NEOTECTONICS NEAR THE NW CORNER OF THE PACIFIC PLATE: MARINE TERRACES, EARTHQUAKES AND TSUNAMIS ON OZERNOI PENINSULA, KAMCHATKA, RUSSIA

The Ozernoi Peninsula, on the southern Bering Sea coast of Kamchatka, is tectonically enigmatic. The simplest tectonic configuration for this region incorporates three plates, Pacific, North America and Komandorsky Island Block (KIB). The Komandorsky Basin lies north of the KIB and is characterized by transtensional and extensional elements: high heat flow, magnetic lineaments, and transcurrent faults. The Ozernoi Peninsula lies on the eastern margin of the Komandorsky Basin, north of the active subduction zone. The western half of the peninsula principally comprises a dissected Quaternary volcano, Nachikinsky. Eastern Ozernoi is undergoing active uplift and in 1969 experienced a Mw 7.7-7.8 compressional earthquake. Possible sources for this compression and uplift include 1) shear distributed from the Pacific Plate, north of the KIB, 2) convergence generated from a spreading Komandorsky Basin, or 3) convergence generated by a rotating Bering Block.

On the NE Ozernoi Peninsula, one Holocene and two Pleistocene marine terraces are generally present, as well as remnants of at least two older terraces. Uplift rates range from 0.1 to 0.3 mm/yr, rates comparable to, if less than, rates on the Kamchatskiy Peninsula, a small collisional orogen at the terminus of the Aleutian chain. Moreover, Ozernoi exhibits seismic activity consistent with a compressional regime. The 22 November 1969 Ozernoi earthquake had an offshore epicenter at 57.8N, 163.6E and generated a tsunami of magnitude 7.7. Using a tide gage record and runup data from observations and tsunami deposits, we have calculated a co-seismic displacement of about 4 m, or about 3.5 m of shortening. From the local record of pre-historic tsunami deposits, the recurrence interval for 1969-like events is ~100-200 years. If we use 200 years, and a 3-m shortening as for 1969, the “Bering Block” would be moving at a rate of 15 mm/y toward Kamchatka, about half the rate of the KIB.

We cannot currently distinguish among the causal mechanisms noted above. Whereas the behavior of the Ozernoi Peninsula coast can hardly be called a subduction zone, it exhibits tectonic activity and Quaternary volcanism that have yet to be fully explained.