Fault slip rate is one of the most crucial parameters to characterize earthquake occurrence in fault‐based seismic hazard assessments (SHA). Accordingly, paleoseismic studies have increasingly ...focused on constraining this parameter in active faults worldwide. We present a comprehensive paleoseismic study in the Alhama de Murcia Fault (AMF), one of the most active faults in SE Spain and source of destructing earthquakes such as the 2011 Mw 5.2 Lorca event. Contrasting with previous studies, we integrate paleoseismic data from four fault strands in the AMF and, based on trench slip analysis and numerical dates, we derive slip rate estimates of each strand over the whole transect and assess their time variability. The AMF has a minimum net slip rate between 1.35+0.16/−0.10 and 1.64+0.16/−0.11 mm/yr for the past 18 ± 1 to 15.2 ± 1.1 ka. These results prove the importance of accounting for the complete sections of a geological structure as they are almost twice the previous estimates for a single fault branch. Slip rate variability is identified in the AMF, with cyclic acceleration‐quiescence patterns that could be related to stress field changes driven by fault interaction or synchronicity with neighboring faults (e.g., Carrascoy). We hope that the data presented here motivates their inclusion into forthcoming fault‐based SHAs. In this regard, limitations related to the lack of paleoseismic data for one fault strand, along with poor characterization of the strike component of slip and insufficient age control of the units for another strand are highlighted and need to be accounted for by modelers.
Key Points
Multi‐site paleoseismic surveys allow to improve the estimations of the geological slip rates in the Alhama de Murcia Fault (AMF)
The AMF shows a geological slip rate between 1.35 and 1.64 mm/yr for the past 18–15 ka
Late Quaternary slip rate fluctuations suggest strain rate variations and highlight possible synchronicities with neighboring faults
Fault slip rates inform models of strain accumulation and release, which over geologic time may vary or remain constant depending on factors like structural complexity, fault strength, deformation ...rates, and proximity to other faults. In this study, we present a Late Pleistocene–Holocene slip history based on four new geologic slip rates for the Agua Blanca Fault (ABF), which transfers Pacific‐North American dextral plate boundary motion across the Peninsular Ranges of northern Baja California. Time‐averaged slip rates from three sites are 2.8 + 0.8/−0.6 mm/a since ~65.1 ka, 3.0 + 1.4/−0.8 mm/a since ~21.8 ka, 3.2 + 1.0/−0.6 mm/a since ~12.5 ka, and 3.5 + 5.1/−2.0 mm/a since ~1.4 ka; however, the actual slip rate may be closer to 4 mm/a when off‐fault slip and age interpretation uncertainties are considered. Significantly, although the ABF has more in common in terms of length, net offset, and slip rate with known variable slip rate faults, the most straightforward age and offset interpretations for the ABF suggest constant slip rates over ~10 kyr time scales. As with other constant slip rate faults, comparable neighboring faults that might modulate the ABF slip rate are absent, suggesting that fault interaction, or lack thereof, may be a more significant factor controlling fault behavior on this and potentially other faults. The new rates indicate that the ABF accommodates at least half of total slip across the Peninsular Ranges, clarifying strain partitioning for seismic forecasting models that previously lacked modern geologic slip rate constraints for this domain of the plate boundary.
Key Points
New geologic slip rates clarify strain partitioning and seismic hazard within the southernmost Pacific‐North American plate boundary system
Four slip rates from three sites document time‐invariant rates of ~3–4 mm/a since ~65, 22, 12.5, and 1.4 ka
The absence of parallel faults and thus of appreciable fault interaction may be the key factor holding slip rates constant
The Black Mango fault is a structural discontinuity that transforms motion between two segments of the active Himalayan Frontal Thrust (HFT) in northwestern India. The Black Mango fault displays ...evidence of two large surface rupture earthquakes during the past 650 years, subsequent to 1294 A.D. and 1423 A.D., and possibly another rupture at about 260 A.D. Displacement during the last two earthquakes was at minimum 4.6 meters and 2.4 to 4.0 meters, respectively, and possibly larger for the 260 A.D. event. Abandoned terraces of the adjacent Markanda River record uplift due to slip on the underlying HFT of 4.8 ± 0.9 millimeters per year or greater since the mid-Holocene. The uplift rate is equivalent to rates of fault slip and crustal shortening of 9.6-3.5
+7.0millimeters per year and 8.4-3.6
+7.3millimeters per year, respectively, when it is assumed that the HFT dips 30° ± 10°.
The east and west rupture directions of the 1943 and 1944 earthquakes on the North Anatolian Fault (NAF) are hypothesized to represent, respectively, long term preferred propagation directions on the ...corresponding sections of the NAF. Fault sections with preferred rupture direction are expected to have an asymmetric damage structure with respect to the slipping zone. To test the above hypothesis, we study geological and geomorphologic manifestations of structural asymmetry with respect to the active trace of the NAF along the 1943 and 1944 sections. The following fault zone elements are mapped: gouge fabric in the cm scale, fault core structure in a metre scale, and secondary faults and fault rocks in tens of metres scale. Mapping results at three sites on the 1943 rupture and one site on the 1944 rupture are consistent with accumulation of more rock damage on the south side of the 1943 section and on the north side of the 1944 section. Erosion patterns adjacent to the fault that are not correlated with the distribution of intrinsic and extrinsic erosion-controlling variables (e.g. rock type) are interpreted as morphologic responses to the damage content of rocks and its impact on rock erodibility. The valleys of 11 rivers are parallel to the studied fault sections. About 75 per cent of the total river valleys length along the 1943 rupture is on the south side of the fault, and about 89 per cent of the total river valleys length along the 1944 rupture is on the north side of the fault. Morphometric analysis of watersheds in two correlative terrains displaced along the 1944 rupture section shows that stream erosion is considerably more intense in the terrain north of the fault, with drainage density values almost double in the north compare to the south. Badland topography at two sites along the 1943 rupture section is substantially more developed at the ∼100 m scale on the south side of fault. Our observations along the 1943–1944 rupture sections, including various types of signals that span a large range of scales, are systematically compatible with an opposite sense of damage asymmetry between the two fault sections. These observations are consistent with opposite preferred direction of ruptures for the two sections, similar to the propagation directions of the two recent earthquakes. If those rupture directions are dictated by the velocity structure at depth, we infer that the south side of the 1943 rupture has faster seismic velocity at seismogenic depth than the north side, and that the sense of velocity contrast is reversed along the 1944 rupture zone.
In this paper we investigate the dynamic behavior of a system of interconnected faults in the Brawley Seismic Zone (BSZ) in southern California. The system of faults includes the southern San Andreas ...Fault (SSAF), the Imperial Fault (IF), and a set of cross faults in the BSZ that may serve as connecting structures between the two larger faults. Geological and seismic evidence imply that the SSAF and IF may have buried extensions that link them together in a large‐scale step over, with the cross faults in the BSZ cutting between them. Such a configuration poses the question of whether through‐going rupture across the step over is possible in this region, leading to large, plate‐boundary scale earthquakes. We investigate potential earthquakes in this region through 3‐D dynamic finite element spontaneous rupture modeling. We find that under multiple assumptions about fault stress and fault geometry, through‐going rupture is possible, both from north to south and south to north. Participation of the cross faults is facilitated by two factors: absence of rupture on one of the main two faults and a contrast in prestress between the main faults and the cross faults, leading to slow propagation speed on the main faults while maintaining ease of failure on the cross faults. The pattern of rupture propagation and slip is strongly affected by fault‐to‐fault dynamic stress interactions during the rupture process. The results may have implications for both potential earthquakes in this region, as well as for understanding the dynamics of geometrically complex/branched faults in general.
Key Points
Based on our current assumptions about fault geometry and prestress, through‐going rupture might be possible in the Brawley Seismic Zone
Slip on the cross faults can hinder rupture propagation on the main faults
Cross‐fault rupture appears to be facilitated by a contrast in stress initial conditions with the main fault segments
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