WARN Help

Overview

WARN stands for Web-enabled Awareness Research Network. (see high-level overview)

The WARN module within Oceans 3.0 provides subscribers with early notifications of tsunamis and earthquakes as part of Ocean Networks Canada's Smart Oceans initiative. It utilizes existing the Oceans 3.0 data acquisition infrastructure while incorporating some new instruments and new event detection algorithms and software. Tsunami detection uses data from pressure sensors such as the existing Bottom Pressure Recorders (BPR) for far-field tsunamis and a planned coastal radar system for near-field events. Earthquake detection uses new sensors providing p-wave data that were deployed in late 2014 but will be upgraded in early 2015 with more sensitive instruments. While WARN was implemented as a research/prototype with limited sensors and select early adopters for now, it is being designed to be expandable to greater numbers and types of instruments and subscribers.

WARN receives instrument data, and returns intrinsic event data such as tsunami detection time and wave height, and earthquake magnitude and epicenter, which can be used in impact assessments by subscribers. Subscribers can subscribe to be notified by any of the following means:

e-mail (generally only useful for test purposes)
direct server notification
Apple push notification using our prototype iPhone application (not available to the public yet).

At this time, in order to subscribe, users must create an account on the Ocean Networks Canada “Oceans 3.0” website. They also require permission from Ocean Networks Canada in order to subscribe, because this project is still under development and we wish to limit access to ONC staff only at this time.

Further details are available in the attached presentation:

WARN Overview

Earthquake Detection

Earthquake detection is done using a number of accelerometers located on land and on the seafloor offshore Vancouver Island. Accelerometers normally only output a characteristic amplitude value, the Japanese Meteorological Agency's (JMA) instrumental intensity, which is not used at the moment. Instead, WARN's driver software, which runs on a computer co-located with the accelerometer, analyzes the accelerometer data on all three axes and looks for the signature of a "P-wave", which is the initial compression wave emanated by the earthquake. This P-wave does not cause damage but travels much faster than the "S-wave" that therefore comes later and is at the source of potential damage. If a P-wave is detected, the driver outputs the time of detection of the P-wave. Then the driver analyzes the first few seconds of motion and determines the maximum displacement of the vertical component of the acceleration, which we call Pd, and the maximum period of the vertical component, which we call Tau. The Pd and Tau are then sent out by the driver computer to the Oceans 3.0 software running at the University of Victoria.

The P-wave time, Pd, and Tau are analyzed by two algorithms to determine the epicenter and time of origin, and two different algorithms to determine the magnitude (the JMA algorithm is not used at this time) - see diagram below.

The epicenter is determined provided at least three accelerometers have reported a P-wave detection. The P-wave detection times are analyzed using a Direct Grid Search algorithm where a pre-defined region is divided into cells of equal size (9,800 for the current area, configurable). Each cell is analyzed by triangulation, knowing the location of the center of the cell and the relative detection times of the P-wave at each of the sensors, plus the location of the sensors. The cells are first analyzed on a coarse grid basis to determine the cell with the highest probability of being the epicenter of the earthquake. This cell is then further analyzed on a fine grid using the same method. In the end the center of the most probable fine grid cell is used as the epicenter latitude and longitude, with a resolution of +/- 0.05 degrees. If the quality of the solution is considered high enough, an earthquake is declared by the Earthquake Correlator and the results are passed to the Event Notification component.

If the quality is not high enough the software waits until a fourth accelerometer reports detection of a P-wave. Then the Direct Grid Search runs again. In addition another algorithm called Linearized Least Squares also analyzes the four P-wave times. The mean value of the output of both algorithms is reported as the epicenter. Knowing the distance from the epicenter to the sensors, the time of origin of the earthquake can be determined.

The magnitude of the earthquake is estimated using two methods. One method compares the Tau (period) values reported by the accelerometers, which is expected to be the same at all sensors. The magnitude is proportional to the Tau on a logarithmic scale. The other method uses the Pd or maximum vertical acceleration reported by each of the sensors. The Pd is attenuated by the distance to the sensor on a logarithmic scale. So after factoring in the attenuation due to distance, the magnitude is logarithmically proportional to the Pd.

The epicenter coordinates, time of origin, and magnitude are passed to the Event Notification software.

Tsunami Detection

Tsunami detection is done using Bottom Pressure Recorders (BPRs) which are located on the seafloor at various locations west of Vancouver Island. These are highly sensitive instruments that can measure the weight of the water (and atmosphere) above them and can detect changes in the water depth of as little as 1mm in some cases. Although tsunamis can be quite large at the shoreline, they are quite a bit smaller in the deep ocean.

For each of the BPRs configured for WARN, the following processing occurs for every data sample received, and a new sample is received every second. See diagram below.

First the effects of the fixed depth and the effects of the tides must be removed. The "detider" analyzes the most recent pressure readings as well as samples from each of the past 3 hours. The data are analyzed using a third-order polynomial curve fitting algorithm in order to predict the next sample. The difference between the predicted value and the actual value is the output of the detider. Typically those values are close to 0 but when a tsunami arrives a non-zero signal is seen.

There are four algorithms that take the detider output as their input, and they run in parallel. Their purpose is to determine whether a tsunami should be declared by this particular BPR. Note that WARN requires more than one BPR to detect a tsunami, but more about that later.

The first algorithm is called Direct Amplitude Thresholding and is very simple. It checks each detided pressure sample and reports if any sample (absolute value) crosses over a threshold.

The second algorithm is called STA/LTA, which stands for Short Term Average over Long Term Average. It computes the average of the most recent 200 (squared) detided samples divided by the average of the most recent (squared) 2400 detided samples. This improves the signal-to-noise ratio. The output is compared with a threshold.

The third algorithm is called Kurtosis. It computes the kurtosis value of the detided data, which is a statistical parameter that measures the "peakedness" of the signal. The output is compared with a threshold.

The final algorithm is a "not-a-tsunami" detector called a Seismic Wave Detector. Knowing that earthquakes cause shaking of the seafloor, which will cause the BPRs to shake and effectively create their own signals similar to what would be caused by ocean waves, this detector distinguishes between false alarms caused by seismic ocean crust waves and tsunami waves on the ocean surface. The first step is to compute a high pass filter to detect the seismic waves which have a higher frequency than tsunami waves. The output signal is improved using an STA/LTA method. The output of that is compared with a threshold.

For any single BPR, a tsunami is declared if any 2 out of 3 tsunami detection algorithms exceed their thresholds within a certain time window and the Seismic Detector does not detect a seismic wave at the same time. This is the function of the Watcher.

The above process is repeated on each of the BPRs. Note that the BPRs are spaced far apart so they detect the tsunami at different times. The Correlator watches for a tsunami declaration from at least 3 BPRs. The correlation has two conditions - 1) the BPRs that have reported must be at least 15km apart from each other (to rule out false alarms), and 2) the arrival time of the tsunami at each BPR must occur within a certain time window relative to all other reporting BPRs. The time window is determined by the longest possible travel time of a tsunami between each BPR pair knowing the distance and the bathymetry between them, since the travel time of a tsunami depends on the bathymetry or the profile of the ocean depth. If all of these conditions are met the Correlator passes on the detection timestamp, the height of the tsunami wave at the time of detection, and the maximum height over a certain time window after detection, to the Event Notification process.

Event Notification

The Event Notification component allows ONC staff to define and activate events, to manually test events, and to allow others to subscribe to certain events and to decide how they wish to be notified. For WARN the two events that are most important are WARN Earthquake Detection and WARN Tsunami Detection.

At the moment there are three ways that a user can be notified of an event:

by server notification, where we directly call the subscriber's URL and the subscriber has written software to accept our call and take some action.
by e-mail. This method is mainly useful for test purposes as it cannot be relied upon to be fast enough in most cases.
by Apple Push notification, which will notify anyone running the WARNAlert iPhone application. Note that this was only developed for demonstration purposes and there is no intention at this time to release this to the public.

As of April 2015 all event subscriptions are restricted to 1) those who have created an account in Oceans 3.0 (http://dmas.uvic.ca) AND 2) they have been given software permissions by ONC to allow them to subscribe to events (ie. a member of the Event Subscription group). Those meeting these conditions can subscribe to events on the Event Maintenance page of Oceans 3.0 at http://dmas.uvic.ca/EventMaintenance under the Event Subscription tab - see below. Note that in the screenshot below you will see other WARN events. These are mainly for test purposes to analyze the output from the algorithms that contribute to the earthquake or tsunami detection. Unless you are trying to understand the internal workings of WARN we recommend that you only subscribe to WARN Earthquake Detection and/or WARN Tsunami Detection.

All events are logged and can be viewed on the Event Maintenance page under the Event Log tab. Here is an example of some test earthquake events. Note that the OriginTimes are in millisecond since 1970, also known as Unix Time. There are converters on the internet that can convert this to human readable date and time.

Notification Contents

The e-mails are sent using the CAP (Common Alerting Protocol) format. Here is an example. Note the fifth line, which will either use the word "Actual" for a real event, or "Test" for a manually generated test.
----

<?xml version="1.0" encoding="UTF-8" standalone="no"?> <alert xmlns="urn:oasis:names:tc:emergency:cap:1.2">

<sender>qatsk03.dc.local</sender>

<status>Actual</status>

<msgType>Alert</msgType>

<scope>Public</scope>

<info>

<event>Earthquake</event>

<urgency>Expected</urgency>

<severity>Unknown</severity>

<certainty>Observed</certainty>

<valueName>Event</valueName>

<value>earthquake</value>

</eventCode>

<valueName>OriginTime</valueName>

</parameter>

<valueName>Epicentre</valueName>

</parameter>

<valueName>Magnitude</valueName>

</parameter>

<area>

<areaDesc>NORTHEAST PACIFIC</areaDesc>

</area>

</info>

</alert>

Earthquake notifications include:

coordinates of the epicenter in degrees latitude and longitude
time of origin at the epicenter, in UTC
magnitude in Richter units

Tsunami notifications include:

Time of detection by the third BPR
coordinates of all the BPRs that made the detection
(additional information is contained in the logs including wave heights)

Software in the device receiving the message (eg. client application or WARNAlert iPhone application) can further process this information. By knowing the GPS coordinates of the client the software can determine the distance to the epicenter and hence predict the arrival time of the S-waves at that location. The client software can also predict the level of shaking at the client location.

Licencing

No licencing is required to become a subscriber of WARN events. However at the present time subscription to earthquake and tsunami events is only possible by invitation and only to those with an account to Oceans 3.0.

Page tree

WARN Help

Overview

Earthquake Detection

Tsunami Detection

Event Notification

Notification Contents

Licencing