Geophysical Journal International

SummaryFollowing reanalysis of data from 8 seismic networks that operated in the region surrounding the Pampean flat slab during the past several decades, we generated 3D images of Vp, Vs, and Vp/Vs from a combination of arrival times of P and S waves from local earthquakes, and Rayleigh wave dispersion curves from both ambient noise and existing shear wave models. Among the robust features in these images is a low velocity, root-like structure that extends beneath the high Andes to a deflection in the flat slab, which suggests the presence of an overthickened Andean crust rather than a hypothesized continental lithospheric root. Most of the larger scale features observed in both the subducted Nazca plate and the overriding continental lithosphere are related to the intense seismic activity in and around the Juan Fernandez Ridge Seismic Zone (JFRSZ). Vp/Vs ratios beneath, within, and above the JFRSZ are generally lower (∼1.65–1.68) than those in the surrounding Nazca and continental lithosphere (∼1.74–1.80). While the higher continental lithosphere ratios are due to reduced Vs and likely a result of hydration, the lower JFRSZ related ratios are due to reduced Vp and can be explained by increased silica and CO2 originating from beneath the slab, perhaps in concert with supercritical fluid located within the fracture and fault networks associated with the JFR. These and related features such as a region of high Vp and Vs observed at the leading edge of the JFRSZ are consistent with a basal displacement model previously proposed for the Laramide flat-slab event, in which the eroded base of the continental lithosphere accumulates as a keel at the front end of the flat slab while compressional horizontal stresses cause it to buckle. An initial concave up bend in the slab facilitates the infiltration of silica and CO2-rich melts from beneath the slab in a manner analogous to petit spot volcanism, while a second, concave down bend, releases CO2 and supercritical fluid into the overlying continental lithosphere.

SummaryTomographic inversion of traveltime picks from both P-wave and S-wave wide-angle seismic data acquired along and across the Louisville Ridge Seamount Chain (LRSC) provides key insights into its magmatic construction and subsequent subduction-related deformation. Our P-wave velocity-depth models reveal that each seamount along the LRSC comprises an intrusive mafic-ultramafic core that rises within the crust to within 1–2 km of the seabed summit (P-wave velocity, Vp = 5.5–6.5 km s−1; S-wave velocity, Vs < 3.6 km s−1), with each underlain by a crustal root ∼4–5 km thick. Notably, Canopus seamount comprises two adjacent eruptive centres, and our modelling shows that the more northern is currently being internally deformed as it rides up (ascends) the Tonga-Kermadec Trench (TKT)-related plate bending outer rise. Lateral variation in Vs within models along and across the LRSC also primarily reflects subduction-related deformation, with low-velocity regions corresponding to large-scale faulting constrained within the crust. Comparison of pre- and post-LRSC-TKT collision forearc crustal structure indicates that bulk Vp properties recover within ∼50 kyr, whereas Vs structure retains it fault-related fabric for at least ∼740 kyr. Vp/Vs ratios (1.75–1.85) confirm a magmatic origin for all LRSC seamounts, with evidence of localized water-filled cracks due to seawater infiltration along faults, particularly beneath the TKT-ward side of the Osbourn seamount. Estimated water content within the upper crust ranges from 12–15 per cent by weight, decreasing to < 10 per cent in the mid-lower crust, with no evidence of > 12 per cent water content within the Pacific crust being subducted. In comparison with post-collision subduction further north, where the observed upper mantle velocity suggests up to 30 per cent water content, our models suggest that, although deformed and faulted as part of subduction, the LRSC appears more resistant to this deformation than the background Pacific crust adjacent. Our findings provide new constraints on the mechanical and compositional evolution of the LRSC, both prior to and during its collision with the overriding Indo-Australian plate.

SummaryThis research had an initial goal to quantitatively fit and then separate an induced polarization (IP) contribution to extensive ground electromagnetic (EM) data from the Girrilambone area, NSW. A secondary goal identified during the study was to explain why inversion of data from two different EM systems covering the same area each consistently predicted different IP time-constants and chargeabilities. The mineral exploration area was originally surveyed by a 6.25 Hz central loop SIROTEM survey measuring dB/dt. The area was later resurveyed with 1 Hz base-frequency Slingram survey using a Landtem B field sensor. The targets were economic sulphides at depth, with expected signatures being slowly decaying EM responses of small amplitude. Most of the data was affected by inductive IP effects of negative sign, with potential late-delay time EM responses of positive sign obscured. The Girrilambone area surveyed includes the Tritton Mine, discovered in 1995 as a result of the 6.25 Hz SIROTEM survey. To enable the subtraction of IP effects from the EM data, our primary goal, we used the EM data to predict Cole-Cole IP parameters that are consistent with documented values associated with extensive in-situ regolith clay resulting from weathering. The data sets were inverted using a polarisable thin-sheet model that estimated regolith conductivity-thickness or conductance S, chargeability m, IP frequency dependence c and conductivity IP time constant τσ. The thin sheet model was generally able to fit the observed responses, with the fitted IP contribution subtracted from the observed data to produce an ‘IP corrected’ data set of EM data more suitable for the detection of slow decays indicative of sulphide targets. The 6.25 Hz dB/dt data was however modelled with quite different parameters to the1 Hz B field data. The 6.25 Hz IP conductivity time constant was smaller by a factor of 10 while the chargeability was smaller by a factor of more than 2. This initial goal of the research was achieved in that subtraction of the fitted IP contributions in either case improved the capability to identify deeper conductive targets. We are confident that the systematic differences in fitted IP conductivity time constant and chargeability are not due to data or system description error, or to inversion constraints. We conclude that TEM systems will not accurately estimate intrinsic IP conductivity time-constants as rigorously defined from wideband laboratory physical property measurements but rather estimate an IP time-constant whose characteristic frequency (inverse of IP time constant) lies within the bandwidth of the TEM system used. Further, the chargeability estimate will reflect only that fraction of polarizable material whose response is within the bandwidth of the system.

SummaryThe magnitudes of earthquakes are generally described by an empirical relation called the Gutenberg-Richter law. This relation corresponds to a well-known statistical distribution, i.e. the exponential distribution. In this work, we verify the validity of the Gutenberg-Richter law using a 44-year-long worldwide seismic catalog of strong (Mw ≥ 6.5) events, by testing the exponentiality and the independence of the magnitudes. Moreover, we suggest a new way to visualize the distribution of the magnitudes, which complements the classical magnitude frequency distribution plot.