The theme for this week’s sustainability research is EARTHQUAKE
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Research in Details
Research #1
Late Quaternary transpressional earthquakes on a long-lived intraplate fault: A case study of the Southern Yangsan Fault, SE Korea
Highlights
The NNE–SSW-striking Yangsan Fault in southeastern is one of the most prominent seismogenic structure.
Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicates recent rupture(s)
Contact of the oldest dated unit that has at least 7.5 m of vertical separation is used to calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr
Authors:Youngbeom Cheon, Jin-Hyuck Choi, Namgwon Kima, Hoil Lee, Iyre Choi, Hankyung Bae, Thomas K. Rockwell, Seung Ryeol Lee, Chung-Ryul Ryoo, Hanwoo Choi, Tae-Ho Lee
Date of publication: 25 JULY 2020
Summary
The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula on the basis of instrumental and historical seismicity, and paleoseismic records along the fault zone. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We conducted a detailed paleoseismic investigation, including an 8 m-deep excavation, in order to understand recent earthquake-faulting events along the Southern Yangsan Fault. Our observations indicate that at least two and possibly four dominantly strike-slip surface-faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late Pleistocene. Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicate that the most recent rupture(s) occurred after 29.2 ± 1.4 ka and the timing of earlier ruptures are constrained to between 70.0 ± 3.7 ka and 29.2 ± 1.4 ka. Using a contact of the oldest dated unit that has at least 7.5 m of vertical separation, we calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr. The minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core. We also propose that the late Quaternary earthquake-faulting kinematics along the Yangsan Fault, expressed as contractional dextral slip with east-side-up geometry, is strongly dependent on a pre-existing fault zone architecture with a strike of N10–20°E that dips to the east, and the direction of neotectonic maximum horizontal stress (ENE–WSW to E–W).
Keywords: Yangsan fault, Late quaternary, Paleo-earthquake, OSL age, Transpression
Research #2
Fault geometry beneath the western and Central Marmara Sea, Turkey, based on ocean bottom seismographic observations: Implications for future large earthquakes
Highlights:
A segmentation boundary on the Main Marmara Fault is identified beneath the Central Basin.
Both inner and outer boundary faults of the Central Basin are developed in the western Central Basin.
Zones of no seismicity beneath the Kumburgaz Basin have the potential for a large earthquake.
The focal area of the September 2019 M 5.7 event was close to the Main Marmara Fault.
Authors: Yojiro Yamamoto, Dogan Kalafat, Ali Pinar, Narumi Takahashi, Zeynep Coskun, Remzi Polat, Yoshiyuki Kaneda, Haluk Ozener
Date of publication: 21 JULY 2020
Summary
Beneath the Marmara Sea, Turkey, the Main Marmara Fault (MMF), the offshore part of the North Anatolian Fault (NAF), is a well-known seismic gap for future M > 7 earthquakes. However, its detailed fault geometry and microearthquake activity have been debated for several decades. Using data acquired from long-term ocean bottom seismograph (OBS) observations, we made precise hypocenter estimations based on 3-D Vp and Vs velocity structures and assessed the fault geometry beneath the western and central parts of the MMF. The results indicate a segmentation boundary between the near-vertical western part and the south-dipping eastern part located around 28.10°E. Enriched OBS locations indicate microseismicity along both the inner and outer boundary faults of the Central Basin, especially on the western side. A comparison with previously published results suggests that the seismicity pattern has not changed for at least two years, between 2014 and 2016. Using a combined dataset of this and previous studies, lateral variations in the dip angle along the MMF fault segment from 27.4°E to 28.8°E were investigated. Based on this, we depicted on-fault seismicity along the MMF and defined three inactive areas of microseismicity. Two are located in the western segment, corresponding to the rupture area of the 1912 Ms. 7.4 earthquake, and the other, the largest, is located on the eastern segment. From a comparison of previous seismic and geodetic studies, it is considered that this area is a fully locked zone and has the potential for large earthquakes. Having compared the difference between hypocenter locations determined from OBSs and land-based stations, it is proposed that the epicentral locations of the mainshock and aftershocks of the September 26, 2019, M 5.7 earthquake are located much closer to the MMF than locations reported from only land-based results.
Keywords: Main Marmara Fault, Seismicity, Fault geometry, Ocean bottom seismographic observation
Research #3
Liquefaction source layer for sand blows induced by the 2016 megathrust earthquake (Mw 7.8) in Ecuador (Boca de Briceño)
Highlights
Sand boil composition provides constraints in the recognition of buried source layer.
Earthquake-induced liquefaction affects angular sand with abundant non-plastic silt.
The study contributes in evaluating the success of liquefaction mitigation project.
Authors: Aura C. Salocchi, Luca Minarelli, Stefano Lugli, Sara Amorosob, Kyle M.Rollins, Daniela Fontana
Date of publication: 16 JULY 2020
Summary
Numerous sand boils were generated in the alluvial plain at the mouth of the Rio Briceño valley (Ecuador) during the Mw 7.8 earthquake of April 2016. The area is characterized by a series of raised marine terraces formed as a consequence of the rapid tectonic coastal uplift during the Quaternary. Boreholes and geotechnical investigations were carried during post-earthquake surveys and for the purpose of mitigating the liquefaction effects. Five lithological units were identified at a site of embankment, which represented continental-marine and transitional sedimentation since the Last Glacial Maximum. A comprehensive study of texture and petrographic composition of sand boils has been performed and compared with sandy silts and silty sands of the buried sedimentary sequence in order to identify the source levels for liquefaction. The petrographic components, in particular the low content of bioclasts and carbonate fragments of the sand boils, allow to pinpoint a source layer made up of fine-grained silty sands located between 2 and 4.5 m depth (Unit 2) whereas the deeper marine sands, richer in bioclasts, were not involved. The results support the idea that earthquake-induced liquefaction phenomena are not restricted to clean sands and well-sorted deposits, but may affect sand layers with significant amount of non-plastic silt.
Keywords: Earthquake-induced liquefaction, Sand blows, Ecuadorian coast, Sand composition, Holocene depositional sequences
Research #4
Impact of topography on earthquake static slip estimates
Highlights:
Topography has a non-negligible impact on estimated coseismic slip models.
Impact of topography is greater when data constraints are limited.
Accounting for topography has a significant effect where gradients are high.
Zeroth-order correction is not effective when the topographic effect is significant.
Authors: Leah Langer, Théa Ragonb, Anthony Sladen, Jeroen Tromp
Date of publication: 16 JULY 2020
Summary
Our understanding of earthquakes is limited by our knowledge, and our description, of the physics of the Earth. When solving for subsurface fault slip, it is common practice to assume minimum complexity for characteristics such as topography, fault geometry and elastic properties. These characteristics are rarely accounted for because our knowledge of them is often partial and they can be difficult to include in simulations. However, topography and bathymetry are known all over the Earth's surface, and recently developed software packages such as SPECFEM-X have simplified the process of including them in calculations. Here, we explore the impact of topography on static slip estimates. We also investigate whether the influence of topography can be accounted for with a zeroth-order correction which accounts for variations in distance between subfaults and the surface of the domain. To this end, we analyze the 2015 Mw 7.5 Gorkha, Nepal, and the 2010 Mw 8.8 Maule, Chile earthquakes within a Bayesian framework. The regions affected by these events represent different types of topography. Chile, which contains both a deep trench and a major orogen, the Andes, has a greater overall elevation range and steeper gradients than Nepal, where the primary topographic feature is the Himalayan mountain range. Additionally, the slip of the continental Nepal event is well-constrained, whereas observations are less informative in a subduction context. We show that topography has a non-negligible impact on inferred slip models. Our results suggest that the effect of topography on slip estimates increases with limited observational constraints and high elevation gradients. In particular, we find that accounting for topography improves slip estimates where topographic gradients are large. When topography has a significant impact on slip, the zeroth-order correction is not sufficient.
Keywords: Earthquake source observations, Inverse theory, Probability distributions, Earthquake modeling, Topography, Maule earthquake, Gorkha earthquake
Research #5
Impact of power-law rheology on the viscoelastic relaxation pattern and afterslip distribution following the 2010 Mw 8.8 Maule earthquake
Highlights
Six years of postseismic displacements after the 2010 Maule earthquake from GPS data.
Combination of 3D forward modelling with power-law rheology and afterslip inversion.
Inverted deep afterslip patterns strongly depend on the choice of rheology model.
Continental lower crustal viscoelastic relaxation reduces need for deep afterslip.
Use of aftershock activity to discriminate simulations.
Authors: Carlos Peña, Oliver Heidbach, Marcos Moreno, Jonathan Bedford, Moritz Ziegler, Andrés Tassara, Onno Oncken
Date of publication: 15 JULY 2020
Summary
After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip or simply afterslip processes. Below approximately 60 km depth, the seismic moment release at the plate interface is quite small indicating that the shear strength is low and stable sliding is the prevailing process. This agrees with the lack of significant interseismic locking at deeper segments (>60 km) resulting from the inversion of geodetic data and thus low afterslip can be expected. However, inversion models that employ linear viscoelastic mantle rheology and an elastic crust result in significant afterslip at depths >60 km. In this paper, we present a combination of a 3D forward geomechanical model with power-law rheology that simulates postseismic relaxation with dislocation creep processes in the crust and upper mantle and an afterslip inversion. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 Mw 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between the observed displacements from the continuous GPS (cGPS) and the ones from the forward modelling. We investigate five simulations, four with different dislocation creep parameters for the crust, slab, and upper mantle and one with elastic properties for the crust and slab, and a linear viscoelastic upper mantle for comparison. Our preferred simulation considers a weak crust since it shows the best fit to the cumulative cGPS postseismic displacements, a good fit to the time-series, and, in particular, a good spatial correlation between afterslip and aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths >60 km, which correlates to the low cumulative seismic moment that is released from aftershocks at these depths. Furthermore, the cumulative afterslip moment release from this simulation corresponds to 10% of the main shock in six years, which is approximately half of the moment release that results from models with an elastic crust and linear viscosity in the upper mantle. We conclude that an integrated analysis by considering power-rheology with dislocation creep processes in the continental crust and upper mantle along with aftershock activity may be used to constrain location and magnitude postseismic relaxation processes better.
Keywords: 2010 Maule earthquake, postseismic deformation, numerical modelling, afterslip, power-law rheology, GPS