Spatiotemporal Evolution of Ground-Motion Intensity at the Irpinia Near-Fault Observatory, Southern Italy

ABSTRACT Fault zones are major sources of hazard for many populated regions around the world. Earthquakes still occur unanticipated, and research has started to observe fault properties with increasing spatial and temporal resolution, having the goal of detecting signs of stress accumulation and strength weakening that may anticipate the rupture. The common practice is monitoring source parameters retrieved from measurements; however, model dependence and strong uncertainty propagation hamper their usage for small and microearthquakes. Here, we decipher the ground motion (i.e., ground shaking) variability associated with microseismicity detected by dense seismic networks at a near-fault observatory in Irpinia, Southern Italy, and obtain an unprecedentedly sharp picture of the fault properties evolution both in time and space. We discuss the link between the ground-motion intensity and the source parameters of the considered microseismicity, showing a coherent spatial distribution of the ground-motion intensity with that of corner frequency, stress drop, and radiation efficiency. Our analysis reveals that the ground-motion intensity presents an annual cycle in agreement with independent geodetic displacement observations from two Global Navigation Satellite System stations in the area. The temporal and spatial analyses also reveal a heterogeneous behavior of adjacent fault segments in a high seismic risk Italian area. Concerning the temporal evolution of fault properties, we highlight that the fault segment where the 1980 Ms 6.9 Irpinia earthquake nucleated shows changes in the event-specific signature of ground-motion signals since 2013, suggesting changes in their frictional properties. This evidence, combined with complementary information on the earthquake frequency–magnitude distribution, reveals differences in fault segment response to tectonic loading, suggesting rupture scenarios of future moderate and large earthquakes for seismic hazard assessment.

Identification of Ransiki fault segment in South Manokwari Regency, West Papua Province, Indonesia based on analysis of a high-resolution of global gravity field: Implications on the Earthquake Source Parameters

The province of West Papua in Indonesia is an area crossed by three major faults, including Sorong, Koor, and Ransiki, leading to the collision of Australia, the Pacific, and Eurasia. In the past, there have been strong and damaging earthquakes on these faults, manly Ransiki fault in the South Manokwari regency. Identification of the Ransiki fault segment was conducted by geological subsurface modeling using the earth gravity field of the Global Gravity Map (GGM) based on satellite measurements implicates for earthquake source parameters. The GGM is seen as a solution for the unavailability of direct measurements in the region. The gravity field analysis begins with data reduction using SRTM2gravity as modern terrain correction to obtain a complete Bouguer anomaly. Furthermore, the gravity gradient approach through vertical and horizontal gradients, analytical signal, and the tilt angle are applied to emphasize a contact or fault structures that are not visible using a 2D fast Fourier transform. Overall, the gravity gradient analysis obtained results that were compatible with the alignment of the Ransiki fault segment which direction of the northwest to south. The gravity inversion produces a geological subsurface model that clearly shows the Ransiki fault segment, associated with a low rock density distribution, thought to the Befoor formation and quaternary sediments, located between high-density rocks correlated to Arfak volcanic rocks as a basement.

The Mount Hood fault zone, active faulting at the crest of the dynamic Cascade Range, north-central Oregon, USA

ABSTRACT The Mount Hood fault zone is a N-trending, ~55-km-long zone of active faulting along the western margin of the Hood River graben in north-central Oregon. The Mount Hood fault zone occurs along the crest of the Cascade Range and consists of multiple active fault segments. It is presently unclear how much Hood River graben extension is actively accommodated on the fault zone, and how Cascade intra-arc extension accommodates regional patterns of clockwise rotation and northwest translation of crustal blocks in the Pacific Northwest region of the United States. Evidence for Holocene activity on the Mount Hood fault zone was discovered in 2009 after acquisition of high-resolution lidar topography of the area. This trip will visit sites displaying evidence of Holocene surface rupture on fault strands within the Mount Hood fault zone. Day 1 starts with a two-hour drive from Portland to Mount Hood, a 3429-m-high glaciated active volcano, where we will visit sites south of the summit along the Twin Lakes fault segment, including several fault scarps and two sites where dating of offset buried soils constrains the timing of the most recent surface-rupturing event to the Holocene. Day 1 includes two hikes of ~1 km and will be partly cross-country. The trip will overnight at the historic Timberline Lodge, an architectural masterpiece from the Civilian Conservation Corps (1933–1942) era, located at tree line on the southern flank of Mount Hood. Day 2 will visit sites north of the summit, stopping along the Blue Ridge fault segment to view the site of 2011 paleoseismic trenches and an offset glacial moraine. We will visit an unusual uphill-facing scarp in coarse talus along the Gate Creek fault segment near the north end of the Mount Hood fault zone. We will conclude Day 2 with a short hike into the Mark O. Hatfield Wilderness along the Gate Creek fault segment to view evidence of a surface-rupturing earthquake that occurred only a few centuries ago, illuminated by a nearby paleoseismic trench hand-dug in 2020. Our neotectonic and paleoseismic data are among the first efforts to document and characterize seismic sources within the Mount Hood fault zone. However, even with our new age data, fault slip rates and earthquake recurrence remain poorly constrained. With our limited earthquake timing data, it is not clear whether all segments of the Mount Hood fault zone rupture together as a ≥ M 7 earthquake, or alternatively, if the fault segments rupture independently in a sequence of smaller ~M 6–sized events.

The Río Grío–Pancrudo Fault Zone (central Iberian Chain, Spain): recent extensional activity revealed by drainage reversal

The NNW–SSE-trending extensional Río Grío–Pancrudo Fault Zone is a large-scale structure that obliquely cuts the Neogene NW–SE Calatayud Basin. Its negative inversion during the Neogene–Quaternary extension gave rise to structural and geomorphological rearrangement of the basin margin. Geological mapping has allowed two right-relayed fault segments to be distinguished, whose recent extensional activity has been mainly characterized using a deformed planation surface (Fundamental Erosion Surface (FES) 3; 3.5 Ma) as a geomorphic marker. Normal slip along the Río Grío–Lanzuela Fault Segment has induced hanging-wall tilting, subsequent drainage reversal at the Güeimil valley after the Pliocene–Pleistocene transition, as well as morphological scarps and surficial ruptures in Pleistocene materials. In this sector, an offset of FES3 indicates a total throw of c. 240 m, resulting in a slip rate of 0.07 mm a–1, while retrodeformation of hanging-wall tilting affecting a younger piedmont surface allows the calculation of a minimum throw in the range of 140–220 m after the Pliocene–Pleistocene transition, with a minimum slip rate of 0.07–0.11 mm a–1. For the late Pleistocene period, vertical displacement of c. 20 m of a sedimentary level dated to 66.6 ± 6.5 ka yields a slip rate approaching 0.30–0.36 mm a–1. At the Cucalón–Pancrudo Fault Segment, the offset of FES3 allows the calculation of a maximum vertical slip of 300 m for the last 3.5 Ma, and hence a net slip rate close to 0.09 mm a–1. Totalling c. 88 km in length, the Río Grío–Pancrudo Fault Zone could be the largest recent macrostructure in the Iberian Chain, probably active, with the corresponding undeniable seismogenic potential.

Seismic and Geodetic Imaging (DInSAR) Investigation of the March 2021 Strong Earthquake Sequence in Thessaly, Central Greece

Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms and of InSAR data, and moment tensor solutions to produce three fault models. The first shock (Mw = 6.3) occurred in a fault segment of strike 314° and dip NE41°. It caused surface subsidence −40 cm and seismic slip 1.2–1.5 m at depth ~10 km. The second earthquake (Mw = 6.2) occurred to the NW on an antithetic subparallel fault segment (strike 123°, dip SW44°). Seismic slip of 1.2 m occurred at depth of ~7 km, while surface subsidence −10 cm was determined. Possibly the same fault was ruptured further to the NW on 12 March (Mw = 5.7, strike 112°, dip SSW42°) that caused ground subsidence −5 cm and seismic slip of 1.0 m at depth ~10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which forms a graben-like structure in the area of northwest Thessaly.

Nonlinear fault damage zone scaling revealed through analog modeling

Fault damage zones strongly influence fluid flow and seismogenic behavior of faults and are thought to scale linearly with fault displacement until reaching a threshold thickness. Using analog modeling with different frictional layer thicknesses, we investigate damage zone dynamic evolution during normal fault growth. We show that experimental damage zone growth with displacement is not linear but progressively tends toward a threshold thickness, being larger in the thicker models. This threshold thickness increases significantly at fault segment relay zones. As the thickness threshold is approached, the failure mode progressively transitions from dilational shear to isochoric shear. This process affects the whole layer thickness and develops as a consequence of fault segment linkage as inferred in nature when the fault matures. These findings suggest that fault damage zone widths are limited both by different scales of mechanical unit thickness and the evolution of failure modes, ultimately controlled in nature by lithology and deformation conditions.

The Traverse Ridge Paleoseismic Site and Ruptures Crossing the Boundary Between the Provo and Salt Lake City Segments of the Wasatch Fault Zone, Utah, United States

How structural segment boundaries modulate earthquake behavior is an important scientific and societal question, especially for the Wasatch fault zone (WFZ) where urban areas lie along multiple fault segments. The extent to which segment boundaries arrest ruptures, host moderate magnitude earthquakes, or transmit ruptures to adjacent fault segments is critical for understanding seismic hazard. To help address this outstanding issue, we conducted a paleoseismic investigation at the Traverse Ridge paleoseismic site (TR site) along the ∼7-km-long Fort Canyon segment boundary, which links the Provo (59 km) and Salt Lake City (40 km) segments of the WFZ. At the TR site, we logged two trenches which were cut across sub-parallel traces of the fault, separated by ∼175 m. Evidence from these exposures leads us to infer that at least 3 to 4 earthquakes have ruptured across the segment boundary in the Holocene. Radiocarbon dating of soil material developed below and above fault scarp colluvial packages and within a filled fissure constrains the age of the events. The most recent event ruptured the southern fault trace between 0.2 and 0.4 ka, the penultimate event ruptured the northern fault trace between 0.6 and 3.4 ka, and two prior events occurred between 1.4 and 6.2 ka (on the southern fault trace) and 7.2 and 8.1 ka (northern fault trace). Colluvial wedge heights of these events ranged from 0.7 to 1.2 m, indicating the segment boundary experiences surface ruptures with more than 1 m of vertical displacement. Given these estimates, we infer that these events were greater than Mw 6.7, with rupture extending across the entire segment boundary and portions of one or both adjacent fault segments. The Holocene recurrence of events at the TR site is lower than the closest paleoseismic sites at the adjacent fault segment endpoints. The contrasts in recurrence rates observed within 15 km of the Fort Canyon fault segment boundary may be explained conceptually by a leaky segment boundary model which permits spillover events, ruptures centered on the segment boundary, and segmented ruptures. The TR site demonstrates the utility of paleoseismology within segment boundaries which, through corroboration of displacement data, can demonstrate rupture connectivity between fault segments and test the validity of rupture models.