Seismica

Population displacement after earthquakes: benchmarking predictions based on housing damage

Nicole Paul, Carmine Galasso, Vitor Silva, Jack Baker — Mon, 28 Oct 2024 00:00:00 -0400

In the aftermath of an earthquake, the number of residents whose housing was destroyed is often used to approximate the number of people displaced (i.e., rendered homeless) after the event. While this metric can provide rapid situational awareness regarding potential long-term housing needs, more recent research highlights the importance of additional factors beyond housing damage within the scope of household displacement and return (e.g., utility disruption, tenure, place attachment). This study benchmarks population displacement estimates using this simplified conventional approach (i.e., only considering housing destruction) through three scenario models for recent earthquakes in Haiti, Japan, and Nepal. These model predictions are compared with officially reported values and alternate mobile location data-based estimates from the literature. The results highlight the promise of scenario models to realistically estimate population displacement and potential long-term housing needs after earthquakes, but also highlight a large range of uncertainty in the predicted values. Furthermore, purely basing displacement estimates on housing damage offers no view on how the displaced population counts vary with time as compared to more comprehensive models that include other factors influencing population return or alternative approaches, such as using mobile location data.

Performance of synthetic DAS as a function of array geometry

Thomas Luckie, Robert Porritt — Thu, 31 Oct 2024 00:00:00 -0400

Distributed Acoustic Sensing (DAS) can record acoustic wavefields at high sampling rates and with dense spatial resolution difficult to achieve with seismometers. Using optical scattering induced by cable deformation, DAS can record strain fields with ones of meters spatial resolution. However, many experiments utilizing DAS have relied on unused, dark telecommunication fibers. As a result, the geophysical community has not fully explored DAS survey parameters to characterize the ideal array design. This limits our understanding of guiding principles in array design to deploy DAS effectively and efficiently in the field. A better quantitative understanding of DAS array behavior can help improve the quality of the data recorded by guiding the DAS array design. Here we use array response functions as well as beamforming and back-projection results from forward modelling calculations to assess the performance of varying DAS array geometries to record regional and local sources. A regular heptagon DAS array demonstrated improved capabilities for recording regional sources over segmented linear arrays, with potential improvements in recording and locating local sources. These results reveal DAS array performance as a function of geometry and can guide future DAS deployments.

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

Tarryn Cawood, James Dolan — Mon, 22 Jul 2024 00:00:00 -0400

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.

Earthquake Moment Magnitudes from Peak Ground Displacements and Synthetic Green's Functions

Torsten Dahm, Daniela Kühn, Simone Cesca, Marius Isken, Sebastian Heimann — Thu, 19 Dec 2024 00:00:00 -0500

We suggest an approach employing full waveforms from synthetic seismograms to estimate moment magnitudes and their uncertainties from peak amplitudes. The new method is theoretically derived. It does not change the established routines of traditional procedures for magnitude determination, while overcoming some of the limitations such as saturation, scattering and source complexity. Attenuation functions are derived on-the-fly for each source-station combination from synthetic seismograms using Green's function databases representing various velocity models if required. In a bootstrap approach, source depth, geometry, dynamic and kinematic parameters are randomly selected within a realistic range. After calibration with observations, attenuation functions can be extrapolated to distances, depths, regions and magnitudes for which no observations exist. Additionally, individual frequency filters and sensor types can be mixed independently of any definition of traditional magnitude scales. Uncertainties of attenuation functions are estimated for every source-station geometry including the sensor characteristics and its potential frequency saturation. Therefore, realistic uncertainties of mean magnitudes can be estimated even in case of only few measurements. The method is especially useful to estimate local and moment magnitudes for temporary deployments or for monitoring induced seismicity in regions with only few tectonic events.

Heterogeneous high frequency seismic radiation from complex ruptures

Sara Cebry, Greg McLaskey — Tue, 17 Sep 2024 00:00:00 -0400

Fault geometric heterogeneities such as roughness, stepovers, or other irregularities are known to affect the spectra of radiated waves during an earthquake. To investigate the effect of normal stress heterogeneity on radiated spectra, we utilized a poly(methyl methacrylate) (PMMA) laboratory fault with a single, localized bump. By varying the normal stress on the bump and the fault-average normal stress, we produced earthquake-like ruptures that ranged from smooth, continuous ruptures to complex ruptures with variable rupture propagation velocity, slip distribution, and stress drop. High prominence bumps produced complex events that radiated more high frequency energy, relative to low frequency energy, than continuous events without a bump. In complex ruptures, the high frequency energy showed significant spatial variation correlated with heterogeneous peak slip rate and maximum local stress drop caused by the bump. Continuous ruptures emitted spatially uniform bursts of high frequency energy. Near-field peak ground acceleration (PGA) measurements of complex ruptures show nearly an order-of-magnitude higher PGA near the bump than elsewhere. We propose that for natural faults, geometric heterogeneities may be a plausible explanation for commonly observed order-of-magnitude variations in near-fault PGA.

Putting faults in the northern Chilean subduction margin into motion: evidence for remote dynamic earthquake triggering on the plate interface and within the forearc

Rebecca Harrington, Debi Kilb, Marco Roth, Pia Victor, Alessandro Verdecchia — Wed, 06 Nov 2024 00:00:00 -0500

Dynamic stresses on the order of ~1 kPa from passing waves of mainshock earthquakes can trigger aftershocks at remote distances. Here, we investigate the prevalence of remote earthquake triggering in northern Chile, where aseismic-slip triggering has been documented. Our twofold approach to quantify triggerability includes a statistical difference-of-means test to quantify seismicity-rate changes bracketing candidate mainshock times, and a waveform-based approach to look for triggered earthquakes missing from the local catalog. We find no persistent, statistically-significant seismicity-rate increases associated with any of the candidate mainshocks when considering the local catalog in aggregate. However, catalog statistics reveal evidence for localized triggering both on the subduction interface and within the shallower forearc faults. Waveforms reveal local, uncataloged earthquakes only visible using a high-pass filter that removes the mainshock signal that otherwise overprints the local signals. Based on Japan mainshocks, we cannot rule out antipodal triggering. Areas showing higher triggerability are consistent with regions of low locking inferred from GNSS models and regions of observed aseismic slip. The spatial coincidence of triggering and low-locking, combined with the absence of a stress-triggering threshold, requires non-linear triggering mechanisms, such as altered frictional strength or aseismic-slip triggering, to be consistent with the observations.

The Pattern of Earthquake Magnitude Clustering Based on Interevent Distance and Time

Derreck Gossett, Michael R. Brudzinski, Qiquan Xiong, Jesse C. Hampton — Thu, 29 Aug 2024 00:00:00 -0400

The clustering of earthquake magnitudes is poorly understood compared to spatial and temporal clustering. Better understanding of correlations between earthquake magnitudes could provide insight into the mechanisms of earthquake rupture and fault interactions, and improve earthquake forecasting models. In this study we present a novel method of examining how seismic magnitude clustering occurs beyond the next event in the catalog and evolves with time and space between earthquake events. We first evaluate the clustering signature over time and space using double-difference located catalogs from Southern and Northern California. The strength of magnitude clustering appears to decay linearly with distance between events and logarithmically with time. The signature persists for longer distances (more than 50km) and times (several days) than previously thought, indicating that magnitude clustering is not driven solely by repeated rupture of an identical fault patch or Omori aftershock processes. The decay patterns occur in all magnitude ranges of the catalog and are demonstrated across multiple methodologies of study. These patterns are also shown to be present in laboratory rock fracture catalogs but absent in ETAS synthetic catalogs. Incorporating magnitude clustering decay patterns into earthquake forecasting models such as ETAS could improve their accuracy.

Spatiotemporal Variability of Fin Whale and Blue Whale Calls Detected by Land Seismometers Along the Lower St. Lawrence Seaway

Mon, 16 Sep 2024 00:00:00 -0400

The Lower St. Lawrence Seaway (LSLS) is critical to Canada’s economy both as part of a major marine shipping corridor and a site of intensive fishing. Every year, fin whales and blue whales frequent the LSLS feeding ground. Understanding the mechanisms driving whale habitat usage is key for making informed decisions on shipping and fishing, reducing whale collision risks and mitigating noise pollution. We detect whales in the LSLS with land seismometers by using a method that relies on the intervals of the regularly repeating low frequency calls. The resulting catalogue contains 14,076 fin whale detections and 3,739 blue whale detections between February 2020 and January 2022. These detections follow the overall pattern of hydrophones, with most detections from fall to early winter in the Estuary and until mid-winter/spring in the Gulf. High detection rates in the Northwest Gulf throughout the winter months demonstrate that this region is potentially utilized year-round. This labelled catalogue may be suitable for developing a deep learning-based whale call detection algorithm. Making use of seismometers and deep learning can increase whale monitoring coverage within the LSLS and elsewhere.

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

Dannielle Fougere, James Dolan, Edward Rhodes, Sally McGill — Fri, 05 Jul 2024 00:00:00 -0400

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.

Along-strike extent of earthquakes on multi-segment reverse faults; insights from the Nevis-Cardrona Fault, Aotearoa New Zealand

Thu, 31 Oct 2024 00:00:00 -0400

Evaluating fault segmentation is important for our understanding of seismic hazard assessment and fault growth. However, it is still unclear what controls if reverse fault earthquakes will rupture across segment boundaries. Here, we combine fault mapping and trench data from the low slip rate (0.04-0.15 mm/yr) multi-segment Nevis-Cardrona Fault (NCF) in the South Island of Aotearoa New Zealand to assess if it has ruptured in single or multi-segment earthquakes during the late Quaternary. Two new trenches on its Nevis segment provide stratigraphic evidence for two surface rupturing earthquakes, which through Optically Stimulated Luminscence dating and OxCal modelling, are constrained to have occurred at 28.9 +12.9 -9.1 ka and 12.8 ± 4.9 ka. The most recent timing is only weakly correlated to surface rupture timings from two trenches along the NCF's NW Cardrona segment. Furthermore, the 2 ± 1 m Nevis segment single event displacements we estimate would be unusually low for a ~85 km long NCF multi-segment rupture. We therefore surmise that late Quaternary NCF surface rupturing earthquakes did not rupture through ~30-50° bends that link these segments. Our trench data and fault mapping also indicate lower slip rates on the Nevis segment than previous studies (0.04-0.1 mm/yr vs 0.4 mm/yr).

An Open Source Hydroacoustic Benchmarking Framework for Geophonic Signal Detection

Mon, 14 Oct 2024 00:00:00 -0400

Passive hydroacoustic studies have underscored the efficiency and relevance of deploying autonomous hydrophones for the surveillance of underwater geophony. In particular, monitoring networks have been deployed for detecting SOFAR-propagating hydroacoustic waves generated by seismic events and locating their sources. The technique has been extended to study other hydroacoustic signals, such as P-waves from teleseismic events or impulsive waves generated by sea water-lava interactions. A significant challenge in this endeavor lies in the time required for the manual detection and annotation of these signals in long-term records. To address this issue, we tested the feasibility of implementing automated algorithms based on machine learning to detect and identify these various signals, and obtained satisfying classification and time picking accuracies. We incorporated those models in a benchmarking framework, proposing a training dataset, two evaluation datasets, two tasks to solve and the evaluations of the mentionned models on them. The goal of this framework is to foster the development of new models in the community, as it gives a clear way to evaluate them.

Insolation Cycles Control the Timing and Pattern of Resonance Frequency Drifts at a Natural Rock Tower, Utah, USA

Jeffrey Moore, Erin Jensen, Brendon Quirk, Guglielmo Grechi, Alex Dzubay — Mon, 25 Nov 2024 00:00:00 -0500

Resonance frequency monitoring can detect structural changes during progressive rock slope failure; however, reversible environmentally-driven frequency drifts may inhibit identification of permanent changes. Frequency drifts are commonly correlated with air temperature, lagging temperature changes by zero to 35–60 days. Here we report observations from two years of monitoring at a rock tower in Utah, USA where annual resonance frequency changes appear to precede air temperature cycles by ~35 days. Using correlations with meteorological data supplemented by numerical modeling, we identify changes in insolation as the primary driver of annual frequency drifts, giving rise to the negative lag time. Sparse in-situ insolation data show that the daily frequency increase lags sunrise by several hours, while frequencies decrease at sunset, responses we attribute to the west facing aspect of the tower. Modeled daily insolation patterns match frequency data for months when in-situ measurements are not available. Numerical models offer the advantage of predicting insolation patterns for different aspects of the rock tower, such as the west facing cliff where measurements would be challenging. Our study highlights the value of long-term datasets in identifying mechanisms driving environmentally associated frequency drifts, understanding that is crucial to facilitate detection of permanent changes during progressive failure.

Sparse fault representation based on moment tensor interpolation

Julien Thurin — Mon, 09 Dec 2024 00:00:00 -0500

Accurate representation of large earthquake sources is required for understanding rupture dynamics and improving seismic hazard assessments. While capable of capturing complex spatio-temporal slip scenarios, traditional finite-fault models often suffer from over-parameterization, require strong regularization, and pose significant computational challenges, especially in rapid-response scenarios. Conversely, multiple point source (MPS) models reduce the rupture as a sequence of point sources but are inadequate to simulate short-period wavefield and static displacement. We introduce a hybrid source representation that leverages moment tensor interpolation to bridge the gap between these models. By treating moment tensors as "key" centroids of a tensor field, we construct geometrically coherent slip models that retain the spatial complexity of finite-fault models while maintaining MPS's computational efficiency and simplicity. Our method extends existing 2D tensor-field reconstruction techniques to moment tensors, allowing source-type-preserving interpolation and enabling sparse model approximation and source upscaling for numerical simulations. We demonstrate how our approach can benefit both the inverse and forward problems on the January 2024 Noto earthquake, computing a sparse approximation of the USGS NEIC source model with fewer than ten key tensors and computing full wavefield and static deformation from upscaled source distributions in a realistic 3D regional tomographic model using spectral-elements method.

Seismic site conditions of RESNOM network

Tue, 22 Oct 2024 00:00:00 -0400

The Northwest Seismic Network of Mexico (RESNOM) is operated by personnel from the Center for Scientific Research and Higher Education of Ensenada, Baja California (CICESE), which supervises station installation, improvement, and maintenance. We employed seismic noise and the Horizontal to Vertical Spectral Ratio (HVSR) method to determine, for each station, the following site condition parameters: the depth of the rock layer (H_{eng_bed}), and the geotechnical parameter V_S30, obtained from 1D shear wave velocity models. Other parameters as the fundamental frequency (f₀) and the average amplitude at the fundamental frequency (A₀) were also estimated. Our results show clear differences between the values obtained for the Mexicali Valley and the Peninsular ranges regions. The V_S30 obtained for stations of the Mexicali Valley region falls in the range from 173 m/s to 535 m/s, while for the Peninsular Ranges region is between 213 m/s and 958 m/s. Regarding the H_{eng_bed} parameter, the values are similar between both regions, from 23 m to 850 m for the Peninsular and from 42 m to 926 m for the Mexicali Valley. Additionally, from the V_S30 values, we propose the site classification according to the U.S. National Earthquake Hazards Reduction Program (NEHRP).

Statistical distribution of static stress resolved onto geometrically-rough faults

Jeremy Maurer — Tue, 23 Jul 2024 00:00:00 -0400

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.

DeepRFQC: automating quality control for P-wave receiver function analysis using a U-net inspired network

Sina Sabermahani, Andrew Frederiksen — Tue, 12 Nov 2024 00:00:00 -0500

This paper introduces DeepRFQC, an automated method for quality control in P-wave receiver function analysis. Leveraging a U-Net inspired deep learning model, which has previously shown promise in denoising and phase detection, DeepRFQC efficiently distinguishes usable from noisy receiver functions. We examine a Proterozoic Trans-Hudson Orogen dataset from northern Canada, including seismic events from 1990 to 2023, which is expanded for training purposes by data augmentation techniques. With 1,508,449 trainable parameters, the DeepRFQC model attains a commendable 96.6% validation accuracy, on a test dataset from the X5 seismic network; tests on stations from different tectonic environments indicate that the model is effective even in environments very different from the training set. Validation through the H-κ stacking method shows consistent and plausible results. As manual quality control is a major bottleneck in receiver-function processing, automated methods such as this one will allow for efficient examination of large data sets.

Imaging microearthquake rupture processes using a dense array in Oklahoma

Harrison Burnett, Wenyuan Fan — Tue, 03 Sep 2024 00:00:00 -0400

Both large and small earthquakes rupture in complex ways. However, microearthquakes are often simplified as point sources and their rupture properties are challenging to resolve. We leverage seismic wavefields recorded by a dense array in Oklahoma to image microearthquake rupture processes. We construct machine-learning enabled catalogs and identify four spatially disconnected seismic clusters. These clusters likely delineate near-vertical strike-slip faults. We develop a new approach to use the maximum absolute SH-wave amplitude distributions (S-wave wavefields) to compare microearthquake rupture processes. We focus on one cluster with earthquakes located beneath the dense array and have a local magnitude range of -1.3 to 2.3. The S-wave wavefields of single earthquakes are generally coherent but differ slightly between the low-frequency (<12 Hz) and high-frequency (>12 Hz) bands. The S-wave wavefields are coherent between different earthquakes at low frequencies with average correlation coefficients greater than 0.95. However, the wavefield coherence decreases with increasing frequency for different earthquakes. This reduced coherence is likely due to the rupture differences among individual earthquakes. Our results suggest that earthquake slip of the microearthquakes dominates the radiated S-wave wavefields at higher frequencies. Our method suggests a new direction in resolving small earthquake source attributes using dense seismic arrays without assuming a rupture model.

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

Mon, 15 Jul 2024 00:00:00 -0400

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.

Strong asymmetry in near-fault ground velocity during an oblique strike-slip earthquake revealed by waveform particle motions and dynamic rupture simulations

Fri, 16 Aug 2024 00:00:00 -0400

The 2022 Mw 7.0 Chihshang (Taiwan) earthquake, captured by almost a dozen near-fault strong-motion seismometers, high-rate GPS and satellite data, offers a rare opportunity to examine dynamic fault rupture in detail. Using dynamic rupture simulations, we investigate the particle motions recorded at near-fault strong-motion and 1 Hz GPS stations surrounding the main asperity. Some of these stations were as close as 250 m from the fault trace as determined by sub-pixel correlation of Sentinel-2 images. Our model reproduces the observed strong asymmetry in the ground motions on either side of the fault rupture, which results from along-dip spatial variability in rake angle on the steeply dipping fault (70°) at shallow depth (2 km). Observed near-fault, pulse-like fault-parallel ground velocity larger than fault-normal velocity can be explained by a model with a sub-shear rupture speed, which may be due to shallow rupture propagation within low-velocity material and to free surface reflections. In addition, we estimate a slip-weakening distance D_c of ~0.7-0.9m from strong-motion seismogram recorded at Station F073, which is located ~250 m from the fault rupture, and the results of dynamic rupture modeling. The inferred D_c is similar to other empirically derived estimates found for crustal earthquakes. These results have important implications for near-fault ground-motion hazard.

The influence of ground shaking on the distribution and size of coseismic landslides from the Mw 7.6 2005 Kashmir earthquake

Mon, 29 Jul 2024 00:00:00 -0400

Understanding the conditions that governed the distribution of coseismic landslide frequency and size from past earthquakes is imperative for quantifying the hazard potential of future events. However, it remains a challenge to evaluate the many factors controlling coseismic landsliding including ground shaking, topography, rock strength, and hydrology, among others, for any given earthquake, partly due to the lack of direct seismic observations in high mountain regions. To address the dearth of ground motion observations near triggered landslides, we develop simulated ground motions, including topographic amplification, to investigate these key factors that control the distribution of coseismic landslides from the M_w 7.6 2005 Kashmir earthquake. We show that the combination of strong peak ground motions, steep slopes, proximity to faults and rivers, and lithology control the overall spatial distribution of landslides. We also investigate the role of topographic amplification in triggering the largest landslide induced by this earthquake, the Hattian Bala landslide, finding that it is amplified at the landslide initiation point due to the trapping of energy within the ridge kink as it changes orientation from E to NE. This focusing effect combined with predisposing conditions for hillslope failure may have influenced the location and size of this devastating landslide.

Microseismicity at the Time of a Large Creep Event on the Calaveras Fault is Unresponsive to Stress Changes

Litong Huang, Susan Y Schwartz, Emily E Brodsky — Wed, 02 Oct 2024 00:00:00 -0400

The potential relationship between surface creep and deeper geological processes is unclear, even on one of the world’s best-studied faults. From June to August 2021, a large creep event with surface slip of more than 16 mm was recorded on the Calaveras fault in California, part of the San Andreas fault system. This event initially appeared to be accompanied by along-fault migration of seismicity, suggesting it penetrated to depth. Other studies have suggested that surface creep events are likely a shallow feature, unrelated to deep seismicity. To provide more detail on the relationship between earthquakes, surface creep, and potential aseismic slip at seismogenic depth, we tripled the number of earthquakes in the Northern California Earthquake Catalog in the region of the creep event for all of 2021. This was accomplished by implementing earthquake detection techniques based on both template matching (EQCorrscan) and AI-based automatic earthquake phase picking (PhaseNet). After manual inspection, the detected earthquakes were first located using Hypoinverse and subsequently relocated via GrowClust. Our enhanced catalog indicates that the spatiotemporal pattern of earthquakes here is not strongly influenced by the creep event and is better explained by structural heterogeneity than transient stress changes, indicating a decoupling of seismicity rate and surficial creep on this major fault.

Inelastic deformation accrued over multiple seismic cycles: Insights from an elastic-plastic slider-and-springboard model

Pierre Dublanchet, Jean-Arthur Olive — Tue, 15 Oct 2024 00:00:00 -0400

We study a toy model designed to build physical insight into the problem of slow accumulation of non-recoverable strain in fault blocks over multiple earthquake cycles. The model consists of a thin, horizontal elastic-plastic plate (springboard) in frictional contact with a vertical, rigid wall moving downward at a steady speed. Our model produces stick-slip cycles consisting of interseismic plate downwarping and coseismic plate upwarping as long as the moment of the frictional force at the contact does not exceed the maximum (purely plastic) bending moment the plate can sustain. We show that the duration of individual earthquake cycles and the spatial pattern of interseismic deflection are controlled by two stress ratios involving the peak yield stress of the plate, the frictional strength of the fault and the coseismic stress drop. We show that non-recoverable plate deflection accumulates over successive earthquake cycles if the plate’s yield strength decreases through time, causing a progressive decrease of the aforementioned stress ratios. We derive scaling relations between the rate of accumulation of inelastic deformation, the relative tectonic plate velocity, and the rate of lithospheric weakening. Our results are consistent with observations of long-term permanent deformation of natural fault regions.

A model of the earthquake cycle along the Gofar oceanic transform faults

Meng (Matt) Wei, Lingchao He, Bridget Smith-Konter — Wed, 06 Nov 2024 00:00:00 -0500

The Gofar oceanic transform fault at the East Pacific Rise has one of the best seismic cycles recorded by modern instruments. The timing, location, and magnitude of major earthquakes (Mw>5.5) have been well constrained by data from global seismic networks for the past 30 years. The earthquake interval is short, about 3-5 years. Several segments have already experienced 5 cycles since 1995, when the seismic network was good enough for surface wave relocation. Two ocean bottom seismometer deployments (2008-2009, 2021-2023) also provide constraints on the seismic properties on the fault. This makes Gofar an ideal place to study earthquake cycles. Here, we developed a model for the seismic cycle along the Gofar transform fault using a semi-analytical approach for rapidly calculating 3D time-dependent deformation and stress caused by screw dislocations embedded within an elastic layer overlying a Maxwell viscoelastic half-space. The 160-km long fault is divided into three major segments with six asperities. Our model simulates the earthquake pattern on this fault for the past 30 years. Most of the time, each asperity ruptured as a large earthquake every 3-5 years. Most segments have a nearly constant Coulomb stress threshold of 2-3 MPa, providing optimal conditions for the forecasting of future earthquakes along Gofar. For three cases that deviated from this simple regular pattern, a large earthquake occurred with a centroid location between two asperities. This is likely due to concurrent rupture that involved both asperities. We also modeled surface deformation with different elastic layer thicknesses and mantle viscosities. Even though most deformation is in the horizontal direction, the difference in both horizontal and vertical directions between models can be as large as a few centimeters per year. Several seafloor geodesy methods can be used to differentiate between models, and seafloor pressure might be the most appropriate one at this remote location.

Chasing the ghost of fracking in the Vaca Muerta Formation: Induced seismicity in the Neuquén Basin, Argentina

Wed, 27 Nov 2024 00:00:00 -0500

Earthquakes are known to be induced by a variety of anthropogenic causes, such as hydraulic fracturing. In the Neuquén Basin of Argentina, hydraulic fracturing has been used to produce hydrocarbons trapped in the shales of the Vaca Muerta Formation. Correspondingly, incidences of seismicity there have increased. We collect information on well stimulations and earthquakes to perform statistical analysis linking these two datasets together. Spatiotemporal association filters suggest that the catalogue of events is biased towards hydraulic fracturing operations. After accounting for false-positives, we estimate that ~0.5% of operations are associated with earthquakes. These associated event-operation pairs show highly correlated temporal signals (>99.99% confidence) between seismicity/injection rates. Based on this evidence, we argue that many of these earthquakes are induced. We support this argument by comparing the geological setting of the Neuquén Basin against conditions needed for fault reactivation in other susceptible/seismogenic basins. This recognition adds to the growing list of (hydraulic fracturing) induced seismicity.

Modeling ground motions and crustal deformation from tsunami earthquakes: Rupture parameter constraints from the 2010 Mentawai event

Tara Nye, Valerie J. Sahakaian, Diego Melgar — Sat, 28 Sep 2024 00:00:00 -0400

We use a combination of near-field simulated and observational data to constrain the rise time, rupture velocity, and high frequency stress parameter for the 2010 M7.8 Mentawai tsunami earthquake. Tsunami earthquakes, which are shallow-rupturing events generating exceptionally large seafloor displacements, are challenging for current tsunami early warning systems. A combination of near-field high-rate GNSS and seismic data can be used for early-discrimination, but the dearth of data from these events limits testing of such an implementation in a real-time scenario. In lieu of near-field data, models with realistic rupture physics can be leveraged to improve local tsunami warning. We develop recommendations for such parameters based on observations of near-field data from the 2010 M7.8 Mentawai earthquake. We find that rise time and rupture velocity covary, and that rise time–rupture velocity combinations ranging from 5.4 s–1.23 km/s to 12 s–1.6 km/s adequately model the long duration of the Mentawai event. We find that a stress parameter of 1.43 MPa best models the high frequency deficiency. We present equations which can be used to determine reasonable parameter values for simulating tsunami earthquakes, and we find that simulated data generated with the recommended parameters capture defining characteristics of tsunami earthquakes.

Site characterization of Sikkim Himalaya using HVSR

Mita Uthaman, Chandrani Singh, Arun Singh — Wed, 20 Nov 2024 00:00:00 -0500

The northeastern state of Sikkim lying in central segment of the Himalayan orogen is a seismically active region which was plagued by the recent 2011 M_w6.9 earthquake. Analysis of local earthquakes recorded at the recently deployed seismic network of 27 broadband seismic stations revealed seismogenic zone extending down to lower crustal depths with a predominant strike-slip faulting mechanism. Persistent seismicity in a region with complex tectonic setting makes it imperative to study the site characteristics crucial for determining the local site conditions. Here, we harness the noise and local earthquakes records from the Sikkim network to compute horizontal-to-vertical spectral ratio (HVSR) for site characterization. Local geology and topography are observed to incite distinctly intricate trends in the HVSR curves. The thick sedimentary deposit of the Himalayan foreland basin causes high amplification (∼7) at low resonant frequencies (<1 Hz). The HVSR curves in the western section of Main Central Thrust Zone exhibits distinct double amplification peaks (∼2.5 at 1 Hz and 5 Hz) under the influence of the parallely dipping sheets of the duplex structure. Whereas, the eastern section of Main Central Thrust zone exhibit a rather irregular trend owing to its proximity to the transitioning lithological unit. The central section prone to landslides has characteristic peaks at 2 Hz and 8 Hz, indicative of the geometry of the sliding surface. Effects of towering topography and high wind speeds at corresponding elevations are observed to result in anomalously high amplification (∼25) at low frequencies (< 1 Hz). Directional amplification along discrete azimuth signifies the pronounced effect of topography and geometry of lithotectonic units in site response. Locally varying site response with prevalent seismicity amplifies the seismic hazard risk potential of Sikkim Himalaya.

Earthquake source inversion by integrated fiber-optic sensing

Nils Müller, Sebastian Noe, Dominik Husmann, Jacques Morel, Andreas Fichtner — Mon, 22 Jul 2024 00:00:00 -0400

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.

Source characterization of the 20th May 2024 MD 4.4 Campi Flegrei caldera earthquake through a joint source-propagation probabilistic inversion

Mon, 05 Aug 2024 00:00:00 -0400

On May 20^th, 2024, an earthquake of magnitude M_D 4.4 nucleated at shallow depth (2.6 km) in the Campi Flegrei caldera (Southern Italy), a densely populated area where an increase in seismic activity has been observed since 2019 attributable to an on-going unrest episode. While the magnitude was moderate, the event produced a strong ground shaking with an observed maximum peak ground acceleration of 3.58 m s^-2, and several buildings were damaged. Here, we characterize the earthquake source using a probabilistic joint source-propagation spectral inversion in the Fourier space. We estimate a moment magnitude M_w = 3.70 ± 0.13 and a corner frequency f_c = 1.11 ± 0.19 Hz. Assuming a circular rupture model, we estimate a source radius r = 400 ± 70 m and a stress drop Δσ = 3.2 ± 2.2 MPa. The estimated stress drop suggests that future earthquakes in the hypocentral region, considering a possible rupture length of 3 km suggested by previous studies, can have magnitude increased by 1.2 ± 0.3 units with respect to May 20^thevent. A systematic source characterization of the recent seismicity in the caldera would hep in estimating the expected ground motions from future large-magnitude events.

Development and Comparison of 3D Seismic Geology and Shear-wave Velocity Models of Metro Vancouver

Sujan Adhikari, Sheri Molnar — Thu, 12 Sep 2024 00:00:00 -0400

This study presents a 3D regional modeling of seismic geology and shear wave velocity (Vs) in Metro Vancouver for seismic microzonation and hazard prediction. Leveraging an extensive geodatabase compiled from invasive and non-invasive in situ data, including lithological logs and seismic field data, we delineated four major geological units: Holocene post-glacial and Pleistocene inter/glacial sediments, and Tertiary sedimentary and Pre-Tertiary Coast Mountain plutonic rocks.

Seismic geology model integrates the four primary geological formations, leveraging significant impedance-based surfaces derived from meticulously analyzed borehole stratigraphic logs and Vs depth profiles sourced from 2333 georecords, enhancing its depth and accuracy. Through a meticulous comparison with established interpreted geological cross-sections, we have reaffirmed the robustness and reliability of our seismic geology modeling approach. A numerical 3D “geotechnical layer” Vs model with 11 isovelocity surfaces was developed using 688 Vs depth profiles. Comparison with microtremor amplification spectra confirms our 3D models' reliable use in predicting site amplification. We find that the combination of local geology (thicknesses) and Vs information outperforms prediction in fundamental peak frequency compared to using only local geology combined with regional Vs information. Our study contributes to advancing understanding of seismic hazards in Metro Vancouver, highlighting the importance of incorporating localized seismic site conditions for precise regional seismic hazard assessments.

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

Thu, 25 Jul 2024 00:00:00 -0400

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.

DASCore: a Python Library for Distributed Fiber Optic Sensing

Tue, 30 Jul 2024 00:00:00 -0400

In the past decade, distributed acoustic sensing (DAS) has enabled many new monitoring applications in diverse fields including hydrocarbon exploration and extraction; induced, local, regional, and global seismology; infrastructure and urban monitoring; and several others. However, to date, the open-source software ecosystem for handling DAS data is relatively immature. Here we introduce DASCore, a Python library for analyzing, visualizing, and managing DAS data. DASCore implements an object-oriented interface for performing common data processing and transformations, reading and writing various DAS file types, creating simple visualizations, and managing file system-based DAS archives. DASCore also integrates with other Python-based tools which enable the processing of massive data sets in cloud environments. DASCore is the foundational package for the broader DAS data analysis ecosystem (DASDAE), and as such its main goal is to facilitate the development of other DAS libraries and applications.

B3AM: A beamforming toolbox for three-component ambient seismic noise analysis

Katrin Löer, Claudia Finger — Fri, 15 Nov 2024 00:00:00 -0500

We introduce the MATLAB toolbox B3AM for beamforming of three-component ambient noise array data. We explain the theory behind three-component beamforming and polarisation analysis in particular, provide an overview of the workflow, and discuss the output using a worked example. The strength of the presented code package is the analysis of multiple beam response maps from multiple time windows. Hence, it provides statistical information about the ambient noise wavefield recorded over a period of time, such as the ratio of surface to body waves, average dispersion velocities, or dominant propagation direction. It can be used to validate assumptions made about the ambient noise wavefield in a particular location, helping to interpret results from other techniques, such as the analysis of horizontal-to-vertical spectral ratios or ambient noise interferometry, and enabling more precise monitoring of specific wavefield components. While designed initially with seismic networks in mind, B3AM is applicable over a wide range of frequencies and array sizes and can thus be adapted also for laboratory settings or civil engineering applications.