Partitioning Ground Motion Uncertainty When Conditioned on Station Data

ABSTRACT Rapid estimation of earthquake ground shaking and proper accounting of associated uncertainties in such estimates when conditioned on strong-motion station data or macroseismic intensity observations are crucial for downstream applications such as ground failure and loss estimation. The U.S. Geological Survey ShakeMap system is called upon to fulfill this objective in light of increased near-real-time access to strong-motion records from around the world. Although the station data provide a direct constraint on shaking estimates at specific locations, these data also heavily influence the uncertainty quantification at other locations. This investigation demonstrates methods to partition the within- (phi) and between-event (tau) uncertainty estimates under the observational constraints, especially when between-event uncertainties are heteroscedastic. The procedure allows the end users of ShakeMap to create separate between- and within-event realizations of ground-motion fields for downstream loss modeling applications in a manner that preserves the structure of the underlying random spatial processes.

Seismic Source Processes of 25 Earthquakes (Mw>5) in the Gulf of California

ABSTRACT The Gulf of California (GoC) is a complex tectonic boundary that has been instrumented in the past several decades to record broadband seismograms. This volume of data has allowed us to study several source parameters systematically. Before, only a few source parameters of earthquakes greater than magnitude five had been studied in the GoC area. We re-examined the focal mechanisms of several earthquakes in the southern GoC that occurred over the last 20 yr using local–regional distance broadband seismograms. These focal mechanisms were then used as input data to retrieve the time–space history of the rupture for each earthquake. This work contributes to the study of 25 rupture-process models computed with the method proposed by Yagi et al. (1999). To investigate more about the nature of the seismicity in the GoC, we also calculated the non-double-couple component of moment tensors for 45 earthquakes. Previous studies (e.g., Ortega et al., 2013, 2016) have shown that non-double-couple components from moment tensors in this region are associated with complex faulting, suggesting that oblique faults or several parallel faults are interacting simultaneously. Our results show that, at least for moderate earthquakes (5 < M < 6), rupture processes in the GoC show a complex interaction between fault systems. It is revealed on the important contribution of non-double-couple component obtained in the full moment tensor analysis.

Alert Optimization of the PLUM Earthquake Early Warning Algorithm for the Western United States

ABSTRACT We determine an optimal alerting configuration for the propagation of local undamped motion (PLUM) earthquake early warning (EEW) algorithm for use by the U.S. ShakeAlert system covering California, Oregon, and Washington. All EEW systems should balance the primary goal of providing timely alerts for impactful or potentially damaging shaking while limiting alerts for shaking that is too low to be of concern (precautionary alerts). The PLUM EEW algorithm forward predicts observed ground motions to nearby sites within a defined radius without accounting for attenuation, avoiding the earthquake source parameter estimation step of most EEW algorithms. PLUM was originally developed in Japan where the alert regions and ground motions for which alerts are issued differ from those implemented by ShakeAlert. We compare predicted ground motions from PLUM to ShakeMap-reported ground motions for a set of 22 U.S. West Coast earthquakes of magnitude 4.4–7.2 and evaluate available warning times. We examine a range of prediction radii (20–100 km), thresholds used to issue an alert (alert threshold), and levels of impactful or potentially damaging shaking (target threshold). We find optimal performance when the alert threshold is close to the target threshold, although higher target ground motions benefit from somewhat lower alert thresholds to ensure timely alerts. We also find that performance, measured as the cost reduction that a user can achieve, depends on the user’s tolerance for precautionary alerts. Users with a low target threshold and high tolerance for precautionary alerts achieve optimal performance when larger prediction radii (60–100 km) are used. In contrast, users with high target thresholds and low tolerance for precautionary alerts achieve better performance for smaller prediction radii (30–60 km). Therefore, setting the PLUM prediction radius to 60 km balances the needs of many users and provides warning times of up to ∼20 s.

NIOM Analysis of Earthquake Observation Records in a Rockfill Dam

ABSTRACT We applied the normalized input–output minimization method (a method developed for the analysis of propagation times in vertical array records) to long-term earthquake observation records from Aratozawa Dam (in Kurihara, Miyagi prefecture, Japan), spanning the period from July 1992 to December 2019 to determine the propagation velocity of seismic waves in the embankment, and investigated changes in soil properties. As a result, we showed that (1) the velocities of S and P waves in the upper section were 449 and 993 m/s, respectively, prior to the strong earthquake motions derived from earthquake records from January 1997 through October 2001, whereas 608 and 1538, respectively, in the lower section, (2) in the Iwate–Miyagi Nairiku earthquake, the S-wave velocity in the upper section decreased to 158 m/s in the principal shock, and (3) in subsequent minor earthquakes the propagation velocity increased more or less in proportion with the logarithm of the number of elapsed days, requiring three years or longer to return to the initial value, (4) although similar changes were observed in the Great East Japan earthquake of 2011, the reduction in propagation velocity that remained after the principal shock was smaller than in the case of the Iwate–Miyagi Nairiku earthquake, and it was judged that there were no large effects on the dam body such as those that occurred in the Iwate–Miyagi Nairiku earthquake, and furthermore (5) in the principal shock of the Iwate–Miyagi Nairiku earthquake, the shear modulus in the upper part of the dam body decreased from 400 to 50 MPa (with a maximum shear strain of 10−3), resulting in more pronounced changes than in the lower section, whereas the damping ratio increased by at least 10% in the lower section during the principal shock of the Iwate–Miyagi Nairiku earthquake, resulting in much greater changes than in the upper section.

Synthesis of Recent Paleoseismic Research on Quaternary Faulting in the Eastern Tennessee Seismic Zone, Eastern North America: Implications for Seismic Hazard and Intraplate Seismicity

ABSTRACT Causes of intraplate seismicity remain a great unsolved problem, in contrast with plate-boundary seismicity. Modern seismicity records frequent seismic activity in plate-boundary seismic zones, but in fault zones where seismic activity is not frequent, plate boundary or intraplate, resolution of prehistoric earthquake activity is critical for estimating earthquake recurrence interval and maximum expected magnitude. Thus, documenting prehistoric earthquakes is crucial for assessing earthquake hazard posed to infrastructure, including nuclear reactors and large dams. The ∼400 km long eastern Tennessee seismic zone (ETSZ), United States, is the third most active seismic zone east of the Rocky Mountains in North America, although the largest recorded ETSZ earthquake is only Mw 4.8. Ironically, it is the least studied major eastern U.S. seismic zone. Recent ETSZ field surveys revealed an 80 km long, 060°-trending corridor containing northeast-striking Quaternary thrust, strike slip, and normal faults with displacements ≥1 m. It partially overlaps a parallel trend of seismicity that extends 30 km farther southwest, suggesting this active faulting zone may extend ∼110 km within part of the ETSZ. Near Dandridge, Tennessee, a thrust fault in French Broad River alluvium records two earthquakes in the last 40,000 yr. About 50 km southwest near Alcoa, Tennessee, a thrust fault cuts Little River alluvium and records two earthquakes between 15,000 and 10,000 yr ago. About 30 km farther southwest at Vonore, Tennessee, a thrust fault displaces bedrock ≥2 m over colluvium, and alluvium is normal faulted >2 m. This corridor, just west of the Blue Ridge escarpment, overlies a steep gradient in midcrustal S-wave velocities, consistent with a basement fault at hypocentral depths. The corridor faults may be connected to a basement fault or localized coseismic faults above a blind basement fault. Our current data suggest at least two Mw≥6.5 surface rupturing events in the last 40,000 yr.

Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence

ABSTRACT Low-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (>∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.

Are Synthetic Accelerograms Suitable for Local Seismic Response Analyses at Near-Field Sites?

ABSTRACT Local seismic response (LSR) studies are considerably conditioned by the seismic input features due to the nonlinear soil behavior under dynamic loading and the subsurface site conditions (e.g., mechanical properties of soils and rocks and geological setting). The selection of the most suitable seismic input is a key point in LSR. Unfortunately, few recordings data are available at seismic stations in near-field areas. Then, synthetic accelerograms can be helpful in LSR analysis in urbanized near-field territories. Synthetic accelerograms are generated by simulation procedures that consider adequately supported hypotheses about the source mechanism at the seismotectonic region and the wave propagation path toward the surface. Hereafter, mainshocks recorded accelerograms at near-field seismic stations during the 2016–2017 Central Italy seismic sequence have been compared with synthetic accelerograms calculated by an extended finite-fault ground-motion simulation algorithm code. The outcomes show that synthetic seismograms can reproduce the high-frequency content of seismic waves at near-field areas. Then, in urbanized near-field areas, synthetic accelerograms can be fruitfully used in microzonation studies.

Recalibration of the Local Magnitude (ML) Scale for Earthquakes in the Yellowstone Volcanic Region

ABSTRACT The Yellowstone volcanic region is one of the most seismically active areas in the western United States. Assigning magnitudes (M) to Yellowstone earthquakes is a critical component of monitoring this geologically dynamic zone. The University of Utah Seismograph Stations (UUSS) has assigned M to 46,767 earthquakes in Yellowstone that occurred between 1 January 1984 and 31 December 2020. Here, we recalibrate the local magnitude (ML) distance and station corrections for the Yellowstone volcanic region. This revision takes advantage of the large catalog of earthquakes and an increase in broadband stations installed by the UUSS since the last ML update in 2007. Using a nonparametric method, we invert 7728 high-quality, analyst-reviewed amplitude measurements from 1383 spatially distributed earthquakes for 39 distance corrections and 20 station corrections. The inversion is constrained with four moment magnitude (Mw) values determined from time-domain inversion of regional-distance broadband waveforms by the UUSS. Overall, the new distance corrections indicate relatively high attenuation of amplitudes with distance. The distance corrections decrease with hypocentral distance from 3 km to a local minimum at 80 km, rise to a broad peak at 110 km, and then decrease again out to 180 km. The broad peak may result from superposition of direct arrivals with near-critical Moho reflections. Our ML inversion doubles the number of stations with ML corrections in and near the Yellowstone volcanic region. We estimate that the additional station corrections will nearly triple the number of Yellowstone earthquakes that can be assigned an ML. The new ML distance and station corrections will also reduce uncertainties in the mean MLs for Yellowstone earthquakes. The new MLs are ∼0.07 (±0.18) magnitude units smaller than the previous MLs and have better agreement with 12 Mws (3.15–4.49) determined by the UUSS and Saint Louis University.

Postseismic Relaxation Following the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACT The 2019 Ridgecrest, California, earthquake sequence involved predominantly right-lateral strike slip on a northwest–southeast-trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left-lateral strike slip on a northeast–southwest-trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey-mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right-lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼1.3×1017 Pa s, and the adjacent Central Valley transient mantle asthenosphere viscosity is ∼7×1017 Pa s, about five times higher and consistent with an asymmetry in postseismic horizontal motions across the mainshock surface rupture.

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.