Earthquake-induced hydrogeological changes in New Zealand

<p>Earthquakes redistribute fluids and change associated flow paths in the subsurface. Earthquake hydrology is an evolving discipline that studies such phenomena, providing novel information on crustal processes, natural hazards and water resources. This thesis uses the internationally significant New Zealand "hydroseismicity" dataset, in a regional-scale multi-site multi-earthquake study which includes the occurrence and the absence of responses, spanning a decade. Earthquake-induced groundwater level and tidal behaviour changes were examined in a range of aquifers, rock types and hydrogeological settings. Monitoring wells were within one (near-field) to several (intermediate- field) ruptured fault lengths of a variety of earthquakes that had a range of shaking intensities. This thesis presents three studies on the seismic and hydrogeological controls on earthquake-induced groundwater level changes. Water level changes were recorded New Zealand-wide within compositionally diverse, young shallow aquifers, in 433 monitoring wells at distances between 4 and 850 km from the 2016 Mw 7.8 Kaikoura earthquake epicentre. Water level changes are inconsistent with static stress changes, but do correlate with peak ground acceleration (PGA). At PGAs exceeding ~2 m/s2, water level changes predominantly increased persistently, which may have resulted from shear-induced consolidation. At lower PGAs there were approximately equal numbers of persistent water level increases and decreases, which are thought to have resulted from permeability enhancement. Water level changes also occurred more frequently north of the epicentre, due to the northward directivity of the Kaikoura earthquake rupture. Local hydrogeological conditions also contributed to the observed responses, with larger water level changes occurring in deeper wells and in well-consolidated rocks at equivalent PGA levels. Earthquakes have previously been inferred to induce hydrological changes in aquifers on the basis of changes to well tidal behaviour and water level, but the relationship between these changes have been unclear. Earthquake-induced changes to tidal behaviour and groundwater levels were quantified in 161 monitoring wells screened in gravel aquifers in Canterbury, New Zealand. In the near-field of the Canterbury earthquake sequence of 2010 and 2011, permeability reduction detected by tidal behaviour changes and increased water levels supports the hypothesis of shear-induced consolidation. Water level changes that occurred with no change in tidal behaviour re-equilibrated at a new post-seismic level within ~50 minutes possibly due to high permeability, good well-aquifer coupling, and/or small permeability changes in the local aquifer. Water level changes that occurred with tidal behaviour changes took from ~240 minutes to ~10 days to re-equilibrate, thought to represent permeability changes on a larger scale. Recent studies commonly utilise a general metric for earthquake-induced hydrological responses based on epicentral distance, earthquake magnitude and seismic energy density. A logistic regression model with random effects was applied to a dataset of binary responses of 495 monitoring well water levels to 11 Mw 5.4 or larger earthquakes. Within the model, earthquake shaking (represented by peak ground velocity), degree of confinement (depth) and rock strength (site average shear wave velocity in the shallow subsurface) were incorporated. For practical applications, the probabilistic framework was converted into the Modified Mercalli (MM) intensity scale. The model shows that water level changes are unlikely below MM intensity VI. At an MM intensity VII, water level changes are about as likely as not to very likely. At MM intensity VIII, the likelihood rises to very likely to virtually certain. This study was the first attempt we are aware of worldwide at incorporating both seismic and hydrogeological factors into a probabilistic framework for earthquake-induced groundwater level changes. The framework is a novel and more universal approach in quantifying responses than previous metrics using epicentral distance, magnitude and seismic energy density. It has potential to enable better comparison of international studies and inform practitioners making decisions around investment to mitigate risk to, and to increase the resilience of, water supply infrastructure.</p>

Earthquake-induced hydrogeological changes in New Zealand

<p>Earthquakes redistribute fluids and change associated flow paths in the subsurface. Earthquake hydrology is an evolving discipline that studies such phenomena, providing novel information on crustal processes, natural hazards and water resources. This thesis uses the internationally significant New Zealand "hydroseismicity" dataset, in a regional-scale multi-site multi-earthquake study which includes the occurrence and the absence of responses, spanning a decade. Earthquake-induced groundwater level and tidal behaviour changes were examined in a range of aquifers, rock types and hydrogeological settings. Monitoring wells were within one (near-field) to several (intermediate- field) ruptured fault lengths of a variety of earthquakes that had a range of shaking intensities. This thesis presents three studies on the seismic and hydrogeological controls on earthquake-induced groundwater level changes. Water level changes were recorded New Zealand-wide within compositionally diverse, young shallow aquifers, in 433 monitoring wells at distances between 4 and 850 km from the 2016 Mw 7.8 Kaikoura earthquake epicentre. Water level changes are inconsistent with static stress changes, but do correlate with peak ground acceleration (PGA). At PGAs exceeding ~2 m/s2, water level changes predominantly increased persistently, which may have resulted from shear-induced consolidation. At lower PGAs there were approximately equal numbers of persistent water level increases and decreases, which are thought to have resulted from permeability enhancement. Water level changes also occurred more frequently north of the epicentre, due to the northward directivity of the Kaikoura earthquake rupture. Local hydrogeological conditions also contributed to the observed responses, with larger water level changes occurring in deeper wells and in well-consolidated rocks at equivalent PGA levels. Earthquakes have previously been inferred to induce hydrological changes in aquifers on the basis of changes to well tidal behaviour and water level, but the relationship between these changes have been unclear. Earthquake-induced changes to tidal behaviour and groundwater levels were quantified in 161 monitoring wells screened in gravel aquifers in Canterbury, New Zealand. In the near-field of the Canterbury earthquake sequence of 2010 and 2011, permeability reduction detected by tidal behaviour changes and increased water levels supports the hypothesis of shear-induced consolidation. Water level changes that occurred with no change in tidal behaviour re-equilibrated at a new post-seismic level within ~50 minutes possibly due to high permeability, good well-aquifer coupling, and/or small permeability changes in the local aquifer. Water level changes that occurred with tidal behaviour changes took from ~240 minutes to ~10 days to re-equilibrate, thought to represent permeability changes on a larger scale. Recent studies commonly utilise a general metric for earthquake-induced hydrological responses based on epicentral distance, earthquake magnitude and seismic energy density. A logistic regression model with random effects was applied to a dataset of binary responses of 495 monitoring well water levels to 11 Mw 5.4 or larger earthquakes. Within the model, earthquake shaking (represented by peak ground velocity), degree of confinement (depth) and rock strength (site average shear wave velocity in the shallow subsurface) were incorporated. For practical applications, the probabilistic framework was converted into the Modified Mercalli (MM) intensity scale. The model shows that water level changes are unlikely below MM intensity VI. At an MM intensity VII, water level changes are about as likely as not to very likely. At MM intensity VIII, the likelihood rises to very likely to virtually certain. This study was the first attempt we are aware of worldwide at incorporating both seismic and hydrogeological factors into a probabilistic framework for earthquake-induced groundwater level changes. The framework is a novel and more universal approach in quantifying responses than previous metrics using epicentral distance, magnitude and seismic energy density. It has potential to enable better comparison of international studies and inform practitioners making decisions around investment to mitigate risk to, and to increase the resilience of, water supply infrastructure.</p>

Long-term monitoring of dynamic characteristics of high-rise and super high-rise buildings using strong motion records

Seismic behavior of a structure is directly related to its dynamic characteristics, which include natural frequency, damping ratio, and mode shape. This study focuses on the long-term monitoring of dynamic characteristics of six selected target structures. The covariance-driven stochastic subspace identification (SSI) approach is used to estimate the fundamental natural frequency and damping ratio of target buildings based on long-term motion records in order to examine the temporal variation of dynamic properties. The fundamental natural frequency and damping ratio variations over time are first discussed. It is found that the fundamental natural frequency of some structures reduces dramatically after the 2011 Tohoku Earthquake, accompanied by a rise in damping ratio. Then, regression analysis is used to assess the relationship between dynamic characteristics and ground motion parameters (Peak ground acceleration (PGA), magnitude, focal depth, and epicentral distance) and structural response (root mean square acceleration, maximum response amplitude). It is discovered that the identified natural frequency has no clear correlation with the focal depth, a slight negative correlation with the epicentral distance, and a strong negative correlation with the magnitude and PGA. The root mean square acceleration and the maximum response amplitude are negatively correlated to the target buildings’ natural frequencies. Finally, the influence of environmental factors on dynamic properties is investigated.

Ground motion models for shallow crustal and deep earthquakes in Hawaii and analyses of the 2018 M 6.9 Kalapana sequence

The damaging 4 May 2018 M 6.9 Kalapana earthquake and its aftershocks have provided the largest suite of strong motion records ever produced for an earthquake sequence in Hawaii exceeding the number of records obtained in the deep 2006 M 6.7 Kiholo Bay earthquake. These records provided the best opportunity to understand the processes of strong ground shaking in Hawaii from shallow crustal (< 20 km) earthquakes. There were four foreshocks and more than 100 aftershocks of M 4.0 and greater recorded by the seismic stations. The mainshock produced only a modest horizontal peak ground acceleration (PGA) of 0.24 g at an epicentral distance of 21.5 km. In this study, we evaluated the 2018 strong motion data as well as previously recorded shallow crustal earthquakes on the Big Island. There are still insufficient strong motion data to develop an empirical ground motion model (GMM) and so we developed a GMM using the stochastic numerical modeling approach similar to what we had done for deep Hawaiian (>20 km) earthquakes. To provide inputs into the stochastic model, we performed an inversion to estimate kappa, stress drops, Ro, and Q(f) using the shallow crustal earthquake database. The GMM is valid from M 4.0 to 8.0 and at Joyner–Boore (RJB) distances up to 400 km. Models were developed for eight VS30 (time-averaged shear-wave velocity in the top 30 m) values corresponding to the National Earthquake Hazards Reduction Program (NEHRP) site bins: A (1500 m/s), B (1080 m/s), B/C (760 m/s), C (530 m/s), C/D (365 m/s), D (260 m/s), D/E (185 m/s), and E (150 m/s). The GMM is for PGA, peak horizontal ground velocity (PGV), and 5%-damped pseudo-spectral acceleration (SA) at 26 periods from 0.01 to 10 s. In addition, we updated our GMM for deep earthquakes (>20 km) to include the same NEHRP site bins using the same approach for the crustal earthquake GMM.

Hydrogeochemical Characteristics of Hot Springs and Their Short-Term Seismic Precursor Anomalies along the Xiaojiang Fault Zone, Southeast Tibet Plateau

Significant hydrogeochemical changes may occur prior- and post-earthquakes. The Xiaojiang fault zone (XJF), situated in a highly deformed area of the southeastern margin of the Tibetan Plateau, is one of the active seismic areas. In this study, major and trace elements, and hydrogen and oxygen isotopes of 28 sites in hot springs along the XJF were investigated from June 2015 to April 2019. The meteoric water acts as the primary water source of the hot spring in the XJF and recharged elevations ranged from 1.8 to 4.5 km. Most of the hot spring water in the study area was immature water and the water–rock reaction degree was weak. The temperature range was inferred from an equation based on the SiO2 concentration and chemical geothermal modeling: 24.3~96.0 °C. The circulation depth for the springs was estimated from 0.45 to 4.04 km. We speculated the meteoric water firstly infiltrated underground and became heated by heat sources, and later circulated to the earth’s surface along the fault and fracture and finally constituted hot spring recharge. Additionally, a continuous monitoring was conducted every three days in the Xundian hot spring since April 2019, and in Panxi and Qujiang hot springs since June 2019. There were short-term (4–35 d) seismic precursor anomalies of the hydrochemical compositions prior to the Xundian ML4.2, Dongchuan ML4.2, and Shuangbai ML5.1 earthquakes. The epicentral distance of anomalous sites ranged from 19.1 to 192.8 km. The anomalous amplitudes were all over 2 times the anomaly threshold. The concentrations of Na+, Cl−, and SO42− are sensitive to the increase of stress in the XJF. Modeling on hydrology cycles of hot springs can provide a plausible physicochemical basis to explain geochemical anomalies in water and the hydrogeochemical anomaly may be useful in future earthquake prediction research of the study area.

Observations of PKKPab Diffraction Waves Well Beyond Cutoff Distance

The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance (PKKPab) is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted PKKPab (denoted as PKKPabdiff) can be used to constrain the P-wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the PKKPabdiff waves are in the epicentral distance range of 95°–105° (minor arc convention) (PKKPabdiff diffraction length less than 10°). However, high-frequency (∼1 Hz) synthetic seismograms show that the PKKPabdiff waveforms could be observable at distance down to 65°, which indicates that the PKKPabdiff signals could be detected at distances less than 95° in observations. To explore the distance ranges in which PKKPabdiff is observable, we collected global three-component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate PKKPabdiff signals, and found 95 events with clear PKKPabdiffsignals, with nearly 60% of the events show PKKPabdiff diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified PKKPabdiff waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.

Magnitude Estimation for Earthquake Early Warning with Multiple Parameter Inputs and a Support Vector Machine

Accurately estimating the magnitude within the initial seconds after the P-wave arrival is of great significance in earthquake early warning (EEW). Over the past few decades, single-parameter approaches such as the τc and Pd methods have been applied to EEW magnitude estimation studies considering the first 3 s after the P-wave onset. However, these methods present considerable scatter and are affected by the signal-to-noise ratio (SNR) and epicentral distance. In this study, using Japanese K-NET strong-motion data, we propose a machine-learning method comprising multiple parameter inputs, namely, the support vector machine magnitude estimation (SVM-M) model, to determine earthquake magnitudes and resolve the aforementioned problems. Our results using a single seismological station record show that the standard deviation of the magnitude prediction errors of the SVM-M model is 0.297, which is less than those of the τc (1.637) and Pd (0.425) methods. The magnitudes estimated by the SVM-M model within 3 s after the P-wave arrival are not obviously affected by the SNR or epicentral distance, and not overestimated for MJMA≤5. In addition, in an offline EEW application, the magnitude estimation error of the SVM-M model gradually decreases with increasing time after the first station is triggered, and the underestimation of event magnitudes for 6.5≤MJMA gradually improves. These results demonstrate that the proposed SVM-M model can robustly estimate earthquake magnitudes and has potential for EEW.

Shallow soil effects for contrasting PGAs at nearby strong-motion stations during the 2017, Mj 5.2, southern Akita Prefecture earthquake

On September 8, 2017, an earthquake of Mj 5.2 occurred with the epicenter in southern Akita Prefecture, Japan, at 22:23 local time. According to the Japan Meteorological Agency (JMA), the focal depth was 9 km. Many strong-motion stations of K-NET and KiK-net recorded ground motions from the earthquake. The maximum horizontal vector peak ground acceleration (PGA) of approximately 136 cm/s2 was recorded at one of the KiK-net stations at an epicentral distance of about 8 km. However, despite being 37 km and 53 km far from the epicenter, two stations recorded PGAs of approximately 126 and 113 cm/s2, respectively, similar to that near the epicenter. Even though these PGAs are not rare, we found that the PGAs at the two sites strongly deviated from the median values suggested by a ground motion prediction equation (GMPE), while the nearby sites generally followed the GMPE. Available velocity models showed that shallow shear wave velocities, especially in the top 5 m, were lower (i.e., the soils were softer) at the two sites compared to those at their nearest neighboring sites. We compared the ratios of the PGAs and peak ground velocities (PGVs) at the two sites with respect to their neighboring sites for many earthquakes covering a wide range of magnitudes and azimuths. We found that the PGAs and PGVs at the two sites were systematically larger than those at the adjacent sites. Linear theoretical site amplifications using the available soil models gave peak frequencies around 6-8 Hz at the larger PGA sites. Bandpass-filtered records showed significantly larger PGAs around these frequencies at the larger PGA sites. The above results showed that local site condition is one of the major contributing factors to induce large PGAs. Furthermore, softer sites experience more substantial nonlinear site amplification than the stiffer sites when input motions exceed some threshold PGAs. This latter effect means that the softer sites can produce a variety of ground motion spectra. Nevertheless, the degree of damage to built structures depends on several factors, including the design and quality of construction. We expect that this study contributes to developing improved microzonation maps for earthquake disaster mitigation.

Pre-Earthquake Ionospheric Anomalies Associated with the 2015 Gorkha Earthquake of Nepal Detected by GPS-TEC Measurements

The present study analyses the variations in the ionospheric total electron content (TEC) prior to and during the 2015 Gorkha Earthquake in Nepal (Mw = 7.8) on 25 April 2015, utilising data from the widely distributed Global Positioning System (GPS) network. This study aimed to determine the association between ionospheric TEC anomalies and the occurrence of earthquakes. The finding shows that anomalous TEC changes occurred several days to a few hours prior to the major impending events. The results reveal that deviations in vertical total electron content (VTEC) at distant locations from the epicentre are less than those observed at the epicentre, implying that variation in ionospheric VTEC is nearly inversely proportional to the distance of GPS stations from the epicentre. In view of the solar-terrestrial environment, the pre-earthquake ionospheric anomalies could be associated with the 2015 Gorkha Earthquake. The VTEC anomaly was identified when it crosses the upper bound (UB) or lower bound (LB). The outcomes additionally show that TEC variation was dominant in the vicinity of the earthquake epicentre. We also observed contrast in TEC throughout the globe using global ionospheric maps at regular 2-hour UT intervals, the day before, during and after the earthquake. As a result, we observed that areas heavily influenced by TEC were found to be transposed from eastern sectors to western sectors through the equatorial plane. TEC Maps indicate that most of the Indian regions, Northern China, Nepal, Bhutan, were heavily affected, indicating the earthquake's onset influence on the day of the event. Furthermore, we examined the cross-correlation of the SGOC station's TEC with the rest of the stations and discovered that the correlation increased gradually with epicentral distance from the surrounding stations, which was an intriguing result.