Postseismic Displacements Research Articles

Rapid afterslip following the 1999 Chi‐Chi, Taiwan Earthquake

Postseismic displacements of as much as 14 cm were recorded by GPS measurements in the 3 months following the MW 7.6 1999 Chi‐Chi, Taiwan earthquake. Data from 35 continuous and 90 campaign‐surveyed stations, which show continued east over west thrusting, are analyzed to estimate the postseismic slip distribution and fault geometry. Assuming the shallow fault dips 24° E, as determined by numerous studies of the mainshock, we invert for the deeper fault structure. Our results show that the fault dip shallows with depth below the hypocenter, merging into a nearly horizontal decollement at a depth of 8–12 km. The afterslip distribution shows a maximum slip of 25 cm in the hypocentral region at 7–12 km depth as well as significant slip on the lower decollement. Afterslip is notably absent in the region of maximum coseismic slip, consistent with the afterslip being driven by the mainshock stress change.

Coseismic Displacements from the Hector Mine, California, Earthquake: Results from Survey-Mode Global Positioning System Measurements

We describe the collection and processing of Global Positioning System (GPS) data from 77 locations around the Hector Mine earthquake, which we use to estimate coseismic displacements related to this shock. The existence of pre-event GPS data, some collected to monitor postseismic displacements from the 1992 Landers earthquake and some to establish survey control in the meizoseismal area, provided a relatively dense coverage close to the rupture zone. The data available were collected mostly within the 2 years prior to the 1999 earthquake; we reobserved many points within a few days after the shock, and all within 6 months after. We include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake. The displacements in general display the pattern expected for a strike-slip fault, though a few show significant vertical motion. The maximum horizontal displacement observed was 2 m; one station between fault ruptures showed little horizontal motion, but significant uplift.

Post-seismic viscoelastic deformation

The 3D quasi-static displacements during and following an earthquake provide a wealth of information on the internal structure and rheological properties of the Earth. If an earthquake occurs in a region that has shallow-viscosity zones inside the crust or at the top of the mantle (asthenosphere), then post-seismic displacements of the crust on the order of millimeters per year are possible. These can be detected by means of permanent GPS stations in the years following a faulting event. In this paper we systematically study the influence of viscoelastic simulation model restrictions on co- and post-seismic deformation. Examples include stratification of the Earth model, modeling of the seismic source, influence of compressibility, effects of position and viscosities of shallow low-viscosity layers. It will be shown that some of these model restrictions or assumptions can have non-negligible influences on the simulated 3D co- and post-seismic displacements.

Active displacement field in the Suez–Sinai area: the role of postseismic deformation

By means of a spherical viscoelastic model of co- and postseismic deformation we compute the postseismic displacement associated with the November, 1995, M w=7.2, Aqaba (Jordan) earthquake. We compare our results with the observational data obtained by a GPS campaign performed in a wide portion of Sinai between November 1997 and May 1998. Though the original purpose of the campaign was to explain the tectonic features of the region, the displacement values deduced from the GPS observations are hardly reconcilable with any of the proposed tectonic models of Sinai. Our hypothesis is that the detected deformation field could be due more to the postseismic relaxation following the Aqaba earthquake rather than to tectonic movements. Our results show that both GPS observational data and the postseismic simulations predict a compressional regime in the Gulf of Suez though the contraction detected by GPS is generally larger. The best agreement with the data is obtained for an asthenospheric and lower crust viscosity of 10 18 Pa s and an absence of aseismic afterslip on the fault plane. We conclude that the contribution coming from the postseismic viscoelastic relaxation of the ductile shallow layers following the 1995 Aqaba earthquake is likely to play an important role in determining the present deformation field in Sinai. The transient postseismic relaxation seems to play a more important role than the secular tectonic deformation.

Coseismic and postseismic displacements related with the 1997 Earthquake Sequence in Umbria‐Marche (Central Italy)

We study the coseismic and postseismic displacements related with the 1997 Umbria‐Marche earthquake sequence by means of leveling lines along a deformed aqueduct located in the epicentral area. Comparing the 1960 and 10/1997 measurements we obtain 0.49±0.10 m of coseismic displacement distributed along 3 km across the normal fault zone. Modeling of the coseismic surface dislocation is obtained from a combination of low angle (38°) faults at depth and high angle (80°) upper fault branches. The best fit model indicates that the upper branches stop at 0.4 km below the ground surface and have 60% of slip with respect to the lower faults. The postseismic displacement measured during 1998 is 0.18 m and represents 36% of the apparent coseismic deformation. Moderate earthquakes in the Apennines and related surface deformation may thus result from curved faults that reflect the brittle‐elastic properties of the uppermost crustal structures.

Stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 Northridge, California, Earthquake

Following the Mw 6.7 Northridge earthquake, significant postseismic displacements were resolved with GPS. Using a three‐dimensional viscoelastic model, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several weeks of the main shock, may have been triggered by continuous stress loading from viscous flow. The long‐term decay time of aftershocks (about 2 years) approximately matches the decay of viscoelastic loading, and thus is controlled by the viscosity of the lower crust. Our model provides a physical interpretation of the observed correlation between after‐shock decay rate and surface heat flow.

南海トラフ沿いのプレート間カップリングと九州南部の南東向き運動について

Inter-plate coupling on the convergent plate boundary along the Nankai Trough is analyzed on the basis of GPS measurement data of horizontal displacements in southwest Japan for a period between March 1996 and April 1998. During this period, five moderate-size earthquakes occurred in the study area: M6.6 event offshore Tanegashima in October 1996, two M6.6 events in Hyuganada in October and December 1996, and two M6.1 events in Kagoshima in March and May 1997. Co-seismic displacements associated with these earthquakes are first removed from the GPS displacements. Then, a back-slip model is applied to estimate slips on the coupling interface. High ratios of back-slip relative to plate motion (approximately 100%) are obtained along the eastern part of the Nankai Trough. On the contrary, offshore Kyushu, the ratios are negative, suggesting southeastward motion of the southern part of Kyushu relative to the Philippine Sea plate. We regard these negative values of the inter-plate coupling as a result of the post-seismic deformation associated with two Hyuganada earthquakes in 1996. The stress relaxation model of the asthenosphere is tested using a 3-D finite element method, and it is found that the effect of the relaxation is too small to explain the observed displacements during the post-seismic period. A feasible model adopted to explain the displacements is the after-slip, which possibly occurred on the focal region or its neighboring extension on the plate boundary. The post-seismic slips in Tanegashima and Hyuganada region thus obtained amount to 145 and 208mm, respectively. Generally, the after-slip can explain the observed post-seismic displacements in Kyushu. However, in the western part of Kyushu, the eastward displacement residuals still remain. To elucidate these displacements, different mechanisms such as a back-arc spreading of the Okinawa Trough, or an internal deformation of the continental plate, should be considered.

Poroelastic rebound along the Landers 1992 earthquake surface rupture

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS‐1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large‐scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson‐Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10–20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [Peltzer et al., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60‐km‐long transect across the Emerson fault [Savage and Svarc, 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio Vp/Vs of 2.1, consistent with the observed low Vs values of fault zone guided waves at shallow depth [Li et al., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post‐seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault.

Numerical Simulation of Displacement and Stress Fields Associated with the 1993 Kushiro-oki, Japan, Earthquake

—We constructed a three-dimensional finite element model to simulate coseismic and postseismic displacement and stress fields associated with the 1993 Kushiro-oki earthquake, which was a very large intermediate-depth earthquake that occurred within the subducted Pacific plate at a depth of 107 km beneath the southeastern part of Hokkaido, Japan. Taking the configuration of the subducted Pacific plate into account, we constructed a realistic model with lateral heterogeneity of viscoelastic structure. We assigned a variable slip distribution to the fault plane, which was obtained from inversion analysis of near-field seismic waveforms. The result shows that elastic deformation associated with the faulting reflects the assigned inhomogeneous slip distribution on the fault plane near the fault region, while it does not reflect the distribution on the free surface of the model. The calculated postseismic deformation does not reflect the slip distribution, but shows symmetric spatial patterns concerning the dipping direction of the fault both near the fault region and on the model surface. For the next 20 years following the earthquake, the amount of the calculated deformation is a fraction of the coseismic deformation. The calculated coeseismic deformation is large just above and below the fault plane, reaching 1 m, while the postseismic deformation is dominant near the upper and lower material boundaries between the subducted plate and the surrounding asthenosphere. The spatial distribution of maximum shear stress near the fault plane corresponds to the assigned slip distribution, amounting to 32 MPa. The directions of principal stress-change axes represent reverse fault type in the SSE region of the fault, whereas normal fault type is dominant in the NNW region with the exception of some asymmetrical spatial patterns of the principal stress-change axes on the fault due to the inhomogeneous slip distribution. Time variations both in the amount and the directions of stresses are minor, suggesting that the coseismic state of the stress would remain unchanged for two decades after the event.

Monitoring of the Palu‐Koro Fault (Sulawesi) by GPS

5 years of GPS measurements across the Palu‐Koro fault (Sulawesi, Indonesia) state left lateral strike‐slip (3.4 cm/yr) with a small normal component (0.4 cm/yr). The measurements on intermediate points of the transect show that the fault is locked, at a depth estimated around 8–16 km by a very simple model. The best measured baseline linking the two most separate sites (Watatu and Toboli) shows very interestingly a constant rate of 2.6 cm/yr from 1992 to 1995, and an increased rate of 6.3 cm/yr since then. This increase is due to an earthquake on January 1, 1996 in the Minahasa trench, presumably affecting station Toboli's position by 2 cm of co‐seismic and 2 cm of post‐seismic displacement, which is added to its long term velocity. We interpret this phenomenon as additional stress loading on the Palu fault, driven by the slip during the earthquake 100 km to the North‐East.

Deformation of the 1995 North Sakhalin earthquake detected by JERS-1/SAR interferometry

We present a map of the coseismic displacement field resulting from the North Sakhalin, Russia, May 28, 1995, earthquake. Raw radar signal data from the JERS-1 synthetic aperture radar instrument acquired in 28 April, 11 June, 25 July, and 7 September 1995 are used to generate a high-resolution, wide area map of the displacements by the two-pass differential interferometry method. The interferogram shows that an area of 60 km (EW) by 80 km (NS) experienced crustal deformations. The slip mechanism with fine structure is inverted using the displacement field of the SAR interferogram. This inversion of the SAR interferogram shows, 1) the rupture area extends to the south of the southernmost rupture trace on the surface, 2) slip varies from one to seven meters from the south to the northern part on the fault plane. A theoretical fringe pattern from the model of the earthquake motion matches the observations closely. We generate a full scene digital elevation model (DEM) by JERS-1 SAR interferometry and synthesize a four-pass interferogram. Comparison of the differential interferometry between the four-pass and two-pass interferograms shows that they are very similar and the both methods are reliable to show surface displacements using JERS-1 SAR. Correlation study of the Sakhalin interferograms indicates that low surface temperatures, below the freezing point, reduced the correlation probably because a freeze would alter the dielectric constant of the surface. In these Sakhalin interferograms, we find few spurious fringes due to the usual heterogeneous distribution of weather-related atmospheric and surface conditions, probably because of the low temperatures. We also present a map of the postseismic displacement field with the SAR interferometry.

Rate change observed at JPLM after the Northridge Earthquake

Geodetic time series determined with the Global Positioning System indicate that the geodetic rate of a permanent site in Pasadena, California (JPLM) changed significantly after the 17 January 1994 Northridge California earthquake. Subtracting the pre‐quake rate and co‐seismic offset leaves 30±4 mm of integrated eastward excess motion observed in the three years following the earthquake. North and vertical components show excess motion of −11 plusmn;3 mm and 25±11 mm respectively. Local surveys to three additional points near JPLM changed by no more than 6 mm E, 3 mm N, and 15 mm V during the two years after the earthquake, ruling out the possibility of a local effect at the JPLM monument. The direction and size of the post‐seismic displacements at JPLM are not consistent with additional slip on the fault which ruptured. The most rapid accumulation of excess motion occurs immediately after the earthquake, suggesting a relationship between the two events.

Time‐dependent residual deformations associated with the June 9, 1994 Bolivia Earthquake

By means of a spherical Earth model with Maxwell rheology we compute the coseismic and the postseismic displacements associated with the June 9, 1994 Bolivia earthquake. Our results are compared with the coseismic uplift detected by an array of broadband seismic stations in Southern Bolivia and with previous theoretical predictions. Our solution method has been recently employed to study the global deformations due to large lithospheric earthquakes. In contrast to purely elastic Earth models, our method includes the time‐dependent effects associated with mantle relaxation. The results presented here show that the surface deformation due to a deep‐focus earthquake are sensibly magnified by the delayed viscoelastic mantle relaxation. The time‐scales which characterize mantle relaxation and the time evolution of surface observables depend on the rheological profile of the upper mantle, and particularly on the thickness of the viscoelastic asthenosphere.

Effective relaxation time and viscosity of the earth inferred from the postseismic vertical deformations of the 1990 M S=7.0 Gonghe earthquake

The postseismic vertical deformation rates of the 1990 Gonghe M S=7.0 earthquake appears to have decreased exponentially. Based on Okada’s coseismic surface displacement solution caused by a uniform fault slip in an elastic homogeneous half space, we derived its postseismic surface displacement by using a single-layer standard linear solid model, and further derived a simplified formula for determining the effective relaxation time and viscosity of the earth, which is independent of the dislocation parameters of the causative fault. From the postseismic vertical deformation of the 1990 Gonghe earthquake, we inferred that the effective relaxation time defined by τ=η/μ is 2.6 years, and the effective viscosity η is about 1018 Pa · s.

Southern California permanent GPS geodetic array: Spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 Landers earthquake

The June 28, 1992 (Mw=7.3) Landers, California, earthquake was the first earthquake to be surveyed by a continuously operating Global Positioning System (GPS) array. The coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake. We employ a new spatial filtering technique that removes common‐mode errors from the coordinate time series. This approach provides precise estimates of site‐specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations. All sites indicate significant coseismic horizontal displacements of 5–65 mm with uncertainties of 1–2 mm. Horizontal displacements are in general agreement with elastic dislocation models, in particular for sites closer to the epicenter. Vertical displacements range from −13 to +7 mm with uncertainties of 2–4 mm. The observed vertical displacements in all cases show 5–10 mm more subsidence than expected from geodetic and seismic/geologic models. Significant postseismic horizontal displacements totaling 6±2 mm (10–20% of the coseismic displacement) are detected at the three sites closest to the epicenter. These displacements are modeled as a short‐term exponential relaxation with a decay time of 22±10 days superimposed on a longer‐term linear interseismic trend. Scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain, which agrees well with the direction and magnitude determined from more precise laser strain meter data. The time series do not show any detectable preseismic displacements.

Silent fault slip following an interplate thrust earthquake at the Japan Trench

Recent global space geodetic measurements have revealed that the velocities of tectonic plates over timescales as short as a decade1 are consistent with models of velocities averaged over the past few million years. The slip inferred from interplate thrust earthquakes at deep sea trenches and and number of earthquakes, however, often falls short of that predicted from these observed plate convergence rates2,3. Here we report transient crustal movements recorded by a permanent Global Positioning System (GPS) network in northeastern Japan following a typical interplate earthquake that occurred in December 1994 at the Japan Trench. Cumulative fault slip was estimated from the postseismic displacements at the GPS points over the first year after the event, and the inferred amount of seismic moment released by the afterslip was comparable to that released in the high-speed rupture. Such seismically 'invisible' slip may therefore account for the shortage of seismic slip relative to that required by time-averaged plate velocities.

Global postseismic rebound of a viscoelastic Earth: Theory for finite faults and application to the 1964 Alaska earthquake

A spherically symmetric Earth model with viscoelastic rheology is used to study the postseismic rebound associated with finite lithospheric dislocations. We perform a systematic study of surface deformations due to sources characterized by two‐ and three‐dimensional faults, modeled by a linear and planar distribution of point sources. Our approach is based on the normal mode technique for a layered Earth with linear viscoelastic rheology and allows for a self‐consistent description of the time evolution of postseismic displacements due to strike‐ and dip‐slip faults. As a case study, we compare the predicted horizontal displacements due to the 1964 Alaska earthquake with very long baseline interferometry (VLBI) baselines changes observed in this region in the period 1984–1989. Although subduction and postglacial isostatic adjustment are expected to contribute significantly to present‐day velocities, our results show that the postseismic rebound due to the 1964 Alaska earthquake plays a relevant role in the changes of several VLBI baselines. The last section is devoted to the analysis of the evolution of other North American baselines. As suggested by recent investigations based on forward approaches, isostatic readjustment of the Earth's crust in response to the melting of the last Pleistocene ice sheets is a major contributor to baseline variations in this area. Our results indicate that a correct interpretation of VLBI baseline changes in North America should account also for the effects of postseismic deformation.

Surface deformation due to a strike‐slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half‐space

We extend a technique previously used to model surface displacements resulting from thrust faulting in an elastic‐gravitational layer over a viscoelastic‐gravitational half‐space to the case of strike‐slip faulting. The method involves the calculation of the Green's functions for a strike‐slip point source contained in an elastic‐gravitational layer over a viscoelastic‐gravitational half‐space. The correspondence principle of linear viscoelasticity is applied to introduce time dependence. The resulting Green's functions are then integrated over the source region to obtain the near‐field displacements. Several sample calculations are presented involving 90° and 30° dipping faults and ruptures completely and partially through the elastic layer. We also illustrate the time dependent deformation due to a buried fault. Results show that the use of a viscoelastic half‐space underlying an elastic layer introduces a long wavelength component into the deformation field [Cohen and Kramer, 1984], even in cases of non vertical strike‐slip fault and inclusion of gravitational effect, that cannot be modeled by purely elastic techniques. Calculations have shown that vertical postseismic displacement is insignificant and that the horizontal movement is about the same magnitude as the coseismic strike‐slip displacement. The inclusion of gravity affects the horizontal displacement due to vertical strike‐slip faulting in far field and the vertical displacement for dipping strike‐slip faulting in near‐field. The computed results have been fit to the Global Positioning System measurements of the Landers earthquake taken shortly after the main shock, assuming a relaxation time of the order of days. This relaxation time is considerably shorter than times of the order of years to decades found in previous studies. The major differences between this detailed three‐dimensional and simplified two‐dimensional model are the decay of magnitude in displacement field and the distinct displacement pattern in the regions beyond the fault tip. The displacement field due to the cyclic earthquakes was constructed by considering the finite fault length and inclusion of gravity. It is found that the displacement field is dominated by the plate motion in the case of short recurrence time. On the other hand, a “looping” and migrating pattern in the displacement field is found in the case of very long recurrence time, which is not seen in those 2D simplified models.

Postseismic deformation following the Landers earthquake, California, 28 June 1992

AbstractAccelerated strain followed the Landers and Big Bear earthquakes, returning to the normal rate only after a period of several months. We observed this strain throughout most of southern California using the Global Positioning System (GPS). Three GPS receivers operating continuously in fixed positions at Pinyon Flat, Jet Propulsion Laboratory (Pasadena), and Goldstone all recorded postseismic deformation in a relative sense. In addition, we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes. Anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault. We modeled the displacements, using dislocation theory, as due to variable slip on the faults that were displaced at the times of the earthquakes. The model suggests that the postseismic strain released the equivalent of about 15% of the seismic moment of the mainshock. While the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks, the major contribution of strain release comes from the lower layer, below 10-km depth. Significant afterslip or viscous relaxation must have occurred below 10-km depth to explain the observed deformation more than 100 km from the fault. One interpretation is that high stress on the margin of the co-seismic rupture zone drives the rupture to extend itself into urbroken rock below and along the initial rupture zone.

Continuous measurements of crustal deformation for the 1992 Landers earthquake sequence

Abstract We describe, and attempt to interpret, continuous measurements of strains and tilts made at Pinon Flat Observatory (PFO) before, during, and after the Landers and Joshua Tree earthquake sequences. These data show substantial transient deformation following the Landers mainshock, with a total amplitude of several percent of the co-seismic deformation, and a decay time of at least several days. Comparing data from the many types of instruments at PFO allows us to infer possible sources for this deformation. The immediate postseismic transient was nearly the same size on three long-base strainmeters, suggesting either broadscale deformation or local motion near one part of the observatory. The latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source. Possible mechanisms for broad-scale deformation include postseismic fault slip, time-dependent creep in near-surface rocks, and elastic or thermal responses to water-table changes. The first two agree best with the observations from PFO, but if postseismic fault slip is the source, it must have been distributed differently than the co-seismic slip, and may have included faults other than those that ruptured seismically. If one of the other mechanisms is the main source, the PFO data imply that the postseismic-slip must have been very much smaller than the seismic slip, perhaps 2% or less. No significant preseismic deformation was observed, at a level of 2 × 10 −3 of the co-seismic deformation, for the days to minutes before the earthquake.