Possible origin of memory in earthquakes: Real catalogs and an epidemic-type aftershock sequence model.

Earthquakes are one of the most devastating natural disasters that plague society. Skilled, reliable earthquake forecasting remains the ultimate goal for seismologists. Using the detrended fluctuation analysis (DFA) and conditional probability (CP) methods, we find that memory exists not only in interoccurrence seismic records but also in released energy as well as in the series of the number of events per unit time. Analysis of a standard epidemic-type aftershock sequences (ETAS) earthquake model indicates that the empirically observed earthquake memory can be reproduced only for a narrow range of the model's parameters. This finding therefore provides tight constraints on the model's parameters and can serve as a testbed for existing earthquake forecasting models. Furthermore, we show that by implementing DFA and CP results, the ETAS model can significantly improve the short-term forecasting rate for the real (Italian) earthquake catalog.

Non quasi-Hemispherical Seismological Pattern of the Earth's Uppermost Inner Core.

We assembled a database consisting of 5,404 PKIKP/PKiKP observations from 555 events, where PKIKP is the phase sampling the inner core (IC) and PKiKP is the phase reflected at the inner core boundary (ICB). Around 138° distances, their differential arrival times and amplitude ratio are mostly sensitive to the seismic velocity and attenuation structure in the uppermost IC (UIC), respectively. Our observations do not support a large-scale anisotropy in the UIC, but do not exclude its presence in some restricted areas. A robust inversion for the isotropic P-wave velocity perturbations shows a higher velocity cap with a radius of ~60°, approximately centered beneath the Northern Sumatra, with a local low velocity zone beneath the central Indian Ocean. The rest of the UIC, including the Northern part of Eurasia and of the Atlantic Ocean, exhibits mostly lower velocity. Amplitude ratio values of PKIKIP/PKiKP (observed vs. computed) from 548 high signal-to-noise (>5) recordings show a large variance, suggesting only a faint correlation between higher velocity and lower attenuation in the UIC. Our results provide better constraints to the models invoking a heat transfer in the UIC, with a complex temperature pattern near ICB.