In this paper, we prove the existence of a new type of relaxation oscillation occurring in a one-block Burridge-Knopoﬀ model with Ruina rateand-state friction law. In the relevant parameter regime, the system is a slowfast ordinary diﬀerential equation with two slow variables and one fast. The oscillation is special for several reasons: Firstly, its singular limit is unbounded, the amplitude of the cycle growing like log ε−1 as ε → 0. As this estimate reﬂects, the unboundedness of the cycle – for this non-polynomial system – cannot be captured by a simple -dependent scaling of the variables, in contrast to e.g. . We therefore obtain its limit on the Poincar´e sphere. Here we ﬁnd that the singular limit consists of a slow part on an attracting critical manifold, and a fast part on the equator (i.e. at∞) of the Poincar´e sphere, which includes motion along a center manifold. The reduced ﬂow on this center manifold runs out along the manifold’s boundary, in a special way, leading to a complex return to the slow manifold. We prove the existence of the limit cycle by showing that a return map is a contraction. The main technical diﬃculty lies in the fact that the critical manifold loses hyperbolicity at an exponential rate at inﬁnity. We therefore use the method in , applying the standard blowup technique in an extended phase space. In this way, we identify a singular cycle, consisting of 12 pieces, all with desirable hyperbolicity properties, that enables the perturbation into an actual limit cycle for 0 < ε << 1. The result proves a conjecture in . The reference  also includes a preliminary analysis based on the approach in  but several details were missing. We provide all the details in the present manuscript and lay out the geometry of the problem, detailing all of the many blowup steps.