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Volcanowatch

October 5, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


More than 19,000 earthquakes catalogued in Hawai`i since 1823

Earthquakes occur every day on the Island of Hawai`i. Most are small, less than magnitude 3, but typically a couple are felt by someone each week. Damaging earthquakes occur every few years, and very large ones take place every few decades or more often. The two largest earthquakes in the past 200 years are those of 1868 (estimated magnitude of 7.9) and 1975 (magnitude of 7.2). These are large by even California standards.

HVO has had a reasonably complete network of seismic stations since about 1960. The network encompasses the entire island and was extended a year ago to include east Maui. Currently about 70 stations are operating. For the past 40 years, the network has enabled the detailed analysis of the earthquakes, including locations, depths, and magnitudes.

What about before 1960? There were plenty of earthquakes then, but the lack of adequate instrumentation made it difficult to estimate locations, depths, and magnitudes very reliably. Yet it is important to know about these historical earthquakes, in order to determine whether seismic patterns of activity have changed with time. Perhaps there are long-term trends that only the study of past earthquakes can reveal.

A stunning new publication--actually a labor of love--by ex-HVO staff members Fred Klein and Tom Wright goes a long way toward making study of the old, historical earthquakes both possible and rewarding. The research publication is titled "Catalog of Hawaiian earthquakes, 1823-1959"; it can be viewed, or a paper copy ordered, on the web at http://geopubs.wr.usgs.gov/prof-paper/pp1623/.

The catalog contains information about 19,464 earthquakes in the Hawaiian Islands, mostly on the Island of Hawai`i, since 1823. The information results from a remarkable combination of sleuthing and insight.

The sleuthing involved pouring over newspaper stories, diaries, and other obscure writings, starting with William Ellis' journal of his 1823 trip around Hawai`i and ending with newspaper accounts from the 1950s. The Lyman diary (1833-1917), written by the missionary family in Hilo, is a notable but incomplete source. Fourteen different newspapers published in Honolulu, Hilo, and Wailuku, were examined, dating from 1856. Felt reports sent to HVO in 1932-1941 and 1951-1958 proved of great value.

The written accounts provide far more information than has previously been assembled, particularly about 19th-century earthquake activity. Approximate locations for many of the earthquakes can be estimated, and rough magnitudes can, in some cases, be assigned on the basis of the felt reports. However, the information is necessarily limited by the lack of instrumental recordings.

The first seismograph in Hawai`i was installed in 1899 at O`ahu College (now Punahou School), but the first systematic published recordings were begun in April 1903 at the Honolulu Magnetic Observatory. A seismograph began operation at HVO in 1912. With the advent of these instruments, much more quantitative information became available. Nonetheless, the lack of a network of several stations made the calculation of precise locations and magnitudes virtually impossible.

Klein and Wright devised an insightful method to assign location and magnitude for all earthquakes at a known or reasonably estimated distance from HVO, or for those events that were widely felt. For the instrumental period from 1903 to 1959, they summarize information on location and magnitude for more than 800 earthquakes of magnitude 4 or above; during this same period, they list 17,917 earthquakes in total! The pre-instrumental record is much sparser, but Klein and Wright found 1,547 earthquakes that were felt between 1823 and 1903.

The authors refrain from attempting any interpretations of the data, preferring to publish them separately. They do caution, however, about accepting seemingly obvious trends, owing to the poor quality of the historical record. Such trends, they warn, "are like the shadow of an object that reveals a hint of shape but nothing about its structure."

Take a look at this remarkable catalogue on the web. Though technical, the tables are readily understandable and tell it all.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week, following a brief surge two weeks ago. Lava is erupting from Pu`u `O`o and flowing southeast through the old tube system for 1.5 km (0.9 mi) to the 2,300-foot elevation. A new tube system within the breakout flows from the 2,300-foot elevation routes the lava down to the coastal flats below Pulama pali and to the coast near Kamokuna. Lava is entering the ocean along a broad, 600-meter (2,000-foot) front located 1.6 km (1 mi) west-southwest of Waha`ula. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

There were no earthquakes reported felt during the week ending on October 5.


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Updated: October 10, 2000