Structure-controlled asperity on the generation of large earthquakes

Abstract

The occurrence of large earthquakes is not only related to the fault geometry and state of stress accumulation, but also closely related to the structure of surrounding rocks. With the continuous enrichment of observation methods and development of techniques in recent decades, various studies have been conducted to explore the fine structure of large earthquake source regions and link the observations to the mechanism of large earthquakes. Seismologists generally use the asperity model to explain the generation mechanism of large earthquakes, that is, the process from locking to sudden slip of the seismogenic faults. The detailed geometrical shape of the fault planes and accurate state of stress accumulation are usually tough to depict during the process, while the structure features of the surrounding rocks are available through seismic tomography and magnetotelluric studies. In this paper, we aim to systematically summarize the results of structure studies in the focal areas of large earthquakes, and discuss the relationship between structure features and asperities. We therefore made statistics on the studies of asperity in the source regions of worldwide large earthquakes with magnitude larger than 6.0. There are totally 123 events with different focal mechanism, among which 54 events are intraplate earthquakes, while the other 69 events are interplate earthquakes. These large earthquakes are well studied by structure and/or source rupture process researches. Structure studies carried on 17 intraplate and 14 interplate earthquakes suggest the presence of high strength (tomographic high-velocity and/or magnetotelluric high-resistance) bodies surrounding the seismogenic faults. These anomalous bodies are considered to be the asperities, which play an important role in the generation of large earthquakes. Among the 31 large earthquakes, there are 4 events that took place on subducted seamounts, which are characterized as patches of anomalous structure features as well as elevated effective normal stress and are asperities with high strength in the evolution of these large earthquakes. The structure studies provide direct evidence for the important role of asperity in the generation of large earthquakes. The indirect evidences come from the source rupture studies of 92 large earthquakes, which show large coseismic slip zones near the seismogenic faults, manifesting the existence of asperities with high strength. The predominance of unilateral rupture in earthquakes has also been revealed, which has a close relationship with the structural heterogeneity. Through statistical analysis of asperity mechanism studies on large earthquakes, we find the asperity characterized by high velocity and resistivity plays an important role in the generation of large earthquakes. The asperities are strong in mechanical strength and could accumulate tectonic stress more easily during the long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between asperity and high-velocity and high-resistivity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies, which will provide important clues for earthquake prevention and disaster reduction. Furthermore, the asperity model is irrelevant to focal mechanism and plays a same important role in thrusting, normal and strike-slip faults.

Keywords

• structure
• asperity
• large earthquake
• seismogenic system