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Gonna do a quick thing on earthquake magnitude here on #SeismogramSaturday. There's a lot to say, so this is just a small part. I'll chime in more about magnitude in later weeks, so keep watching this space.
Meantime, here's today's lesson:
Meantime, here's today's lesson:
Magnitude is a measure of the raw size of the earthquake. It doesn't relate to how strong it felt or how much damage it did, because those things depend on factors like distance from the quake and the quality of infrastructure.
(those things feed in to a parameter called "intensity" which is different from magnitude...we'll discuss that later too).
I was appropriately beaten to the punch by @earthquakeguy for this #seismogramsaturday, and he's much more of an expert on this topic than I, so please look at his post first. I'll add a few of my own figures below.
Today we're talking about the tremor pulses that go hand-in-hand with slow slip during Episodic Tremor and Slip (ETS) events. These are fascinating events in subduction zones that are well described here: earthquakescanada.nrcan.gc.ca/pprs-pprp/pubs…
Some of you may have seen the tremor maps created by seismologist extraordinaire Aaron Wech, and shown on the @PNSN1 page at pnsn.org/tremor.
Today, #SeismogramSaturday is actually #SpectrogramSaturday. I'm going to talk about another way that seismologists might look at seismic data.
The key background information here is that earthquake waves, like all waves, have a quality called "frequency", which describes how many times the wave oscillates per second (a unit called Hertz, or Hz).
Sound waves are a good way to explain frequency--if the air oscillates many times per second it creates a high frequency wave, which is a high note. Low frequency waves are low notes. Humans can hear in the range of about 20-20,000 Hz. Middle C is 256 Hz.
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). 
For #SeismogramSaturday #SeismoTwitter I thought it would be interesting to talk about a lesser known seismic phenomenon called “slow-rupture tsunami earthquakes, or simply “tsunami earthquakes”.
There’s been some great info about the M 7.5 Ecuador earthquake a couple days ago, and this topic will discuss frequency content of earthquakes. Notably why earthquakes with the same magnitude may be very different from one another.
Tsunami earthquakes differ from other M 9+ tsunamigenic earthquakes such as the 2004 Indian Ocean earthquake and the 2011 Japanese earthquake. Tsunami earthquakes have more moderate-size magnitudes but produce large tsunami for their size.
Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior?
In the past couple of weeks we’ve discussed body waves (P and S waves, which move through the body of the Earth) and surface waves (Love and Rayleigh which, unsurprisingly, only travel across Earth’s surface).
But in some seismograms, notably those recorded far away from the earthquake epicenter, we see lots of other arrivals. What are these, and why do they have such strange names?
Okay, #SeismoTwitter, it's (almost) #SeismogramSaturday! I'll be offline for most of Saturday, so I'm posting a little early. Here's today's seismogram!
Today, we have show shaking caused by the M9.0 Tohoku earthquake on March 11, 2011. This figure shows shaking at a station in North Dakota (caused by a quake in Japan!).
The three wiggles here show how the ground shook in an east-west manner (top panel), north-south (middle) and vertical (bottom).
Hey #SeismoTwitter! I propose that we do #SeismogramSaturday, to teach about the world of our wiggles. Feel free to pick your favorite (or least favorite) seismogram to tell us about. We'll do it next week too.
I'll start. This is a normal little quake. Info follows:
I'll start. This is a normal little quake. Info follows:
First, a seismogram literally shows us how the ground shook when seismic waves reached the seismometer. The vertical axis is a proxy for this (I'll explain the units later).
This image shows a magnitude 2.0 earthquake that took place near Monroe, WA, a few days ago. No one felt it, since it was tiny and took place about 17 miles underground.
