Spatiotemporal Complexity of Continental Intraplate Seismicity: Insights from Geodynamic Modeling and Implications for Seismic Hazard Estimation by Qingsong Li, Mian Liu, and Seth Stein Abstract Continental intraplate seismicity seems often episodic, clustered, and mi- grating. The observed seismicity shows both spatial clustering in seismic zones and scattering across large plate interiors, temporal clustering followed by long periods of quiescence, and migration of seismicity from one seismic zone to another. Here, we explore the complex spatiotemporal patterns of intraplate seismicity using a 3D visco- elasto-plastic finite-element model. The model simulates tectonic loading, crustal failure in earthquakes, and coseismic and postseismic stress evolution. For a later- ally homogeneous lithosphere with randomly prespecified perturbations of crustal strength, the model predicts various spatiotemporal patterns of seismicity at different timescales: spatial clustering in narrow belts and scattering across large regions over hundreds of years, connected seismic belts over thousands of years, and widely scat- tered seismicity over tens of thousands of years. The orientation of seismic belts coincides with the optimal failure directions associated with the assumed tectonic loading. Stress triggering and migration cause spatiotemporal clustering of earth- quakes. When weak zones are included in the model the predicted seismicity initiates within the weak zones but then extends far beyond them. If a fault zone is weakened following a large earthquake, repeated large earthquakes can occur on the same fault zone even in the absence of strong tectonic loading. These complex spatiotemporal patterns of intraplate seismicity predicted in this simple model suggest that assessment of earthquake hazard based on the limited historic record may be biased toward over- estimating the hazard in regions of recent large earthquakes and underestimating the hazard where seismicity has been low during the historic record. Introduction The distribution of earthquakes in space and time within continental interiors is far more complex than on major plate boundary faults. Earthquakes on plate boundaries result from strain produced by steady relative plate motions. Hence, plate boundaries remain the loci of large quasi-periodic earthquakes for long intervals until the plate boundary ge- ometry changes. As a result, the long-term pattern of seis- micity inferred from geological fault studies is generally consistent with that observed in large earthquakes in historic/ instrumental records (e.g., Marco et al., 1996; Weldon et al., 2004; Cisternas et al., 2005). Moreover, the direction and rate of geodetically observed strain accumulation is generally consistent with the mechanisms and recurrence of large earthquakes (Stein and Freymueller, 2002). The situation is quite different within continental plate interiors, where earthquakes appear to be clustered, episodic, and migrating in space (Fig. 1). Paleoseismic data show that intraplate earthquakes often occur in temporal clusters on faults that remain active for some time and then have long quiescent periods during which other fault zones are active (Crone et al., 2003; Camelbeeck et al., 2007). Thus, some faults without historically recorded earthquakes, such as the Meers fault in Oklahoma (Crone and Luza, 1990), appear to have had prehistoric large events (Clark and McCue, 2003). Besides individual faults, earthquakes in continental in- traplate regions also appear to occur in temporal clusters. For example, earthquakes in North China’ s Weihe-Shanxi graben are temporally clustered before 1700 (Fig. 2a). Similarly, large earthquakes (M> 6) in the North China Plain are clus- tered between 1900 and 2000 (Fig. 2b). Moreover, the locus of seismicity seems to be migrating from the Weihe-Shanxi graben eastward to the North China Plain in the past 200 yr (Fig. 1b). The spatial patterns of continental intraplate seismicity are also complex, with both spatial clustering of earthquakes in seismic zones and scattered activity over large regions 52 Bulletin of the Seismological Society of America, Vol. 99, No. 1, pp. 52–60, February 2009, doi: 10.1785/0120080005