Pn first arrivals recorded by 832 seismic stations at a distance range of 180–1500 km from the epicenter. The tomography highlighted that "Pn velocities are found to be lowest in eastern Turkey (<7.6 km s-1) and highest in the eastern Mediterranean Sea and Zagros Suture (>8.3 km s-1). Large Pn anisotropy is observed in the Aegean, central Anatolia and along the southern coast of Anatolia. [...] Large crustal thicknesses are observed along the Dinarides-Hellenides and along the southern coast of Anatolia." Rocks Many of the rocks exposed in Turkey were formed long before this process began. Turkey contains outcrops of Precambrian rocks, (more than 540 million years old). During the Mesozoic era (about 250 to 66 million years ago) a large ocean (Tethys Ocean), floored by oceanic lithosphere existed in-between the supercontinents of Gondwana and Laurasia (which lay to the south and north respectively). This large oceanic plate was consumed at subduction zones. At the subduction trenches the sedimentary rock layers that were deposited within the prehistoric Tethys Ocean were folded, faulted and tectonically mixed with huge blocks of crystalline basement rocks of the oceanic lithosphere. These blocks form a very complex mixture or mélange of rocks that include mainly serpentinite, basalt, dolerite, and chert. The Eurasian margin, now preserved in the Pontides (the Pontic Mountains along the Black Sea coast), is thought to have been geologically similar to the Western Pacific region today. Volcanic arcs and back-arc basins formed and were emplaced onto Eurasia as ophiolites as they collided with microcontinents (literally relatively small plates of continental lithosphere). These microcontinents had been pulled away from the Gondwanan continent further south. Turkey is therefore made up from several different prehistorical microcontinents. During the Cenozoic (Tertiary about 66 to 1.6 million years) folding, faulting and uplifting, accompanied by volcanic activity and intrusion of igneous rocks was related to major continental collision between the larger Arabian and Eurasian plates. Earthquakes Turkey's most severe earthquake in the twentieth century occurred in Erzincan on the night of 1939-12-27; it devastated most of the city and caused an estimated 30,000 deaths. Earthquakes of moderate intensity often continue with sporadic aftershocks over periods of several days or even weeks. Seismicity in Turkey is more likely to happen in the North Anatolian Fault Zone, East Anatolian Fault Zone and in the subduction region of the Aegean Plate between the Anatolian plate. See also List of earthquakes in