Sedimentary features of seismoturbidites triggered by the 1983 and older historical earthquakes in the eastern margin of the Japan Sea
Abstract
Both mass failure deposits and turbidites are widely distributed in the epicentral area of the 1983 earthquake (Mw 7.9) in the eastern margin of the Japan Sea. This basin is semi-enclosed with no active conduits for the entrance of coarse detrital sediments. Seismic reflection surveys and sediment cores reveal that mass-failure deposits, which are distributed on the continental slopes, merge into turbidites in the basin plain. Hence, the mass wasting process is a major transportation process in this region. One gravity core, GH93-816, was recovered from the southeastern margin of the Japan Basin, close to the epicentral area of the 1983 Japan Sea earthquake. Sedimentological results show that five turbidite beds are intercalated in the top 68 cm of the column. Occurrence of the B-Tm tephra at the depth of 68-71 cm indicates that these turbidite beds were developed during the last 1000 years. The high 137Cs concentration in the top 20 cm of the core implies that the uppermost turbidite bed was deposited after 1954. Based on these age limits for the top and the bottom of the studied interval, we estimate the recurrence intervals of turbidites as 160-330 years. We suggest that these turbidites were generated by submarine slides which were triggered by the 1983 and older historical earthquakes in this region. These seismoturbidites are characterized by amalgamated beds, irregular structure sequences, grain-size breaks/fluctuations, abrupt changes in composition within bed, and variable composition among beds. From these features, we infer that deposition resulted from a series of turbidity currents associated with multiple or line sources, for example multiple shallow slides that were regionally initiated by the same earthquake. The sedimentary features described herein may be useful as criteria for identifying modern seismoturbidites in other settings as well as seismoturbidites in the geologic record. However, further study is needed for the distinction of this type of turbidites from other types (e.g. tempestites, reflected turbidites).
- Publication:
-
Sedimentary Geology
- Pub Date:
- September 2000
- DOI:
- 10.1016/S0037-0738(00)00059-2
- Bibcode:
- 2000SedG..135....1N
- Keywords:
-
- Seismoturbidites;
- eastern margin of the Japan Sea;
- 1983 Japan Sea earthquake;
- sedimentary features;
- amalgamation;
- line source