Coseismic Static Stress Changes Research Articles

On the Patterns and Scaling Properties of the 2021–2022 Arkalochori Earthquake Sequence (Central Crete, Greece) Based on Seismological, Geophysical and Satellite Observations

The 27 September 2021 damaging mainshock (Mw6.0) close to Arkalochori village is the strongest earthquake that was recorded during the instrumental period of seismicity in Central Crete (Greece). The mainshock was preceded by a significant number of foreshocks that lasted nearly four months. Maximum ground subsidence of about 18 cm was estimated from InSAR processing. The aftershock sequence is located in an almost NE-SW direction and divided into two main clusters, the southern and the northern ones. The foreshock activity, the deformation area, and the strongest aftershocks are located within the southern cluster. Based on body-wave travel times, a 3-D velocity model was developed, while using combined space and ground-based geodetic techniques, the co-seismic ground deformation is presented. Moreover, we examined the co-seismic static stress changes with respect to the aftershocks’ spatial distribution during the major events of the foreshocks, the Mw = 6.0 main event as well as the largest aftershock. Both the foreshock and the aftershock sequences obey the scaling law for the frequency-magnitude distribution as derived from the framework of non-extensive statistical physics (NESP). The aftershock production rate decays according to the modified Omori scaling law, exhibiting various Omori regimes due to the generation of secondary aftershock sequences. The analysis of the inter-event time distribution, based on NESP, further indicates asymptotic power-law scaling and long-range correlations among the events. The spatiotemporal evolution of the aftershock sequence indicates triggering by co-seismic stress transfer, while its slow migration towards the outer edges of the area of the aftershocks, related to the logarithm of time, further indicates a possible afterslip.

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

ABSTRACTStrong mining-induced earthquakes are often followed by aftershocks, similar to natural earthquakes. Although the magnitudes of such in-mine aftershocks are not high, they may pose a threat to mining infrastructure, production, and primarily, people working underground. The existing post-earthquake mining procedures usually do not consider any aspects of the physics of the mainshock. This work aims to estimate the rate and distribution of aftershocks following mining-induced seismic events by applying the rate-and-state model of fault friction, which is commonly used in natural earthquake studies. It was found that both the pre-mainshock level of seismicity and the coseismic stress change following the mainshock rupture have strong effects on the aftershock sequence. For mining-induced seismicity, however, we need to additionally account for the constantly changing stress state caused by the ongoing exploitation. Here, we attempt to model the aftershock sequence, its rate, and distribution of two M≈2 events in iron ore Kiruna mine, Sweden. We could appropriately estimate the aftershock sequence for one of the events because both the modeled rate and distribution of aftershocks matched the observed activity; however, the model underestimated the rate of aftershocks for the other event. The results of modeling showed that aftershocks following mining events occur in the areas of pre-mainshock activity influenced by the positive coulomb stress changes, according to the model’s assumptions. However, we also noted that some additional process not incorporated in the rate-and-state model may influence the aftershock sequence. Nevertheless, this type of modeling is a good tool for evaluating the risk areas in mines following a strong seismic event.

Local stress perturbations associated with the 2008 Wenchuan M 8.0 earthquake near the Longmenshan fault zone in the eastern margin of the Tibetan Plateau

The 2008 Wenchuan M 8.0 earthquake induced local stress perturbations near the Longmenshan (LMS) fault zone. Using in situ measurements and stress tensor inversions from focal mechanisms, we investigated the post-earthquake perturbed stress field at shallow depths (≤1.2 km), in the upper crust above a basal attachment (~5–18 km) as well as in the middle crust beneath a basal attachment (~19–32 km). At shallow depths in the Beichuan-Jiangyou section, the maximum principal stress (σ1) orientations changed from a pre-earthquake orientation of NWW to a post-earthquake orientation of NEE. However, near the Baoxing-An'xian and Qingchuan-Mianxian sections, the pre-earthquake σ1 orientation of NW-NWW was largely unaltered. Before the mainshock, in the upper crust above the basal detachment, the entire LMS fault zone exhibited a consistent σ1 orientation of NW-NWW. However, after the earthquake, some sections, such as the Beichuan-Nanba section, achieved σ1 orientations of NEE that were distinctly different from the pre-seismic stress field. In the middle crust beneath the basal detachment, the stress field was mostly unchanged. The crust beneath the basal detachment, along the eastern margin of the Tibetan Plateau, is dominated by NWW-oriented maximum principle stress and strain directions and a thrust faulting stress regime along the entire LMS fault zone; these observations are representative of the southeastward motion of the Bayan Har block and its ongoing collision with the South China block along the LMS thrust belt. The spatially heterogeneous stress field that occurred in the upper crust (≤18 km) after the mainshock was most likely caused by the co-seismic static stress changes associated with the Wenchuan earthquake. Prior to the onset of seismic activity, the deviatoric stress magnitudes were estimated at less than 22 MPa and 4 MPa at depths of 10 km and 20 km, respectively, suggesting that the mainshock fault zone has a relatively low fault strength to overcome, in order to initiate significant co-seismic rupture and aftershocks, especially in the crust beneath the basal detachment. These smaller deviatoric magnitudes are indicative of the weaker fault materials that resides in the LMS region.

Physical interpretation of moment tensor and the energetics of shear faulting

Moment tensor is a basic concept of source representation in seismology, established in the 1970s. On the basis of theoretical consideration to the equation of motion in continuum mechanics, we clarified the physical meaning of the Backus-Mulcahy moment tensor and derived a fundamental equation that the moment tensor of a seismic event is mathematically equivalent to the volume integral of coseismic static stress changes over the whole region. This equation provides new perspectives on the interaction between the inelastic source process and the elastic process in the surrounding region. As an example, we applied it to the energetics of shear faulting and obtained a quantitative relation between the work done for shear faulting and the static change in elastic strain energy in the surrounding region. With this energy equation, we elucidated the theoretical background of the Wallace-Bott hypothesis; that is, when the level of background tectonic stress is much higher than coseismic stress changes, the most probable orientation of slip on a given fault is parallel to the maximum resolved shear stress there in the sense of the efficiency of shear-strain energy release in the surrounding region.

Seismic loading of fault-controlled fluid seepage systems by great subduction earthquakes

Various types of fluid expulsion features (mud volcanoes, pockmarks, authigenic carbonate mounds and associated gas pipes, etc.) are often found above subduction zones, which have the highest seismic potential on Earth. Faults potentially control the liberation of deep-seated greenhouse gases into the feeder systems of seepage features located above subduction thrusts. These feeder systems could be stressed by large earthquakes, yet the mechanisms that can drive episodic mobilization of stored hydrocarbon gases remain poorly understood. Here I address the potential stress loading on fluid expulsion systems created by past earthquakes nucleated at both accretionary and erosive subduction margins. The most significant effects occur in the epicentral area where subduction earthquakes can produce normal stress changes as high as 20–100 bar, although these are generally restricted to relatively small regions. Coseismic normal stress changes and elastic strain relaxation upon a ruptured subduction thrust could increase crustal permeability by dilating fault-controlled conduits, and channelling fluids to the seafloor. Fluid pressure pulses released during subduction earthquakes can greatly contribute to the rupture of fluid pathways that have been brought closer to failure from coseismic static stress changes, although the inaccessible location of most submarine seepage systems has so far hampered probing these relationships.

Stress evolution and seismic hazard on the Maqin-Maqu segment of East Kunlun Fault zone from co-, post- and interseismic stress changes

The East Kunlun Fault zone, striking E–W to WNW–ESE, has been recognized as one of the largest and most active left-lateral strike-slip faults in the China continent. Presently, the Maqin-Maqu segment (MMS) is recognized as a seismic gap on the East Kunlun Fault. Since several highly populated counties are close to this region, understanding stress transfer and accumulation along this segment is important for hazard assessment along the MMS. In this study, we calculated the stress evolution along the MMS of the East Kunlun Fault zone during 1879–2008 by integrating coseismic effects, viscoelastic relaxation and tectonic loading. It is observed that the stress accumulation on the western part of the Maqin segment has been effected by the 1937 Tuosuo Lake earthquake, the stress on the eastern part of the Maqin segment. Also, the western part of the Maqu segment was relaxed by the 1947 Dari earthquake, and the stress loading on the eastern part of Maqu segment was increased by both the 1879 Wudu and 2008 Wenchuan earthquakes. It is observed that, compared to coseismic static stress changes, the post-seismic viscoelastic relaxation process has played a more important role on stress accumulation in the Maqu segment. The increased stress on the Maqin and Maqu segment is consistent with tectonic loading over 160 and 250 yr, respectively, which we expect will lead to future earthquakes and associated seismic hazard on these segments.

The role of triggering by static stress transfer during geothermal reservoir stimulation

During creation of an Enhanced Geothermal System, massive fluid injections are conducted to induce fracture shear which generates reservoir permeability. In this study we analyze coseismic static stress transfer caused by induced seismic events during a stimulation at the European research project at Soultz‐sous‐Forêts (Alsace, France). For this purpose we developed an efficient method to calculate coseismic static stress changes from an elliptical slip distribution on a circular fracture using superposition of rectangular sources, which enables us to apply an analytical solution for fast computation. This method is applied on a data set of 715 focal mechanisms derived from seismic recordings of the stimulation of the well GPK2 to calculate temporal evolution of static stress transfer. We find that the resulting structure of coseismic stress changes can be divided into three parts: a quiet zone where no spreading of seismicity occurs, an active zone within the created reservoir with ongoing fracturing and a process zone where the growth of the reservoir occurs. Static stress changes in the active zone are of the order of 1 MPa, both positive and negative, but may exceed this value considerably on a local scale. Analysis of stress changes from a cluster of events that occurred after shut‐in lets us conclude that triggering by coseismic static stress changes is possible for some events. Our analysis shows that triggering by static stress transfer plays a minor role for injection induced seismicity in a volumetric reservoir, whereas it can be quite effective for rupture propagation along single large fault zones.

Triggering Effect of M 4-5 Earthquakes on the Earthquake Cycle of Repeating Events at Parkfield, California

Stress perturbations influence earthquake recurrence and are of funda- mental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M 0:4 to ∼1:7 repeating earthquake sequences from the High Resolution Seismic Network cat- alog, we find that the time to recurrence of repeating events subsequent to nearby M 4-5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of ∼5 km, corresponding to static coseismic stress changes of > 0:6 26:6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s. Online Material: Figures of event chronologies of RES, estimates of dt, and coseismic static stress changes.

Triggered and Primary Surface Ruptures along the Camp Rock Fault, Eastern California Shear Zone

During the 1992 M w 7.3 Landers earthquake in the eastern California shear zone, only the shallowest part of the central 8 km of the Camp Rock fault apparently ruptured, triggered by coseismic static stress changes. Our detailed tectonic-geomorphic analysis of a 2 km long stretch of the central Camp Rock fault reveals that the 1992 triggered surface rupture differs markedly from earlier primary surface ruptures in terms of its vertical-slip function. We interpret this to be related to a difference in stress directions that induced fault slips: the 1992 static stress changes decreased normal stress on the central Camp Rock fault, thereby inducing a normal component of slip, whereas a minor reverse-slip component is associated with more typical primary seismogenic surface ruptures due to regional north–south compression. Our findings suggest that a detailed geomorphic analysis of a key locality may allow for isolation of triggered surface ruptures from repeated primary surface ruptures identified in paleoseismic studies.

The St Ives mesothermal gold system, Western Australia—a case of golden aftershocks?

The late orogenic evolution of the Archaean greenstone sequence in the Eastern Goldfields Province of the Yilgarn Craton in Western Australia is characterised by strike-slip tectonics and locally very high, fault-controlled, fluid fluxes. Fluid flow was associated with the formation of many fault-hosted and shear-zone-hosted gold deposits, which are commonly clustered adjacent to high displacement faults or shear zones. In the St Ives goldfield, near Kambalda, fluid flow in a gold-producing hydrothermal system was localised within arrays of low displacement faults and shear zones, which form part of the NNW trending, crustal-scale, Boulder–Lefroy fault system (BLFS). The numerous ore-hosting structures are kinematically related to sinistral to sinistral–oblique slip on the Playa Fault, which is a 20-km-long splay of the 200-km-long Boulder–Lefroy Fault. Most of the known gold mineralisation at St Ives occurs within an area of 20 km 2 immediately south-west of the Playa Fault. The distribution of low displacement faults and shear zones that host gold mineralisation is related to the presence of a kilometre-scale contractional jog (the Victory jog) and an associated imbricate thrust array on the Playa Fault. By analogy with modern seismogenic systems, the low displacement structures that localised fluid flow and gold mineralisation in the St Ives goldfield are interpreted as aftershock structures whose development was driven by major slip events on the BLFS. For large slip events on the BLFS, and mainshock rupture arrest at the Victory jog, modelling of co-seismic static stress changes indicates that most ore-hosting structures are localised within a crustal volume whose stress state was closer to failure as a consequence of Coulomb stress transfer. The modelling supports an interpretation that aftershock networks can form a high permeability damage zone that localises fluid flow and gold mineralisation within particular parts of crustal-scale fault systems. Both co-seismic stress transfer and time-dependent changes in fluid pressures, during post-seismic fluid redistribution, are implicated in driving the growth of low displacement, gold-hosting fault networks in the St Ives goldfield. Stress transfer modelling has application for area selection in exploration programs targeting mesothermal gold systems. Clustering of deposits hosted by aftershock fracture networks is favoured by the presence of major, long-lived jogs or bends that can repeatedly arrest ruptures propagating along high displacement faults.

Earthquake triggering by transient and static deformations

Observational evidence for both static and transient near‐field and far‐field triggered seismicity are explained in terms of a frictional instability model, based on a single degree of freedom spring‐slider system and rate‐ and state‐dependent frictional constitutive equations. In this study a triggered earthquake is one whose failure time has been advanced by Δt (clock advance) due to a stress perturbation. Triggering stress perturbations considered include square‐wave transients and step functions, analogous to seismic waves and coseismic static stress changes, respectively. Perturbations are superimposed on a constant background stressing rate which represents the tectonic stressing rate. The normal stress is assumed to be constant. Approximate, closed‐form solutions of the rate‐and‐state equations are derived for these triggering and background loads, building on the work of Dieterich [1992, 1994]. These solutions can be used to simulate the effects of static and transient stresses as a function of amplitude, onset time t0, and in the case of square waves, duration. The accuracies of the approximate closed‐form solutions are also evaluated with respect to the full numerical solution and t0. The approximate solutions underpredict the full solutions, although the difference decreases as t0 approaches the end of the earthquake cycle. The relationship between Δt and t0 differs for transient and static loads: a static stress step imposed late in the cycle causes less clock advance than an equal step imposed earlier, whereas a later applied transient causes greater clock advance than an equal one imposed earlier. For equal Δt, transient amplitudes must be greater than static loads by factors of several tens to hundreds depending on t0. We show that the rate‐and‐state model requires that the total slip at failure is a constant, regardless of the loading history. Thus a static load applied early in the cycle, or a transient applied at any time, reduces the stress at the initiation of failure, whereas static loads that are applied sufficiently late raise it. Rate‐and‐state friction predictions differ markedly from those based on Coulomb failure stress changes (ΔCFS) in which Δt equals the amplitude of the static stress change divided by the background stressing rate. The ΔCFS model assumes a stress failure threshold, while the rate‐and‐state equations require a slip failure threshold. The complete rate‐and‐state equations predict larger Δt than the ΔCFS model does for static stress steps at small t0, and smaller Δt than the ΔCFS model for stress steps at large t0. The ΔCFS model predicts nonzero Δt only for transient loads that raise the stress to failure stress levels during the transient. In contrast, the rate‐and‐state model predicts nonzero Δt for smaller loads, and triggered failure may occur well after the transient is finished. We consider heuristically the effects of triggering on a population of faults, as these effects might be evident in seismicity data. Triggering is manifest as an initial increase in seismicity rate that may be followed by a quiescence or by a return to the background rate. Available seismicity data are insufficient to discriminate whether triggered earthquakes are “new” or clock advanced. However, if triggering indeed results from advancing the failure time of inevitable earthquakes, then our modeling suggests that a quiescence always follows transient triggering and that the duration of increased seismicity also cannot exceed the duration of a triggering transient load. Quiescence follows static triggering only if the population of available faults is finite.