Paleoseismology of the Marquesado-La Rinconada thrust system, Eastern Precordillera of Argentina

Excavated trenches at two sites across the Marquesado–La Rinconada fault system along the eastern Precordilleran front south of San Juan, Argentina, reveal the earthquake history of this rapidly urbanizing region. Interpretation of earthquakes is based on both the generation of colluvial wedges and upward fault terminations, as well as folding events in fine-grained alluvium ponded behind upslope-facing fault scarps. The ages of the past five interpreted earthquakes at the Loma Negra site are E1 at 2.8 ± 2.8 ka, E2 at 7.1 ± 1.5 ka, E3 at 9.6 ± 1.3 ka, E4 at 14.4 ± 2.1 ka, and E5 at 17.2 ± 3.1 ka. At the Jejenes sites, we documented event ages of 2.7 ± 0.1 ka, 3.9 ± 0.6 ka, 5.9 ± 1.3 ka, and 11.4 ± 4 ka. These results indicate that the recurrence interval along the Marquesado–La Rinconada fault zone averages several thousand years. The inferred displacements at the Jejenes site are about 1.1 m for E1, E3, and E4 and 2.1 m for event E2, whereas the displacements at Loma Negra averaged about 1 m, but the most recent event displays less slip. Notably, the older events seem to have been larger and emergent, whereas the youngest event appears to have been smaller and blind in the ponded sediment; this may partially explain the poor expression of classic colluvial wedges associated with some events. Despite the fact that active surface faulting has an uncertain relationship with the primary seismic sources at depth in the crust, past and future events of Mw ∼7.5 are consistent with the length scale of active deformation, the ∼1–2 m slip per event scale of these ruptures, and the size of historical earthquakes

Subsurface signatures and timing of extreme wave events along the Southeast Indian coast

Written history's limitation becomes apparent when attempting to document the predecessors of extreme coastal events in the Indian Ocean, from 550-700 years in Thailand and 1000 years in Indonesia. Detailed ground-penetrating radar (GPR) surveys in Mahabalipuram, southeast India, complemented with sedimentological analyses, magnetic susceptibility measurements, and optical dating provide strong evidence of extreme wave events during the past 3700 years. The diagnostic event signatures include the extent and elevation of the deposits, as well as morphologic similarity of buried erosional scarps to those reported in northern Sumatra region. Optical ages immediately overlying the imaged discontinuities that coincides with high concentration of heavy minerals date the erosional events to 340 ± 35, 350 ± 20, 490 ± 30, 880 ± 40, 1080 ± 60, 1175 ± 188, 2193 ± 266, 2235 ± 881, 2489 ± 293, 2450 ± 130, 2585 ± 609, 3710 ± 200 years ago. These evidences are crucial in reconstructing paleo extreme wave events and will pave the way for regional correlation of erosional horizons along the northern margin of Indian Ocean

Accelerating slip rates on the Puente Hills blind thrust fault system beneath metropolitan Los Angeles, California, USA

Slip rates represent the average displacement across a fault over time and are essential to estimating earthquake recurrence for probabilistic seismic hazard assessments. We demonstrate that the slip rate on the western segment of the Puente Hills blind thrust fault system, which is beneath downtown Los Angeles, California (USA), has accelerated from ∼0.22 mm/yr in the late Pleistocene to ∼1.33 mm/yr in the Holocene. Our analysis is based on syntectonic strata derived from the Los Angeles River, which has continuously buried a fold scarp above the blind thrust. Slip on the fault beneath our field site began during the late-middle Pleistocene and progressively increased into the Holocene. This increase in rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time. This challenges the characteristic earthquake model and presents an evolving and potentially increasing seismic hazard to metropolitan Los Angeles

Towards quantifying beta microdosimetric effects in single-grain quartz dose distribution

In luminescence dating, the single grain approach offers a promise for identification of the most bleached grains for age calculations. A proper interpretation of single grain dose distribution however is still awaited. Ideally the palaeodose distribution should be a sharply peaked distribution with its width determined by the experimental errors. However, a large range of values of the relative standard deviation (RSD), even for well-bleached samples, indicates the presence of additional effects, not considered so far in the literature. We suggest that microscopic fluctuations in the spatial distribution of feldspar containing 40K β emitters (termed as hotspots) can cause heterogeneous distribution of dose rate. This paper models these fluctuations and quantifies their effect on the distribution of doses in quartz. In this approach the dose to a quartz grain from a given spatial configuration of the hotspots within a sphere of maximum beta range is computed and this is then averaged over all possible configurations, to arrive at the dose rate/palaeodose distribution. The dose distribution is positively skewed. The net effect of such a distribution is estimated as a function of potassium concentration (lower the potassium, higher the hotspot heterogeneity and larger is the palaeodose distribution). For well bleached samples with low potassium content, the computations suggest that: (a) the probability of grains receiving zero doses is negligible and (b) distribution of doses is large. We suggest a minimum dose concept through a percentile analysis of the distribution function, viz., as the lowest doses received by a small fraction of the grains. This study therefore may imply a paradigm shift in the manner, in which the ages are computed from single grain paleodose distributions

DataSheet1_Paleoseismology of the Marquesado-La Rinconada thrust system, Eastern Precordillera of Argentina.PDF

Excavated trenches at two sites across the Marquesado–La Rinconada fault system along the eastern Precordilleran front south of San Juan, Argentina, reveal the earthquake history of this rapidly urbanizing region. Interpretation of earthquakes is based on both the generation of colluvial wedges and upward fault terminations, as well as folding events in fine-grained alluvium ponded behind upslope-facing fault scarps. The ages of the past five interpreted earthquakes at the Loma Negra site are E1 at 2.8 ± 2.8 ka, E2 at 7.1 ± 1.5 ka, E3 at 9.6 ± 1.3 ka, E4 at 14.4 ± 2.1 ka, and E5 at 17.2 ± 3.1 ka. At the Jejenes sites, we documented event ages of 2.7 ± 0.1 ka, 3.9 ± 0.6 ka, 5.9 ± 1.3 ka, and 11.4 ± 4 ka. These results indicate that the recurrence interval along the Marquesado–La Rinconada fault zone averages several thousand years. The inferred displacements at the Jejenes site are about 1.1 m for E1, E3, and E4 and 2.1 m for event E2, whereas the displacements at Loma Negra averaged about 1 m, but the most recent event displays less slip. Notably, the older events seem to have been larger and emergent, whereas the youngest event appears to have been smaller and blind in the ponded sediment; this may partially explain the poor expression of classic colluvial wedges associated with some events. Despite the fact that active surface faulting has an uncertain relationship with the primary seismic sources at depth in the crust, past and future events of Mw ∼7.5 are consistent with the length scale of active deformation, the ∼1–2 m slip per event scale of these ruptures, and the size of historical earthquakes.</p