Microseismic background ?noise? is low amplitude ground motion or vibration that is commonly caused by wind, waves or even vehicle traffic. This energy has long been known to be propagated as surface waves that travel along the Earth?s surface. Less well known is that some of this energy travels through the Earth?s interior as body waves generated largely by storm waves, which are useful for studying the structure of the Earth. This phenomenon and its potential applications have been little studied. This project will apply beam forming and back projection methods can be usefully applied to analyze P-wave microseismic data from a variety of seismic arrays. These techniques could prove to be transformative for studies of Earth structure, particularly in regions of the interior that are not well sampled by seismic waves from earthquakes. Broader impacts of the work include support for a graduate student and involving an undergraduate student through the UCSD STARS program, and a unique feature of the work is its highly interdisciplinary nature; related field of study include oceanography, ocean acoustics, and seismology.
This project study seismic noise and nontraditional ocean wave generated seismic sources using advanced array processing algorithms that can extract information from weak signals. Our work will help to characterize seismic noise sources and develop new methods to use them for resolving Earth structure. Our main focus will be on P wave microseisms from storms. Although thousands of papers have been published on surface wave microseisms (including 1200 before 1964 [Haubrich and McCamy, 1969]), fewer than 10 have been published on body wave microseisms. Thus, using dense arrays and powerful new processing methods to study these underexplored phases is likely to lead to novel and unanticipated results. One exciting aspect of our research is that noise analysis methods have the potential to be very useful in improving body wave tomography for Earth’s structure, just as noise crosscorrelation methods have recently proven successful in surface wave tomography. Preliminary tests examining teleseismic P waves recorded in southern California show that similar arrival time anomalies can be obtained from both direct P waves from a natural earthquake and P wave noise generated under large storms. In the latter case, the noise can be processed using waveform crosscorrelation among different station pairs, and optimal P relative arrivaltime estimates can be computed using the same approach traditionally used to analyze earthquake arrival times. Microseisms, low amplitude wave-induced signals that are ubiquitous in seismic recordings, have long been regarded as a nuisance in the study of transient seismic signals, such as earthquakes and active sources. However, recent advances in array processing tomography using diffuse ambient noise have resulted in a resurgence of interest in microseisms and their generation. The spectrum of ocean wave-induced seismicity contains three peaks. The most energetic peak is at double the frequency of ocean waves, the double-frequency (DF) microseism peak. Nearly opposing waves of the same frequency generate standing waves at double the frequency of the initial waves with energy independent of depth, while ocean surface gravity waves decay exponentially with depth. The DF standing waves have very small wave numbers and thus couple efficiently to seismic modes that have lower wave numbers than ocean surface waves. A smaller peak, the primary microseism (PM) peak, is centered at the frequency of ocean surface waves. This peak is attributed to the direct interaction of ocean waves with the seabed. Although the ocean surface waves have much higher wave numbers than the seismic modes they excite, in environments with varying ocean depth the ocean wave spectrum varies spatially and can excite low-wave number fluctuations in the seabed. As ocean waves decay exponentially with depth, this generation process is limited to shallow water; however, the average observed primary microseism peak is only 20 dB less than the DF peak and thus represents a significant fraction of observed microseism energy. The smallest peak, the Earth’s seismic hum, is centered at frequencies much lower than those of ocean surface gravity waves. The Earth’s seismic hum is usually attributed to infragravity waves, ocean waves of exceptionally low frequency that are generated by the interactions of ocean surface-gravity waves with the seabed in shallow water. The Earth’s hum has been considered a distinct process from microseisms (DF and PM) and most theoretical and observational work has considered them separately.
