Xiaopeng Tong

The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy

Radar interferometry from the ALOS satellite captured the coseismic ground deformation associated with the 2010 Mw 8.8 Maule, Chile earthquake. The ALOS interferograms reveal a sharp transition in fringe pattern at ~150 km from the trench axis that is diagnostic of the downdip rupture limit of the Maule earthquake. An elastic dislocation model based on ascending and descending ALOS interferograms and 13 near-field 3-component GPS measurements reveals that the coseismic slip decreases more or less linearly from a maximum of 17 m (along-strike average of 6.5 m) at 18 km depth to near zero at 43–48 km depth, quantitatively indicating the downdip limit of the seismogenic zone. The depth at which slip drops to near zero appears to be at the intersection of the subducting plate with the continental Moho. Our model also suggests that the depth where coseismic slip vanishes is nearly uniform along the strike direction for a rupture length of ~600 km. The average coseismic slip vector and the interseismic velocity vector are not parallel, which can be interpreted as a deficit in strike-slip moment release.

Localized fault slip to the trench in the 2010 Maule, Chile Mw = 8.8 earthquake from joint inversion of high-rate GPS, teleseismic body waves, InSAR, campaign GPS, and tsunami observations

The 27 February 2010, Mw 8.8 Maule earthquake ruptured ~500 km along the plate boundary offshore central Chile between 34°S and 38.5°S. Establishing whether coseismic fault offset extended to the trench is important for interpreting both shallow frictional behavior and potential for tsunami earthquakes in the region. Joint inversion of high-rate GPS, teleseismic body waves, interferometric synthetic aperture radar (InSAR), campaign GPS, and tsunami observations yields a kinematic rupture model with improved resolution of slip near the trench. Bilateral rupture expansion is resolved in our model with relatively uniform slip of 5–10 m downdip beneath the coast and two near-trench high-slip patches with >12 m displacements. The peak slip is ~17 m at a depth of ~15 km on the central megathrust, located ~200 km north from the hypocenter and overlapping the rupture zone of the 1928 M ~8 event. The updip slip is ~16 m near the trench. Another shallow near-trench patch is located ~150 km southwest of the hypocenter, with a peak slip of 12 m. Checkerboard resolution tests demonstrate that correctly modeled tsunami data are critical to resolution of slip near the trench, with other data sets allowing, but not requiring slip far offshore. Large interplate aftershocks have a complementary distribution to the coseismic slip pattern, filling in gaps or outlining edges of large-slip zones. Two clusters of normal faulting events locate seaward along the plate motion direction from the localized regions of large near-trench slip, suggesting that proximity of slip to the trench enhanced extensional faulting in the underthrusting plate.

Is there a discrepancy between geological and geodetic slip rates along the San Andreas Fault System

Previous inversions for slip rate along the San Andreas Fault System (SAFS), based on elastic half-space models, show a discrepancy between the geologic and geodetic slip rates along a few major fault segments. In this study, we use an earthquake cycle model representing an elastic plate over a viscoelastic half-space to demonstrate that there is no significant discrepancy between long-term geologic and geodetic slip rates. The California statewide model includes 41 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known historical ruptures and geologic recurrence intervals. The slip rates are constrained by 1981 secular velocity measurements from GPS and L-band intereferometric synthetic aperture radar. A model with a thick elastic layer (60 km) and half-space viscosity of 1019Pa s is preferred because it produces the smallest misfit to both the geologic and the geodetic data. We find that the geodetic slip rates from the thick plate model agrees to within the bounds of the geologic slip rates, while the rates from the elastic half-space model disagree on specific important fault segments such as the Mojave and the North Coast segment of the San Andreas Fault. The viscoelastic earthquake cycle models have generally higher slip rates than the half-space model because most of the faults along the SAFS are late in the earthquake cycle, so today they are moving slower than the long-term cycle-averaged velocity as governed by the viscoelastic relaxation process.

GMTSAR: An InSAR Processing System Based on Generic Mapping Tools

GMTSAR is an open source (GNU General Public License) InSAR processing system designed for users familiar with Generic Mapping Tools (GMT). The code is written in C and will compile on any computer where GMT and NETCDF are installed. The system has three main components: 1) a preprocessor for each satellite data type (e.g., ERS, Envisat, and ALOS) to convert the native format and orbital information into a generic format; 2) an InSAR processor to focus and align stacks of images, map topography into phase, and form the complex interferogram; 3) a postprocessor, mostly based on GMT, to filter the interferogram and construct interferometric products of phase, coherence, phase gradient, and line-of-sight displacement in both radar and geographic coordinates. GMT is used to display all the products as postscript files and kml-images for Google Earth. A set of C-shell scripts has been developed for standard 2- pass processing as well as image alignment for stacking and time series. ScanSAR processing is also possible but requires a knowledgeable user. Users are welcome to contribute to this effort.

Tectonic and Anthropogenic Deformation at the Cerro Prieto Geothermal Step-Over Revealed by Sentinel-1A InSAR

The Cerro Prieto geothermal field (CPGF) lies at the step-over between the imperial and the Cerro Prieto faults in northern Baja California, Mexico. While tectonically this is the most active section of the southern San Andreas Fault system, the spatial and temporal deformation in the area is poorly resolved by the sparse global positioning system (GPS) network coverage. Moreover, interferograms from satellite observations spanning more than a few months are decorrelated due to the extensive agricultural activity in this region. Here we investigate the use of frequent, short temporal baseline interferograms offered by the new Sentinel-1A satellite to recover two components of deformation time series across these faults. Following previous studies, we developed a purely geometric approach for image alignment that achieves better than 1/200 pixel alignment needed for accurate phase recovery. We construct interferometric synthetic aperture radar time series using a coherence-based small baseline subset method with atmospheric corrections by means of common-point stacking. We did not apply enhanced spectral diversity because the burst discontinuities are generally small (<1.4 mm) and can be effectively captured during the atmospheric corrections. With these algorithms, the subsidence at CPGF is clearly resolved. The maximum subsidence rate of 160 mm/yr, due to extraction of geothermal fluids and heat, dominates the ~40 mm/yr deformation across the proximal ends of the imperial, the Cerro Prieto, and the indiviso faults.

Correction to “Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake”

[1] In the paper “Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake” by Fred F. Pollitz et al. (Geophysical Research Letters, 38, L09309, doi:10.1029/2011GL047065, 2011) the captions for Figure 1–3 are ordered incorrectly. The correct captions for Figures 1–3 appear here. [2] Figure 1. Rupture areas associated with historic earthquakes along the Andean megathrust and aftershocks of the February 27, 2010 event. Historical epicenters are provided by Centro Regional de Sismologia para America del Sur and aftershock locations are provided by the National Earthquake Information Center. White lines indicate the surface projection of the fault plane used to model the 2010 event. Nazca–South America relative plate motion vector is from Ruegg et al. [2009]. [3] Figure 2. Observed coseismic GPS offsets (black vectors) with 95% uncertainties compared with model horizontal offsets using the coseismic slip model obtained by the joint InSAR/GPS inversion, which is contoured in gray (values in meters). White lines indicate the surface projection of the fault plane. [4] Figure 3. Coseismic slip models derived from (a) InSAR data only, (b) GPS data only, and (c) combined GPS and InSAR data, with seismic moment indicated for each model. Contour interval is 3 m. Star symbol indicates the Global CMT epicenter.