies), and jointly with VLF-R data (the apparent resistivity and phase) using very fast simulated annealing (VFSA) as a tool in global inversion are considered in this paper. Synthetic responses from two different models, which represent typical subsurface structures in the shield areas, were studied. Various models achieved after 10 VFSA runs were used to compute the mean model and the corresponding covariance and correlation matrices which were then used to estimate the uncertainties in the mean model parameters and correlations between the model parameters. An individual inversion of either real or imaginary anomalies cannot yield a model that would fit both observations. However, the joint inversion of the real and imaginary data sets will yield a model, which fits both observations. The models obtained either by the individual or by joint inversion of VLF data do not show good agreement with the VLF-R observations. Generally, the models obtained from VLF inversions show better agreement with the apparent resistivity than with the phase data. Conversely, the joint inversion of the apparent resistivity and phase data yields models which give good agreement with the real and imaginary anomalies. The results obtained after the joint inversion of all the VLF and VLF-R data give good agreement with all the data sets. Individual inversion of any response cannot yield a reliable subsurface structure no matter how well a particular observed response is fitted by the computed responses. Field data from three profiles traversing different geological structures were also inverted to demonstrate the efficacy of the approach for delineating the geometry and properties of 2-D structures. The VLF-R inversions invariably give superior results in comparison with the VLF inversions. However, the most reliable models are obtained from the joint inversion of both VLF and VLF-R data sets.