Slip, Differentiate, Observe: State and Parameter Estimation for Rate and State Friction From Noisy Data

ABSTRACT Quantifying frictional properties of interfaces remains a major challenge in both terrestrial and extraterrestrial geomechanics, where available samples, laboratory apparatuses, and geophysical observations are inherently limited. We introduce an analytic and numerical framework, grounded in nonlinear control theory, to infer the emergent frictional behavior of seismic faults. From noisy slip measurements, we first reconstruct the slip rate and frictional response in finite time using a Robust Exact Filtering Differentiator (REFD) that attenuates measurement noise. Building on these reconstructions, we design an exponentially convergent adaptive‐gain observer that estimates the internal state variable and the key parameters () and of the rate‐and‐state friction (RSF) law, widely used in fault mechanics. Numerical experiments show that, in fast‐slip regimes where data are sufficiently rich, the method recovers RSF parameters with errors on the order of 20 % and accurately tracks the RSF state variable despite noise contamination, whereas slowly varying sliding periods lack the observability required for reliable estimation. We also establish observability and identifiability conditions for the extended system, enabling the determination of additional parameters and outlining pathways for more advanced control‐theoretic approaches to friction and state identification in fault systems. Although we apply the approach to a reduced spring–slider analog, it improves on classical RSF calibration methods that depend on laboratory access to shear and normal stress. It also offers convergence guarantees and explicit error bounds, and it can further support model‐based inversions that embed RSF in forward simulations.