Corné Kreemer

A geodetic plate motion and Global Strain Rate Model

We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ∼2.5 in areas with dense data coverage. We determined 6739 velocities from time series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15,772 velocities from 233 (mostly) published studies onto our core solution to obtain 22,511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25° longitude by 0.2° latitude in dimension). The remainder of the Earths surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS-derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid-body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self-consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no-net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.

Active deformation in eastern Indonesia and the Philippines from GPS and seismicity data

In this study we combine Global Positioning System (GPS) velocities with information on the style of regional seismicity to obtain a self-consistent model velocity and strain rate field for the entire eastern Indonesia and Philippines region. In the process of interpolating 93 previously published GPS velocities, the style and direction of the seismic strain rate field, inferred from earthquakes with M0 < 1 × 1020 N m (from the Harvard centroid moment tensor catalog), are used as constraints on the style and direction of model strain rates within the plate boundary zones. The style and direction of the seismic strain rate field are found to be self-similar for earthquakes up to M0 = 1 × 1020 N m (equivalent to Mw < 7.3). Our inversion result shows the following: The Java Trench, which lacks any significant (historic) seismicity, delineates the Australian plate (AU) - Sunda block (Sunda) plate boundary west of the island of Sumba. East of Sumba, convergence is distributed over the back arc and Banda Sea, and there is no subduction at the Timor Trough, suggesting that the northern boundary of the AU plate runs north of this part of the Banda arc through the Banda Sea. In New Guinea most motion is taken up as strike-slip deformation in the northern part of the island, delineating the Pacific plate (PA) - AU boundary. However, some trench-normal convergence is occurring at the New Guinea Trench, evidence that the strain is partitioned in order to accommodate oblique PA-AU motion. PA-AU motion is consistent with NUVEL-1A direction, but ∼ 8 mm yr−1 slower than the NUVEL-1A estimate for PA-AU motion. The Sulawesi Trench and Molucca Sea delineate zones of high strain rates, consistent with high levels of active seismicity. The Sulawesi Trench may take up some of the AU-Sunda motion. Philippine Sea plate motion is in a direction slightly northward of the NUVEL-IA estimate and is partitioned in some strike-slip strain rates along the Philippine Fault and relatively larger trench-normal convergence along the Philippine Trench and on the Philippine mainland in the southern Philippines and along the Manila Trench in the northern Philippine islands. The high level of strain rate along the Manila Trench is not released by any significant (historic) seismic activity. For the entire eastern Indonesia-Philippines region, seismicity since 1963 has taken up ∼40% of the total moment rate inferred from our model.

On the determination of a global strain rate model

The objective of this paper is to outline the fundamental concepts underlying the estimation of a global strain rate model. We use a variant of the method first introduced by Haines and Holt (1993) to estimate the strain rate tensor field within all of the Earth’s deforming regions. Currently the observables used are ~1650 geodetic velocities, seismic moment tensors from the Harvard CMT catalog, and Quaternary fault slip rate data. A model strain rate field and velocity field are obtained in a least-squares fit to both the geodetic velocities and the observed strain rates inferred from fault slip rates. Seismic moment tensors are used to provide a priori constraints on the style and direction (not magnitude) of the model strain rate field for regions where no fault slip rate data are available. The model will soon be expanded to include spreading rates, ocean transform azimuths, and more fault slip rate data. We present a first estimate of the second invariant of the global model strain rate field. We also present Euler poles obtained by fitting geodetic vectors located on defined rigid plates. We find that 17% of the total model moment rate is accommodated in zones of (diffuse) continental deformation.

Comparison of Seismic and Geodetic Scalar Moment Rates across the Basin and Range Province

Scalar moment rates estimated from a 146-year seismicity catalog agree, within uncertainties, with the deformation rate of the Basin and Range province determined by using space geodesy. Seismic-moment rates have been estimated from a new catalog of earthquakes complete for M 5. The catalog was compiled from 15 preexisting catalogs, supplemented by the review of 44 journal articles. Through- out the catalog compilation, care was taken to obtain the moment magnitude or a reasonable, and not inflated, equivalent. Seventy-six percent of the moment release occurred during 10 earthquakes of magnitude MW 6.79. The spatial distribution of earthquakes and their moment release matches the geodetic pattern of deformation. All three are concentrated in a 200-km-wide zone along the western boundary of the Great Basin, with this zone widening to the north. Several techniques, ultimately traceable to Kostrov and Brune, are used to translate the geodetic strain rates into rates of seismic-moment release. The agreement between geodetic and seismic- moment rate suggests that, within uncertainties, the rate of historic earthquakes within the Basin and Range province, taken as a whole, provides a reasonable estimate for the future rate of seismicity. These results support the hypothesis that even a few years of detailed geodetic monitoring can provide a good constraint on earthquake occurrence rate estimates for large-enough regions.

Western Mediterranean Ridge mud belt correlates with active shear strain at the prism-backstop geological contact

A high-resolution swath-mapping survey conducted in the deep waters of the eastern Mediterranean Sea allowed mapping of active faults and mud volcanism along a sizable portion of the Mediterranean Ridge. Active shear is localized at the prism-backstop con- tact, a major dextral flower structure and a site of massive mud expulsion. We investigate the relationship between the mud output rate and horizontal strain rate by combining the mud volume estimate from sea-bottom reflectivity with kinematic modeling based on far- field global positioning system data and local fault and strain patterns. We find a direct correlation between maxima of mud output and maxima of the shear component of strain at the backstop contact. Mud volcanism may reflect the abundance of solid (mud) and fluid (methane) sources combined with a favorable tectonic regime established at the prism-backstop contact in post-Pliocene time, in relation to plate tectonic changes.

Contemporary uplift of the Sierra Nevada, western United States, from GPS and InSAR measurements

Modern space geodesy has recently enabled the direct observation of slow geological processes that move and shape Earth’s surface, including plate tectonics and crustal strain accumulation that leads to earthquakes. More elusive has been the direct observation of active mountain growth, because geodetic measurements have larger uncertainties in the vertical direction, while mountain growth is typically very slow. For the Sierra Nevada of California and Nevada, western United States, the history of elevation is complex, exhibiting features of both ancient (40–60 Ma) and relatively young (<3 Ma) elevation. Here we exploit the complementary strengths of high-precision three-component point positions from the GPS and blanket coverage line-of-sight measurements from interferometric synthetic aperture radar (InSAR) to show that contemporary vertical motion of the Sierra Nevada is between 1 and 2 mm/yr. The motion is upward with respect to Earth’s center of mass and with respect to a relatively stable eastern Nevada, indicating generation of relief and uplift against gravity. Uplift is distributed along the entire length of the range, between latitude 35°N and 40°N, and is not focused near localized, seismically imaged mantle downwellings. These results indicate that the modern episode of Sierra Nevada uplift is still active and could have generated the entire modern range in <3 m.y.

A no‐net‐rotation model of present‐day surface motions

A significant portion of the Earths surface consists of zones of diffuse deformation. The interior regions of these diffuse zones of deformation move at distinctly different velocities from that of adjacent plates, and, because of their complexities, have been ignored in previous no-net-rotation (NNR) models. We have calculated a new NNR model from a continuous velocity field that incorporates both rigid plate motion and velocity gradients within plate boundary zones. When compared with earlier NNR models we find significantly different angular velocities for many plates. Differences between the NNR model presented here and earlier NNR models can be attributed to the effect of including velocity gradients in diffuse plate boundary zones as well as to the actual differences between geodetically derived surface motions and geologic estimates.

MIDAS robust trend estimator for accurate GPS station velocities without step detection

Abstract Automatic estimation of velocities from GPS coordinate time series is becoming required to cope with the exponentially increasing flood of available data, but problems detectable to the human eye are often overlooked. This motivates us to find an automatic and accurate estimator of trend that is resistant to common problems such as step discontinuities, outliers, seasonality, skewness, and heteroscedasticity. Developed here, Median Interannual Difference Adjusted for Skewness (MIDAS) is a variant of the Theil‐Sen median trend estimator, for which the ordinary version is the median of slopes vij = (xj–xi)/(tj–ti) computed between all data pairs i > j. For normally distributed data, Theil‐Sen and least squares trend estimates are statistically identical, but unlike least squares, Theil‐Sen is resistant to undetected data problems. To mitigate both seasonality and step discontinuities, MIDAS selects data pairs separated by 1 year. This condition is relaxed for time series with gaps so that all data are used. Slopes from data pairs spanning a step function produce one‐sided outliers that can bias the median. To reduce bias, MIDAS removes outliers and recomputes the median. MIDAS also computes a robust and realistic estimate of trend uncertainty. Statistical tests using GPS data in the rigid North American plate interior show ±0.23 mm/yr root‐mean‐square (RMS) accuracy in horizontal velocity. In blind tests using synthetic data, MIDAS velocities have an RMS accuracy of ±0.33 mm/yr horizontal, ±1.1 mm/yr up, with a 5th percentile range smaller than all 20 automatic estimators tested. Considering its general nature, MIDAS has the potential for broader application in the geosciences.

Global deformation from the great 2004 Sumatra-Andaman Earthquake observed by GPS: Implications for rupture process and global reference frame

Static coseismic offsets > 1 mm are observed up to 7800 km away from the great Sumatra-Andaman earthquake of 26 Dec. 2004 using global GPS network data. We investigate the rupture process based on far-field continuous GPS data. To reduce error in the coseismic offset estimates due to post-seismic deformation in the days following the main shock, we simultaneously fit a model of co- and postseismic offsets for nearby stations SAMP (500 km) and NTUS (900 km). The 3-month cumulative postseismic displacement for station SAMP amounts to 20% of the coseismic displacement, and can be well modeled by velocity-strengthening afterslip. We find that coseismic slip on the northern rupture segment is ∼3 m, which is consistent with seismic estimates. Our best estimate of the moment magnitude is Mw = 9.13 if we take into account the expected increase of the shear modulus with depth (for uniform μ = 30 GPa, the moment-magnitude would only be 8.97). Our geodetic results, and thus our inferred rupture model, are different from a similar study using far-field data of Banerjee et al. (2005). These differences highlight the challenge in earthquake studies on a global scale in terms of the sensitivity of far-field offset estimates to the analysis strategy and reference frame treatment. Our predicted coseismic offsets from this event are at least 1 mm across almost the entire globe. This warrants a reconsideration of how to maintain the global terrestrial reference frame affected by earthquakes of Mw > 9.0.

Geodetic constraints on areal changes in the Pacific–North America plate boundary zone: What controls Basin and Range extension?

Using ∼1500 geodetic velocities we model the present-day spatial patterns of areal changes inside the Pacific–North America plate boundary zone. From this model we show that between the central Gulf of California and the Queen Charlotte Islands there is no significant net change in surface area. This zero net areal-change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone. We find that areal growth of the Basin and Range province (BRP) and its eastern margin (∼5.2 ± 0.1 × 10 3 m 2 /yr) is balanced by areal reduction near northwestern California between 38°N and 42°N. The San Andreas fault system south of 38°N and the plate boundary zone north of ∼42°N (including the Juan de Fuca and Gorda Ridge systems) each have no significant net areal change. Our results suggest a kinematic relationship between extension in the BRP and contraction near the northern California Coast Ranges and Klamath Mountains. From these observations we propose that, although BRP extension may be caused by internal forces, the southernmost Cascadia subduction zone provides a “window of escape” that acts as a stress guide to BRP extension as well as northwestward Sierra Nevada motion. Such a dynamic model is consistent with independent findings that (1) the least principal horizontal stress orientations in the BRP are toward northern California, (2) extension directions in the BRP have changed orientation to track the northward migration of the Mendocino triple junction, and (3) the southernmost Cascadia subduction zone is a relatively weak plate boundary.