DOI: 10.21455/GPB2017.4-8

RING-SHAPED SEISMICITY STRUCTURES
IN THE CASCADIA SUBDUCTION ZONE:
POSSIBLE NUCLEATION OF LARGE EARTHQUAKES

Yu.F. Kopnichev1,  I.N. Sokolova2

1 Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia

2 Institute of Geophysical Research, Ministry of Energy of Republic of Kazakhstan, Almaty, Kazakhstan

Abstract. We have been considering seismicity characteristics in the Cascadia subduction zone, located in the west of the North America. It was established, that since 1964 shallow (h = 0–33 km) ring-shaped seismicity structures with threshold magnitudes Mt of 4.4 and 4.6 respectively have been formed prior to two large earthquakes (17.08.1991, Mw = 7.1 and 25.04.1992, Mw = 7.2). Note that deep ring-shaped structures (h = 34–70 km) were not observed here, in contrast to many other subduction zones. At the same time deep seismicity strips are formed in some cases in this zone. We have been identified two new ring-shaped structures with values Mt of 5.0 and 5.2, which began to form since 1973. It is supposed that large earthquakes can be nucleated in these areas. On the basis of correlation dependences of ring sizes and Mt values on magnitudes of major events, obtained earlier for the east of Pacific, we estimated magnitudes for possible large earthquakes: Mw = 7.8 and 7.9. It follows from this data that the giant earthquake with Mw ~ 9, analog of the 1700 event is not being nucleated now in the subduction zone. At the same time a realization of large earthquake (Mw = 7.9±0.1) is most probable in the area of Vancouver island during the nearest years. It is supposed that the formation of the shallow seismicity rings and deep strips is connected with deep-seated fluid migration, similarly to other regions.

Keywords: lithosphere, ring-shaped seismicity structures, large earthquakes, deep-seated fluids.

KOPNICHEV Yuri F. – dr. of ph. & math., professor, chief research scientist, Schmidt Institute of Physics of the Earth, Russian Academy of Sciences. Moscow, Russia. Tel.: +7 (7272) 956-350. E-mail: yufk777@mail.ru

SOKOLOVA Inna N. – dr. of ph. & math., senior scientist, Institute of Geophysical Research, Ministry of Energy of Republic of Kazakhstan. Almaty, Kazakhstan. Tel.: +7 (7272) 631-330.
E-mail: sokolova.inessa@mail.ru