Regional Velocity Field Research Articles

Active Tectonic Deformation of the Qilian Shan, Northeastern Tibetan Plateau

Abstract —The Qilian Shan (or Qilian Mountains), located on the northeastern margin of the Tibetan Plateau, is an actively growing orogenic belt resulting from the far-field impact of the India–Eurasia collision. The northward penetration of the Indian Plate is responsible for intense crustal shortening in the Qilian Shan. However, the tectonic deformation pattern in response to the crustal shortening remains unclear. In this study, we present the regional seismicity, fault activity, and GPS crustal movement velocity field to characterize the active tectonic deformation of the Qilian Shan based on historical data over the past two decades. The results suggest that the western Qilian Shan is characterized by distributed north–south crustal shortening, while the eastern Qilian Shan is dominated by blocklike eastward extrusion of crust along major strike-slip faults coupled with clockwise rotation. North–south crustal shortening and east–west lateral extrusion, two deformation modes responding to the India–Eurasia convergence, match the crustal deformation in the Qilian Shan. The tectonic deformation of the western Qilian Shan is largely in agreement with the former, while the eastern Qilian Shan corresponds closely to the latter. Lower crustal flow beneath the central Tibetan Plateau provides the potential driving force to induce the eastward extrusion of crustal material out of the plateau and the growth of some boundary mountain ranges, such as the Qilian Shan.

Intradecadal Fluctuations and Three‐Dimensional Crustal Kinematic Deformation of the Tianshan and Pamir Derived From Multi‐Geodetic Imaging

AbstractThe uplift of the Tianshan and Pamir Mountains/Areas are caused by active intracontinental tectonic movements. The dynamic mechanism is controversial and has aroused great interest among scholars. In this study, we investigate the present three‐dimensional (3‐D) crustal deformation in the Tianshan and Pamir areas from multi‐geodetic observations from 2002 to 2021. The continuous water deficit throughout Tianshan and Pamir locates mainly in glacier‐covered areas, at a total rate of −0.8 cm w.e./yr, inducing spatial surface uplifting at rates of ∼0.2–0.5 mm/yr. These Global Positioning System (GPS)‐derived velocities in the vertical deformation of the Tianshan and Pamir were corrected based on surface elastic loading models and the Gravity Recovery and Climate Experiment Follow‐On (GRACE/GFO)‐inferred hydrological loading deformation, and interpolated using the GPS imaging method for a higher 3‐D crustal deformation spatial resolution. In conclusion, surface hydrology is one of the driven factors affecting the regional vertical velocity field observed by GPS. The kinematic crustal shortening and vertical tectonic uplift are integrated within different blocks, which illustrate the contemporary dynamics throughout the Tianshan and Pamir. The relevant spatial characteristics between the 3‐D crustal deformation and dynamic tomography along the Tianshan indicate that the relatively weaker lithosphere beneath the central Tianshan is subject to strong compression, which induces the present‐day crustal uplift.

Stress and Strain Characteristics before the Mw 7.3 Maduo Earthquake and Its Implications for Future Earthquakes on the Maqin–Maqu Fault

ABSTRACT To determine the crustal deformation and the stress–strain accumulation characteristics preceding the Mw 7.3 Maduo earthquake and the seismic potential of the Maqin–Maqu fault segment, the seismic data from small earthquake events and Global Positioning System (GPS) observations stations are collected. The b-value, the GPS velocity field, the GPS baseline, and the GPS strain parameters for multiple areas are calculated and analyzed. The results show a low b-value anomaly in the epicenter and its adjacent areas before the earthquake, which suggest that the stress level of the medium in this area was high before the earthquake. The regional GPS velocity field indicates that the earthquake epicenter is in an area where strain energy accumulates at varying slip rates and orientations. The deformation rates of the northeast-oriented baselines on the east side of the fault and the northwest-oriented baselines on the west side of the fault are lower than those observed near the three near north–south-oriented baselines, reflecting a small compressive deformation that occurred at the epicenter before the earthquake. Meanwhile, the motion of baselines QHYS-QHMD, QHYS-QHMQ, and QHGE-SCSP noticeably decreased, indicating that the area near the fault had already stored a great deal of strain energy prior to the earthquake. The rate of plane strain reduced in the epicenter region before the earthquake gradually since the second half of 2019, illustrating the deficit points to high stress conditions in the epicenter area. Furthermore, the area near the epicenter was characterized by high strain accumulation before the earthquake. The Maqin–Maqu segment of the East Kunlun fault is likely to be capable of generating strong earthquakes in the near future, because the current state of the area is similar to that of the Maduo epicenter before the onset of the earthquake.

Sparse Hardy function model of regional velocity field from GNSS data

Classical Hardy function interpolation method is often necessary in establishment of regional velocity field. In this work, a regularization method, namely the L1-norm regularization, also called the least absolute shrinkage selection operator (Lasso), is employed to improve the traditional method. With the new method, a sparse model can be obtained with many zero elements in the parameter vector. Compared to L2-norm regularization, which is also called the Tikhonov regularization, the L1-norm regularization will select the best model automatically by making the coefficient of the unnecessary kernels zero, through solving a convex optimization problem. In this paper, the velocity field dataset derived from global navigation satellite system data is used to establish models in different directions. By comparing the interpolation accuracy of velocity at the same unknown points of Hardy function improved by Lasso and Tikhonov regularizations respectively, the feasibility of the former is verified. The results show that L1-norm regularization method has a slightly worse interpolation accuracy than Tikhonov regularization method in North and Up directions, but in East direction, the interpolation effect is much better and all directions can get great sparse performance. More specifically, the prediction accuracy of the proposed method in the East direction is improved by 17.3%, but in North and Up directions is decreased by 5.4% and 6.3% respectively in comparison with that of Tikhonov regularization method, though the sparse rates of them are over 60%. In addition, besides selecting the root mean square error as the evaluation standard for model selection, we can also select the sparse rate as the evaluation criteria to make the model much sparser if necessary. A sparse model would be beneficial in terms of better interpretability and improved variable assigning efficiency. To summarize, L1-norm regularization can be viewed as a potential alternative in velocity field modelling.

Rapid Geodetic Observations of Spatiotemporally Varying Postseismic Deformation Following the Ridgecrest Earthquake Sequence: The U.S. Geological Survey Response

Abstract The U.S. Geological Survey’s geodetic response to the 4–5 July 2019 (Pacific time) Ridgecrest earthquake sequence comprised primarily the installation and/or reoccupation of Global Navigation Satellite System (GNSS) monumentation. Our response focused primarily on the United States’ Navy’s China Lake Naval Air Weapons Station base (NAWSCL). This focus was because much of the surface rupture occurred on the NAWSCL and because of NAWSCL access restrictions only permitting Federal and State of California personnel. In total, we measured or are still measuring at 24 sites, 14 of which were on the NAWSCL and, as of this writing, operational. The majority of sites were set up as continuous stations logging at either 1 sample per second or 1 sample per 15 s. Two stations were recording a 200 m cross-rupture aperture starting ∼10 hr after the M 6.4 event, and they recorded the coseismic displacements of the M 7.1. Approximately, 1 hr after the M 7.1 event, two new stations were recording a ∼200 m cross-rupture aperture of the surface rupture. In the days following, we established the rest of the stations ranging to a distance of ∼15 km from the M 7.1 principal rupture trace. The lack of differential displacement across the M 6.4 rupture during the M 7.1 event suggests that it did not reactivate the M 6.4 plane. The lack of differential cross-fault displacement for both events suggests that rapid shallow afterslip did not occur at those two locations. The postseismic time series from these stations shows centimeters of horizontal displacement over periods of a few months. They record a mixture of fault-parallel and fault-normal displacements that, in conjunction with analysis of more spatially complete Interferometric Synthetic Aperture Radar displacement fields, suggest that both poroelastic and afterslip phenomena occur along the M 6.4 and 7.1 rupture planes. Using preliminary data from these and other regional stations, we also explore the Ridgecrest sequence’s effect on regional GNSS time series and the differentiation of long-term postseismic motions and secular deformation rates. We find that redefining a common-mode noise filter using different GNSS stations that are assumed to be unaffected by the earthquakes results in small but systematic differences in the regional velocity field estimate.

A creeping intracontinental thrust fault: past and present slip-rates on the Northern edge of the Tien Shan, Kazakhstan

We demonstrate that a continental interior reverse fault is deforming by aseismic creep, presently, and likely also in the long term. The Karkara Rangefront Fault, part of the larger Main Terskey Front, forms the northern boundary of the high Terskey Tien Shan in southeastern Kazakhstan and is a mature structure with evidence for high slip rates throughout the late Cenozoic. Combining field studies with a satellite stereo-image derived digital elevation model (2 m resolution), we map a series of fluvial terraces along the rangefront which are uplifted by up to ∼300 m above the present river level. Radiocarbon ages from one catchment constrain the ages of the lowermost two terraces to be ∼4–5 ka and ∼10–15 ka, consistent with prominent, regionally extensive terraces observed elsewhere in the Tien Shan. Based on conservative estimates for the fault dip under the displaced terraces, we estimate a slip rate along the fault plane of 3.5+1.7−0.4 mm yr–1 on the Karkara Rangefront Fault and a further >0.8 mm yr–1 on a fold structure in the Kegen basin that we infer is driven by a detachment from the main rangefront. We therefore estimate a minimum shortening rate across the rangefront of 1.1–3.3 mm yr–1. Elastic modelling of the regional GPS velocity field suggests that the fault is presently creeping at ∼3 mm yr–1 (horizontal shortening), consistent with the upper limit of our Late Quaternary slip rate estimate. This is the fastest known slip rate in the northern Tien Shan and the only individual structure resolved in the regional velocity field. At present the fault is accumulating minimal strain, and there is evidence in the geomorphology that this creep is sustained in the long term, but whether or not it is also capable of generating earthquakes requires further study.

Current block motions and strain accumulation on active faults in the Caribbean

AbstractThe Caribbean plate and its boundaries with north and south America, marked by subduction and large intra‐arc strike‐slip faults, are a natural laboratory for the study of strain partitioning and interseismic plate coupling in relation to large earthquakes. Here we use most of the available campaign and continuous GPS measurements in the Caribbean to derive a regional velocity field expressed in a consistent reference frame. We use this velocity field as input to a kinematic model where surface velocities results from the rotation of rigid blocks bounded by locked faults accumulating interseismic strain, while allowing for partial locking along the Lesser Antilles, Puerto Rico, and Hispaniola subduction. We test various block geometries, guided by previous regional kinematic models and geological information on active faults. Our findings refine a number of previously established results, in particular slip rates on the strike‐slip faults systems bounding the Caribbean plate to the north and south, and the kinematics of the Gonave microplate. Our much‐improved GPS velocity field in the Lesser Antilles compared to previous studies does not require the existence of a distinct Northern Lesser Antilles block and excludes more than 3 mm/yr of strain accumulation on the Lesser Antilles‐Puerto Rico subduction plate interface, which appears essentially uncoupled. The transition from a coupled to an uncoupled subduction coincides with a transition in the long‐term geological behavior of the Caribbean plate margin from compressional (Hispaniola) to extensional (Puerto Rico and Lesser Antilles), a characteristics shared with several other subduction systems.

Geodetic investigation into the deformation of the Salton Trough

AbstractThe Salton Trough represents a complex transition between the spreading center in Baja California and the strike‐slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high‐resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008–2009 and previous surveys between 2000 and 2005. We also investigate small‐scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11 mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15 mm/yr of left‐lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17 mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10°/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.

Modern movement and deformation in the South China Sea shown by GPS measurements and numerical simulation

To better understand the crustal deformation of the South China Sea Basin, we produce a mechanically consistent 2-dimensional model for observing regional velocity field in the South China Sea (SCS). We studied the dominating regional tectonic stress field by geodetic measurements and finite element analysis, the spatial variations of velocity field and strain field, and relative movements among different blocks, using a 2-dimensional model describing crustal deformation of the South China Sea Basin. Strain results show that the SCS is extending at present. The western part of SCS is opening gradually in NWSE direction from its northern margin to the south, but the eastern part of SCS is opening gradually from its central part to the north and south. In addition, we analyzed the plate kinematics to the deformation of the SCS, using a two-dimensional finite element model. Our simulations results are well explained by available geodetic data. The movement of SCS is resulted from interactions among Indian Plate, Pacific Plate, Philippine Sea Plate, and Eurasian Plate.

Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador

New GPS measurements in Chiapas (Mexico), Guatemala and El Salvador are used to constrain the fault kinematics in the North America (NA), Caribbean (CA) and Cocos (CO) plates triple junction area. The regional GPS velocity field is first analysed in terms of strain partitioning across the major volcano-tectonic structures, using elastic half-space modelling, then inverted through a block model. We show the dominant role of the Motagua Fault with respect to the Polochic Fault in the accommodation of the present-day deformation associated with the NA and CA relative motion. The NA/CA motion decreases from 18–22 mmyr-’1 in eastern Guatemala to 14–20 mmyr-’1 in central Guatemala (assuming a uniform locking depth of 14–28 km), down to a few millimetres per year in western Guatemala. As a consequence, the western tip of the CA Plate deforms internally, with ≃9 mmyr-’1 of east–west extension (≃5 mmyr-’1 across the Guatemala city graben alone). Up to 15 mmyr-’1 of dextral motion can be accommodated across the volcanic arc in El Salvador and southeastern Guatemala. The arc seems to mark the northern boundary of an independent forearc sliver (AR), pinned to the NA plate. The inversion of the velocity field shows that a four-block (NA, CA, CO and AR) model, that combines relative block rotations with elastic deformation at the block boundaries, can account for most of the GPS observations and constrain the overall kinematics of the active structures. This regional modelling also evidences lateral variations of coupling at the CO subduction interface, with a fairly high-coupling (≃0.6) offshore Chiapas and low-coupling (≃0.25) offshore Guatemala and El Salvador.

Unmodelled Effects in the Horizontal Velocity Field Determination: ASG-EUPOS Case Study

Recent space and satellite technologies offer great opportunities to determine crustal movements in the single, global reference frame. The densification of the global network through local area networks leads to increase the resolution of modelled deformations both horizontal and vertical. However, the credibility of the obtained velocity field is limited by several factors associated with unmodelled (or mismodelled) effects at the stage of GNSS data adjustment. Some of them are periodic (the influence of local atmospheric or hydrological impacts), some temporary (natural or anthropogenic seismicity) or related to local influences (snow load or effects associated with the freezing of the ground). This paper presents the usefulness of ASG-EUPOS time series for determination of the regional velocity field. The system has been operating since mid-2008, so the velocities obtained through the processing of 3-year time series are supposed to be reliable. The paper also presents comparison of the velocity determinations to the geological NNR-NUVEL-1A

Right-lateral shear across Iran and kinematic change in the Arabia-Eurasia collision zone

New offset determinations for right-lateral strike-slip faults in Iran revise the kinematics of the Arabia–Eurasia collision, by indicating along-strike lengthening of the collision zone before a change to the present kinematic regime at ∼5 Ma. A series of right-lateral strike-slip faults is present across the Turkish—Iranian plateau between 48°E and 57°E. Fault strikes vary between NW—SE and NNW—SSE. Several of the faults are seismically active and/or have geomorphic evidence for Holocene slip. None of the faults affects the GPS-derived regional velocity field, indicating active slip rates are ≤2 mm yr−1. We estimate total offsets for these faults from displaced geological and geomorphic markers, based on observations from satellite imagery, digital topography, geology maps and our own fieldwork observations, and combine these results with published estimates for fault displacement. Total right-lateral offset of the Dehu, Anar, Deh Shir, Kashan, Ab-Shirin-Shurab, Kousht Nousrat, Qom, Bid Hand, Indes, Soltanieh and Takab faults is ∼250 km. Other faults (North Zanjan, Saveh, Jorjafk, Rafsanjan, Kuh Banan and Behabad) have unknown or highly uncertain amounts of slip. Collectively, these faults are inferred to have accommodated part of the Arabia–Eurasia convergence. Three roles are possible, which are not mutually exclusive: (1) shortening via anticlockwise, vertical axis rotations; (2) northward movement of Iranian crust with respect to stable Afghanistan to the east; (3) combination with coeval NW—SE thrusts in the Turkish–Iranian plateau, to produce north–south plate convergence (‘strain partitioning’). This strike-slip faulting across Iran requires along-strike lengthening of the collision zone. This was possible until the Pliocene (≤ 5 Ma), when the Afghan crust collided with the western margin of the Indian plate, thereby sealing off a free face at the eastern side of the Arabia–Eurasia collision zone. Continuing Arabia–Eurasia plate convergence had to be accommodated in new ways and new areas, leading to the present pattern of faulting from eastern Iran to western Turkey, and involving the westward transport (‘escape’) of Anatolia and the concentration of thrusting in the Zagros and Alborz mountains.

Effect of Kunlun Ms 8.1 earthquake on crustal deformation in northeastern edge region of Qinghal-Tibet plateau

Seismic fault parameters can be inversed with Okada model based on deformation data before and after earthquakes in focal region and its adjacent area. Co-seismic displacements can be simulated by using these parameters, and then regional velocity field obtained by deducting the co-seismic displacements from the observed displacements by GPS method. We processed and analyzed the data in the northeastern edge region of the Qinghai-Tibet plateau observed during 2001 – 2003 in two steps: firstly, the displacements generated by Kunlun M s8.1 earthquake of 2001 in this region was simulated, and secondly, deducted the co-seismic displacements from it and obtained the horizontal crustal velocity field. The results reveal: 1) the effect of Kunlun M s8.1 earthquake on crustal deformation in this region is significant; 2) the velocity field obtained with this method is better than the original GPS velocity field in reflecting the status of regional crustal movement and strain.

Quantitative analysis and visualization of nonplanar fault surfaces using terrestrial laser scanning (LIDAR)--The Arkitsa fault, central Greece, as a case study

Many fault surfaces are noticeably nonplanar, often containing irregular asperities and more regular corrugations and open warping. Terrestrial laser scanning (light detection and ranging, LIDAR) is a powerful and versatile tool that is highly suitable for acquisition of very detailed, precise measurements of slip-surface geometry from well-exposed faults. Quantitative analysis of the LIDAR data, combined with three-dimensional visualization software, allows the spatial variation in various geometrical properties across the fault surface to be clearly shown. Plotting the variation in distance of points from the mean fault plane is an effective way to identify culminations and depressions on the fault surface. Plots showing the spatial variation of surface orientation are useful in highlighting corrugations and warps of different wavelengths, as well as cross faults and fault bifurcations. Analysis of different curvature properties, including normal and Gaussian curvature, provides the best plots for quantitative measurements of the geometry of corrugations and folds. However, curvature analysis is highly scale dependent, so requires careful filtering and smoothing of the data to be able to analyze structures at a given wavelength. Three well-exposed fault panels from the Arkitsa fault zone in central Greece were scanned and analyzed in detail. Each panel is markedly nonplanar, and shows significant variation in surface orientation, with spreads of ~±20°–25° in strike and ±10° in dip. Most of the variation in orientation reflects decimeter- and meter-scale corrugations and longer wavelength warps of the fault panels. Average wavelengths of corrugations measured on two of the panels are 4.04 m and 4.43 m. Whereas the orientations of the three fault surfaces show significant variations, the orientations of fault striae are very similar between the panels, and are tightly clustered within each panel. Fault-slip analysis from each panel shows that the local stress field is consistent with the regional velocity field derived from global positioning system data. The oblique slip lineations observed on the fault panels represent a combination of two contemporaneous strain components: north-northeast–south-southwest extension across the Gulf of Evia and sinistral strike slip along the west-northwest–east-southeast Arkitsa fault zone.

Fore‐arc motion and Cocos Ridge collision in Central America

We present the first regional surface velocity field for Central America, showing crustal response to interaction of the Cocos and Caribbean plates. Elastic half‐space models for interseismic strain accumulation on the dipping subduction plate boundary fit the GPS data well and show strain accumulation offshore and beneath the Nicoya and Osa peninsulas in Costa Rica but not in Nicaragua. Since large subduction zone earthquakes occur in Nicaragua, we suggest that interseismic locking in Nicaragua and some other parts of Central America occurs but is mainly shallow, <20 km depth, too far offshore to be detected by our on‐land GPS measurements. Our data also show significant trench‐parallel motion for most of the region, generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates. However, trench‐parallel motion is also observed in central Costa Rica, where plate convergence is normal to the trench, and in the Nicaraguan fore arc, where trench‐parallel motion is fast, up to 9 mm a−1, but mechanical coupling is low. A finite element model of collision (as opposed to subduction) involving the aseismic Cocos Ridge also fits the GPS surface velocity field, most significantly reproducing the pattern of trench‐parallel motion. We infer that buoyant, thickened CNS‐2‐Cocos Ridge crust resists normal subduction and instead acts as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica and contributing to trench‐parallel fore‐arc motion in Costa Rica and perhaps Nicaragua as a type of tectonic escape.

Regional versus detailed velocity analysis to quantify hydrate and free gas in marine sediments: the South Shetland Margin case study

Abstract The presence of gas hydrate and free gas within marine sediments deposited along the South Shetland margin, offshore the Antarctic Peninsula, was confirmed by low and high resolution geophysical data, acquired during three research cruises. Seismic data analysis has revealed the presence of a bottom-simulating reflector that is very strong and continuous in the eastern part of the margin. This area can be considered as a useful site to study the seismic characteristics of sediments containing gas hydrate, with a particular focus on the estimation of gas hydrate and free gas amounts in the pore space. Pre-stack depth migration and tomographic inversion were performed to produce a regional velocity field of gas-phase bearing sediments and to obtain information about the average thickness of gas hydrate and free gas layers. Using these data and theoretical models, the gas hydrate and free gas concentrations can be estimated. Moreover, the common image gather semblance analysis revealed the presence of detailed features, such as layers with small thickness characterized by low velocity alternating with high velocity layers, below and above the bottom-simulating reflector. These layers are associated with free gas trapped within the hydrate stability zone and deeper sediments. Thus, the use of the detailed and the regional velocity field analysis is important to give a more reliable estimate of gas content in the marine sediments.

Geo-kinematic investigations in Slovakia and the region of Central and Southeastern Europe based on combination of permanent and epoch-wise GPS networks

Geo-kinematic investigations in Slovakia and the region of Central and Southeastern Europe based on combination of permanent and epoch-wise GPS networksThe region of Central and Southeastern Europe is covered by numerous GPS networks investigating geo-kinematical behavior of this area. These activities started in early nineties within the several projects as CERGOP and CERGOP-2/Environment. In this paper we describe the process of combination of four networks located in the region of Central and Southeastern Europe using the CATREF (Combination and Analysis of Terrestrial REFerence Frame) software developed in Institute Geographique National in France. We were particularly interested in the CEPER (Central European Permanent Network), CEGRN (Central European Geodynaminc Reference Network), SGRN (Slovak Geodynamic Reference Network) and local network TATRY shortly described in this paper. Homogeneous velocity field obtained from the final combination is for the purpose of better interpretation divided into three parts: Central Europe, Slovakia and Tatra Mountains. Main interest is focused on the territory of Slovakia where the regional velocity field is not so frequently discussed in scientific community as the case of Central and Southeastern Europe.

Ice thinning, upstream advection, and non-climatic biases for the upper 89% of the EDML ice core from a nested model of the Antarctic ice sheet

Abstract. A nested ice flow model was developed for eastern Dronning Maud Land to assist with the dating and interpretation of the EDML deep ice core. The model consists of a high-resolution higher-order ice dynamic flow model that was nested into a comprehensive 3-D thermomechanical model of the whole Antarctic ice sheet. As the drill site is on a flank position the calculations specifically take into account the effects of horizontal advection as deeper ice in the core originated from higher inland. First the regional velocity field and ice sheet geometry is obtained from a forward experiment over the last 8 glacial cycles. The result is subsequently employed in a Lagrangian backtracing algorithm to provide particle paths back to their time and place of deposition. The procedure directly yields the depth-age distribution, surface conditions at particle origin, and a suite of relevant parameters such as initial annual layer thickness. This paper discusses the method and the main results of the experiment, including the ice core chronology, the non-climatic corrections needed to extract the climatic part of the signal, and the thinning function. The focus is on the upper 89% of the ice core (appr. 170 kyears) as the dating below that is increasingly less robust owing to the unknown value of the geothermal heat flux. It is found that the temperature biases resulting from variations of surface elevation are up to half of the magnitude of the climatic changes themselves.

Evolution of stress in Southern California for the past 200 years from coseismic, postseismic and interseismic stress changes

SummaryThe seismicity of southern California results from stresses that arise from the relative motion of the Pacific and North American Plates being accommodated along the San Andreas Fault (SAF) system and the Eastern California Shear Zone (ECSZ). Here we calculate how the stress field in southern California has evolved over the past two centuries due to interseismic loading, as inferred from current GPS observations of surface velocities, from redistributions of static stress induced by large (Mw ≥ 6.5) earthquakes since the 1812 Wrightwood quake, and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep, warm, lower crust and upper mantle to the overlying seismogenic upper crust. We calculate Coulomb stress changes on vertical strike-slip faults striking parallel to the SAF and at the hypocenters on the rupture planes of all Mw ≥ 6 events over the past two centuries. Our results suggest that the 1857 Mw = 8.2 Fort Tejon earthquake, by far the largest event to have occurred in the region over the past two centuries, had a profound influence on the state of stress in Southern California during the 19th century, inducing significant stress increases to the north (Parkfield region and adjoining creeping SAF) and south (southern SAF and San Jacinto fault), and stress relief across the southern ECSZ. These stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century. Slow interseismic build-up of stress further loads all major strike-slip faults and works to reload the areas of the ECSZ where stress was relieved by the 1857 quake. Our calculations suggest that only 56% of hypocenters were pushed closer to failure by preceding coseismic stress changes, suggesting that the occurrence of large earthquakes is not strongly determined by coseismic Coulomb stress changes. This percentage rises to 70% when postseismic stress changes are also considered. Our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes. We find, however, that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern California.

Three-dimensional finite-element modeling of the glacial isostatic adjustment in Fennoscandia

During the last ice age cycles, large ice sheets have covered North America, Northern Eurasia, Greenland and Antarctica. The Earth's crust and mantle has been depressed by the weight of these ice sheets by several hundreds of meters. At the end of the last ice age cycle, the ice sheets have vanished around 6000 yr ago, and the Earth's surface rebounded. However, due to the time-dependent viscoelastic relaxation of the Earth's mantle, the rebound, also termed glacial isostatic adjustment (GIA), is still observable today. In Fennoscandia, a key region of GIA, numerous observations such as paleo-strandlines, present-day crustal deformations monitored by GPS observations, and present-day changes in the gravity field seen by satellite missions, provide a detailed picture of the past and ongoing deformation. We model the GIA process in Fennoscandia by means of the finite-element technique. We employ a three-dimensional viscosity structure in the Earth's mantle derived from seismic shear-wave tomography models, and we use thermodynamic considerations to convert the shear-wave perturbations into viscosity variations. We then compare the results based on the three-dimensional Earth's structure with a simpler earth model, where viscosity depends on the vertical direction only. Our results indicate significant differences between three- and one-dimensional modeling. The vertical crustal velocities reveal differences up to 7 mm/yr, and horizontal crustal velocities are effected even stronger. The typical divergent motions of the latter observed for one-dimensional earth models are no longer present for three-dimensional viscosity models. Instead, a regional velocity field with movements away from the Norwegian coast towards the old Baltic Shield is observed. In a sensitivity analysis we show that the dramatic change in the horizontal flow pattern has its origin deeper in the upper mantle, between 450 and 670 km depth. We also confirm that the observed GIA process in Fennoscandia is not very sensitive to the viscosity structure in the lower mantle. However, a comparison with BIFROST data reveals a best-fit with the simple, one-dimensional model, which requires a revision of our three-dimensional models in a future analysis.