Centroid Source Research Articles

Spatial and temporal variations of stress field in the Longmenshan Fault Zone after the 2008 Wenchuan, China earthquake

Summary We investigate the temporal and spatial patterns of the regional stress field along the Longmenshan Fault Zone (LFZ) following the 2008 Wenchuan Mw7.9 earthquake. Regional broadband seismic records are used to determine the focal mechanisms and centroid source depths of >400 aftershocks of magnitudes 3.3 ≤ Mw ≤ 5.9. The focal mechanisms are then used to invert for the temporal and spatial variations of the regional stress field. Results show clear spatial changes in the directions of principle stress axes as well as the faulting types along the LFZ. The regional stress field is in an overall thrust regime with nearly horizontal maximum compressional axis. In the southern section of the LFZ, in addition to the primarily thrust background with maximum compressional axis perpendicular to the main fault zone, strike-slip component is also present. In the middle section, the maximum compressional axis varies spatially, changing abruptly from perpendicular to parallel to the LFZ. In the northernmost section, the stress environment becomes primarily strike-slip. Our result suggests that the collision of Indian and Eurasian plates led to the first-order feature of the stress field in the study area. The geometry of blocks and faults, the lower crustal flow as well as the shallow crustal property difference are the major factors to controlling the local stress field. Temporal variations of stress field suggest that the mainshock caused a large stress disturbance at early stage in certain areas of LFZ, and the stress field variations with depth imply a different deformation pattern and different stress background in the shallow and lower crust.

Source Rupture Models for the Mw 9.0 2011 Tohoku Earthquake from Joint Inversions of High-Rate Geodetic and Seismic Data

Research Article| May 01, 2013 Source Rupture Models for the Mw 9.0 2011 Tohoku Earthquake from Joint Inversions of High‐Rate Geodetic and Seismic Data Han Yue; Han Yue Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Thorne Lay Thorne Lay Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2013) 103 (2B): 1242–1255. https://doi.org/10.1785/0120120119 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Han Yue, Thorne Lay; Source Rupture Models for the Mw 9.0 2011 Tohoku Earthquake from Joint Inversions of High‐Rate Geodetic and Seismic Data. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1242–1255. doi: https://doi.org/10.1785/0120120119 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The space–time history of fault slip during the 11 March 2011 Tohoku earthquake (Mw 9.0) is determined using high sample rate three‐component Global Positioning System (GPS) recordings from regional stations across Japan, teleseismic broadband P waves, global R1 source time functions determined by empirical Green’s function deconvolutions of short‐arc Rayleigh waves, and ocean‐bottom deformation observations. Least‐squares inversions are performed for models with prescribed rupture‐front expansion velocity. Joint inversion yields improved resolution of slip compared with inversions using any single data type for both checkerboard rupture simulations and the actual data. Joint inversion stabilizes inversions with respect to some key parameters, such as the rupture expansion velocity and subfault total rupture durations, due to lower dependence on these parameters of some datasets (mainly the high sample rate [1 sample/s] three‐component GPS recordings [hr‐GPS] data). The preferred joint inversion model has a seismic moment estimate of 4.2×1022 N·m (Mw 9.0), with a primary large‐slip patch with maximum slip of ∼50–60 m located up‐dip of the hypocenter on the shallow megathrust and distributed slip of 20–30 m near the hypocenter. A down‐dip low‐slip extension to the south is also resolved, with centroid source time later than 110 s.Online Material: Figures of inverted slip distributions and associated source time functions and centroid times, and waveform fits. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Feasibility of Real-Time Broadband Waveform Inversion for Simultaneous Moment Tensor and Centroid Location Determination

We present a feasibility study for an automated system to simulta- neously determine centroid source location and seismic moment tensor (MT) for regional earthquakes. This system uses continuous real-time waveforms in a time window that is continuously shifted forward with a short time interval (20 sec) from a sparse network of broadband seismic instruments, and unlike the current standard method, it performs inversion without prior knowledge of the location and origin time information. We tested 68 earthquakes with ML 4.2 that occurred in northern and central California between 1993 and 1999 and that have well-calibrated solutions obtained by an established procedure of MT determination. The solutions determined by the new system are compared with the well-calibrated solutions for performance appraisal. Results show that onshore earthquakes with Mw 4.5 can be detected and adequately characterized in terms of MT and approximate centroid lo- cation. The threshold value of variance reduction to detect an event is about 70%. In contrast, events that occurred off the Mendocino coast are not as reliably con- strained, reflecting the unmodeled complexity of the transitional structure from ocean to the continent using the current 1D model as well as the gap in azimuthal coverage. Significant improvement of the computational efficiency is expected if the system is fully configured on a powerful PC with multiple CPUs of approximately 1 Gbyte memory and the monitoring of event location and MT determination over the grid can be updated within the time frame of shifting the time window (20 sec).

Broadband waveform modelling of anomalous strong ground motion in the 1989 Loma Prieta Earthquake using three‐dimensional geologic structures

Strong motion recordings of the 1989 Loma Prieta earthquake show a large, coherent SH wave displacement pulse across much of the San Francisco Bay area. This distinctive ground motion has been modelled using approximate, broadband synthetics for a double‐couple centroid source in a 3‐D crustal model for the region. The synthetics indicate that (1) horizontal bending and focusing of SH waves by the lateral crustal velocity contrast across the San Andreas fault and (2) the source radiation pattern, contribute significantly to the observed azimuthal variation of the SH pulse and hence to the regional variations of strong ground shaking in the period range of 1 to 8 sec. In addition, the scattering of surface‐reflected shear waves by deep basins under the Santa Clara Valley may contribute to increased coda amplitude and duration observed for sites around Oakland.The wave modelling also supports the earlier conclusion that an increase in intensity in portions of San Francisco and Oakland is in part the result of energy reflected from the base of the crust [Somerville and Yoshimura, 1990].

Source mechanism of the 1987 Vatnafjöll Earthquake in south Iceland

The May 25, 1987, Vatnafjöll earthquake occurred on the Mid‐Atlantic plate boundary, at a junction between a transform (the South Iceland Seismic Zone) and a spreading segment (the Eastern Volcanic Zone). The South Iceland Seismic Zone has had the most destructive earthquakes in the history of Iceland with average intervals of 80–100 years and estimated maximum magnitudes around 7. The 1987 Vatnafjöll earthquake was the largest earthquake (mb = 5.8; MS = 5.8; Mw = 5.9) in south Iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region. Foreshocks and aftershocks of the Vatnafjöll earthquake were located with relatively good accuracy. They define an elongated N‐S epicentral area, 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km. We determined centroid source parameters of the Vatnafjöll earthquake by inverting long‐period teleseismic P and SH waves. The centroid source mechanism and foreshock and aftershock distribution indicate N‐S right‐lateral strike‐slip faulting on a near‐vertical plane. The rupture initiated near the base of the crust and propagated upward along an approximately 10‐km‐long fault but did not reach the surface. The centroid depth is well constrained and is between 5 and 8 km depth at the 95% confidence level. The seismic moment is 9.1×1024dyn cm. The source has relatively short duration of 3 s, which may indicate higher stress drop than global average of earthquakes of this size. The South Iceland Seismic Zone is a 70‐km‐long E‐W striking transform zone between two overlapping rift zones, the Eastern and Western Volcanic Zones. It is characterized by “bookshelf” type of tectonics; that is, there is no surface faulting in the direction of the transform. Instead, N‐S right‐lateral strike‐slip faults are distributed along and perpendicular to the transform zone. The Vatnafjöll earthquake has characteristics of earthquakes in the South Iceland Seismic Zone and is probably not related directly with magmatic activity in the Eastern Volcanic Zone. The transform zone thus extends farther into the rift zone than was previously recognized.