OK, you felt it, but did it move you?

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Every week Hawai`i residents feel earthquakes. Some weeks the U.S. Geological Survey's Hawaiian Volcano Observatory receives only a few reports, but the largest earthquakes can generate thousands of felt earthquake reports from the community.

These are Community Internet Intensity Maps of August 13, 2007, Magnitude-5.4 Earthquake.

Community Internet Intensity Maps (CIIM) of August 13, 2007, Magnitude-5.4 Earthquake.

(Public domain.)

We received 3,110 reports for the October 15, 2006, Kiholo Bay magnitude-6.7 earthquake. More recently, the November 23, 2006, Kiholo Bay magnitude-5.0 earthquake was reported by 2,067 people, and the August 13, 2007, magnitude-5.4 earthquake west of Kalapana was reported by 903 people.

The USGS utilizes these felt earthquake reports to produce Community Internet Intensity Maps (CIIM), which are a summary of questionnaire responses provided by internet users. An intensity of shaking value is assigned to each community from which a filled-out report is received, and then the intensity of shaking values are color-coded to form a map. Whereas the magnitude of an earthquake is a measure of its energy, the assigned intensity reflects the effects of the earthquake-induced shaking on people and structures in the community.

The internet earthquake reports also contain additional descriptions, such as observations of lamps swaying, windows and doors rattling, and sensations of the floor moving (for instance, from side to side as if the house were rocking like a boat). These descriptions help determine the earthquake intensity.

Felt reports often describe two distinct types of ground motion: the first is a sharp jolt or bump, and the second is a side-to-side swaying. These two types of ground motion are two of the fundamental ways that seismic energy is radiated from an earthquake. The first form of seismic energy is called the primary, or P, wave. It travels the fastest, at speeds between 1.5 and 8 kilometers per second (3,350-17,900 miles per hour) in the Earth's crust, and its motions are compressional, because it shakes the ground in the direction in which the wave is traveling. The second form of seismic energy is known as secondary, or S, waves (also referred to as shear waves). They travel more slowly, usually at 60-70 percent of the speed of P waves, and they shake at a perpendicular, or in a transverse, angle to the direction of travel.

People also describe moving from side to side over what seems to be several inches or even a foot or two, but how much does the ground actually move during an earthquake? HVO operates seismic recording instruments that make precise measurements of ground motion across the state of Hawai`i. Data from these instruments are used to rapidly produce Shakemaps, a geographic representation of the amount of ground shaking produced by an earthquake.

Shakemaps show the local peak ground accelerations, which are usually presented as a percentage of gravity, recorded during the earthquake. Accelerations are proportional to the shaking forces that we feel. If the local vertical acceleration reaches greater than 100 percent of gravity, it is enough to throw you up off of the ground! On October 15, 2006, ground acceleration near the earthquake epicenter reached greater than 50 percent of gravity, and more recently, during the August 13, 2007, earthquake ground acceleration near the epicenter was over 15 percent of gravity.

For the October 15, 2006, earthquake, the side-to-side ground motions (displacements) were over 9 cm (3.5 inches) near the earthquake epicenter, and for the August 13, 2007, earthquake, they were greater than 3 cm (just over 1 inch). The amount the ground actually moves at a given site depends on a number of factors, including your distance from the earthquake and the local geology. If your house is built on hard rock, it will move much less than if it is built on thick soil made of soft sediments or ash. You are accustomed to the stability of the ground beneath your feet, so when it moves, even if only a few inches, it feels as if you have suddenly found yourself standing on a surfboard heading for shore.

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Volcano Activity Update

Kīlauea summit and Pu`u `O`o continued to deflate slowly, although the changes have been quite small over the past week. Seismic tremor levels continued to be low. Earthquakes were located mostly beneath Halema`uma`u Crater and the south flank faults.

Episode 58 (the July 21 eruption) continues to supply lava into a perched channel near the lower end of eruptive fissure D, 2.3 km (1.4 mi) northeast of Pu`u `O`o. The lava forms a north-northeast-trending molten stream about 1.4 km (0.9 mi) long. The stream width is not consistent, but pinches and swells irregularly, from 90 m (300 ft) to 20 m (70 ft), along the channel length. Frequent, brief overflows from both sides of the channel continue to build up the channel walls, thus elevating the channel even more. Overflows draining from the end of the open channel have built a distributary apron, or fan. From there, pahoehoe is oozing northeast for another 1 km or so. Small `a`a flows continue to seep from the lower northwest flank of the channel to nestle against 23-year-old lava from Pu`u `O`o's early days. New seeps formed on the southeast flank of the channel in the past week. These are feeding lava flows that are piling up at the northern base of the Kupaianaha shield. Most of the active lava flows remain almost entirely within the area covered since July 21.

At Pu`u `O`o, no incandescence has been seen on the Webcam at night for the last several weeks. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure. Sloughing of Pu`u `O`o into its own crater since late August has left numerous fresh cracks on the north rim and south flank of the cone.

Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).

Three earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.2 earthquake occurred at 3:11 p.m. H.s.t on Monday, October 22, 2007, and was located 9 km (5 miles) southwest of Kīlauea summit at a depth of 33 km (20 miles). A magnitude-3.1 earthquake occurred at 8:06 p.m. on Tuesday, October 23, and was located 17 km (10 miles) east of Mauna Loa summit at a depth of 7 km (4 miles). A magnitude-2.6 earthquake occurred at 5:39 p.m. on Wednesday, October 24, and was located 8 km (5 miles) southeast of Pahala at a depth of 40 km (25 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.