Earthquake Fault Slip Research Articles

Fault locking, block rotation and crustal deformation in the Pacific Northwest

SummaryWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

Numerical simulation for 1975 Haicheng and 1999 Xiuyan earthquake processes by DDA+FEM

In this paper, using discontinuous deformation analysis, we simulate numerically the processes of the 1975 Haicheng earthquake and the 1999 Xiuyan earthquake in the tectonic environment of North China where blocks restrict each other by DDA+FEM. Based on the research on the distribution of foreshocks and recent aftershocks of Xiuyan earthquake and the results from our study and other study on Haicheng earthquake, we present a tectonic block model of Haicheng and Xiuyan earthquakes. The results from numerical simulation include the variation of principal stress field released by these two earthquakes, contour patterns of maximum shear stress change, variation patterns of displacement vectors before and after the earthquakes and variation of the earthquake fault slip with time, which agree basically with the focal mechanisms, macroscopic isoseismal and horizontal displacement characters from observations.

Present-Day crustal deformation field in China continent inverted from GPS observations, earthquake moment tensor and quaternary fault slip rates

Based on the bi-cubic Bessel spline function method, we have inverted the present-day crustal horizontal velocity field and deformation field in China continent by combining with 410 GPS observations, 327 seismic moment tensors of earthquakes and fault slip rates in China and its neighboring areas, and considered the geological and geophysical parameters at the same time. The results reveal that the crustal movement mostly takes on compression in SN-NE direction and extension in EW-NW direction in China continent. The continent is rotating southeast and takes on a clockwise rotation image from the west to the east, particularly in the southeast of Tibet and Chuan-dian area. Different blocks have different deformation in China continent. The south China, Ordos and Northeast block have good integrity. The deformation in the western part of China is obviously stronger than that in the eastern part. The strong strains are focused on the Himalaya, east Tibet, west Tian Shan and Chuandian block etc. The deformation at the joint of blocks is stronger than that within the blocks. The China continent deformation not only has the strike-slip faulting and extrusion feature but also has the crustal shortening and thickening feature.

Interplate Earthquake Fault Slip During Periodic Earthquake Cycles in a Viscoelastic Medium at a Subduction Zone

Interplate Earthquake Fault Slip During Periodic Earthquake Cycles in a Viscoelastic Medium at a Subduction Zone

Earthquake Processes: Physical Modelling, Numerical Simulation and Data Analysis

A. Earthquake Cycles, Crustal Deformation and Plate Dynamics.- 3-D Simulation of Earthquake Generation Cycles and Evolution of Fault Constitutive Properties.- Interplate Earthquake Fault Slip During Periodic Earthquake Cycles in a Viscoelastic Medium at a Subduction Zone.- Development of a Finite Element Simulator for Crustal Deformation with Large Fault Slipping.- 3-D Viscoelastic FEM Modeling of Crustal Deformation in Northeast Japan.- Combined GPS and InSAR Models of Postseismic Deformation from the Northridge Earthquake.- A Hidden Markov Model Based Tool for Geophysical Data Exploration.- Geophysical Applications of Multidimensional Filtering with Wavelets.- Large Amplitude Folding in Finely Layered Viscoelastic Rock Structures.- Mantle Convection Modeling with Viscoelastic/Brittle Lithosphere: Numerical Methodology and Plate Tectonic Modeling.- GEM Plate Boundary Simulations for the Plate Boundary Observatory: A Program for Understanding the Physics of Earthquakes on Complex Fault Networks via Observations, Theory and Numerical Simulation.- B. Seismicity Change and its Physical Interpretation.- Accelerated Seismic Release and Related Aspects of Seismicity Patterns on Earthquake Faults.- Stress Correlation Function Evolution in Lattice Solid Elasto-dynamic Models of Shear and Fracture Zones and Earthquake Prediction.- Pattern Dynamics and Forecast Methods in Seismically Active Regions.- Long-range Automaton Models of Earthquakes: Power-law Accelerations, Correlation Evolution, and Mode-switching.- Critical Sensitivity and Trans-scale Fluctuations in Catastrophic Rupture.- Load-Unload Response Ratio and Accelerating Moment/Energy Release Critical Region Scaling and Earthquake Prediction.- Simulation of the Load-Unload Response Ratio and Critical Sensitivity in the Lattice Solid Model.

Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin

We describe the prehistoric–historic activity of the main and minor volcano-tectonic structures of the northeastern flank of Mt. Etna, giving new data which provide a clearer view of how destabilized volcanoes react to magma pressure and gravity forces. Data are mostly based on new lithostratigraphic surveys and on offset measurements of late Pleistocene–Holocene deposits and historic man-made features. The North-East Rift (NE Rift) is a volcanic rift that has guided mostly explosive, but also secondary effusive, activity since the end of the Pleistocene. Monogenetic cones and explosive craters are strongly elongated and aligned along 66 main surface fractures and several others, hidden, but inferred, from sinkhole alignments. The opening direction is locally oblique, with a left-lateral component in the north-eastern segment of the rift. At the NE tip of the rift, the movements are relieved by the normal left-lateral Pernicana Fault (PF), an 11-km-long structure comprising E–W fault segments connected by right-stepping en échelon faults. Our study demonstrates that the very high slip rates at a centennial scale of these structures are well constrained and similar (NE Rift=0.6–1.8 cm/a, PF=0.5–2.2 cm/a) and have been also recognized in the Latest Pleistocene–Holocene (2.7 cm/a). The E–W Fiumefreddo Fault (FF) has normal left-lateral kinematics and a historic slip rate of 0.1 cm/a. Ripa della Naca Faults strike NE and are covered by unfaulted <15-ka-old lavas. Relative sinking to the SE gives a late Pleistocene slip rate of 0.2 cm/a. Newly recognized NNE- to NE-striking active normal faults interfere at the PF–FF junction zone. The active East-North-East Rift, which is described in detail for the first time, is another volcanic rift affecting the eastern flank of Mt. Etna. This rift shows left-lateral normal motions and has guided the location of several pyroclastic cones since at least the end of the Pleistocene. Stratigraphic and structural data prove the presence of a North–South Rift which has existed across the Mt. Etna summit zone during at least the last 14 ka. Our data coupled with published seismological data indicate that these structures are the surface expressions of the seaward sliding of the eastern flank of Etna along two slip surfaces in a two-component principal stress field, produced by gravity loading and magma intrusion. A shallower slide surface can be activated following dominance of gravity instability triggered by the mass of the volcano flank, or movements can occur along a deeper surface triggered by injection phases, or the two surfaces can move at the same time by a combination of effects. The existence of this double system of deformation can explain several apparently contradictory phenomena, such as discontinuous correlation of deformation, earthquakes and eruptions, and different earthquake foci, focal mechanisms and fault slip rates.

On relating apparent stress to the stress causing earthquake fault slip

Apparent stress τa is defined as , where is the average shear stress loading the fault plane to cause slip and η is the seismic efficiency, defined as Ea/ W, where Ea is the energy radiated seismically and W is the total energy released by the earthquake. The results of a recent study in which apparent stresses of mining‐induced earthquakes were compared to those measured for laboratory stick‐slip friction events led to the hypothesis that . This hypothesis is tested here against a substantially augmented data set of earthquakes for which can be estimated, mostly from in situ stress measurements, for comparison with τa. The expanded data set, which includes earthquakes artificially triggered at a depth of 9 km in the German Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) borehole and natural tectonic earthquakes, covers a broad range of hypocentral depths, rock types, pore pressures, and tectonic settings. Nonetheless, over ∼14 orders of magnitude in seismic moment, apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of ∼0.06, consistent with the hypothesis proposed before. This behavior of τa and η can be expressed in terms of two parameters measured for stick‐slip friction events in the laboratory: the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot. Typical values for these two parameters yield seismic efficiencies of ∼0.06. In contrast to efficiencies for laboratory events for which η is always near 0.06, those for earthquakes tend to be less than this bounding value because Ea for earthquakes is usually underestimated due to factors such as band‐limited recording. Thus upper bounds on appear to be controlled by just a few fundamental aspects of frictional stick‐slip behavior that are common to shallow earthquakes everywhere. Estimates of from measurements of τa for suites of earthquakes, using , are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths.

Coupling of tectonic loading and earthquake fault slips at subduction zones

Because of the viscoelastic behaviour of the earth, accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales. If the lithospheric plate is acted upon by distant tectonic forces, strain accumulation must occur in a broad region. However, an earthquake releases strain only in a region comparable to the size of the rupture area. A two-dimensional finite-element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips. A fault lock-and-unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed, but determined by the accumulated stress. The amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation, as well as the rheology of the rock material. The larger the region of strain accumulation is compared to the earthquake rupture, the smaller is the earthquake fault slip. The reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate. Since rapid permanent plate shortening is not observed at subduction zones, there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles. The former can be achieved only by significant aseismic fault slip between large subduction earthquakes. The most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault, which requires significant aseismic fault slip between asperities.

Mapping of the high‐frequency source radiation for the Loma Prieta Earthquake, California

Waveform inversion has been successfully applied to map the spatial distribution of earthquake fault slip from strong motion records. But its application has been restricted to a relatively lower frequency range of the data (ƒ&lt; 2.0–3.0 Hz) essentially due to the difficulties in calculating the high‐frequency Green's function; these difficulties arise specifically from the small‐scale complexity of source, path and station site, and insufficient numbers of observations. However, we can avoid these difficulties by simply looking at the seismic wave energy envelopes instead of the complicated high‐frequency wave phases. By assuming that the slip responsible for high‐frequency wave (e.g.,ƒ&gt; 5 Hz) generation is incoherent between neighboring patches on the fault, we formulated an analytical model for calculating the envelope time history of the squared high‐frequency displacement seismogram. This envelope time history is found through the line integration of the squared energy radiation intensity over the isochron. Using a recursive stochastic inversion scheme, we determined both the lower‐frequency slip distributions from the low‐pass‐filtered strong motion displacement data and the higher‐frequency energy radiation intensity from the envelope of the squared high‐frequency displacement data on an extended fault for the 1989 Loma Prieta earthquake, respectively. Our resulting lower‐frequency slip model shows that there are two main slip patches and little slip directly updip of the hypocenter. One major slip zone located 10 km northwest of the hypocenter with predominant dip‐slip and the other located 12 km southeast of the hypocenter with predominant oblique slip. Our preliminary results of the high‐frequency energy radiation intensity inversion indicate three large high‐frequency sources and few smaller ones located on the outer periphery of the large slip zones. This suggests that the high‐frequency energy sources are located along or near the boundaries of large slip zones, which is consistent with the theoretical consideration that high frequencies are primarily generated from the rupture stopping areas or places with large slip variation.

Estimating the probability of occurrences of surface faulting earthquakes on the Wasatch fault zone, Utah

abstract Although geological and geomorphic evidence strongly suggests that the Wasatch fault zone has generated large-magnitude earthquakes in late Quaternary time, the fault zone has not been associated with earthquakes greater than magnitude 512 in the past 133 yr. Therefore, realistic estimates of the likelihood of future damaging earthquakes must be based on more than historical seismicity data. The data base can be expanded by collecting site-specific geological information on earthquake recurrence and fault slip rates and by using this information in a model of the earthquake generation process. Uncertainties in both the physical basis for the model and in the geological parameters dictate a probabilistic approach. A semi-Markov model provides real-time probabilities of occurrence of at least one moderate to large (magnitude 612 or larger) earthquake at either of two sites for given elapsed times. Probabilities derived for the entire Wasatch fault zone are based on earthquake recurrence data on individual fault segments and are especially sensitive to elapsed times on individual segments.