Dynamic nucleation process of shallow earthquake faulting in a fault zone

Two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes: low-frequency and low-amplitude waves following the onset of a P wave ( P ₁) are interrupted by the arrival of the second impulsive phase P₂ enriched with high-frequency components. This observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation.
We propose a theoretical model that can naturally explain the above nucleation behaviour. The model is 2-D and the deformation is assumed to be anti-plane. A key clement in our model is the assumption of a zone in which numbers of pre-existing cracks are densely distributed; this cracked zone is a model for the fault zone. Dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions, which exert two conflicting effects: one tends to accelerate the crack growth, and the other tends to decelerate it. The accelerating and decelerating effects are generally ascribable to coplanar and non-coplanar crack interactions, respectively. We rigorously treat the multiple interactions among the cracks, using the boundary integral equation method (BIEM), and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation.
Our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non-coplanar interactions. This process radiates the P₁ phase. If the crack continues to grow, coalescence with adjacent coplanar cracks occurs after a short time. Then, coplanar interactions suddenly begin to prevail and crack growth is accelerated; the P₂ phase is emitted in this process. It is interpreted that the two distinct phases appear in the process of the transition from non-coplanar to coplanar interaction predominance.     

Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering

10 M ≥ 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939–1992, providing an unsurpassed opportunity to study how one large shock sets up the next. We use the mapped surface slip and fault geometry to infer the transfer of stress throughout the sequence. Calculations of the change in Coulomb failure stress reveal that nine out of 10 ruptures were brought closer to failure by the preceding shocks, typically by 1–10 bar, equivalent to 3–30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake-nucleation constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the stress changes dominate during the mean 10 yr period between triggering and subsequent rupturing shocks in the Anatolia sequence. The stress changes result in an average three-fold gain in the net earthquake probability during the decade after each event. Stress is calculated to be high today at several isolated sites along the fault. During the next 30 years, we estimate a 15 per cent probability of a M ≥ 6.7 earthquake east of the major eastern centre of Ercinzan, and a 12 per cent probability for a large event south of the major western port city of Izmit. Such stress-based probability calculations may thus be useful to assess and update earthquake hazards elsewhere.     

Excitation of the normal modes of a rotating earth model by an earthquake fault

Summary. The motion excited in a rotating earth model by a kinematically prescribed earthquake fault is solved for in closed form. In addition, expressions for the total energy released and the energy dissipated by bodily friction subsequent to faulting are obtained in terms of the normal-mode excitation amplitudes.     

The 2003 Mw 7.2 Fiordland subduction earthquake, New Zealand: aftershock distribution, main shock fault plane and static stress changes on the overlying Alpine Fault

The 2003 August 21 Fiordland earthquake ( M _L7.0, M _W7.2) was the largest earthquake to occur in New Zealand for 35 yr and the fifth of M6+ associated with shallow subduction in Fiordland in the last 15 yr. The aftershocks are diffuse and do not distinguish between the two possible main shock fault planes implied by the Harvard CMT solution, one corresponding to subduction interface thrusting and the other corresponding to steeply seaward dipping thrusting. The distinction is important for calculating the induced stress changes on the overlying Alpine Fault which has a history of very large earthquakes, the last possibly in 1717. We have relocated the aftershocks, using data from temporary seismographs in the epicentral region and the double difference technique. We then use the correlation between aftershock hypocentres and regions of positive changes in Coulomb Failure Stress (CFS) due to various candidate main shock fault planes to argue for concentrated slip on the shallow landward dipping subduction interface. Average changes in CFS on the offshore segments of the Alpine Fault are then negative, retarding any future large events. In our models the change in CFS is evaluated on faults of optimal orientation in the regional stress field as determined by inversion of P -wave polarities.