lies at the northern edge of the Beijing-Tianjin-Tangshan Plain, an alluvial plain that stretches from Beijing to the Sea of Bohai. This plain – the northeastern corner of the great North China Plain – is where sediments eroded from the Yanshan mountains to the north have filled in the ancient Sea of Bohai, with Tangshan near where the shore was about 4,000 years ago. To the south these sediments have formed a layer of weak soils as much as three kilometers thick. At Tangshan and northward these sediments are thinner where the underlying strata crops out to form isolated hills. This underlying strata is a thick (typically 10 km) layer of mainly sedimentary strata such as limestone and sandstone, with large deposits of coal. Tangshan is located particularly over a northeast oriented syncline, a fold in the sedimentary strata that has brought massive deposits of coal close enough to the surface to be mined. In this area the overlying alluvium varies in thickness from several meters to around . Underlying all this is the ancient bedrock of different kinds of metamorphic rock (such as schist, gneiss, quartzite, granulite, etc.) that form the Eastern Block of the North China Craton. This craton (see figure) was formed approximately two billion years ago by the collision of two major crustal blocks that left a belt of uplifted mountains – the Central (China) Orogenic Belt (COB) – that crosses China approximately southwest to northeast, passing just west and north of Beijing. Just north of Zunhua another orogenic belt, the east–west trending Yanshan mountain fault-fold belt (also known as the Yanshan seismic belt) marks the northern edge of the North China Craton (and of the alluvial plain). It is also the location of over half of the destructive earthquakes in Hebei province, as under the plain several fault zones (oriented parallel to the Central Orogenic Belt) terminate against the Yanshan mountains. Many of these faults are ancient, but have been reactivated by the force transmitted from the collision of the Indian Plate against the Eurasian Plate, making the Eastern Block unusually active seismically, accounting for six of the ten deadliest earthquakes in recorded history. The Tangshan fault that ruptured 28 July runs right under the center of Tangshan City. One of three faults in the Changdong fault zone, it runs approximately east-northeast (ENE) about 36 km to where it terminates against the north–south trending fault