USGS Hawaiian Volcano Observatory
on January 26, 2018

Why do some earthquakes have negative depths?

Astute visitors to the USGS Hawaiian Volcano Observatory (HVO) website may have noticed that some recent earthquakes have negative depths. This does not indicate a change in seismicity but, rather, an upgrade in HVO’s seismic data processing system.

The new system reports earthquake depths with respect to the common reference elevation, or datum, of sea level. When set to display earthquakes by depth, the HVO website map now includes a dark red color to indicate earthquakes that occur above sea level but below the ground surface. In the earthquake list to the right of the map, some events are now reported with negative depths.

To understand negative depth, imagine a number line with zero in the middle, positive numbers going one direction, and negative numbers going the opposite direction. For Hawaii earthquake depths, the zero point is now sea level. Positive depths indicate downward from sea level, and negative depths indicate upward from sea level.

This common reference elevation is known as the geoid, an imaginary surface that approximates the ocean surface influenced by Earth’s gravity and rotation. This surface extends inland to where sea level would be if land were not present. Thus, the geoid is equivalent to sea level, and the new earthquake depths are called “geoid depths.”

Prior to the new system, HVO reported “model depths” with respect to the ground surface above the earthquake hypocenter (point of origin). This model surface was not the actual ground elevation but, instead, was the average elevation of the five closest seismic stations.

Since the earth’s surface is not flat, model depth approximations did not always represent the true depth of an earthquake below ground. More importantly, it meant that there was no uniform frame of reference for comparing depths of different earthquakes. The zero elevation was different for every earthquake. This made it challenging to rigorously compare shallow earthquakes in HVO’s earthquake catalog.

To illustrate the difference between model and geoid depths, consider an earthquake beneath Mauna Loa, with its summit about 4 km (2.5 mi) above sea level. The model depth of this earthquake would have been previously reported as 3 km (1.9 mi), but with our new system, the geoid depth is now 3 km minus 4 km, or negative 1 km (‒0.6 mi).

One advantage of geoid depths is that systematic bias caused by mountain topography is corrected. We can now essentially “straighten out” depth profiles beneath the island to more accurately present where earthquakes occur.

By adopting the common reference datum of sea level, earthquakes reported nationwide are now more consistent and comparable. Regional seismic networks around the country have been migrating from model depth to geoid depth over the past few years. HVO’s adoption of the sea level reference brings it in line with this standard.

It’s important to note that the absolute location of earthquakes being computed in three-dimensional space has not changed. The only difference is the point at which we assign zero depth. All that has changed is how we describe the depths.

It is also important to remember that earthquake locations are mathematical models of where an earthquake occurs within the earth. Accurate locations depend on precise measurements of seismic wave arrival times at seismometers, accurate positions of those seismometers, and a realistic model of the speed at which seismic waves travel through the Earth. HVO’s seismic processing system receives real-time seismic data from about 100 stations and computes locations automatically to track seismicity in Hawaii around the clock.

You may occasionally notice that automatically-posted earthquakes with negative depths seem to be “floating” above the ground surface. This, of course, is not physically possible. Rather, it is a reflection of our imperfect Earth velocity models, which we intend to improve in the months and years to come. Once a seismologist reviews these events, the depths should go beneath the surface, where earthquakes really occur!

Earthquake depth is an important parameter for volcano monitoring because it can be a clue to changing magmatic activity. A shallowing of earthquakes with time can indicate magma moving toward the surface to erupt. This is why we want to accurately determine and describe earthquake depths and inform you about what’s shaking beneath our island.