Ground motions from subduction-zone earthquakes

A total of 258 horizontal components of accelerogram data recorded at soil sites during thrust, normal, and strike-slip earthquakes occurring in seven subduction zones around the Pacific Ocean were analyzed. The results of statistical analyses of 5 per cent damped pseudo-velocities (PSV), computed from these accelerograms at 10 periods ( T ) between 0.1 and 4 sec, indicated no significant differences in the average PSV levels of the thrust, normal, and strike-slip data from the Northern Honshu zone. Each of these groups of Northern Honshu data, which together comprised one-half of the total data base, were fit equally well by the same attenuation model. The analyses also revealed that at intermediate periods (0.8 ≦ T < 3 sec), the PSV observed at stiff-soil sites within the Northern Honshu, Nankai, Kuril, Mexico, and Alaska zones were, on the average, significantly greater than the PSV observed at similar sites in the Peru/Northern Chile and New Britain/Bougainville zones. This observation indicates that differences in the source and/or travel-path characteristics between the two groups accounted for the differences in PSV. Independent evidence supporting source differences is the correlation noted between PSV in this period band and the characteristic source complexities for these zones, which Hartzell and Heaton inferred from the teleseismic data of the larger magnitude earthquakes. Certain anomalous tectonic characteristics of the New Britain/Bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff-soil sites within this zone. Some of the differences in the PSV among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics. Geology greatly influenced the Mexican PSV data from soft-soil sites. To a lesser extent, geology probably affected the Alaskan data, which were recorded mostly on deep stiff soils, and the New Britain/Bougainville data recorded at Yonki, which is underlain by softer soil deposits.