Search

We present a new method to locate low-frequency earthquakes (LFEs) within tectonic tremor episodes based on time-reverse imaging techniques. The modified time-reverse imaging technique presented here is the first method that locates individual LFEs within tremor episodes within 5 km uncertainty without relying on high-amplitude P-wave arrivals and that produces similar hypocentral locations to methods that locate events by stacking hundreds of LFEs without having to assume event co-location. In contrast to classic time-reverse imaging algorithms, we implement a modification to the method that searches for phase coherence over a short time period rather than identifying the maximum amplitude of a superpositioned wavefield. The method is independent of amplitude and can help constrain event origin time. The method uses individual LFE origin times, but does not rely on a priori information on LFE templates and families. We apply the method to locate 34 individual LFEs within tremor episodes that occur between 2010 and 2011 on the San Andreas Fault, near Cholame, California. Individual LFE location accuracies range from 2.6 to 5 km horizontally and 4.8 km vertically. Other methods that have been able to locate individual LFEs with accuracy of less than 5 km have mainly used large-amplitude events where a P-phase arrival can be identified. The method described here has the potential to locate a larger number of individual low-amplitude events with only the S-phase arrival. Location accuracy is controlled by the velocity model resolution and the wavelength of the dominant energy of the signal. Location results are also dependent on the number of stations used and are negligibly correlated with other factors such as the maximum gap in azimuthal coverage, source-station distance and signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The article focuses on the EarthScope project examining the San Andreas fault in California. It states that a major element of the project, a suite of instruments and detectors buried deep in the San Andreas fault, stopped functioning. Geophysicist Mark Zoback said that the instruments stopped functioning a few days after they were installed in the fault line. It mentions that there is insufficient funding to recover the detectors, which included seismometers and accelerometers.

In this article the authors have analyzed seismic noise to obtain new information on the structure of Earth's crust. By correlating the data from a month of ambient noise due in part to wave-wave interactions in the ocean 2 detected by 62 long-period seismograph stations in southern California, they determined the seismic response that they would have obtained from Earth's crust if they had applied forces at each of their stations. In particular, they measured the times that it took for seismic surface waves to propagate between every pair of stations. They then used lomographic techniques to create a map of seismic wave velocity with an unprecedented horizontal resolution of 75 to 100 km.

The observation of continuously excited free oscillations of the Earth, in the absence of earthquakes, was first made by Japanese scientists in 1998. Since then, attention has focused on elucidating the physical mechanism responsible for them. The mechanism must be shallow, as fundamental modes appear to be preferentially excited and it shows seasonal variability. An array-based method has been developed to detect and locate sources of very long period surface wave energy, utilizing the dispersive properties of Rayleigh waves and data from two large aperture arrays of very long period seismometers, in California and in Japan. It is shown that, for each array, there is a well defined preferential direction, which is stable over one season but changes significantly from winter to summer. The fluctuations as a function of time of the maximum stack amplitudes are correlated across the two arrays and point to the northern Pacific Ocean in the northern hemisphere winter and the southern oceans in the summer, correlating with the distribution of maximum wave height. It is inferred that the background oscillations originate primarily in the oceans, and are caused by a non-linear coupling mechanism involving the atmosphere (winds), the oceans (infragravity waves) and the seafloor. [ABSTRACT FROM AUTHOR]

The article reports that every day, several of California's more than 1000 shifting faults shake some part of the state. This clearinghouse of earthquake data keeps check on the ones that rattle northern California. Sponsored by the University of California, Berkeley, and the U.S. Geological Survey, the site catalogs magnitude, depth, location, and other measurements for earthquakes dating back to 1967. One should check out near-real-time recordings from a network of seismographs scattered around northern and central California.