Abstract
Seismic attenuation properties were tested as indicators of lateral variation in geological structures and detection of faults within poorly reflective oceanic crust, on a seismic survey line along the Nankai Trough. We can specify both sedimentary structures by configuration of reflections and faults by offsetting of reflections on seismic reflection profiles. This procedure is often applied to analyze geological structures and existence of faults within sedimentary layers; however, it is almost impossible to analyze them within igneous oceanic crust because seismic reflections are inherently invisible there. Therefore, we applied seismic attenuation profiling to visualize geological structures and faults within poorly reflective oceanic crust. As a result, oceanic crust altered by late-coming volcanisms as well as damaged by intraplate earthquakes was imaged as extremely high-attenuation property, which was clearly distinguished from normal oceanic crust. Many faults were observed in the sedimentary unit on the seismic reflection profile, whereas possible lower segments of the faults were imaged as high-attenuation stripes in the oceanic crust on the seismic attenuation profile. Thus, the effectiveness of seismic attenuation profiling to structural and fault imaging within poorly reflective oceanic crust was successfully demonstrated.
Highlights
The Nankai Trough (NT) is located at the northern margin of the Philippine Sea Plate (PSP) subducting below the Eurasian Plate (EP) (Fig. 1), where large earthquakes with Mw > 8 have occurred with a recurrence interval of 100–200 years (Ando 1975)
This paper presents the results of visualization of lateral variation in geological structure and fault, constructed using the seismic attenuation profiling (SAP) method, within poorly reflective oceanic crust of the seaward slope of the NT
Are the results: The oceanic crust altered by volcanism as well as damaged by intraplate earthquakes was visualized as the extremely high-attenuation zone, being distinguished from the normal oceanic crust (HAZ and LAZ)
Summary
The Nankai Trough (NT) is located at the northern margin of the Philippine Sea Plate (PSP) subducting below the Eurasian Plate (EP) (Fig. 1), where large earthquakes with Mw > 8 have occurred with a recurrence interval of 100–200 years (Ando 1975). This paper presents the results of visualization of lateral variation in geological structure and fault, constructed using the SAP method, within poorly reflective oceanic crust of the seaward slope of the NT. As input data of SAP calculation, we used a post-stack time migration record in the present study, because post-stack data have an advantage in S/N ratio because random noise and remaining multiples are strongly suppressed by stacking effect It has a disadvantage in preservation of original frequency contents because NMO correction distorts frequency of reflection. We used post-stack seismic data for Q computation by spectral ratio method and discuss lateral variation in attenuation property in the igneous oceanic crust. We defined the average Q values having negative percentage perturbation (yellow to red) as ‘high-attenuation’ and those having positive percentage perturbation (yellowish green to blue) as ‘low-attenuation.’
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