The Asperity Model and the Nature of Large Subduction Zone Earthquakes

Abstract

Regional variations in the rupture characteristics of large shallow earthquakes in circum-Pacific subduction zones are interpreted in the context of the asperity model of heterogeneous stress distribution on the fault plane. It is assumed that the degree of seismic coupling between the downgoing and overriding plates is reflected in the maximum earthquake rupture dimensions in each region, and that gross features of the regional stress distribution can be inferred from the rupture process of large earthquakes. The results of numerous studies of the historic record and detailed source process of large subduction zone events are summarized for each region. The systematic variation in maximum rupture extent in different zones indicates that four fundamental categories of behavior are observed. These are (1) the Chile-type regular occurrence of great ruptures spanning more than 500 km; (2) the Aleutian-type variation in rupture extent with occasional ruptures reaching 500 km in length, and temporal clustering of large events; (3) the Kurile-type repeated failure over a limited zone of 100-300 km in length in isolated events; and (4) the Marianas-type absence of large ruptures. The rupture processes associated with each category have distinctive features. The great earthquakes of category (1), such as the 1964 Alaskan event, are often preceded by a prolonged period of increased seismicity, and the body wave source process is characterized by a long duration (~ 120 sec) time function. The earthquakes in category (2) are usually preceded by seismic quiescence and may occur as doublets or multiplets. The body wave source time functions of these events tend to consist of several long duration (30 to 60 sec) discrete ruptures. Category (3) events commonly have a precursory quiescence followed by extensive foreshock activity, and the body waves are complicated since they result from a sequence of short duration (< 30 sec) ruptures. Interpreting these features with the asperity model indicates that the stress distribution in categories 1,2,3, and 4 are characterized by very large asperities and strong coupling, large but discrete asperities, numerous smaller asperities, and an absence of significant asperities, and large component of aseismic slip, respectively.