February 27, 2003
A weekly feature provided by scientists at the Hawaiian VolcanoObservatory.
Collecting Data on Earthquake Effects
One morning, you're awakened by a violent shaking of your bed; you hear the glass jalousies rattle, and the dog barks hysterically. In the seconds it takes to put your wits together, the sensation is over, and all is calm again. Now that you're wide awake, what do you do? For many Hawai`i Island residents, the next step is to go online and check the "current earthquake maps" accessible through the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) website (http://hvo.wr.usgs.gov).
People experiencing an earthquake will usually find the epicenter plotted on the maps within about five minutes, although some felt earthquakes manage to escape the automated processing routines that feed the web displays. Such earthquakes are often poorly recorded by our seismic network due to their small magnitude, shallow depth, insufficient station coverage, or a combination of these factors. Communications failure between computers can also prevent an earthquake from appearing on the maps.
The HVO website allows users to share their earthquake experience with staff members by providing an earthquake felt report form. In the past, felt reports were largely collected from callers leaving messages on HVO's main telephone line (967-7328). While this means of reporting earthquakes remains available, the web-based form provides a standardized method of collecting intensity information and is much easier to summarize, especially for widely felt earthquakes where multiple reports from the same location are received.
The questions arise, "Doesn't HVO already have the magnitudes for their earthquakes?" and "Why do they want to know how strongly I felt it?" As many have learned from past "Volcano Watch" articles, magnitude and intensity are not the same.
Earthquake magnitude is an instrumental measure of the size of an earthquake. Characteristics of the recorded earthquake, such as its maximum amplitude or signal duration, are measured. These measurements are used in a mathematical formula that corrects for differences in the local geology beneath the station and the distance between the earthquake source and the station. Normally, magnitudes are computed for many stations, and an average or median is reported.
Intensity describes the effects of an earthquake at a given location. These effects may be manifested at the earth's surface (ground cracks, landslides, etc.) or appear as damage to man-made structures. Generally, intensity values decrease with distance away from the epicenter, although this is not always the case, due to variations in local geology, building construction, and the directional nature of the earthquake source. The Modified Mercalli Intensity Scale, developed by Harry Wood and Frank Neumann in 1931, is currently used in the U.S. Values of Modified Mercalli intensity range from I (barely perceptible) to XII (catastrophic destruction).
Collection of intensity data is arguably much more demanding than determining earthquake magnitude, in that it requires collecting more data per earthquake. In Japan, hundreds of "intensity meters" were recently installed to provide a nationwide map of earthquake effects within minutes after an earthquake occurs. In the case of a damaging earthquake, this can direct emergency response teams to the most affected areas. This kind of effort is best accomplished in relatively small countries with significant resources. Here in the U.S., reports from individuals continue to provide the overwhelming majority of collected earthquake intensity data.
When assigning an intensity value to an earthquake, it is important to remember three points. First, establish as accurately as possible the time of the earthquake. Occasionally, several earthquakes may be felt over a short period, making it difficult to determine the earthquake belonging to the submitted report. Second, remember to note the location the earthquake was felt, whether it is a town, village, or subdivision. Finally, be as accurate in your assessment of the shaking as possible. Your input provides valuable data to HVO's earthquake monitoring efforts and may be used in studies to evaluate the earthquake hazard in Hawai`i. We thank those who have submitted intensity reports in the past and encourage your continued participation in this effort.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Scattered surface breakouts from the western "Kohola" lobe of the Mother's Day flow are seen mauka of the old roadway. The new Kohola ocean entry is barely limping along, but lava is pouring into the ocean from multiple entries of the old reliable West Highcastle delta.
The public should be aware that the ocean entry areas could collapse at any time, potentially generating large explosions in the process. The steam clouds rising from the entry areas are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary onto the lava deltas and benches. Even the intervening beaches are susceptible to large waves suddenly generated during delta collapse; these beaches should be avoided.
Two earthquakes were reported felt during the past week. Shortly after midnight on February 22, a resident of Leilani Estates felt the earth move. The magnitude-2.2 event was located 6 km (3.6 mi) east of Pu`ulena Crater at a depth of 2 km (1.2 mi). Residents of Hilo, Wainaku, and Papa`ikou felt an earthquake at 6:46 p.m. on February 25. The magnitude-3.5 earthquake was located 5 km (3 mi) southeast of Pu`u `O`o at a depth of 9 km (5.4 mi).
Mauna Loa is not erupting. The summit region continues to inflate, though the rate of inflation has slowed gradually during the past month or two. The earthquake activity is low, with only four earthquakes located in the summit area during the last seven days.
Updated: March 4, 2003 (pnf)