Earthquake Early Warning relies on rapid detection and transmission of ground motions from an on-going earthquake. Current EEW systems use ground motions measured by seismometers installed across California. These seismic stations are part of the California Integrated Seismic Network (CISN), a collaborative organization of state and federal government agencies as well as universities. Additional information is provided below on the following topics:
- Introduction to the CISN
- Seismic Instrumentation Used in EEW
- System Delays Affecting Earthquake Detection
- Future Sources of Ground Motion Measurements
INTRODUCTION TO THE CISN
The CISN goals are:
- Operate a reliable and robust statewide system to record earthquake ground motions over the relevant range of frequencies and shaking levels
- Distribute information about earthquakes rapidly after their occurrence for emergency response and public information
- Create an easily accessible archive of California earthquake data for engineering and seismological research, including waveform data and derived products
- Maintain CISN infrastructure as a reliable state-of-the-art data collection, processing, and information distribution system
- Apply the latest research and technology to develop new algorithms for analyzing earthquake data and extracting more detailed information for new user products
- Maximize the use and benefit of real-time seismic information and other rapidly evolving tools and technologies through technology transfer to the user community
Six organizations have collaborated to form the CISN in order to further the goals of earthquake monitoring. The founding members of the CISN include: California Geological Survey, Caltech Seismological Laboratory, Berkeley Seismological Laboratory, USGS Menlo Park, USGS Pasadena, and the California Governor’s Office of Emergency Services. In addition to the core members, several organizations contribute data that enhances the capabilities of the CISN. Contributing members of the CISN include: University of California, Santa Barbara, University of California, San Diego, University of Nevada, Reno, University of Washington, California Department of Water Resources, Lawrence Livermore National Lab, and Pacific Gas and Electric.
SEISMIC INSTRUMENTATION USED IN EEW
The CISN consists of several different types of seismic instrumentation, reflecting the significant changes in technology over the last 30 years as well as the multiple goals of seismic monitoring. The network includes different types of sensors (short-period, broadband, and strong-motion) in different environments (surface and/or free field, structures, boreholes) with different recording and communications systems. Data from approximately 400 seismic stations across California are received in real time.
Figure 1. Map of seismic stations (dots) that form the California Integrated Seismic Network (CISN). Colors indicate operating agency.
SYSTEM DELAYS AFFECTING EARTHQUAKE DETECTION
The time required to detect and issue a warning for an earthquake is dependent on several factors:
- Distance between the earthquake source and the closest station. It takes a finite amount of time for seismic waves to travel from the source (e.g. the point on a fault that is breaking) to the seismic station. The first waves to arrive at a station are the lower amplitude P waves that travel at 5-6 km/s, on average. The large amplitude (more damaging) S waves travel at ~3 km/s. Therefore, the closer a station is to the fault, the more rapidly the earthquake can be detected. Accurate detections often depend on multiple ground motion measurements from more than one station; so, increasing the density of stations near the fault can improve detection times.
- Transfer of information to the regional networks. Data from multiple must be collected and analyzed at the regional seismic networks, so ground motion information must be transferred from the station to the central network. The CISN uses a variety of methods to send data back to the server to improve robustness, including radio links, phone lines, public/private internet, and satellite links. In addition, delays with packaging and sending the data from the station must be reduced. Recent upgrades to the CISN have been completed to reduce latencies in packaging and sending ground motion observations useful for EEW.
FUTURE SOURCE OF GROUND MOTION MEASUREMENTS
In the future, additional sources of ground motion observations may be integrated in the EEW algorithms. Additional data may be able to help reduce the time to detection and improve early estimates of earthquake magnitude and location. Some examples include:
- Real-time GPS displacements. Throughout California there over a hundred high sample rate Global Positioning System sensors that provide very accurate measurements of ground displacement. Measurements of ground displacement can be very useful for identifying large earthquakes that can have centimeters to meters of ground displacement. It can be challenging to recover displacements in real time because very accurate information is needed on the orbits of the GPS satellites. Several groups are currently developing algorithms to estimate GPS positions accurately in real-time and methods to integrate the information into existing EEW algorithms.
- Low-cost sensors hosted in homes, businesses, and schools. New sensor technologies have greatly reduced the cost of lower-resolution strong motion seismometers. These sensors use micro-electro-mechanical systems (MEMS) accelerometers that are contained on a single computer chip. Several groups have been exploring ways to utilize theses sensors to increase the number of strong motion sensors in urban areas. Two examples include the Quake-Catcher Network and the Community Seismic Network which connect sensors to personal computers installed in homes, businesses, and schools.