J. A. Eakins

Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ;318N latitude. Depth to the Moho is ;33 (63) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ;40 (64) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ;15-18 (62) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ;258. This range of Moho depths within the Peninsula Ranges, as well as the sharp ;east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

Regional crustal thickness variations of the Peninsular Ranges, southern California

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from ∼37 km beneath the western Peninsular Ranges batholith to ∼27 km at the western edge of the Salton trough, an average apparent dip of ∼10° to the west over a lateral distance of 60 km. We previously obtained a similar result for a profile ∼100 km to the south (a Moho dip of ∼20° over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes. These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Model Update May 2016: Upper‐Mantle Heterogeneity beneath North America from Travel‐Time Tomography with Global and USArray Data

ABSTRACT P ‐wave travel‐time residuals from USArray helped improve the scale and consistency with which the mantle beneath North America is resolved. Beginning in 2008, we published a series of P ‐wave velocity models based on a global ray theoretical inversion of USArray and global catalog data. Here, we present the final model update, MITP_USA_2016MAY, which includes the complete set of travel‐time residuals from USArray Transportable Array (TA) in the contiguous United States. In this model, the area of high resolution extends to the eastern margin of the continent, allowing us to better estimate the location and extent of slow features in Central Virginia and New England. An increasing number of data from the TA in Alaska also allows us to recover the structure of subducting Pacific plate and Yakutat terrane. In addition to highlighting new features in the final model, we visualize and discuss the improvements to the model due to the addition of USArray data through time.

The USArray Transportable Array as a Platform for Weather Observation and Research

AbstractThe USArray Transportable Array (TA), a component of the National Science Foundation’s EarthScope Initiative, has proven to be a successful model for large-scale real-time monitoring networks. Approximately 400 stations are deployed simultaneously in the continental United States on a nominal Cartesian grid across an area of approximately 2,000,000 km2. Each station was originally designed to operate autonomously as a seismic observing platform capable of recording and transmitting data at 1 and 40 samples per second in real time. The expansion of onboard instrumentation to include surface atmospheric pressure sensors improved the USArray’s real-time capability in monitoring the atmosphere and weather phenomena at the same sample rates. Though not a traditional weather monitoring station, the combination of these seismic and pressure sensors at each TA station can contribute to the observation of surface weather phenomena and has facilitated broader-scale observational applications. Twenty-five TA...

An SOA-based Framework for Instrument Management for Large-scale Observing Systems (USArray Case Study)

Large-scale observing systems are poised to become the dominant means of study for a variety of natural phenomena. These systems are comprised of hundreds to thousands of instruments that must be queried, managed, and shared in a scalable fashion. Services-oriented architectures (SOAs) are widely recognized as the preferred framework for building scalable and extensible cyber infrastructure. By applying SOA concepts, we created a framework for organizing observing system resources. Guided by this framework, we developed Web services, custom workflow applications, and an integrated user interface of monitors and controls for managing instruments in large-scale sensor network observing systems. In this paper we present our approach and discuss its application to the NSF EarthScope USArray large-scale seismic observing system

Seismicity around the Hawaiian Islands Recorded by the PLUME Seismometer Networks: Insight into Faulting near Maui, Molokai, and Oahu

Instrumental limitations have long prevented the detailed characteriza- tion of offshore earthquakes around the Hawaiian Islands, and little is known about the spatial distribution of earthquakes in regions outside the vicinity of the well- monitored island of Hawaii. Here, we analyze data from the deployment of two successive networks of ocean-bottom seismometers (OBSs) as part of the Plume- Lithosphere Undersea Melt Experiment (PLUME) to better determine seismicity pat- terns along the Hawaiian Islands and their offshore regions. We find that earthquake detection rates are improved when seismograms are high-pass filtered above ∼5 Hz to reduce the background seismic noise. Hypocentral solutions have been determined for 1147 previously undetected microearthquakes, and an additional 2880 events corre- spond to earthquakes already in the catalog of the United States Geological Survey (USGS) Hawaiian Volcano Observatory (HVO). The spatial patterns of earthquakes identified solely on the PLUME network provide complementary information to pat- terns identified by the HVO network. A diffuse pattern of seismicity is found to the southeast of the island of Hawaii, and clusters of earthquakes are located west of the island. Many microearthquakes are observed in the vicinity of Maui and Molokai, including some located at mantle depths. A small number of microearthquakes are found to occur near Oahu. There is no evidence from our analyses that the Molokai fracture zone (MFZ) is seismically active at this time, and no evidence was found of a previously hypothesized Diamond Head fault (DHF) near Oahu. However, on the basis of both the PLUME and HVO locations, there is a northeast-southwest-trending swath of epicenters extending northeastward of Oahu that may indicate the locus of moderate-sized historic earthquakes attributed to the Oahu region.

Large Teleseismic P Wavefront Deflections Observed with Broadband Arrays

We measure the plane wavefront incidence azimuth for teleseismic P at large-aperture (∼50 km) broadband arrays. The incidence azimuth is determined by crosscorrelation of the P arrivals on the vertical component seismograms filtered in successive frequency bands. The periods considered range from 10 to 35 sec. At the Anza array in southern California, the plane wave direction is deflected from the great circle azimuth of the event by up to 20°. In addition, we find a surprisingly strong frequency dependence of the same magnitude and a striking antisymmetric pattern of the deflection as a function of backazimuth, whereas the curvature of the wavefront is small. Similar characteristics are found at the Grafenberg array in Germany and the NORSAR array in Norway, however, with much weaker amplitudes of ∼5°. We ascribe the behavior at Anza to structure in the lower crust and uppermost mantle beneath the array, given that the observations are only a function of source backazimuth and not of source depth and source mechanism, that the wavelengths under consideration range from 50 to 270 km, and that the sign of the deviation is opposite to that predicted from shallow crustal structure and Moho topography. We are able to reproduce the magnitude and frequency dependence of the wavefront deflection using finite difference numerical modeling of plane wave propagation through simple 2D structures.