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We use survey and continuous GPS observations during the period 1997-2010 to investigate active deformation in the Isparta Angle region of SW Anatolia, Turkey. This region, bordered by the Fethiye Burdur Fault Zone (FBFZ) in the west and the SE extension of the Aksehir Simav Fault Zone (AKSFZ) in the east, accommodates a part of the active deformation of W Turkey. Our results show that the Isparta Angle region rotates counter-clockwise (CCW) with respect to Anatolia. Both the FBFZ and the AKSFZ are predominantly transtensional boundaries that accommodate southward motion of the Isparta region with respect to Anatolia. The FBFZ has left-lateral strike-slip behaviour along its SW segment that changes to right-lateral strike-slip along its NE extension. This change in the sense of strike-slip motion is accommodated by extension on a NW-SE striking normal fault system that is associated with the Menderes Graben system. Transtensional fault systems along the boundaries of the Isparta Angle with Anatolia are inconsistent with extrusion models for present-day southward motion. An increase in motion rates towards the Hellenic and Cyprus arc subduction systems, and CCW of the Isparta region, supports dynamic models involving active rollback of the subducting African Plate, toroidal mantle flow around the eastern edge of the subducting African slab near the Hellenic arc/Florence Rise junction, and/or gravitational collapse of the overriding Anatolia Plate.

The potential for large, shallow earthquakes and their associated seismic hazard in the eastern Caucasus, an area of dense population and sensitive industrial infrastructure, remains speculative based on historical precedent and current geologic and seismologic observations. Here we present updated and expanded results from a GPS network between the northern edge of the Lesser Caucasus and Greater Caucasus, providing geodetic constraints to the problem. A significant strain rate is observed in a profile over a distance of about 150 km across the Kura Basin. We attribute this to inter-seismic strain accumulation on buried fault structures and present simple elastic dislocation models for their plausible geometry and slip rate based on the known geology, seismicity and the GPS velocities. Due to the close proximity of the strain anomaly to Baku, further observations are needed to determine whether observed contraction is due to inter-seismically locked faults and, if so, implications for the seismic hazard in the region.

GPS survey sites in the Sinai Peninsula show northerly motion relative to Africa (Nubia) at 1.4F0.8 mm/yr north and 0.4F0.8 mm/yr west. Continuous IGS GPS sites in Israel, west of the Dead Sea fault show a similar northerly sense of motion relative to Nubia (2.4F0.6 mm/yr north and 0.04F0.7 mm/yr east), suggesting that the entire Sinai Block south of Lebanon is characterized by northward translation relative to the Nubian plate. We develop an elastic block model constrained by the GPS results that is consistent with the regional tectonics and allows us to estimate slip rates for Sinai bounding faults, including the Gulf of Aqaba–southern Dead Sea fault system (~4.4F0.3 mm/yr, left lateral), the Gulf of Suez (1.9F0.3 mm/yr left lateral, and 1.5F0.4 mm/yr extension), and the Cyprus Arc (predominantly convergence at 8.9F0.4 mm/yr along the western segment, and ~6.0F0.4 mm/yr left lateral, strike slip along the eastern segment). These observations imply that the Sinai Peninsula and Levant region comprise a separate sub-plate sandwiched between the Arabian and Nubian plates. D 2005 Elsevier B.V. All rights reserved.

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.

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 ± 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 ± 2, 9 ± 2, and 6 ± 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 ± 2 and 42 ± 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake.

International audience; The North Anatolian Fault Zone (NAFZ) is the major transform system that accommodates thewestward movement of the relatively rigid Anatolian block with respect to Eurasia. Mitigating the hazard associated with devastating earthquakes requires understanding how the NAFZ accumulates and releases the potential energy of elastic deformation both in space and in time. In this study, we focus on the central bend of the NAFZ where the strike of the North Anatolian Fault (NAF) changes from N75◦to N105◦within less than 100 km, and where a secondary fault system veers southwards within the interior of Anatolia. We present interseismic velocity fields obtained from a Persistent-Scatterers (PS) Interferometric radar analysis of ERS and Envisat radar archives. Despite the high vegetation cover, the spatial density of measurementsis high (∼10 PS/km2 in average). Interseismic velocities presented below indicate a velocitychange of∼6–8 mm/yr along the satellite line-of-sight (LOS) mainly centred on the NAF surface trace, and are in good agreement with the GPS velocity field published previously. The observed deformation is accommodated within a zone of ∼20 to 30 km width, in this area where no surface creep has been reported, contrary to the Ismetpasa segment located ∼30 km to the west of this study zone. Although less conspicuous, ∼2–3 mm/yr (∼1 mm/yra long the LOS) of the total deformation seems to be localized along the Lacin Fault. Theoverall agreement with horizontal GPS measurements suggests that the vertical component of the ground deformation is minor. This is confirmed, over the western part of our study zone, by the 3-D estimation of the ground deformation from the combination of the GPS-and the PS-mean velocity fields. However, a specific pattern of the PS velocity fields suggests that an area, enclosed between two faults with roughly south–north orientation, experiences uplift. The PS analyses of radar time-series both prior and posterior to the Izmit and Dûzce earthquakes indicate that these events did not induce detectable velocity changes in this central bend. The only temporal changes we identify are due to a moderate local earthquake (Mw5.7,1996 August 14) whose precise location and coseismic deformation are determined here. Finally, we propose a model of slip-rate distribution at depth along the NAF from the joint inversion of the GPS and PS mean velocity fields. This model suggests a long-term slip-rate of ∼20 mm/yr for a rather uniform locking depth in the range of 15–20 km. However, thelocking depth increases to ∼25–30 km in the section comprised between longitudes E34◦20’and E34◦50’. This lateral evolution is in general agreement with the earthquake distributionat depth from three different catalogues.

International audience; Understanding the behavior of active faults in response to tectonic stresses is a critical issue in earthquake physics and seismic hazard assessments (Carpenter et al., 2011). While most active faults are unstable and move abruptly, releasing in seconds to a few minutes the strain accumulated around them for decades to centuries, some faults slip continuously, storing little or no strain and are therefore considered to be unlikely to produce signifi cant earthquakes as long as this behavior persists (Bürgmann et al., 2000). Although aseismic fault creep was fi rst reported over half a century ago along some major plate boundary faults, including the San Andreas fault in California (United States) and the North Anatolian fault in Turkey, the origin and physical processes of fault creep remain subjects of debate (Schleicher et al., 2010). Two primary, complementary, and competing mechanisms have been proposed to explain the mechanics of aseismic fault creep: (1) the presence of weak mineral phases such as clay and phyllosilicates in the bulk composition of fault zones (Carpenter et al., 2011) or the formation of highly localized, phyllosilicate-rich interconnected networks of shear planes or foliations (Collettini et al., 2009) via chemical and mechanical processes, and/ or (2) trapped fl uid overpressures within fault zones (Byerlee, 1990). Although recent observations (Lockner et al., 2011) support the idea that aseismic creep is controlled by the presence of weak rocks in the fault zone rather than by high fl uid pressures, analysis of rock samples from active (Holdsworth et al., 2011) and exhumed (Warr and Cox, 2001) fault zones suggests that transient fl uid overpressures do occur in many fault zones and are most likely generated during coseismic faulting due to shear heating, low per-meability, high water content, and low strength of clay-rich shear zones. Whether due to mineralogy and/or fl uid pressure affects, how or when stable aseismic surface creep initiates on active faults remains uncertain. The fact that creeping segments of the North Anatolian fault at Ismetpasa and the San Andreas fault in the San Francisco Bay area also rupture coseismically suggests to us that fault creep may be initiated as postseismic after-slip (Çakir et al., 2005; Schmidt et al., 2005), a mechanism that could not be confi rmed previously due to the lack of pre-and post-earthquake observations on these fault sections. In this study we use space geodetic data and recent fi eld observations along the A.D. 1999 Izmit earthquake rupture (Fig. 1) to demonstrate continuing afterslip that appears to grade into steady-state creep on a >60-km-long segment of the Izmit coseismic fault, supporting seismic slip as one of the possible mechanisms for initiating steady-state creep on mature strike-slip faults. THE 1999 IZMIT EARTHQUAKE The M w 7.4 Izmit earthquake on 17 August 1999 occurred on the North Anatolian fault

We present evidence that GPS velocity estimates of plate motions and fault slip rates agree to within uncertainties with geologic estimates during the most recent phase of the geologic evolution of the E Mediterranean region (post-Late Miocene). On this basis, we use the GPS differential velocities to estimate the timing of initiation of the principal structures in NW Turkey, the N Aegean Sea, and central Greece, including, the Marmara Sea, the Gulfs of Evia (GoE) and Corinth (GoC). and the Kephalonia Transform fault (KTF). We interpret these ages to indicate that the North Anatolian fault propagated across the N Aegean, opening the GoE and GoC and initiating the KTF, during the past 1-4 Ma. We further suggest that Aegean extension that was earlier more distributed across the Aegean Basin became focused on this new fault system allowing the southern Aegean and Peloponnisos to translate SW with little internal deformation, as observed today with GPS. This change in tectonic configuration may account for the clear geologic evidence for crustal thinning throughout the S Aegean in apparent contradiction with low present-day strain rates. We further show that the low present-day strain rate along the southern edge of the Aegean micro-plate requires substantial aseismic slip along the plate interface below Crete, consistent with the low level of historic, subduction-type earthquakes along this segment of the subduction zone. (C) 2009 Elsevier B.V. All rights reserved.

[1] Deformation measured by regional GPS networks in continental plateaus reflects the geologic and tectonic variability of the plateaus. For two collisional plateaus (Tibet and Anatolia) and one noncollisional (the Altiplano), we analyze the regional strain and rotation rate by inverting GPS velocities to calculate the full two-dimensional velocity gradient tensor. To test the method, we use gridded velocities determined from an elastic block model for the eastern Mediterranean/Middle East region and show that to a first order, the deformation calculated directly from the GPS vectors provides an accurate description of regional deformation patterns. Principal shortening and extension rate axes, vertical axis rotation, and two-dimensional (2-D) volume strain (dilatation) are very consistent with long-term geological features over large areas, indicating that the GPS velocity fields reflect processes responsible for the recent geologic evolution of the plateaus. Differences between geological and GPS descriptions of deformation can be attributed either to GPS networks that are too sparse to capture local interseismic deformation, or to permanent deformation that accrues during strong earthquakes. The Altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2-D dilatation everywhere. Vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau. In contrast, the collisional plateaus have large regions of quasi-rigid body rotation bounded by strike-slip faults with the opposite rotation sense from the rotating blocks. Tibet and Anatolia are the mirror images of each other; both have regions of positive dilatation on the outboard sides of the rotating blocks. Positive dilatation in the Aegean correlates with a region of crustal thinning, whereas that in eastern Tibet and Yunnan province in China is associated with an area of vertical uplift. Rollback of the Hellenic trench clearly facilitates the rotation of Anatolia; rollback of the Sumatra–Burma trench probably also enables rotation about the eastern syntaxis of Tibet.

Geophysicists are using the Global Positioning System (GPS), a highly precise satellite navigation system, to monitor the slow movements (cm/yr) of the crustal plates composing the Earth's surface and the deformations where such plates interact. We report the results of repeated GPS measurements in the complex zone of interaction between the Eurasian, Arabian, and African plates in Turkey. The GPS observations made between 1988 and 1992 indicate that western, central, and east central Turkey are decoupled from the Eurasian plate and are moving as a more or less coherent unit about an axis located north of the Sinai peninsula. Other space-based measurements of crustal motion in Greece and along the Hellenic arc [Smith et al., 1994] suggest that this coherent motion includes southern Greece and the south central Aegean Sea. We refer to this region (shaded in Figure 1) as the Anatolian plate.

We model the geodetically observed secular velocity field in north- western Turkey with a block model that accounts for recoverable elastic-strain ac- cumulation. The block model allows us to estimate internally consistent fault slip rates and locking depths. The northern strand of the North Anatolian fault zone (NAFZ) carries approximately four times as much right-lateral motion (24 mm/yr) as does the southern strand. In the Marmara Sea region, the data show strain accu- mulation to be highly localized. We find that a straight fault geometry with a shallow locking depth of 6-7 km fits the observed Global Positioning System velocities better than does a stepped fault geometry that follows the northern and eastern edges of the sea. This shallow locking depth suggests that the moment release associated with an earthquake on these faults should be smaller, by a factor of 2.3, than previously inferred assuming a locking depth of 15 km. Online material: an updated version of velocity-field data.

This paper reviews the main tectonic features of the Eastern Mediterranean region combining the recent information obtained from GPS measurements, seismicity and neotectonic studies. GPS measurements reveal that the Arabian plate moves northward with respect to Eurasia at a rate of 23 ± 1 mm/yr, 10 mm/yr of this rate is taken up by shortening in the Caucasus. The internal deformation in Eastern Anatolia by conjugate strike-slip faulting and E-W trending thrusts, including the Bitlis frontal thrust, accommodates approximately a 15 mm/yr slip rate. The Northeast Anatolian fault, which extends from the Erzincan basin to Caucasus accommodates about 8 ± 5 mm/yr of left-lateral motion. The neotectonic fault pattern in Eastern Anatolia suggests that the NE Anatolian block moves in an E-ENE direction towards the South Caspian Sea. According to the same data, the Anatolian-Aegean block is undergoing a counter-clockwise rotation. However, from the residuals it appears that this solution can only be taken as a preliminary approximation. The Eulerian rotation pole indicates that slip rate along the North Anatolian fault is about 26 ± 3 mm/yr. This value is 10 mm/yr higher than slip rates obtained from geological data and historical earthquake records and it includes westward drift of the Pontides of a few millimetres/year or more. GPS measurements reveal that the East Anatolian fault accommodates an 11 ± 1 mm/yr relative motion. GPS data suggest that Central Anatolia behaves as a rigid block, but from neotectonic studies, it clearly appears that it is sliced by a number of conjugate strike-slip faults. The Isparta Angle area might be considered a major obstacle for the westward motion of the Anatolian block (Central and Eastern Anatolia). The western flank of this geological structure, the Fethiye-Burdur fault zone appears to be a major boundary with a slip rate of 15-20 mm/yr. The Western Anatolian grabens take up a total of 15 mm/yr NE-SW extension. The fact that motions in Central Anatolia relative to Eurasia, are 15-20 mm/yr while in Western Anatolia and Aegean Sea they are 30-40 mm/yr could suggest that Western Anatolia decouples from Central Anatolia and the Isparta Angle by the Fethiye-Burdur fault zone and Eski?ehir fault. It is also hypothesized that the differentiation of tectonic styles and velocities in the Anatolian-Aegean block are related to differences between the slabs lying under the Cyprus and Hellenic arcs.

Surface deformation in Morocco, derived from five years of global positioning system (GPS) survey observations of a 22-station network, four continuously recording GPS (CGPS) stations, and four International GNSS Service (IGS) stations in Iberia, indicates roughly southward motion (∼3 mm/yr) of the Rif Mountains, Morocco, relative to stable Africa. Motion of the Rif is approximately normal to the direction of Africa-Eurasia relative motion, which is predominantly strike slip, and results in shortening of the Rif and subsequent crustal extension of the adjacent Alboran Sea region. The sense, and the N-S asymmetry of the observed deformation (i.e., no evidence for north-directed shortening in the Betic Mountains north of the Alboran Sea) cannot be easily explained in terms of crustal plate interactions, suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western Mediterranean. The model that best fits the observations involves delamination and southward rollback of the African lithospheric mantle under the Alboran and Rif domains.

Large station displacements observed from Imperial Valley Global Positioning System (GPS) compaigns are attributed to the November 24, 1987 Superstition Hills earthquake sequence. Thirty sites from a 42 station GPS network established in 1986 were reoccupied during 1988 and/or 1990. Displacements at three sites within 3 kilometers of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (M(sub S) 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 + or - 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 + or - 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 x 10 (exp 25) dyne-cm and 2.3 x 10 (exp 25) dyne-cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest the post seismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using Singular Value Decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of non-seismic displacement is observed across the Imperial Valley, which is attributed in part to interseismic plate-boundary deformation.

The Global Positioning System (GPS) data collected in southern California from 1986 to 1989 indicate considerable strain accumulation across the Imperial Valley. Displacements are computed at 29 stations in and near the valley from 1986 to 1988, and at 11 sites from 1988 to 1989. The earlier measurements indicate 5.9 =/- 1.0 cm/yr right-lateral differential velocity across the valley, although the data are heavily influenced by the 1987 Superstition Hills earthquake sequence. Some measurements, especially the east-trending displacements, are suspects for large errors. The 1988 to 1989 GPS displacements are best modeled by 5.2 =/- 0.9 cm/yr of valley crossing deformation, but rates calculated from conventional geodetic measurements (3.4 to 4.3 cm/yr) fit the data nearly as well. There is evidence from GPS and Very Long Base Interferometry (VLBI) observations that the present slip rate along the southern San Andreas fault is smaller than the long-term geologic estimate, suggesting a lower earthquake potential than is currently assumed. Correspondingly, a higher earthquake potential is indicated for the San Jacinto fault. The Imperial Valley GPS sites form part of a 183 station network in southern California and northern Baja California, which spans a cross-section of the North American-Pacific plate boundary.

The National Geodetic Survey completed approximately 170 km of releveling in the Diablo Plateau-Salt Basin region during the fall of 1977. The survey was specifically designed to detect possible vertical crustal movements in the area. The new releveling measurements confirm contemporary uplift of the region and suggest that uplift has been continuous for the entire time the movements were monitored (1934-1977). Although the precise mechanism responsible for the observed uplift is unknown, it is almost certainly of tectonic origin. The pattern of uparching is perturbed at the Salt Basin graben, indicating 4.2 + or - 0.3 cm of subsidence relative to its margins between 1958 and 1977.

Repeated levelings in southwestern Montana, the western portion of Yellowstone National Park and the Snake River Plain provide information on the pattern of relative vertical crustal movement throughout this region. Except for the coseismic deformation associated with the 1959 Hebgen Lake earthquake, the most outstanding and best-defined feature of the data is contemporary doming at a rate of 3–5 mm/yr, involving approximately 8000 km 2 including the epicentral area and after-shock zone of the 1959 Hebgen Lake earthquake. Based on observations over different time intervals, doming appears to have continued throughout the time the movements were monitored, beginning at least 25 years prior to the 1959 earthquake and continuing for at least 1 year after the earthquake. The character of the coseismic deformation associated with the 1959 earthquake and the high regional elevation are consistent with the observed doming. It is suggested that doming preceded the earthquake for a considerable time (on the order of hundreds to thousands of years, perhaps longer), giving rise to tensional stresses in the upper crust. When these stresses exceeded some critical value, faulting and collapse in response to gravity occurred, resulting in the 1959 earthquake. The voluminous Tertiary and younger volcanics in the vicinity of the doming region suggest that magma intrusion into the crust is the most likely cause of the observed uplift. The proximity of the doming region to the thermally active Yellowstone area supports this suggestion. Secondary features of the data include: 1. (1) A spatial correlation between tilting and historic seismic activity. 2. (2) Uplift within the Norris—Mammoth corridor in Yellowstone National Park relative to nearby bench marks to the north and south. 3. (3) Regional subsidence of the eastern Snake River Plain relative to points north and west of this physiographic province, including subsidence of the Pleistocene Island Park caldera floor relative to its rim fractures. 4. (4) Rapid tilting in the vicinity of 4.1. (a) the Continental fault east of Butte, 4.2. (b) the intersection of the Gardiner, Mammoth and Reese faults just .north of Yellowstone National Park, and 4.3. (c) the Madison Range fault in eastern Idaho.

Relative vertical displacements of bench marks in extreme western Kentucky have been determined by comparison of successive leveling surveys in 1947 and 1968. The resulting pattern of apparent surface deformation shows steep offset which can be closely modeled by a normal fault buried in an elastic half-space. The offset is located near the northern boundary of the Mississippi Embayment and the New Madrid seismic zone, an area where faults have previously been inferred on the basis of both geological and geophysical evidence. If the apparent movement is due to slip along a fault, several lines of evidence (regional structure, earthquake data, and lineations) suggest that the postulated fault trends NNE. Thirteen earthquakes were recorded in this area between the times of leveling; focal mechanisms exist for three of these. The nearest of these three focal mechanisms to the leveling offset implies normal faulting. The magnitude of the earthquake, however, appears to be too small to account for the amount of slip required by the fault model. Thus the apparent deformation may have accumulated with several undetected small earthquakes, or gradually as aseismic creep.

Results of four leveling surveys carried out by the National Geodetic Survey between Anchorage and Whittier, Alaska, combined with an analysis of sea level measurements at Anchorage, indicate as much as 0.55 m of land uplift in the decade following the 1964 Prince William Sound earthquake. The pattern of uplift is parabolic in shape, convex upward, and reaches a maximum approximately halfway between Anchorage and Whittier, or about 300 km northwest of the Aleutian trench axis. The data suggest that the position of maximum uplift is migrating away from Anchorage, i.e., toward the Aleutian trench. The observed uplift occurs in a region which subsided as much as 1.9 m during the earthquake. The rate of uplift has decreased exponentially since the time of the 1964 earthquake. These movements appear to represent postseismic deformation associated with the 1964 Alaska earthquake. The observations are most easily explained by creep along the downdip extension of the fault which ruptured during the 1964 earthquake, although viscoelastic rebound and long-term elastic strain accumulation mechanisms may play a part. There is no evidence supporting magma intrusion or dilatancy mechanisms. These results provide new constraints for models of tectonic processes at convergent plate margins.

Repeated first-order leveling surveys conducted by the National Geodetic Survey (NGS) in 1972, 1974, 1976, 1978, and 1981 provide evidence of contemporary relative uplift near the junction of the San Andreas fault and the Brawley Seismic Zone. Uplift, which extends over a distance of about 5 km where crossed by the leveling line, apparently developed progressively between 1972 (possibly before) and 1978. Maximum relative uplift during this period reached 58 ± 4 mm. This spatially and temporally coherent pattern of uplift was interrupted between 1978 and 1981 possibly as a result of the 1979, M6.6 Imperial Valley earthquake. While the cause of the observed uplift is unknown, given the location, one interpretation is that it represents a zone of concentrated strain possibly associated with fault activity.

Analysis of repeated leveling observations in the vicinity of the San Gabriel Fault in Southern California indicate subsidence immediately south of the Fault relative to points to the north, south and east. These observations were previously interpreted as reflecting tectonic motions associated with either the 'Palmdale Bulge' or with preseismic effects of the San Fernando earthquake. Relative subsidence between 1953 and 1964 reaches approximately 9 cm and extends over a distance of more than 20 km. Subsidence occurs directly above the Saugus aquifer and shows a temporal correlation with the history of water level decline within the aquifer. The degree of subsidence of individual benchmarks is roughly proportional to the product of aquifer thickness and water level decline at the location of the benchmarks. Thses observations strongly suggest that movements of the surface near the San Gabriel Fault, previously inferred to be of tectonic origin, actually result from near surface sediment compaction within the Saugus basin.

We have reduced and analyzed over 90% of existing U.S. National Geodetic Survey releveling observations, in order to evaluate the implications of these measurements for contemporary crustal movements. The data are displayed both in profile form (movement versus distance along route) and, where adequate coverage is available, as maps of apparent elevation change. Our analysis involves: 1. (1) evaluating the reliability of the leveling measurements using modified geodetic checks as well as through comparison with independent measurements of crustal movement (e.g., tide and lake level measurements, seismological evidence, tilt and strain observations); 2. (2) interpreting what appear to be real movements in light of other geological and geophysical information on crustal dynamics. Empirical analysis of the U.S. releveling data base indicates that apparent tilts of less than 3 microradians are pervasive and must be regarded as typical signal levels and/or as typical levels of systematic error. Systematic leveling errors are suggested in some cases by close correlations between apparent elevation change and topography, disagreements between leveling and tide gauge measurements, and internal inconsistencies in leveling circuits. Apparent movements which correlate with topography are observed in many areas of the U.S. Such apparent movements often reach 35 mm per 100 m of relief. Disagreements between leveling and tide gauge measurements along the east and west coasts and internal inconsistencies in leveling circuits suggest errors reaching 1 mm/km which can accumulate monotonically for distances exceeding 1000 km. Regional maps of apparent elevation change for the Eastern U.S. show serious inconsistencies with similar maps for adjacent portions of Canada suggesting that such maps may be more indicative of systematic leveling errors than real crustal movements. The largest signals (apparent tilts greater than 4.5 microradians) are for the most part associated with: 1. (1) coseismic and postseismic movements of major (M>6) dip-slip earthquakes, 2. (2) regions of contemporary magmatic activity, 3. (3) subsidence due to fluid withdrawal, 4. (4) topography-correlated apparent movements. Coseismic movements reported for eleven U.S. earthquakes are roughly consistent with simple models of elastic rebound. At least some postseismic movements appear consistent with after-slip on the fault or an extension of the fault that ruptured during the earthquake, although other explanations have been proposed (e.g., viscoelastic relaxation of the lithosphere-asthenosphere). Deformations possibly associated with contemporary magmatic processes are not restricted to volcanically active areas (e.g. Hawaii) having been observed in the Rio Grande rift and in Yellowstone National Park as well. Subsidence due to fluid withdrawal, well known in areas of intense pumping, is considerably more widespread than previously reported occuring throughout much of the Atlantic and Gulf coastal plains, along the Mississippi Valley and within many smaller sedimentary basins which have been exploited for groundwater. The association of many of the largest signals with real surface movements further demonstrates the ability of historic leveling to detect relatively subtle deformation.

Analysis of repeated leveling surveys in the Rio Grande rift shows a pronounced zone of relative subsidence north of Espanola, New Mexico, which appears to be associated spatially with anomalous seismic activity. Maximum subsidence relative to nearby bench marks was 4.9 cm between September 1934 and March 1939. Observed subsidence occurs over 19 km and extends from 4 km north to 23 km north-northwest of Espanola. The leveling anomaly does not appear to be associated with either ground-water effects or leveling errors and thus most likely represents crustal deformation. The zone of relative subsidence occurs near some of the few late Tertiary dikes mapped in the rift and lies within the zone of high heat flow (> 2.5 HFU) along the western part of the rift. The crustal movement anomaly appears qualitatively similar but opposite in sign to movements associated with a crustal magma body that have been observed in the Socorro, New Mexico, area of the Rio Grande rift. The movements reported here appear consistent with either deflation of a shallow (

Recent releveling measurements conducted in the Socorro area of the Rio Grande rift specifically to study possible crustal deformation indicate uplift of the central part of the rift (Socorro) relative to bench marks to the west (Magdalena). Total relative uplift measured along the newly releveled line between 1934 and 1978.7 reached 10 ± 1 cm. Combining this new information with previously reported releveling data suggests a roughly elliptical uplift affecting at least 7,000 km 2 . Maximum observed uplift near the center of this area reaches about 20 cm relative to the periphery. The uplift occurs directly above an unusual crustal discontinuity at a depth of about 20 km below this part of the rift. This discontinuity, discovered on the basis of seismic evidence, has been inferred to be the upper boundary of an extensive magma body. The spatial coincidence of the zone of uplift and the presumed magma body, the persistent microearth-quake activity, and the modeling results strongly suggest that the observed movements result from expansion of the Socorro magma body. Although available data are not sufficient to determine uniquely the temporal behavior of the doming, the observations are consistent with more or less continuous uplift averaging about 5 mm/yr during the time the movements were monitored (1909 to 1979). The average inflation rate within the magma body required to produce the observed uplift is on the order of 1 to 2 × 10 −2 km 3 /yr and is consistent with measurements in volcanic regions. Although less clearly defined, the leveling data also suggest relative uplift within the Albuquerque-Belen Basin well beyond the presumed northern boundary of the Socorro magma body. If this uplift is associated with magmatic activity, the Socorro magma body extends considerably farther north than previously expected, or another magma body lies beneath the Albuquerque-Belen Basin.

A 1983 releveling survey conducted by the National Geodetic Survey, combined with previous surveys in 1923, 1960, 1967, and 1975, provides evidence for ongoing vertical deformation of a broad region (diameter of ∼150 km) surrounding the site of the 1959 Hebgen Lake earthquake. Deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault. Maximum observed elevation change during the postseismic period (1960–1983) exceeds 30 cm. Assuming that the entire uplift (i.e., 1923–1983) occurred following the 1959 earthquake (i.e., uplift measured between 1923 and 1960 was entirely postseismic), the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years. The spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer (thickness of ∼30–40 km) overlying a viscoelastic half-space (viscosity of ∼1019 Pa s). This model may also account for the anomalously large horizontal strain around the Hebgen Lake region measured by repeated trilateration surveys during the period 1973–1984. The releveling measurements in the Hebgen Lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the United States.

Elevation changes in the Imperial Valley, California, derived from repeated leveling surveys for the time period including the 1979 M = 6.6 earthquake, provide some constraints on fault geometry and slip distribution associated with this event. Many of the first-order features of the observed vertical movements are well matched by simple models consisting of variable slip on planar faults in an elastic half-space using fault offsets inferred from strong ground motion observations (Archuleta, 1984) and measured afterslip. The geodetic and seismic observations suggest that significant slip is confined to depths above 13 km with maximum right-lateral offset reaching about 1.5 m on the Imperial fault. Dip slip occurs predominantly in the sediments on the upper 5 km of the Imperial fault and on the Brawley fault. Right-lateral afterslip is confined to the upper 5 km of the Imperial fault and reaches about 30 cm for the period 1979–1981. In contrast, observed elevation changes near the 1979 hypocenter (on southern end of Imperial fault) and in the Brawley Seismic Zone show significant deviations from those predicted by models of fault slip inferred from strong ground motion measurements. Specifically, the geodetic data suggest that slip on the Imperial fault is significantly lower in the vicinity of the earthquake hypocenter than along the central and northern sections of the fault. In addition, there is marginal evidence that the dip of the Imperial fault changes along strike from approximately vertical just north of the U.S.-Mexico border to between 70° and 80° near the northernmost extent of the 1979 surface break. This change in dip may be related to a change in local strike along the fault. Large elevation changes (>15 cm) also occur within the Brawley Seismic Zone well north of the primary surface faulting. While these movements are consistent with a number of possible fault models, our prefered interpretation based on geodetic and seismic observations (aftershock locations and focal mechanisms) involves right-lateral, aseismic slip on a northwest striking fault along the east side of the Brawley Seismic Zone and conjugate left-lateral faulting on a northeast striking fault (possibly associated with an M = 5.8 aftershock). Buried creep on this same right-lateral fault in the Brawley Seismic Zone can also account for vertical deformation during the postseismic period of the 1940, M = 7.1 Imperial Valley earthquake as well as deformation during the interseismic period between the 1940 and 1979 events. Substantial right-lateral slip in the Brawley Seismic Zone suggests that a significant part of the shear strain released during and following the 1940 and 1979 earthquakes on the Imperial fault is transferred through the Brawley Seismic Zone to the southern end of the San Andreas fault.

The historic geodetic data base in the eastern United States dates back over 100 years. Although instrumentation and hence measurement precision has improved since the initial observations, the basic techniques [leveling, triangulation, water level gauging] remained unchanged until the recent introduction of space geodesy [Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Positioning System (GPS)]. Because of this, it has been possible to compare repeated observations to identify apparent changes in relative positions. Such “changes” can reflect observational errors, non-tectonic movements (water level effects, monument instability, loading, etc.) or tectonic/isostatic deformation. A number of possible deformation features in the eastern U. S. have been reported in the literature, including uplift of the southern Appalachian and Adirondack mountains, doming of the Gulf coast inland of the Mississippi delta, subsidence of Chesapeak Bay and along the coast of Maine, horizontal deformation in New York and Connecticut, and possible fault related deformation near Charleston and in the New Madrid area. Unfortunately, it is not dear to what extent any of these features represent tectonic deformations and hence what their significance may be for the earthquake problem. An important recent development in monitoring regional deformation has been the establishment of the eastern U. S. GPS strain network by the National Geodetic Survey. The network consists of roughly 45 sites uniformly distributed east of the Rocky Mountains. The network was established in 1987 and will be reobserved in 1989. Subsequent reobservations will be made at 2 to 5 year intervals depending on analysis of the early measurements. Given the precision of the GPS measurements (few cm in 3-D relative positions), a considerable time period will be required to detect the subtle deformations expected for this intraplate region. Perhaps more immediate information will result from recent progress in recognizing and correcting systematic errors in the historic geodetic data base. This, together with the newly automated historic data base, and improved techniques for integrating and analyzing these extensive observations, provide the necessary basis for effectively evaluating the deformational features listed above. Given the potential importance of these data for understanding tectonic processes (and the huge expense of collecting these measurements over the pst century), such studies should be an integral part of future earthquake studies in the eastern U. S.