Advanced seismic waveform analysis in the Marmara region: uncovering stress accumulation and release patterns with python

The effect of periods on the accumulation and release of stress in GaAs/AlInAs superlattices structure is reported here. It is observed that in GaAs/AlInAs superlattices, when the Indium (In) content is greater than 10%, stress accumulates monotonically as the number of period increases. In GaAs/AlInAs superlattices with an In content of 5%, the accumulated stress is larger when the number of periods is less than 10. However when the number of periods exceeds 10, it was observed that suddenly there is a significant increase in defects and stress release. However, with any further increase in period number, there is once again an accumulation of stress.

The measurement of relative plate velocities during the past few years is a signal accomplishment in earth science, leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins. Regrettably, deformation premonitory to an earthquake has yet to be detected with confidence. Delineation of the spatial and temporal buildup of strain between earthquakes, however, has put limits on models of the earthquake cycle. A diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike‐slip faults. In contrast, the geometry of thrusts and normal faults, revealed by earthquake deformation, has been found to differ markedly from expectations. Geodetic observations have proved vital to monitor the ascent of magma through the Earth's crust and to predict volcanic eruptions at the Earth's surface. Episodic vertical and horizontal deformation in southern California remains a subject of dispute; if anything, it is less certain than it once seemed.

We constructed a 3-D physical model of tectonic loading at transcurrent plate boundaries by considering viscoelastic stress relaxation in the asthenosphere and spatial variation in frictional properties (peak strength and critical weakening displacement) of faults. With this model we simulated the process of stress accumulation and release at a seismogenic region with relatively high strength on the plate interface. In low strength regions surrounding the seismogenic region, quasi-static fault slip gradually proceeds with the progress of relative plate motion. The increase of slip deficits in the seismogenic region brings about stress concentration at its margin. The stress accumulation rate is roughly proportional to the inverse of the effective fault length. The accumulated stress is released by unstable dynamic rupture if the critical weakening displacement D c is small, and by stable fault slip if D c is very large. When a fault system consists of two adjacent seismogenic regions, sudden stress release in one region accelerates the stress accumulation process in another region through transient viscoelastic stress transfer as well as instantaneous elastic stress transfer. This indicates the importance of elastic and viscoelastic interaction between adjacent seismic faults even in stress accumulation processes.

In this work we have analyzed the results of temporal geodetic variations associated with tectonic deformation on the Andean backarc in southern San Juan province, Argentina. This area is affected by the subduction of the Juan Fernandez Ridge and has the greatest seismic hazard in Argentina. Since 2000, we have monitored the variations of gravity annually with high precision (g4D) in the active deformation belt, around the city of San Juan (31°S to 32°S, 68°W to 69°W). Temporal gradients of g4D have been associated with active structures and interpreted as subsurface mass redistributions as a result of accumulation and release of stress. These data, in addition to the topographic deformations measured by the Global Positioning System (GPS) and the identification of structures in the crystalline basement from the geophysical models, allow us interpret mechanisms of deformation in the contact between the Precordillera and Pampean Ranges in the foreland of San Juan. In this area, after more than a decade of low activity, in 2012 the Tulum Fault System (TFS) was reactivated, generating crustal events of moderate magnitude including an earthquake of Mw 5.0 (January 18, 2012). According to our interpretation, the fault system accumulated elastic deformation for more than 10 years expressed as height increases at gravity stations (decrease of g4D) until the year 2011. The January 18, 2012 earthquake released part of the energy at the same time as a coseismic increase of g4D. This should be reflected in regional tectonic subsidence from the point of view of conventional gravimetric models. However, GPS reoccupation of the benchmarks located in this fault system indicates a generalized increase in altitude of the fault system in the last 15 years. TFS behavior could be assumed as topographic flexural displacements induced by a system of blind faults. We analyzed the time series of a permanent Global Navigation Satellite System (GNSS) station located to the north of the TFS. The GRLS station (31.61oS; 68.32oW) presents slow variation in the vertical and north components. The variation began on December 20, 2011 reaching its maximum amplitude on January 18, 2012. The geodetic variations coincide with the period of seismic reactivation.

Finite element methods are used to model the dynamics of deformation along complex transform plate boundaries, specifically the San Andreas fault system, California. Effects of mantle rheology and fault geometry on the stress buildup and release are investigated. No prior knowledge of the earthquake cycle time or amount of fault slip is assumed and the results suggest that the San Andreas fault slips at low shear stress (≈15 MPa). Although the maximum stress on the fault is 15 MPa, models with an upper mantle shear zone deforming entirely by dislocation creep accumulate stresses that exceed 100 MPa, a stress level high enough to drive localized dynamic recrystallization and a shift in dominant deformation mechanism to diffusion creep. Models in which the mantle shear zone deform locally by diffusion creep reach a dynamic steady state where lithospheric shear stresses never exceed the specified fault stress anywhere in the model and indicate that the strength of the upper mantle is an important parameter in the dynamics of plate boundary deformation.

Temporal change in plate coupling and long-term slow slip events in southwestern Japan

Deep earthquakes of up to moderate size have been analyzed by a long‐period waveform inversion of Global Digital Seismographic Network data where the use of a point source model proved to be adequate, but the use of a standard earth model was inadequate without modification. Modifications include calculations in an earth model based on PREM but with different source and receiver structures and the introduction of time delays which were determined together with the moment tensor components of the source by an iterative correlation method. The moment tensor components are in accordance With the pattern of stress release in other work, but results for the seismic moment differ somewhat and there is no compelling evidence for deviations from a double couple mechanism. The time delays were inverted to find the source centroid depths and time shifts and the station residuals. There is no baseline effect in the times of source duration which could be explained by a simple circular source model, but there is such an effect in the station residuals together with a distance trend especially for S. Part of the baselines may be an effect of bias in origin times, but the pattern of S residuals in particular suggests also possible modifications of the standard model.

Postseismic stress diffusion in Chile and South Peru

We present results of a study of the peculiarities of the seismicity and electrical conductivity distribution beneath the Ukrainian Eastern Carpathians. Based on the analysis of seismic data for the years 1999–2016, specific zones of concentration of earthquake sources related to the principal fault systems and their intersections have been distinguished. This paper covers two zones, one linked to the contact of the Outer Carpathians and the Carpathian Foredeep and another one linked to the fault system transverse to the Carpathians strike. Both belts of earthquake sources concentration correlate well with the geoelectric models of the studied area obtained as a result of 2D and quasi-3D inversion. Most of the seismic events occur at the intersection of the mentioned seismic zones, at shallower depths, than the main conductive structures appear, concentrated at their marginal parts. The interrelation of both phenomena suggests their common explanation by processes occurring in active fault systems: fracturing, shear deformation, migration of highly mineralized fluids, high porous pressure, accumulation and release of tectonic stress.

Abstract. Earthquake is a major natural disaster triggered by the accumulation and release of crustal stress, and the accurate extraction of pre-seismic anomaly signals is crucial to improve the earthquake prediction capability. In this study, an anomaly detection method for borehole strain data based on the combination of Segmented Variational Modal Decomposition (SVMD) and Informer network is proposed, and a pre-seismic anomaly extraction study is carried out for the 2021 Maduo Ms 7.4 earthquake in Qinghai. The SVMD method effectively solves the memory limitation problem of traditional Variational Modal Decomposition (VMD) when dealing with large-scale data through the sliding-window mechanism, and at the same time maintains the correlation between the data. The Informer network significantly reduces the computational complexity of the long-series prediction and realizes the high-precision one-time long-time series prediction by utilizing its ProbSparse self-attention mechanism and self-attention distillation. By analyzing the borehole strain data from the Menyuan station, this study identifies the accelerated anomaly accumulation phenomenon in the two stages before the Maduo earthquake: in the first stage, the number of anomalous days shows an accelerated growth starting from about 3 months before the earthquake (13 February 2021); in the second stage, the anomalous accumulation tendency is further intensified since the second month before the earthquake (the end of March 2021), and the accumulation curve shows a typical S-shape growth characteristic. The results are highly consistent with the time windows of the index of microwave radiation anomaly (IMRA), outward long-wave radiation (OLR) and geoelectric field anomalies, and with the subsurface-to-atmosphere multilayer anomalies (e.g., Benionff strain, CO concentration, electron concentration anomalies), which indicate that the borehole strain anomalies are closely related to the gestation process of the Maduo earthquake. This study provides a new method for the extraction of pre-seismic anomalies based on machine learning, and provides an important basis for understanding earthquake precursors.

Transient deformation resulting from postseismic stress relaxation in underlying viscoelastic layers is examined. The main purpose of the present study is to understand spatial and temporal modes of postseismic deformation especially due to strike-slip faulting with a realistic geometry. The fault size as well as the viscoelastic structure of the earth proved to have large effects on the amount of postseismic deformation. Therefore, it should be taken into serious consideration in modeling the faulting. It is noteworthy that the concentration of shear strain into the near field continues long after the faulting. This strain concentration is similar to the preseismic strain accumulation and is contrary to the coseismic strain release. The viscoelastic structure of the earth, however, is hard to determine solely by the surface measurements performed at long time intervals. Crustal movements in the Inner Zone of Southwest Japan, in particular, those associated with the 1927 Tango earthquake of M=7.5, are then investigated in connection with postseismic stress relaxation. The discrepancies between the observed and calculated postseismic deformations can be reduced if the effect of tectonic stress is added to the calculated deformation field. It is widely accepted that the basic model to explain the physical behavior of the earth's crust and upper mantle generally consists of three layers with different rheological properties; namely, the uppermost elastic and brittle lithosphere, the intervenient ductile asthenosphere with low viscosity, and the substratum with relatively high viscosity. In addition, it is also recognized that there exist considerable regional heterogeneities in both thicknesses and properties of the layered structure of the earth. These heterogeneities probably characterize the mode of accumulation and release of tectonic stress in the region, and this mode is reflected in the crustal movements and seismic activity peculiar to the region.

Two asoismicallyc …. pin faults are taken to be altuated in a visco-elastic half-space, representiDl tbo IIthOlphor.· .. thenOlphere systOlD. Solution. are obtalned for Ih. dl.plac:emenls, atreues and aURins, usioS a technique involving the use of Green’s functioDi aDd intogral transforms, for three possible cases-the case of the absence of any fault creep, the case in which one fault is creeping and tbe other is locked and lbe …. In which both the faull. are creepi .. , taking inlo accounllhe displace· ments and stresses present initially. and assuming that the tectonic torce. maintain a cODstant shear stress Car away from tbe faulla. The types of lault creep Cor which the displacements. stresses and strains are finite everywhore In lhe model near the faullS are idenlified, and Ihe condition. salisfi.d by these types of rault creep are determined in a simple form. Fault creep across a fault is pneraUy found to reduce the rate of accumulation of shear stress near lbelf. The effect of aseismic creeP across one fault on the shear stress ncar tho other faull is found 10 depend on the distanco, dimensions, reJative positIon and other characteristics of the two faults. Fault creep acrObS one fault is aeneraUy fOUnd to reduce the rate of shear stress accumulation near the other neiahbour .. inB fault in the theoretical model considered. Under suitable circumstancea. aseismic creep across one of the raules is roulld to result in aseismic release of ahear stress near both the fauhs, thus reducing prosressh·ely the possibility of a sudden Cault movement, generating an earthquake. The inHuence of one creep.. iog Cault OD another is found to decrease quito rapidly with increase in tho distance between Ihe faults. The po. sible useS of Ihe mod.1 in the sludy of lhe interaction between neiahbouriog strike-slip faults in the lithosphre and in the estimation of return times of earthquakes is examined

Resolution of earthquake source characteristics is limited by structural complexities in the source region. This study explores the effects of dipping near‐source bathymetry and structure on long‐period teleseismic body waveforms from shallow subduction zone earthquakes. Ray theory synthetics calculated for shallow double‐couple sources and velocity structures incorporating planar dipping interfaces suggest that P waveforms are most sensitive to water depth and sea bottom orientation. For sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry. Sediment thickness and the dip of the sediment‐crust and crust‐mantle interfaces generally have lesser effects. The high‐amplitude arrivals result largely from changes in the source takeoff angles of the reverberations. These arrivals present difficulties for source depth and time function determination from P waveforms and may cause spurious complexity in the derived pattern of earthquake moment release. SH waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events, suggesting that SH waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure. Effects of nonhorizontal structure are observed in the P waveforms from shallow Kurile Trench events (Ms 5.8–7.2), which show prominent later arrivals not observed on SH waveforms. Synthetics calculated for a dipping water‐crust interface provide a better fit to these arrivals than those calculated for a horizontal interface, suggesting that the P waveforms show systematic effects of seafloor dip. The synthetics are highly sensitive to the assumed orientation of the seafloor; the data are fit best for seafloor orientations consistent with the dip of the inner trench slope. The distorting effect of the water layer gives rise to systematic differences in P wave complexity between shallow events near the seismic front, which occur beneath a thick water layer and show numerous later arrivals, and deeper, arcward events which occur beneath a thin water layer and show simple waveforms. Large reverberations in the P waveforms of the October 20, 1963 and June 10, 1975 Kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture; however, SH waveforms suggest simple source time functions with durations of 25 and 43 s. Inversion of P and SH waveforms and amplitudes yields shallowly dipping thrust mechanisms, with no conclusive evidence for a previously proposed steepening of the fault dip during rupture. The large P wave reverberations cannot be fit using a point source and a horizontal seafloor; detailed modeling of the 1963 event as an upward propagating rupture extending through low‐velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched. The anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments.

We simulated dynamic rupture propagation along various nonplanar fault models of the 1999 Izmit, Turkey, earthquake using a boundary integral equation method. These models were inferred from geological and geodetic observations. Based on these results, we modeled seismic-wave propagation around the fault system using a finite difference method. We focused on the effect of different fault geometries on the rupture process and seismic-wave propagation. Numerical simulation results imply a rapid and continuous rupture propagation from the Izmit–Sapanca Lake segment to the Sapanca–Akyazi segment. The rupture under Sapanca Lake appears to have propagated not on a disconnected fault segment but along a smooth fault structure with a bend of only a few degrees. The observational complexity of the surface breaks, however, can be best simulated by a highly segmented fault model. This infers that fault geometric characters observed in the field reflect near-surface structure and that seismological and geodetic features are controlled by global fault structure at depth. Then we investigated the effect of frictional parameters and the initial stress field. In order to explain near-field seismograms at station SKR, located a distance of a few kilometers from the fault, we had to force the rupture to propagate at shallow depth close to the station. In order to obtain this, we had to introduce a finite cohesive force in the friction law that allows stress accumulation and release in the shallow crust. The external stress field had to be large enough for the rupture to propagate at very rapid speed. Our simulation results show that it is important to include detailed fault geometry in the numerical simulation, and to constrain frictional parameters and the initial stress field, for understanding of the full dynamic process of an earthquake.

Stress accumulation and release on the San Andreas fault