Fossil earthquakes preserved on fossils: New examples

The coastal plain of Miaoli is classified as part of the Outer Foothill of the Western Foothill in Taiwan. Around six million years ago, the initiation of the Penglai Orogeny set off significant tectonic activities in the Miaoli region, resulting in the formation of numerous faults and folds. These geological structures have played a concurrent role in influencing the occurrence and preservation of fossil echinoids in the area.

Unlike the commonly reported deformed fossils found in Mesozoic and older strata, this study documents new instances of deformed fossils from the Pleistocene strata in Taiwan. Through micro-CT tomographic imaging, the 3D geometry of fault planes in deformed sand dollars is revealed. Thin sections expose additional tectonic structures, including pressure solutions, box folds, and monoclines. Some deformations, both ductile and brittle, such as fractures along the taphonomically weak ossicle boundaries, may have originated from sedimentary processes. However, fault features triggered by earthquakes are unmistakably preserved in rare specimens.

Notably, a thorough analysis of earthquake epicenters in Miaoli since 1997 indicates that no earthquakes occurred in proximity to the fossil localities. Consequently, it is deduced that the deformed specimens are a result of ancient or fossilized earthquakes. This study presents novel and distinctive evidence contributing to the comprehension of neotectonics regarding the collision between the Philippine Sea Plate and Eurasia Plate in Taiwan, as inferred from fossils.