Thread by @geophysichick

Today's #SeismogramSaturday is devoted to my favorite category of earthquakes--volcanic quakes. These are events that occur within volcanoes, due to a variety of causes. We'll discuss three types of volcano quakes today.

In general, all quakes occur when some type of stress causes rock to break or slip. In tectonic environments, the driving force is plate tectonics. In volcanoes, the forces are different--usually magma, but also fluids or the weight of the mountain itself.

The first category of quake is shown below. It has a clear start and end, it shows lovely P and S wave arrivals, and it shakes at frequencies of ~2-25 Hz. This M2.6 quake took place beneath Hualalai volcano (HI) last summer (it was not erupting).

Quakes like this almost exactly like tectonic quakes you'd see in a subduction setting or the San Andreas fault. For this reason, we often call them "volcano tectonic" or VT events, which is to say that they are regular quakes, but just happen to be in a volcano.

There are a lot of fluids in volcanoes, including magma, volcanic gas, or hydrothermal fluid (water). As these flow, they can also shake the ground. But this isn't the same as rock slipping, so they look a little different. Here's one at Shishaldin volcano (Alaska) last year:

These events shake at 1-5 Hz, so we call them low frequency, or often "long period" (LP) events. They don't really have clear S-waves. Because they're related to fluids, they often precede eruptions (magma or volcanic gas may be on the move!)

...but they don't always indicate that an eruption is imminent, so on their own LPs are not cause for alarm.

Sometimes, volcanoes hum with a signal we called "tremor". These signals are longer in duration--sometimes they last a few 10s of seconds, and sometimes tremor lasts days or weeks. Tremor is a continuous (but tiny) ground shaking. Sometimes it occurs in pulses, like this:

Like LP activity, tremor is probably related to fluid motion. There are a lot of suggestions for what causes it, but we're not entirely sure. Tremor is often recorded during or before eruptive activity.

By the way, the signal above was recorded at Veniaminof volcano (Alaska).

Tremor can also be "harmonic", which means that it shakes at a few very specific frequencies, that are multiples of one another. We'll get into that another time...maybe next week.

There are a ton of other types of volcanic quakes, because anything a volcano does may shake the ground. Explosions cause quake-like signals, for example. There are also "hybrid" quakes, which have both VT and LP characteristics.

And there's tons of debate about what these quakes should be called (high frequency? low-frequency? A-type? B-type?). If you want to hear a volcano seismologist develop attitude, ask them how they think volcanic earthquakes should be categorized.

An important thing to know about volcanic quakes is that they are almost always small. Earthquake size scales with the size of the fault and since volcanoes are small (compared to tectonic plates), they generally produce small quakes. Even a magnitude 4 is rare at a volcano.

(there are exceptions to this, notably in Hawai`i, but that's because Hawaiian volcanoes are HUGE).

Got questions about volcanic earthquakes? Ask 'em here! Happy #SeismogramSaturday to all. May your volcanoes be seismically exciting but harmless.