Coseismic Vertical Displacements Research Articles

Late quaternary faulted landforms and recent activity of the eastern segment of the Nanbei River–Boli Fault

The Nanbei River–Boli Fault is one of the largest NW-trending faults in Northeast China. However, limited data on the style and timing of recent earthquake activity on this fault greatly restrict the accuracy of seismic risk assessment. Here, we provide evidence of late Pleistocene to Holocene activity of the eastern segment of this fault for the first time. Specifically, the eastern segment of the Nanbei River–Boli Fault has experienced multiple earthquakes since the late Quaternary. Trench excavations indicate a recent strong earthquake at 7.28–2.11 ka, with approximately 1 m of co-seismic vertical displacement and preliminary estimates of the minimum vertical slip rates of 0.07–0.12 mm/a. According to a comprehensive analysis, we estimate that the eastern segment of the Nanbei River–Boli Fault can generate strong earthquakes of magnitude 7.0. This research provides important information for seismic risk assessments and seismic zoning, which can help seismic hazard assessment and mitigate their effects in Northeast China.

Linked and fully coupled 3D earthquake dynamic rupture and tsunami modeling for the Húsavík–Flatey Fault Zone in North Iceland

Abstract. Tsunamigenic earthquakes pose considerable risks, both economically and socially, yet earthquake and tsunami hazard assessments are typically conducted separately. Earthquakes associated with unexpected tsunamis, such as the 2018 Mw 7.5 strike-slip Sulawesi earthquake, emphasize the need to study the tsunami potential of active submarine faults in different tectonic settings. Here, we investigate physics-based scenarios combining simulations of 3D earthquake dynamic rupture and seismic wave propagation with tsunami generation and propagation. We present time-dependent modeling of one-way linked and 3D fully coupled earthquakes and tsunamis for the ∼ 100 km long Húsavík–Flatey Fault Zone (HFFZ) in North Iceland. Our analysis shows that the HFFZ has the potential to generate sizable tsunamis. The six dynamic rupture models sourcing our tsunami scenarios vary regarding hypocenter location, spatiotemporal evolution, fault slip, and fault structure complexity but coincide with historical earthquake magnitudes. Earthquake dynamic rupture scenarios on a less segmented fault system, particularly with a hypocenter location in the eastern part of the fault system, have a larger potential for local tsunami generation. Here, dynamically evolving large shallow fault slip (∼ 8 m), near-surface rake rotation (± 20∘), and significant coseismic vertical displacements of the local bathymetry (± 1 m) facilitate strike-slip faulting tsunami generation. We model tsunami crest to trough differences (total wave heights) of up to ∼ 0.9 m near the town Ólafsfjörður. In contrast, none of our scenarios endanger the town of Akureyri, which is shielded by multiple reflections within the narrow Eyjafjörður bay and by Hrísey island. We compare the modeled one-way linked tsunami waveforms with simulation results using a 3D fully coupled approach. We find good agreement in the tsunami arrival times and location of maximum tsunami heights. While seismic waves result in transient motions of the sea surface and affect the ocean response, they do not appear to contribute to tsunami generation. However, complex source effects arise in the fully coupled simulations, such as tsunami dispersion effects and the complex superposition of seismic and acoustic waves within the shallow continental shelf of North Iceland. We find that the vertical velocity amplitudes of near-source acoustic waves are unexpectedly high – larger than those corresponding to the actual tsunami – which may serve as a rapid indicator of surface dynamic rupture. Our results have important implications for understanding the tsunamigenic potential of strike-slip fault systems worldwide and the coseismic acoustic wave excitation during tsunami generation and may help to inform future tsunami early warning systems.

Slip-rates and time recurrences of the seismogenic Sant’Eufemia normal fault (SE Tyrrhenian Sea), a multiscale and multidisciplinary approach

Time recurrences and slip rates of paleoearthquakes along offshore seismogenic faults represent one of the key challenges that the scientific community is facing off. Here we provide an integrated study combining geophysical, sedimentological and geochemical data in order to define slip rates and time recurrences of the Sant’Eufemia normal Fault (SE Tyrrhenian Sea). We have documented the paleo-events associated to this geological structure and reconstructed the paleoenvironmental history. From post-LGM (ca. 20,484 yr BP) to Present, four main events (EQ1, EQ2, EQ3, and EQ4) and the historical 1905 Calabria earthquake have been recognized. Time recurrence ranges from 4.68 ± 0.94 to 7.322 ± 1.84 ky (mean value of 5.4 ky), while the co-seismic vertical displacement ranges from 0.21 to 1.085 m (mean value of 0.68 m), indicating that slip rates change along the fault plane moving from the central part to the lateral edges. In this work we have defined a new class of active fault, here named “ultra-slow”, that allowed to extend the range of time recurrences previously provided for the main earthquakes affecting the Southern Apennines. Phenomena that may be related with the oceanic slab sinking beneath the Calabrian arc.

Surface ruptures and off-fault deformation of the October 2016 central Italy earthquakes from DInSAR data

Large magnitude earthquakes produce complex surface deformations, which are typically mapped by field geologists within the months following the mainshock. We present detailed maps of the surface deformation pattern produced by the M. Vettore Fault System during the October 2016 earthquakes in central Italy, derived from ALOS-2 SAR data, via DInSAR technique. On these maps, we trace a set of cross-sections to analyse the coseismic vertical displacement, essential to identify both surface fault ruptures and off-fault deformations. At a local scale, we identify a large number of surface ruptures, in agreement with those observed in the field. At a larger scale, the inferred coseismic deformation shows a typical long-wavelength convex curvature of the subsiding block, not directly recognizable in the field. The detection of deformation patterns from DInSAR technique can furnish important constraints on the activated fault segments, their spatial distribution and interaction soon after the seismic events. Thanks to the large availability of satellite SAR acquisitions, the proposed methodological approach can be potentially applied to worldwide earthquakes (according to the environmental characteristics of the sensed scene) to provide a wider and faster picture of surface ruptures. Thus, the derived information can be crucial for emergency management by civil protection and helpful to drive and support the geological field surveys during an ongoing seismic crisis.

Co-Seismic Ionospheric Disturbances Following the 2016 West Sumatra and 2018 Palu Earthquakes from GPS and GLONASS Measurements

The study of ionospheric disturbances associated with the two large strike-slip earthquakes in Indonesia was investigated, which are West Sumatra on 2 March 2016 (Mw = 7.8), and Palu on 28 September 2018 (Mw = 7.5). The anomalies were observed by measuring co-seismic ionospheric disturbances (CIDs) using the Global Navigation Satellite System (GNSS). The results show positive and negative CIDs polarization changes for the 2016 West Sumatra earthquake, depending on the position of the satellite line-of-sight, while the 2018 Palu earthquake shows negative changes only due to differences in co-seismic vertical crustal displacement. The 2016 West Sumatra earthquake caused uplift and subsidence, while the 2018 Palu earthquake was dominated by subsidence. TEC anomalies occurred about 10 to 15 min after the two earthquakes with amplitude of 2.9 TECU and 0.4 TECU, respectively. The TEC anomaly amplitude was also affected by the magnitude of the earthquake moment. The disturbance signal propagated with a velocity of ~1–1.72 km s−1 for the 2016 West Sumatra earthquake and ~0.97–1.08 km s−1 for the 2018 Palu mainshock earthquake, which are consistent with acoustic waves. The wave also caused an oscillation signal of ∼4 mHz, and their azimuthal asymmetry of propagation confirmed the phenomena in the Southern Hemisphere. The CID signal could be identified at a distance of around 400–1500 km from the epicenter in the southwestern direction.

Insights on the small tsunami from January 28, 2020, Caribbean Sea MW7.7 earthquake by numerical simulation and spectral analysis

A major left-lateral strike-slip Mw7.7 earthquake occurred in the vicinity of the Caribbean Sea on January 28, 2020. As a result, a small-scale tsunami was generated. The properties of the seismogenic source were described using observational data gathered for the earthquake and tsunami, as well as information on the regional tectonic setting. The tsunami was simulated with the COMCOT model and Okada’s dislocation model from finite fault solutions for MW7.7 Caribbean Sea earthquakes published by the United States Geological Survey. The simulation results were compared to tide gauge records to validate whether the seafloor’s vertical displacement generated by the strike-slip fault caused a small-scale tsunami. We conducted a spectral analysis of the tsunami to better understand the characteristics of tsunami records. The tsunami simulation results showed that the co-seismic vertical displacement caused by a strike-slip MW7.7 earthquake could have contributed to the small-scale tsunami, but the anomalously large high-frequency tsunami waves recorded by the George Town tide gauge 11 min after the earthquake were unrelated to the earthquake-generated tsunami. According to the spectrum analysis, the predominant period of noticeable high-frequency tsunami waves recorded by the George Town tide gauge occurred only two minutes after the earthquake. This indicates that the source of the small-scale tsunami was close to the George Town station and the possible tsunami source was 150 km away from George Town station. These facts suggest that a submarine landslide was caused by the strike-slip earthquake. The comprehensive analysis showed that the small-scale tsunami was not caused solely by co-seismic seafloor deformation from the strike-slip event but that an earthquake-triggered submarine landslide was the primary cause. Therefore, the combined impact of two sources led to the small-scale tsunami.

Interpretation of spatiotemporal gravity changes accompanying the earthquake of 21 August 2017 on Ischia (Italy)

We analyse spatiotemporal gravity changes observed on the Ischia island (Italy) accompanying the destructive earthquake of 21 August 2017. The 29 May 2016 to 22 September 2017 time-lapse gravity changes observed at 18 benchmarks of the Ischia gravimetric network are first corrected for the gravitational effect of the surface deformation using the deformation-induced topographic effect (DITE) correction. The co-seismic DITE is computed by Newtonian volumetric integration using the Toposk software, a high-resolution LiDAR DEM and the co-seismic vertical displacement field derived from Sentinel-1 InSAR data. We compare numerically the DITE field with its commonly used Bouguer approximation over the island of Ischia with the outcome that the Bouguer approximation of DITE is adequate and accurate in this case. The residual gravity changes are then computed at gravity benchmarks by correcting the observed gravity changes for the planar Bouguer effect of the elevation changes at benchmarks over the same period. The residual gravity changes are then inverted using an inversion approach based on model exploration and growing source bodies, making use of the Growth-dg inversion tool. The found inversion model, given as subsurface time-lapse density changes, is then interpreted as mainly due to a co-seismic or post-seismic disturbance of the hydrothermal system of the island. Pros and weak points of such interpretation are discussed.

Distributed displacement on the Papatea fault from the 2016 Mw 7.8 Kaikōura earthquake and implications for hazard planning

ABSTRACT The distribution of ground motion, displacement, and secondary hazards around active faults is defined by the interaction of tectonic and site characteristics including fault kinematics. As a result of this complexity, recommendations for fault setback distances or avoidance zones are necessarily simplified. We observed distributed vertical coseismic displacement from the 2016 Mw 7.8 Kaikōura earthquake up to c. 500 m on either side of the sinistral-reverse Papatea fault. On average, c. 32% (2.13 m) of vertical displacement was measured at distances >50 m from the Papatea fault. Additionally, in places, there is strong asymmetry where displacement was accommodated over a wider area in the hanging wall of the fault, a common observation around many dip-slip fault ruptures globally. We compare the distribution of displacement around the Papatea fault to the current fault avoidance zone and find that increasing the size of the fault avoidance zone by 10 m captures 11% more area with a high gradient of vertical displacement. Given these results and similar findings for other faults globally, we recommend that existing standards for fault avoidance zones be evaluated to ensure their ongoing efficacy.

Middle to Late Pleistocene faulting history of the Heerlerheide fault, Roer Valley Rift System, influenced by glacio-isostasy and mining-induced displacement

Faults of the Roer Valley Rift System (RVRS) are characterized by seismicity, scarps and displaced fluvial terraces, showing that they are active. The Heerlerheide fault is part of the southern boundary fault system of the RVRS, the Feldbiss fault zone (FFZ). During the late 19th and first half of the 20th century coal was mined in the subsurface south of the FFZ. As a result, general subsidence, sinkholes, and fault scarplets appeared on the surface. One of the induced fault scarps coincides with the location of the Heerlerheide fault, indicating that the upper part of the fault was reactivated by the mining. A trench was opened across the fault to study the mining-induced fault reactivation as well as the tectonic fault displacement history.The Heerlerheide fault offsets a 340 ka Meuse river terrace overlain by loess and loess-like deposits of Eemian to Late Pleniglacial age, including the Rocourt soil, the Eben/Patina discordance and the Nagelbeek soil complex. At least three tectonic faulting events were reconstructed, which are most likely surface rupturing earthquakes. However, the two oldest displacements could represent multiple faulting events. The age of the youngest event is well constrained between 17 and 15 ka. The duration of the time interval between the penultimate and youngest events is at least 8 ky (the inter-event time is an important parameter for assessing seismic hazard). The vertical coseismic displacement of the youngest event was around ∼0.25 m; the estimated moment magnitude is around 6.2. This event is more or less synchronous with the age of events found in other fault trench studies of the FFZ. The timing is roughly contemporaneous with surface rupturing earthquakes along the northern boundary fault zone of the RVRS, the Peel Boundary fault zone (PBFZ), and a phase of volcanism in the nearby Eifel area, suggesting a common mechanism. The timing also corresponds to the start of the glacio-isostatic forebulge collapse, which has been invoked before to explain earthquake events in the RVRS, and in northern Germany and Denmark.Previous studies provided evidence for a Holocene surface rupturing earthquake with an offset of about 1 m at the Geleen fault in the Meuse valley on Belgian territory, taking place between 2.5 ± 0.3 and 3.1 ± 0.3 ka. The Geleen and Feldbiss faults on Dutch territory experienced faulting during the Late Glacial – Holocene. The incision and deposition history of a brook crossing these faults suggests an age around 7.5 ka for this event. However, in contrast, our results for the Heerlerheide fault show no evidence for Late Glacial – Holocene tectonic fault activity.We also observed 0.34 m of vertical displacement of the base of the plough layer, which corresponds to the amount to what was observed at the surface during the mining in 1936, indicating that no fault motions have occurred afterwards, despite re-flooding of the mines and consequent surface rebound. However, the fact that the fault was reactivated by subsurface mining shows that it is a weakness zone in the subsurface, and therefore fault reactivation might occur due to still ongoing rebound. In contrast to the tectonic fault displacement, the mining-induced offset was accompanied by downslope movement of the upper part of the hanging wall, resulting in splaying of the fault tip and crack formation.

Seismic source characteristics of the 2016 Pedernales-Ecuador earthquake (Mw 7.8)

The 2016 Pedernales earthquake (Mw 7.8), is part of the sequence of large earthquakes in Ecuador and Colombia: 1906 (~8.8 Mw), 1942 (Mw 7.8), 1958 (Mw 7.7), 1979 (Mw 8.2), all these events with a focal mechanism of the thrust fault type. In this research, we have obtained the seismic source distribution (indicating the presence of 2 main asperities), from inversion of teleseismic signals, as well as the parameters of the seismic rupture process: focal mechanism that indicates a type of inverse fault (strike = 25∘, dip = 12∘, rake = 118∘) for a focal depth of 23 km and the source time function indicating a composite event, the duration of the rupture process was around 70 s, assuming an average rupture velocity of 3.0 km/s. The seismic moment was calculated at 5.77×1020 Nm, equivalent to a magnitude of Mw 7.8. The smallest asperity is located around the epicentre and the main asperity is located around the coastline to the south-west from the epicentre. The maximum dislocation or slip was 3.75 m. The coseismic vertical displacement is not conductive to the generation of a large tsunami, as it is around 1 m only. The zone of Pedernales have a Coulomb stress transfer of 22 bar, therefore, this area is loaded with stresses and is a potentially seismic zone for an upcoming earthquake.

Role of landslides on the volume balance of the Nepal 2015 earthquake sequence

The 7.8 Mw earthquake that struck Nepal on April 25th, 2015 triggered over 21,000 landslides over an area of more than 25,000 km2. These landslides contributed to mass wasting, partially compensating the tectonic uplift by the earthquake. In this paper we quantify the volume balance resulting from the 2015 earthquake uplift (or subsidence) and landslide erosion. Starting from a new complete earthquake-induced landslide inventory, we calculated landslide volume by adopting different strategies for low-mobility and high-mobility landslides, considering also the potential supply of sediments to the drainage network. The results show that the contribution of earthquake-induced landslides to erosion is about one order of magnitude smaller than the vertical coseismic displacement. We found landslide volume values, due to the 2015 Nepal earthquake, ranging between 251 (− 15/ + 16) Mm3 up to 1503 (− 183/ + 210) Mm3 based on the adopted method, and a volume due to coseismic vertical displacement of 2134 (± 1269) Mm3 for the whole area. The volume balance of the 2015 Nepal earthquake is strongly dominated by tectonic displacement. We show that these estimates depend on several uncertainties. We identified and quantified uncertainties related to: (1) the choice of empirical volume-area scaling relationships and their parameters; (2) the completeness and quality of landslide inventory through comparison with available inventories; (3) the approach adopted for the assessment of elongated landslide volume; (4) the InSAR displacement data.

Seiche Effects in Lake Tekapo, New Zealand, in an Mw8.2 Alpine Fault Earthquake

This study investigates the potential for seismic seiches in Lake Tekapo, New Zealand, triggered by ground shaking from an Mw8.2 Alpine Fault earthquake. Synthetic ground motions are used as a forcing boundary to drive lake water motions by further developing a tsunami simulation model—COMCOT—and coupling it with earthquake simulation model outputs. Our modelling results reveal that lake water oscillations are mobilised immediately by the ground movement and further amplified by cross-lake seiches. Amplitudes of lake oscillations can reach up to 4.0 m in the lake’s narrow southern arm, over 1.0 m along the shore of Lake Tekapo township, and about 1.5–2.5 m along many other parts of the lake shore. Large-amplitude water oscillations quickly attenuate in the first 5–10 min after the earthquake due to their relatively short periods, while long-period oscillations continue for a long time, albeit with much smaller amplitudes. Spectral analysis clearly reveals that the ground motions trigger both fundamental and higher modes in the lake whose oscillation periods are consistent with theoretical estimates. We find that large-amplitude lake water oscillations are better correlated with low-frequency, less energetic ground motion content than with high-frequency large-amplitude ground motions. Ground motion-triggered lake oscillations are large enough to pose potential threat to tourists, residents, boats and infrastructure both in the lake water and onshore near the waterfront. In contrast, vertical co-seismic displacements in the lake area, the conventional mechanism for tsunami generation, are too small to trigger tsunami waves of concern.

The Zagreb (Croatia) M5.5 Earthquake on 22 March 2020

On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16–0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area.

Seismic history of a bedrock fault scarp using quantitative morphology together with multiple dating methods: A case study of the Luoyunshan piedmont fault, southwestern Shanxi Rift, China

The study of bedrock fault scarps is a useful approach to identify paleo-earthquakes and acquire the faulting history in bedrock areas where traditional trench techniques are not applicable. Here, we report a study on the Luoyunshan piedmont fault (LYSPF), southwestern Shanxi Rift, China. Although several studies have analyzed the LYSPF, its faulting behavior is still under debate. For example, the tectonic geomorphology fails to determine the frequency of past slips and the elapsed time. To address these problems, we investigated two bedrock fault surfaces, Luoyuncun (northern segment) and Weicun (southern segment) using quantitative morphology analyses together with multiple Quaternary dating techniques. Quantitative morphology analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation. This kind of segmentation indicates that the fault surfaces are exhumed intermittently, likely owing to periodic faulting earthquakes. Therefore, earthquake events can be identified as morphological segments, and the corresponding co-seismic displacement can be determined by the height of these segments. According to the results from the quantitative morphology and dating techniques, four seismic events, with a co-seismic vertical displacement of 0.9 m, 0.82 m, 1.64 to 2.38 m and 2.30 to 2.54 m, on the northern segment occurred during the late Pleistocene, whereas, two seismic events, with a co-seismic vertical displacement of ~1.33 m and 1 to 1.08 m are identified on the southern segment occurred during the Holocene. Combined with the results of knickpoint series and trench techniques of the LYSPF, eight paleo-earthquake events were identified since 45 ka, with an average recurrence interval of 5.8 kyr. The more complete paleo-earthquake sequence also showed the possibility that the variation of the slip rate along the LYSPF depends on the segment, and that the displacement produced by the Linfen M 7½ earthquake of 649 CE has been recorded at the WC fault surface.

熊本県西原村布田における布田川断層の2016年熊本地震（Mw7.0）時上下変位と活動性

熊本県西原村布田における布田川断層の2016年熊本地震（Mw7.0）時上下変位と活動性

The influence of topography and crustal heterogeneity on coseismic deformation of the 2008 Mw7.9 Wenchuan Earthquake, China

The Longmen Shan Thrust Belt, which lies at the convergence of the eastern Tibetan Plateau and Sichuan Basin, is characterized by a topographic relief of 4000 m over a distance of 100 km. While the rock underlying the Sichuan Basin is strong, the crust underlying the Tibetan Plateau is weak. Four models that simulate deformation caused by distributions of dislocations along a geometrically complex fault system are developed to investigate how the contrast in elevational and crustal heterogeneity influences coseismic deformation of the 2008 Mw7.9 Wenchuan earthquake. The models are configured with an assembly precise ramp-décollement structures and planar fault geometries. The coseismic slip distribution is estimated by inverting three component GPS observations with Green's functions calculated for each model configuration. By comparing the slip distribution and residuals among the displacement predictions from these four models and measurements of GPS, the crustal heterogeneity model robustly explained most of the observed GPS horizontal and vertical coseismic displacements. Results revealed four composite dislocation asperities distributed in the shallow crust and a slip of 4–8 m on the sub-surface décollement fault with a depth of ~20 km. The resulting geodetic moment is 8.10 × 1020 Nm, which is in agreement with the seismological estimates. The deep slip predictions obtained with the ramp-décollement fault geometry indicate that the eastern Tibetan Plateau was thrust over the Sichuan Basin rather than the plateau being uplifted owing to the underlying weak crust.

Introducing a vertical land motion model for improving estimates of sea level rates derived from tide gauge records affected by earthquakes

We reassess the absolute and relative sea level changes at 38 tide gauge stations in the earthquake-affected Western North Pacific for the 1993–2015 period, focusing on the vertical land motion (VLM) which is crucial for narrowing the gap between these estimates. In this area, simply discarding all earthquake-affected sites, one overestimates the average regional sea level rise by more than 0.5 mm/year. Disregarding VLM would lead to misestimating local sea level trends between 0.2 and 7.6 mm/year. If accounted for, but modeled as linear during the entire time span, VLM leads to regional absolute sea level rise errors of up to 0.4 mm/year. Therefore, we introduce a new methodology that better represents the Global Positioning System (GPS)-derived nonlinear VLM by accounting for co-seismic offsets, changes in the vertical velocities and post-seismic transient. Also, for the first time, a combination of white and power-law noises is added to this nonlinear model to derive proper uncertainties of VLM. We find a maximum difference of 15.3 mm/year between pre- and post-seismic vertical velocities. The GPS-sensed vertical co-seismic displacement approaches 36 mm. Assuming the changes in vertical velocities and displacement due to the tectonic movements is not accounted for, and then, estimating absolute sea level rise from tide gauges can result in an error of 10 mm/year. Introducing a new nonlinear VLM model improves absolute tide gauge sea level estimates by 20% on average. Finally, for the reconstructed Western North Pacific sea level, altimetry agrees best with tide gauge data corrected employing the new nonlinear VLM model.

Co-seismic change in ocean bottom topography: Implication to absolute global mean sea level change

Earthquakes perturb both the ocean bottom topography due to displacements of sea floor and the geoid due to mass redistribution, which induces the relative sea level (RSL) change. However, the relative global mean sea level (GMSL) change is zero in that sea water mass is conserved. But the absolute GMSL change is not zero because earthquakes displace total ocean mass with respect to the Earth's center of mass (CM) which remains unchanged after an earthquake. This displacement, i.e. the absolute GMSL change, may be detectable by altimetry since the satellites are orbiting around CM. In this paper, we proposed a method to estimate co-seismic absolute GMSL change caused by earthquakes based on the point dislocation theory for a spherically symmetric, non-rotating, elastic and isotropic (SNREI) Earth. This change can be directly connected to the perturbation of ocean bottom topography. We first computed co-seismic displacements as well as the change in geo-potential and solved the sea level equation to validate the insignificance of the oceans' feedback, i.e. the loading effect due to RSL change, to co-seismic displacements. The results imply that the loading effect due to RSL change is negligible on displacements while is considerable on geoid. We then computed the absolute GMSL change caused by co-seismic vertical and horizontal displacements by making use of the integrated Green's function method. The numerical results show that a large earthquake may raise the absolute GMSL by magnitude of sub-millimeter and the recent three large events cause GMSL to rise about one millimeter, in which the contribution from horizontal displacement is non-negligible.

Volume unbalance on the 2016 Amatrice - Norcia (Central Italy) seismic sequence and insights on normal fault earthquake mechanism

We analyse the Mw 6.5, 2016 Amatrice-Norcia (Central Italy) seismic sequence by means of InSAR, GPS, seismological and geologic data. The >1000 km2 area affected by deformation is involving a volume of about 6000 km3 and the relocated seismicity is widely distributed in the hangingwall of the master fault system and the conjugate antithetic faults. Noteworthy, the coseismically subsided hangingwall volume is about 0.12 km3, whereas the uplifted adjacent volumes uplifted only 0.016 km3. Therefore, the subsided volume was about 7.5 times larger than the uplifted one. The coseismic motion requires equivalent volume at depth absorbing the hangingwall downward movement. This unbalance regularly occurs in normal fault-related earthquakes and can be inferred as a significant contribution to coseismic strain accomodated by a stress-drop driven collapse of precursory dilatancy. The vertical coseismic displacement is in fact larger than the horizontal component, consistent with the vertical orientation of the maximum lithostatic stress tensor.

Investigating paleoseismicity using fault scarp morphology of the Dushanzi Reverse Fault in the northern Tian Shan, China

This study uses the morphology of fault scarps to study the paleoseismic history of the Dushanzi Reverse Fault (DRF) in the northern Tian Shan region, China. Using high-resolution LiDAR DEMs to create a large number of scarp profiles, we detected slope breaks in the fault scarp morphology to quantify the number of paleoearthquakes that occurred on the scarps and obtained the along-strike scarp height distribution to determine the cumulative and single-event vertical displacements. Our results revealed four surface-rupturing events with similar co-seismic vertical displacements of ~1.4 m via the statistical analysis of displacement datasets and slope breaks, which were in agreement with the findings from paleoseismic trenching. We then assessed the paleomagnitudes using the empirical scaling relationships between moment magnitude and rupture parameters and the recurrence interval for surface-rupturing events using the formation ages of the displaced terrace surfaces. The results indicate that the possible magnitudes range from Mw 6.5 to 6.7, even reaching Mw ~7.2 in the case of a multi-fault rupture of the entire anticline-fault belt, with an average recurrence interval of ~2.4–3.0 ka. This study also demonstrates the importance of using high-resolution topographic data in paleoseismic investigations, which enables the determination of fine-scale tectono-geomorphic interpretations and detailed offset measurements over large regions along the fault.