July 11, 1997

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

How big was it? Magnitude and Intensity of an Earthquake

Following the June 30th M5.3 Kalapana earthquake, there seemed to be a misunderstanding of what the magnitude of an earthquake really means.

Magnitude is an instrumental measure of the size of an earthquake, based on maximum amplitudes of motions recorded on a seismographic network. It is a property of the earthquake, determined after effects related to a particular recording site like distance from the earthquake and local geology are accounted for. The Richter Scale was originally used by Professor Charles Richter to determine earthquake magnitudes in southern California by reading the maximum amplitude from a particular seismograph and correcting for the distance of that seismograph from the earthquake. Here's how it's done. The Richter Scale has been generalized to apply to other regions and other instruments besides the ones originally used by Richter to determine the local earthquake magnitude. Another commonly used method to determine magnitudes is by duration. Duration magnitude is calculated by relating the length, in seconds, of a recorded seismograph signal to local earthquake magnitude.

Intensity is the description of how strongly an earthquake was felt at a given site. It is a classification of changes in the earth's surface and/or damage to man-made structures caused by an earthquake. Intensity ratings for a given earthquake will vary from one site to another. The Modified Mercalli Intensity Scale is a common rating used to describe the results of an earthquake. The use of this description is a very important part of the reported information of an earthquake and its effects. It is critical at times when instrumental data are not available.

HVO has continued to receive numerous felt reports from the public. We appreciate your cooperation and hope that you will continue to respond.

Kilauea Eruption Status--July 11, 1997

Kilauea's east rift zone eruptive activity continued during the past week. The level of the lava lake in Pu`u O`o fluctuated with activity from the spatter cone vent within the crater. Lava flowed over the western rim for brief periods twice in the last ten days. A sustained lava flow has traveled five miles to the southeast and is within a mile of entering the ocean.

Recent Big Island Earthquakes

A M3.2 earthquake on July 7 at 9:01 a.m. was centered about 3 miles west of Honoka'a. The earthquake was felt by residents in Pa'auilo and in Honoka'a with intensity ratings of IV.