Full waveform inversion (FWI) based methods are getting more attractive in passive seismic (microseismic) monitoring studies. A high resolution knowledge of the medium in which the events occur is crucial to proper monitoring. In most cases, not only the velocity, but also the anisotropy has a large influence on locating passive events related to hydraulic fracturing. Due to the inherent anisotropy nature of most rocks associated with unconventional reservoirs, accounting for anisotropy is even more important in such investigations. We propose a 3D acoustic orthorhombic FWI method for passive seismic events to invert for the source image, source function and the model parameters, without any a prior knowledge about source location or source function in time. In order to mitigate the effect of the unknown source ignition time, we convolve reference traces with the observed and modeled data. A total variation regularization is applied to improve the robustness of the model parameters inversion considering the limited sources and illumination angles of microseismic experiments. The adjoint-state method is used to derive the gradient for the source image, source function and the anisotropic model parameters. The proposed method produces good estimates of the source location and the model structures for the modified orthorhombic 3D SEG/EAGE overthrust model.