Large Strike-slip Earthquakes Research Articles

Finite source properties of large strike-slip earthquakes

SUMMARY Earthquake ruptures are complex physical processes that may vary with the structure and tectonics of the region in which they occur. Characterizing the factors controlling this variability would provide fundamental constraints on the physics of earthquakes and faults. We investigate this by determining finite source properties from second moments of the stress glut for a global data set of large strike-slip earthquakes. Our approach uses a Bayesian inverse formulation with teleseismic body and surface waves, which yields a low-dimensional probabilistic description of rupture properties including the spatial deviation, directivity and temporal deviation of the source. This technique is useful for comparing events because it makes only minor geometric constraints, avoids bias due to rupture velocity parametrization and yields a full ensemble of possible solutions given the uncertainties of the data. We apply this framework to all great strike-slip earthquakes of the past three decades, and we use the resultant second moments to compare source quantities like directivity ratio, rectilinearity, average moment density and vertical deviation. We find that most strike-slip earthquakes have a large component of unilateral directivity, and many of these earthquakes show a mixture of unilateral and bilateral behaviour. We notice that oceanic intraplate earthquakes usually rupture a much larger width of the seismogenic zone than other strike-slip earthquakes, suggesting these earthquakes may often breach the expected thermal boundary for oceanic ruptures. We also use these second moments to resolve nodal plane ambiguity for the large oceanic intraplate earthquakes and find that the rupture orientation is usually unaligned with encompassing fossil fracture zones.

Effect of real-world frictional strengthening layer near the Earth's free surface on rupture characteristics with different friction laws: Implication for scarcity of supershear earthquakes

More and more supershear rupture earthquakes, with rupture propagation velocity larger than the local shear wave speed, have been identified in recent years. However, the number of supershear events is scarce compared with large strike-slip earthquakes. This problem remains unresolved although several previous workers have offered some explanations. In this study, we utilize finite element method to simulate spontaneous dynamic rupture propagations in three dimensional half-space model with and without velocity strengthening friction layer (VSFL) near the Earth's free surface employing different frictional constitutive relations, including linear slip weakening law and rate- and state-dependent relations with aging form and slip form. The simulated results showed that the VSFL suppresses supershear rupture transition at the Earth's free surface occurring in the absence of a VSFL. In particular, we found that the minimum thickness of VSFL to prevent supershear rupture transition is ~1.4-km. Meanwhile, we reviewed a great deal of previous work on thickness of VSFL along mature faults in nature, and its thickness generally exceeds ~3-km, which is enough to suppress the transition to supershear rupture in the real-world. The presence of VSFL may be the primary reason why the supershear rupture earthquakes in nature are so scarce. In addition, the modelling results indicated that spatial heterogeneity of VSFL on the surface along fault strike affects the supershear rupture transition to some extent. Therefore, this study contributes to understanding the earthquake source dynamics and seismic hazard mitigation.

A revised paleoseismological record of late Holocene ruptures on the Kekerengu Fault following the 2016 Kaikōura earthquake

ABSTRACT The Mw 7.8 Kaikōura earthquake of November 14th, 2016 provided rare opportunities to evaluate ground deformation during a large strike-slip earthquake. Following the earthquake, both halves of a displaced paleoseismic trench were re-excavated and extended to test, refine, and extend the known late Holocene chronology of surface rupturing earthquakes on the Kekerengu Fault. 28 organic-bearing samples were collected during these excavations. Of these, six samples provided new 14C ages that could be superimposed on the preferred age model of (Little VDR, Kearse J, Norton K, Benson A, Wang N. 2018. Kekerengu fault, New Zealand: Timing and size of Late Holocene surface ruptures. Bulletin of the Seismological Society of America. 108(3B):1556–1572) to derive an expanded, updated age model of earthquake events on the fault that is now based on 16 dated samples. Including the 2016 earthquake, we recognise six surface rupturing earthquakes on the Kekerengu Fault since ∼2000 cal. B.P. Based on the last five events, our analysis yields an updated estimate of the mean recurrence interval for surface rupturing on the fault of 375 ± 32 yrs (1σ) since ∼1650 cal. B.P. An older, sixth event (E5) was not included in the preferred age model due to uncertainties in interpretation; however, incorporating this event into an alternative, six-event age model would adjust the recurrence interval estimate to 433 ± 22 yrs (1σ) since ∼2000 cal. B.P.

Co-Seismic Ionospheric Disturbances Following the 2016 West Sumatra and 2018 Palu Earthquakes from GPS and GLONASS Measurements

The study of ionospheric disturbances associated with the two large strike-slip earthquakes in Indonesia was investigated, which are West Sumatra on 2 March 2016 (Mw = 7.8), and Palu on 28 September 2018 (Mw = 7.5). The anomalies were observed by measuring co-seismic ionospheric disturbances (CIDs) using the Global Navigation Satellite System (GNSS). The results show positive and negative CIDs polarization changes for the 2016 West Sumatra earthquake, depending on the position of the satellite line-of-sight, while the 2018 Palu earthquake shows negative changes only due to differences in co-seismic vertical crustal displacement. The 2016 West Sumatra earthquake caused uplift and subsidence, while the 2018 Palu earthquake was dominated by subsidence. TEC anomalies occurred about 10 to 15 min after the two earthquakes with amplitude of 2.9 TECU and 0.4 TECU, respectively. The TEC anomaly amplitude was also affected by the magnitude of the earthquake moment. The disturbance signal propagated with a velocity of ~1–1.72 km s−1 for the 2016 West Sumatra earthquake and ~0.97–1.08 km s−1 for the 2018 Palu mainshock earthquake, which are consistent with acoustic waves. The wave also caused an oscillation signal of ∼4 mHz, and their azimuthal asymmetry of propagation confirmed the phenomena in the Southern Hemisphere. The CID signal could be identified at a distance of around 400–1500 km from the epicenter in the southwestern direction.

Origin of the shallow slip deficit on a strike slip fault: Influence of elastic structure, topography, data coverage, and noise

Numerous fault slip models for several large strike-slip earthquakes indicate a reduction of slip along the uppermost part of the fault (Shallow Slip Deficit, SSD). In this paper, we investigate the possibility that the SSD is an artifact due to the simplifications made when we model the medium. Using a set of synthetic data for the 1992 Landers earthquake, we show that while neglecting the topography of the Landers area has a moderate impact on the retrieved slip model, neglecting variations in elastic properties, in particular the variation with depth, leads to significant bias on the slip model and can produce 25% of artificial SSD (whereas neglecting the fault damage zone has a smaller impact). We further show that a lack of data coverage around the fault further increases the amount of artificial SSD. Moreover, we find that correlated data noise with maximum amplitude as low as 5% of the maximum surface displacement can also produce significant artificial SSD. We conclude that simple elastic forward modeling in a homogeneous medium may be inappropriate for resolving slip-variability with depth, even in areas of limited topography or lateral variation in elastic properties. Finally, we argue that the estimation of model uncertainty due to data error should be systematic in inversion studies to avoid any misinterpretation of the slip distribution.

Imaging the rupture process of the 10 January 2018 MW7.5 Swan island, Honduras earthquake*

The 10 January 2018 MW7.5 Swan island, Honduras earthquake occurred on the Swan island fault, which is a transform plate boundary between the North American and Caribbean plates. Here we back-project the rupture process of the earthquake using dense seismic stations in Alaska, and find that the earthquake ruptured at least three faults (three stages) for a duration of ~40 s. The rupture speed for the longest fault (stage 3) is as fast as 5 km/s, which is much faster than the local shear wave velocity of ~4 km/s. Supershear rupture was incidentally observed on long and straight strike-slip faults. This study shows a supershear rupture that occured on a strike-slip fault with moderate length, implying that supershear rupture might commonly occur on large strike-slip earthquakes. The common occurrence of supershear rupture on strike-slip earthquakes will challenge present understanding of crack physics, as well as strong ground motion evaluation in earthquake engineering.

Dyke intrusion and stress-induced collapse of volcano flanks: The example of the 2018 event at Mt. Etna (Sicily, Italy)

Magmatic intrusions, eruptions and flank collapses are frequent processes of volcano dynamics, inter-connected at different space and time scales. The December 2018 recrudescent episode at Mt. Etna is an exemplary case where a sudden intrusive event culminated with a short eruption, intense seismicity and a shallow large strike-slip earthquake at the edge of the eastern sliding flank. Here, we show that high resolution velocity models and transient changes of VP and VP/VS resolve the magma intrusion through a dyke and local stress increase at the base of the unstable flank, inducing the collapse. Episodic brittle faulting occurs at the edge of the sliding sector, locally contributed by high fluid pressure. The feedback between magma ascent, stress changes and flank collapse is driving the volcano dynamics, with processes ranging from long term to transient episodes.

Fracture energies at the rupture nucleation points of large strike-slip earthquakes on the Xianshuihe fault, southwestern China

Earthquake cycles along a pure strike-slip fault were numerically simulated using a rate- and state-dependent friction law to obtain the fracture energies at the rupture nucleation points. In the model, deep aseismic slip is imposed on the fault, which generates recurrent earthquakes in the shallower velocity-weakening friction region. The fracture energy at the rupture nucleation point for each simulated earthquake was calculated using the relation between shear stress and slip, which indicates slip-weakening behavior. The simulation results show that the relation between the fracture energy at the nucleation point and other source parameters is consistent with a theoretical approach based on fracture mechanics, in that an earthquake occurs when the energy release rate at the tip of the aseismic slip zone first exceeds the fracture energy. Because the energy release rate is proportional to the square of the amount of deep aseismic slip during the interseismic period, which can be estimated from the recurrence interval of earthquakes and the deep aseismic slip rate, the fracture energies for strike-slip earthquakes can be calculated. Using this result, we estimated the fracture energies at the nucleation points of large earthquakes on selected segments of the Xianshuihe fault, southwestern China. We find that the estimated fracture energies at the rupture nucleation points are generally smaller than the values of average fracture energy for developed ruptures as estimated in previous studies, suggesting that the fracture energy tends to increase with the rupture propagation distance.

Fast rupture propagation for large strike-slip earthquakes

Abstract Studying rupture speeds of shallow earthquakes is of broad interest because it has a large effect on the strong near-field shaking that causes damage during earthquakes, and it is an important parameter that reflects stress levels and energy on a slipping fault. However, resolving rupture speed is difficult in standard waveform inversion methods due to limited near-field observations and the tradeoff between rupture speed and fault size for teleseismic observations. Here we applied back-projection methods to estimate the rupture speeds of 15 M w ≥ 7.8 dip-slip and 8 M w ≥ 7.5 strike-slip earthquakes for which direct P waves are well recorded in Japan on Hi-net, or in North America on USArray. We found that all strike-slip events had very fast average rupture speeds of 3.0–5.0 km/s, which are near or greater than the local shear wave velocity (supershear). These values are faster than for thrust and normal faulting earthquakes that generally rupture with speeds of 1.0–3.0 km/s.

Postseismic Deformation Following the 2010 $$M = 7.2$$ M = 7.2 El Mayor-Cucapah Earthquake: Observations, Kinematic Inversions, and Dynamic Models

Due to its location on a transtensional section of the Pacific-North American plate boundary, the Salton Trough is a region featuring large strike-slip earthquakes within a regime of shallow asthenosphere, high heat flow, and complex faulting, and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface. The 2010 M=7.2 El Mayor-Cucapah earthquake was the largest shock in the Salton Trough since 1892 and occurred close to the US-Mexico border, and so the postseismic deformation recorded by the continuous GPS network of southern California provides an opportunity to study the rheology of this region. Three-year postseismic transients extracted from GPS displacement time-series show four key features: (1) 1–2 cm of cumulative uplift in the Imperial Valley and ∼ 1 cm of subsidence in the Peninsular Ranges, (2) relatively large cumulative horizontal displacements > 150 km from the rupture in the Peninsular Ranges, (3) rapidly decaying horizontal displacement rates in the first few months after the earthquake in the Imperial Valley, and (4) sustained horizontal velocities, following the rapid early motions, that were still visibly ongoing 3 years after the earthquake. Kinematic inversions show that the cumulative 3-year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture, though these solutions require afterslip with a total moment equivalent to at least a M=7.2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes. Forward modeling shows that stress-driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the Imperial Valley, while Newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the Imperial Valley and the subsidence and large westward displacements in the Peninsular Ranges. We present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well: a model combining afterslip in the lower crust, Newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity, and Newtonian viscoelastic relaxation in the asthenosphere; and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress-dependent viscosity in the mantle. The rheology of this high-heat-flow, high-strain-rate region may incorporate elements of both these models and may well be more complex than either of them.

Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: Mw dependence of the disturbance amplitudes

We studied ionospheric responses to the 2012 April 11 Mw 8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CIDs) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (∼1 km s–1). Resonant atmospheric oscillation with a frequency ∼4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes world-wide with moment magnitudes (Mw) 6.6–9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the Mw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, that is small vertical crustal movements for their magnitudes.

Rupture velocity inferred from near-field shear strain analysis

AbstractWe propose a new technique to determine the rupture velocity of large strike slip earthquakes. By means of simple numerical ground motion simulations, we show that when the rupture penetrates a shallow layer of sediment or fractured rock, shock waves propagate along the surface fault trace in the forward rupture direction. Such shock waves, which are insensitive to the complexity of slip over the fault plane, propagate at a phase velocity equal to the rupture speed. We show that those shock waves can be easily isolated in the frequency domain, and that phase velocity can then be simply obtained from shear strain.

Strike-slip earthquakes can also be detected in the ionosphere

It is generally assumed that co-seismic ionospheric disturbances are generated by large vertical static displacements of the ground during an earthquake. Consequently, it is expected that co-seismic ionospheric disturbances are only observable after earthquakes with a significant dip–slip component. Therefore, earthquakes dominated by strike-slip motion, i.e. with very little vertical co-seismic component, are not expected to generate ionospheric perturbations. In this work, we use total electron content (TEC) measurements from ground-based GNSS-receivers to study ionospheric response to six recent largest strike-slip earthquakes: the Mw7.8 Kunlun earthquake of 14 November 2001, the Mw8.1 Macquarie earthquake of 23 December 2004, the Sumatra earthquake doublet, Mw8.6 and Mw8.2, of 11 April 2012, the Mw7.7 Balochistan earthquake of 24 September 2013 and the Mw 7.7 Scotia Sea earthquake of 17 November 2013. We show that large strike-slip earthquakes generate large ionospheric perturbations of amplitude comparable with those induced by dip–slip earthquakes of equivalent magnitude. We consider that in the absence of significant vertical static co-seismic displacements of the ground, other seismological parameters (primarily the magnitude of co-seismic horizontal displacements, seismic fault dimensions, seismic slip) may contribute in generation of large-amplitude ionospheric perturbations.

Complementary slip distributions of the August 4, 2003 Mw 7.6 and November 17, 2013 Mw 7.8 South Scotia Ridge earthquakes

The South Scotia Ridge Transform (SSRT) plate boundary between the Scotia and Antarctic plates experienced large strike-slip earthquakes on August 4, 2003 (Mw 7.6) and November 17, 2013 (Mw 7.8). These events have overlapping aftershock zones, which is unusual. A 36°–45° southward dipping fault zone ruptured with left-lateral displacements in each event along the northern margin of the South Orkney micro-continent near 60°S. Slip distributions for the two events are determined using teleseismic body and surface wave recordings along with constraints from GPS ground motion recordings at station BORC on Laurie Island (South Orkney Islands), just south of the SSRT. The aftershock distributions, high-frequency back-projections, and unconstrained body wave finite-fault inversions permit significant overlap of the 2003 and 2013 slip zones; however, the GPS static displacements resolve differences in the large-slip regions of the two ruptures. The 2013 earthquake sequence along the SSRT initiated with Mw 6.1 (November 13) and Mw 6.8 (November 16) foreshocks located ∼50 km west of the mainshock hypocenter, and had aftershocks extending ∼250 km eastward. The rupture spread primarily eastward at ∼2.5 km/s with a total rupture duration of about 120 s, with two distinct patches of large-slip located northwest and northeast of the South Orkney Islands. The rupture swept past BORC, with high-rate GPS (HRGPS) ground motion recordings capturing the time-varying slip history of the faulting. Traditional GPS data require that the largest-slip region of the shorter rupture in 2003 is located in the gap NNE of BORC between the two patches that ruptured in 2013. There appears to be some overlap of lower slip regions. The complementary slip distributions comprise a relatively uniform offset along this portion of the SSRT, which is one of the most seismically active regions of the entire Antarctic plate boundary.

How well do surface slip measurements track slip at depth in large strike-slip earthquakes? The importance of fault structural maturity in controlling on-fault slip versus off-fault surface deformation

Comparisons of observed surface displacements with geodetically inferred slip at depth in six large (Mw⩾7.1) strike-slip earthquakes reveal a correlation between the structural maturity of the fault and the ratio of deep slip to surface slip that occurs on localized zones of surface rupture. Specifically, structurally immature faults (⩽25 km of total displacement) manifest only ∼50–60% of total slip on narrow fault surface traces versus ∼85–95% for structurally mature faults (⩾85 km total slip). The same pattern holds when structurally simple parts of a surface rupture are analyzed separately from parts that exhibit obvious structural complexity (e.g., discontinuous or multiple traces or significant dip slip). These results imply that geologic measurements of surface slip along structurally immature faults are likely to significantly underestimate the true slip at depth in large earthquakes. This observation has implications for a number of important problems, including determination of fault slip rates, which are based on surface offsets; earthquake probability assessments, which are based on geologic fault slip rates; comparisons of geologic and geodetic rate data in the search for strain transients; the structural evolution of fault zones; estimation of paleo-earthquake magnitudes based on geomorphic offsets; analyses of the relative importance of faulting vs. distributed deformation in accommodating relative plate motions in continental crust; the design and construction of infrastructure built near active faults; and possibly for the prediction of strong ground motions, which may at least partially depend on the degree of slip localization to the surface.

Aftershocks due to property variations in the fault zone: A mechanical model

Studies of many large strike-slip earthquakes have shown that the seismicity rate increases in quadrants in which the Coulomb stress is increased by the earthquake rupture. This mechanism explains the concentration of aftershocks near the tips of the fault rupture, but cannot explain the aftershocks on the sides of the rupture zone, where the aftershocks are commonly abundant. We suggest that these aftershocks may be induced by variations of fault-zone properties not included in the previous models. In a finite-element model that treats the rupture plane as a zone with heterogeneous physical properties, as suggested by field observations and laboratory experiments, the Coulomb-stress changes triggered by the mainshock no longer uniformly decrease on the sides of the rupture zone, but show complex patterns of negative and positive changes. These results may explain some of the aftershocks on the sides of the mainshock rupture.

Earthquake Fault Scaling: Self-Consistent Relating of Rupture Length, Width, Average Displacement, and Moment Release

In this paper, I propose the scaling relation W = C 1 L β (where β ≈2/3) to describe the scaling of rupture width with rupture length. I also propose a new displacement relation ![Graphic][1] , where A is the area ( LW ). By substituting these equations into the definition of seismic moment (![Graphic][2] ), I have developed a series of self-consistent equations that describe the scaling between seismic moment, rupture area, length, width, and average displacement. In addition to β , the equations have only two variables, C 1 and C 2, which have been estimated empirically for different tectonic settings. The relations predict linear log–log relationships, the slope of which depends only on β . These new scaling relations, unlike previous relations, are self-consistent, in that they enable moment, rupture length, width, area, and displacement to be estimated from each other and with these estimates all being consistent with the definition of seismic moment. I interpret C 1 as depending on the size at which a rupture transitions from having a constant aspect ratio to following a power law and C 2 as depending on the displacement per unit area of fault rupture and so static stress drop. It is likely that these variables differ between tectonic environments; this might explain much of the scatter in the empirical data. I suggest that these relations apply to all faults. For small earthquakes ( M ∼5) earthquakes β =2/3, so L 2.5 applies. For dip-slip earthquakes this scaling applies up to the largest events. For very large ( M >∼7.2) strike-slip earthquakes, which are fault width-limited, β =0 and assuming ![Graphic][3] , then L 1.5 scaling applies. In all cases, M ∝ A 1.5 fault scaling applies. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif

When an oceanic tectonic plate cracks

Analyses of two recent earthquakes of great magnitude show how complex the breaking of the oceanic lithosphere can be, how it is linked to earlier great events and how it triggers seismicity worldwide. See Letters p.240, p.245 & p.250 On 11 April 2012, two of the largest strike-slip earthquakes ever recorded — at magnitudes of 8.7 and 8.2 — occurred in the northeastern Indian Ocean, a few hundred kilometres off the coast of Sumatra. Three groups now report the analysis of seismic data from the days and months before and after these events, as well as the events themselves. Matthias Delescluse and co-authors show that these earthquakes are part of the ongoing boost of intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes. They conclude that the Australian plate, driven by slab-pull forces at the Sunda trench, is gradually detaching from the Indian plate. Han Yue and colleagues show that the 11 April event involved a complex four-fault rupture lasting several minutes, followed two hours later by a magnitude-8.2 aftershock. These great ruptures on a lattice of strike-slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the Indian and Australian plates. Fred Pollitz and colleagues show that, in the six days following 11 April, the global rate of remote earthquakes with magnitudes greater than 5.5 increased nearly fivefold, and events up to magnitude 7 seem to have been triggered. The unprecedented delayed triggering power of this earthquake may arise from its strike-slip source geometry, or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure.

Shallow slip deficit due to large strike-slip earthquakes in dynamic rupture simulations with elasto-plastic off-fault response

SUMMARY Slip inversions of geodetic data from several large (magnitude ∼7) strike-slip earthquakes point to coseismic slip deficit at shallow depths (<3–4 km), that is, coseismic slip appears to decrease towards the Earth surface. While the inferred slip distribution may be consistent with laboratory-derived rate and state friction laws suggesting that the uppermost brittle crust may be velocity strengthening, there remains a question of how the coseismic slip deficit is accommodated throughout the earthquake cycle. The consequence of velocity-strengthening fault friction at shallow depths is that the deficit of coseismic slip is relieved by post-seismic afterslip and interseismic creep. However, many seismic events with inferred shallow slip deficit were not associated with either resolvable shallow interseismic creep or robust shallow afterslip. Hence, the origin of shallow ‘slip deficit’ remains uncertain. In this study, we investigate whether inelastic failure in the shallow crust due to dynamic earthquake rupture can explain the inferred deficit of shallow slip. Evidence for such failure is emerging from geologic, seismic and geodetic observations. We find that the amount of shallow slip deficit is proportional to the amount of inelastic deformation near the Earth surface. Such deformation occurs under a wide range of parameters that characterize rock strength in the upper crust. However, the largest magnitude of slip deficit in models accounting for off-fault yielding is 2–4 times smaller than that inferred from kinematic inversions of geodetic data. To explain this discrepancy, we further explore to what extent assumptions in the kinematic inversions may bias the inferred slip distributions. Inelastic deformation in the shallow crust reduces coseismic strain near the fault, introducing an additional ‘artificial’ deficit of up to 10 per cent of the maximum slip in inversions of geodetic data that are based on purely elastic models. The largest magnitude of slip deficit in our models combined with the bias in inversions accounts for up to 25 per cent of shallow slip deficit, which is comparable, but still smaller than 30– 60 per cent deficit inferred from kinematic inversions. We discuss potential mechanisms that may account for the remaining discrepancy between slip deficit predicted by elasto-plastic rupture models and that inferred from inversions of space geodetic data.

Regions of Reduced Static Stress Drop near Fault Tips for Large Strike-Slip Earthquakes

Abstract We present a case study of slip distributions for the 1992 Landers ( M w 7.3) and the 1999 Hector Mine ( M w 7.1) earthquakes in California’s Mojave Desert. Slip distributions, as determined from geophysical inversion of geodetic, strong ground motion, and teleseismic data, are complex, heterogeneous, and often exhibit linearly tapering or concave-upward patterns toward segment tips, distinctly different from the elliptical slip distributions characteristic of uniform stress drop. Mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the Camp Rock/Emerson fault segment of the Landers rupture; however, numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations. Numerical models demonstrate that long (5–25 km) zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes. Zones of reduced stress drop are implemented as regions of increased Coulomb strength. Slip distributions are improved 30%–70% relative to models with uniform stress drop. Regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment. Similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop. Plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction, spatial or temporal changes in pore pressure, geometric complexity of the fault surface, heterogeneity of normal tractions resolved on the fault surface, inelastic deformation, and dynamic rupture effects.