Modern Digital Seismology: Instrumentation, and Small Amplitude Studies in the Engineering World

The recording of ground motions is a fundamental part of both seismology and earthquake engineering. The current state-of-the-art 24-bit continuously recording seismic station is described, with particular attention to the frequency range and dynamic range of the seismic sensors typically installed. An alternative method of recording the strong-motions would be to deploy a velocity sensor rather than an accelerometer. This instrument has the required ability to measure the strongest earth motions, with enhanced long period sensitivity. An existing strong motion velocity sensor from Japan was tested for potential use in US seismic networks. It was found to be incapable of recording strong motions typically observed in the near field of even moderate earthquakes. The instrument was widely deployed near the M8.3 Sept 2003 Tokachi-Oki earthquake. The dataset corroborated our laboratory observations of low velocity saturations. The dataset also served to show all inertial sensors are equally sensitive to tilting, which is widespread in large earthquakes. High-rate GPS data is also recorded during the event. Co-locating high-rate GPS with strong motion sensors is suggested to be currently the optimal method by which the complete and unambiguous deformation field at a station can be recorded. A new application of the modern seismic station is to locate them inside structures. A test station on the 9th floor of Millikan Library is analysed. The continuous data-stream facilitates analysis of the building response to ambient weather, forced vibration tests, and small earthquakes that have occurred during its lifetime. The structure's natural frequencies are shown to be sensitive not only to earthquake excitation, but rainfall, temperature and wind. This has important implications on structural health monitoring, which assumes the natural frequencies of a structure do not vary significantly unless there is structural damage. Moderate to small earthquakes are now regularly recorded by dense, high dynamic range networks. This enhanced recording of the earthquake and its aftershock sequences makes possible the development of a Green's Function deconvolution approach for determining rupture parameters

Results of Millikan Library Forced Vibration Testing

This report documents an investigation into the dynamic properties of Millikan Library under forced excitation. On July 10, 2002, we performed frequency sweeps from 1 Hz to 9.7 Hz in both the East-West (E-W) and North-South (N-S) directions using a roof level vibration generator. Natural frequencies were identified at 1.14 Hz (E-W fundamental mode), 1.67 Hz (N-S fundamental mode), 2.38 Hz (Torsional fundamental mode), 4.93 Hz (1st E-Wovertone), 6.57 Hz (1st Torsional overtone), 7.22 Hz (1st N-S overtone), and at 7.83 Hz (2nd E-Wovertone). The damping was estimated at 2.28% for the fundamental E-W mode and 2.39% for the N-S fundamental mode. On August 28, 2002, a modal analysis of each natural frequency was performed using the dense instrumentation network located in the building. For both the E-W and N-S fundamental modes, we observe a nearly linear increase in displacement with height, except at the ground floor which appears to act as a hinge. We observed little basement movement for the E-W mode, while in the N-S mode 30% of the roof displacement was due to basement rocking and translation. Both the E-W and N-S fundamental modes are best modeled by the first mode of a theoretical bending beam. The higher modes are more complex and not well represented by a simple structural system

Potential Advantages of a Strong-motion Velocity Meter over a Strong-motion Accelerometer

This study examines whether it would be better to deploy a velocity-recording strong-motion instrument in place of existing force-balance accelerometers. The proposed instrument would be comparable to a low-gain version of existing broadband seismometers. Using a large suite of Earth signals, we compare such a hypothetical long-period low-gain velocity seismometer (with a clipping level set to ±5 m/s) with the existing ±2 g clipping Kinemetrics FBA-23 accelerometer. We show that there are significant advantages in the deployment of the proposed instrument over an accelerometer

An Evaluation of the SCSN Moment Tensor Solutions: Robustness of the M_w Magnitude Scale, Style of Faulting, and Automation of the Method

We have generated moment tensor solutions and moment magnitudes (M_w) for >1700 earthquakes of local magnitude (M_L) >3.0 that occurred from September 1999 to November 2005 in southern California. The method is running as an automated real-time component of the Southern California Seismic Network (SCSN), with solutions available within 12 min of event nucleation. For local events, the method can reliably obtain good-quality solutions for M_w with M_L >3.5, and for the moment tensor for events with M_L >4.0. The method uses the 1D Time-Domain INVerse Code (TDMT_INVC) software package (Dreger, 2003). The Green’s functions have been predetermined for various velocity profiles in southern California, which are used in the inversion of observed three-component broadband waveforms (10–100 sec), using data from at least four stations. Moment tensor solutions have an assigned quality factor dependent on the number of stations in the inversion, and the goodness of fit between synthetic and observed data. If a minimum quality factor is attained, the M_L or M_w is 5.0 or greater, and if the event is in the southern California reporting regions, the M_w will be the official SCSN/CISN magnitude. The M_w from the high-quality solutions determined from our method generally correlate very well with reviewed M_L, except in regions at the perimeter of the network. The M_w reported here indicates the SCSN M_L systematically underestimates the magnitude in the Mexicali region of Baja California, Mono Lakes area, Coso region, and the Brawley seismic zone, and overestimates the magnitude in the Coastal Ranges. Comparisons of the moment tensors determined using this model are made with Harvard Centroid Moment Tensors generated for larger earthquakes in the California region, and recent 3D models for events in the Los Angeles region, with excellent correlation. Most of the earthquakes with good-quality solutions exhibit strike-slip faulting, in particular, along the major late Quaternary strike-slip faults. Thrust faulting on east– west-striking planes is observed along the southern edge of the Transverse Ranges, while northwest-striking thrust faulting is observed in the Coastal Ranges. Normal faulting is most common in Baja California and southern Sierra Nevada including the western Basin and Range region. Poor-quality solutions with unreliable M_w are caused by excessive background noise in the waveforms. Small events (M_L < 4.0) can be affected by ambient noise or teleseisms, but larger events can also have unreliable solutions if they follow a recent large regional event

Moment tensor inversions of icequakes on Gornergletscher, Switzerland

We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green’s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M_w-1.5 using a full-waveform inversion method usually applied to moderate seismic events (M_w>4) recorded at local to regional distances (≈50–700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting

Evidence for Universal Earthquake Rupture Initiation Behavior

Earthquake onsets provide a unique opportunity to study physical rupture processes because they are more easily observable than later rupture stages. Despite this relative simplicity, the observational basis for rupture onsets is unclear. Numerous reports of evidence for magnitude-dependent rupture onsets (which imply deterministic rupture behavior, e.g. Colombelli et al., 2014) stand in contradiction to a large body of physics-based rupture modeling efforts, which are mostly based on inherently non-deterministic principles (e.g. Rice, 1993). Here we make use of the abundance of short-distance recordings available today; a magnitude-dependency of onsets should appear most prominently in such recordings. We use a simple method to demonstrate that all ruptures in the studied magnitude range (4 < M < 8) share a universal initial rupture behavior and discuss ensuing implications for physical rupture processes and earthquake early warning

The Observed Wander of the Natural Frequencies in a Structure

The Southern California Seismic Network (scsn) has recently installed seismic stations in two buildings on the Caltech campus (Millikan Library and the Broad Center). Continuous real-time accelerometer data from these structures are now freely available to the community. This dataset provides a new opportunity to observe, and better understand, the variances in the primary dynamic property of a building system, its natural frequencies. Historical data (triggered strong-motion records, ambient and forced vibration tests) from the well-studied Millikan Library show dramatic decreases in natural frequencies, attributed mainly to moderately large local earthquakes. The current forced vibration east–west fundamental frequency is 22% lower than that originally measured in 1968. Analysis of the new continuous data stream allows the examination of other previously unrecognized sources of measurable change in the fundamental frequencies, such as weather (wind, rain, and temperature), as well as nonlinear building vibrations from small local and moderate regional earthquakes. Understanding these nonlinear shifts is one of the long-term goals of real-time building instrumentation and is critical if these systems are to be used as a postearthquake damage assessment tool

The neural processing of moral sensitivity to issues of justice and care.

The empirical and theoretical consideration of ethical decision making has focused on the process of moral judgment; however, a precondition to judgment is moral sensitivity, the ability to detect and evaluate moral issues [Rest, J. R. (1984). The major components of morality. In W. Kurtines & J. Gewirtz (Eds.), Morality, moral behaviour, and moral development (pp. 24–38). New York, NY: Wiley]. Using functional magnetic resonance imaging (fMRI) and contextually standardized, real life moral issues, we demonstrate that sensitivity to moral issues is associated with activation of the polar medial prefrontal cortex, dorsal posterior cingulate cortex, and posterior superior temporal sulcus (STS). These activations suggest that moral sensitivity is related to access to knowledge unique to one\u27s self, supported by autobiographical memory retrieval and social perspective taking. We also assessed whether sensitivity to rule-based or “justice” moral issues versus social situational or “care” moral issues is associated with dissociable neural processing events. Sensitivity to justice issues was associated with greater activation of the left intraparietal sulcus, whereas sensitivity to care issues was associated with greater activation of the ventral posterior cingulate cortex, ventromedial and dorsolateral prefrontal cortex, and thalamus. These results suggest a role for access to self histories and identities and social perspectives in sensitivity to moral issues, provide neural representations of the subcomponent process of moral sensitivity originally proposed by Rest, and support differing neural information processing for the interpretive recognition of justice and care moral issues