Overview

A user of ShakeAlert™ receives a message like this on the screen of his computer. The message alerts the user to how many seconds before the shaking waves arrive at their location and the expected intensity of shaking at that site. The shaking intensity follows the Modified Mercalli scale; an intensity of VI, as shown here, would mean the shaking is felt by everyone, people find it difficult to stand, and structures may suffer some damage. The warning message also displays a map with the location of the epicenter, the magnitude of the quake, and the current position of the P and S waves. In this example, the alert is for the ShakeOut scenario earthquake. (Public domain.)

Earthquake early warning systems use earthquake science and the technology of monitoring systems to alert devices and people when shaking waves generated by an earthquake are expected to arrive at their location. The seconds to tens of seconds of advance warning can allow people and systems to take actions to protect life and property from destructive shaking.

Even a few seconds of warning can enable protective actions such as:

• Public: Citizens, including schoolchildren, drop, cover, and hold on; turn off stoves, safely stop vehicles.
• Businesses: Personnel move to safe locations, automated systems ensure elevators doors open, production lines are shut down, sensitive equipment is placed in a safe mode.
• Medical services: Surgeons, dentists, and others stop delicate procedures.
• Emergency responders: Open firehouse doors, personnel prepare and prioritize response decisions.
• Power infrastructure: Protect power stations and grid facilities from strong shaking.

EEW systems are currently operating in several countries, and others are building them. Since 2006 the USGS has been working to develop EEW for the United States, with the help of several cooperating organizations including the California Geological Survey (CGS), the California Institute of Technology (Caltech), the California Office of Emergency Services (CalOES), the Moore Foundation, the University of California, Berkeley, the University of Washington, and the University of Oregon. The goal is to create and operate an EEW system for the highest risk areas of the United States beginning with the West Coast states: California, Washington, and Oregon.

A demonstration EEW system called ShakeAlert™ began sending test notifications to selected users in California in January 2012. The system detects earthquakes using the California Integrated Seismic Network (CISN),an existing network of about 400 high-quality ground motion sensors. CISN is a partnership between the USGS, State of California, Caltech, and University of California, Berkeley, and is one of seven regional networks that make up the Advanced National Seismic System (ANSS).

In February of 2016 the USGS, along with its partners, rolled-out the next-generation ShakeAlert™ early warning test system in California. This “production prototype” has been designed for redundant, reliable operations. The system includes geographically distributed servers, and allows for automatic fail-over if connection is lost. This next-generation system will not yet support public warnings but will allow selected early adopters to develop and deploy pilot implementations that take protective actions triggered by the ShakeAlert™ warnings in areas with sufficient coverage. The USGS has published an Implementation Plan with the steps needed to complete the system and begin issuing public alerts.

Earthquake early warning systems like ShakeAlert™ work because the warning message can be transmitted almost instantaneously, whereas the shaking waves from the earthquake travel through the shallow layers of the Earth at speeds of one to a few kilometers per second (0.5 to 3 miles per second). This diagram shows how such a system would operate. When an earthquake occurs, both compressional (P) waves and transverse (S) waves radiate outward from the epicenter. The P wave, which travels fastest, trips sensors placed in the landscape, causing alert signals to be sent ahead, giving people and automated electronic systems some time (seconds to minutes) to take precautionary actions before damage can begin with the arrival of the slower but stronger S waves and later-arriving surface waves. Computers and mobile phones receiving the alert message calculate the expected arrival time and intensity of shaking at your location. USGS image created by Erin Burkett (USGS) and Jeff Goertzen (Orange County Register). (Public domain.)

Earthquake Early Warning Around the World

Earthquake Early Warning systems are operational in several countries around the world, including Mexico, Japan, Turkey, Romania, China, Italy, and Taiwan. All of these systems rapidly detect earthquakes and track their evolution to provide warnings of pending ground shaking. Systems can vary depending on the local faults and the specific ground motion data available.

Examples of Early Warning Systems

• Mexico City has an EEW system that warns of strong shaking from large earthquakes that occur off of the country’s coast. The system consists of a series of sensors located along the coast that detect shaking from a large earthquake and rapidly determine the location and magnitude. Since Mexico City is located several hundred miles from the main plate boundary they can receive up to a minute or more of warning of the impending shaking for subduction zone earthquakes, but warning times are shorter for earthquakes that occur closer to the city. This system has been in operation since 1991.

• Japan currently has the most sophisticated early warning systems in the world. The warnings were initially developed for use in slowing and stopping high-speed trains prior to strong shaking. The success of that program in addition to the devastating effects of the 1995 Kobe earthquake paved the way for building a nationwide early warning system. Japan has built a dense network of seismic instruments to rapidly detect earthquakes. They have been issuing public warnings since 2007.

Time to Detect an Event

An earthquake early warning system on the west coast of the United States could provide up to tens of seconds of warning prior to shaking arriving. The time required to detect and issue a warning for an earthquake is dependent on several factors:

1. Distance between the earthquake source and the closest seismic network seismometer (station). It takes a finite amount of time (3–4 miles per second) for the first seismic waves to travel from the source (e.g. the point on a fault that is breaking) to the seismic station. Therefore, the closer a station is to where an earthquake begins, 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.
2. Transfer of information to the regional networks. Data from multiple stations is collected and analyzed by the regional seismic networks, so ground motion information must be transferred from the station to the processing center. Existing networks use 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 from packaging and sending the data from the station must be minimized to provide useful warning times.
3. Detection and characterization of an earthquake. Ground motion records received from the stations in real time are used to detect an earthquake and rapidly determine an initial location and magnitude of the event. We are developing multiple algorithms to estimate the earthquake information as rapidly as possible. Earthquakes can continue to grow in size over many seconds (the larger the earthquake generally the longer it takes to get to the final size), so magnitude estimates can also change through time as the evolving earthquake is tracked.
4. Shaking intensity threshold used to issue an alert. Alerts are issued for a region when the expected ground shaking intensity is above a minimum threshold. Alerts can be provided more quickly for low thresholds of ground shaking because the system doesn’t need to wait as long for the earthquake magnitude to grow (larger earthquakes produce high ground shaking intensities).

Current Status

Since 2008, the USGS Earthquake Hazards Program has supported research and development on earthquake early warning in partnership with Caltech, University of California Berkeley, and others, with goals to develop methods that would allow rapid detection of earthquakes in the western United States, to test and improve those methods using the California Integrated Seismic Network (CISN), to define the network improvements that would be needed to support a fully operational system, and to build a prototype system capable of providing early warnings to certain test users. Continued funding of and by the USGS complements funding from a private foundation recently awarded to the academic institutions (partners in the Advanced National Seismic System (ANSS) for research on and development of EEW components. This increase is part of an $8.6 million initiative to improve USGS disaster response capabilities through preparedness and robust monitoring.

The earthquake early warning system under development leverages federal and state investments already made in the ANSS that monitors earthquake activity in the US. By utilizing an existing, active seismic network, the early warning system can be tested and monitored daily through existing operations. Any infrastructure improvements required for EEW will also result in improved information for emergency response and aftershock forecasting. For instance, in 2011, upgrades to stations across California have been completed to reduce latencies in packaging and sending ground motion observations useful for EEW.

Testing the current system

Example of what an earthquake early warning may look like. (Public domain.)

As part of the collaboration with our university partners, the USGS is currently working with a group of trusted partners to determine real-world performance of ShakeAlert™, the prototype notification system that delivers the earthquake warnings, as well as document feedback for research and development by identifying the following:

• Industry-by-industry assessment of uses for early warning
• Cost-benefit analyses in the context of shaking risk
• Potential limitations of system
• Cost to the business or individual to implement ShakeAlert™

Beta Test User Group participants in Northern and Southern California have been selected using following criteria:

• User group must be pre-identified organizations that will be key users of early warning and able to provide the time, manpower and equipment necessary to perform the necessary tests of ShakeAlert™ and provide quality feedback regarding the operation, usage and delivery mechanism of the system. Beta Test Users need to utilize ShakeAlert™ in their daily operations.
• Identify key personnel within the organization to approve and facilitate a plan to activate ShakeAlert™ and provide feedback from their team of users.
• Test user must understand that the ShakeAlert™ system is under development and that there are limitations of the current system (robustness, networks coverage) and uncertainties in earthquake parameters and reporting (false positives and negatives).

Next Steps

Earthquake sensor density: California versus Japan. New sensors need to be added in California to shorten the CISN sensors spacing to approximately 12 miles to facilitate timely EEW. The shorter the station spacing, the smaller the blind zone will be because warnings can be issued faster. (Public domain.)

Improving the sensor network

The most important component of an earthquake early warning system is a dense network of seismic and geodetic stations with robust communications. Future development of the warning system will include the installation of larger numbers of seismic stations and upgrading station telecommunications. The current seismic station densities in California are currently much lower than the Japanese public alert system. New sensors are needed in California to reduce earthquake detection times allowing warnings to be issued faster.

Additional Sources of Ground Motion Measurements

In the future, additional sources of ground motion observations can 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:

1. Real-time GPS displacements. Throughout California there are over a hundred high sample rate Global Positioning System (GPS) sensors that provide very accurate measurements of ground displacement. Data is collected from several regional GPS network including from the Southern California Integrated GPS Network and Bay Area Regional Deformation Network. 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 research groups within the USGS and at collaborating universities are currently developing algorithms to estimate GPS positions accurately in real-time and methods to integrate the information into existing EEW algorithms.
2. 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. Scientists 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 that install low-cost sensors in homes, businesses, and schools.

A GPS antenna in southern California. Many of the GPS stations in California are currently being converted to send data in real time back to the networks. And, algorithms are being developed to process and use these data in the earthquake early warning alerts. (Public domain.)

MEMS accelerometer being tested for earthquake detection. These lower cost sensors may be used in urban areas to better map variation in shaking amplitudes across a region. In addition, the data may be useful for reducing the time needed to detect an earthquake. (Public domain.)

Issuing Warnings

Every available technology will be used to ensure that EEW messages reach as many people and as quickly as possible. Most currently available mass messaging technologies are too slow for EEW. Unlike the Japanese system, here in the US we are unable to send messages to large numbers of cell phones without delays. However, many promising technologies are on the horizon like broadcast text messaging, smartphone apps and recent upgrades to the national Integrated Public Alert and Warning System (IPAWS). EEW may also open the door to many public/private partnerships.

Public Outreach

The EEW system must be connected with users of the warning ahead of time, and therefore requires a public outreach effort upon implementation to make people aware of the system and how to respond to it. Responses are most effective when automated and pre-established so the recipients know what action to take when they get a warning.