Firearm violence rates have increased in U.S. cities in 2020 and into 2021. We investigate contagious and non-contagious space-time clustering in shooting events in four U.S. cities (Chicago, Los Angeles, New York and Philadelphia) from 2016 to 2020. We estimate the dynamic reproduction number (Rt) of shootings, a measure of contagion, using a Hawkes point process. We also measure concentration over time using a spatial Gini index. We find that the contagious spread of violence increased in 2020 in several, but not all, of the cities we considered. In all four cities, non-contagious (Poisson) events comprised the majority of shootings across time (including 2020). We also find that the spatial location and concentration of shooting hot spots remained stable across all years. We discuss the implications of our findings and directions for future research.

2020 年和 2021 年美国城市的枪支暴力发生率有所增加。我们调查了 2016 年至 2020 年美国四个城市（芝加哥、洛杉矶、纽约和费城）枪击事件中传染性和非传染性的时空聚集情况。我们估计动态再生数（Rt) 的枪击事件，一种传染性度量，使用霍克斯点过程。我们还使用空间基尼指数测量一段时间内的浓度。我们发现，2020 年，在我们考虑的几个（但不是全部）城市中，暴力的传染性传播有所增加。在所有四个城市中，非传染性（泊松）事件构成了整个时间（包括 2020 年）的大部分枪击事件。我们还发现，拍摄热点的空间位置和集中度在所有年份都保持稳定。我们讨论了我们的发现和未来研究方向的影响。