Numerical modeling of crustal block-and-fault dynamics, earthquakes and slip rates in the Tibet-Himalayan region

The Tibetan plateau and Himalayans have resulted from the continuous Indian and Eurasian plate convergence following their initial collision about 55 million years ago. Earthquakes in the region occur mainly in response to the crustal motion and stress localization associated with this convergence. To understand the basic features of the crustal motion and seismicity in the Tibet-Himalayan region, we develop a numerical model of block-and-fault dynamics. The model structure is composed of six major upper crustal blocks separated by fault planes. These blocks are assumed to be perfectly rigid and move as a consequence of the Indian plate push and of a flow of the lower crust. Deformations take place along the fault planes separating the blocks. The interaction of the blocks along the fault planes is visco-elastic as long as the ratio of the shear stress to the difference between the pore pressure and normal stress remains below a critical strength level. When the critical level is exceeded in some part of a fault plane, an earthquake (stress-drop) occurs causing also failures in adjacent parts of the fault plane. The stress-drop-affected parts of the fault plane enter in a state of creep immediately after the earthquake, and the creep lasts until the stress falls below a certain level. We develop several sets of numerical experiments to analyze the earthquake clustering, frequency-to-magnitude relationships, earthquake focal mechanisms, and fault slip rates in the model. Large events in the numerical experiments cluster on the fault segments associated with the Himalayan Frontal Thrust as well as at some internal faults of the Tibetan plateau. The clustering of earthquakes on a given fault is a consequence of the dynamics of the regional fault system rather than that of the fault only. We show that variations in the relationship of magnitude to frequency of the events are associated with changes in the motion of the upper crustal blocks and depend on the rheological properties of fault plane zones. The focal mechanisms of model events are found to be consistent with that of earthquakes in the region. The synthetic moment rate captures the observed rate of seismic moment release. We demonstrate in the model that the present crustal motion in the region is indeed governed by the north-northeastern movement of India toward Eurasia and the movement of the lower crust. Variations in the rheological properties of fault plane zones and/or in the motion of the lower crust influence rates of the crustal block displacements and slips at the faults separating the blocks. This can explain the discrepancies in estimates of slip rates over short and long time scales at major faults in the region.