Total‐moment spectra of fourteen large earthquakes

Total‐moment spectra M T (ω) = ‖M(ω)‖/ , where M is the moment rate tensor, are computed for 14 large earthquakes recorded by the International Deployment of Accelerometers (IDA) network using the scalar‐moment retrieval method proposed by Silver and Jordan 1982. For each event we obtain estimates of M T averaged over the 10 disjunct, 1‐mHz intervals in the low frequency band 1–11 mHz; typical IDA record sets from events with M T ≳ 0.2 A (1 A = 10 27 dyne · cm) yield standard errors on the 1‐mHz averages that are generally less than 20%. Our multiple‐band estimates of M T are usually consistent with comparable single‐band values found by other investigators. From the total‐moment spectra we derive the zero‐frequency (static) moment ≡ M T (0) and the characteristic source duration τ c ≡ 2 (Var f ( t )) ½ , where f ( t ) is the time function of the moment rate tensor, assumed to be the same for all components. The parameter τ C formally depends on the second‐degree temporal, spatial, and mixed spatial‐temporal moments of an extended source, but calculations with realistic source geometries indicate that the interpretation of T C strictly in terms of the second temporal moment leads to very little error. A plot of τ c against shows considerable scatter; some events lie significantly above the empirical scaling curve of Kanamori and Given 1981 (‘slow earthquakes’) and some below (‘fast earthquakes’). Examples of the former include all three deep‐focus events analyzed here as well as the 1970 Colombia and 1963 Peru‐Bolivia deep‐focus earthquakes, whose total‐moment spectra are calculated from the moment tensor solutions of Gilbert and Dziewonski 1975. An example of the latter is the great Sumbawa earthquake of August 9, 1977 ( = 24±3 A), which is inferred to have a relatively shallow spatial centroid (< 20 km) and a nearly flat moment spectrum, suggesting that the faulting was confined to the upper, more brittle portion of the oceanic lithosphere. Three other large earthquakes—Tonga (June 22, 1977; = 23±2 A), Kuril Islands (December 6, 1978; = 3.6±0.2 A), and Tumaco (December 12, 1979; = 25±2 A)—exhibit moment spectra which significantly decrease toward higher frequencies, indicating larger values of τ c . The aftershocks of both the Tonga and Kuril Islands events are distributed over a considerable range of depths, consistent with rupture into the lower, more ductile portions of the oceanic lithosphere. We speculate that the characteristic source duration may be causally related to the depth of coseismic rupturing within the lithosphere.