Here we analyze the rupture process of the December 29 th , 2020 MW6.4 Petrinja earthquake (Croatia), the largest event recorded in this area characterized by a moderate strain-rate intraplate setting. We use foreshocks and aftershocks, recorded at more than 80 broadband stations located 70km to 420km from the earthquake, as empirical Green's functions (EGFs) to separate source effects from propagation and local site effects. First, we deconvolve the mainshock P-wave time windows from the EGFs in the frequency domain to obtain the corner frequency (fc). Spectral analysis based on a Brune's source model reveals a large stress drop of 24 MPa. Next, by deconvolving the Love waves in the time domain, we calculate the Apparent Source Time Functions (ASTFs). We find that the average duration of the source is ~5 s, with no significant directivity effects, indicating a bilateral rupture. To extract physical rupture parameters such as rupture velocity, slip distribution and rise time, we deploy two techniques: (1) Bayesian inversion and (2) backprojection onto isochrones of ASTFs. Both techniques show a low rupture velocity (40-50% of the shear wave velocity) and a rupture length of less than 10 km, i.e. much less than would typically be expected for a magnitude 6.4 earthquake. This apparent anticorrelation between stress drop and rupture velocity may be attributed to the complex and segmented fault system characteristic of immature intraplate settings.