# Visualisation of earthquakes

For asset management purposes we monitor the effects of earthquakes on all kind of structures. We also operate a number of ground sensors to record the movements at the upper surface levels.

Over the past years we were confronted with two different types of earthquakes:

1. Locally induced earthquakes due to gas exploration
2. Geophysical earthquakes due to geophysical processes e.g. continental plate drift

What we record are the effects of earthquakes at surface level. We are (or better our customers) confronted with the effects of seismic activity on a daily basis.

To explain how we visualize the effects of earthquakes we will use two examples, for each type of quake mentioned above, one example.

The content of this explanation is limited to the measurements taken by the tilt sensors.

## Tilt data processing

#### Explanation based on an intercontinental earthquake

Every sensor measures the tilt (the local gravitational direction) in two directions perpendicular to each other. An X- and Y-direction.

The direction of the local gravitational force is measured with a sample rate of 100Hz. For a predefined period, under standard operating conditions that is 15 seconds, but may vary depending on the project type, the sensor buffers this data. Every 15 seconds only three values of both directions are uploaded to the central server system:

1. The average of the 1500 measurements
2. The Highest value of the 1500 measurements
3. The Lowest value of the 1500 measurements.

In the raw-data graphs these three values form three different colored lines:

1. The average value = black
2. The highest value = red
3. The lowest value = blue

On measuring an earthquake the long term data provides us with an envelope of the recorded data. This envelope is clearly visible in the central graph in the image above.

In order to make the data accessible to the public, we spit up the results in two different graphs per direction:

1. Tilt
2. The Vibrations

The tilt lines are the recorded average values every 15 seconds.  The Vibrations lines being the difference between the recorded maximum values and the recorded minimum values every 15 seconds.

In this way, a single line represents the amount of motion recorded every 15 seconds. The result of this simplification is the upper right graph in the image above.

#### Inside the envelope

In the long term data the amount of motion is represented by a single (red) line named Vibrations. Depending on the need, it might be of interest to know what is happened inside the envelope.

Therefore we equipped our sensors with some basic intelligence to self asses if the current amount of motion should be determined as exceptional. If rated as exceptional, the sensor stores all data during a certain period. Generally this period covers somewhere from 30 seconds up to 2 minutes.

Once a block of this high speed data is completed, the sensor transfers is to our central server systems.

Via our web application you can browse trough all recorded high speed data blocks. One of the many high speed datablocks inside the envelope shown above is visible in the lower left graph of the image.

# Layout of standard long term graphs

Below a description of the layout of our standard reporting graphs. The graphs exists of 3 basic sections and an optional fourth one. Each section containing two of more plots.

1. Tilt
• Average tilt in X direction (Tilt mean)
• Average tilt in Y direction (Tilt mean)
2. Vibrations
• The amount of rotation in X direction (the total arc / sweep)
• The amount of rotation in Y direction (the total arc / sweep)
3. Accelerations
• Peak acceleration in X direction.
• Peak acceleration in Y direction.
• Peak acceleration in Z direction.
4. Environmental parameters (optional section, not in image below)
• Temperature