Geology topics

Ellen Yu

Publications and source records attributed to Ellen Yu.

5 recordsLinked to original sources

USGS2025

Overview of The SCEC/USGS Community Stress Drop Validation Study using the 2019 Ridgecrest earthquake sequence

We present initial findings from the ongoing Community Stress Drop Validation Study to compare spectral stress‐drop estimates for earthquakes in the 2019 Ridgecrest, California, sequence. This study uses a unified dataset to independently estimate earthquake source parameters through various methods. Stress drop, which denotes the change in average shear stress along a fault during earthquake rupture, is a critical parameter in earthquake science, impacting ground motion, rupture simulation, and source physics. Spectral stress drop is commonly derived by fitting the amplitude‐spectrum shape, but estimates can vary substantially across studies for individual earthquakes. Sponsored jointly by the U.S. Geological Survey and the Statewide (previously, Southern) California Earthquake Center our community study aims to elucidate sources of variability and uncertainty in earthquake spectral stress‐drop estimates through quantitative comparison of submitted results from independent analyses. The dataset includes nearly 13,000 earthquakes ranging from M 1 to 7 during a two‐week period of the 2019 Ridgecrest sequence, recorded within a 1° radius. In this article, we report on 56 unique submissions received from 20 different groups, detailing spectral corner frequencies (or source durations), moment magnitudes, and estimated spectral stress drops. Methods employed encompass spectral ratio analysis, spectral decomposition and inversion, finite‐fault modeling, ground‐motion‐based approaches, and combined methods. Initial analysis reveals significant scatter across submitted spectral stress drops spanning over six orders of magnitude. However, we can identify between‐method trends and offsets within the data to mitigate this variability. Averaging submissions for a prioritized subset of 56 events shows reduced variability of spectral stress drop, indicating overall consistency in recovered spectral stress‐drop values.

California↗

USGS2020

VS30 and Dominant Site Frequency (⁠fd⁠) as Provisional Station ML Corrections (⁠dML⁠) in California

New seismic stations added to a regional seismic network cannot be used to calculate local magnitude ( ⁠ M L "> M L ⁠ ) until a revised regionwide amplitude decay function is developed. Each station must record a minimum number of local and regional earthquakes that meet specific amplitude requirements prior to recalibration of the amplitude decay function. Station component adjustments ( ⁠ d M L "> d M L ⁠ ; Uhrhammer et al. , 2011 ) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of d M L "> d M L ⁠ for long‐running stations. Therefore, there can be significant delay between when a new station starts contributing real‐time waveform packets and when data can be included in magnitude determinations. We propose the use of known estimates of seismic site conditions such as the time‐averaged shear‐wave velocity ( ⁠ V S "> V S ⁠ ) of the upper 30 m ( ⁠ V S 30 "> V S 30 ⁠ ) and the site dominant frequency ( ⁠ f d "> f d ⁠ ) to calculate d M L "> d M L ⁠ ⁠ . Previously established d M L "> d M L ⁠ ⁠ , measured V S 30 "> V S 30 ⁠ ⁠ , and f d "> f d data are available for between 126 and 458 horizontal components (east–west and north–south) at 81 seismic stations in the California Integrated Seismic Network; d M L "> d M L ⁠ d M L "> data range from −1.10 to 0.39, V S 30 "> V S 30 ⁠ values range from 202 to 1464    m / s "> 1464 m / s ⁠ , and 440 f d "> f d values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find V S 30 "> V S 30 ⁠ and d M L "> d M L ⁠ exhibit a positive coefficient of determination ( ⁠ R = 0.59 "> R = 0.59 ⁠ ), indicating that as V S 30 "> V S 30 ⁠ increases, d M L "> d M L ⁠ increases. This implies that greater site amplification (lower V S 30 "> V S 30 ⁠ ⁠ ) results in smaller d M L "> d M L ⁠ ⁠ . f d "> f d and d M L "> d M L ⁠ also generally exhibit a positive correlation ( ⁠ R 2 < 0.56 "> R 2 < 0.56 ⁠ ), which implies lower d M L "> d M L values are related to site resonance at depth‐dependent frequencies. Using the developed relationships, V S 30 "> V S 30 ⁠ or f d "> f d measurements can be used to establish a provisional d M L "> d M L ⁠ for newly established stations. This procedure allows new stations to contribute to regional network M L "> M L determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.

California↗

USGS2020

Caltech/USGS Southern California Seismic Network (SCSN) and Southern California Earthquake Data Center (SCEDC): Data availability for the 2019 Ridgecrest sequence

The 2019 M6.4 and M7.1 Ridgecrest earthquake sequence occurred in the eastern California shear zone (ECSZ). The mainshock ruptured the Little Lake fault zone and aftershocks extended from the Garlock fault in the south, to the southern end of the 1872 M7.5 Owens Valley earthquake rupture in the north. We present data from the Southern California Seismic Network (SCSN) and partner seismic networks recorded by the SCSN in the region. These time series data and related products such as the SCSN earthquake picks and catalogs, available from the Southern California Earthquake Data Center (SCEDC), provide the most comprehensive seismic datasets for the 2019 Ridgecrest earthquake sequence.

California↗

USGS2017

The Station Information System (SIS): A centralized seismic station repository for populating, managing, and distributing metadata

Creating, maintaining, and archiving accurate station metadata is critical for successful seismic network operations, data discovery, and research. The Station Information System (SIS) is a centralized repository of seismic station equipment inventory, instrument response, and site information of stations operated by regional seismic networks (RSNs) of the Advanced National Seismic System (ANSS; Filson and Arabasz, 2017 ). It has a web‐based user interface that enables the creation and manipulation of the corresponding metadata. The system can track the installation, maintenance, and removal of equipment from a site, which often results in the creation of new metadata epochs. SIS also computes the overall response, including gain, of a data channel by combining the responses of the underlying hardware components. SIS distributes this information in standard formats such as Federation of Digital Seismic Networks StationXML and dataless Standard for the Exchange of Earthquake Data. SIS can also be used to manage inventory of field equipment such as power, telemetry, or Global Positioning System antenna, as well as links to other site‐related repositories external to SIS to give the network operator the most complete view of a site and the overall network. This article summarizes the main features in SIS. We present its basic infrastructure, holdings, workflow, and how RSNs retrieve data from it. We also explain the reasoning to pursue one centralized repository and why it supports the goals of SIS and the ANSS. We demonstrate that by providing the ANSS network operator with a comprehensive site view, SIS enables the production of high‐quality metadata, a necessary prerequisite for producing high‐quality seismic data.

Seismological Research Letters↗

USGS2004

Geotechnical characterization of TriNet sites: A status report

The TriNet project, launched in 1997, created an improved, real-time seismic monitoring network in Southern California. Planning of the network began in 1995 ( e.g. , Heaton et al. , 1996 ), building on the success of the earlier TERRAscope network, which included 24 digital broadband and strong-motion instruments throughout Southern California ( e.g. , Kanamori et al. , 1993 ). At the end of the five-year TriNet project the network comprised 150 real-time digital broadband stations and another 400 strong-motion sensors, 50 of which were also real-time. This network is now recording digital broadband data for Southern California earthquakes at an unprecedented rate, data that are already proving valuable for investigations of earthquake sources and regional wave propagation, as well as earthquake response. In this report we describe an ongoing effort aimed at a full geotechnical characterization of the newly installed TriNet sets. Shallow geologic structure is known to play a substantial role in controlling the ground motions recorded at any site. Documentation of amplified ground motion at soft-sediment sites can be found among even early macroseismic observations of strong ground motions ( e.g. , Drake, 1815 ). Seismic waves are also now known to be strongly affected by deep basin structure as well ( e.g. , Frankel et al. , 1991 ; Field, 2000 ; Joyner, 2000 ). It will be necessary to understand these effects to exploit fully the rich data set being recorded at TriNet sites. Clearly, the nature of site conditions and site response at the recording sites must be understood for studies focused on ground motions and hazard from future large earthquakes, but it is also necessary to understand these effects to conduct earthquake source studies of both large and small earthquakes. Our multifaceted site characterization project involves geological/geotechnical site investigations, database development, and investigation of empirical amplification factors determined from broadband and strong-motion data recorded to date. Our goal is to complete a first-order geologic site characterization by 2005 and then employ appropriate methods to obtain direct constraint on shallow shear-wave velocity structure at each site. In the latter effort we will endeavor to find the most accurate and cost-effective methods to quantify geotechnical parameters at sites that have ranges of geologic site conditions and cultural settings. Our long-term goal is to obtain direct estimates of the average shear-wave velocity in the upper 30 m, Vs30, at each site. If velocity information is available to greater depths, for example at sites characterized under the ROSRINE project ( http://geoinfo.usc.edu/rosrine/ ), this information will be included in the database as well. The original motivation for using Vs30 to characterize near-surface velocity was pragmatic, determined by such factors as the typical reach of a drill rig in a single day. The parameter has become the accepted standard with which many seismic recording sites are characterized, however. In a recent study using mainshock and aftershock recordings at sites in the Los Angeles region, Wald and Mori ( 2000 ) observed good correlation between Vs30 and amplification at 1-7 Hz, albeit with significant scatter. The purpose of this report is threefold. First, we describe our ongoing efforts and present site characterization results collected to date for 62 broadband and strong-motion stations in and around the greater Los Angeles metropolitan region. Second, we present preliminary results that illustrate how improved geologic site characterizations can improve the correlation between site conditions and site response. Finally, we invite feedback from the community to guide our future investigations, in particular with respect to our ongoing database development efforts.

California↗