Apparent Clustering and Apparent Background Earthquakes Biased by Undetected Seismicity

In models of triggered seismicity and in their inversion with empirical data, the detection threshold md is commonly equated to the magnitude m0 of the smallest triggering earthquake. This unjustified assumption neglects the possibility of shocks below the detection threshold triggering observable events. We introduce a formalism that distinguishes between the detection threshold md and the minimum triggering earthquake m0 ≤ md. By considering the branching structure of one complete cascade of triggered events, we derive the apparent branching ratio na (which is the apparent fraction of aftershocks in a given catalog) and the apparent background source Sa that are observed when only the structure above the detection threshold md is known due to the presence of smaller undetected events that are capable of triggering larger events. If earthquake triggering is controlled in large part by the smallest magnitudes as several recent analyses have shown, this implies that previous estimates of the clustering parameters may significantly underestimate the true values: for instance, an observed fraction of 55% of aftershocks is renormalized into a true value of 75% of triggered events.