scholarly journals Seismic velocity recovery in the subsurface: transient damage and groundwater drainage following the 2015 Gorkha earthquake, Nepal

2022 ◽  
Author(s):  
Luc Illien ◽  
Christoph Sens-Schönfelder ◽  
Christoph Andermann ◽  
Odin Marc ◽  
Kristen Cook ◽  
...  
Keyword(s):  
Main Shock ◽  
Seismic Velocity ◽  
Gorkha Earthquake ◽  
Ground Shaking ◽  
Transient Behaviour ◽  
Shallow Subsurface ◽  
2015 Gorkha Earthquake ◽  
Unique Maximum ◽  
Velocity Variations ◽  
Groundwater Drainage

Shallow earthquakes frequently disturb the hydrological and mechanical state of the subsurface, with consequences for hazard and water management. Transient post-seismic hydrological behaviour has been widely reported, suggesting that the recovery of material properties (relaxation) following ground shaking may impact groundwater fluctuations. However, the monitoring of seismic velocity variations associated with earthquake damage and hydrological variations are often done assuming that both effects are independent. In a field site prone to highly variable hydrological conditions, we disentangle the different forcing of the relative seismic velocity variations $\delta v$ retrieved from a small dense seismic array in Nepal in the aftermath of the 2015 Mw 7.8 Gorkha earthquake. We successfully model transient damage effects by introducing a universal relaxation function that contains a unique maximum relaxation timescale for the main shock and the aftershocks, independent of the ground shaking levels. Next, we remove the modeled velocity from the raw data and test whether the corresponding residuals agree with a background hydrological behaviour we inferred from a previously calibrated groundwater model. The fitting of the $\delta v$ data with this model is improved when we introduce transient hydrological properties in the phase immediately following the main shock. This transient behaviour, interpreted as an enhanced permeability in the shallow subsurface, lasts for $\sim$ 6 months and is shorter than the damage relaxation ($\sim$ 1 year). Thus, we demonstrate the capability of seismic interferometry to deconvolve transient hydrological properties after earthquakes from non-linear mechanical recovery.

Seismic velocity recovery following the 2015 Mw 7.8 Gorkha earthquake, Nepal: Towards a coupled vision of damage and hydrological-induced velocity variations

2021 ◽  
Author(s):  
Luc Illien ◽  
Christoph Sens-Schönfelder ◽  
Christoff Andermann ◽  
Odin Marc ◽  
Kristen Cook ◽  
...  
Keyword(s):  
Seismic Waves ◽  
Seismic Velocity ◽  
Monsoon Season ◽  
Seismic Interferometry ◽  
Linear Superposition ◽  
Gorkha Earthquake ◽  
Ground Shaking ◽  
Velocity Variations ◽  
Velocity Changes ◽  
Induced Velocity

<p>Following the passage of seismic waves, most geomaterials experience non-linear mesoscopic elasticity (<em>NLME</em>). This is described by a drop in elastic moduli that precedes a subsequent recovery of physical properties over a relaxation timescale. Thanks to the development of seismic interferometry techniques that allows for the continuous monitoring of relative seismic velocity changes <em>δv</em> in the subsurface, observations of <em>NLME</em> (<em>δv</em><sub><em>NLME</em></sub>) in the field are now numerous. In parallel, a growing community uses seismic interferometry to monitor velocity changes induced by seasonal hydrological variations (<em>δv<sub>hydro</sub></em>). Monitoring of these variations are often independently done and a linear superposition of both effects is mostly assumed when decomposing the observed <em>δv</em> signal (<em>δv</em> =  <em>δv<sub>NLME</sub></em> + <em>δv<sub>hydro</sub></em>). However, transient hydrological behaviour following co-seismic ground shaking has been widely reported in boreholes measurements and streamflow, which suggests that  <em>δv<sub>hydro</sub></em> may be impacted by the transient variation of material properties caused by <em>NLME</em>. In this presentation, we attempt to characterize the relative seismic velocity variations <em>δv</em> retrieved from a small dense seismic array in Nepal that was deployed in the aftermath of the  2015 Mw 7.8 Gorkha earthquake and that is prone to highly variable hydrological conditions. We first investigated the effect of aftershocks in computing <em>δv</em> at a 10-minute resolution centered around significant ground shaking events. After correcting <em>δv</em> for <em>NLME</em> caused by the Gorkha earthquake and its subsequent aftershocks, we test whether the corresponding residuals are in agreement with the background hydrological behaviour which we inferred from a calibrated hydrological model. This is not the case and we find that transient hydrological properties improve the data description in the early phase after the mainshock. We report three distinct relaxation time scales that are relevant for the recovery of seismic velocity at our field site:  <strong>1.</strong> A long time scale activated by the main shock of the Gorkha earthquake (~1 year) <strong>2.</strong> A relatively short timescale (1-3 days) that occurs after moderate aftershocks. <strong>3.</strong> An intermediate timescale (4-6 months) during the 2015 monsoon season that corresponds to the recovery of the hydrological system. This timescale could correspond to an enhanced permeability caused by Gorkha ground shaking. Our study demonstrates the capability of seismic interferometry to monitor transient hydrological properties after earthquakes at a spatial scale that is not available with classical hydrological measurements. This investigation demands calibrated hydrological models and a framework in which the different forcing of <em>δv</em> are coupled.</p>

scholarly journals The Influence of Surface Topography on the Weak Ground Shaking in Kathmandu Valley during the 2015 Gorkha Earthquake, Nepal

Sensors ◽  
2020 ◽  
Vol 20 (3) ◽  
pp. 678
Author(s):  
Mark van der Meijde ◽  
Md Ashrafuzzaman ◽  
Norman Kerle ◽  
Saad Khan ◽  
Harald van der Werff
Keyword(s):  
Numerical Simulations ◽  
Surface Topography ◽  
Seismic Energy ◽  
Spectral Element ◽  
Kathmandu Valley ◽  
Ground Displacement ◽  
Gorkha Earthquake ◽  
Ground Shaking ◽  
Earthquake Scenarios ◽  
2015 Gorkha Earthquake

It remains elusive why there was only weak and limited ground shaking in Kathmandu valley during the 25 April 2015 Mw 7.8 Gorkha, Nepal, earthquake. Our spectral element numerical simulations show that, during this earthquake, surface topography restricted the propagation of seismic energy into the valley. The mountains diverted the incoming seismic wave mostly to the eastern and western margins of the valley. As a result, we find de-amplification of peak ground displacement in most of the valley interior. Modeling of alternative earthquake scenarios of the same magnitude occurring at different locations shows that these will affect the Kathmandu valley much more strongly, up to 2–3 times more, than the 2015 Gorkha earthquake did. This indicates that surface topography contributed to the reduced seismic shaking for this specific earthquake and lessened the earthquake impact within the valley.

Site Characteristics of Kathmandu Valley from Array Microtremor Observations

2017 ◽  
Vol 33 (1_suppl) ◽  
pp. 85-93 ◽  
Author(s):  
Nakhorn Poovarodom ◽  
Deepak Chamlagain ◽  
Amorntep Jirasakjamroonsri ◽  
Pennung Warnitchai
Keyword(s):  
Main Shock ◽  
Site Response ◽  
Horizontal Component ◽  
Response Analysis ◽  
Kathmandu Valley ◽  
Gorkha Earthquake ◽  
Velocity Models ◽  
Site Characteristics ◽  
2015 Gorkha Earthquake ◽  
Good Agreement

Array microtremor observations were conducted in Kathmandu Valley close to six seismic stations. Sedimentary layers from surface to deep basement rock were modeled according to the derived velocity structures for site response analysis. The records in horizontal component from the 2015 Gorkha earthquake main shock at deep sedimentary sites were compared with the predictions from analysis using the records from a shallow sedimentary site as input motions. Generally, the comparisons are in good agreement where spectral amplification at long periods and suppression at short periods could be justified by the velocity models.

Contribution of Fault Geometry on Probabilistic Seismic Hazard Assessment in Intermontane Basin: An example from Kathmandu Valley

2020 ◽  
Vol 60 ◽  
pp. 21-36
Author(s):  
Deepak Chamlagain ◽  
Govinda Prasad Niroula
Keyword(s):  
Seismic Hazard ◽  
Hazard Assessment ◽  
Main Shock ◽  
Seismic Hazard Assessment ◽  
Probabilistic Seismic Hazard Assessment ◽  
Kathmandu Valley ◽  
Probabilistic Seismic Hazard ◽  
Gorkha Earthquake ◽  
2015 Gorkha Earthquake ◽  
The Himalaya

The intermontane basins of the Himalaya are prone to damaging earthquakes as they are located roughly 10-15 km above the Main Himalayan Thrust (MHT), a major seismogenic thrust fault in the Himalaya.  After the Mw 7.8 2015 Gorkha earthquake, the geometry of the MHT has been investigated using different approaches. Two contrasting models with a single ramp and double ramp geometries are proposed. However, the contribution of these geometries on seismic hazard has not been investigated yet. In this contribution, therefore, a probabilistic seismic hazard assessment is carried out using both models for Kathmandu valley and the obtained results are compared with the measured strong ground motion data of main shock of the 2015 Gorkha seismic sequence at Kirtipur, Kathmandu (rock site). It is found that the areal sources have the least contribution indicating sole contribution of MHT to relatively higher level of seismic hazard in the valley located on the up-dip locked portion of the MHT. The Peak Ground Acceleration (PGA) of the main shock of the 2015 Gorkha earthquake and PGA for 760 yr (exposure period of 50 yr and probability of exceedance 6.36%) of return period adopting both single and double ramp models are approximately same with error level of ± 3.84%. The results indicate that the adopted seismic model fairly represents the seismo-tectonic of the region, particularly of MHT. Considering this as the best fit results, the spatial distribution of the seismic hazard is analysed using double ramp model. It is found that the PGA values in the valley for 760 yr return period vary from 0.24 g to 0.28 g. The PGA values are higher in the southern part and gradually decrease towards north. Such decrease in PGA is consistent with the decrease in locking level of the MHT towards north. The study, therefore, emphasizes detailed geometrical characterization of the MHT while carrying out the seismic hazard assessment in the Himalaya.

Cross-Correlation Analysis of Long-Term Ambient Seismic-Noise Recordings in the Caribbean Netherlands to Monitor the Volcanoes on Saba and St. Eustatius

2020 ◽  
Vol 110 (5) ◽  
pp. 2541-2558
Author(s):  
Reinoud Sleeman ◽  
Elske de Zeeuw-van Dalfsen
Keyword(s):  
Seismic Velocity ◽  
Sine Wave ◽  
Peak Ground Velocity ◽  
Data Set ◽  
Ground Shaking ◽  
Daily Monitoring ◽  
Cross Correlation Analysis ◽  
Single Station ◽  
Velocity Variations ◽  
Different Characteristics

ABSTRACT The continuous recordings of broadband seismometers on Saba and St. Eustatius in the Lesser Antilles provide a unique and long data set to measure temporal seismic velocity variations (dv/v) at two active but quiescent volcanoes (Mt. Scenery and The Quill). We compare results from single-station cross-component (SC) correlations with cross-station cross-component (CC) correlations and achieve the best similarities within the frequency band 1.3–2.1 Hz, with average correlations of 0.82 for Saba and 0.36 for St. Eustatius, justifying the use of SC as proxy for CC at these frequencies. Temporal dv/v variations derived from 13 yr of data show different characteristics at both islands. At St. Eustatius dv/v highly correlates (0.72) with air temperature and can be modeled by a simple sine wave with a period of 1 yr. Remaining residuals reveal cohurricane dv/v drops, thus at times of the passage of a hurricane. At Saba, subsurface velocity variations show temporal coseismic changes, up to −0.49% compared with −0.19% at St. Eustatius, and thus show a higher sensitivity to ground shaking. Our data set, although limited, shows a linear relation (correlation 0.78) between the coseismic dv/v drop and peak ground velocity at Saba around 1.3 Hz. We model the associated seismic velocity recovery with an exponential decay function and we estimate the recovery time at 2 yr. After subtracting the coseismic drop and recovery model, dv/v at Saba obtained from CC data correlates with the sine model (correlation 0.71). SC may be an appealing alternative for CC for monitoring purposes; however, the use of a small network is preferred to reduce the variance in dv/v (at St. Eustatius from 0.12% to 0.05%) and to detect dv/v variations unrelated to volcanic activity (e.g., hurricane). We continue work on the implementation of CC in the daily monitoring for Mt. Scenery and The Quill.

scholarly journals Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

2021 ◽  
Vol 13 (14) ◽  
pp. 2684
Author(s):  
Eldert Fokker ◽  
Elmer Ruigrok ◽  
Rhys Hawkins ◽  
Jeannot Trampert
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Pressure Head ◽  
Groundwater Table ◽  
Surface Velocity ◽  
Seismic Velocities ◽  
Stress Monitoring ◽  
Near Surface ◽  
Velocity Variations ◽  
Velocity Changes

Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

Characterizing the Kathmandu Valley sediment response through strong motion recordings of the 2015 Gorkha earthquake sequence

2017 ◽  
Vol 714-715 ◽  
pp. 146-157 ◽  
Author(s):  
S. Rajaure ◽  
D. Asimaki ◽  
E.M. Thompson ◽  
S. Hough ◽  
S. Martin ◽  
...  
Keyword(s):  
Strong Motion ◽  
Earthquake Sequence ◽  
Kathmandu Valley ◽  
Gorkha Earthquake ◽  
2015 Gorkha Earthquake
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