1. We estimate Walker Lane fault slip rates using a dense filtered and gridded geodetic velocity field and a robust multi-block model approach 2. The geodetic slip rates are independent of geologic slip rates, but 80% agree with them to within uncertainties 3. The method images off-fault deformation and vertical axis rotations providing more insight into how crustal motion drives earthquakes Hammond et al., 2023

The predominant approach for modeling faults in the Earth’s crust represents them as elastic dislocations, extending downdip into the lower crust, where the faults slip continuously. The resulting surface deformation features strain accumulation concentrated across locked faults during the interseismic period. An alternative model proposes faults confined to the elastic crust, with surface deformation driven by a wide zone of distributed shear underneath. Using high-precision GPS data, we analyze deformation profiles across the Walker Lane (WL), USA. The WL is a transtensional region of complex faulting, which delineates the western edge of the Basin and Range province and accommodates a significant portion of the Pacific-North American plate boundary deformation budget. Despite a dense geodetic network surveyed collectively for nearly 20 years, horizontal velocities reveal no evidence of localized strain rate accumulation across fault surface expressions. Instead, deformation within the shear zone is uniformly linear, suggesting that the surface velocities reflect distributed shear within the ductile crust rather than discrete fault deformation. This implies no downdip fault extension below the seismogenic layer. The shear zone, bound by the Sierra Nevada crest in the west, is 172±6 km wide in the northernmost WL narrowing to 116±4 km in the central WL. This study’s conclusion challenges the assumption of the presence of dislocations in the lower crust when estimating geodetic slip rates, suggesting that slip rates are instead controlled by the fault’s position and orientation within the shear zone. This has important implications for quantifying seismic hazards in regions with complex fault systems.

It has been known for decades that the present-day shortening rates across the Western Transverse Ranges (WTR) in southern California are rapid, reaching 10-15 mm/yr near the heavily populated Los Angeles area. However, only recently have geodetic measurements of vertical motion in the WTR been sufficiently dense to resolve a tectonic vertical signal. In this study, we show that much of the geodetically-derived vertical velocity field in the WTR can be attributed to the interseismic signal of strain accumulation on reverse faults. We invert geodetic and geologic data for slip rate and interseismic coupling on faults using a kinematic model consisting of faults embedded in an elastic crust over an inviscid mantle. This method allows us to infer the permanent, long-term component of vertical motions from recoverable, short term motions. We infer that much of the geodetically observed 3-4 mm/yr of differential vertical motion across the WTR, involving subsidence along the Santa Barbara coastline and uplift of the Santa Ynez Range, can be attributed to recoverable elastic deformation associated with interseismic locking on faults dipping under the WTR. The sum of dip-slip rates across the WTR decreases from 10.5-14.6 mm/yr on the east side near Ventura, California to 5-6.2 mm/yr across the western side of the Santa Barbara Channel. The total moment accumulation rate in both the Santa Barbara Channel and the combined San Fernando Valley-LA Basin regions is equivalent to about two M=7 earthquakes every 100 years.

The 2020 Monte Cristo Earthquake sequence in western Nevada began with a M6.5 shock on 5/15/20, and was the largest to occur in Nevada since 1954. The event exhibited left-lateral slip along an eastward extension of the Candelaria fault and extensive distributed surface faulting in the epicentral area. Groundwater monitoring and strain analysis were conducted to evaluate hydrochemical effects on the regional groundwater systems following the initial event. Physio-chemical monitoring, (started on 5/16 and still ongoing) includes measurements of temperature (temp), pH, specific conductance (SpC), flow rate, alkalinity and collection of samples for major ions and trace element analysis. Since sites had not been monitored prior to the initial shock, measurements were evaluated against a year of post-event data to gauge response to seismicity. Four sites were monitored: a well from Columbus Marsh (CM) located 5 km from the epicenter; an artesian thermal well from Fish Lake Valley (FL); a well at Willow Ranch (WR) tapping cool water above the FL waters; and a spring along Mina Dump Road (MD) located 15 km north of the Candelaria fault on the Benton Springs Fault. GPS and InSAR measurements were used to create a model of the slip from which we estimated coseismic strain at each sampling location. All but one sample site, MD, experienced positive dilation and CM experienced the greatest amount of strain (15-17 microstrains). Hydrologic and chemical changes were observed following the initial shock, varying between sites. CM had significantly lower SpC values in the week following the event, as well as changes in major ion composition. Other sites showed minor changes; MD showed fluctuations in pH values and FL experienced a slight drop in temp. These waters showed minimal changes in major ions and trace elemental composition. Clear responses were observed throughout three >M5 aftershocks (6/30/20, 11/13/20, and 12/1/20), especially in SpC and alkalinity. A remarkable change in elemental concentration (an increase in Ca, K, SO4, Fe, and decrease in Na, Cl, Li, and Ba) was observed in CM. WR experienced a transient increase in temp measured two weeks prior to the 11/13/20 earthquake. Strain analyses of the smaller (>M5) events are planned to further evaluate observed responses and to clarify factors affecting groundwater response.

We use a newly updated GPS dataset and the GPS Imaging technique to show that the relief of the Apennines Mountain chain in Italy is currently increasing along its entire length by 1-2 mm/yr. We image positive uplift along the entire length of the Apennine crest including the northern Apennines, Calabria and northern Sicily. The maximum uplift rate is aligned with the topographic drainage divide, the greatest elevations and the zone of horizontal extension accommodating east-northeast translation of the Adriatic microplate relative to the Tyrrhenian Basin. Uplift occurs in a 100 - 150 km wide zone with a profile similar to the long wavelength topography, but not to shorter wavelength topography generated by active faulting and erosion. A zone of lower amplitude uplift aligns with the restive volcanic fields and high geothermal potential west of the Apennines, e.g., at Campi Flegrei, Alban Hills, and Monte Amiata-Larderello. Several factors including consistency of the geodetic rate with geologic uplift rates, and incompatibility with transient hydrological or earthquake cycle effects imply that it is a long-lived feature. Uplift occurs despite that the expected consequence of extension is crustal thinning and subsidence, suggesting a causal relationship between gravitational forces and active extension. Anomalies in gravity and upper mantle seismic velocity suggest that elevation gain is driven by forces originating in the mantle. We use these observations to address the hypothesis that these forces result from upward flow of asthenosphere beneath the Apennines, although the spatial and temporal scale of the mantle circulation is unclear.