Heterogeneity in Microseismicity and Stress Near Rupture‐Limiting Section Boundaries Along the Late‐Interseismic Alpine Fault

Abstract

Paleoseismic evidence from the late‐interseismic Alpine Fault suggests key section boundaries conditionally inhibit rupture. We utilize a year of data from a two‐part seismometer network (Dense Westland Arrays Researching Fault Segmentation) to characterize ∼7,500 earthquakes (−0.7 ≤ MLv ≤ 4.2) and ∼800 focal mechanisms, producing high‐resolution structural images of these boundaries to study effects of material and structural heterogeneities on mode‐switching rupture behavior. Lithologically‐controlled frictional behavior and crustal strength appear to influence lateral and vertical on‐fault seismicity distributions. Ultramafic hanging‐wall serpentinite and serpentinite‐related fault core minerals along the South Westland (SW) boundary, result in a locally shallow seismogenic cuttoff (∼8 km) and abundant on‐fault seismicity. Maximum horizontal compressive stress rotations (14° anti‐clockwise and 20° clockwise near the SW and North Westland (NW) boundaries, respectively, relative to the Central Section), coupled with spatially variable fault frictional properties, are more important than geometry alone in controlling Sections' relative frictional stability. Whereas the SW and Central Sections are well‐oriented for failure, the NW Section is severely misoriented compared with favorably oriented faults of the Marlborough Fault Zone, which possibly facilitate a preferred rupture route. Geometrically, a 40° dip change at the SW boundary may be accommodated either by a single through‐going fault plane ‐ a difficult geometry across which to obtain multi‐segment earthquakes when considering rupture dynamics ‐ or by a deeper vertical fault strand truncated by a shallower listric plane. Our new observations have implications for Alpine Fault rupture scenarios and highlight the need to consider a range of spatially heterogeneous, interdependent physical factors when evaluating controls on rupture segmentation.

Oblique Blind Faulting Underneath the Luzon Volcanic Arc During the 2022 Mw 7.0 Abra Earthquake, the Philippines

“…The estimated seismogenic layer is thicker than most other continental strike-slip fault systems (Donzé et al, 2021). For instance, the seismogenic thickness is ∼15 km surrounding the San Andreas fault (Hauksson & Meier, 2019;Zuza & Cao, 2020), ∼15 km for north-central Anatolia (Yolsal-Çevikbilen et al, 2012), and ∼8 km for the Alpine fault (Warren-Smith et al, 2022). Hsu et al (2016) used GNSS velocities and a fault locking depth of 20 km to assess the seismic potential of inland faults in Luzon ranging from M w 6.9 to 7.6 over a centennial recurrence interval.…”

“…The estimated seismogenic layer is thicker than most other continental strike‐slip fault systems (Donzé et al., 2021). For instance, the seismogenic thickness is ∼15 km surrounding the San Andreas fault (Hauksson & Meier, 2019; Zuza & Cao, 2020), ∼15 km for north‐central Anatolia (Yolsal‐Çevikbilen et al., 2012), and ∼8 km for the Alpine fault (Warren‐Smith et al., 2022). Hsu et al.…”

Parametric Testing of EQTransformer’s Performance against a High-Quality, Manually Picked Catalog for Reliable and Accurate Seismic Phase Picking

“…The Southern Alps of New Zealand and the Alpine fault that bounds them have been the focus of several recent microearthquake studies using temporary networks (e.g., Boese et al, 2012;Bourguignon et al, 2015;Michailos et al, 2019;Warren-Smith et al, 2022;and references therein). The Southern Alps Micro-earthquake Borehole Array (SAMBA; Boese et al, 2012) is one such network adjacent to the central Alpine fault.…”

“…In this workflow, reliable earthquake detection and accurate phase arrival estimation are essential for obtaining an accurate catalog of earthquake hypocenters. Comprehensive seismicity catalogs provide first-order constraints on the geometry, segmentation, seismogenic extent, and kinematics of active faults (e.g., Waldhauser and Schaff, 2008;Shelly et al, 2015;Woo et al, 2019;Liu et al, 2022;Warren-Smith et al, 2022), and thus on their mechanical behavior and the hazards they pose.…”

Synthesis of Current Seismicity and Tectonics Along the 1857 Mw7.9 Fort Tejon Earthquake Rupture and the Southernmost San Andreas Fault, California, USA

“…Seismicity quiescence was reported to have started a decade prior to the 1999 M w 7.4 Izmit, Turkey earthquake that ruptured the North Anatolian fault for a distance of 125 km (Bulut, 2015). The more than 450 km long New Zealand Alpine fault that last ruptured in 1717 also exhibits complex seismicity patterns due to the presence of geometrical complexities as well as along some parts, the presence of serpentinite‐type fault core minerals (Warren‐Smith et al., 2022). In particular, the central section of the Alpine fault exhibits relative seismic quiescence in its fault core flanked by nearby extended‐damage zone seismicity as compared with the south and north sections that are more active and may accommodate minor seismicity in the fault core (Warren‐Smith et al., 2022).…”