An earthquake occurred this morning at 7:24 a.m. located 12 miles east-northeast of the city of Barstow in the Calico Mountains (map view). It had a local magnitude of 5.4.

An Aftershock of Landers. The event was located at the southern end of a cluster of earthquakes, called the Barstow cluster, that are part of the aftershock sequence of the M7.3 Landers earthquake of June 28, 1992. We therefore are calling this earthquake an aftershock the 1992 event. An aftershock is defined as an earthquake that

• occurs near the fault that produced the mainshock
  
• occurs while the rate of earthquakes is greater than it was before the mainshock
  

Fault. The earthquake was located at the northern end of the Calico fault. The focal mechanism shows right-lateral strike-slip motion on a fault striking N30°W, the strike of the Calico fault. The earthquake's hypocenter was relatively shallow, around 6 km depth.

Ground Motions. The ground motions produced by this earthquake were recorded at over 60 stations of the new digital seismographic network in southern California called TriNet (a cooperative project of Caltech, U.S. Geological Survey, and the State Department of Conservation Division of Mines and Geology). The ground motions are defined as a percentage of the force of gravity. An acceleration of 1%g means the ground is pushing on you with 1% of the force that the gravity of the earth pulls on you. The maximum acceleration recorded in this earthquake is 8.2%g at Flash Peak, 19 km west of the mainshock.

Peak ground values from the TriNet stations were used in an automated algorithm to produce shaking maps (ground velocity and ground acceleration) several minutes after the earthquake origin time. These maps display which regions had the strongest shaking and thus which regions should be first examined for damage. The maps are placed on the World Wide Web as soon as they are generated for general access.

Aftershocks of an Aftershock. Aftershocks are earthquakes and will produce their own aftershocks. As of 11:30 a.m., 11 aftershocks to the M5.4 have been recorded, ranging in magnitude from 1.8 to 3.7. An average California earthquake of M5.4 would have 2 aftershocks of M4 or greater within the first week, and 16 aftershocks of M3 or greater in the first week. In addition, any earthquake has a small probability (about 5-10%) of being followed by an even larger earthquake within one week.

Southern California Earthquake Center Data Center Home Page

U.S. Geological Survey - Pasadena Home Page