On a possible origin of the anomalous effects that were observed during the May 24, 2013 earthquake in the Sea of Okhotsk

The deep-focus Sea of Okhotsk earthquake that occurred on May 24, 2013 (h = 630 km, M w = 8.3) was accompanied by anomalous effects that were unknown previously. A combined analysis of published data concerning the source rupture evolution and some features of the deep structure provided an explanation of some anomalous effects, such as the large number of aftershocks and the low level of ground shaking in the epicentral area. However, GPS observations revealed high coseismic vertical displacements in the area. The seafloor uplift in the Sea of Okhotsk and the adjacent coasts was 3–12 mm, peaking at the approximate center of the sea, while Kamchatka and the North Kuril Islands subsided by 3–18 mm, peaking at the Apacha station 190 km east of the earthquake epicenter. These maximum estimates are 1.2–1.8 times the analogous values (10 mm) for the Chile mega-earthquake of May 20, 1960 (M w ~ 9.5). It is known that the large distances at which ground shaking is felt during deep-focus earthquakes are due to the fact that the body waves travel through the high-Q lower mantle. However, this does not explain the paradox of the present earthquake in the Sea of Okhotsk, viz., a constant intensity of shaking (two grades) in the range of epicentral distances between 1300 and 9500 km. The explanation requires consideration of the earth’s free oscillations excited by the earthquake.