Glenn Thompson

Near-real-time seismic monitoring of Shishaldin volcano, January-April 1999

This web page is intended to be a collection of IceWeb plots describing the main events preceding, during, and immediately after the main eruption on April 19th. Further information can be found on the AVO public page.

Introduction

After volcanic tremor was identified in February 1999, AVO went into 24-hour monitoring mode. Near-real-time spectrograms and reduced displacement plots, produced by the IceWeb system, were the most valuable monitoring data available to AVO during this period. On April 19th, 1999, a VEI 3 eruption occured at Shishaldin volcano sending ash up to a height of ~50,000 feet and endangering air traffic on routes between North America and Asia. This eruption was preceded by a rapid increase in reduced displacement, which enabled AVO to respond to this eruption before ash entered the air routes.

Contents

Tremor emerges

During January 1999 reduced displacement at Shishaldin increased gradually and volcanic tremor was suspected. During February this tremor signal was intermittent. After early March, reduced displacement showed a linear trend until around April 5th. This increase may have been related to the March 4th earthquake.

M5.2 earthquake near Shishaldin, March 4th 1999

The March 4 earthquake produced hundreds of aftershocks, the occurence of which is shown very well on spectrograms and pseudo-helicorder plots. However, this aftershock sequence was unusual and much later there were several M>4 events, suggesting these earthquakes were in some way related to the activity at Shishaldin.

Tremor hits 3cm^2 - April 1, 1999

In the last few days of March reduced displacement rose fairly significantly. From March 28th till April 2nd three bursts of tremor can be seen, each with increasing intensity. Spectrograms for this period shows tremor, which peaks at around 1 or 2 Hz, strengthening significantly from March 28th till April 4th.

Tremor reaches 10cm^2 - April 7, 1999

Around 0400 UT on April 7th reduced displacement rose sharply from 1-2 cm^2 up to 10 cm^2 on SSLN, but returned to low levels again by 0800 UT. However, at 1552 UT a much stronger episode of tremor began, probably coinciding with a small eruption, and persisted at high levels for several hours.

April 8-13th: Tremor back at background

After 1200 UT on April 8th reduced displacement returned to background levels and remained between 1 and 2 cm^2 until the beginning of April 14th.

Another tremor burst - April 14, 1999

On April 14th reduced displacement rose sharply again to levels of around 5 cm^2 on SSLN. This activity persisted until the early part of April 15th, shown here on spectrograms and pseudo-helicorder plots. Strombolian activity was first observed around this time.

April 16-18th: Tremor back at background

From the beginning of April 16th, reduced displacement returned to background levels, suggesting any low-level eruptive activity had ceased.

April 19th eruption

This eruption was preceded by a steady rise in reduced displacement for approximately 60 hours. Closer examination reveals the rise is linear rather than exponential, but that the gradient increased at 0000 UT on April 19th. Between 0000 UT and 1800 UT on April 19th, reduced displacement doubled on all stations. This gradual increase is shown well on a spectrograms.

The first indication that an significant eruption was occuring was a rapid increase in reduced displacement. The onset of the eruption was probably 1939 UT as this corresponds to a rapid increase in seismicity seen on the following high resolution Dr plot and spectrograms.

This page is under development - Glenn Thompson, 12 May 1999.

Some extra data...