A document by Dan Sandiford. Click on the document to view its contents.

Earthquake moment tensors and centroid locations in the catalogue of the Global CMT (gCMT) project, formerly the Harvard CMT project, have become an essential and extraordinarily valuable resource for studying active global tectonics, used by many solid-Earth researchers. The catalogue’s quality, long duration (1976–present), ease of access and global coverage of earthquakes larger than about Mw~5.5 has transformed our ability to study regional patterns of earthquake locations and focal mechanisms. It also allows researchers to easily identify earthquakes with anomalous mechanisms and depths that stand out from the global or regional patterns, some of which require us to look more closely at accepted interpretations of geodynamics, tectonics or rheology. But, as in all catalogues that are, to some extent and necessarily, produced in a semi-routine fashion, the catalogue may contain anomalies that are in fact errors. Thus, before re-assessing geodynamic, tectonic or rheological understanding on the basis of anomalous earthquake locations or mechanisms in the gCMT catalogue, it is first prudent to check those anomalies are real. The purpose of this paper is to illustrate that necessity in the eastern Himalayas and SE Tibet, where two earthquakes that would otherwise require a radical revision of current geodynamic understanding are shown, in fact, to have gCMT depths (and, in one case, also focal mechanism) that are incorrect — in spite of the overwhelming majority of gCMT solutions in that region being unremarkable and likely to be approximately correct.

We report the first detection of unrest at Socompa, Northern Chile, a stratovolcano which has recorded no eruptions since ~7,200 years ago. We measure deformation at and around Socompa using Interferometric Synthetic Aperture Radar (InSAR) observations between Jan 2018 and Oct 2021. We find that, whilst initially inactive, Socompa shows a steady uplift (17.5 mm/yr) from Dec 2019, independently recorded by near-field continuous Global Positioning System (GPS) data. The data can be fit with pressure increase in an ellipsoidal source region stretching from 1.9 to 9.5 km, with a volume change rate of ~5.8×106 m3/yr. Our observations of the onset of uplift preclude the possibility that a nearby Mw 6.8 deep intraslab earthquake on 3rd June 2020 triggered the unrest. The deformation signal we detect indicates the initiation of unrest at Socompa, after at least two decades without measurable deformation, and many thousands of years without volcanic activity.

Internal deformation within the downgoing plate in subduction zones to accommodate the bending of the plate as it starts to subduct is reflected in widespread intraplate seismicity. This seismicity, extending from the outer rise and outer trench slope, down to intermediate depths within the slab, is dominated by the combination of both normal- and thrust-faulting earthquakes reflecting the accumulation and recovery of down-dip curvature. In the idealised case, where all internal deformation is recovered and slabs descend as a straight plate into the deeper mantle, we might expect the seismic moment released in both extension and compression to balance. However, a number of factors may complicate this: the thermal, compositional, and rheological evolution of the slab as it subducts, changes in the proportion of deformation accommodated seismically, and whether the slab undergoes any permanent deformation (e.g., slab necking). Here, we assess earthquake moment release in intraslab settings around the world, focusing on those subduction systems with relatively simple slab geometries. Whilst moment balances for individual regions are often heavily dependent on extreme large-magnitude events, considering the combination of numerous regions around the western Pacific and eastern Indian Ocean indicates that substantially more deformation is accommodated seismically during bending than during unbending, and that in both settings, significantly more moment release reflects down-dip extension than down-dip compression. This suggests that, although the location of seismicity is clearly related to changes in slab curvature, there is a component of permanent, unrecovered down-dip extension in many subducting slabs.