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The conversion from seismic to ocean-acoustic waves occurs in different places on the bottom of the ocean, often hundreds to thousands of kilometers away from the epicenter. Here, we investigate this conversion process by studying 15 large-magnitude earthquakes that occurred between 2014 and 2022 along the Kermadec Arc in the southwestern Pacific Ocean. To pinpoint the location where seismic-to-acoustic conversion takes places, we analyze hydroacoustic signals recorded by a hydrophone triplet station of the International Monitoring System in the Juan Fernández archipelago. Results from direction-of-arrival and travel-time calculations indicate that the location of the conversion zone largely matches segments of the Louisville Seamount Chain, its lateral extent ranging from approximately 300 to 1800 km, and its location depending on the geometry between earthquake epicenter and the seamounts. [ABSTRACT FROM AUTHOR]

Accurate regional seismic travel-time (RSTT) predictions rely on regional phases (e.g., Pg, Lg, Pn, Sn) to account for 3D effects in the crust and upper mantle that are not captured by 1D models traditionally used for real-time location. The RSTT prediction model accounts for regional-scale crust and upper mantle structure globally by incorporating regional seismic phases into its travel-time calculations. Previous versions of the RSTT model have used a constant grid cell size of 1°. To improve the tomographic accuracy of recovering velocity structure at regional scales, we perform data-driven grid refinement on the RSTT model down to a 0.125° grid (~14 km) in pursuit of two main goals: (1) to test the limits of RSTT capability and accuracy of determined velocity structure through variable grid refinement and (2) to image smaller structures in Israel and the Middle East and illuminate upper mantle dynamics operating in this complex tectonic area. We investigate the effects of model parameterization as grid cell size decreases and the trade-offs between recovered velocity structures. Our final dataset includes 4751 events and 499 stations that recorded 79,344 Pn and 7489 Pg. The variable grid refinement method allows recovery of finer-scale velocity structures and reduces travel-time residuals in areas with the highest data coverage. At smaller grid cell sizes, longer paths need to be upweighted to stabilize the inversion. Results illuminate tectonic features undefined in coarser grid-size models; in particular, we observe mantle perturbations related to the subduction zone around the Cyprian arc and crustal anomalies near the Dead Sea fault and throughout the Anatolian plate. [ABSTRACT FROM AUTHOR]

The task of monitoring the Earth for nuclear explosions relies heavily on seismic data to detect, locate, and characterize suspected nuclear tests. Motivated by the need to locate suspected explosions as accurately and precisely as possible, we developed a tomographic model of the compressional wave slowness in the Earth's mantle with primary focus on the accuracy and precision of travel-time predictions for P and Pn ray paths through the model. Path-dependent travel-time prediction uncertainties are obtained by computing the full 3D model covariance matrix and then integrating slowness variance and covariance along ray paths from source to receiver. Path-dependent travel-time prediction uncertainties reflect the amount of seismic data that was used in tomography with very low values for paths represented by abundant data in the tomographic data set and very high values for paths through portions of the model that were poorly sampled by the tomography data set. The pattern of travel-time prediction uncertainty is a direct result of the off-diagonal terms of the model covariance matrix and underscores the importance of incorporating the full model covariance matrix in the determination of travel-time prediction uncertainty. The computed pattern of uncertainty differs significantly from that of 1D distance-dependent travel-time uncertainties computed using traditional methods, which are only appropriate for use with travel times computed through 1D velocity models. [ABSTRACT FROM AUTHOR]

We develop a regional seismic travel-time (RSTT) model and method for use in routine seismic analysis. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3D crust and laterally variable upper-mantle velocity. The upper-mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel-time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global, our first RSTT tomographic effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals. Ten percent of the data set is randomly selected and set aside for testing purposes. Travel-time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from i7.3 km (ak135 model) to 9.3 km (RSTT model) after tomography. The median uncertainty ellipse area is reduced by 68% from 3070 km² (ak135) to 994 km² (RSTT), and the number of ellipses with area less than 1000 km², which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% (ak135) to 51% (RSTT). [ABSTRACT FROM AUTHOR]

We studied a sequence of ∼150 earthquakes (magnitude [ML, MD] < 0.8-4.0) that began in December 1998 in an area of complex faulting in the Santa Cruz Mountains, California. Epicenters plot west of the San Andreas Fault, east of the San Gregorio Fault, and just south of the Zayante Fault. Data for systematic hypocenter relocations and focal mechanisms were acquired from temporary deployment of eight IRIS/PASSCAL digital three-component 1-2 Hz seismographs and Northern California Seismic Network data. Our event relocations indicate this sequence is linked to the Zayante fault zone, along a segment previously considered inactive. In addition to detecting the 51 events listed by the Northern California Earthquake Data Center, 99 uncataloged events, presumably of lesser magnitude, were also recorded by our instruments. All data were processed using a waveform correlation program and shear-wave rotation for high-resolution P- and S-wave arrival times. Relocations were done using the master event method. Relocated hypocenters group tightly within approximately ± 1 km about latitude 37° 04.62', longitude 122° 03.26', and range in depth from 2.1 to 8.0 km with a mean depth of 5.1 km. Epicenters plot in a roughly N60-70° W alignment ∼3.5 km south of the main branch of the mapped Zayante Fault. A plot of hypocenters in 3D also shows a N60-70° W alignment. Aftershocks occurred in rapid succession, with most of the early events within seconds of each other. Spatiotemporal analysis shows a rapid expansion of the aftershock field. Focal mechanisms are primarily thrust with small components of lateral slip. Average nodal plane strike is N37° W ± 15°, dipping 54° ± 8° southwest (range of 46-65°), and N82° W ± 17°, dipping 47° ± 14° northeast. We interpret the former as representing the fault plane, which is more consistent with the geology, and the resultant... [ABSTRACT FROM AUTHOR]