Seismology

Background

Earthquakes in volcanic systems

Seismology is the study of earthquakes—how and where they originate, and how the waves they generate travel through the Earth. The primary tool of a seismologist is a seismometer, which are sensitive enough to detect even the small vibrations caused by very small—micro—earthquakes. By taking such records across a number of instruments (a 'network'), we are able to accurately determine exactly when and where an earthquake originated. This has proven to be an exceptionally powerful tool for investigating volcanic systems and tracking how bodies of melt migrate through the subsurface in the lead up to, and during, an eruption.

It is also possible to create maps of the subsurface—specifically the speeds at which seismic waves propagate through it—by comparing our observations of earthquakes to sophisticated models that incorporate information about the source and the velocity structure.

One avenue for forecasting eruptions is to look for changes in the seismic data. Upticks in the number or intensity of earthquakes, evolving hypocentres, or changes to the average amplitude of vibrations at the surface, can indicate changes in the volcanic system that might indicate an impending eruption.

Seismic data

Our instruments

We are using Trillium Compact (120s post-hole) seismometers manufactured by Nanometrics. For more information, please see the Nanometrics webpage. The voltage signals output by the analogue seismometers are digitised using the 24-bit Nanometrics Centaur datalogger, which we also use to digitise the signals from our analogue fluxgate magnetometers.

Example data plots

An example of a drumplot (/helicorder) showing a day of seismic data.

Drum plot showing seismic data at AV.CLNE, an AVO site on Cleveland volcano.

Data access

Weekly and monthly views of the seismic data recorded at each of our stations can be viewed on the respective site pages, which can be browsed from the respective project page. A limited amount of seismic data can also be viewed interactively via our data API.

Data recorded by our 3-component broadband seismometers are archived with the IRIS Data Management Center (now under EarthScope Consortium) and can be requested via the FDSN web services API. Note: in order to reconstitute the recorded data from digitised counts to physically meaningful units, you will also need to request the response files (also available via the IRIS DMC) and remove the response from the data. ObsPy provides a suite of tools for data downloading, response removal, and general manipulation of the resulting seismic data.

            
    from obspy.clients.fdsn import Client


    client = Client("IRIS")

    # Get the instrument response inventory for a single station
    inventory = Inventory()
    inventory += client.get_stations(
        network="AV",
        station="CLNE",
        starttime=obspy.UTCDateTime("2022-09-21"),
        endtime=obspy.UTCDateTime.utcnow(),
        level="response",
    )
    inventory.write("CLNE_mag_response.xml", format="STATIONXML")

    # Get a day of waveform data from the data center
    stream = client.get_waveforms(
        network="AV",
        station="CLNE",
        location="*",
        channel="BH*",
        starttime=obspy.UTCDateTime("2022-09-21"),
        endtime=obspy.UTCDateTime("2022-09-22"),
    )
    stream.merge(method=-1)

    # Write each component to a separate miniSEED file
    for component in "ENZ":
        component_stream = stream.select(component=component)
        component_stream.write(f"AV.CLNE_{component}.m", format="MSEED")
            
        

Data attribution

The instruments deployed as part of the Alaska project are archived at the IRIS Data Management Center under the AV network (DOI: 10.7914/SN/AV), which covers the Alaska Volcano Observatory instrument network.