Increased earthquake rate prior to mainshocks

According to the Omori-Utsu law, the rate of aftershocks after a mainshock decays as a power law with an exponent close to 1. This well-established law was intensively used in the past to study and model the statistical properties of earthquakes. Moreover, according to the so-called inverse Omori law, the rate of earthquakes should also increase prior to a mainshock -- this law has received much less attention due to its large uncertainty. Here, we mainly study the inverse Omori law based on a highly detailed Southern California earthquake catalog, which is complete for magnitudes larger than M>0.3. First, we develop a technique to identify mainshocks, foreshocks, and aftershocks. We then find, based on a statistical procedure we developed, that the rate of earthquakes is higher a few days prior to a mainshock. We find that this increase is much smaller for a catalog with a magnitude threshold of m over 2.5 and for the Epidemic-Type Aftershocks Sequence (ETAS) model catalogs, even when used with a small magnitude threshold. We also analyze the rate of aftershocks after mainshocks and find that the Omori-Utsu law does not hold for many individual mainshocks and that it may be valid only statistically when considering many mainshocks together. Yet, the analysis of the ETAS model based on the Omori-Utsu law exhibits similar behavior as that of the real catalogs, indicating the validity of this law.Comment: 19 pages, 7 figure

Scaling of Horizontal and Vertical Fixational Eye Movements

Eye movements during fixation of a stationary target prevent the adaptation of the photoreceptors to continuous illumination and inhibit fading of the image. These random, involuntary, small, movements are restricted at long time scales so as to keep the target at the center of the field of view. Here we use the Detrended Fluctuation Analysis (DFA) in order to study the properties of fixational eye movements at different time scales. Results show different scaling behavior between horizontal and vertical movements. When the small ballistics movements, i.e. micro-saccades, are removed, the scaling exponents in both directions become similar. Our findings suggest that micro-saccades enhance the persistence at short time scales mostly in the horizontal component and much less in the vertical component. This difference may be due to the need of continuously moving the eyes in the horizontal plane, in order to match the stereoscopic image for different viewing distance.Comment: 5 pages, 4 figure

Higher neuronal discharge rate in the motor area of the subthalamic nucleus of Parkinsonian patients

International audienceIn Parkinson's disease, pathological synchronous oscillations divide the subthalamic nucleus (STN) of patients into a dorsolateral oscillatory region and ventromedial nonoscillatory region. This bipartite division reflects the motor vs. the nonmotor (associative/limbic) subthalamic areas, respectively. However, significant topographic differences in the neuronal discharge rate between these two STN subregions in Parkinsonian patients is still controversial. In this study, 119 STN microelectrode trajectories (STN length > 2 mm, mean = 5.32 mm) with discernible oscillatory and nonoscillatory regions were carried on 60 patients undergoing deep brain stimulation surgery for Parkinson's disease. 2,137 and 2,152 multiunit stable signals were recorded (recording duration > 10 s, mean = 21.25 s) within the oscillatory and nonoscillatory STN regions, respectively. Spike detection and sorting were applied offline on every multiunit stable signal using an automatic method with systematic quantification of the isolation quality (range = 0–1) of the identified units. In all, 3,094 and 3,130 units were identified in the oscillatory and nonoscillatory regions, respectively. On average, the discharge rate of better-isolated neurons (isolation score > 0.70) was higher in the oscillatory region than the nonoscillatory region (44.55 ± 0.87 vs. 39.97 ± 0.77 spikes/s, N = 665 and 761, respectively). The discharge rate of the STN neurons was positively correlated to the strength of their own and their surrounding 13- to 30-Hz beta oscillatory activity. Therefore, in the Parkinsonian STN, beta oscillations and higher neuronal discharge rate are correlated and coexist in the motor area of the STN compared with its associative/limbic area