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Faults & earthquakes

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USGS1997

Postseismic strain following the 1989 Loma Prieta earthquake from GPS and leveling measurements

Postseismic deformation in the 5 years following the 1989 Loma Prieta earthquake has been measured with the Global Positioning System and precise leveling. Postearthquake velocities at distances greater than ∼20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake. However, velocities at stations within ∼20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the San Andreas fault system. A combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2.9 cm/yr on the San Andreas fault and/or the Loma Prieta rupture zone and 2.4 cm/yr reverse slip along a buried fault within the Foothills thrust belt. The best fitting sources of postseismic deformation are all located at depths of less than 15 km. We find no evidence for accelerated flow or shear below the Loma Prieta rupture in the first 5 years following the earthquake. The inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture.

Journal of Geophysical Research B: Solid Earth↗

USGS1997

Post seismic deformation associated with the 1992 Mω = 7.3 Landers earthquake, southern California

Following the 1992 M ω =7.3 Landers earthquake, a linear array of 10 geodetic monuments at roughly 5‐km spacing was established across the Emerson fault segment of the Landers rupture. The array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it. The array was surveyed by Global Positioning System 0.034, 0.048, 0.381, 1.27, 1.88, 2.60, and 3.42 years after the Landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation. The temporal behavior is described roughly by a short‐term (decay time 84±23 days) exponential relaxation superimposed upon an apparently linear trend. Because the linear trend represents motions much more rapid than the observed preseismic motions, we attribute that trend to a slower (decay time greater than 5 years) postseismic relaxation, the curvature of which cannot be resolved in the short run (3.4 years) of postseismic data. About 100 mm of right‐lateral displacement and 50 mm of fault‐normal displacement accumulated across the geodetic array in the 3.4‐year interval covered by the postseismic surveys. Those displacements are attributed to postseismic, right‐lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace. The right‐lateral slip amounted to about 1 m directly beneath the geodetic array, and the fault‐normal displacement is apparently primarily a consequence of the curvature of the rupture. These conclusions are based upon dislocation models fit to the observed deformation. However, no dislocation model was found with rms residuals as small as the expected observational error.

California↗

USGS1998

Deformation following the 1994 Northridge Earthquake (M=6.7), Southern California

Following the 1994 M w =6.7 Northridge earthquake, a 65‐km‐long, north‐south array of 11 geodetic monuments was established across the rupture. The array was surveyed with GPS ten times in the 4.25 yr after the earthquake. Although there is evidence for modest nonlinear postseismic relaxation in the first few weeks after the Northridge earthquake, the deformation in the subsequent four years can be adequately described by constant station velocities. The observed S70°E velocity components are consistent with the deformation expected from steady strain accumulation on the San Andreas fault. The N20°E velocity components indicate that the southern Northridge fault block is moving almost as a unit N20°E with repect to the northern fault block, the motion being accommodated by a zone of convergence (width 20 km) at the north end of the Northridge rupture.

California↗

USGS1998

The coseismic slip distributions of the 1940 and 1979 Imperial Valley, California, earthquakes and their implications

Geodetic arrays observed by the U.S. Coast and Geodetic Survey span the Imperial fault in southern California. For the 1940 M 7.1 Imperial Valley earthquake, a 1934–1941 triangulation network has sufficient resolution to allow inversion for the coseismic slip distribution on fault segments 5 to 25 km long extending from the surface to a depth of 9 km. The estimated right-lateral slip is 0.8 to 1.7 m on the northern 30 km of the main trace of the Imperial fault, 4.8±0.2 m on a 10-km-long segment straddling the United States - Mexico border, and 1.3±0.4 m on a southern 25-km-long segment in Mexico. Fixing this strike-slip model and inverting 1940 leveling data only for dip slip yields 0.1 m of east-side-down dip slip. The seismic moment for this model is M 0 = (3.2±0.3) ×10 19 N m. The 1979 geodetic data set, mostly elevation changes from leveling routes, has insufficient resolution for inversion. However, it is possible to use this geodetic data set and results published by others to infer that the 1940 and 1979 earthquakes may be similar on the rupture zone common to both events. Our preferred 1940 model is similar to the 1979 geodetic results of Crook [1984] on the segments where both networks have good resolution. Elevation changes from 1940 and 1979 leveling data are very similar. Thus the geodetic data corroborate the surface slip evidence of Sharp [1982b] that the 1940 and 1979 slip distributions are examples of “characteristic slip” on the northern Imperial fault.

California↗

USGS2012

Coseismic stress changes induced by the 1989 Loma Prieta, California Earthquake

Earthquake focal mechanisms from before and after the 1989 Loma Prieta, California earthquake are used to infer the coseismic stress change. Before the main shock, most earthquakes correspond to right lateral slip on planes sub-parallel to the San Andreas fault, and imply a generally N-S most compressional stress axis and a vertical intermediate stress axis. Aftershocks within the main shock rupture zone, however, display almost every style and orientation of faulting, implying an extremely heterogeneous stress field. This suggests that the main shock relieved most, if not all, of the shear stress acting on its fault plane. Aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states, but only when considered in three groups: north, south, and above the main shock rupture area. In each of these areas the stress state may reflect stress transfer by the main shock.

California↗

USGS1991

Seismic quiescence at Parkfield due to detachment faulting

On the San Andreas fault near Parkfield, California, rates of seismicity 1,2 and geodetic line shortening 3,4 have been lower since 1986 than before. Wyss et al . 1,3 interpret the rate decreases as precursors to an imminent moderate earthquake (magnitude M = 5.5−6) and estimate the earthquake time to be March 1991 ± 1 yr. The earthquake was previously predicted to occur in 1988 ± 5 yr based on five of six similar earthquakes which occurred at intervals ∼ 22 yr, the last one in 1966 5 . Here I present a model that attributes the rate changes to the initiation of slip on a subhorizontal detachment fault under the pending rupture area. In this model, the slip starts late in the seismic cycle, and diminishes the loading rate on the nearby San Andreas, thus acting as a buffer.

California↗

USGS2013

Frictional-faulting model for harmonic tremor before Redoubt Volcano eruptions

Seismic unrest, indicative of subsurface magma transport and pressure changes within fluid-filled cracks and conduits, often precedes volcanic eruptions. An intriguing form of volcano seismicity is harmonic tremor, that is, sustained vibrations in the range of 0.5–5 Hz. Many source processes can generate harmonic tremor. Harmonic tremor in the 2009 eruption of Redoubt Volcano, Alaska, has been linked to repeating earthquakes of magnitudes around 0.5–1.5 that occur a few kilometres beneath the vent. Before many explosions in that eruption, these small earthquakes occurred in such rapid succession—up to 30 events per second—that distinct seismic wave arrivals blurred into continuous, high-frequency tremor. Tremor abruptly ceased about 30 s before the explosions. Here we introduce a frictional-faulting model to evaluate the credibility and implications of this tremor mechanism. We find that the fault stressing rates rise to values ten orders of magnitude higher than in typical tectonic settings. At that point, inertial effects stabilize fault sliding and the earthquakes cease. Our model of the Redoubt Volcano observations implies that the onset of volcanic explosions is preceded by active deformation and extreme stressing within a localized region of the volcano conduit, at a depth of several kilometres.

Alaska↗

USGS2021

Earthquakes indicated magma viscosity during Kīlauea’s 2018 eruption

Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. The identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards 1 . The unanticipated May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai‘i 2 displayed exceptional chemical and thermal variability in erupted lavas, leading to unpredictable effusion rates and explosivity. Here, using an integrated analysis of seismicity and magma rheology, we show that the orientation of fault-plane solutions (which indicate a fault’s orientation and sense of movement) for earthquakes preceding and accompanying the 2018 eruption indicate a 90-degree local stress-field rotation from background, a phenomenon previously observed only at high-viscosity eruptions 3 , and never before at Kīlauea 4,5,6,7,8 . Experimentally obtained viscosities for 2018 products and earlier lavas from the Pu‘u ‘Ō‘ō vents tightly constrain the viscosity threshold required for local stress-field reorientation. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves magma of heightened viscosity, and thus real-time monitoring of the orientations of fault-plane solutions could provide critical information about the style of an impending eruption. Furthermore, our results provide insight into the fundamental nature of coupled failure and flow in complex multiphase systems.

Hawaii↗

USGS2004

The threat of silent earthquakes

Not all earthquakes shake the ground. The so-called silent types are forcing scientists to rethink their understanding of the way quake-prone faults behave. In rare instances, silent earthquakes that occur along the flakes of seaside volcanoes may cascade into monstrous landslides that crash into the sea and trigger towering tsunamis. Silent earthquakes that take place within fault zones created by one tectonic plate diving under another may increase the chance of ground-shaking shocks. In other locations, however, silent slip may decrease the likelihood of destructive quakes, because they release stress along faults that might otherwise seem ready to snap.

Scientific American↗

USGS2026

Low-frequency earthquakes track the motion of a captured slab fragment

Accurate tectonic models are essential for assessing seismic hazard and fault interactions. However, the plate configuration at the complex Mendocino triple junction, where the San Andreas Fault and the Cascadia subduction zone meet, remains uncertain. We analyzed fault slip associated with a recently identified zone of tectonic tremor and low-frequency earthquakes (LFEs) near the southern edge of the subducting Gorda slab. Based on tidal sensitivity and P-wave first motions, we show that the LFEs are generated by dipping, strike-slip motion. This suggests that a former Farallon slab fragment, now captured by the Pacific plate, is translating northward beneath westernmost North America. This geometry effectively extends the slab interface fault, challenging prevailing interpretations of slab window formation and creating a potential unaccounted earthquake hazard in this region.

Science↗

USGS1970

Mechanism of the Chilean Earthquakes of May 21 and 22, 1960

The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

GSA Bulletin↗

USGS1997

Growth of a tectonic ridge during the Landers earthquake

The formation of tectonic ridges by localized vertical uplift along strike-slip faults has long been suspected, but the actual growth of a tectonic ridge during an earthquake has never been documented. During the 1992 Landers, California, earthquake sequence, an awl-shaped, dome-like topographic ridge along the Emerson fault zone increased its height at least 1 m concurrently with 3 m of right-lateral shift across the fault zone containing the ridge. Five deformation vectors within the ridge reveal dilatant behavior in addition to the uplift and shift on boundary faults.

Geology↗

USGS2009

Recent developments in understanding the tectonic evolution of the Southern California offshore area: Implications for earthquake-hazard analysis

During late Mesozoic and Cenozoic time, three main tectonic episodes affected the Southern California offshore area. Each episode imposed its unique structural imprint such that early-formed structures controlled or at least influenced the location and development of later ones. This cascaded structural inheritance greatly complicates analysis of the extent, orientation, and activity of modern faults. These fault attributes play key roles in estimates of earthquake magnitude and recurrence interval. Hence, understanding the earthquake hazard posed by offshore and coastal faults requires an understanding of the history of structural inheritance and modifi-cation. In this report we review recent (mainly since 1987) findings about the tectonic development of the Southern California offshore area and use analog models of fault deformation as guides to comprehend the bewildering variety of offshore structures that developed over time. This report also provides a background in regional tectonics for other chapters in this section that deal with the threat from offshore geologic hazards in Southern California. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2021

Subducting oceanic basement roughness impacts on upper plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the 2nd largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977 – 1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Further surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ~3 to 5 km thick sediment. Sediment on the subducting plate modulates the plate interface relief. The accreted prism’s imbricate thrust faults have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Further upslope, another fault extends >100 km along-strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5 D plate interface images show modest relief whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading/landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Alaska↗

USGS2002

Identifying a large landslide with small displacements in a zone of coseismic tectonic deformation; the Villa Del Monte landslide triggered by the 1989 Loma Prieta, California, earthquake

The Villa Del Monte landslide was one of 20 large and complex landslides triggered by the 1989 Loma Prieta , California , earthquake in a zone of pervasive coseismic ground cracking near the fault rupture. The landslide was approximately 980 m long, 870 m wide, and encompassed an area of approximately 68 ha. Drilling data suggested that movement may have extended to depths as great as 85 m below the ground surface. Even though the landslide moved <1 m, it caused substantial damage to numerous dwellings and other structures, primarily as a result of differential displacements and internal Assuring. Surface cracks, scarps, and compression features delineating the Villa Del Monte landslide were discontinuous, probably because coseismic displacements were small ; such discontinuous features were also characteristic of the other large , coseismic landslides in the area, which also moved only short distances during the earthquake . Because features marking landslide boundaries were discontinuous and because other types of coseismic ground cracks were widespread in the area, identification of the landslides required detailed mapping and analysis. Recognition that landslides such as that at Villa Del Monte may occur near earthquake-generating fault ruptures should aid in future hazard evaluations of areas along active faults.

Reviews in Engineering Geology↗

USGS1996

The enigma of the New Madrid earthquakes of 1811-1812

Continental North America's greatest earthquake sequence struck on the western frontier of the United States. The frontier was not then California but the valley of the continent's greatest river, the Mississippi, and the sequence was the New Madrid earthquakes of the winter of 1811–1812. Their described impacts on the land and the river were so dramatic as to produce widespread modern disbelief. However, geological, geophysical, and historical research, carried out mostly in the past two decades, has verified much in the historical accounts. The sequence included at least six (possibly nine) events of estimated moment magnitude M ≥ 7 and two of M ≃ 8. The faulting was in the intruded crust of a failed intracontinental rift, beneath the saturated alluvium of the river valley, and its violent shaking resulted in massive and extensive liquefaction. The largest earthquakes ruptured at least six (and possibly more than seven) intersecting fault segments, one of which broke the surface as a thrust fault that disrupted the bed of the Mississippi River in at least 2 (and possibly four) places.

Arkansas, Kentucky, Missouri, Tennessee↗

USGS2002

A bilinear source-scaling model for M-log a observations of continental earthquakes

The Wells and Coppersmith (1994) M-log A data set for continental earthquakes (where M is moment magnitude and A is fault area) and the regression lines derived from it are widely used in seismic hazard analysis for estimating M, given A. Their relations are well determined, whether for the full data set of all mechanism types or for the subset of strike-slip earthquakes. Because the coefficient of the log A term is essentially 1 in both their relations, they are equivalent to constant stress-drop scaling, at least for M ??? 7, where most of the data lie. For M > 7, however, both relations increasingly underestimate the observations with increasing M. This feature, at least for strike-slip earthquakes, is strongly suggestive of L-model scaling at large M. Using constant stress-drop scaling (???? = 26.7 bars) for M ??? 6.63 and L-model scaling (average fault slip u?? = ??L, where L is fault length and ?? = 2.19 &times 10-5) at larger M, we obtain the relations M = log A + 3.98 ?? 0.03, A ??? 537 km2 and M = 4/3 log A + 3.07 ?? 0.04, A > 537 km2. These prediction equations of our bilinear model fit the Wells and Coppersmith (1994) data set well in their respective ranges of validity, the transition magnitude corresponding to A = 537 km2 being M = 6.71.

Bulletin of the Seismological Society of America↗

USGS2004

Observed and simulated ground motions in the San Bernardino basin region for the Hector Mine, California, earthquake

During the MW 7.1 Hector Mine earthquake, peak ground velocities recorded at sites in the central San Bernardino basin region were up to 2 times larger and had significantly longer durations of strong shaking than sites just outside the basin. To better understand the effects of 3D structure on the long-period ground-motion response in this region, we have performed finite-difference simulations for this earthquake. The simulations are numerically accurate for periods of 2 sec and longer and incorporate the detailed spatial and temporal heterogeneity of source rupture, as well as complex 3D basin structure. Here, we analyze three models of the San Bernardino basin: model A (with structural constraints from gravity and seismic reflection data), model F (water well and seismic refraction data), and the Southern California Earthquake Center version 3 model (hydrologic and seismic refraction data). Models A and F are characterized by a gradual increase in sediment thickness toward the south with an abrupt step-up in the basement surface across the San Jacinto fault. The basin structure in the SCEC version 3 model has a nearly uniform sediment thickness of 1 km with little basement topography along the San Jacinto fault. In models A and F, we impose a layered velocity structure within the sediments based on the seismic refraction data and an assumed depth-dependent Vp/Vs ratio. Sediment velocities within the SCEC version 3 model are given by a smoothly varying rule-based function that is calibrated to the seismic refraction measurements. Due to computational limitations, the minimum shear-wave velocity is fixed at 600 m/sec in all of the models. Ground-motion simulations for both models A and F provide a reasonably good match to the amplitude and waveform characteristics of the recorded motions. In these models, surface waves are generated as energy enters the basin through the gradually sloping northern margin. Due to the basement step along the San Jacinto fault, the surface wave energy is confined to the region north of this structure, consistent with the observations. The SCEC version 3 model, lacking the basin geometry complexity present in the other two models, fails to provide a satisfactory match to the characteristics of the observed motions. Our study demonstrates the importance of using detailed and accurate basin geometry for predicting ground motions and also highlights the utility of integrating geological, geophysical, and seismological observations in the development and validation of 3D velocity models.

California↗