## 1 Introduction  
A typical aspect of the observed seismicity in the northern and central Apennines, and in the whole Italian region more generally, is the occurrence of earthquake sequences characterized by multiple, similarly large mainshocks. An example of this behavior is the quantitative model \"Every Earthquake Precursory According to Scale\" (EEPAS), applied by [PERSON] and [PERSON] [1, 2, 3]. According to their quantitative definition, introduced by [PERSON] and [PERSON] [4], swarms are seismic sequences constituted by at least three earthquakes whose magnitudes are linked to each other by empirical rules.  
In this study we define as a multiplet a set of two or more earthquakes, with the following conditions: (a) the first event has a magnitude equal to or larger than a given threshold; (b) the others occur within a time difference and distance defined by the [PERSON] and [PERSON] [5] criterion from each other; and (c) within a given magnitude range. This definition is different from that usually applied for common seismicity patterns such as foreshock-aftershock sequences and clusters (e.g., [PERSON] and [PERSON] [6]).  
Building upon a previous paper ([PERSON] et al. [7]), in which we examined the aspect of multiple mainshocks in central Italy, in this study we aim at verifying if a synthetic catalog reproduces this kind of earthquake clustering. For this purpose, we apply a new version of the simulator algorithm, in which the role of stress transfer among elementsof an expanding rupture is enhanced. Moreover, we give also examples of other space-time seismic features exhibited by synthetic catalogs, both in short- (days-months) and long-term (years-centuries), some of which were observed in real earthquake catalogs.  
In Section 2 we present a brief description of the algorithm used for detecting multiple events in an earthquake catalog, based on the previously cited (a), (b), and (c) criteria. This algorithm is applied for providing a possible metric for comparing real observations with simulations.  
Section 3 gives an outline of the seismotectonic model of our study area of northern and central Apennines, along with examples of recent and historical sequences of multiple mainshocks observed in this region.  
In Section 4, after a short introduction of the new version of the simulator employed in this study, we show the results obtained applying this simulation code to the above mentioned seismotectonic model of the study area. Having tried three choices for the two main free parameters present in the algorithm, for a total of nine different combinations, we chose one of them by a criterion based on the analysis of the multiplets in the synthetic catalog of 100,000 years. Some features of this preferred simulated catalog are then compared in several ways with a real set of observations lasting only 370 years in the same seismogenic area.  
Section 5 reports other results of spatio-temporal analysis of the same 100,000 years simulated catalog that appear to be consistent in reasonable way with real seismicity patterns not strictly related to our study area. In particular, we show that the use of simulators allows testing hypotheses of seismogenic models in a way that is not possible on the basis of real observations, due to lack of completeness and homogeneity of these observations in the long-term.