Margaret Hellweg

Real‐time earthquake detection and hazard assessment by ElarmS across California

[1]xa0ElarmS is a network-based methodology for rapid earthquake detection, location and hazard assessment in the form of magnitude estimation and peak ground motion prediction. The methodology is currently being tested as part of the real-time seismic system in California leveraging the resources of the California Integrated Seismic Network (CISN) and the Advanced National Seismic System. A total of 603 velocity and acceleration sensors at 383 sites across the state stream waveform data to ElarmS processing modules at three network processing centers where waveforms are reduced to a few parameters. These parameters are then collected and processed at UC Berkeley to provide a single statewide prediction of future ground shaking that is updated every second. The system successfully detected the Mw 5.4 Alum Rock earthquake in northern California for which it generated an accurate hazard prediction before peak shaking began in San Francisco. It also detected the Mw 5.4 Chino Hills earthquake in southern California. The median system latency is currently 11.8 sec; the median waveform data latency is 6.5 sec.

Designing a Network‐Based Earthquake Early Warning Algorithm for California: ElarmS‐2

The California Integrated Seismic Network (CISN) is developing an earthquake early warning (EEW) demonstration system for the state of California. Within this CISN ShakeAlert project, three algorithms are being tested, one of which is the network-based Earthquake Alarm Systems (ElarmS) EEW system. Over the last three years, the ElarmS algorithms have undergone a large-scale reassessment and have been recoded to solve technological and methodological challenges. The improved algorithms in the new production-grade version of the ElarmS version 2 (referred to as ElarmS-2 or E2) code maximize the current seismic networks configuration, hardware, and software per- formance capabilities, improving both the speed of the early warning processing and the accuracy of the warnings. E2 is designed as a modular code and consists of a new event monitor module with an improved associator that allows for more rapid association with fewer triggers, while also adding several new alert filter checks that help minimize false alarms. Here, we outline the methodology and summarize the performance of this new online real-time system. The online performance from 2 October 2012 to 15 February 2013 shows, on average, ElarmS currently issues an alert 8:68 � 3:73 sa fter the first P-wave detection for all events across California. This time is reduced by 2 s in regions with dense station instrumentation. Standard deviations of magnitude, origin time are 0.4 magnitude units, 1.2 s, and the median location errors is 3.8 km. E2 successfully detected 26 of 29 earthquakes (MANSS >3:5) across California, while issuing two false alarms. E2 is now delivering alerts to ShakeAlert, which in turn distributes warnings to test users.

CISN ShakeAlert: An Earthquake Early Warning Demonstration System for California

To demonstrate the feasibility of earthquake early warning (EEW) in California, we have developed and implemented the CISN ShakeAlert demonstration system. A Decision Module combines estimates and uncertainties determined by three algorithms implemented in parallel, (tau _mathrm{{c}}-mathrm{{P}}_mathrm{{d}}) Onsite, Virtual Seismologist, and ElarmS, to calculate and report at a given time the most probable earthquake magnitude and location, as well as the likelihood of correct alarm. A User Display receives the alert messages in real-time, calculates the expected local shaking intensity, and displays the information on a map. Currently, CISN ShakeAlert is being tested by (sim )70 individuals and test users from industries and emergency response organizations in California. During the next 3 years we plan to expand this demonstration warning system to the entire US West Coast.

Performance of Several Low-Cost Accelerometers

Several groups are implementing low-cost host-operated systems of strong-motion accelerographs to support the somewhat divergent needs of seismologists and earthquake engineers. The Advanced National Seismic System Technical Implementation Committee (ANSS TIC, 2002), managed by the U.S. Geological Survey (USGS) in cooperation with other network operators, is exploring the efficacy of such systems if used in ANSS networks. To this end, ANSS convened a working group to explore available Class C strong-motion accelerometers (defined later), and to consider operational and quality control issues, and the means of annotating, storing, and using such data in ANSS networks. The working group members are largely coincident with our author list, and this report informs instrument-performance matters in the working group’s report to ANSS. Present examples of operational networks of such devices are the Community Seismic Network (CSN; csn.caltech.edu), operated by the California Institute of Technology, and Quake-Catcher Network (QCN; Cochran et al., 2009; qcn.stanford.edu; November 2013), jointly operated by Stanford University and the USGS. Several similar efforts are in development at other institutions. The overarching goals of such efforts are to add spatial density to existing Class-A and Class-B (see next paragraph) networks at low cost, and to include many additional people so they become invested in the issues of earthquakes, their measurement, and the damage they cause.

The polarization of volcanic seismic signals: medium or source?

Abstract With the proliferation of three-component seismometers on volcanoes, the temptation is great to use polarization analysis, as we do in earthquake seismology, to determine propagation direction and/or wavetype in order to locate and characterize the wave’s source. In volcano seismology, there are two impediments to such a procedure: the complexity of the volcano’s structure and the often long-lasting volcanic seismic signals. I develop a simple model of acoustic scattering and apply it to three simple, theoretical source signals, which represent the classes of signals encountered at volcanoes. When the medium is strongly scattering, the polarization at a receiver location for impulsive, sinusoidal or square-wave sources mimics the characteristics for volcanic shocks, tornillos with a single frequency and multichromatic tremor or tornillos. The polarization observed for the square-wave source resembles that observed for multichromatic tremor at Lascar Volcano, Chile, and multichromatic tornillos at Galeras Volcano, Colombia. At low frequencies, the particle motion is fairly linear and nearly constant, but cannot necessarily be used directly to indicate the direction to the source. At all frequencies, the particle motion exhibits some characteristics of Rayleigh waves, namely ellipticity of varying degrees and an ‘ RZ ’ product that oscillates with twice the frequency of the mode. If this model is correct, the lack of change of fundamental wavefield parameters, such as the polarization, for low frequencies during any given tornillo and from one tornillo to the next implies that the location of the source is stable to within a resolution of about 200 m.

California Integrated Seismic Network (CISN) Local Magnitude Determination in California and Vicinity

Determining local magnitude (M_L) in a manner that is uniform and internally consistent for earthquakes throughout California and the vicinity is an important component of the California Integrated Seismic Network (CISN). We present a new local magnitude attenuation function and corresponding station adjustments that are valid throughout California. The new attenuation function is an analytic function of the radial hypocentral distance between 1 and 500 km. Associated station adjustments are also available for 1185 horizontal seismometer and accelerometer channels from five seismic networks operating in California. The new attenuation function and adjustments provide several advantages to CISN. They allow a more robust M_L computation, the M_Ls are more consistent between northern and southern California than they have been in the past, and because adjustments are now available for more station-network-channel-location codes (SNCLs), M_Ls can be computed for small earthquakes in more locations than was previously possible. In addition to describing our method for calibrating the new CISN M_L, we also present a tool for adding adjustments for new or upgraded stations.

The multiparameter station at Galeras Volcano (Colombia): concept and realization

Abstract Volcanoes are complex systems, in which the interaction of many different physical and chemical factors and processes contribute to changes in activity. In the past 40 years, our ability to observe and quantify short-term changes in a volcano’s activity has improved due to the installation of seismometers and tiltmeters and the continuous records they provide. However, due to instrumental limitations, the observations have mainly been used phenomenologically, to draw inferences about possible changes on the basis of previous experience. Since 1995, the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR) and the Instituto de Investigacion e Informacion Geocientifica, Minero-Ambiental y Nuclear (INGEOMINAS) have been working to develop and deploy a multiparameter (MP) station on Galeras Volcano, Colombia. This station is designed to concurrently measure various geophysical and geochemical parameters. It includes three broadband seismometers at the crater rim, as well as a more remotely located, broadband seismic reference. At other locations in the crater or on the rim, electromagnetic probes, an infrasound sensor and a weather station are operating. The data from these sensors are digitized at each site with 24-bit digitizers and transmitted by spread-spectrum radio, via repeater when necessary, to the Observatorio Vulcanologico y Sismologico (OVP) in the city of Pasto. There they are received and displayed on a networked personal computer and recorded continuously. The data flow into the routine analysis procedures of the OVP and the continuous data are archived on CD. In addition to the other sensors, a system of specially developed sensors continuously monitors the chemistry and physics of the gases at fumaroles on the active cone. The data from this system are also transmitted in realtime to OVP and recorded. The continuous recordings of the MP station are supplemented by regular thermographic measurements of the surface temperature in the crater using an infrared camera. Joint analysis and interpretation of the data streams from the many sensors of the MP station will improve our understanding of the physical processes occurring in Galeras Volcano.

Parameterization of multichromatic tornillo signals observed at Galeras Volcano (Colombia)

In the past decade several of the ash eruptions at Galeras Volcano, Colombia, have been preceded by tornillos. These unusual tremor wavelets have quasi-sinusoidal waveforms with screw-like envelope profiles and can last up to several minutes. A swarm of tornillos occurred at Galeras Volcano between 8 December 1999 and 12 February 2000. These tornillos appear to be more complex than those previously recorded with the broadband instruments or with the short-period network of the Observatorio Vulcanologico y Sismologico in Pasto. They are multichromatic with a varying number of narrow spectral peaks between 1 and 20 Hz. We describe a procedure for parameterizing the tornillo signals in the time and frequency domains to determine signal parameters. In addition to wavelets like the tornillos, the procedure can be applied to random signals such as volcanic tremor. We derive distribution and correlation functions for the signal parameters determined from the swarm. These provide, along with the signal signature, constraints for modelling variations of the source process. From these observations we derive qualitative conclusions about the characteristics of differential equations which describe the underlying processes and excitation mechanisms as forced or self-excited oscillators.

Local Magnitude Tomography in California

Lateral variation in crustal attenuation of California is calculated by inverting 25,330 synthetic Wood–Anderson amplitudes from the California Integrated Seismic Network (CISN) for site, source, and path effects. Two-dimensional attenuation ( q or 1/ Q ) is derived from the path term, which is calculated via an iterative least-squares inversion that also solves for perturbations to the site and source terms. Source terms agree well with initial CISN M L s, and site terms agree well with a prior regression analysis; q ranges from low attenuation at 0.001 ( Q =1000) to high attenuation at 0.015 ( Q =66), with an average of 0.07 ( Q =143). The average q is consistent with an amplitude decay function (log A ) for California when q is combined with a simple geometrical spreading rate. Attenuation in California is consistent with the tectonic structure of California, with low attenuation in the Sierra batholith and high attenuation at The Geysers, at Long Valley, and in the Salton trough possibly due to geothermal effects. Also, path terms are an order of magnitude smaller than site and source terms, suggesting that they are not as important in correcting for M L .