Geology topics

Antony Martin

Publications and source records attributed to Antony Martin.

8 recordsLinked to original sources

USGS2024

Are the horizontal-to-vertical spectral ratios of earthquakes and microtremors the same?

We consider the similarities and differences between earthquake and microtremor horizontal‐to‐vertical spectral ratios (eHVSR and mHVSR, respectively) using a dataset of 161 sites in southern California. Quantitative comparisons are made in terms of the eHVSR and mHVSR lognormal median curves, as well as the frequencies and amplitudes associated with the fundamental‐ and higher‐mode resonances where present. The results show only 58% of the eHVSR–mHVSR pairs agree in terms of their median curve and only 25% of the eHVSR–mHVSR pairs agree in terms of shared resonances, which increases to 68% if flat HVSRs are considered equivalent. Furthermore, while the shared resonances match very well in terms of frequency (root mean square error, RMSE, <0.11 Hz), the amplitudes of those resonances do not agree (RMSE >1.6). These findings demonstrate that while eHVSR and mHVSR agree at some sites, they are not equivalent at all sites. To investigate if the agreement between eHVSR and mHVSR could be related to features of the microtremor data, earthquake recordings, and/or the site conditions, three machine learning (ML) models at varying levels of interpretability are presented. The ML models—which include multivariate logistic regression, gradient‐boosted trees, and support vector machines—show only partial success at using site‐specific data to predict whether eHVSR and mHVSR will likely agree in terms of their median curve (accuracy of 78%) and number of resonances (accuracy of 84%). Therefore, we conclude that while eHVSR and mHVSR can be quite similar in terms of resonant frequencies at some sites, they are not identical at all sites. Furthermore, preliminary evidence shows that the agreement of eHVSR and mHVSR can be predicted a priori given features of the microtremor measurements, earthquake recordings, and site conditions, although a larger dataset will be necessary for developing a robust predictive model.

California↗

USGS2022

Flexible multimethod approach for seismic site characterization

We describe the flexible multimethod seismic site characterization technique for obtaining shear-wave velocity ( V S ) profiles and derivative information, such as the time-averaged V S of the upper 30 m ( V S30 ). Simply stated, the multimethod approach relies on the application of multiple independent noninvasive site characterization acquisition and analysis techniques utilized in a flexible field-based approach through a decision process based on primary factors such as the investigator’s available equipment, field logistics (e.g., available array deployment space, urban versus rural), geologic setting, and funding among other primary factors. In a multimethod approach, sites ideally should be characterized using both active and passive noninvasive (i.e., no drilling and only minimal disturbance to the ground surface) methods. Almost without exception, we recommend the use of at least one active-source technique for analyzing surface waves, which in the current state of practice would include one or more of the following: spectral analysis of surface waves (SASW; commonly Rayleigh waves) and multichannel analysis of surface waves (MASW; Rayleigh and/or Love waves). In addition, passive-source microtremor array methods (MAMs) using linear (one-dimensional) and two-dimensional arrays may be appropriate or even required for characterizing a given site. Similarly, the microtremor horizontal-to-vertical spectral ratio (mHVSR) method can be valuable for identifying shallow rock sites. Finally, P- and SH-wave seismic refraction methods warrant consideration, especially at rock and shallow-rock sites.

Journal of Seismology↗

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

Graphical Dispersion Plot Editor (DPE) for seismic-site characterization by using multiple surface-wave methods

Introduction To understand the behavior of potentially damaging ground motions during earthquakes, seismic-site effects are routinely characterized by using the dispersion of surface waves. Many methods exist to measure dispersion; these methods have various advantages and disadvantages, but they all yield dispersion data that must be inverted for shear-wave velocity. This report presents a graphical tool for efficiently removing spurious data as well as combining data from multiple methods prior to inversion. The Dispersion Plot Editor (DPE) program presented here (version 1.5) is coded in Python 3, which is open source and platform independent. DPE accepts input dispersion data as one or more delimited text files. The program plots the data in useful forms, including both scattered points and an interpolated heat map. The user selects points to delete by drawing arbitrary shapes with the mouse cursor. After the spurious data are removed, the user may represent the acceptable data with a dispersion curve. The acceptable data and the representative dispersion curve are output as separate comma-delimited text files. Images of the plotted data and the representative dispersion curve may also be saved.

Open-File Report↗

USGS2019

Precision of VS30 values derived from noninvasive surface wave methods at 31 sites in California

We study the inter- and intra-method variability of VS30 results by inverting/forward-modeling individual dispersion data for 31 seismographic stations located in California where combinations of surface-wave methods were applied and the minimum recorded wavelength from each method satisfies the 30-meter depth criteria. These methods consist of noninvasive geophysical (active and passive surface-wave techniques) multi-station approaches, including the Multi-channel Analysis of Surface Waves (MASW; Rayleigh and Love waves), Spectral Analysis of Surface Waves (SASW), Microtremor Array [using Extended Spatial Autocorrelation (ESAC) processing methods], and Refraction Microtremor (ReMi) methods. Depending on the apparent geologic or seismic complexity of the site, field crews applied one or a combination of these methods whenever economically feasible to estimate the one-dimensional shear-wave velocity (VS) profile and calculate VS30, the time-averaged VS to a depth of 30 m. For each of the 31 sites, we find both types of variability in VS30 estimates generally remain insignificant (arithmetic mean of 5 % difference). We also find similar results (3 %) when we evaluate individual-method based VS30 estimates against composite-method based estimates. We note that VS30 values vary insignificantly when using a combination of complementary methods, e.g., active MASW data combined with passive MAM data, and that the most reliable results are also based on close fitting of the theoretical dispersion data to the representative (experimental) dispersion data.

California↗

USGS2017

Geophysical characterization of seismic station sites in the United States – The importance of a flexible, multi-method approach

Noninvasive geophysical site characterization methods were used in two recent projects to obtain shear-wave velocity (VS) profiles to a minimum depth of 30 m and the time-averaged VS of the upper 30 meters (VS30) at seismic station sites. These projects include the 2009 American Recovery and Reinvestment Act (ARRA) funded U.S. Geological Survey site characterization project for 191 sites in California and the Central-eastern United States (CEUS), and the 2012 Electric Power Research Institute (EPRI) funded project for 33 additional CEUS sites. These sites are located in rural to urban settings with topographic conditions ranging from relatively flat sedimentary basins to mountaintop ridges. About 60 percent of the ARRA sites and 80 percent of the EPRI sites are located on rock or have thin sediment cover over rock, including Quaternary volcanic rock, Tertiary sediments and sedimentary rock, and Mesozoic (or older) crystalline or sedimentary rock. The remaining sites consist of thick sequences of Quaternary sediments overlying older sediments and rock. ARRA sites were characterized using non-invasive active and passive surface-wave methods, including the horizontal-tovertical spectral ratio (HVSR) method and one or more of the following: spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW; Rayleigh and Love waves) and, occasionally, array microtremor (linear and 2-D arrays) methods. P-wave seismic refraction data were also acquired at rock and shallow-rock sites. S-wave seismic refraction and/or Love-wave MASW methods were applied at sites where characterization proved difficult with Rayleighwave methods. Based on our experience from the ARRA project, we acquired Rayleigh- and Love-wave based MASW and P- and S-wave refraction data for the EPRI project at CEUS sites. The HVSR method was found to be useful for identifying shallow-rock sites and for evaluating the relative variability of the depth-to-rock interface beneath the seismic station and the testing array(s). The fundamental mode modeling assumption was generally valid at most of these sites; nevertheless, multi-mode or effective-mode modeling routines were occasionally required, particularly in the case of shallow high-velocity layers. Deep sediment sites were characterized using active and, when appropriate, passive surface-wave based methods. Rock and shallow sediment sites were generally more challenging to characterize than deep sediment sites. About 10 percent of rock sites could not be characterized using surface wave methods, thus these sites were characterized using body-wave refraction methods. Love wave methods were found to be more effective than Rayleigh wave methods at some rock and shallow-rock sites (e.g., sites with shallow rock and sites with a thin low-velocity, highly attenuating surface layer). Lateral velocity variability was found to be very common at rock and shallow-rock sites, often causing significant scatter in the surface-wave dispersion data. Seismic refraction models have demonstrated that it may not be unusual for VS30 to vary by 20 percent, or more, over small distances (several tens of meters) at such sites. Based on these experiences, it is important to consider the application of combinations of methods when using noninvasive geophysical approaches to characterize seismic site conditions.

Conference Paper↗

USGS2014

Advantages of active love wave techniques in geophysical characterizations of seismographic station - Case studies in California and the central and eastern United States

Active-source Love waves, recorded by the multi-channel analysis of surface wave (MASLW) technique, were recently analyzed in two site characterization projects. Between 2010 and 2012, the 2009 American Recovery and Reinvestment Act (ARRA) funded GEOVision to conduct geophysical investigations at 191 seismographic stations in California and the Central Eastern U.S. (CEUS). The original project plan was to utilize active and passive Rayleigh wave-based techniques to obtain shear-wave velocity (VS) profiles to a minimum depth of 30 m and the time-averaged VS of the upper 30 meters (VS30). Early in this investigation it became clear that Rayleigh wave techniques, such as multi-channel analysis of surface waves (MASRW), were not suited for characterizing all sites. Shear-wave seismic refraction and MASLW techniques were therefore applied. In 2012, the Electric Power Research Institute funded characterization of 33 CEUS station sites. Based on experience from the ARRA investigation, both MASRW and MASLW data were acquired by GEOVision at 24 CEUS sites. At shallow rock sites, sites with steep velocity gradients, and, sites with a thin, low velocity, surficial soil layer overlying stiffer sediments, Love wave techniques generally were found to be easier to interpret, i.e., Love wave data typically yielded unambiguous fundamental mode dispersion curves and thus, reduce uncertainty in the resultant VS model. These types of velocity structure often excite dominant higher modes in Rayleigh wave data, but not in the Love wave data. It is possible to model Rayleigh wave data using multi- or effective-mode techniques; however, extraction of Rayleigh wave dispersion data was found to be difficult in many cases. These results imply that field procedures should include careful scrutiny of Rayleigh wave-based dispersion data in order to also collect Love wave data when warranted.

Conference Paper↗

USGS2013

ARRA-funded V S30 measurements using multi-technique approach at strong-motion stations in California and central-eastern United States

Funded by the 2009 American Recovery and Reinvestment Act (ARRA), we conducted geophysical site characterizations at 191 strong-motion stations: 187 in California and 4 in the Central-Eastern United States (CEUS). The geophysical methods used at each site included passive and active surface-wave and body-wave techniques. Multiple techniques were used at most sites, with the goal of robustly determining VS (shear-wave velocity) profiles and V S30 (the time-averaged shear-wave velocity in the upper 30 meters depth). These techniques included: horizontal-to-vertical spectral ratio (HVSR), two-dimensional (2-D) array microtremor (AM), refraction microtremor (ReMi™), spectral analysis of surface wave (SASW), multi-channel analysis of surface waves (Rayleigh wave: MAS R W; and Love wave: MAS L W), and compressional- and shear-wave refraction. Of the selected sites, 47 percent have crystalline, volcanic, or sedimentary rock at the surface or at relatively shallow depth, and 53 percent are of Quaternary sediments located in either rural or urban environments. Calculated values of VS30 span almost the full range of the National Earthquake Hazards Reduction Program (NEHRP) Site Classes, from D (stiff soils) to B (rock). The NEHRP Site Classes based on V S30 range from being consistent with the Class expected from analysis of surficial geology, to being one or two Site Classes below expected. In a few cases where differences between the observed and expected Site Class occurred, it was the consequence of inaccurate or coarse geologic mapping, as well as considerable degradation of the near-surface rock. Additionally, several sites mapped as rock have Site Class D (stiff soil) velocities, which is due to the extensive weathering of the surficial rock.

California↗