Seeking Repeating Anthropogenic Seismic Sources: Implications for Seismic Velocity Monitoring at Fault Zones.

Seismic velocities in rocks are highly sensitive to changes in permanent deformation and fluid content. The temporal variation of seismic velocity during the preparation phase of earthquakes has been well documented in laboratories but rarely observed in nature. It has been recently found that some anthropogenic, high-frequency (>1 Hz) seismic sources are powerful enough to generate body waves that travel down to a few kilometers and can be used to monitor fault zones at seismogenic depth. Anthropogenic seismic sources typically have fixed spatial distribution and provide new perspectives for velocity monitoring. In this work, we propose a systematic workflow to seek such powerful seismic sources in a rapid and straightforward manner. We tackle the problem from a statistical point of view, considering that persistent, powerful seismic sources yield highly coherent correlation functions (CFs) between pairs of seismic sensors. The algorithm is tested in California and Japan. Multiple sites close to fault zones show high-frequency CFs stable for an extended period of time. These findings have great potential for monitoring fault zones, including the San Jacinto Fault and the Ridgecrest area in Southern California, Napa in Northern California, and faults in central Japan. However, extra steps, such as beamforming or polarization analysis, are required to determine the dominant seismic sources and study the source characteristics, which are crucial to interpreting the velocity monitoring results. Train tremors identified by the present approach have been successfully used for seismic velocity monitoring of the San Jacinto Fault in previous studies.

Monitoring Seismic Velocity Changes Across the San Jacinto Fault Using Train-Generated Seismic Tremors.

Microseismic noise has been used for seismic velocity monitoring. However, such signals are dominated by low-frequency surface waves that are not ideal for detecting changes associated with small tectonic processes. Here we show that it is possible to extract stable, high-frequency body waves using seismic tremors generated by freight trains. Such body waves allow us to focus on small velocity perturbations in the crust with high spatial resolution. We report on 10 years of seismic velocity temporal changes at the San Jacinto Fault. We observe and map a two-month-long episode of velocity changes with complex spatial distribution and interpret the velocity perturbation as produced by a previously undocumented slow-slip event. We verify the hypothesis through numerical simulations and locate this event along a fault segment believed to be locked. Such a slow-slip event stresses its surroundings and may trigger a major earthquake on a fault section approaching failure.

Train Traffic as a Powerful Noise Source for Monitoring Active Faults With Seismic Interferometry.

Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train-generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4-km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California.

Correlation of clinical characteristics and small bowel histopathology in celiac disease.

BACKGROUND: Literature information regarding clinical and histological correlates in celiac disease is limited. The present study was designed to assess the value of various clinical parameters in predicting the severity of small bowel histopathology. METHODS: Small bowel biopsy specimens of 59 children with established celiac disease (ESPGAN criteria) were evaluated blindly. Morphology was evaluated based on a common histopathology score. The following clinical variables were evaluated: age at diagnosis, duration of symptoms, severity score of clinical symptoms, severity score of physical signs, and growth parameters (height and weight Z scores). Multiple regression analysis was performed to evaluate the relative importance of each clinical parameter. RESULTS: Only three clinical variables revealed a significant correlation with the histopathology score. The symptom severity score (t = 3.883, p = 0.0003) demonstrated a positive correlation. The two others, age at diagnosis (t = 3.076, p = 0.0032) and duration of symptoms (t = -2.987, p = 0.0041), revealed a negative correlation. CONCLUSIONS: We conclude that more severe clinical symptoms of a shorter duration, presented at a younger age, are better predictors of a more severe form of small bowel histopathology in children with celiac disease.

Slip complexity in earthquake fault models.

We summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes. For models of smooth faults between elastically deformable continua, it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state. That results in a finite nucleation size, or coherent slip patch size, h*. Models of smooth faults, using numerical cell size properly small compared to h*, show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self-similar (power law) Gutenberg-Richter frequency-size statistics. This conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences. In contrast, some models of locally heterogeneous faults with quasi-independent fault segments, represented approximately by simulations with cell size larger than h* so that the model becomes "inherently discrete," do show small event complexity of the Gutenberg-Richter type. Models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h* = 0 and hence always fall into the inherently discrete class. We suggest that the small-event complexity that some such models show will not survive regularization of the constitutive description, by inclusion of an appropriate length scale leading to a finite h*, and a corresponding reduction of numerical grid size.

[Circumcision of new immigrants].

Recent immigration from eastern Europe to Israel (1990-1992) has brought to the Negev many uncircumcised newcomers. The rationale for circumcising healthy children has been a matter of controversy, not yet settled. Healthy adults are not usually circumcised except for ritual reasons. In the past 3 years we circumcised 2857 males 1-64 years old, mostly of Russian origin. All were operated on as outpatients on a 1-day, ambulatory service. 75% of the newcomers were operated on during the first 6 months after immigration. 86% of the circumcisions were done under general anesthesia and the rest under local. After stretching the prepuce backwards, the foreskin was excised. Hemostasis was achieved with the aid of electrocautery and the skin was approximated with in absorbable sutures. All patients were re examined 1 week later. There were complications in 50 (1.75%), consisting mostly of postoperative bleeding and wound infections. We conclude that ritual circumcision is a safe procedure in normal adults.

San andreas fault zone head waves near parkfield, california.

Microearthquake seismograms from the borehole seismic network on the San Andreas fault near Parkfield, California, provide three lines of evidence that first P arrivals are "head" waves refracted along the cross-fault material contrast. First, the travel time difference between these arrivals and secondary phases identified as direct P waves scales linearly with the source-receiver distance. Second, these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct P arrivals. Third, the first motion polarities of the emergent arrivals are reversed from those of the direct P waves as predicted by the theory of fault zone head waves for slip on the San Andreas fault. The presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms. The fault zone head waves indicate that the velocity contrast across the San Andreas fault near Parkfield is approximately 4 percent. Further studies of these waves may provide a way of assessing changes in the physical state of the fault system.