Aseismic Research Articles

Effects of the 2022 Mw 7.0 Chihshang Earthquake on the Shallow Creeping of the Central Longitudinal Valley Fault in Eastern Taiwan: Insights from Precise Leveling Measurements

Abstract This study investigated the effects of the 2022 Mw 7.0 Chihshang earthquake in eastern Taiwan on the shallow creeping of the central Longitudinal Valley fault (LVF). Precise leveling surveys were conducted along the Ruisui-Dewu, Yuli, and Dongzhu routes over the LVF. Before the earthquake, previous leveling surveys had detected deformations caused by stable fault creeping of the LVF. Interseismic linear trends and coseismic deformations were removed from the leveling data. Two shallow faults at 1 km were estimated from the corrected-coseismic deformation for the Yuli route. A shallow fault at 1 km and a deep fault at 5 km were estimated from the interseismic deformation. A positive delta Coulomb failure function change was calculated for shallow faults due to the creeping movement on the deep fault before the earthquake, and the creeping movement on the deep fault before the earthquake may have promoted the rupture of the shallow fault. The strong Chihshang earthquake most likely triggered the rupture of the shallow fault, on which strain had been concentrated by the interseismic creeping of the deeper fault. In contrast, the accumulated total strain at the deeper part might be quite low due to a strain release process for a steady creeping movement on deep fault in the interseismic period. The significant slip was not induced in the coseismic period. Although significant slip occurred in the shallower part, the slip might not be induced in the deeper part adjacent to the asperity of the LVF. In the Dewu route, the observed large west-side-uplift is attributed to the coseismic rupture of the Chihshang earthquake. In the Dongzhu route, an interseismic east-side-uplift by a creeping fault and a coseismic west-side-uplift were observed at the same location, indicating a local tensile stress field by the Chihshang earthquake.

Tectonic Control of Aseismic Creep and Potential for Induced Seismicity Along the West Valley Fault in Southeastern Metro Manila, Philippines

Vertical creep along 15 ground ruptures within a 15 km long and 1.5 km wide zone has been occurring along the southeastern part of Metro Manila. Though the unusually high rates of vertical slip point to excessive groundwater withdrawal as the trigger, the evidence presented herein indicates that these may not be simple irregular subsidence fissures. Tectonic control of creep along these traces is suggested by the following: the occurrence of some of these ground ruptures along pre-existing scarps that coincide with topographic and lithologic boundaries, the left-stepping en echelon pattern of surface rupturing, and the distribution of the creeping zone within the dilational gap of the dextral strike-slip West Valley Fault (WVF). Furthermore, interpretation of an exposure across one of the creeping faults indicates reactivation by creep of a pre-existing tectonic fault zone. The paleoseismic evidence also suggests that the pre-creep slips are coseismic and dominantly strike-slip. Recognizing the occurrence of coseismic slip preceding aseismic creep is a primary consideration in assessing the potential of the WVF’s creeping segment and its adjacent segments in generating earthquakes. Tighter groundwater extraction regulations may be necessary to avoid exacerbating the effects of vertical ground deformation and the occurrence of induced seismicity.

Quantifying Creep on the Laohushan Fault Using Dense Continuous GNSS

The interseismic behavior of faults (whether they are locked or creeping) and their quantitative kinematic constraints are critical for assessing the seismic hazards of faults and their surrounding areas. Currently, the creep of the eastern segment of the Laohushan Fault in the Haiyuan Fault Zone at the northeastern margin of the Tibetan Plateau, as revealed by InSAR observations, lacks confirmation from other observational methods, particularly high-precision GNSS studies. In this study, we utilized nearly seven years of observation data from a dense GNSS continuous monitoring profile (with a minimum station spacing of 2 km) that crosses the eastern segment of the Laohushan Fault. This dataset was integrated with GNSS data from regional continuous stations, such as those from the Crustal Movement Observation Network of China, and multiple campaign measurements to calculate GNSS baseline change time series across the Laohushan Fault and to obtain a high spatial resolution horizontal crustal velocity field for the region. A comprehensive analysis of this primary dataset indicates that the Laohushan Fault is currently experiencing left-lateral creep, characterized by a partially locked shallow segment and a deeper locked segment. The fault creep is predominantly concentrated in the shallow crustal region, within a depth range of 0–5.7 ± 3.4 km, exhibiting a creep rate of 1.5 ± 0.7 mm/yr. Conversely, at depths of 5.7 ± 3.4 km to 16.8 ± 4.2 km, the fault remains locked, with a loading rate of 3.9 ± 1.1 mm/yr. The shallow creep is primarily confined within 3 km on either side of the fault. Over the nearly seven-year observation period, the creep movement within approximately 5 km of the fault’s near field has shown no significant time-dependent variation, instead demonstrating a steady-state behavior. This steady-state creep appears unaffected by postseismic effects from historical large earthquakes in the adjacent region, although the deeper (far-field) tectonic deformation of the Laohushan Fault may have been influenced by the postseismic effects of the 1920 Haiyuan M8.5 earthquake.

Creep on the Argentine Precordillera Décollement Following the 2015 Illapel, Chile, Earthquake: Implications for Andean Seismotectonics

AbstractThe Central and South‐Central Andes form a “two‐sided” mountain belt bounded by distinct zones of convergence in the forearc and backarc flanks. Previous geodetic interseismic deformation studies found that the forearc to backarc velocity field is better explained when elastic models allow reverse aseismic slip on the Andes eastern‐flank décollement faults. Here, we extend the earlier interpretation of interseismic motion and argue that normal aseismic creep of the Precordillera décollement is required to explain backarc Global Navigation Satellite System displacements during the co‐ and early postseismic phases of the 2015 Illapel, Chile, earthquake. This model significantly reduces the previously reported overlap between coseismic slip and afterslip on the megathrust of this earthquake, consistent with the expectation that these slip modes are spatially partitioned. These findings have direct implications for estimating recurrence interval and slip rate, and for probabilistic seismic hazard analysis on both sides of the orogen.

Temporal and Spatial Variation of Fault Creep along the Xianshuihe Fault from InSAR Stacking

Temporal and Spatial Variation of Fault Creep along the Xianshuihe Fault from InSAR Stacking

Relocation of the 2018–2022 seismic sequences at the Central Gulf of Corinth: New evidence for north-dipping, low angle faulting

The Gulf of Corinth, Central Greece, is a highly active half-graben, characterized by seismicity which is more intense in its western part, while destructive earthquakes have also occurred towards its eastern end. We herein present an analysis of the seismicity in the Central Gulf of Corinth, for the period from June 2018 to December 2022. We applied the EQTransformer machine-learning model to enhance the initially available data, adding missing P- and S-wave arrival-times or improving existing ones. The events were initially located using a new local velocity model and then relocated using the double-difference method, including waveform cross-correlation data from local stations. The hypocenters, generally distributed at depths between 5 and 15 km, along with the focal mechanisms of significant earthquakes (1965 through 2022) and the geometry of mapped faults on the surface were co-examined to better understand their possible connection. It is shown that major outcropping north-dipping structures, such as the East Helike fault and its eastward offshore extension, match only with the southern bounds of seismicity. The Mw = 5.9, 1970 Antikira and Mw = 5.7, 1992 Galaxidi earthquakes cannot be associated with known mapped faults on the surface and likely occurred on low-angle, north-dipping planes. The variability in slip behavior of the low-angle detachment in the Gulf of Corinth, ranging from seismic slip to aseismic creep, probably accounts for the most part of the N-S extensional deformation. The spatial pattern of the 2018–2022 microseismicity delineates the edges of the rupture planes of major events that occurred during the instrumental era, including the Mw = 6.3, 1995 Aigion earthquake. The lack of aftershocks for significant earthquakes, including the Mw = 5.0, 8 October 2022 event, south of Desfina, is interpreted in terms of different pore pressure conditions, variations in fault-rock strength, and the preferred accumulation of high stress inside the upper crust.

Deformation mechanisms and slip behaviors of tectonically deformed conglomerates from the Central Apennines fold-and-thrust belt: Implications for shallow aseismic and seismic slip

Fold-and-thrust belts and accretionary prisms exhibit considerable variability in slip behaviors along active faults. While numerous studies have investigated fault mechanics and slip behaviors in carbonate-, shale-, and sandstone-hosted faults as well as clay-rich portions of shallow accretionary prisms, the deformation mechanisms and slip behaviors of deformed, thrust-top molasse-type conglomerates remain poorly understood. To fill this knowledge gap, we conducted structural and microstructural analyses on exhumed and deformed conglomerates in the footwall of an out-of-sequence thrust from the Central Apennines fold-and-thrust belt (Italy). Our findings aim to constrain conglomerate deformation mechanisms and infer possible slip behaviors. We observed flattened pebbles and an intense foliation comprising densely spaced, thrust-parallel, clay-rich stylolites that indicate slow deformation attributed to carbonate dissolution during aseismic creep. In contrast, quartz clasts with calcite micro veins and micrometer-thick slip increments in slickenfibers suggest fast brittle failure and fluid overpressure in competent pebbles, while the matrix continues to deform aseismically through pressure solution. The general structure is similar to block-in-matrix textures observed in tectonic mélanges from exhumed subduction zones and accretionary prisms. Our results provide new insights into the slip behaviors of shallow (<∼1 km) portions of fold-and-thrust belts, complementing existing understandings of deformation mechanisms and slip behaviors across different depth ranges in fold-and-thrust belts and accretionary prisms.

Active shortening and aseismic slip along the Cephalonia Plate Boundary (Paliki Peninsula, Greece): Evidence from InSAR and GNSS data

We present a comprehensive analysis of geodetic data, including InSAR and GNSS, to assess the interseismic deformation of the Paliki peninsula in western Cephalonia, Greece. The region is prone to frequent earthquakes, due to its proximity to the Cephalonia Transform Fault (CTF), a 140 km long dextral strike-slip fault (striking NNE-SSW) that accommodates the relative motion between the Apulian and Aegean lithospheric plates. Our analysis covers the period from 2016 to 2022 and leverages LiCSBAS, an open-source package, for InSAR time series analysis with the N-SBAS method. The results of the InSAR analysis demonstrate deformation rates between 2 and 5 mm/yr in the line-of-sight (LOS) direction of the satellite. We constructed E-W velocity profiles that provided a velocity gradient terminating against the outcropping trace of the seismic fault of the 3 February 2014 earthquake (M6.1) near Atheras. This geodetic evidence indicates a ∼ 1 mm/yr minimum aseismic slip (fault creep) motion along the February 2014 seismic fault, during 2016–2022. Positive LOS values in both satellite imaging geometries show that the coastal town of Lixouri experiences uplift of 1 mm/yr. East-West velocity cross-sections across western Cephalonia including Gulf of Argostoli reveal several velocity discontinuities, possibly bounded by active faults and/or landslides. The E-W shortening rate between Lixouri and Argostoli areas amounts to 1.5 mm/yr corresponding to −187 ns/yr (nanostrain/yr) of tectonic strain. Our results suggest a complex deformation pattern on the Paliki peninsula with strain accumulation along strike-slip and reverse-slip faults. We also inverted GNSS velocities from Italy and Greece, across the Cephalonia segment of CTF (assuming elastic half-space) and obtained a locking depth of 13 km, and a slip rate of 17.3 ± 0.8 mm/yr.

3D architecture and complex behavior along the simple central San Andreas fault

The central San Andreas Fault (CSAF) exhibits a simple linear large-scale fault geometry, yet seismic and aseismic deformation features vary in a complex way along the fault. Here we investigate fault zone behaviors using geodetic observation, seismicity and microearthquake focal mechanisms. We employ an improved focal-mechanism characterization method using relative earthquake radiation patterns on 75,164 Ml ≥ 1 earthquakes along a 2-km-wide, 190-km-long segment of the CSAF, from 1984 to 2015. The data reveal the 3D fine-scale structure and interseismic kinematics of the CSAF. Our findings indicate that the first-order spatial variations in interseismic fault creep rate, creep direction, and the fault zone stress field can be explained by a simple fault coupling model. The inferred 3D mechanical properties of a mechanically weak and poorly coupled fault zone provide a unified understanding of the complex fine-scale kinematics, indicating distributed slip deficits facilitating small-to-moderate earthquakes, localized stress heterogeneities, and complex multi-scale ruptures along the fault. Through this detailed mapping, we aim to relate the fine-scale fault architecture to potential future faulting behavior along the CSAF.

A strainmeter array as the fulcrum of novel observatory sites along the Alto Tiberina Near Fault Observatory

Abstract. Fault slip is a complex natural phenomenon involving multiple spatiotemporal scales from seconds to days to weeks. To understand the physical and chemical processes responsible for the full fault slip spectrum, a multidisciplinary approach is highly recommended. The Near Fault Observatories (NFOs) aim at providing high-precision and spatiotemporally dense multidisciplinary near-fault data, enabling the generation of new original observations and innovative scientific products. The Alto Tiberina Near Fault Observatory is a permanent monitoring infrastructure established around the Alto Tiberina fault (ATF), a 60 km long low-angle normal fault (mean dip 20°), located along a sector of the Northern Apennines (central Italy) undergoing an extension at a rate of about 3 mm yr−1. The presence of repeating earthquakes on the ATF and a steep gradient in crustal velocities measured across the ATF by GNSS stations suggest large and deep (5–12 km) portions of the ATF undergoing aseismic creep. Both laboratory and theoretical studies indicate that any given patch of a fault can creep, nucleate slow earthquakes, and host large earthquakes, as also documented in nature for certain ruptures (e.g., Iquique in 2014, Tōhoku in 2011, and Parkfield in 2004). Nonetheless, how a fault patch switches from one mode of slip to another, as well as the interaction between creep, slow slip, and regular earthquakes, is still poorly documented by near-field observation. With the strainmeter array along the Alto Tiberina fault system (STAR) project, we build a series of six geophysical observatory sites consisting of 80–160 m deep vertical boreholes instrumented with strainmeters and seismometers as well as meteorological and GNSS antennas and additional seismometers at the surface. By covering the portions of the ATF that exhibits repeated earthquakes at shallow depth (above 4 km) with these new observatory sites, we aim to collect unique open-access data to answer fundamental questions about the relationship between creep, slow slip, dynamic earthquake rupture, and tectonic faulting.

Fault-network geometry influences earthquake frictional behaviour.

Understanding the factors governing the stability of fault slip is a crucial problem in fault mechanics1-3. The importance of fault geometry and roughness on fault-slip behaviour has been highlighted in recent lab experiments4-7 and numerical models8-11, and emerging evidence suggests that large-scale complexities in fault networks have a vital role in the fault-rupture process12-18. Here we present a new perspective on fault creep by investigating the link between fault-network geometry and surface creep rates in California, USA. Our analysis reveals that fault groups exhibiting creeping behaviour show smaller misalignment in their fault-network geometry. The observation indicates that the surface fault traces of creeping regions tend to be simple, whereas locked regions tend to be more complex. We propose that the presence of complex fault-network geometries results in geometric locking that promotes stick-slip behaviour characterized by earthquakes, whereas simpler geometries facilitate smooth fault creep. Our findings challenge traditional hypotheses on the physical origins of fault creep explained primarily in terms of fault friction19-21 and demonstrate the potential for a new framework in which large-scale earthquake frictional behaviour is determined by a combination of geometric factors and rheological yielding properties.

Kinematic Evolution of the Santa Bárbara System in the Foreland of the Central Andes of Northwestern Argentina (26°S)

AbstractCompared to the thin‐skinned Subandean foreland fold‐and‐thrust belt of northern Argentina and Bolivia, the tectonically active morphotectonic province of the Santa Bárbara System in the Andean broken foreland of northwestern Argentina is characterized by a temporally and spatially disparate deformation style. Although there is no well‐defined orogenic deformation front associated with the uplift of these basement‐cored, reverse‐fault bounded mountain ranges, there has been an overall eastward trend in Andean compressional deformation since the Miocene. While reactivation of basement anisotropies, such as early Paleozoic metamorphic fabrics and Cretaceous normal faults, has been proposed to have profoundly affected deformation processes in the Santa Bárbara System, other mechanisms involving Mesozoic and Cenozoic cover rocks may also have played an important role in orogenic deformation in this sector of the Andean foreland. We present structural field observations, interpretations of seismic reflection profiles, and kinematic modeling from the basins bordering the Candelaria Range in the southern sector of the Santa Bárbara System at 26°S. Our analysis features a 110‐km‐long structural cross section that images and links deep‐seated structures with shallow neotectonic faults. We find that regional shortening has been facilitated by several faults associated with a regional detachment at a depth of 23 km, resulting in total horizontal shortening of about 17% (18.4 km). While the deep‐seated first‐order structures seem closely linked to seismogenic processes in the Andean foreland, shallow second‐order structures within the sedimentary cover modify the intermontane landscapes and appear to be associated with aseismic creep.

Seismogenic structures and active creep in the Granada Basin (S-Spain)

The Granada Basin is a slowly deforming intramountain basin in the Betic Cordillera (S-Spain). Despite historical and paleoseismological evidence for M6 earthquakes, instrumental seismicity lacks large events and the seismotectonic model must be built from small earthquakes (M < 5). Here, we reanalyze 35 years of data from the Granada Basin short period network, and further seismic stations with shorter operating time, to identify seismogenic structures and understand their relationships. We sort events with similar waveforms into multiplet clusters and perform relative location to image active fault patches. Cluster orientations are used as a priori constraint for inverting focal mechanisms from composite cluster polarity measurements. We further estimate moment tensor solutions from full waveform inversion using local and regional broadband stations. We can identify four groups of structures at different positions in the basin: 1) in the northeast sector, we observe shallow, NW-SE striking, high-angle normal faulting, often related with known fault structures; 2) the southern sector shows high-angle normal faulting on unmapped, ∼E-W striking structures at mid-crustal depths; 3) between both groups, we image sub-horizontal fault patches, associated with the basal detachment beneath the basin, showing SSW transport direction; 4) at the western limit of the basin, we find ∼N-S trending, left-lateral strike-slip faults. Groups 2 and 3 are characterized by clusters with overall constant event production rate, indicating ongoing and largely aseismic creep of the basal detachment over the last 35 years. Seismological evidence suggests a locking depth of ∼10 km and a brittle-ductile transition near 15 km, according to the depth range of clusters in groups 2 and 3.

Active Faults, Kinematics, and Seismotectonic Evolution during Tajogaite Eruption 2021 (La Palma, Canary Islands, Spain)

During the 2021 La Palma strombolian and fissure eruption, two faults were identified that controlled the spatial distribution of earthquake hypocenters and effusive eruptive vents. One of these faults has a NW-SE trend (Tazacorte Fault: TZF) and the other one shows an ENE-WSW trend (Mazo Fault: MZF). Previous works on fault structural analysis in La Palma indicated that the eruption zone was compatible with an extensional tectonic strain ellipsoid which activated normal-strike-slip directional faults at the confluence of TZF and MZF. These fractures were activated during the 2021 Tajogaite eruption, determining the NW-SE and WSW-ENE spatial distribution of vents. Both faults were mapped in real time during the volcanic eruption from fieldwork and remote sensing imagery (aerial drone images). We have collected more than 300 fracture data associated with the effusive vents and post-eruption seismic creep. Since the affected area was densely inhabited, most of these fractures affect houses and infrastructures. Some of the houses affected by the TZF were damaged 9 months after the eruption, although they were not damaged during the eruption. Surprisingly, these houses already had repairs made to the same fractures since 1980, giving information of previous fault creep movement. During the 2021 Tajogaite eruption, shallow seismicity was spatially related to both faults, suggesting a seismic behavior instead of the precedent creep movement. However, the lack of seismicity after the eruption indicates that the faults went back to creep aseismic behavior, similarly to 1980. The mapping and monitoring of these faults (TZF and MZF) is relevant bearing in mind that they have been active since 1980 and the post-eruptive phase of the 2021 volcanic eruption, which has to be included in the land use planning in areas affected by the volcanic eruption and creep movement. Furthermore, both faults could act as seismogenic sources triggering volcanic earthquakes with potential high macroseismic intensities and mass movements. The data presented here show the importance of having this type of study before the onset of the eruption, thus allowing a better interpretation of seismic data during volcanic unrest.

Fluids control along-strike variations in the Alaska megathrust slip

Interseismic and coseismic slip on the subduction zone megathrust show complex along-strike variations and the controlling factors remain debated. Here we image seismic velocity anomalies to infer fluid distribution in the Alaska subduction zone (Alaska Peninsula section) with seismic tomography. The weakly locked Shumagin segment is characterized by abundant fluids on the plate interface and in the overriding plate, whereas the moderately-to-highly locked Chignik and Chirikof segments that hosted M > 8 earthquakes are relatively dry. The Kodiak segment, which was ruptured by the 1964 M9.2 Great Alaska earthquake and is presently fully locked near the trench, is characterized by a fluid-rich plate interface but a fluid-poor and inferred low-porosity overriding plate. Multiple large earthquakes have nucleated in fluid-rich regions at the plate interface. Our study highlights the important roles of fluids, particularly in the overriding plate, in controlling interseismic slip deficit and earthquake rupture. We propose that a fluid-rich overriding plate means that there has been a high sustained flux of fluids across the plate interface, which enhances the formation of clay minerals that promote fault creep.

A study of fluid overpressure microstructures from the creeping segment of the San Andreas fault

Evidence of episodic fluid overpressure events noted in samples from the San Andreas Fault Observatory at Depth (SAFOD) have remained largely uncorrelated in terms of their collective significance for seismic history of the fault zone. The compositional and microstructural correlations sought in this study could shed light on questions about potential for major seismic events in the creeping segment of the SAF in central California. We used quantitative energy dispersive spectroscopy (EDS), Cathodoluminescence (CL) and Scanning Electron Microscope (SEM) imaging, and electron backscatter diffraction (EBSD) analysis to acquire geochemical and microstructural data from a suite of twenty SAFOD core samples including the damage zone and the active core of the fault. The results indicate intermittent coseismic fluid overpressure events that overprint the background aseismic creep across the fault. Analysis of trace elements and deformation in the coseismic calcite vein generations and their associated hydrothermal mineral phases indicate progressive uplift and exhumation followed by an asymmetric incursion of meteoric water into the damage zone. The same analysis suggests that the actively creeping intervals act as permeability barriers. Our results are in overall agreement with recent studies of the SAF in central California that indicate large seismic events have occurred intermittent with aseismic creep in recent geological time or suggest future potential for such events.

Sapanca Gölü Yakınında Tek Bir Sismik İstasyonda Tekrarlayan Deprem Gözlemleri

The North Anatolian Fault Zone (NAFZ), which forms the plate boundary between Anatolian Plate and Eurasian Plate, is one of the most active transform fault zones in the world. Following two consecutive magnitude M&gt;7 earthquakes in 1999, an intensified monitoring of western portion of NAF is commenced. Dense networks of onshore/offshore seismic, acoustic, geodetic sensors and surface creep and strain sensors were installed. A single seismic sensor among these, which is located at the midpoint of 1999 ruptures, near Lake Sapanca, exhibits some unusual seismic activity. On a fault segment where creep is known to be present, a series of minor seismic events was observed with identical locations and a recurrence time of three years. These events are quite short in duration and highly similar in their waveforms. Using a single station approach, their angle of incidence and back azimuth were found to coincide with the location of two M2.3 and M2.1 events. At this stage, it is not clear whether these events reflect fault creep at seismogenic depth. Nevertheless, these initial observations emphasize the necessity of monitoring this segment more densely, where recurrent minor earthquakes are likely to be observed.

Ionospheric compensation in L-band InSAR time-series: Performance evaluation for slow deformation contexts in equatorial regions

Multi-temporal Synthetic Aperture Radar Interferometry (MT-InSAR) is the only geodetic technique allowing to measure ground deformation down to mm/yr over continuous areas. Vegetation cover in equatorial regions favors the use of L-band SAR data to improve interferometric coherence. However, the electron content of ionosphere, affecting the propagation of the SAR signal, shows particularly strong spatio-temporal variations near the equator, while the dispersive nature of the ionosphere makes its effect stronger on low-frequencies, such as L-band signals. To tackle this problem, range split-spectrum method can be implemented to compensate the ionospheric phase contribution. Here, we apply this technique for time-series of ALOS-PALSAR data, and propose optimizations for low-coherence areas. To evaluate the efficiency of this method to retrieve subtle deformation rates in equatorial regions, we compute time-series using four ALOS-PALSAR datasets in contexts of low to medium coherence, showing slow deformation rates (mm/yr to cm/yr). The processed tracks are located in Ecuador, Trinidad and Sumatra, and feature 15 to 19 acquisitions including very high, dominating ionospheric noise, corresponding to equivalent displacements of up to 2 m. The correction method performs well and allows to reduce drastically the noise level due to ionosphere, with significant improvement compared with a simple plane fitting method. This is due to frequent highly non-linear patterns of perturbation, characterizing equatorial TEC distribution. We use semivariograms to quantify the uncertainty of the corrected time-series, highlighting its dependence on spatial distance. Thus, using ALOS-PALSAR-like archive, one can expect a detection threshold on the Line-of-Sight velocity ranging between 3 and 6 mm/yr, depending on the spatial wavelength of the signal to be observed. These values are consistent with the accuracy derived from the comparison of velocities between two tracks in their overlapping area. In the case studies that we processed, the time-series corrected from ionosphere allows to retrieve accurately fault creep and volcanic signal but it is still too noisy for retrieving tiny long-wavelength signals such as slow (mm/yr) interseismic strain accumulation.

Integrated geologic and geophysical modeling across the Bartlett Springs fault zone, northern California (USA): Implications for fault creep and regional structure

Abstract The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpent ine mineral stable at depth, in fault gouge near Lake Pillsbury.

Deformation Behavior and Reactivation Mechanism of the Dandu Ancient Landslide Triggered by Seasonal Rainfall: A Case Study from the East Tibetan Plateau, China

In recent years, numerous ancient landslides initially triggered by historic earthquakes on the eastern Tibetan Plateau have been reactivated by fault activity and heavy rainfall, causing severe human and economic losses. Previous studies have indicated that short-term heavy rainfall plays a crucial role in the reactivation of ancient landslides. However, the deformation behavior and reactivation mechanisms of seasonal rainfall-induced ancient landslides remain poorly understood. In this paper, taking the Dandu ancient landslide as an example, field investigations, ring shear experiments, and interferometric synthetic aperture radar (InSAR) deformation monitoring were performed. The cracks in the landslide, formed by fault creeping and seismic activity, provide pathways for rainwater infiltration, ultimately reducing the shear resistance of the slip zone and causing reactivation and deformation of the Dandu landslide. The deformation behavior of landslides is very responsive to seasonal rainfall, with sliding movements beginning to accelerate sharply during the rainy season and decelerating during the dry season. However, this response generally lags by several weeks, indicating that rainfall takes time to infiltrate into the slip zone. These research results could help us better understand the reactivation mechanism of ancient landslides triggered by seasonal rainfall. Furthermore, these findings explain why many slope failures take place in the dry season, which typically occurs approximately a month after the rainy season, rather than in the rainy season itself.