An Iterative Inversion Technique to Compute Structural Martian Models for Refining Event Locations

The InSight mission will deploy a single seismic station on Mars in 2018. The main task of the MarsQuake Service within the project includes detecting and locating quakes on Mars, and managing the event catalog. In preparation for the mission, we continually calibrate single station event location algorithms, employing seismic phase travel times computed for a suite of structural models. However, our knowledge about the interior structure of Mars is limited, which in turn will affect our ability to locate events accurately. Here, we present an iterative method to invert for the interior structure of Mars and revise event locations, consecutively. We first locate seismic events using differential arrival times (with respect to the first phase arrival) of all possible seismic phases, computed for a priori initial structural models. These models are built considering a one-dimensional average crust and current estimates of bulk mantle chemistry and areotherm. Phase picks and uncertainty assignments are done manually. Then, we invert for the interior structure employing the arrival times for the picked phases, and generate an updated suite of models, which are further used to revise the initial phase picks, and relocate events. We repeat this sequence for each additional and new entry in the travel time database to improve event locations and models for average Martian structure. In order to test our approach, we simulate the operational conditions we will encounter in practice: We compute synthetic waveforms for a realistic event catalog of 120 events, with magnitudes between 2.5 and 5.0 and double-couple source mechanisms only. 1-Hz seismograms are computed using AxiSEM and Instaseis, employing two Martian models with a thin (30 km) and thick (80 km) crust, both with and without seismic surface noise. The waveforms are hosted at the ETH servers, and are publicly accessible via FDSN web services.