archive
Crustal Deformation
Coseismic Deformation
The 2010 Mw 7.2 El-Mayor Cucapah earthquake in Baja California, Mexico
The 4th of April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake in Baja California and Sonora, Mexico had primarily right-lateral strike-slip motion and a minor normal-slip component. The surface rupture extended about 120 km in a NW-SE direction, west of the Cerro Prieto fault. Here we use geodetic measurements including near- to far-field GPS, InSAR (Interferometric Synthetic Aperture Radar), and sub-pixel offset measurements of radar and optical images to characterize the fault slip during the EMC event. We use dislocation inversion methods and determine an optimal nine-segment fault geometry, as well as a sub-fault slip distribution from the geodetic measurements. With systematic perturbation of the fault dip angles, randomly removing one geodetic data constraint, or different data combinations, we are able to explore the robustness of the inferred slip distribution along fault strike and depth. The model-fitting residuals imply contributions of early postseismic deformation to the InSAR measurements as well as lateral heterogeneity in the crustal elastic structure between the Peninsular Ranges and the Salton Trough. We also find that with incorporation of near-field geodetic data and finer fault patch size, the shallow slip deficit is reduced in the EMC event by reductions in the level of smoothing. These results show that the outcomes of coseismic inversions can vary greatly depending on model parameterization and methodology.
The 2010 Mw 6.3 JiaShian earthquake in SW Taiwan
The 2010 March 4, Jia-Shian (Mw 6.3) earthquake in SW Taiwan caused moderate damage and no surface rupture was observed, reflecting a deep source that is relatively rare in western Taiwan. We develop finite-source models using a combination of seismic waveform (strong motion and broadband), Global Positioning System (GPS) and synthetic aperture radar inter- ferometry (InSAR) data to understand the rupture process and slip distribution of this event. The rupture centroid source depth is 19 km based on a series of moment tensor solution tests with improved 1-D Green’s functions. The preferred fault model strikes 322◦ and dips 27◦ to the NE and the mainshock is a thrust event with a small left-lateral component. The finite- source model shows a primary slip asperity that is about 20 km in diameter at a depth range from 22 to 13 km, with peak slip of 42.5 cm, a total scalar seismic moment of 3.25 × 1018 N m (Mw 6.34) and with an average static stress drop of 0.24 MPa. The rupture velocity of this event is faster than the mid-crustal shear wave velocity in Taiwan, which suggests the possibility of a supershear event which has not been previously observed in Taiwan. Systematic resolution and sensitivity tests are performed to confirm the slip distribution, rupture velocity, the choice of weighting and smoothing for the joint inversions, and the consistency of the slip distribution. The first 24 hours of aftershocks appeared along the upper periphery of the main coseismic slip asperity. Both the mainshock and aftershocks are located in a transition zone where the depth of seismicity and an inferred regional basal décollement increases from central to southern Taiwan. The difference between the current orientation of plate convergence in Taiwan (120o) and the P axis of this event (052o) and nearby measurements of recent crustal strain directions (050◦ to 080◦), as well as the relatively low static stress drop, suggest that the Jia-Shian event involves the reactivation of a deep and weak pre-existing NW–SE geological structure.
Postseismic Deformation
Twenty-five years of postseismic deformation following he 1989 Mw 6.9 Loma Prieta earthquake in California
The October 17, 1989 M w 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 Mw 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the Loma Prieta earthquake postseismic displacements from 1989 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 yr of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe GPS horizontal displacements at mean rates of 1–4 mm/yr toward Loma Prieta Mountain until 2000, and ∼2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002 shown by InSAR, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989–1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994. The best fitting model shows an elastic 19-km-thick upper crust above an 11-km-thick viscoelastic lower crust with viscosity of ∼6 × 1018 Pa s, underlain by a viscous upper mantle with viscosity between 3 × 1018 and 2×1019 Pas. The millimeter-scale postseismic deformation does not resolve the viscosity in the different layers very well, and the lower-crustal relaxation may be localized in a narrow shear zone. However, the inferred lithospheric rheology is consistent with previous estimates based on post-1906 San Francisco earthquake measurements along the San Andreas fault system. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.
The 2010 Mw 7.2 El Mayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the cold Mesozoic arc crust of the Peninsular Ranges and newly-formed, hot, extending lithosphere within the Salton Trough. We use five years of GPS measurements to invert for afterslip and constrain a 3D finite element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicenter). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the LAB is ~65 km below the Peninsular Ranges and ~32 km beneath the Salton Trough. These depths are shallower then the corresponding seismic LAB, but deeper than the long-term LAB inferred from coherence analysis of gravity and topography. The seismic LAB corresponds well with the depth at which the geotherm is inferred to become adiabatic within the stability field of partial molten peridotite beneath the Peninsular Ranges. Beneath the Salton Trough, the mechanical LAB corresponds well with the onset of mantle partial melting, whereas the seismic LAB corresponds with the onset of an adiabatic geotherm. This supports the idea that the inferred depth of the LAB is directly associated with partial melting of the mantle, and is dependent on the time frame of the observational constraints—the longer that time frame, the shallower the LAB, as rocks with higher viscosities have more time to relax.
Interseismic Deformation
Interseismic and seasonal crustal deformation in SW Taiwan based on ten years of InSAR time series
Geodetically measured surface displacements are produced by a combination of underlying deformation processes acting at different spatial and temporal scales. A complete history of surface measurements in an area can help discriminate contributions from tectonic, hydrologic, and anthropogenic processes. In this study, we use Synthetic Aperture Radar (SAR) images of the active mountain front and adjacent coastal plain of southwestern Taiwan to generate time series of surface deformation from 1995 to 2001 and from 2005 to 2008 based on the InSAR small baseline method. The InSAR measurements agree well with LOS motions estimated from continuous GPS measurements of 3D displacements between 2006 and 2008. A significant range increase in line of sight (LOS) is dominated by land subsidence in the coastal area of western and southwestern Taiwan. Subsidence rates vary with annual periods and are highly correlated with seasonal precipitation, which are likely associated with groundwater recharge and withdrawal. The long-term deformation is dominated by long-term tectonic loading in SW Taiwan during the interseismic period of the earthquake cycle, including elastic strain along or continuous creep on the active faults. Our results show the ability of InSAR to reveal spatiotemporal crustal deformation in western Taiwan with high spatial resolution and accuracy, which is potentially important for evaluating seismic hazards.
Geomorphology & other research
The Sacramento-San Joaquin Delta, like other heavily engineered deltas throughout the world, has undergone substantial subsidence since the late 1800s when the natural estuary was leveed to form islands. Today, islands within the Delta have subsided to the point where most lie below mean sea level. Long-term sustainability of the Delta requires reversal of subsidence, but spatially comprehensive maps of subsidence have not been available to inform and monitor remediation. Reported here is the first spatially dense map of recent subsidence rates across the Delta, based on synthetic aperture radar interferometry and constrained by GNSS observations. The analysis uses a temporally dense and spatially overlapping set of data acquired by the Uninhabited Aerial Vehicle SAR (UAVSAR) sensor in 2009–2015. On average, the Delta is subsiding by 9.2 ± 4.4 mm/yr with high variability even at the sub-island scale, including peak subsidence of 160 ± 4.4 mm/yr occurring just inland of the levee of the westernmost island and average subsidence of 11 mm/yr along the planned path of water conveyance tunnels through the Delta. The results are compared to local sea level rise (SLR), measured at a tidal gauge in San Francisco Bay, to show that subsidence is currently the dominant contributor to local relative sea level rise (RSLR) by nearly a factor of five. Based on recently measured and modeled acceleration of SLR, subsidence in the Delta is likely to dominate RSLR until the middle of the 21th century.
ACTIVE FAULTING WITHIN THE CHURACHANDPUR-MAO FAULT SYSTEM; INDO-BURMAN RANGES, NORTHEAST INDIA (Chiama, Betka, Huang)
The Indo-Burman Ranges (IBR) comprise a ~375 km-wide accretionary prism, located at the northern end of the Sunda subduction zone, which accommodates oblique convergence (~70º; ~46 mm/yr) of the Ganges-Brahmaputra delta on the Indian Plate with the Shan Plateau of SE Asia. GPS measurements indicate that the northward component of convergence is mostly partitioned among two north-striking dextral faults, known as the Sagaing and Churachandpur-Mao faults (CMF). The eastward component of convergence (~13-17 mm/yr) is absorbed by a fold-thrust belt propagating westward at the front of the IBR. While the fold-thrust belt, Sagaing fault, and northern segment (26-24ºN lat., 93.4-94ºE long.) of the CMF are well constrained by GPS measurements and field studies, the extent, structure and significance of active faulting along CMF south of ~24ºN is largely unknown. Using an ~12 m shaded relief DEM, we identified several structural lineaments which cross-cut the IBR fold-thrust belt within an ~35 km wide zone along-strike of the CMF south of 24ºN. The mapped features include an array of NW-striking subvertical fault scarps, N-striking scarps that are subparallel to the CMF, and NE-striking subvertical features that appear to be joints. Preliminary fault-slip data suggest both dip-slip and strike-slip motion along the NW-striking faults. The relative orientation of the mapped faults and joints suggests a Riedel shear geometry compatible with dextral slip on the CMF if the NW-striking faults are dominantly sinistral. Alternatively, the NW features are in a favorable orientation relative to the oblique N20E convergence direction to be reverse faults. We combined field measurements, tectonic geomorphology, fault-slip analysis, and InSAR to distinguish between these models. Preliminary analysis reveals several sets of previously unrecognized faults and suggests the CMF expands south of ~24ºN to form a 10s of km wide fault zone defined by kinematically compatible arrays of secondary faults. Field measurements, tectonic geomorphology, and fault-slip analysis along 4 NW-trending fault strands reveals sinistral motion and rotation of fold axes within fault blocks. InSAR using Sentinel-1 data ranging from 2015 to 2020 along one NW-trending fault reveals local subsidence on the northern side of the fault, indicating a potential dip-slip component. We propose that these structures reflect distributed dextral shear and the active southward propagation of the CMF. Our study contributes new mapping of the potentially seismogenic CMF and describes how oblique convergence is accommodated across a subduction-accretion wedge.