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Seismic signals from ocean‐solid Earth interactions are ubiquitously recorded on our planet. However, these wavefields are typically incoherent in the time domain limiting their utilization for understanding ocean dynamics or solid Earth properties. In contrast, we find that during large storms such as hurricanes and Nor'easters the interaction of long‐period ocean waves with shallow seafloor features located near the edge of continental shelves, known as ocean banks, excites coherent transcontinental Rayleigh wave packets in the 20‐ to 50‐s period band. These "stormquakes" migrate coincident with the storms but are effectively spatiotemporally focused seismic point sources with equivalent earthquake magnitudes that can be greater than 3.5. Stormquakes thus provide new coherent sources to investigate Earth structure in locations that typically lack both seismic instrumentation and earthquakes. Moreover, they provide a new geophysical observable with high spatial and temporal resolution with which to investigate ocean wave dynamics during large storms. Plain Language Summary: Large storms such as hurricanes and Nor'easters generate strong long‐period ocean waves, which can interact with shallow seafloor features located near the edge of continental shelves known as ocean banks. Such interactions produce seismic sources with equivalent earthquake magnitudes that can be greater than 3.5. These seismic sources are termed "stormquakes," and they can excite coherent seismic wave fields that are well recorded across the North American continent. Stormquake is a newly identified geophysical phenomenon, which involves interactions of atmosphere, ocean, and the solid Earth. Therefore, stormquakes can provide useful information to investigate the Earth structure and ocean wave dynamics. Key Points: Energetic storms can generate stormquakes exciting coherent transcontinental surface wavefieldsThese stormquakes are effective point sources with equivalent earthquake magnitudes that can be greater than 3.5Large continental shelves, ocean banks, and strong storms are the three necessary factors for stormquake generation [ABSTRACT FROM AUTHOR]

Very accurate timing of seismic recordings is critical for modern processing techniques. Clock synchronization among the instruments constituting an array is, however, difficult without direct communication between them. Synchronization to Global Positioning System (GPS) time is one option for on-land deployments, but not for underwater surveys as electromagnetic signals do not propagate efficiently in water. If clock drift is linear, time corrections for ocean-bottom seismometer (OBS) deployments can be estimated through GPS synchronization before and after the deployment, but this is not sufficient for many applications as the nonlinear component of the drift can reach tens to hundreds of milliseconds for long-duration experiments. We present two techniques to retrieve timing differences between simultaneous recordings at ocean-bottom instruments after deployment has ended. Both techniques are based on the analysis of the cross correlation of ambient seismic noise and are effective even if clock drift is nonlinear. The first, called time symmetry analysis, is easy to apply but requires a proper illumination so that the noise cross-correlation functions are symmetric in time. The second is based on the doublet analysis method and does not have this restriction. Advantages and drawbacks of both approaches are discussed. Application to two OBS data sets shows that both can achieve synchronization of recordings down to about five milliseconds (a few percent of the main period used). [ABSTRACT FROM AUTHOR]

Abstract: The rate of earthquake occurrence varies by many orders of magnitude in a given region due to variations in the stress state of the crust. Our focus here is on variations in seismicity rate triggered by transient aseismic processes such as fluid flow, fault creep or magma intrusion. While these processes have been shown to trigger earthquakes, converting observed seismicity variations into estimates of stress rate variations has been challenging. Essentially aftershock sequences often obscure changes in the background seismicity rate resulting from aseismic processes. Two common approaches for estimating the time dependence of the underlying driving mechanisms are the stochastic Epidemic Type Aftershock Sequence model (ETAS) [Ogata, Y., (1988), Statistical models for earthquake occurrences and residual analysis for point processes, J. Am. Stat. Assoc., 83, 9–27.] and a physical approach based on the rate- and state-model of fault friction [Dieterich, J., (1994), A constitutive law for rate of earthquake production and its application to earthquake clustering, J. Geophys. Res., 99, 2601–2618.]. The models have different strengths that could be combined to allow more quantitative studies of earthquake triggering. To accomplish this, we identify the parameters that relate to one another in the two models and examine their dependence on stressing rate. A particular conflict arises because the rate–state model predicts that aftershock productivity scales with stressing rate while the ETAS model assumes that it is time independent. To resolve this issue, we estimate triggering parameters for 4 earthquake swarms contemporaneous with geodetically observed deformation transients in various tectonic environments. We find that stressing rate transients increase the background seismicity rate without affecting aftershock productivity. We then specify a combined model for seismicity rate variations that will allow future studies to invert seismicity catalogs for variations in aseismic stressing rates. [Copyright &y& Elsevier]

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominately aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion. [ABSTRACT FROM AUTHOR]