Earthquake clustering (grouping in space and time) is a widely observed mode of strain release in the upper crust, although this behavior on individual faults is a departure from classic elastic rebound theory. In this study, we consider factors responsible for a cluster of earthquakes on the Bear River fault zone (BRF), a recently activated, 44-km-long normal fault on the eastern margin of Basin and Range extension in the Rocky Mountains. The entire surface-rupturing history of the BRF, as gleaned from paleoseismic and geomorphic observations, began only 4500 years ago and consists of at least three large events. Rupture of the BRF is spatially complex and is clearly conditioned by preexisting structure. In particular, where the south end of the fault intersects older thrust faults and upturned strata along the south-dipping flank of the Precambrian basement-cored Uinta arch, the main trace ends abruptly in a set of orthogonal splays that accommodate down-dropping of a large hanging-wall graben against the arch. We hypothesize that the geomechanically strong Uinta arch crustal block impeded the development of the BRF and, over time, enabled a significant accumulation of elastic strain energy, eventually giving rise to a pulse of strain release in the mid- to late Holocene. We surmise that variations in fault strength, both in space and time, is a cause of earthquake clustering on the BRF and on other faults that are structurally and tectonically immature. The first two earthquakes on the BRF occurred during the same period of time as a regional cluster of earthquakes in the Middle Rocky Mountains, suggesting that isolated faults in this slowly extending region interact through widespread changes in stress conditions.

地震聚类（在空间和时间上分组）是上地壳应变释放的一种广泛观察到的模式，尽管这种对单个断层的行为与经典的弹性回弹理论背道而驰。在这项研究中，我们考虑了引起熊河断裂带（BRF）上的一系列地震的因素，该断裂带是盆地东部边缘最近活动的，长达44公里的正断层，以及落基山脉的山脉延伸。从古地震和地貌观测中收集到的BRF的整个表面破裂历史仅在4500年前就开始了，并且至少由三个大事件组成。BRF的破裂在空间上是复杂的，并且显然受到预先存在的结构的限制。尤其是，断层南端与前冲断层相交，并沿前寒武纪基底带芯的Uinta拱南倾侧翼相交，上层地层向上倾斜，主要迹线突然终止于一组正交八角形，可容纳大型垂悬物的下垂。墙抓住拱门。我们假设地质力学上很强的Uinta拱形地壳块阻碍了BRF的发展，并且随着时间的推移，使得弹性应变能得以大量积累，最终在全新世中期至晚期引起了应变释放的脉冲。我们推测，断层强度在空间和时间上的变化是造成BRF以及在结构和构造上不成熟的其他断层上发生地震聚集的原因。