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.

Inferring Critical Slip-Weakening Distance from Near-Fault Accelerogram of the 2014 Mw 6.2 Ludian Earthquake

To better assess potential earthquake hazards requires a better understanding of fault friction and rupture dynamics. Critical slip-weakening distance (Dc) as one of the key friction parameters, however, is hard to determine on natural faults. For strike-slip earthquakes, we may directly estimate the Dc from Dc″—the double near-fault ground displacement at the time of the peak velocity (Fukuyama and Mikumo, 2007). Yet near-fault observations are very few, and, thus, there were only limited earthquakes with such Dc″ estimation. In 2014, an Mw 6.2 strike-slip event—the Ludian earthquake—occurred in southwest China. The strong-motion station (LLT) that is ∼0.45 km from the fault recorded the earthquake and enabled us to estimate Dc″ from the accelerograms. We inspect the polarity of the accelerometers and compare the integrated velocities with waveforms of nearby broadband stations. We also analyze the particle motion at the LLT station and retrieve the earthquake initiation at the intersection of the conjugated faults. We then apply the baseline correction to the seismograms, recover the ground velocities and displacements, and obtain the value of Dc″=0.1 m at the station. The recovered final displacements are compared with the predicted ground displacements of a finite-fault model. The discrepancy of fault-parallel displacements might imply limited underestimation of Dc″, and the estimated upper limit is 0.3 m. Comparison between the Dc″ and final slip on the fault patch follows the scaling of previous larger earthquakes. Analysis of the near-fault accelerometer data enhances our understanding on the earthquake source of the Ludian earthquake. This case extends the lower magnitude boundary of the Dc″ values obtained from natural faults and opens a window into the friction property in the seismically active region.

Spatial analysis for an evaluation of monitoring networks: examples from the Italian seismic and accelerometric networks

In this work, we propose a statistical approach to evaluate the coverage of a network based on the spatial distribution of its nodes and the target information, including all those data related to the final objectives of the network itself. This statistical approach encompasses descriptive spatial statistics in combination with point pattern techniques. As case studies, we evaluate the spatial arrangements of the stations within the Italian National Seismic Network and the Italian Strong Motion Network. Seismic networks are essential tools for observing earthquakes and assessing seismic hazards, while strong motion (accelerometric) networks allow us to describe seismic shaking and to measure the expected effects on buildings and infrastructures. The capability of both networks is a function of an adequate number of optimally distributed stations. We compare the seismic network with the spatial distributions of historical and instrument seismicity and with the distribution of well-known seismogenic sources, and we compare the strong motion station distribution with seismic hazard maps and the population distribution. This simple and reliable methodological approach is able to provide quantitative information on the coverage of any type of network and is able to identify critical areas that require optimization and therefore address areas of future development.

Comparison of Coseismic Displacement Obtained from GEONET and Seismic Networks

It is generally recognized that permanent displacements estimated by the double integration of acceleration records need a suitable baseline correction. Current baseline correction methods have been validated by comparing the displacements with those from the Global Positioning System (GPS) records nearby, but GPS stations that are sufficiently close to a strong-motion station are scarce. Because the [Formula: see text] Tohoku-Oki earthquake produced geodetic displacements in a wide area and because dense strong-motion and GPS networks are available in Japan, we interpolated the displacements calculated from GPS records to estimate the permanent displacements at 508 strong-motion stations. The estimated results were used to evaluate uncertainties in permanent displacements obtained using two baseline correction methods, and results were found to be reliable only for KiK-net’s borehole acceleration records. A new joint parameter search method for the surface and borehole records was further proposed, and reliable results were obtained for KiK-net’s surface records.

Systematic Ground Motion Observations in the Canterbury Earthquakes and Region-Specific Non-Ergodic Empirical Ground Motion Modeling

This paper presents an examination of ground motion observations from 20 near-source strong motion stations during the most significant ten events in the 2010–2011 Canterbury earthquake sequence to examine region-specific systematic effects based on relaxing the conventional ergodic assumption. On the basis of similar site-to-site residuals, surfical geology, and geographical proximity, 15 of the 20 stations are grouped into four sub-regions: the Central Business District; and Western, Eastern, and Northern suburbs. Mean site-to-site residuals for these sub-regions then allows for the possibility of non-ergodic ground motion prediction over these sub-regions of Canterbury, rather than only at strong motion station locations. The ratio of the total non-ergodic vs. ergodic standard deviation is found to be, on average, consistent with previous studies, however it is emphasized that on a site-by-site basis the non-ergodic standard deviation can easily vary by ±20%.

Comparison of a Christchurch-specific CPT-Vs correlation and Vs derived from surface wave analysis for strong motion station velocity characterisation

The Christchurch-specific empirical correlation between shear wave velocity (Vs) and cone penetration test (CPT) data developed by McGann et al. [1-3] for the non-gravel soils of the Christchurch and Springston Formations is evaluated through comparison to Vs profiles obtained using surface wave analysis techniques at twelve Christchurch strong motion stations. These comparisons highlight the similarities and differences between the Vs profiles obtained from each approach, and allow for an assessment of the relative strengths and weaknesses of each. It is shown that, with known differences, the results of the surface wave analysis and CPT correlation compare well in terms of their independently obtained Vs magnitudes. The sources of the differences between the results of each method are identified and discussed.

Near-Surface Geophysical Profiling Near Former Location of K-NET Tsukidate Strong Motion Station in Miyagi Prefecture, Japan

During the 2011 Earthquake off the Pacific Coast of Tohoku, high acceleration records with a PGA of 2.7 G were reported at the K-NET Tsukidate station (MYG004), where a maximum seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale was observed. However, no major damage to the wooden houses in the area surrounding the station was reported. The objectives of this study are to obtain a 2D shallow soil profile of the area around the Tsukidate strong motion station (MYG004) located on the top of a 5 m cliff, and also to provide basic material for a detailed understanding of the high accelerations during the earthquake. We conducted a seismic refraction survey west of the station, and we used a fullwaveform inversion of the acquired seismic data to retrieve a 2D shallow soil profile. The inverted 2D soil model underlines a clear lateral S-wave velocity variation in the surface layer, and comparisons to results of the microtremor measurements using an array and horizontal-to-vertical ratio conducted along the seismic survey line show significant similarities to the lateral velocity variation revealed by our 2D inversion. We also examined the effect of this lack of velocity homogeneity on the soil response, and we found that it could play an important role in amplifying the content of the high frequencies.

Dynamic site characterization of Christchurch strong motion stations

This paper details efforts to characterize the small-strain dynamic properties of 13 strong motion station (SMS) sites in the greater Christchurch, New Zealand area. These SMS recorded a unique set of ground motions (GM) from the 2010-2011 Canterbury earthquakes. Currently, little information about the subsurface layering and dynamic characteristics at these 13 SMS is available. Information provided by GeoNet consists only of generalised layering based on regional geological characteristics and nearby well logs, with no information on dynamic properties. Consequently, the seismic site classifications of these sites were largely based on assumptions. To better define the site classifications, we performed active- and passive-source surface wave testing to obtain shear wave velocity (Vs) profiles at each site. The Vs profiles were used to calculate the average Vs over the top 30 m of the subsurface and to estimate the natural period of vibration (Tn). Additionally, estimates of Tn were obtained by computing the horizontal-to-vertical spectral ratios from recorded GM at each SMS. Based on this new information, we have updated the site classifications at the 13 SMS sites tested; 10 of which ended up with a slightly different site classification than the original assumption (often one site class lower).

Data Analysis of the Euroseistest Strong Motion Array in Volvi (Greece): Standard and Horizontal-to-Vertical Spectral Ratio Techniques

In this study, the standard spectral ratio and the horizontal-to-vertical spectral ratio techniques are applied in order to study their effectiveness in investigating and quantifying the influence of geological conditions on strong ground motion. For this purpose, an accelerogram data set recorded at the Euroseistest array in the Mygdonia graben (lake Volvi area) near Thessaloniki, Greece, during the period April 1994 to June 1996 is used. Both experimental techniques show similar spectral ratio shapes with comparable fundamental resonant frequencies, which are well correlated with the well known geotechnical-geological conditions. Namely, the resonant frequency at the center of the valley is shifted to lower values, less than 1 Hz, while at the edge it is shifted to higher values, greater than 2 Hz. The horizontal-to-vertical spectral ratio technique is an effective method to estimate some basic characteristics of local site effects using a single accelerograph station. It reveals the fundamental resonant frequency of alluvial deposits by using only a single strong motion station, while the absolute level of the horizontal-to-vertical spectral ratio method tends to underestimate the amplification level compared to the standard spectral ratio technique.