Tectonic Loading Rate Research Articles

The Effects of Characteristic Weakening Distance on Earthquake Nucleation Styles in Fully Dynamic Seismic Cycle Simulations

AbstractEarthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate‐and‐state friction (RSF), it was found that the ratios of to parameters control whether earthquakes nucleate as an expanding crack or with a fixed length prior to the dynamic instability. However, the characteristic weakening distance controls the weakening efficiency of state variables in RSF and can influence the nucleation styles as well. Here we investigate the effects of on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. A larger (>0.75) is needed to produce expanding crack nucleation styles for relatively small , which suggests that fixed length nucleation styles may dominate on natural and laboratory faults. Furthermore, we find a more complex nucleation style when the nucleation site is not in the center of the asperity and identify a twin‐like nucleation style which includes two initial acceleration phases. We conclude that the earthquake nucleation style is strongly controlled by the value of . The possible dominance of fixed length nucleation styles suggests that the minimum size of earthquake rupture may be estimated at the early stage of the nucleation phase.

Late Quaternary right-lateral slip rate along the Tangyuan segment of the Yilan–Yitong Fault Zone on the NE China Plain

The slip rate of strike-slip active faults is crucial for fault rupture behavior analysis and seismic hazard assessment. Although many segments of the Yilan–Yitong Fault Zone (YYFZ) in NE China have been strongly deformed since the late Quaternary, little progress has been made on its slip rate. With the help of high-resolution satellite images, detailed field investigations, seismic reflections, and Quaternary chronological dating, we mainly studied the late Quaternary right-slip rate of the YYFZ. Field investigations revealed an ∼15 km long by ∼1–2 m high-surface scarp belt extending along the Tangyuan graben interior, with a series of sag ponds and small parallel bulges. Research has revealed that the most recent paleoearthquake (∼M 7.0) occurred between 2,800± 600 a BP and 1,700 ± 200 a BP, with evidence of coseismic surface rupture. The T2 terrace abandonment age of the Heijin River is approximately 55.13 ± 1.78 ka (OSL), and the maximum cumulative right-lateral offset may reach 110 ± 5 m. Thus, the maximum right-slip rate of the Tangyuan segment of the YYFZ since the late Quaternary is constrained to 1–2 mm/a according to the upper terrace model. This study suggests that the presence of a new fault in the basin interior merits more attention when assessing the influential surface range and earthquake potential along the YYFZ, and the features of “low tectonic loading rate, activity migration in space, and clustering in time separated by ten thousand years of seismic quiescence” observed along the YYFZ are highly important for earthquake model construction and tectonic deformation studies in stable continental regions (SCRs).

Earthquakes Induced by Rapid Loading of Faults During Pulichintala Reservoir Impoundment in the Stable Continental Region of India

AbstractThe first stage impoundment of the Pulichintala reservoir in the Proterozoic Cuddapah basin in the southeastern Indian shield began in August 2014. The second stage of impoundment started in 2019 when the height of the dam was increased and the filling of the reservoir was rapid. Soon after earthquakes started occurring which continued for a few months with a maximum magnitude of ML 4.6 on 25 January 2020. Two distinct clusters within the shallow crust were identified, which follow the faults/shear zones in the region. The estimated focal mechanisms with predominant strike slip motion on steep planes and the derived stress state are consistent with the regional stress and plate motion of the Indian plate. We simulated the influence of reservoir impoundment on the inferred seismogenic faults and found that the stress and pore pressure due to reservoir load indeed promoted failure, thus inducing earthquakes. We ascribe the induced earthquakes to rapid filling, longer and higher retention of high‐water levels during 2019–2020, which caused a stress change of 4–8 kPa. This stress change was at least two orders of magnitude higher than the typical tectonic loading rate in the stable continental region of the Indian Shield.

Newly discovered shallow creep along the Gozha Co fault in northwestern Tibet: Spatial extent, rate and temporal evolution

Whereas most continental faults are interseismically locked, some exceptions have been observed to exhibit aseismic slip over a range of depths in the Earth's upper crust, i.e. shallow creep. This unusual slip behaviour helps probe frictional properties of natural faults. Here, we investigate the kinematic features of a newly discovered shallow creeping fault, the Gozha Co fault in northwestern Tibet, using Sentinel-1 radar interferometry. High-resolution velocities derived from 4 tracks of Sentinel-1 data reveal a 120-km-long creeping section on the Gozha Co fault, with a mean surface fault-parallel rate of ∼3 mm/yr. We use a simple elastic dislocation model to invert for the rate and depth extent of creep. Results show that creep occurs at all depths in the crust at the tectonic loading rate, ∼5 mm/yr, indicating that the discovered shallow creep is steady-state in response to continued tectonic loading, which is supported by the analysis of surface creep time-series that yields a time-independent creep rate. The Sentinel-1 velocities also show that the Longmu Co fault, part of the commonly called Longmu-Gozha Co fault system, has no measurable motion, and therefore may be inactive. Present-day deformation within the Tianshuihai Terrane, 3.8±0.8 mm/yr E-W shear and 0.6±0.6 mm/yr N-S shortening as the GPS velocities show, is in fact accommodated by motion on the Gozha Co fault.

How the Earth’s upper crust deforms in a viscous or brittle manner and how these behaviors interact and evolve over time: the crucial role of pressure solution creep and sealing processes1

Observations of the geological deformation of the Earth’s upper crust show both brittle behavior (faults) and viscous behavior (folds, shear zones). This paper explains the crucial role of pressure solution creep and sealing processes in these contrasting behaviors and in their evolutions over time. A description of natural deformation by pressure solution shows that the pressure solution creep process can accommodate large ductile deformation without any faults. This process can also accommodate near-stable ductile deformation through the coupling of pressure solution and fracturing. Even if pressure solution creep cannot accommodate the tectonic loading rate and earthquakes consequently occur, the post-seismic evolution is largely controlled by pressure solution processes such as post-seismic creep and fault healing and sealing. Some key experiments are presented that allow evaluating the thermodynamics and kinetics of these processes. Various models are then presented that could help engineers integrate pressure solution creep and sealing processes into predictions of the long-term behavior of rock deformation in underground storage and geo-energy facilities.

Geometric Control on Seismic Rupture and Earthquake Sequence Along the Yingxiu‐Beichuan Fault With Implications for the 2008 Wenchuan Earthquake

AbstractThe 2008 Mw 7.9 Wenchuan earthquake is the most disastrous seismic event in China since 1976. Both field and seismological investigations suggest a multi‐stage coseismic rupture with most damage associated with the Yingxiu‐Beichuan Fault (YBF) of spatially variable fault strike and dip angles. To investigate the effect of fault geometric complexity on coseismic rupture and paleoseismic pattern on the YBF, we perform earthquake sequence modeling on 3D fault geometry in the framework of rate‐and‐state friction law. Our model produces a long‐term earthquake sequence with quasi‐regular recurrences of whole‐fault ruptures and segmented ruptures. The along‐strike rupture segmentation, earthquake propagation speed, and slip rate are mainly controlled by along‐strike variations of fault dip and strike angles. Particularly, the YBF can be divided into two segments: the southern segment featured by large events (Mw > 8.0) and the northern segment with smaller events (Mw < 7.5), with recurrence intervals primarily determined by the tectonic loading rate. In a whole‐rupture event, the rupture speed mainly correlates with the fault dip angle; a smaller dip angle results in a wider seismogenic zone and higher rupture speeds, whereas the small‐scale variation in rupture speed is regulated by the fault strike angle. The effect of strike variation on the total coseismic slip amount is more pronounced due to the large strike angle gradient along the YBF. After varying the slip vector to reflect the northward transition from the dominant thrust‐slip to strike‐slip faulting during the Wenchuan coseismic rupture, we obtain reasonably good agreement of model simulated coseismic surface displacements with GPS observations.

Localization of large intraplate earthquakes along faulted density-contrast boundaries: Insights from the 2017 Mw6.5 Botswana earthquake

The major controls on the localization of deep crustal intraplate earthquakes remain enigmatic due to their deep hypocentral depths and rarity of coseismic surface ruptures. Here, we investigate the 3-D crustal density structure of the 2017 Mw 6.5 Botswana earthquake epicentral region, a strong lower-crustal (∼24–29 km) event which is suspected to have reactivated a Precambrian structure via normal faulting. We perform a 3-D inversion of the gravity data using published geological constraints, then integrate the resulting density model with aftershock hypocenter distribution and use as constraints in a 3-D thermo-mechanical geodynamic model implemented in ASPECT. Our results reveal crustal blocks of density anomalies, with the aftershocks clustering along a prominent NW-trending, NE-dipping density contrast separating a high-density (>2708 kg/m3) footwall and lower-density (2670–2700 kg/m3) hanging wall blocks. Additionally, a secondary density contrast boundary in the hanging wall coincides with a splay of aftershock clusters at depth. Our observations suggest that the 2017 Mw6.5 Botswana earthquake nucleated near a fault intersection in the lower crust and is associated with brittle normal faulting reactivation of a long-lived basement fault that follows a prominent deep-reaching density contrast boundary. Further, as demonstrated by geodynamic modeling results, we argue that in regions of low tectonic loading rates, where stress perturbations are high enough, faulted crustal-scale density contrast boundaries are preferential concentrators of strain that may localize intraplate earthquakes.

Stress evolution on major faults in Tien Shan and implications for seismic hazard

Tien Shan tectonic belt has experienced intense seismicity and a series of destructive strong earthquakes. However, earthquake triggering effects and faulting interactions in this area are poorly understood. A 3D finite element model of Tien Shan tectonic belt is constructed, to investigate stress evolutions on major faulting zones driven by interseismic tectonic loading and historical strong earthquakes with M≥ 6.0 since 1900. The numerical results show Tien Shan is dominated by nearly N-S compression, with higher tectonic loading rate in southwest Tien Shan. 1906 Manas M7.7 earthquake exerted pronounced Coulomb stress increase on its adjacent faulting zones, especially in the epicenter of 2016 Hutubi M6.0 earthquake. And three large earthquakes with M≥ 8.0, e.g., Chilik M8.3 earthquake in 1889, Kemin M8.0 earthquake in 1911 and Atushi M8.2 earthquake in 1902, increased the Coulomb stress by above 100 kPa in the epicenter of 1991 Keping M6.0 earthquake. While, stress perturbations by other strong earthquakes are limited, with slight Coulomb stress changes in the epicenters of their subsequent earthquakes. Overall, strong earthquakes with M> 7.0 in Tien Shan, induced substantial Coulomb stress changes on the adjacent faulting zones. Stress evolutions on major faults reveal higher stress accumulation in southwest Tien Shan, east KQX fault, west BoA fault, and HMT fault, indicating higher seismic risk.

Possible influence of static and viscoelastic stress perturbations in Musgrave block (Central Australia) earthquake sequence

The spatial and temporal distribution of earthquakes inside continental interiors, where tectonic loading rate is negligible, is much more complicated than plate boundary regions. On 30 March 1986, the moment magnitude (Mw) 5.7 Marryat Creek earthquake occurred in the Musgrave Block of central Australia. Subsequent earthquakes include the1989 Ml 5.6 Uluru, 2012 Mw 5.2 Pukatja, 2013 Mw 5.6 Mulga Park, and 2016 Mw 6.0 Petermann earthquakes. In this study, I explore whether coseismic and viscoelastic Coulomb stress changes following the 1986 Marryat Creek earthquake could have played a role in influencing the basic spatiotemporal characteristics of this earthquake sequence. I find that coseismic stress change does not successfully explain earthquake triggering in the area. I then explore this issue using a viscoelastic stress change model with a range of rheology parameters. In some models, the magnitude of viscoelastic stress change is higher than coseismic stress changes and may be of sufficient magnitude to be considered within an earthquake triggering context. Moreover, I also hypothesize that these earthquakes may have occurred due to dynamic stress triggering or other processes such as fluid migration, nonlinear friction, and/or aseismic deformation, which due to the small distribution of seismic networks across the area I have not been able to investigate in detail. However, I cannot dismiss the possibility that these earthquakes occurred randomly, with no identifiable stress triggering relationships among them.

Geodetic imaging of shallow creep along the Xianshuihe fault and its frictional properties

The Xianshuihe fault is one of the most tectonically active faults in the eastern Tibetan plateau. Understanding its slip behaviour and frictional properties is essential for determining the seismic potential. We use Sentinel-1A/B data from ascending and descending tracks to estimate the deformation field and retrieve three-dimensional velocities during the period of 2014-2019. We find that shallow creep is widely distributed along the entire Xianshuihe fault. Elastic dislocation modelling based on fault-parallel velocities reveals that the tectonic loading rate along the Xianshuihe fault is in the range of 8.8-17.9 mm/yr with a locking depth of 7.6-18.5 km. The shallow aseismic slip rate ranges from 3.3-7.8 mm/yr on the Daofu and Qianning segments, to 16.3-19.8 mm/yr on the western Kangding segment; the high aseismic slip rate on the western Kangding segment is likely due to postseismic relaxation of the 2014 Kangding Mw 5.6 and 5.9 earthquakes. The seismic moment accumulated on the Qianning segment since the last major event is equivalent to a Mw 6.6 earthquake. Analysis of the evolution of aseismic slip in the framework of rate-and-state friction shows that the Kangding segment has a much smaller (a-b) (0.0030) than the Daofu segment, (a-b) = 0.0162. The difference is potentially controlled by different rock types between the Kangding segment (granite) and other segments of the Xianshuihe fault (quartz-rich sandstone).

Validation of a Multilayered Analog Model Integrating Crust‐Mantle Visco‐Elastic Coupling to Investigate Subduction Megathrust Earthquake Cycle

AbstractWe have developed a scaled analog model of a subduction zone simulating seismic cycle deformation phases. Its rheology is based on multilayered visco‐elasto‐plastic materials to account for the mechanical behavior of a continental lithospheric plate overriding a subducting oceanic plate. The seismogenic zone displays unstable slip behavior, extending at depth into a weak interface with stable slip properties. The model succeeds in reproducing interseismic phases interrupted by coseismic ruptures and followed by after‐slip. The experimental data catalog shows a broad variability of slip events from aseismic slow slips to fast dynamic lab quakes. Results also show the occurrence of both isolated and precursory slow‐slip events arising before the mainshocks. Given the absence of fluids in the model, the broad variability in slip event velocity can be attributed to fault roughness complexity. The model rheology induces also a key visco‐elastic coupling between the elastic overriding plate and the mantle wedge allowing, for the first time, to reproduce experimentally a realistic postseismic visco‐elastic relaxation phase. Preliminary results reveal that the tectonic loading rate modulates this visco‐elastic coupling. A low loading rate weakens it, which increase the amount of storable interseismic elastic deformation, and favors the occurrence of large megathrust events. A high loading rate strengthens it, which minimize the accumulation of interseismic elastic deformation, the slip‐event sizes, and promote aseismic creep. This new scaled‐analog subduction model is a complementary tool to investigate earthquake mechanics and improve the interpretation of geodetic and seismological records.

Maximum Earthquake Size and Seismicity Rate from an ETAS Model with Slip Budget

ABSTRACTWe define a seismicity model based on (1) the epidemic-type aftershock sequence model that accounts for earthquake clustering, and (2) a closed slip budget at long timescale. This is achieved by not permitting an earthquake to have a seismic moment greater than the current seismic moment deficit. This causes the Gutenberg–Richter law to be modulated by a smooth upper cutoff, the location of which can be predicted from the model parameters. We investigate the various regimes of this model that more particularly include a regime in which the activity does not die off even with a vanishingly small spontaneous (i.e., background) earthquake rate and one that bears strong statistical similarities with repeating earthquake time series. Finally, this model relates the earthquake rate and the geodetic moment rate and, therefore, allows to make sense of this relationship in terms of fundamental empirical law (the Gutenberg–Richter law, the productivity law, and the Omori law) and physical parameters (seismic coupling, tectonic loading rate).

Earthquake Nucleation Size: Evidence of Loading Rate Dependence in Laboratory Faults.

Recent Global Positioning System observations of major earthquakes such as the 2014 Chile megathrust show a slow preslip phase releasing a significant portion of the total moment (Ruiz et al., 2014, https://doi.org/10.1126/science.1256074). Despite advances from theoretical stability analysis (Rubin & Ampuero, 2005, https://doi.org/10.1029/2005JB003686; Ruina, 1983, https://doi.org/10.1029/jb088ib12p10359) and modeling (Kaneko et al., 2017, https://doi.org/10.1002/2016GL071569), it is not fully understood what controls the prevalence and the amount of slip in the nucleation process. Here we present laboratory observations of slow slip preceding dynamic rupture, where we observe a dependence of nucleation size and position on the loading rate (laboratory equivalent of tectonic loading rate). The setup is composed of two polycarbonate plates under direct shear with a 30‐cm long slip interface. The results of our laboratory experiments are in agreement with the preslip model outlined by Ellsworth and Beroza (1995, https://doi.org/10.1126/science.268.5212.851) and observed in laboratory experiments (Latour et al., 2013, https://doi.org/10.1002/grl.50974; Nielsen et al., 2010, https://doi.org/10.1111/j.1365-246x.2009.04444.x; Ohnaka & Kuwahara, 1990, https://doi.org/10.1016/0040-1951(90)90138-X), which show a slow slip followed by an acceleration up to dynamic rupture velocity. However, further complexity arises from the effect of (1) rate of shear loading and (2) inhomogeneities on the fault surface. In particular, we show that when the loading rate is increased from 10−2 to 6 MPa/s, the nucleation length can shrink by a factor of 3, and the rupture nucleates consistently on higher shear stress areas. The nucleation lengths measured fall within the range of the theoretical limits L b and L∞ derived by Rubin and Ampuero (2005, https://doi.org/10.1029/2005JB003686) for rate‐and‐state friction laws.

A new paradigm for large earthquakes in stable continental plate interiors

AbstractLarge earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earth's seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occurring when the accrual of tectonic stress localized at long‐lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a prestressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do not apply. As a consequence, seismic hazard in SCRs is likely more spatially distributed than indicated by paleoearthquakes, current seismicity, or geodetic strain rates.

Strain accumulation in the New Madrid and Wabash Valley seismic zones from 14 years of continuous GPS observation

AbstractThe mechanical behavior—and hence earthquake potential—of faults in continental interiors is an issue of critical importance for the resultant seismic hazard, but no consensus has yet been reached on this controversial topic. The debate has focused on the central and eastern United States, in particular, the New Madrid Seismic Zone, struck by four magnitude 7 or greater earthquakes in 1811–1812, and to a lesser extent the Wabash Valley Seismic Zone just to the north. A key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults, a quantity that remains debated. Here we address this issue with an analysis of up to 14.6 years of continuous GPS data from a network of 200 sites in the central United States centered on the New Madrid and Wabash Valley seismic zones. We find that the high‐quality sites in these regions show motions that are consistently within the 95% confidence limit of zero deformation. These results place an upper bound on strain accrual on faults of 0.2 mm/yr and 0.6 mm/yr in the New Madrid and Wabash Valley Seismic Zones, respectively. For the New Madrid region, where a paleoseismic record is available for the past ∼5000 years, we argue that strain accrual—if any—does not permit the 500–900 year repeat time of paleo‐earthquakes observed in the Upper Mississippi Embayment. These results, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicate that either tectonic loading rates or fault properties vary with time in the New Madrid Seismic Zone and possibly plate wide.

Renewal models and coseismic stress transfer in the Corinth Gulf, Greece, fault system

AbstractWe model interevent times and Coulomb static stress transfer on the rupture segments along the Corinth Gulf extension zone, a region with a wealth of observations on strong‐earthquake recurrence behavior. From the available information on past seismic activity, we have identified eight segments without significant overlapping that are aligned along the southern boundary of the Corinth rift. We aim to test if strong earthquakes on these segments are characterized by some kind of time‐predictable behavior, rather than by complete randomness. The rationale for time‐predictable behavior is based on the characteristic earthquake hypothesis, the necessary ingredients of which are a known faulting geometry and slip rate. The tectonic loading rate is characterized by slip of 6 mm/yr on the westernmost fault segment, diminishing to 4 mm/yr on the easternmost segment, based on the most reliable geodetic data. In this study, we employ statistical and physical modeling to account for stress transfer among these fault segments. The statistical modeling is based on the definition of a probability density distribution of the interevent times for each segment. Both the Brownian Passage‐Time (BPT) and Weibull distributions are tested. The time‐dependent hazard rate thus obtained is then modified by the inclusion of a permanent physical effect due to the Coulomb static stress change caused by failure of neighboring faults since the latest characteristic earthquake on the fault of interest. The validity of the renewal model is assessed retrospectively, using the data of the last 300 years, by comparison with a plain time‐independent Poisson model, by means of statistical tools including the Relative Operating Characteristic diagram, the R‐score, the probability gain and the log‐likelihood ratio. We treat the uncertainties in the parameters of each examined fault source, such as linear dimensions, depth of the fault center, focal mechanism, recurrence time, coseismic slip, and aperiodicity of the statistical distribution, by a Monte Carlo technique. The Monte Carlo samples for all these parameters are drawn from a uniform distribution within their uncertainty limits. We find that the BPT and the Weibull renewal models yield comparable results, and both of them perform significantly better than the Poisson hypothesis. No clear performance enhancement is achieved by the introduction of the Coulomb static stress change into the renewal model.

Long-term changes of earthquake inter-event times and low-frequency earthquake recurrence in central California

The temporal evolution of earthquake inter-event time (IET) may provide important clues for the timing of future events and underlying physical mechanisms of earthquake interaction. In this study, we examine ~12yr of local earthquake and low-frequency earthquake (LFE) activity near Parkfield, CA from catalogs of ~50,000 earthquakes and ~730,000 LFEs. We focus on the long-term evolution of IETs after the 2003 Mw6.5 San Simeon and 2004 Mw6.0 Parkfield earthquakes. The IETs of local earthquakes along and to the southwest of the San Andreas fault show clear decreases of several orders of magnitude after the Parkfield and San Simeon earthquakes, followed by recoveries with time scales of ~3yr and >8yr, respectively. We also observe decreases in recurrence times in some of LFE families, followed by long-term recoveries with time scales of ~4 months to several years. The long-term recovery of the earthquake IET is a manifestation of the aftershock decay of the Parkfield and San Simeon earthquakes, and the different recovery time scales likely reflect the different tectonic loading rates in the two regions. The drop in the recurrence times of LFEs after the Parkfield earthquake is likely caused by static and dynamic stresses induced by the Parkfield earthquake, and the long-term recovery in LFE recurrence time could be due to post-seismic relaxation or gradual recovery of fault zone material properties. The recovery time scales for general earthquake IET and LFE recurrence following the Parkfield earthquake are similar to those estimated for repeating earthquake recurrence identified in previous studies, indicating that they could be controlled by similar mechanisms.

Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003Mw= 6.9 Zemmouri, Algeria, earthquake

[1] The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 Mw = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10–20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a Mw = 6.6–6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history.

Time-Variable Deformation in the New Madrid Seismic Zone

New geodetic measurements show that the New Madrid is currently deforming too slowly, if at all, to account for large earthquakes in the region over the past 5000 years. This result, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicates that either tectonic loading rates or fault properties vary over a few thousand years.

Afterslip and aftershocks in the rate‐and‐state friction law

We study how a stress perturbation generated by a main shock affects a fault obeying the rate‐state friction law using a simple slider block system. Depending on the model parameters and on the initial stress, the fault exhibits aftershocks, slow earthquakes, or decaying afterslip. We found several regimes with slip rate decaying as a power law of time, with different characteristic times and exponents. The behavior of the rate‐state friction law is thus far more complex than described by the “steady state” approximation frequently used to fit afterslip data. The fault reaches steady state only at very large times, when slip rate has decreased to the tectonic loading rate. The complexity of the model makes it unrealistic to invert for the friction law parameters from afterslip data. We modeled afterslip measurements for three earthquakes using the complete rate‐and‐state law and found a huge variety of model parameters that can fit the data. In particular, it is impossible to distinguish the stable velocity‐strengthening regime (A > B) from the (potentially) unstable velocity‐weakening regime (A < B). Therefore, it is not necessary to involve small‐scale spatial or temporal fluctuations of friction parameters A or B in order to explain the transition between stable sliding and seismic slip. In addition to B/A and stiffness, the fault behavior is strongly controlled by stress levels following an event. Stress heterogeneity can thus explain part of the variety of postseismic behaviors observed in nature. Afterslip induces a progressive reloading of faults that are not slipping, which can trigger aftershocks. Using the relation between stress and seismicity derived from the rate‐and‐state friction law, we estimate the aftershock rate triggered by coseismic and postseismic slip. Aftershock rate does not simply scale with stress rate but exhibits different characteristic times and sometimes a different power law exponent. Afterslip is thus a possible candidate to explain observations of aftershock rate decaying as a power law of time with an Omori exponent that can be either smaller or larger than 1. Progressive unloading due to afterslip can also produce delayed seismic quiescence.