Sustained serpentinization of peridotite within the oceanic lithosphere requires effective supply of water to systems that experience continuous expansion of the solid volume. Hence, serpentinization preferentially occurs along ridge axes and in subduction zones where tectonic activity is intense and fracturing helps generating and sustaining the permeability required to connect seafloor-near environments to depth. The slowest mid-oceanic ridges produce little melt leading to discontinuous magmatic activity with very thin to no crust along most of the ridge length and up to 8 km thick crust focused around local magmatic centers. Three types of ultra-slow ridge sections can be distinguished: i) amagmatic, characterized by scarce basaltic crust and deep seismic activity, ii) magmatic, characterized by a thin basaltic crust and intermediate depth seismic activity, and iii) volcanic, characterized by a thick basaltic crust and shallow seismic activity. At amagmatic and magmatic ridge types, aseismic zones are identified above the seismic zone. The lower limit of the aseismic zone along amagmatic sections is thermally controlled and follows a 400-500˚C isotherm corresponding to the upper temperature limit for the onset of serpentinization. This observation suggests that the aseismic zone is significantly serpentinized with ample supply of water to the peridotite-serpentine interface. Based on recorded seismic activity, we estimate the associated rock volume affected by brittle damage for the different ultra-slow ridge types. We show that damage produced by seismic activity sustains pervasive serpentinization along amagmatic and magmatic types, while it is limited in the case of volcanic sections.

We examine stress parameters in Southern California with a focus on the region near the South Central Transverse Ranges (SCTR), using a refined stress inversion methodology to 1981-2017 declustered and aftershocks focal mechanisms independently. Comparison between the associated stress parameters provides information on the local dominant loading. The estimated stress parameters are examined in relation to the regional stress regime and local loadings. Over the regional scale, the Strends towards the NNE and the stress ratios vary from transtensional stress regime near the Eastern California Shear Zone (ECSZ), to shear stress near the SCTR, and towards transpression near the Western Transverse Ranges. Detailed analysis of stress parameters near the SCTR indicates deviations from the regional shear stress. The San Bernardino Mountain area shows S direction towards NNW and transpressional stress components likely associated with the relative motion of the San Andreas Fault and ECSZ. The Cajon Pass and San Gorgonio Pass show transpressional stress regime near the bottom of the seismogenic zones likely associated with the elevated topography. In Crafton Hills, rotation of the principal stress plunges and S direction and transtensional stress regime below ~10 km, along with lower estimated apparent friction coefficient suggest a weak fault possibly associated with deep creep. The results reveal effects of local loadings resolved by the performed multi-scale analysis. The study does not show significant temporal variations of stress variations near the SCTR from the average stress parameters in the analyzed 37 years.

▪ Our attenuation inversion method is based on Liu et al. (2015) using linear triplet of stations ▪ Application to the dense 3C linear array crossing the San Jacinto Fault in Ramona, CA ▪ Clear fundamental mode Rayleigh and Love wave phases are extracted ▪ Attenuation tomography maps reveal new information in addition to velocity tomography ▪ Shear velocity radial anisotropy and possible shear attenuation anisotropy

We analyze seismograms recorded by four arrays (B1-B4) with 100-m station spacing and apertures of 4-8 km that cross the surface rupture of the 2019 Mw7.1 Ridgecrest earthquake. The arrays extend from B1 in the northwest to B4 in the southeast of the surface rupture. Delay times between P-wave arrivals associated with ∼1200 local earthquakes and four teleseismic events are used to estimate local velocity variations beneath the arrays. Both teleseismic and local P waves travel faster on the northeast than the southwest side of the fault for ~4.6% and ~7.5% beneath arrays B1 and B4, but the velocity contrast is less significant at arrays B2 and B3. We identify several 1- to 2-km-wide low-velocity zones with more intensely damaged inner cores beneath each array. The damage zone at array B4 generates fault-zone head, reflected, and trapped waves. An automated detector, based on peak ground velocities and durations of high-amplitude waves, identifies candidate fault-zone trapped waves (FZTWs) in a localized zone for ~600 earthquakes. Synthetic waveform modeling of averaged FZTWs, generated by ~30 events with high-quality signals, indicate that the trapping structure at array B4 has a width of ∼300 m, depth of 3-5 km, S-wave velocity reduction of ∼20% with respect to the surrounding rock, Q-value of ∼30, and S-wave velocity contrast of ~4% across the fault (faster on the northeast side). The results show complex fault-zone internal structures that vary along fault strike, in agreement the surface geology (alternating playa and igneous rocks).

Temporal changes of S-wave velocities at shallow depth on Mars are derived using seismic data from the InSight mission. Autocorrelation functions are computed for three-component seismic recordings to retrieve zero-offset reflection seismograms. Observed S-wave reflection phase with two-way travel time of ~1.2 s and its multiples indicate an interface at ~200 m depth. Daily relative travel time changes (dt/t) with ~5% variations are correlated well with the surface temperature. A top ~1m-thick regolith layer produces a delay of about one Martian day between the dt/t and surface temperature. Assuming the travel time changes are produced primarily in the top ~18 m sand layer, the daily velocity variations in that layer are ~40%. The dominant mechanisms driving the changes are thermoelastic strain in the shallow structure generating the time delays and possible material failures in the regolith layer.