Geology topics

E. Robert Engdahl

Publications and source records attributed to E. Robert Engdahl.

4 recordsLinked to original sources

USGS2022

A global catalog of calibrated earthquake locations

We produced a globally distributed catalog of earthquakes and nuclear explosions with calibrated hypocenters, referred to as the Global Catalog of Calibrated Earthquake Locations (GCCEL). This dataset currently contains 18,782 events in 289 clusters with >3.2 million arrival times observed at 19,258 stations. The term “calibrated” refers to the property that the hypocenters are minimally biased by unknown Earth structure. In addition, we calculate uncertainties using empirically determined variability of the arrival‐time data itself, specific to each calibrated cluster of hypocenters. Outliers in the arrival‐time dataset are removed based on measured variability of the data. In each cluster, we estimate the empirically determined uncertainty for each set of station‐phase arrival times. We use a version of the hypocentroidal decomposition multiple event relocation algorithm specifically adapted for calibrated relocations of clusters of seismic events. Most clusters are calibrated by fitting the subset of direct crustal first arrivals ( Pg and Sg ) with a locally appropriate travel‐time model to estimate the cluster hypocentroid. A few clusters are calibrated by aligning the pattern of relative locations in space and time with one or more events for which a ground‐truth hypocenter is available from an independent source with known uncertainty, such as a nuclear explosion. Epicentral uncertainties in GCCEL typically range from 1 to 5 km with a 90% confidence interval. Most events have depth constraint from one or more sources, usually with an uncertainty of ≤5 km. GCCEL is a significant resource for research at local, regional, and global scales because it provides minimally biased absolute hypocenters, meaningful associated error estimates, and curated arrival times as a reference dataset that can be used as prior constraints in the development of new regional, national, and global earthquake catalogs; validation of new location techniques; and the generation of advanced Earth models.

Seismological Research Letters↗

USGS2013

Public release of the ISC-GEM Global Instrumental Earthquake Catalogue (1900-2009)

The International Seismological Centre–Global Earthquake Model (ISC–GEM) Global Instrumental Earthquake Catalogue (1900–2009) is the result of a special effort to substantially extend and improve currently existing global catalogs to serve the requirements of specific user groups who assess and model seismic hazard and risk. The data from the ISC–GEM Catalogue would be used worldwide yet will prove absolutely essential in those regions where a high seismicity level strongly correlates with a high population density.

Seismological Research Letters↗

USGS2013

Intermediate‐depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs

We elaborate on the well-known spatial association between axc volcanoes and Wadati Benioff zones and explore in detail their genetic relationships as dual physical expressions of slab metamorphism of the oceanic crust and uppermost mantle. At hypocentral depths less than 200 km intra slab Wadati-Benioff earthquakes tend to occur near the top surfaces of slabs. Subduction of very young lithosphere (age < 15-25 Ma) with high heat flow (> 75 mW/m 2) produces mainly shallow earthquakes and spaxse or absent arc volcanism. Subduction of older crust with normal heat flow (50-65 mW/m 2) produces markedly deeper intraslab earthquakes and generally normal volcanic vigor. Seismological observations show that the low-seismic-velocity gabbroic mineralogy of the crust may persist to depths of as much as 150 km in old, cold lithosphere but only to depths of 50-60 km in young, warm slabs. Metamorphic processes in the crust and shallow upper mantle of subducting slabs and the reactivation of faults originally created at shallow depths in the ocean basins probably control the occurrence of intraslab earthquakes to depths of as much as 350 kin. A conceptual model for this metamorphism incorporates the likely effects of water liberated by dehydration. Such dehydration facilitates both brittle faulting by fault reactivation and promotes the kinetics of the transformation of the anhydrous gabbro component of the crust to eclogite. Finite-element modelling shows that densification to eclogite is expected to produce extensional stresses in transformed crust and a smaller compression in the underlying mantle. This model helps explain why most intermediate-depth intraslab earthquakes occur just below the top surfaces of slabs and why many have focal mechanisms indicating down-dip extension. Young, warm slabs have mostly shallow intraslab earthquakes and sparse axc volcanoes because dehydration and eclogite formation largely cease before such slabs axe in contact with asthenosphere. These processes are evidently delayed by kinetic hindrance and the high pressure stability of hydrous phazes at low temperatures in older, colder subducting crust, and thus earthquake activity and asthenospheric-wedge melting tend to be focused at depths of 100-170 kin. Anomalous behavior correlated with the subduction of island-and seamount chains appears to be associated with anomalou shallow intraplate faulting and with perturbations of slab metamorphism by these chains.

Geophysical Monograph↗

USGS2012

The 9 June 94 Bolivian Deep Earthquake: An exceptional event in an extraordinary subduction zone

We investigate the physical setting of the Bolivian shock based on the history of the subducting Nazca plate, intraslab seismicity, deep seismic moment release, and seismic tomography. South America has two broad regions of reverse arc curvature. Subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle. Such contortions are observed at intermediate depths “upstream” from the slab source regions of both the great 1970 Colombian and 1994 Bolivian events. Thickening helps explain how the Nazca slab accommodates large seismic source dimensions at depths of 625–650 km.

Geophysical Research Letters↗