# How are marsquake epicenters determined using only one seismometer?

This tweet says:

Cerberus Fossae is the first active seismic zone ever discovered on Mars. Located about 1,600 kilometers east of @NASAInSight lander, this huge extensive tectonic structure was the epicenter of two significant MarsQuakes detected during sols 173 and 235.

Using only a single seismometer, how was the locations of these two, apparently distinct epicenters determined?

• – uhoh
Dec 19, 2019 at 2:08
• I asked myself the same question. Haven't had the time for a search yet, only found a pop science article i will read soon(tm): seis-insight.eu/en/public-2/martian-science/seismic-activity
– user34174
Dec 19, 2019 at 11:25
• The article @ebv links explains it pretty well. Mars' much smaller size compared to earth makes it so that the surface waves can travel around the planet multiple times. Combined with horizontal sensors to determine the direction of the waves, the arrival times of the same surface waves after traveling around the planet multiple times can be used to determine the position. Dec 19, 2019 at 15:00
• @AlexanderVandenberghe it's as if they'd thought of that ahead of time! ;-)
– uhoh
Dec 19, 2019 at 15:14

user34174 linked an article which explains much of the techniques used: https://www.seis-insight.eu/en/public-2/martian-science/seismic-activity

To not have it lost as a volatile comment, I will paraphrase the relevant parts.

1. A marsquake emit both p-waves (faster) and s-waves (slower). Their relative arrival time can be used to calculate the distance within an error of 10%

2. The seismometer can measure waves in all three spatial directions. This allows calculating the direction the waves are coming from, within 10 degrees.

This is two degrees of freedom, sufficiently to estimate a position on a sphere.

In addition, if the quake is large enough, the surface waves travelling in the opposite direction can also reach the seismometer. At a magnitude of over 4.5, it can also observe the waves after travelling an additional round around Mars. These extra wavefronts can be used to increase the accuracy of the measurements.

• Thanks! This helps to clear up the mystery.
– uhoh
Jan 22, 2020 at 15:22