Search

The reliable estimation of earthquake magnitude and stress drop are key in seismology. The novel technology of distributed acoustic sensing (DAS) holds great promise for source parameter inversion owing to the measurements' high spatial density. In this study, I demonstrate the robustness of DAS for magnitude and stress drop estimation using the empirical Green's function deconvolution method. This method is applied to nine co‐located earthquakes recorded in Israel following the 2023 Turkey earthquakes. Spectral ratios were stacked along the fiber, and fitted with a relative Boatwright source spectral model. Excellent fits were obtained even for similar sized earthquakes. Stable seismic moments and stress drops were calculated assuming that the moment of one earthquake is known. DAS derived estimates were found to be more stable and reliable than those obtained using a dense accelerometer network. The results demonstrate the great potential of DAS for source studies. Plain Language Summary: Estimating earthquake magnitude and stress drop, the two most fundamental source parameters, is key for various seismological investigations, including earthquake self‐similarity and hazard mitigation. The extraction of source parameters requires that path and site effects are reliably deconvolved from source contributions. One approach is to deconvolve a small earthquake from a co‐located larger one in the frequency domain such that common path and site functions cancel and relative source function, between both earthquakes, remain. In this study, I apply the deconvolution approach to nine earthquakes that followed the 2023 Turkey earthquakes, recorded using distributed acoustic sensing (DAS) in Israel. DAS converts standard optical fibers into dense arrays, with seismic measurements every few meters along tens‐of‐kilometers long fibers. The deconvolution was calculated for different earthquake pairs and excellent fits were obtained with a relative source spectral model. Stable seismic moments and stress drops were calculated. These parameters were found to be more reliable than those obtained using a dense standard‐accelerometer network. The results demonstrate the great potential of DAS for source studies. Key Points: The spectral ratios approach was applied to nine earthquakes in Israel recorded by distributed acoustic sensing (DAS) and accelerometersAccurate relative magnitudes and stress drops were calculatedDAS can provide reliable source parameter estimates that may outperform accelerometers [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

Extreme temperature and pressure conditions on the surface of Venus present formidable technological challenges against performing ground‐based seismology. Efficient coupling between the Venusian atmosphere and the solid planet theoretically allows the study of seismically generated acoustic waves using balloons in the upper atmosphere, where conditions are far more clement. However, earthquake detection from a balloon has never been demonstrated. We present the first detection of an earthquake from a balloon‐borne microbarometer near Ridgecrest, CA in July 2019 and include a detailed analysis of the dependence of seismic infrasound, as measured from a balloon on earthquake source parameters, topography, and crustal and atmospheric structure. Our comprehensive analysis of seismo‐acoustic phenomenology demonstrates that seismic activity is detectable from a high‐altitude platform on Earth, and that Rayleigh wave‐induced infrasound can be used to constrain subsurface velocities, paving the way for the detection and characterization of such signals on Venus. Plain Language Summary: The interior structure of Venus remains unknown due to lack of in situ seismic observations. Adverse temperature and pressure conditions on the Venusian surface limit the lifetimes of landers to a few hours, which poses a technological challenge against performing ground‐based seismology to detect venusquakes. Seismic energy on Venus, as on Earth, can be transmitted into the atmosphere through mechanical coupling and propagate as low‐frequency sound (infrasound). Infrasound from earthquakes travels long distances and has been detected from ground‐based stations. This mechanism may allow the detection of seismically generated pressure disturbances on Venus using balloons, enabling remote seismology from its upper atmosphere, where temperature and pressure conditions are far more clement and longer mission lifetimes are likely. However, the feasibility of such a technique has yet to be established through the detection of ground motion following an earthquake using a freely floating balloon. We demonstrate the first detection of an earthquake from a high‐altitude balloon. We explore the dependence of the pressure signal seen by the balloon on parameters such as the magnitude, focal mechanism, location of the earthquake, surface topography, and atmospheric structure. We also show how the signal recorded at the balloon can be used to study the subsurface. Key Points: First detection of a natural earthquake using balloon‐borne infrasound dataRayleigh wave‐induced infrasound dispersion characteristics provide constraints on subsurface velocitiesShallow waveguides, focal mechanism, and subwavelength topographic changes control infrasound amplitude and dispersion by weak earthquakes [ABSTRACT FROM AUTHOR]

The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred ∼6 hr apart on 12 August 2018, in the northeastern Brooks Range. The earthquakes were captured by Sentinel‐1 interferometric synthetic aperture radar (InSAR) satellites and Earthscope Transportable Array seismic data, giving insight into the little‐known active tectonic processes of Arctic Alaska, obscured until recently by sparse data availability. In this study, InSAR modeling, teleseismic back projections, calibrated hypocentral relocations, and regional moment tensor solutions resolve two previously unknown, SSW dipping right‐lateral fault segments. These are the first active faults identified as conjugate to the NE trending sinistral Canning displacement zone directly to the west, which is therefore a more complex zone of diffuse faulting than previously thought. The northeastern Brooks Range has been characterized as an area of low to moderate seismic hazard, but these earthquakes illustrate the potential for larger, possibly destructive events in a region earmarked for rapid resource development. Plain Language Summary: The largest earthquakes recorded in northern Alaska (magnitude 6.4 and magnitude 6.0) occurred ∼6 hr apart on 12 August 2018. Few active faults are mapped in this region despite widespread seismicity, and the current tectonic setting remains unclear due to limited available data and the remote location. We use satellite radar images and seismic data to resolve two previously unknown fault segments, along which the magnitude 6.4 earthquake ruptured unilaterally eastward. This fault geometry demonstrates that the Canning displacement zone, the main tectonic feature in the area, is a more complex zone of diffuse faulting than previously thought. These results are also important for reassessing seismic hazard by illustrating the potential for damaging earthquakes on seemingly aseismic faults. Key Points: The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred on previously unknown active faultsInSAR and calibrated hypocenter relocations indicate that the faults are conjugate to the Canning displacement zoneThe Canning displacement zone involves a complex fault network and vertical‐axis block rotations [ABSTRACT FROM AUTHOR]

The 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 > 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. Key Points: We examine postseismic slip following M4‐5 earthquakes using borehole strain dataThe median estimated postseismic moment is between 0.25 and 0.6 times the coseismic momentThe postseismic moment ratio is intermediate between estimates for small and large earthquakes [ABSTRACT FROM AUTHOR]

The 3 September 2017 underground nuclear test (mbNEIC = 6.3) is the largest of six announced North Korean explosions. The event generated many P wave seismograms at global broadband seismic stations with good signal‐to‐noise ratio for periods less than ~5 s. Instrument deconvolution provides 435 stable broadband P wave ground displacement records in the period range 0.1 to 5.0 s. These are stacked in 26 azimuth/distance windows to average path and receiver effects. Waveform stacks and average amplitude of 4‐Hz ground displacements are modeled assuming a Mueller‐Murphy explosion source model for a granite source medium. Nonelastic pP delays consistent with burial depths in the mountainous source topography are considered, and explosion yield and an average constant‐Q attenuation operator are estimated by fitting the waveforms. For a source depth of 750 m in heavily damaged environment, the estimated yield = 230 ± 50 kt and t* = 0.78 ± 0.03 s. Plain Language Summary: Seismic P wave ground motions recorded around the world for the 3 September 2017 underground nuclear test at the North Korean test site are modeled to constrain the explosion yield and the average path attenuation parameter. Broadband ground motion waveforms are modeled along with the amplitude of 4‐Hz ground motions, exploring a wide range of attenuation and yield parameters along with allowing for nonelastic behavior of the P wave reflection (pP) from the free surface above the explosion, which travels down through the damage zone surrounding the explosion‐produced cavity. For a source depth of 750 m, which is reasonable for location of the explosion under the mountain peak, and allowing for an extensive damage zone, the waveform modeling provides an estimated yield of 230 ± 50 kt. Yield estimates of smaller events are made based on calibration of magnitude‐yield relations using this result and range from 0.5 to 17.8 kt. Key Points: Teleseismic broadband and high‐frequency P wave displacements are modeled using the Mueller‐Murphy model to constrain the 2017 yieldP wave displacements for frequencies above 4 Hz have little influence from the free surface reflection pPFor a source depth of 750 m and an extensive damage zone that delays pP, the yield is estimated to be 230 ± 50 kt for t* = 0.78 ± 0.03 s [ABSTRACT FROM AUTHOR]

Fracture damage patterns around faults induced by dynamic earthquake rupture are an invaluable record to clarify the rupture process on complex fault networks. The 2016 Mw 7.8 Kaikōura earthquake in New Zealand has been reported as one of the most complex earthquakes ever documented that ruptured at least 15 crustal faults. High‐resolution optical satellite image displacement maps provide distinctive profiles of displacement across the faults and help visualize the off‐fault damage pattern. They are combined with field observation and coupled with a numerical tool that captures the dynamics of the rupture and simultaneous activation of off‐fault damage to allow the determination of the most likely rupture scenario. This study demonstrates that complex rupture processes can be explained in a rather simple way via a synergetic combination of state‐of‐the‐art observation and first principle physics‐based numerical modeling of off‐fault damage. Plain Language Summary: In a first‐of‐its‐kind work, we combine optical image correlation at a resolution never achieved before (1.8‐m ground resolution), field observations, and a unique dynamic rupture simulation tool, including off‐fault deformation, to unravel, beyond doubt, part of the Kaikōura rupture process. We show that matching observed off‐fault deformation pattern with rupture simulation helps easily identify the earthquake rupture path to propose a kinematically simple earthquake scenario. Thus, this work shows that when building models of seismic hazard, like in Southern California, systematic consideration of off‐fault damage patterns for past events would help understand earthquake scenario and to model future events. Hence, this presents a great potential to narrow down the multitude of rupture scenarios along complex fault networks that need to be considered in seismic hazard model assessment. Key Points: Matching observed off‐fault deformation pattern with rupture simulation helps identify the earthquake rupture pathSystematic consideration of off‐fault damage patterns for past events would help understand earthquake scenario and to model future eventsThis presents a great potential to narrow down the multitude of rupture scenario on complex fault system [ABSTRACT FROM AUTHOR]

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long‐term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992–1999, ENVISAT 2003–2010, and Sentinel‐1 2014–present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2–7 over the past few decades. We consider quasi‐static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long‐lasting effect on SSAF creep rates. Plain Language Summary: There are two significant conclusions from this study. First, we analyzed 25 years of InSAR measurements over the Southern San Andreas Fault system to document a major increase in the average creep rate following the 1992 Mw 7.3 Landers Earthquake which is then followed by creep rate reductions after the 1999 Mw 7.1 Hector Mine Earthquake and the 2010 Mw 7.2 El Major Cucapah Earthquake. Second, we attribute all these creep rate changes to the Coulomb stress variations from these three major Earthquakes. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers and lower rates after the other two events. An implication of this study is that small but steady Coulomb stress changes have a larger impact on shallow creep than the larger dynamic stress changes associated with passing seismic waves. These results illuminate the significance of time scale‐dependent complexity of shallow fault creep and how these behaviors are communicated by stress perturbations from regional earthquakes. Key Points: We provide detailed new observations that constrain variations in average creep rates following regional earthquakesA new model is proposed for understanding dynamic/static stress changes and short‐ and long‐term creep rate variations on the shallow SSAFOur observations suggest that static stress change has a long‐lasting effect on fault creep, even in the presence of transient processes [ABSTRACT FROM AUTHOR]