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A
Sandwell, DT, Myer D, Mellors R, Shimada M, Brooks B, Foster J.  2008.  Accuracy and Resolution of ALOS Interferometry: Vector Deformation Maps of the Father's Day Intrusion at Kilauea. Ieee Transactions on Geoscience and Remote Sensing. 46:3524-3534.   10.1109/tgrs.2008.2000634   AbstractWebsite

We assess the spatial resolution and phase noise of interferograms made from L-band Advanced Land Observing Satellite (ALOS) synthetic-aperture-radar (SAR) data and compare these results with corresponding C-band measurements from European Space Agency Remote Sensing Satellite (ERS). Based on cross-spectral analysis of phase gradients, we find that the spatial resolution of ALOS interferograms is 1.3x better than ERS interferograms. The phase noise of ALOS (i.e., line-of-sight precision in the 100-5000-m wavelength band) is 1.6x worse than ERS (3.3 mm versus 2.1 mm). In both cases, the largest source of error is tropospheric phase delay. Vector deformation maps associated with the June 17, 2007 (Father's day) intrusion along the east rift zone of the Kilauea Volcano were recovered using just four ALOS SAR images from two look directions. Comparisons with deformation vectors from 19 continuous GPS sites show rms line-of-site precision of 14 mm and rms azimuth precision (flight direction) of 71 mm. This azimuth precision is at least 4x better than the corresponding measurements made at C-band. Phase coherence is high even in heavily vegetated areas in agreement with previous results. This improved coherence combined with similar or better accuracy and resolution suggests that L-band ALOS will outperform C-band ERS in the recovery of slow crustal deformation.

E
Neves, MC, Cabral J, Luttrell K, Figueiredo P, Rockwell T, Sandwell D.  2015.  The effect of sea level changes on fault reactivation potential in Portugal. Tectonophysics. 658:206-220.   10.1016/j.tecto.2015.07.023   AbstractWebsite

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5-1 MPa along the Manteigas-Vilarica-Braganca (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas-Vilarica-Braganca fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies. (C) 2015 Elsevier B.V. All rights reserved.

F
Lyons, S, Sandwell D.  2003.  Fault creep along the southern San Andreas from interferometric synthetic aperture radar, permanent scatterers, and stacking. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb001831   AbstractWebsite

[1] Interferometric synthetic aperture radar (InSAR) provides a practical means of mapping creep along major strike-slip faults. The small amplitude of the creep signal (<10 mm/yr), combined with its short wavelength, makes it difficult to extract from long time span interferograms, especially in agricultural or heavily vegetated areas. We utilize two approaches to extract the fault creep signal from 37 ERS SAR images along the southern San Andreas Fault. First, amplitude stacking is utilized to identify permanent scatterers, which are then used to weight the interferogram prior to spatial filtering. This weighting improves correlation and also provides a mask for poorly correlated areas. Second, the unwrapped phase is stacked to reduce tropospheric and other short-wavelength noise. This combined processing enables us to recover the near-field (&SIM;200 m) slip signal across the fault due to shallow creep. Displacement maps from 60 interferograms reveal a diffuse secular strain buildup, punctuated by localized interseismic creep of 4-6 mm/yr line of sight (LOS, 12-18 mm/yr horizontal). With the exception of Durmid Hill, this entire segment of the southern San Andreas experienced right-lateral triggered slip of up to 10 cm during the 3.5-year period spanning the 1992 Landers earthquake. The deformation change following the 1999 Hector Mine earthquake was much smaller (<1 cm) and broader than for the Landers event. Profiles across the fault during the interseismic phase show peak-to-trough amplitude ranging from 15 to 25 mm/yr (horizontal component) and the minimum misfit models show a range of creeping/locking depth values that fit the data.

I
Kaneko, Y, Fialko Y, Sandwell DT, Tong X, Furuya M.  2013.  Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties. Journal of Geophysical Research-Solid Earth. 118:316-331.   10.1029/2012jb009661   AbstractWebsite

We present high-resolution measurements of interseismic deformation along the central section of the North Anatolian Fault (NAF) in Turkey using interferometric synthetic aperture radar data from the Advanced Land Observing Satellite and Envisat missions. We generated maps of satellite line-of-sight velocity using five ascending Advanced Land Observing Satellite tracks and one descending Envisat track covering the NAF between 31.2 degrees E and 34.3 degrees E. The line-of-sight velocity reveals discontinuities of up to similar to 5 mm/yr across the Ismetpasa segment of the NAF, implying surface creep at a rate of similar to 9 mm/yr; this is a large fraction of the inferred slip rate of the NAF (21-25 mm/yr). The lateral extent of significant surface creep is about 75 km. We model the inferred surface velocity and shallow fault creep using numerical simulations of spontaneous earthquake sequences that incorporate laboratory-derived rate and state friction. Our results indicate that frictional behavior in the Ismetpasa segment is velocity strengthening at shallow depths and transitions to velocity weakening at a depth of 3-6 km. The inferred depth extent of shallow fault creep is 5.5-7 km, suggesting that the deeper locked portion of the partially creeping segment is characterized by a higher stressing rate, smaller events, and shorter recurrence interval. We also reproduce surface velocity in a locked segment of the NAF by fault models with velocity-weakening conditions at shallow depth. Our results imply that frictional behavior in a shallow portion of major active faults with little or no shallow creep is mostly velocity weakening. Citation: Kaneko, Y., Y. Fialko, D. T. Sandwell, X. Tong, and M. Furuya (2013), Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties, J. Geophys. Res. Solid Earth, 118, 316-331, doi: 10.1029/2012JB009661.

Gonzalez-Ortega, JA, Gonzalez-Garcia JJ, Sandwell DT.  2018.  Interseismic velocity field and seismic moment release in northern Baja California, Mexico. Seismological Research Letters. 89:526-533.   10.1785/0220170133   AbstractWebsite

We have analyzed all available continuous Global Positioning System (cGPS) and campaign-mode GPS data from northern Baja California, Mexico, covering the 1993.1-2010.1 period to obtain a consistent interseismic velocity field to derive a continuous strain-rate field. The analysis shows concentrations of high strain rate along the Imperial/Cerro Prieto fault system extending from the Salton Sea to the Gulf of California, with strike-slip faulting consistent with principal strain axes direction within the area of largest historical and instrumental seismic release. We translated the strain rate into geodetic moment accumulation rate to evaluate the potential of seismic activity of the region and compare with the actual seismic release of historical and instrumental earthquake catalog. Comparison of regional moment accumulation rate based on geodesy (M-0(g) = 6.3 +/- 1.3 x 10(18) N center dot m/yr) to the corresponding moment release rate by earthquakes (M-0(s) = 2.7 +/- 0.8 x 10(18) N center dot m/yr) highlights a moment rate deficit equivalent to an M-w 7.5-7.8 earthquake. As part of this accumulated moment was released by the recent 2010 M-w 7.2 El Mayor-Cucapah earthquake, these results can provide input constraints on earthquake forecasts for the northern Baja California fault system.

S
Tong, X, Sandwell DT, Schmidt DA.  2018.  Surface creep rate and moment accumulation rate along the Aceh segment of the Sumatran Fault from L-band ALOS-1/PALSAR-1 observations. Geophysical Research Letters. 45:3404-3412.   10.1002/2017gl076723   AbstractWebsite

We analyzed the interferometric synthetic aperture radar data from the ALOS-1/PALSAR-1 satellite to image the interseismic deformation along the Sumatran fault. The interferometric synthetic aperture radar time series analysis reveals up to similar to 20 mm/year of aseismic creep on the Aceh segment along the Northern Sumatran fault. This is a large fraction of the total slip rate across this fault. The spatial extent of the aseismic creep extends for similar to 100 km. The along-strike variation of the aseismic creep has an inverse "U" shape. An analysis of the moment accumulation rate shows that the central part of the creeping section accumulates moment at approximately 50% of the rate of the surrounding locked segments. An initial analysis of temporal variations suggests that the creep rate may be decelerating with time, suggesting that the creep rate is adjusting to a stress perturbation from nearby seismic activity. Our study has implications to the earthquake hazard along the northern Sumatran fault.

Xu, XH, Ward LA, Jiang JL, Smith-Konter B, Tymofyeyeva E, Lindsey EO, Sylvester AG, Sandwell DT.  2018.  Surface creep rate of the southern San Andreas Fault modulated by stress perturbations from nearby large events. Geophysical Research Letters. 45:10259-10268.   10.1029/2018gl080137   AbstractWebsite

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long-term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992-1999, ENVISAT 2003-2010, and Sentinel-1 2014-present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2-7 over the past few decades. We consider quasi-static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long-lasting effect on SSAF creep rates. Plain Language Summary There are two significant conclusions from this study. First, we analyzed 25 years of InSAR measurements over the Southern San Andreas Fault system to document a major increase in the average creep rate following the 1992 Mw 7.3 Landers Earthquake which is then followed by creep rate reductions after the 1999 Mw 7.1 Hector Mine Earthquake and the 2010 Mw 7.2 El Major Cucapah Earthquake. Second, we attribute all these creep rate changes to the Coulomb stress variations from these three major Earthquakes. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers and lower rates after the other two events. An implication of this study is that small but steady Coulomb stress changes have a larger impact on shallow creep than the larger dynamic stress changes associated with passing seismic waves. These results illuminate the significance of time scale-dependent complexity of shallow fault creep and how these behaviors are communicated by stress perturbations from regional earthquakes.