Borehole Strainmeters Research Articles

Stress Reorientation by Earthquakes Near the Ganzi-Yushu Strike-Slip Fault and Interpretation with Discrete Element Modelling

Earthquakes are generally known to alter the stress field near seismogenic faults. Observations using YRY-four-gauge borehole strainmeters within Yushu (YSH) borehole near the Ganzi-Yushu fault in eastern Tibetan Plateau shows that the azimuth variation of maximum horizontal stress (SH) first decreased and then increased substantially when the earthquakes occurred during the measurement period from January 1, 2009 to December 31, 2018. In this period, 38 earthquakes (M ≥ 3) were detected near the fault and the SH orientation showed a drastic change after the 2010 Ms 7.3 Yushu mainshock. We present a discrete element modelling using Particle Flow Code 2D (PFC2D) to simulate a dynamic fault rupturing process and to use the modelling results for interpretation of the stress reorientation. The modelling reveals that dilatation and compression quadrants are formed around a fault rupturing in strike-slip model, resulting in different spatiotemporal changes of the orientation of maximum horizontal stress (Δθ). The value of Δθ in the compression quadrants shows a sharp drop at the time of coseismic slip, then approaches slowly to an asymptotic value. In the dilatation quadrants, Δθ drops by coseismic slip, then increases sharply and finally reaches a stable value. The modelled Δθ by coseismic fault slip agrees with in-situ observations at YSH borehole during 2010 Ms 7.3 Yushu mainshock. It is also found that, the value of Δθ decreases with increasing distance from the rupturing source. We modelled the effect of fault geometry and host rock properties on the Δθ, and found that structural complexity and off-fault damage by coseismic fault slip have significant impact on the stress field alteration near the rupturing source.

Earthquake Magnitudes from Dynamic Strain

ABSTRACTDynamic strains have never played a role in determining local earthquake magnitudes, which are routinely set by displacement waveforms from seismic instrumentation (e.g., ML). We present a magnitude scale for local earthquakes based on broadband dynamic strain waveforms. This scale is derived from the peak root-mean-squared strains (A) in 4589 records of dynamic strain associated with 365 crustal earthquakes and 77 borehole strainmeters along the Pacific-North American plate boundary on the west coast of the United States and Canada. In this data set, catalog moment magnitudes range from 3.5≤Mw≤7.2, and hypocentral distances range from 6≤R≤500 km. The 1D representation of geometrical spreading and attenuation of A common to all strain data is logA0(R)=−0.00072R−1.45log(R). After correcting for instrument gain, site terms, and event terms, the magnitude scale, MDS=logA−logA0(R)−log(3×10−9), scales as ≈0.92Mw with a residual standard deviation of 0.19. This close association with Mw holds for events east of the −124° meridian; west of this boundary, however, a constant correction of 0.41 is needed to adjust for additional along-path attenuation effects. As a check on the accuracy of this magnitude scale, we apply it to dynamic strain records from three strainmeters located in the near field of the 2019 M 6.4 and 7.1 Ridgecrest earthquakes. Results from these six records are in agreement to within 0.5 magnitude units, and five out of six records are in agreement to within 0.34 units.

Aseismic strain episodes at Campi Flegrei Caldera, Italy

Abstract. Since 2004 a research project has been developed to monitor subsurface deformation of Italian volcanoes using borehole strainmeters and long-baseline tiltmeters. Six Sacks-Evertson dilatometers were installed around Campi Flegrei caldera and Vesuvius during 2004–2005 (Scarpa et al., 2007), and in 2008 these instruments were supplemented by two arrays of 28–280 m long water-tube tiltmeters in underground tunnels. Relevant strainmeter and tiltmeter data have been collected and analysed from the instruments installed near Campi Flegrei caldera during the recent unrest episodes. In the period 2004–2005 strain, tilt and GPS data from Campi Flegrei indicate the onset of surface deformation that accompanied a low rate of vertical displacement that continued to 2006, corresponding to an increase of CO2 emission. This strain episode preceded caldera microseismic activity by a few months, as was observed also during a significant inflation episode in 1982. Other transient strain episodes occurred in October 2006, which were accompanied by a swarm of VT (Volcano-Tectonic) and LP (Long Period) events, in 2009, at the time of renewed gas emission activity at Solfatara, and again in March 2010, several minutes before a seismic swarm. The time scale of these transient strain events ranges from some hours to several days, putting tight constraints on the origin of ground uplifts at Campi Flegrei. Their location is compatible with a source inferred from long term deformation signals, located about 4 km beneath Pozzuoli. A proposed mechanism for these aseismic strain episodes is that they are associated with magma growth in reservoirs with occasional pressure relief associated with the leakage of gas.

Abnormal Strain Induced by Heavy Rainfall on Borehole Strainmeters Observed in Taiwan

We found some obvious abnormal strain induced by heavy rainfall from borehole strainmeters deployed in Western Taiwan. The strain induced by rainfall can be divided into two parts, one is the quick response for extra loads of rainwater on the ground, and another one is the slow response for rainwater infiltrating into the strata. The quick and slow rainfall responses of areal strain data are analyzed using the technique of recursive digital filtering. Moreover, the rainfall impact functions of the studied stations are calculated using deconvolution. We found, in most cases, the response strain will reach the maximum in half an hour after heavy rainfall, and then show an exponential decay, it might persist more than 200 h depending on the hydrogeological condition around the station. Whereas the river flowing beside the station will help accelerating the runoff dispersion and reducing rainfall decay time in the hill or mountain region. We also compare the results after calibration in term of isotropic and vertical coupling individually. We found that the response strains are smaller in vertical coupling rather than isotropic coupling. The effects of debris avalanches caused by intensive rainfall in the mountain areas can be viewed as two types of rock deformation: generated only under the influence of rainfall and generated by the increased load in the river channels due to rainfall-induced landslides or debris flow. When the cumulative rainfall exceeds a certain threshold, the strain response curves show a noticeable anomaly likely due to the effects of the debris flow events in places prone to landslides.

Inherited State of Stress as a Key Factor Controlling Slip and Slip Mode: Inference From the Study of a Slow Slip Event in the Longitudinal Valley, Taiwan

AbstractUsing borehole strainmeters, we detected a 13‐day long slow slip event in 2011 with Mw = 5.45 on the Longitudinal Valley Fault, Taiwan. It has been likely promoted by the significant Coulomb stress changes (∼0.5–1 MPa) imparted by a combination of coseismic and postseismic slip of the 2003 Chengkung earthquake. Using kinematic slip models, we infer that the slow event has accommodated about 15%–35% of the slip deficit accumulated from 1997 to 2011 in its source region. Further, we find that inherited state of stress may have controlled the slow event southward propagation. We also highlight a spatiotemporal correlation between the slow event and a cluster of repeating microearthquakes, suggesting a complex interplay between seismic and aseismic processes on the fault, where transient aseismic regions are adjacent to highly coupled zones.

Borehole Strain Observations Based on a State-Space Model and ApNe Analysis Associated With the 2013 Lushan Earthquake

YRY-4 borehole strainmeters have been installed in Sichuan Province, China, since 2008, aimed at monitoring the crustal activities associated with earthquakes. In this study, data from six YRY-4 strainmeters at the southwestern endpoint of the Longmenshan fault zone were analysed, to study the relationship of tectonic strain changes with the 2013 Lushan earthquake. We developed a state-space model to remove the strain response due to air pressure, solid tides and the changes in the water level to preferentially isolate non-tectonic disturbances. Strain responses to each influencing factor were estimated using the environmental coefficients computed in the state-space model by an adaptive Kalman filter with measurement noise. The results were consistent with the expected response of the strainmeter systems. The corrected strain considered to originate from underground tectonics provides new insights into the changes in the pre-earthquake strain. Approximate negentropy (ApNe) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${b}$ </tex-math></inline-formula> value were introduced to quantify the probability distribution of the corrected borehole strain and compared with the local seismic activities. The nearest station and two further stations, almost simultaneously recorded short-term ApNe anomalies six to four months before the earthquake. The anomalous region also had a correspondingly low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${b}$ </tex-math></inline-formula> value. Moreover, the anomaly acceleration rates of each station were dependent on the epicentral distances. Further comparison with the strain of random periods illustrated the significance of the extracted anomalies. Our results indicate that the corrected strain may contain seismogenic information and reflect the accelerated strain accumulation of focal areas before the earthquake.

Teleseismic waves reveal anisotropic poroelastic response of wastewater disposal reservoir

Connecting earthquake nucleation in basement rock to fluid injection in basal, sedimentary reservoirs, depends heavily on choices related to the poroelastic properties of the fluid-rock system, thermo-chemical effects notwithstanding. Direct constraints on these parameters outside of laboratory settings are rare, and it is commonly assumed that the rock layers are isotropic. With the Arbuckle wastewater disposal reservoir in Osage County, Oklahoma, high-frequency formation pressure changes and collocated broadband ground velocities measured during the passing of large teleseismic waves show a poroelastic response of the reservoir that is both azimuthally variable and anisotropic; this includes evidence of static shifts in pressure that presumably relate to changes in local permeability. The azimuthal dependence in both the static response and shear coupling appears related to tectonic stress and strain indicators such as the orientations of the maximum horizontal stress and faults and fractures. Using dynamic strains from a nearby borehole strainmeter, we show that the ratio of shear to volumetric strain coupling is ~0.41 which implies a mean Skempton's coefficient of <inline-formula><tex-math id="M2">\begin{document}$ A = 0.24 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SI6250-Andrew_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SI6250-Andrew_M2.png"/></alternatives></inline-formula> over the plausible range of the undrained Poisson's ratio. Since these observations are made at relatively low confining pressure and differential stress, we suggest that the hydraulically conductive fracture network is a primary control on the coupling between pore pressure diffusion and elastic stresses in response to natural or anthropogenic sources.

The characteristics analysis of strain variation associated with Wenchuan earthquake using principal component analysis

Borehole strainmeters that are installed deeply into bedrock are capable of recording both continuous stress and strain measurements, and have consequently become an important tool for monitoring crustal deformation. A YRY-4 borehole strainmeter installed at the Guza Station recorded anomalous changes in borehole strain data preceding the Wenchuan earthquake on May 12, 2008 (UTC) (=8.0). We apply principal component analysis (PCA) to analyze borehole strain data from the Guza Station. The first principal component eigenvalues and eigenvectors are calculated. The fitted results of the cumulative number of anomalous eigenvalues demonstrate that an acceleration occurred approximately 4 months before the earthquake (from January 2008). The results of the combined eigenvalue and eigenvector analyses show that the spatial distribution of eigenvectors and accelerated occurrence of eigenvalue anomalies represents the stress evolution characteristics of the fault from a steady state to a sub-instability state in rock experiments. We tentatively infer that this process may also be linked to the preparation phase of a large earthquake.

Strain Signals Before and During Paroxysmal Activity at Stromboli Volcano, Italy

AbstractIn the last decades, Mt. Stromboli produced four vulcanian eruptions, in 2003 and 2007 and July and August 2019, recorded by INGV monitoring network. Specifically, last three events are studied through records from borehole strainmeters, which allow to infer details on source dynamics. These events are preceded by a slow strain buildup, starting several minutes before the paroxysms, which can be used in future for early warning. Eruptions consist of two or more strain pulses, with oscillations ranging from several seconds, as in 2007, to some minutes, as in 2019, and lasting from several minutes to 1 hr after the explosions.

Strain-Estimated Ground Motions Associated with Recent Earthquakes in California

ABSTRACTPeak ground velocity (PGV) is a commonly used parameter in earthquake ground-motion models (GMMs) and hazard analyses, because it is closely related to structural damage and felt ground shaking, and is typically measured on broadband seismometers. Here, we demonstrate that strainmeters, which directly measure in situ strain in the bulk rock, can easily be related to ground velocity by a factor of bulk shear-wave velocity and, thus, can be used to measure strain-estimated PGV. We demonstrate the parity of velocity to strain utilizing data from borehole strainmeters deployed along the plate boundaries of the west coast of the United States for nine recent M 4.4–7.1 earthquakes in California, including the largest two events of the July 2019 Ridgecrest earthquake sequence. PGVs derived from maximum horizontal shear strains fall within the range of seismic-estimated values recorded at the same distances. We compare the strain-estimated data with GMMs based on seismic PGVs and find consistency in residual polarity (positive vs. negative; the sign of the difference between observed and modeled data) for certain earthquake–station paths, where some paths indicate an overestimation and others indicate an underestimation of strain-derived PGVs, as compared with the GMMs. We surmise that this may be indicative of over or underestimation of shear-wave velocity along those paths, as compared with the average velocity used to derive PGV from strain measurements, or indicative of repeatable site and path effects that are not accounted for in our analyses. This direct comparison of strain with velocity can highlight physical path effects, as well as improve the density and capability of ground-motion recordings.

Observation and Theory of Strain–Infrasound Coupling during Ground-Coupled Infrasound Generated by Rayleigh Waves in the Longitudinal Valley (Taiwan)

ABSTRACT In this study, changes in atmospheric pressure recorded by absolute microbarometers operating in the Longitudinal Valley (Taiwan) during the passing seismic waves from strong earthquakes (Mw≥6) are systematically analyzed during the 2007–2019 period. Using a continuous wavelet transform analysis, local infrasound signals are detected for 23% of the events (21 events out of 89), with Mw ranging from 6.0 to 9.1 at a radial distance of 15 to about 4000 km from the central Longitudinal Valley. Infrasound signals are observed in the period range from about 1 to 20 s; they have maximal amplitudes ranging from 0.4 to 20 Pa and initiate predominantly during the passage of Rayleigh waves. The atmospheric pressure response to dilatational strain waves during seismoacoustic disturbances is investigated using collocated borehole strainmeter stations, and dynamic interactions between signals are characterized using a sliding windowed time-lagged cross-correlation analysis. The infrasound response shows a phase shift of −60° to −100°, with respect to the dilatation strain signal with a coupling factor of 0.002–0.006 Pa/nϵ for most of the cases (62%). Whereas acoustic pressure fluctuations are generated instantaneously by the vertical seismic velocity, the phase delay is related to the intrinsic nature of the dilatational strain. Observational strain–infrasound coupling parameters are in close agreement with theoretical estimates in the case of ground-coupled acoustic signals generated by Rayleigh waves. The study represents the first attempt to analyze ground-coupled infrasonic waves with strain waves and illustrates the potential of collocated strainmeter–microbarometer stations for basic seismoacoustic studies.

The 24 December 2018 Eruptive Intrusion at Etna Volcano as Revealed by Multidisciplinary Continuous Deformation Networks (CGPS, Borehole Strainmeters and Tiltmeters)

AbstractWe have modeled the fast dike intrusion that started on 24 December 2018 at Mount Etna. The intrusion was accompanied by an intense seismicity swarm that also continued the following day. Since previous studies did not detail the overall chain of events in time during the magma ascent, here we propose a combined analytical and FEM modeling of all available continuous deformation data, focusing on the signals over 2 days (24–25 December) when the continuous deformation networks recorded clear variations directly related to the dike ascent. High‐rate GPS enabled obtaining an early and reliable source model. Borehole instruments (strainmeters and tiltmeters) highlighted clear variations, starting about 1 hr before those recorded by GPS, and moreover, made it possible to improve the dike ascent modeling. In particular, our continuous deformation data clearly revealed not one but two dikes and showed how they evolved in time. We inferred one dike that began propagating in the first kilometer above sea level, continued to rise with a maximum opening of 1.9 m and increased its horizontal dimension until reaching the ground surface. Soon after, continuous deformation networks revealed a new elongated intrusion in the southern flank, matching the south shifted position of the seismic swarm. This second dike, with a thicker opening of 4.9 m, started from a depth of about −3 km (below sea level) but did not reach the ground surface. This proposed multiparametric modeling of continuous deformation data has therefore enabled disentangling the complexity of the real volcanic processes.

The Potential of Using Dynamic Strains in Earthquake Early Warning Applications

Abstract We investigate the potential of using borehole strainmeter data from the Network of the Americas (NOTA) and the U.S. Geological Survey networks to estimate earthquake moment magnitudes for earthquake early warning (EEW) applications. We derive an empirical equation relating peak dynamic strain, earthquake moment magnitude, and hypocentral distance, and investigate the effects of different types of instrument calibration on model misfit. We find that raw (uncalibrated) strains fit the model as accurately as calibrated strains. We test the model by estimating moment magnitudes of the largest two earthquakes in the July 2019 Ridgecrest earthquake sequence—the M 6.4 foreshock and the M 7.1 mainshock—using two strainmeters located within ∼50 km of the rupture. In both the cases, the magnitude based on the dynamic strain component is within ∼0.1–0.4 magnitude units of the catalog moment magnitude. We then compare the temporal evolution of our strain-derived magnitudes for the largest two Ridgecrest events to the real-time performance of the ShakeAlert EEW System (SAS). The final magnitudes from NOTA borehole strainmeters are close to SAS real-time estimates for the M 6.4 foreshock, and significantly more accurate for the M 7.1 mainshock.

Geophysical precursors of the July-August 2019 paroxysmal eruptive phase and their implications for Stromboli volcano (Italy) monitoring

Two paroxysmal explosions occurred at Stromboli volcano in the Summer 2019, the first of which, on July 3, caused one fatality and some injuries. Within the 56 days between the two paroxysmal explosions, effusive activity from vents located in the summit area of the volcano occurred. No significant changes in routinely monitored parameters were detected before the paroxysmal explosions. However, we have calculated the polarization and the fractal dimension time series of the seismic signals from November 15, 2018 to September 15, 2019 and we have recognized variations that preceded the paroxysmal activity. In addition, we have defined a new parameter, based on RSAM estimation, related to the Very Long Period events, called VLP size, by means of which we have noticed significant variations through the whole month preceding the paroxysm of July 3. In the short term, we have analyzed the signals of a borehole strainmeter installed on the island, obtaining automatic triggers 10 minutes and 7.5 minutes before the July 3 and the August 28 paroxysms, respectively. The results of this study highlight mid-term seismic precursors of paroxysmal activity and provide valuable evidence for the development of an early warning system for paroxysmal explosions based on strainmeter measurements.

Kinematic Rupture and 3D Wave Propagation Simulations of the 2019 Mw 7.1 Ridgecrest, California, Earthquake

ABSTRACTWe model the kinematic rupture process of the 2019 Mw 7.1 Ridgecrest, California, earthquake using numerical simulations to reproduce the elastodynamic wave field observed by inertial seismometers, high-rate Global Navigation Satellite System stations, and borehole strainmeters. This was the largest earthquake in Southern California in 20 yr and was widely felt throughout the region. The Mw 7.1 mainshock was part of a large sequence of ∼30,000 aftershocks and was notably preceded by an Mw 6.4 foreshock by 34 hr on fault structures that were once poorly understood. A large number of seismic and geodetic instruments measured the rupture process for both events, with many stations located in the near field. Hence, this is a rare opportunity to better understand complex earthquake processes that arise in an immature fault zone using advanced computing. Of the kinematic rupture models that we tested, our preferred is the simplest one that reproduces signals recorded by the three different geophysical datasets; it is composed of four distinct ruptures that progressively migrate to the southeast with delayed initiation times, and typical rupture speeds. This type of model does a better job at matching the recorded ground motions and deformations than does one composed of a continuous rupture with very low-rupture velocity, as proposed in other studies of this earthquake.

Kinematics of Fault Slip Associated with the 4–6 July 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACT The 2019 Ridgecrest, California, earthquake sequence produced observable crustal deformation over much of central and southern California, as well as surface rupture over several tens of kilometers. To obtain a detailed picture of the fault slip involved in the 4 July M 6.4 foreshock and 6 July M 7.1 mainshock, we combine strong-motion seismic waveforms with crustal deformation observations to obtain kinematic and static slip models of both events. We sample the regional seismic wavefield for both the foreshock and mainshock with three-component records from 31 stations of the California Integrated Seismic Network. The deformation observations include Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and borehole strainmeter recordings of the dynamic strain field. These data collectively constrain the kinematic coseismic slip distributions of the events, with measurements variously observing coseismic slip from one event (e.g., seismic waveforms, kinematic solutions from continuous GPS, and strainmeter time series) or coseismic slip from both events combined (InSAR). We find that the foreshock ruptured two separate faults, one with left-lateral strike slip on a northeast–southwest-trending fault and the other with right-lateral strike slip on an orthogonal fault, with unilateral rupture propagation along both. The mainshock ruptured a series of northwest–southeast-trending faults with right-lateral strike slip concentrated in the uppermost 6 km with exceptionally low-rupture velocity averaging 1.0–1.5 km/s. A possible explanation for the low-rupture velocity is that the mainshock rupture expended relatively high energy, generating secondary fractures in off-fault deformation, which is consistent with field and seismic evidence of plastic deformation on small fault strands adjacent to the main rupture trace.

Investigation on Issuing Earthquake-prediction Related Information for an Unusual Strain Change Observed by a Borehole Strainmeter at the Mikkabi Station, Central Japan in 2003: Implications for the Nankai Trough Earthquake Information

Investigation on Issuing Earthquake-prediction Related Information for an Unusual Strain Change Observed by a Borehole Strainmeter at the Mikkabi Station, Central Japan in 2003: Implications for the Nankai Trough Earthquake Information

Static-dynamic strain response to the 2016 M6.2 Hutubi earthquake (eastern Tien Shan, NW China) recorded in a borehole strainmeter network

The 2016 M6.2 Hutubi Earthquake is an event occurred on a reverse fault located in the northern Chinese Tien Shan, but it is debatable whether the seismogenic fault is a low-angle thrust fault. The coseismic strain responses have been recorded by nine 4-component RZB borehole strainmeters, locating 11–320 km away from the epicenter. The nearest four stations have recorded resolvable static strain responses. The authors used the theoretic tide to calibrate the strains, and relocated the 10.31 km by 6.85 km fault plane through a grid-search method. Two groups of fault parameters 279°/70°/87° and 103°/28°/85° were obtained, and the predictions for 24/26 channels for both groups are consistent with the offsets observed in the near stations by correcting the azimuths. The seismogenic fault is more likely the high-angle backthrust fault based on the comparison with the selected aftershocks. Furthermore, all strainmeters have recorded the dynamic strain waves. The dynamic strains are sensitive to the seismic wave propagation direction, and the peak dynamic strains decay faster in the N-S direction than that in the E-W direction due to the regional structure features. The absolute static strains are comparable to the dynamic strains in the near field, but the ratios between them decrease remarkably in the intermediate and far field. The difference in the dependence on distance was found to be useful to differentiate static strains from dynamic strains.

Cyclic magma recharge pulses detected by high-precision strainmeter data: the case of 2017 inter-eruptive activity at Etna volcano

Unprecedented ultra-small strain changes (~10−8–10−9), preceding and accompanying the 2017 explosive-effusive activity, were revealed by a high precision borehole strainmeter at Etna. No pre- or co-eruptive deformation was detected by the GPS measurements, which often fail to detect ground deformation engendered by short-term small volcanic events due to their limited accuracy (millimetres to few centimetres). Through the analysis and detection of ultra-small strain changes (few tens of nanostrain), revealed by filtering the raw data, a significant time correspondence with the eruptive activity is observed. For the first time, cyclic fast exponential strain changes, preceding the onset of eruptive events, with a timescale of about 2–7 days, were detected. These variations are attributable to the expansion of the shallow magma reservoir, which is replenished with new magma from depth during the inter-eruptive periods. Interpreting the strain changes in terms of pressurization/depressurization of the chamber due to the cyclic influx and withdrawal of magma, allows placing some constraints on the magma recharge volume rate. A Finite Element model has been developed to simulate the temporal evolution of the strain changes generated by the re-pressurization of a spheroidal magma source using a dynamical approach. An average total mass budget of about 1–2 × 109 kg, which is in the range of the erupted mass, is estimated to be accumulated within a shallow vertically elongated magma chamber during the inter-eruptive periods. Such evidence demonstrates that the near-real time analysis of strainmeter records is remarkable for its ability to record small transients and highlight recharging phases preceding eruptive activity, which would go undetected with other current methodologies. Under these conditions, the ability to simulate inter-eruptive periods offers an opportunity to estimate the magma recharge rate with important implications for volcano hazard assessment.

Intermediate‐Magnitude Postseismic Slip Follows Intermediate‐Magnitude (M4 to 5) Earthquakes in California

AbstractThe magnitude of postseismic slip is useful for constraining physical models of fault slip. Here we examine the postseismic slip following intermediate‐magnitude (M 4 to 5) earthquakes by systematically analyzing data from borehole strainmeters in central and northern California. We assess the noise in the data and identify 11 earthquakes that generated interpretable strain records. We estimate the earthquakes' postseismic to coseismic moment ratios by comparing the coseismic strain changes with strain changes induced by afterslip in the following 1.5 days. The median estimated postseismic moment is 0.45 times the coseismic moment, with a 90% confidence interval between 0.25 and 0.60. This postseismic moment is slightly larger than typically observed following large (M &gt; 6) earthquakes but smaller than observed following small (M2 to 4) earthquakes. The intermediate‐magnitude postseismic slip suggests a size dependence in the dynamics of earthquakes or in the properties of fault areas that surround earthquakes.