Borehole Strainmeters Research Articles

Afterslip and Creep in the Rate‐Dependent Framework: Joint Inversion of Borehole Strain and GNSS Displacements for the Mw 7.1 Ridgecrest Earthquake

AbstractThe elusive transition toward afterslip following an earthquake is challenging to capture with typical data resolution limits. A dense geodetic network recorded the Mw 7.1 Ridgecrest earthquake, including 16 Global Navigation Satellite System (GNSS) stations and 3 borehole strainmeters (BSM). The sub‐nanostrain precision and sub‐second sampling rate of BSMs bridges a gap between conventional seismologic and geodetic methods, exemplified by atypical postseismic shear strain reversals observed at nearfield (<2 km) station B921 that remain unexplained. We jointly invert GNSS displacements and BSM strains for coseismic and postseismic slip spanning hours to months over 7 independent periods. Cosiesmically, our model resolves the largest slip magnitudes of up to 6.6 m on the mainshock rupture plane, with similar patterns to other inferred slip distributions. The foreshock fault appears to slip coincidently with mainshock, revealing potential asperities activated during the preceding Mw 6.4 event. Postseismically, the best‐fitting models adhere to mechanical rate‐and‐state expectations of logarithmically decaying slip adjacent to the coseismic rupture terminus, and where deep rheologic conditions favor creep. Most spatial variation occurs in the early postseismic timeframe (<1–2 weeks), with evidence for regional rheologic control and static stress dependence. Triggered creep on the neighboring Garlock Fault unexpectedly persists for >178 days—further highlighting the importance of fault networks in postseismic stress redistribution, critical to assessing future hazard.

Extraction of pre-earthquake anomalies from borehole strain data using Graph WaveNet: a case study of the 2013 Lushan earthquake in China

Abstract. On 20 April 2013, Lushan experienced an earthquake with a magnitude of 7.0. In seismic assessments, borehole strainmeters, recognized for their remarkable sensitivity and inherent reliability in tracking crustal deformation, are extensively employed. However, traditional data-processing methods encounter challenges when handling massive dataset-s. This study proposes using a Graph WaveNet graph neural network to analyze borehole strain data from multiple stations near the earthquake epicenter and establishes a node graph structure using data from four stations near the Lushan epicenter, covering the years 2010–2013. After excluding the potential effects of pressure, temperature, and rainfall, we statistically analyzed the pre-earthquake anomalies. Focusing on the Guza, Xiaomiao, and Luzhou stations, which are the closest to the epicenter, the fitting results revealed two acceleration events of anomalous accumulation that occurred before the earthquake. Occurring approximately 4 months before the earthquake event, the first acceleration event indicated the pre-release of energy from a weak fault section. Conversely, the acceleration event observed a few days before the earthquake indicated a strong fault section that reached an unstable state with accumulating strain. We tentatively infer that these two anomalous cumulative accelerations may be related to the preparation phase for a large earthquake. This study highlights the considerable potential of graph neural networks in conducting multistation studies of pre-earthquake anomalies.

Moho Imaging with Fiber Borehole Strainmeters Based on Ambient Noise Autocorrelation.

Moho tomography is important for studying the deep Earth structure and geodynamics, and fiber borehole strainmeters are broadband, low-noise, and attractive tools for seismic observation. Recently, many studies have shown that fiber optic seismic sensors can be used for subsurface structure imaging based on ambient noise cross-correlation, similar to conventional geophones. However, this array-dependent cross-correlation method is not suitable for fiber borehole strainmeters. Here, we developed a Moho imaging scheme for the characteristics of fiber borehole strainmeters based on ambient noise autocorrelation. S-wave reflection signals were extracted from the ambient noise through a series of processing steps, including phase autocorrelation (PAC), phase-weighted stacking (PWS), etc. Subsequently, the time-to-depth conversion crustal thickness beneath the station was calculated. We applied our scheme to continuous four-component recordings from four fiber borehole strainmeters in Lu'an, Anhui Province, China. The obtained Moho depth was consistent with the previous research results. Our work shows that this method is suitable for Moho imaging with fiber borehole strainmeters without relying on the number of stations.

The Strain State Analysis of 4-component Borehole Strainmeter Observations in Sichuan-Yunnan Region

Analysis of nearly a decade of crustal strain changes state after the Ms8.0 Wenchuan earthquake by using 4-component borehole strain observation data, and combined with the tectonic stress field, this paper studied the regional stress and strain variation characteristics and dynamics. The results show that the main compressive strain direction of the 4-component borehole strain observation is approximately the same as that of tectonic stress field. However, there are also some regional inconsistencies, which should be related to regional structural features.

Observations of Love Wave Dispersion Properties Using Seismometers and Component Strainmeters

With amplitude ratio of two different types of ground motion, the information of phase velocity could be reflected in the frequency-dependent in the assumption of plane-wave propagation. In this paper, we test the possibility of determining local Love waves dispersion properties using transverse velocity and strain measurement. With the strain seismic wave recording of the four-component borehole strainmeters built at the Balun seismic station in Xinjiang, China., we estimated the Love wave phase velocities using amplitude ratios between transverse velocity and shear strain components. The results show the possibility that we could recover general Love wave dispersion characteristics that conformed to the realistic local structure at periods ranging from 20-120 s.

Sensitivity limits for strain detection of hypothetical remote fluid-induced earthquakes (Mw ≥ 4): a case study in Taiwan

Capturing and quantifying the timing of remotely triggered earthquakes and understanding the physical processes responsible for this delay represent major challenges in earthquake forecasting. In this study, we propose a physical framework for the integration of borehole strainmeter observations for the investigation of remote triggering of moderate to large earthquakes (Mw ≥ 4) in Taiwan. Based on the time-delay computation between regional events and global earthquakes, we establish a selection of earthquakes showing fault zone properties (hydraulic diffusivity and nucleation length) that may be compatible with a magnitude-dependent fluid-induced nucleation process. Using theoretical fault zones parameters, we calculate the evolution of fluid pressure transiting along the nucleation region under the assumption of a one-dimensional, homogeneous poroelastic medium. Pore pressure levels reached before earthquake rupture are ranging from about 0.02 kPa to 3 kPa in the case of teleseismic wave-induced elastic pressure ranging from 0.15 kPa to 27.3 kPa. To compute the time-dependent evolution of deformation generated by a remote diffusing pressure front, we model the nucleation region using the analogue volcano source represented by a horizontal circular crack, and calculate synthetic dilatation at the strainmeter location from displacements using a finite-difference approach. In general, predictions are about two to four orders of magnitude smaller than observations (∼ 10–5 to 10–3 nϵ). Therefore, this suggests that detection of pore pressure-related deformation would have required change of volume in the nucleation region that is at least one order of magnitude larger than for the hypothetical cases considered here. The study represents the first attempt to analyze strain time-series for detecting pre-earthquake strain anomalies related to fluid-induced earthquakes and illustrates the challenge for detecting and characterizing intermediate-to far-field earthquake precursors caused by fluid flow in active regions.

The apparent focal depth, emergence angle, and take-off angle of seismic wave measured by YRY-4-type borehole strainmeter as one kind of strain seismograph

Introduction: In theory, the observation objects and principles of strain seismograph and traditional pendulum seismograph are different, and the characteristics of observed signals should also be dissimilar. The observation results of pendulum seismograph show that seismic waves in inhomogeneous media will undergo refraction, reflection, and attenuation. Then, what signal characteristics can be detected by strain seismograph is great significance for understanding and explaining the observation results.Methods: Using YRY-4 type four-gauge borehole strainmeter as one kind of strain seismograph to detect the strain tensor change of the plane seismic wave emitted from the surface, a five-site strain seismograph observation network was built in Shanxi Province, with continuous observation for 2 years at a sampling rate of 100 Hz. In this paper, two local events occurring in the area covered by the strain seismograph observation network are taken as examples. We systematically studied the characteristics of seismic wave signals recorded by strain seismographs at five sites, inverted for the focal depth of the two local earthquakes and the relationship between the wave velocity and the wave velocity gradient of the focal depth, and calculated the apparent focal depth, the emergence angle and the take-off angle of seismic waves.Results: These results show stable uniqueness and apparent regularity, especially since the inverted focal depths are basically consistent with the seismic solutions based on those traditional pendulum seismographs. The observations from this study show that the strain seismograph can be used as an effective supplement to the pendulum seismograph.Discussion: In the future, we will continue to study the rupture process and focal mechanism of moderate-strong earthquakes and teleseismic earthquakes by combining two kinds of observations.

Seasonal Variations of Seismic Travel-Time Changes in Binchuan, Southwestern China, Inferred from Large Volume Air-Gun Source Array Data

AbstractActive sources, especially air-gun sources in the water reservoir, have proven to be powerful tools for detecting regional scale velocity changes. However, the water level change affects the repeatability of the air-gun waveform and, thus, affects the stability of the detection of the velocity changes. This article explores how to make full use of the air-gun signals excited at different water levels from analyzing three years of air-gun data recorded by 20 stations deployed from ∼50 m to ∼25 km from the source. At the same time, by utilizing the poroelastic model, we quantify both vertical and horizontal distances affected by the water level change. More important, supported by the strain data from one borehole strainmeter station, the influence mechanisms of the seasonal variation derived from the three years of air-gun data are also discussed. Results indicate the water level affects the main frequency of the air gun and has a strong influence on the coda wave. When the water level of the reservoir changes abruptly, the dominant effect on the derived delay time change is from the water level change. In this case, the deconvolution method can hardly eliminate the influence of the abrupt water level change. However, when the reservoir's water level changes gently, the delay time varies accordingly rather than inversely with the water level. Other reasons affect the material properties and, thus, influence the derived delay time. The modeled vertical component of poroelastic strain caused by the reservoir water level change is 1×10−7. The observed strain (4×10−7) from the strainmeter is likely associated with thermoelastic strain induced by temperature change. Our results show that although the long-term air-gun signal is affected by water level, there is still much information about changes in the subsurface that is worth mining.

Instrumental response of borehole tensor strainmeters for oblique-incident seismic waves

It has been proven qualitatively that borehole tensor strainmeters (BTSM) could measure three surface-wave-induced strain components in the horizontal-normal plane. But strain waves are usually incident obliquely to borehole axis to induce not only horizontalnormal strain components but also the three other strain components in the plane of vertical normal, which are usually assumed to zero in conventional model of BTSM installed near the earth surface. Therefore quantitative kinematical-parameter measurement of strain waves of wave normal oblique to borehole axis by BTSM is still a challenging problem. In this paper, a scattering model of oblique-incidence seismic waves by an empty borehole is introduced to investigate instrumental responses of BTSM. Besides for the four gauges in conventional BTSM, three other gauges considering the influences of strain components in the plane of vertical normal are also introduced to build dynamic calibration matrix between instrumental and incident strains. It shows that within the effective bandwidth of BTSM, zero-frequency-gain for dynamic calibration matrix could be adopted approximately to build quantitative relation between instrumental strain waves of BTSM and oblique-incidence seismic strain waves. It is shown that the recommended gauge combinations could promise quantitative measurement of kinematic parameters of the incident strain P or S wave if high-resolution insitu calibration of BTSM has been conducted, which is important to the development of early warning system by borehole strainmeters.

Joint Analysis of Regional Strain Variations Based on Borehole Strainmeter and Global Navigation Satellite System Observations

Joint Analysis of Regional Strain Variations Based on Borehole Strainmeter and Global Navigation Satellite System Observations

Two Different Layouts of Sensors of Four-gauge Borehole Strainmeter and Their Influence on Field Measurement

Two Different Layouts of Sensors of Four-gauge Borehole Strainmeter and Their Influence on Field Measurement

Real-time signal processing of high-precision borehole strainmeters at Mt. Etna for volcanic surveillance and eruptive events detection

Real-time signal processing of high-precision borehole strainmeters at Mt. Etna for volcanic surveillance and eruptive events detection

The Potential of Using Fiber Optic Distributed Acoustic Sensing (DAS) in Earthquake Early Warning Applications

ABSTRACT As the seismological community embraces fiber optic distributed acoustic sensing (DAS), DAS arrays are becoming a logical, scalable option to obtain strain and ground-motion data for which the installation of seismometers is not easy or cheap, such as in dense offshore arrays. The potential of strain data in earthquake early warning (EEW) applications has been recently demonstrated using records from borehole strainmeters (BSMs). However, current BSM networks are sparse, installing more BSMs is expensive and often impractical, and BSMs have the same limitations in offshore environments as other traditional seismic instruments. Here, we aim to provide a road map about how DAS data could be used in existing EEW applications, using the ShakeAlert EEW System for the West Coast of the United States as an example. We review the data requirements for EEW systems, examine ways in which strain-derived ground-motion data can be incorporated into such systems without significant modifications, and determine what is still needed for full utilization of DAS data in these applications. Importantly, EEW algorithms require ground-motion amplitude information for rapid earthquake source characterization; thus, accurate strain amplitude observations, not only phase information, are necessary for deriving these ground-motion metrics from DAS data. To obtain high-quality ground-motion observations, EEW-compatible DAS arrays need to be multicomponent, well coupled, and low noise. We suggest ways to achieve such data requirements using existing DAS technology and discuss areas in which further research is needed to optimize DAS array performance for EEW.

A Type‐Curve Approach for Evaluating Aquifer Properties by Interpreting Shallow Strain Measured During Well Tests

AbstractStrains occur at shallow depths in response to pressure changes during well tests in an underlying aquifer, and recent developments in instrumentation have made it feasible to measure essentially the full strain tensor. Simulations using poroelastic analyses indicate that shallow normal strains are approximately proportional to the logarithm of time when a well is injecting into or pumping from a deep aquifer or reservoir. The drawdown is also a linear function of log time, as shown by the classic Cooper‐Jacob type‐curve analysis. The time when the semilog straight line intercepts the zero‐strain axis is similar to the time determined from the Cooper‐Jacob pressure analysis, and it can be used to estimate hydraulic diffusivity, suggesting that horizontal strain data can be used directly to estimate aquifer properties. This approach was validated using measurements from shallow (30‐m deep) borehole strainmeters during an injection test at a 530‐m‐deep sandstone aquifer/reservoir in Oklahoma. The results show intercept times for the shallow normal strain data are essentially the same as for deep pressure data from an equivalent radial distance. The slopes of the semilog plots of the pressure and the strain increase at the same time, suggesting that they both respond to a lateral aquifer boundary. Significantly, though, strain was measured at shallow depths while the pressure data were measured at 530‐m depth. This suggests that strain data from shallow depths could be an effective way to improve the characterization of an underlying aquifer.

Months‐Long Crustal Deformation Driven by Aseismic Slips and Pore Pressure Transients Triggered by Local and Regional Earthquakes

AbstractStrong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation.

Near-Fault Monitoring Reveals Combined Seismic and Slow Activation of a Fault Branch within the Istanbul–Marmara Seismic Gap in Northwest Turkey

Abstract Various geophysical observations show that seismic and aseismic slip on a fault may occur concurrently. We analyze microseismicity recordings from a temporary near-fault seismic network and borehole strainmeter data from the eastern Marmara region in northwest Turkey to track seismic and aseismic deformation around the hypocentral region of an Mw 4.5 earthquake in 2018. A slow transient is observed that lasted about 30 days starting at the time of the Mw 4.5 event. We study about 1200 microseismic events that occurred during 417 days after the Mw 4.5 event around the mainshock fault rupture. The seismicity reveals a strong temporal clustering, including four episodic seismic sequences, each containing more than 30 events per day. Seismicity from the first two sequences displayed typical characteristics driven by aseismic slip and/or fluids, such as the activation of a broader region around the mainshock and swarm-like topology. The third and fourth sequences correspond to typical mainshock–aftershock sequences. These observations suggest that slow slip and potentially fluid diffusion along the fault plane could have controlled the seismicity during the initial 150 days following the Mw 4.5 event. In contrast, stress redistribution and breaking of remaining asperities may have caused the activity after the initial 150 days. Our observation from a newly installed combined dense seismic and borehole strainmeter network follows an earlier observation of a slow transient occurring in conjunction with enhanced local seismic moment release in the same region. This suggests a frequent interaction of seismic and aseismic slip in the Istanbul–Marmara seismic gap.

Anatomy of a Paroxysmal Lava Fountain at Etna Volcano: The Case of the 12 March 2021, Episode

On 13 December 2020, Etna volcano entered a new eruptive phase, giving rise to a number of paroxysmal episodes involving increased Strombolian activity from the summit craters, lava fountains feeding several-km high eruptive columns and ash plumes, as well as lava flows. As of 2 August 2021, 57 such episodes have occurred in 2021, all of them from the New Southeast Crater (NSEC). Each paroxysmal episode lasted a few hours and was sometimes preceded (but more often followed) by lava flow output from the crater rim lasting a few hours. In this paper, we use remote sensing data from the ground and satellite, integrated with ground deformation data recorded by a high precision borehole strainmeter to characterize the 12 March 2021 eruptive episode, which was one of the most powerful (and best recorded) among that occurred since 13 December 2020. We describe the formation and growth of the lava fountains, and the way they feed the eruptive column and the ash plume, using data gathered from the INGV visible and thermal camera monitoring network, compared with satellite images. We show the growth of the lava flow field associated with the explosive phase obtained from a fixed thermal monitoring camera. We estimate the erupted volume of pyroclasts from the heights of the lava fountains measured by the cameras, and the erupted lava flow volume from the satellite-derived radiant heat flux. We compare all erupted volumes (pyroclasts plus lava flows) with the total erupted volume inferred from the volcano deflation recorded by the borehole strainmeter, obtaining a total erupted volume of ~3 × 106 m3 of magma constrained by the strainmeter. This volume comprises ~1.6 × 106 m3 of pyroclasts erupted during the lava fountain and 2.4 × 106 m3 of lava flow, with ~30% of the erupted pyroclasts being remobilized as rootless lava to feed the lava flows. The episode lasted 130 min and resulted in an eruption rate of ~385 m3 s−1 and caused the formation of an ash plume rising from the margins of the lava fountain that rose up to 12.6 km a.s.l. in ~1 h. The maximum elevation of the ash plume was well constrained by an empirical formula that can be used for prompt hazard assessment.

Fifteen Years of Continuous High-Resolution Borehole Strainmeter Measurements in Eastern Taiwan: An Overview and Perspectives

As one of the most sensitive instruments for deformation monitoring in geophysics, borehole strainmeter has the capability to record a large spectrum of tectonic and environmental signals. Sensors are usually deployed near active faults and volcanoes and provide high-resolution continuous recordings of seismic and aseismic signals, hydrological variations (rainfall, groundwater level) and natural hazards (tropical cyclones, landslides, tsunamis). On the occasion of the 50th anniversary of the installation of the first Sacks–Evertson borehole strainmeter, in central Japan, we present an overview of the major scientific contributions and advances enabled by borehole strainmeter measurements in Taiwan since their installation in the mid 2000s. We also propose a set of future research directions that address recent challenges in seismology, hydrology and crustal strain modeling.

Green's function at depth of borehole observation required for precise estimation of the effect of ocean tidal loading near coasts

SUMMARY We calculated Green's functions of displacements and strains caused by a surface load in surface and subsurface observations. Green's functions of displacements and strains at depth became different from those at the surface for a range of angular distances less than 0.01–0.1°. These Green's functions closely agree with the corresponding Boussinesq approximations for tidal loading at angular distances of less than about 0.01–0.001°. We examined the difference between the ocean tidal loading effect estimated by Green's functions at the surface and the effect for subsurface observations, and found a non-negligible difference when the ratio of distance between the coast and observatory to the deployment depth is smaller than 20. We compared amplitudes and phase shifts of areal and shear strains in the M2 tidal constituent observed by borehole strainmeters at 11 observatories with the theoretical strains using the Green's functions at the deployment depth and those at the surface. The theoretical strains using Green's function at the deployment depth are much closer to the observed strains than theoretical strains at the surface at four observatories where the ratios of distance from the coast to the observatories to the deployment depth are small. These results suggest that Green's functions at the deployment depth are needed to estimate theoretical strains precisely near the coast.

Afterslip and Spontaneous Aseismic Slip on the Anza Segment of the San Jacinto Fault Zone, Southern California

AbstractUnderstanding the interplay of seismic and aseismic slip is key in seismic hazard evaluation. It is particularly important to know if the same or nearby fault segments can host different slip modes and understand the transition between modes of slip. We investigate this in the trifurcation area of the Anza segment of the San Jacinto fault where deep creep driving seismicity below the geodetic locking depth has been proposed. We focus on periods following local moderate‐sized earthquakes by combining the occurrence of highly correlated families of earthquakes (i.e., closely spaced or partially overlapping events) that we describe here as near‐repeating earthquakes, seismicity, and borehole strainmeter data. We find that all Mw > 4.5 earthquakes between 2010 and 2020 triggered afterslip with coplanar families of near‐repeating earthquakes and off‐fault microseismicity. These observations include newly detected afterslip following the June 10, 2016 Mw 5.2 and April 4, 2020 Mw 4.9 local earthquakes. Afterslip geometries defined by the near‐repeating earthquake families are consistent with strain change observations. We conclude that families of near‐repeating earthquakes, similar to low‐frequency earthquakes within tremor, can be useful indicators of aseismic slip transients and can reveal faulting complexities during aseismic slip. Further, we identify the first evidence of spontaneous aseismic slip in the Anza region from near‐repeating earthquake families on two minor faults in 2015. Taken together, our observations support a model where deep microseismicity is located in a transitional region at the bottom of the seismogenic zone with spatially heterogeneous frictional properties that produce frequent aseismic slip transients near Anza.