Interseismic Slip Research Articles

Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike‐slip faults

We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface‐cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 Mw = 6.7 Coalinga earthquake brought the subsequent 1983 Mw = 6.0 Nuñez and 1985 Mw = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust‐faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 Mw = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 Mw = 9.5 and 1995 Mw = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2–20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike‐slip systems modulates the stress acting on nearby thrust and strike‐slip faults. We calculate that the 1857 Mw = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault ∼1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw = 7.3 Kern County shock but inhibited slip on the left‐lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales.

Distribution of interseismic slip rates and the potential for significant earthquakes on the Calaveras fault, central California

The Calaveras fault is a major component of the San Andreas fault system in the San Francisco Bay area, that generated 13 earthquakes of ML > 5 since 1850. In most recent ML > 5 events, premain shock and postmain shock microseismicity is sparse in the region of coseismic slip. These aseismic areas are believed to represent locked patches of the fault that are accumulating strain to be released in ML > 5 events. We analyze geodetic data to better characterize the spatial distribution of interseismic slip rates on the Calaveras fault, modeling the slip distribution in the seismogenic zone by inversion of over 25 years of surface deformation data. We use a regional fault model with the seismogenic zone of the Calaveras fault discretized into ∼6 km × 3 km elements, employing a weighted least squares approach with smoothing and positivity constraints. Our discretized fault slip model consistently identifies regions of slip deficit in the seismogenic zone of the Calaveras fault that generally correspond to regions of decreased microseismicity and ruptures of previous moderate earthquakes. In particular, we find correspondence with the 1979 Coyote Lake and 1984 Morgan Hill events, as well as regions where historical earthquakes on the Coyote and the Sunol‐San Ramon segments have occurred. Moment magnitude calculations based on the estimated slip deficit, fault area, and recurrence intervals agree with measured magnitudes of modern events and interpreted historical magnitudes. The results suggest that a combination of geodetically derived fault slip models and microseismicity distribution can be used to characterize seismic hazard.

A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt

We use a Global Positioning System (GPS)-derived surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion (306°). The GPS data are divided into northern and southern regions and the velocities in each region are projected into single profiles. The model fault geometry includes a shallowly dipping décollement, based on the balanced geological cross-sections in the Coastal Plain and Western Foothills, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The décollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 and 80 mm/yr, respectively. The optimal geometry of décollement is subhorizontal (2°∼11°) at a depth of 8∼9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu Fault, while in the southern section, it is located beneath the Chukou Fault. The elastic dislocation model successfully matches the horizontal velocity data, and predicts elastic strain accumulation in the Western Foothills that will presumably be released in future earthquakes. However, considered over multiple earthquake cycles, our model cannot explain the topography of the CR and thus fails to predict the active mountain building process in Taiwan. This failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior.

Geodetic constraints on postseismic deformation following the 1990 Ms 7.8 Luzon earthquake and implications for Luzon tectonics and Philippine Sea plate motion

We have obtained three epochs of high‐quality dual‐frequency GPS data at 15 stations in the Luzon, Philippines, region since the July 16, 1990, Ms 7.8 earthquake, from which we generate horizontal velocity fields at the Earth's surface for the 1993–1996 and 1996–1998 periods. We use these velocities, plus 1996–1998 GPS velocity results reported elsewhere from 13 additional stations, to show that present‐day deformation in Luzon is dominated by strike‐slip motion along the Philippine fault system. The measured strike‐slip rate across central Luzon is faster than the expected long‐term slip rate on the fault, which we attribute to postseismic deformation following the 1990 earthquake. The 1993–1996 and 1996–1998 deformation patterns cannot be statistically distinguished within the ∼10% uncertainties of the data. If the observed deformation is interpreted using an elastic half‐space model with uniform slip below a subsurface locking depth, we find a fault slip rate of 40 mm/yr and a locking depth of ∼15 km. This is too fast to be a steady interseismic rate, as it disagrees with what is known from paleoseismic and other evidence about earthquake recurrence and long‐term slip rate on the Philippine fault system in central and northern Luzon. Also, a 15 km locking depth for steady interseismic slip is probably too shallow compared to the depth of rupture in the 1990 earthquake. If the deformation is instead interpreted with two types of viscoelastic model, we find that the observed velocities can be fit well for a range of values of lower lithosphere viscosity and long‐term slip rate. Using a two dimensional (2‐D) viscoelastic coupling model, the minimum allowable lower lithosphere viscosity is 0.5 × 1019 Pa s, with an associated long‐term slip rate of 15–22 mm/yr. Faster long‐term slip rates also fit well, with correspondingly higher values of viscosity. With a possibly more realistic model that includes 3‐D viscoelastic effects we find preferred long‐term velocities in the range 20–35 mm/yr and viscosities in the range 2–6 × 1019 Pa s. Our preferred viscosities correspond to Maxwell times on the order of 7–20% of the typical earthquake recurrence interval and are several times higher than values found for a number of Californian strike‐slip earthquakes. Other implications of our 1993–1998 surface velocities are that the Philippine Sea‐Eurasia (PH‐EU) Euler vector of Seno et al. [1993] is a better description of the PH‐EU convergence rate than a Euler vector recently estimated from GPS velocities in the northern PH and that the great majority of the normal component of PH‐EU plate convergence is taking place west of Luzon, presumably largely at the Manila Trench. We also find that Luzon is rotating counterclockwise relative to PH at 1°–2°/Myr, only ∼25% of the rate recently suggested by other workers on the basis of limited GPS data.

GPS Constraints on M 7-8 Earthquake Recurrence Times for the New Madrid Seismic Zone

Newman et al. (1999) estimate the time interval between the 1811–1812 earthquake sequence near New Madrid, Missouri and a future similar sequence to be at least 2,500 years, an interval significantly longer than other recently published estimates. To calculate the recurrence time, they assume that slip on a vertical half-plane at depth contributes to the current interseismic motion of GPS benchmarks. Compared to other plausible fault models, the half-plane model gives nearly the maximum rate of ground motion for the same interseismic slip rate. Alternative models with smaller interseismic fault slip area can satisfy the present GPS data by having higher slip rate and thus can have earthquake recurrence times much less than 2,500 years.

A Simple Stick-Slip and Creep-Slip Model for Repeating Earthquakes and its Implication for Microearthquakes at Parkfield

If repeating earthquakes are represented by circular ruptures, have constant stress drops, and experience no aseismic slip, then their recurrence times should vary with seismic moment as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(t\_{\mathrm{r}}{\propto}M\_{0}^{1{/}3}\) \end{document}. In contrast, the observed variation for small, characteristic repeating earthquakes along a creeping segment of the San Andreas fault at Parkfield (Nadeau and Johnson, 1998) is much weaker. Also, the Parkfield repeating earthquakes have much longer recurrence intervals than expected if the static stress drop is 10 MPa and if the loading velocity V L is assumed equal to the geodetically inferred slip rate of the fault V f. To resolve these discrepancies, previous studies have assumed no aseismic slip during the interseismic period, implying either high stress drop or V L ≠ V f. In this study, we show that a model that includes aseismic slip provides a plausible alternative explanation for the Parkfield repeating earthquakes. Our model of a repeating earthquake is a fixed-area fault patch that is allowed to continuously creep and strain harden until reaching a failure threshold stress. The strain hardening is represented by a linear coefficient C , which when much greater than the elastic loading stiffness k leads to relatively small interseismic slip (stick-slip). When C and k are of similar size creep-slip occurs, in which relatively large aseismic slip accrues prior to failure. Because fault-patch stiffness varies with patch radius, if C is independent of radius, then the model predicts that the relative amount of seismic to total slip increases with increasing radius or M , consistent with variations in slip required to explain the Parkfield data. The model predicts a weak variation in t r with M similar to the Parkfield data.

Strain accumulation along the Cascadia Subduction Zone

We combine triangulation, trilateration, and GPS observations to determine horizontal strain rates along the Cascadia subduction zone from Cape Mendocino to the Strait of Juan de Fuca. Shear‐strain rates are significantly greater than zero (95% confidence) in all forearc regions (26–167 nanoradians/yr), and are not significant in the arc and backarc regions. The deformation is primarily uniaxial contraction nearly parallel to Juan de Fuca‐North America plate convergence (N55°–80°E). The strain rates are consistent with an elastic dislocation model for interseismic slip with a shallow 100‐km wide locked zone and a deeper 75‐km transition zone along the entire megathrust, except along the central Oregon coast where relatively lower strain rates are consistent with 30–40 km wide locked and transition zones.

The effect of lateral variations of friction on crustal faulting

We propose that lateral variations in fault friction control the heterogeneity of slip observed in large earthquakes, We model these variations using a rate and state-dependent friction law, where we differentiate velocity-weakening into strong and weak-seismic fields, and velocity-strengthening into compliant and viscous fields. The strong-seismic field comprises the seismic slip concentrations, or asperities. The two «intermediate» frictional fields, weak-seismic and compliant, modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the compliant and viscous regions slip aseismically while the strong-seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak-seismic regions contain most of the interseismic activity and aftershocks, but also «creep seismically», that is, most of the weak-seismic area slips aseismically, actuating the seismicity on the remaining area. This «mixed» frictional behavior can be obtained from a sufficiently heterogenous distribution for the critical slip distance. The interseismic slip provides an inherent rupture resistance: dynamic rupture fronts decelerate as they penetrate into these unloaded compliant or creeping weak-seismic areas, diffusing into broad areas of accelerated afterslip. Aftershocks occur in both the weak-seismic and compliant areas around the fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area, by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the interevent seismicity and aftershocks to the coseismic slip distribution for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes.

An Overview of Southern California Crustal Deformation Studies and Their Tectonic Interpretation: ABSTRACT

Geodectic studies, particularly the high-accuracy results obtained from Global Positioning System (GPS) data, have helped to define the active tectonics of the western Transverse Ranges. Recent results of the Transverse Ranges Experiments (TREX) and related work have documented revisions to earlier published reports on crustal deformation in the region. A review of the earliest to the most recent geodectic results will be followed by a summary of the currently documented rates and styles of deformation. Also, the latest findings from work in the Los Angeles region will be discussed. A challenging aspect of geodectic work is to convert the displacement vectors obtained into, for example, slip rates that are useful as inputs to tectonic or seismic hazard models. Because the faults under study are generally locked to depths of 10 to 20 km, modeling is required to interpret the observed displacement field. The related problems and limitations inherent in obtaining equivalent fault slip rates from a displacement field map will be discussed, and a range of plausible models investigated for the Los Angeles region. Without accurate information on the geometry of faulting at depths >10 km, where interseismic slip occurs, many fault slip models are tenable. Because of this, themore » interpretation of geodectic data in terms of slip rates on active faults will continue to rely in turn on interpretations of deep structures that are based on seismic images, borehole, and surface geological data. It is hoped that resolution of controversies over the nature of deep structure in the western Transverse Ranges will coincide with the development of a more complete geodetic picture of ongoing deformation, and that cooperative work can then lead to a greatly improved seismic hazard model for the region. In the meantime, geodectic information can provide among the strongest constraints on expected moment rate, for computing seismic hazard in areas of blind' or widely distributed faulting.« less

A model of interseismic fault slip in the presence of asperities

SUMMARY A 2-D model which represents a slipping fault with non-uniform Coulomb friction is studied. The fault plane is subject to a uniform ambient shear stress, slowly increasing with time. Aseismic fault creep is assumed to start in a weak zone, when the ambient stress reaches a strength threshold. The solution for the resulting dislocation is worked out analytically using a technique based on Chebyshev polynomials. It is found that the dislocation partially propagates into the adjacent asperities, concentrating stress onto them and preparing the conditions which will produce the asperity failure and the accompanying earthquake. Propagation is not self-similar and occurs at increasing velocity. A non-linear slip hardening effect is reproduced. The nearness to earthquake instability is measured by a dimensionless parameter which depends on Coulomb friction and ambient shear stress and decreases to zero with time. An upper boundary to the critical value of this parameter, at which instability may occur, is estimated and is found to depend on the ratio between the sizes of the asperity and the weak zone.

Historic surface slip along the San Andreas Fault near Parkfield, California

The Parkfield Earthquake Prediction Experiment is focusing close attention on the 44‐km‐long section of the San Andreas fault that last ruptured seismically in 1966 (Ms 6.0). The 20‐km‐long central segment of the 1966 Parkfield rupture, extending from the mainshock epicenter at Middle Mountain southeastward to Gold Hill, forms a 1‐ to 2‐km salient northeastward away from the dominant N40°W strike. Following the 1966 earthquake afterslip, aseismic slip has been nearly constant. Moderate Parkfield earthquakes have recurred on average every 21 years since 1857, when a great earthquake (M≈8) ruptured at least as far north as the southern Parkfield segment. Many measurements of slip have been made near Parkfield since 1966. Nevertheless, much of the history of surface slip remained uncertain, especially the total amount associated with the 1966 event. In 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 Parkfield rupture segment. We interpret net slip on each feature as a sum of event slip (sum of coseismic and rapid preseismic and postseismic slip) from Parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays, creep meters, and trilateration lines. We assumed for each site that event slip was identical for the 1922, 1934, and 1966 Parkfield events and that long‐term average rate of interseismic slip was constant between all events. Two fences on the southern segment, southeast of Gold Hill, indicate event slip of 13 and 15 cm and interseismic slip rate of 0.36 and 0.30 cm/yr since 1959 and 1908, respectively. At these sites, redundant independent data support our assumption that both event and interseismic slip occur uniformly. On the central segment, near Parkfield, both the 1934 and the 1966 ruptures offset a bridge built in 1932. Interseismic slip rate near the bridge has been about 1.1 cm/yr since 1966; thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes. On a parallel fault trace, 1 km to the southwest, slip was about 8 cm in 1966; thus total event slip summed across the entire fault zone near Parkfield was nearly 40 cm. On Middle Mountain, 4 km north of the 1966 mainshock epicenter, an offset fence indicates 17 cm of slip in 1966 and a 2.26‐cm/yr interseismic slip rate since circa 1946. Thus the central segment of the 1966 rupture is characterized by much larger event slip (∼40 cm) than both distal segments (∼15 cm). This amount of surface slip per event is about twice what had been previously assumed. Larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65±10 cm slip on a narrow, buried asperity between Middle Mountain and Gold Hill that has been inferred from the depth distribution of early aftershocks. Assuming our characteristic surface slip model, one can further deduce a deficit in slip since the great 1857 earthquake. Taking the long‐term slip rate as 3.3 cm/yr, the surface slip deficit is 3±0.2 m south of Gold Hill but only 0.3±0.3 m northward from Parkfield.

Slip Deficit on the San Andreas Fault at Parkfield, California, as Revealed by Inversion of Geodetic Data

A network of geodetic lines spanning the San Andreas fault near the rupture zone of the 1966 Parkfield, California, earthquake (magnitude M = 6) has been repeatedly surveyed since 1959. In the study reported here the average rates of line-length change since 1966 were inverted to determine the distribution of interseismic slip rate on the fault. These results indicate that the Parkfield rupture surface has not slipped significantly since 1966. Comparison of the geodetically determined seismic moment of the 1966 earthquake with the interseismic slip-deficit rate suggests that the strain released by the latest shock will most likely be restored between 1984 and 1989, although this may not occur until 1995. These results lend independent support to the earlier forecast of an M = 6 earthquake near Parkfield within 5 years of 1988.