Ambient noise levels and detection threshold in Norway.

Ambient seismic noise is caused by a number of sources in specific frequency bands. The quantification of ambient noise makes it possible to evaluate station and network performance. We evaluate noise levels in Norway from the 2013 data set of the Norwegian National Seismic Network as well as two temporary deployments. Apart from the station performance, we studied the geographical and temporal variations, and developed a local noise model for Norway. The microseism peaks related to the ocean are significant in Norway. We, therefore, investigated the relationship between oceanic weather conditions and noise levels. We find a correlation of low-frequency noise (0.125-0.25 Hz) with wave heights up to 900 km offshore. High (2-10 Hz) and intermediate (0.5-5 Hz) frequency noise correlates only up to 450 km offshore with wave heights. From a geographic perspective, stations in southern Norway show lower noise levels for low frequencies due to a larger distance to the dominant noise sources in the North Atlantic. Finally, we studied the influence of high-frequency noise levels on earthquake detectability and found that a noise level increase of 10 dB decreases the detectability by 0.5 magnitude units. This method provides a practical way to consider noise variations in detection maps.

Computation of uniform wave forms using complex rays.

Complex rays and polynomial phase functions are used to numerically solve the Helmholtz equation in a realistic two-dimensional smoothly varying heterogeneous velocity model with multiple adjacent cusp caustics. Together these two methods allow the determination of global uniformly asymptotic solutions in the presence of arbitrarily many caustics. Two algorithms are introduced to this end: a two-point ray tracing algorithm for complex rays and a perturbation method for constructing polynomial phase functions. Model representation in complex space is performed via discrete cosine transform analysis. Geometrical and uniformly asymptotic solutions are computed for a linear layer test model as well as a velocity model from Yucca Mountain.