SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey May 8, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ May 1 - 7, 1997 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Lucy Jones, U. S. Geological Survey California Institute of Technology For further information, please contact the authors or the Caltech Public Relations Office at 818-395-6327. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, May 1, Universal Time (ie. GMT), to midnight Wednesday night, May 7, Universal Time. In local time, the period of coverage is from 5 p.m., April 30, Pacific Daylight Time, to 5 p.m., May 7, Pacific Daylight Time. We detected approximately 225 earthquakes during the seven-day period covered. First of all, an apology is in order for the inadvertant "downgrade" of the two largish Northridge aftershocks on April 26 and 27. As most regular readers of the Weekly Report know by now, the first attempts at magnitude determination often do not include all of the available data. At this time in the early stages of the TriNet project, we are trying to integrate data from our old analog-telemetry network AND the new digital-telemetry stations. At this stage of the integration, it takes human intervention to include both sets in the magnitude determination. With some people out sick last week and incomplete communication between those who were left, this step did not happen. In fact, when all the new digital data are included, the "real" final magnitudes for these events are M5.1 for the April 26 event and M4.9 for the April 27 event. We are sorry for the confusion and are working to eliminate opportunities for such mistakes in the future. For the readers, however, there is a lesson in magnitude accuracy here. A difference of 0.1 is scientifically insignificant, and such a change can happen easily with the inclusion of more stations. It can, however, be of great social interest, especially when it spans a whole unit change: M5.0 sounds much worse than M4.9). Now for this week's seismicity. Although the total number of events was not particularly high, several of the week's events were felt. The week's largest was a M4.5 on Tuesday afternoon, located in the Sierra Nevada south of Lake Isabella. It occurred in an unpopulated area but was slightly felt in Tehachapi and Ridgecrest. The focal mechanism was strike-slip. It was located within the Southern Sierra Seismic Lineation, a common source of microseismicity. This quake was not an aftershock. Just a few minutes before the Lake Isabella quake, there was a M3.2 Landers aftershock, slightly felt in the Barstow area. As expected for Landers aftershocks, it also had a strike-slip focal mechanism. In addition, there was a little more activity along the northern edge of the Northridge aftershock zone. The two that were reported felt were: a M2.8 last Wednesday night and a M3.5 late Saturday afternoon. These events, like many of the events recorded in the last eleven days, were aftershocks of the M5.1 aftershock (April 26, 1997), which itself was an aftershock of Northridge (Mw6.7 on January 17, 1994). These events are considered Northridge aftershocks because they are all located in the Northridge aftershock zone, and because the earthquake rate this zone is still significantly higher than it was prior to the January 17, 1994 mainshock. This centroid of this little cluster of events is located almost equidistant from Simi Valley, Valencia, and Newhall. The descriptive location is always given with respect to the nearest town, whichever that happens to be. This done, not to confuse people, but so that the computers can generate these listings automatically. The seismicity is, in fact, occurring in one fairly continuous blob, elongated to in the east-west direction along a buried fault. The fault appears to be close to veritical, and is probably one that was activated early in the Northridge sequence, although it is not the fault that produced the Northridge mainshock. There was also another Northridge aftershock felt this week, at a different location, early Monday morning. That quake was a M2.8 near San Fernando, at the northeasternmost edge of the Northridge zone. Table 1 lists the quakes this week that were M2.0 or larger in the central part of the coverage area. Times are local times; if you want Greenwich Mean Time, add 7 hrs to the Pacific Daylight Time or 8 hrs to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 4/30 11:22 pm 34 22.8 118 39.0 2.8 5 mi. WSW of Valencia FELT 4/30 11:22 pm 34 23.9 118 41.2 2.0 8 mi. N of Simi Valley 5/1 10:00 am 33 31.5 118 1.0 2.0 7 mi. SW of Newport Beach 5/1 10:17 am 34 7.4 116 22.8 2.1 3 mi. WSW of Joshua Tree 5/1 1:12 pm 33 57.7 116 49.5 2.1 3 mi. NW of Cabazon 5/1 2:22 pm 35 48.0 117 38.4 2.1 12 mi. N of Ridgecrest 5/1 3:07 pm 36 2.8 120 36.9 2.7 14 mi. NW of Parkfield 5/2 6:28 am 34 18.6 119 13.2 2.0 4 mi. ENE of Ventura 5/2 8:17 am 34 29.0 116 30.7 2.8 25 mi. N of Yucca Valley 5/3 12:03 am 34 22.4 118 39.5 2.5 5 mi. WSW of Valencia 5/3 12:36 am 33 55.1 117 45.3 2.2 3 mi. ENE of Yorba Linda 5/3 5:51 am 34 22.3 118 40.2 3.5 6 mi. NNE of Simi Valley FELT 5/3 2:11 pm 34 19.8 118 27.2 2.1 3 mi. NNW of San Fernando 5/3 7:21 pm 32 18.9 115 19.0 2.5 26 mi. SSE of Calexico 5/3 7:49 pm 35 2.0 119 10.8 2.4 13 mi. W of Wheeler Ridge 5/3 8:10 pm 36 1.4 118 52.1 2.0 9 mi. ESE of Porterville 5/3 11:54 pm 35 6.4 119 6.5 2.2 11 mi. NW of Wheeler Ridge 5/4 3:02 pm 32 56.4 116 13.0 2.0 14 mi. SSW of Ocotillo Wells 5/4 7:08 pm 33 22.2 116 24.7 2.4 8 mi. NNW of Borrego Springs 5/4 10:20 pm 34 23.2 116 27.7 2.0 18 mi. N of Yucca Valley 5/4 11:31 pm 32 54.5 116 16.6 2.0 18 mi. SSW of Ocotillo Wells 5/5 3:30 am 34 17.9 118 25.8 2.8 1 mi. NNE of San Fernando FELT 5/5 3:54 am 36 5.5 117 39.6 2.0 16 mi. E of Coso Junction 5/5 6:30 pm 33 16.2 116 0.5 2.3 2 mi. WSW of Salton City 5/5 10:37 pm 34 58.2 116 48.8 2.5 13 mi. ENE of Barstow 5/6 2:23 am 33 9.2 116 28.9 2.2 8 mi. NE of Julian 5/6 11:53 am 34 58.3 116 48.7 3.2 13 mi. ENE of Barstow 5/6 12:09 pm 34 18.2 118 25.4 2.1 1 mi. NNE of San Fernando 5/6 12:12 pm 35 27.2 118 25.9 4.5 12 mi. S of town of Lake Isabella; FELT 5/6 6:30 pm 35 57.8 117 39.3 2.3 17 mi. ESE of Coso Junction 5/6 8:36 pm 32 13.1 115 48.8 2.8 37 mi. SSE of Ocotillo 5/6 8:53 pm 35 26.9 118 26.1 2.1 13 mi. S of town of Lake Isabella 5/6 10:26 pm 34 16.1 118 26.3 2.3 1 mi. S of Pacoima 5/6 11:39 pm 33 52.5 116 51.3 2.5 3 mi. SSE of Banning 5/6 11:40 pm 33 52.5 116 51.4 2.3 " 5/7 7:27 am 34 19.2 116 29.3 2.2 14 mi. N of Yucca Valley 5/7 7:27 am 34 22.7 118 40.8 2.1 7 mi. NNE of Simi Valley 5/7 7:51 am 35 57.7 117 39.5 2.5 17 mi. ESE of Coso Junction 5/7 3:17 pm 34 58.2 116 48.5 2.4 13 mi. ENE of Barstow -------------------------------------------------------------