Seismica

Statistical distribution of static stress resolved onto geometrically-rough faults

2024-05-03T02:16:07-04:00

The in-situ stress state within fault zones is technically challenging to characterize, requiring the use of indirect methods to estimate. Most work to date has focused on understanding average properties of resolved stress on faults, but fault non-planarity should induce spatial variations in resolved static stress on a single fault. Assuming a particular stochastic model for fault geometry (band-limited fractal) gives an approximate analytic solution for the probability density function (PDF) on fault stress that depends on the mean fault orientation, mean stress ratio, and roughness level. The mean stress is shown to be equal to the planar fault value, while deviations are described by substituting a second-order polynomial expansion of the stress ratio into the inverse distribution on fault slope. The result is an analytical expression for the PDF of shear-to-normal stress ratio on 2-D rough faults in a uniform background stress field. Two end-member distributions exist, one approximately Gaussian when all points on the fault are well away from failure, and one reverse exponential, which occurs when the mean stress ratio approaches the peak. For the range of roughness values expected to apply to crustal faults, stress deviations due to geometry can reach nearly 100% of the background stress level. Consideration of such a distribution of stress on faults suggests that geometric roughness and the resulting stress deviations may play a key role in controlling earthquake behavior.

Seismic characteristics of the 2022-2023 unrest episode at Taupō volcano, Aotearoa New Zealand

2024-07-15T10:22:40-04:00

Taupō is a large caldera volcano located beneath a lake in the centre of the North Island of New Zealand and most recently erupted ~1800 years ago. The volcano has experienced at least 16 periods of unrest since 1872, each of which were characterised by increased seismic activity. Here we detail seismic activity during the most recent period of unrest from May 2022 to May 2023. The unrest was notable for the highest number of earthquakes detected during instrumented unrest episodes, and for one of the largest magnitude earthquakes detected beneath the lake for at least 50 years (M_L 5.7). Relocated earthquakes indicate seismic activity was focused around an area hosting overlapping caldera structures and a hydrothermal system. Moment tensor inversion for the largest earthquake includes a non-negligible inflationary isotropic component. We suggest the seismic unrest was caused by the reactivation of faults due to an intrusion of magma at depth.

Refined Holocene Slip Rate for the Western and Central Segments of the Garlock Fault: Record of Alternating Millennial-Scale Periods of Fast and Slow Fault Slip

2024-07-05T08:08:17-04:00

We use lidar- and field-based mapping coupled with single-grain infrared-stimulated luminescence dating to constrain three new slip rate estimates from the western and central segments of the Garlock fault in southern California, revealing a more complete picture of incremental slip rate in time and space for this major plate-boundary fault. These new rates reinforce and refine previous evidence showing that the Garlock fault experiences significant temporal variations in slip rates that span multiple earthquake cycles, with multi-millennial periods of very fast (13-14 mm/yr) early and late Holocene slip separated by a mid-Holocene period of slow slip (3 mm/yr). Similar ca. 8 ka slip rates for the central Garlock fault of 8.8 ± 1.0 mm/yr and 8.2 +1.0/-0.8 mm/yr for the western Garlock fault demonstrate that the fault has slipped at a faster long-term average rate than suggested by previous studies. These fast rates are consistent with kinematic models in which the western and central Garlock fault segments are driven primarily by lateral extrusion associated with N-S contractional shortening, with additional slip driven by WNW-ENE Basin and Range extension north of the fault and minor rotation of the Garlock within the N-S zone of dextral ECSZ shear.

An exploration of potentially reversible controls on millennial-scale variations in the slip rate of seismogenic faults: Linking structural observations with variable earthquake recurrence patterns

2024-01-19T05:12:13-05:00

Paleoseismic studies show that faults within a fault system may trade off slip over time, with mechanically complementary faults displaying alternating fast- and slow periods. Each of these periods spans multiple seismic cycles, and typically involves ~20-25m of slip. This suggests that the relative strength (or tendency to slip) of individual faults varies, over time and displacement scales larger than those of individual seismic cycles. The mechanisms responsible for these strength variations must: affect rocks in the strongest portion of the fault (the brittle-ductile transition) as this likely controls the overall slip rate of the fault; be reversible (or able to be counteracted) on a cyclical basis; provide a negative feedback that operates to change the fault from its current state; and have a measurable effect on fault strength over a time or length scale that corresponds to the observed fast and slow periods of fault slip. In this paper, we systematically explore 19 potentially weakening and 11 potential strengthening mechanisms and evaluate them in light of these criteria. This analysis reveals a relatively small subset of mechanisms that could account for the observed behavior, leading us to suggest a possible model for fault strength evolution.

Investigation of suspected Holocene fault scarp near Montréal, Québec: The first paleoseismic trench in eastern Canada

2024-07-25T09:16:05-04:00

Québec has experienced historical damaging earthquakes in several seismic zones (e.g. 1732 M5.8 Montréal, 1663 M7 Charlevoix, 1935 M6.2 Témiscamingue). Despite a high seismicity rate, no surface-rupturing faults have been discovered due to a combination of dense vegetation cover, recent glaciation, sparse earthquake records, and low regional strain rates. We manually searched lidar-derived digital elevation models (DEMs) of the region to search for potential post-glacial surface-rupturing faults across southern Québec and identified a scarp ~50km north of Montréal. We performed three geophysical surveys (ground penetrating radar, depth estimates from ambient seismic noise, and refraction seismology) that revealed a buried scarp, confirmed with a <1 m-deep hand-dug test pit. These observations convinced us to excavate the first paleoseismic trench in Québec to test for the presence of a surface-rupturing fault in July 2023. We found a glacial diamict containing no signs of syn- or post-glacial deformation. In this paper, we present the observations that led to the identification of a scarp and hypothesized faulting. We highlight the importance of trenching to confirm recent fault scarps in challenging environments. We hope our study can be used to optimize future paleoseismic research in the province of Québec and similar intracratonic glaciated landscapes.

Earthquake source inversion by integrated fiber-optic sensing

2024-07-22T09:40:26-04:00

We present an earthquake source inversion using a single time series produced by integrated fiber-optic sensing in a phase noise cancellation (PNC) system used for frequency metrology. Operating on a 123 km long fiber between Bern and Basel (Switzerland), the PNC system recorded the Mw3.9 Mulhouse earthquake that occurred on 10 September 2022 around 10 km north-west of the northern fiber end. A generalised least-squares inversion in the 4 - 13 s period band constrains the components of a double-couple moment tensor with an uncertainty that corresponds to around 0.2 moment magnitude units, nearly independent of prior information. Uncertainties for hypocenter location and original time are more variable, ranging between 4 - 20 km and 0.1 - 1 s, respectively, depending on whether injected prior information is realistic or almost absent. This work is a proof of concept that quantifies the resolvability of earthquake source properties under specific conditions using a single-channel stand-alone integrated (non-distributed) fiber-optic measurement. It thereby constitutes a step towards the integration of long-range phase-transmission fiber-optic sensors into existing seismic networks in order to fill significant seismic data gaps, especially in the oceans.