Real-time magnitude estimation and rapid fault characterization with GPS data for Earthquake Early Warning applications

The combined use of seismic and geodetic observations is now a common practice for finite-fault modeling and seismic source parametrization. With the advent of high-rate 1Hz GPS stations the seismological community has begun to look at ways to include GPS data in Earthquake Early Warning (EEW) algorithms. GPS stations record ground displacement without any risk of saturating or need for baseline or other corrections. Thus, geodetic displacement timeseries complement the high-frequency information provided by seismic data. In the standard approaches to early warning, the initial portion of the P-wave signal is used to rapidly characterize the earthquake magnitude and to predict the expected ground shaking at a target site, before damaging waves arrive. Whether the final magnitude of an earthquake can be predicted while the rupture process is underway, still represents a controversial issue; the point is that the limitations of the standard approaches when applied to giant earthquakes have become evident after the experience of the Mw 9.0, 2011 Tohoku-Oki earthquake. Here we explore the application of GPS data to EEW and investigate whether the co-seismic ground deformation can be used to provide fast and reliable magnitude estimations. We implemented an algorithm to extract the permanent static offset from GPS displacement timeseries; the static displacement is then used to invert for the slip distribution on the fault plane, using a constant-slip, rectangular source embedded in a homogeneous half-space. We developed an efficient real-time static slip inversion scheme for both the rapid determination of the event size and for the near real-time estimation of the rupture area. This would allow for a correct evaluation of the expected ground shaking at the target sites, which represents, without doubt, the most important aspect of the practical implementation of an early warning system and the most relevant information to be provided to non-expert end-users. The strategy we propose is fairly robust and does not need any predefined geometry or restrictive prior assumptions; the starting model is a simple fault plane divided into a limited number of patches. The geometry and dimensions are determined as soon as the first magnitude estimation from near-field seismic stations is available. The methodology is expected to be suitable for any seismically active area and can be easily incorporated into a real-time Earthquake Early Warning System.