Inversion of complex body waves—III

We have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes. The rupture pattern is represented as a sequence of subevents distributed on the fault plane. This method is an extension of our earlier method in which the subevent mechanisms were fixed. In the new method, the subevent mechanisms are determined from the data and are allowed to vary during the sequence. When subevent mechanisms are allowed to vary, however, the inversion often becomes unstable because of the complex trade-offs between the mechanism, the timing, and the location of the subevents. Many different subevent sequences can explain the same data equally well, and it is important to determine the range of allowable solutions. Some constraints must be imposed on the solution to stabilize the inversion. We have developed a procedure to explore the range of allowable solutions and appropriate constraints. In this procedure, a network of grid points is constructed on the τ - I plane, where τ and I are, respectively, the onset time and the distance from the epicenter of a subevent; the best-fit subevent is determined at all grid points. Then the correlation is computed between the synthetic waveform for each subevent and the observed waveform. The correlation as a function of τ and I and the best-fit mechanisms computed at each τ - I grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution. The method is illustrated using the data for the 1976 Guatemala earthquake

Inversion of complex body waves

A numerical method to deconvolve complex body waves into a multiple shock sequence is developed. With the assumption that all the constituent events of a multiple shock have identical fault geometry and depth, the far-field source time function is obtained as a superposition of ramp functions. The height and the onset time of the ramp functions are determined by matching the synthetic waveforms with the observed ones in the least-square sense. The individual events are then identified by pairs of ramp functions or discrete trapezoidal pulses in the source time sequence. The method can be used for the analysis of both single and multi-station data. Teleseismic long-period P waves from the 1976 Guatemala earthquake are analyzed as a test of our method. The present method provides a useful tool for a systematic analysis of multiple event sequences

The Shikotan Earthquake of October 4, 1994: Lithospheric earthquake

A large M_w=8.2 earthquake occurred off Shikotan Is., one of the Kurile Is., on October 4, 1994. We inverted 32 body-wave records to determine the rupture pattern using an iterative deconvolution method. The mechanisms of the subevents were allowed to vary during rupture. The source parameters obtained are: the location of the initial break = (43.48°N, 147.40°E); the centroid depth = 56 km; (strike, dip, rake) = (49°, 75°, 125°) for the total source; the seismic moment M_o = 2.6×10^(21) Nm (M_w=8.2); source time duration T = 42 s; the average rupture velocity ν = 2.5 km/s. We also determined the mechanism using long-period Love and Rayleigh waves from 14 stations. The solution for a finite source distributed over a depth range from 0 to 90 km is (strike, dip, rake) = (54°, 76°, 129°) with Mo = 2.3×10^(21) Nm, in good agreement with that from body waves. Referring to the extent of the aftershock area and the subevent distribution, we estimated the fault area S = 120 × 60 km², the average slip D=5.6m, and the stress drop Δσ=11 MPa. We computed synthetic waveforms as well as static displacements using either the steep or the low-angle plane as the fault plane, and found that the steep-dip fault model fits the data better. Our result (the mechanism, large centroid depth, high stress drop) strongly suggests that the 1994 Shikotan earthquake is a lithospheric earthquake: an intra-plate event that ruptures through a substantial part of the subducting oceanic lithosphere. This type of lithospheric earthquake is relatively common

The mechanism of the deep Bolivia Earthquake of June 9, 1994

We analyzed the IRIS broad-band seismograms for the June 9, 1994, Bolivia earthquake, the largest deep-focus earthquake ever recorded. We inverted the record sections with a duration of 330 sec which include P, pP and sP phases, and obtained a dip-slip mechanism: (dip(δ), rake(λ), strike(ф)) = (13°, −4°, 1°) or (89°, −103°, 95°), with a scalar moment of Mo=2.9×10^(21) Nm (Mw=8.2). This mechanism is similar to that obtained from long-period (175 to 250 sec) Rayleigh and Love waves: (δ, λ, ф) =(83°, −100°, 90°); Mo=3.0×10^(21) Nm (Mw=8.3). The main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1.2×10^(20) Nm (Mw=7.3). The main rupture starts at about 20 km east or northeast of the initial break, and propagates horizontally in the east to northeast direction. The source duration is 40 sec, and the rupture dimension is about 40km. If a fault area of 40×40km² is assumed, the stress drop is 110 MPa. An inversion of body waves (P, pP, and sP) with 6 basis moment tensors yielded a best-fit solution with a 10% implosive component. This result is of marginal significance and its confirmation would require more detailed information on the structures near the source region, pP bounce points and stations. The observed amplitude of the radial mode, _0S_0, (1226 sec) is consistent with a double couple model with less than 2 % isotropic component

Rupture Process of the Kobe, Japan, Earthquake of Jan. 17, 1995, Determined from Teleseismic Body Waves

The source process of the 1995 Kobe earthquake is determined using teleseismic body waves. The source parameters obtained for the total source are: focal mechanism [strike, dip, rake] equal to [233°, 86°, 167°], nearly pure strike-slip with an EW compression axis; the seismic moment, M_o= 2.5×10^(19) Nm (M_w= 6.9); and source duration T=11s. The rupture process consists mainly of three subevents with source durations of 5-6 s: two are nearly pure strike-slip with slightly different fault strikes, and the other is dip-slip. Combining the source location with the aftershock distribution, we infer that the first subevent is a bilateral rupture and the later subevents are unilateral propagating NE and reaching the Kobe area. The total fault length is 40 km, the averaged dislocation is 2.1 m, and the averaged stress drop is 8 MPa. These source parameters indicate that the Kobe earthquake was a typical shallow inland earthquake which occurred on a previously mapped Quaternary fault zone. The directivity toward Kobe determined from teleseismic data was probably one of the main factors responsible for the heavy damage. The combined use of regional networks and teleseismic networks as demonstrated in this study will continue to be important for rapidly assessing the overall social impact of an earthquake in the very early stage of sequence

Source complexity of the 1988 Armenian Earthquake: Evidence for a slow after-slip event

We analyzed teleseismic P and S waves using a multiple-event deconvolution method to investigate the source process of the Spitak, Armenia, earthquake of December 7, 1988. Teleseismic long-period body waves exhibit complex waveforms, significantly more complex than those normally seen for an event of this size. We identified two groups of subevents. One is a group of strike-slip events during the first 20 s. The other is a dip-slip event initiating at about 30 s after the initial rupture. The tensor sum of all the subevents is essentially strike-slip (strike=302°, dip=59°, rake=143°) with a scalar moment of 1.47×10^(19) N m. The seismic moment of the dip-slip event (strike=89°, dip=60°, rake=88°) is 5.6 ×10^(18) N m, nearly 40% of the total, and the moment release rate is slower than the other subevents with a strike-slip mechanism. We call this event a slow after-slip event. The mechanism of the after-sup event is consistent with the present-day deformation pattern in the Caucasus where north-south compression is predominant due to the continental collision between the Arabian plate and the Russian Platform. This result suggests that in a region where both strike slip motion and crustal shortening are taking place, like southern California, a large strike-slip event may accompany large thrust events and vice versa. In order to determine the total coseismic deformation pattern, it is essential to resolve these multiple events in time, space, and mechanism

Genome-wide association meta-analyses and fine-mapping elucidate pathways influencing albuminuria

Abstract: Increased levels of the urinary albumin-to-creatinine ratio (UACR) are associated with higher risk of kidney disease progression and cardiovascular events, but underlying mechanisms are incompletely understood. Here, we conduct trans-ethnic (n = 564,257) and European-ancestry specific meta-analyses of genome-wide association studies of UACR, including ancestry- and diabetes-specific analyses, and identify 68 UACR-associated loci. Genetic correlation analyses and risk score associations in an independent electronic medical records database (n = 192,868) reveal connections with proteinuria, hyperlipidemia, gout, and hypertension. Fine-mapping and trans-Omics analyses with gene expression in 47 tissues and plasma protein levels implicate genes potentially operating through differential expression in kidney (including TGFB1, MUC1, PRKCI, and OAF), and allow coupling of UACR associations to altered plasma OAF concentrations. Knockdown of OAF and PRKCI orthologs in Drosophila nephrocytes reduces albumin endocytosis. Silencing fly PRKCI further impairs slit diaphragm formation. These results generate a priority list of genes and pathways for translational research to reduce albuminuria

Implicating genes, pleiotropy, and sexual dimorphism at blood lipid loci through multi-ancestry meta-analysis

Publisher Copyright: © 2022, The Author(s).Background: Genetic variants within nearly 1000 loci are known to contribute to modulation of blood lipid levels. However, the biological pathways underlying these associations are frequently unknown, limiting understanding of these findings and hindering downstream translational efforts such as drug target discovery. Results: To expand our understanding of the underlying biological pathways and mechanisms controlling blood lipid levels, we leverage a large multi-ancestry meta-analysis (N = 1,654,960) of blood lipids to prioritize putative causal genes for 2286 lipid associations using six gene prediction approaches. Using phenome-wide association (PheWAS) scans, we identify relationships of genetically predicted lipid levels to other diseases and conditions. We confirm known pleiotropic associations with cardiovascular phenotypes and determine novel associations, notably with cholelithiasis risk. We perform sex-stratified GWAS meta-analysis of lipid levels and show that 3–5% of autosomal lipid-associated loci demonstrate sex-biased effects. Finally, we report 21 novel lipid loci identified on the X chromosome. Many of the sex-biased autosomal and X chromosome lipid loci show pleiotropic associations with sex hormones, emphasizing the role of hormone regulation in lipid metabolism. Conclusions: Taken together, our findings provide insights into the biological mechanisms through which associated variants lead to altered lipid levels and potentially cardiovascular disease risk.Peer reviewe