Here is the late afternoon update on the Coso swarm, with magnitude revisions. The largest quakes have been: a M4.2 at 3:08 pm yesterday (Monday), a M3.7 at 10:55 am today, a M2.9 at 11:08 am today, and a M4.1 at 2:08 pm today. All were located 19 miles east of Coso Junction, about 3 miles east of the epicenter of a swarm in late 1996 that included a Mw5.3.
The reason for the confusion on magnitudes is the fact that there are multiple methods of magnitude computation. The original values posted by the automatic system, and the final values that you see now, are both ML estimates. ML, or local magnitude, is very similar but not identical to Richter's original method (e.g. we are now using more modern seismometers!). ML works very well for quakes in this size range. The values that were posted were ML's based on only a few stations or MC magnitudes. MC is called "coda magnitude" and is based on how fast the tail of the quake dies off on the record. It tends to work well for smaller quakes, but not so well for M4's.
The Coso swarm continues: at 10:55 am today, there was a M3.5 located 19 miles east of Coso Junction. As mentioned previously, this is in the Coso Range, which is a prolific source of earthquake swarms.
The update from last night apparently did not make it onto the web page: there was a M4.0 at 3:08 pm yesterday at the same location. As expected, these two largish events have been accompanied by a fair number of M2 and M3 smaller shocks, making this a fairly typical small Coso swarm.
A pair of M2.7's occurred late yesterday afternoon 18 miles east of Coso Junction. The times were 3:26 pm and 3:29 pm. This is a very unpopulated area, and we got no inquiries.
There was some earthquake action overnight, in the Imperial Valley and in the Coso Range area. At 3:40:58 am early this morning, a M3.0 occurred 2 miles north-northwest of Brawley. It was followed at 3:41:53 am by a M3.1 at the same location. Both quakes were reported felt. Brawley is about 13 miles north of El Centro.
About 20 minutes later, at 4:02 am, an unrelated M3.3 quake occurred 19 miles east of Coso Junction. This is close to where the Coso swarms occurred in November of 1996.
The week's earthquake list, which is fully based on analyst processed hypocenters, totals 325 this week. Click here to see a summary and map .
A M2.7 quake occurred early this afternoon, at 1:04 pm, 3 miles east of Borrego Springs. We have received no inquiries.
There was a M3.9 Hector aftershock at 2:41 am this morning, located 36 miles south-southwest of Baker, in the northern part of the aftershock zone. It was reported felt in the area east of Barstow.
Since the last update, there have been two M2.9's in the southern Sierra Nevada area. At 2:42 pm Tuesday, there was one 21 miles south-southwest of Coso Junction, and the second one occurred at 5:26 am today, 9 miles northwest of Coso Junction. Neither was felt.
There have been a couple of potentially feelable aftershocks to this morning's Loma Linda quake: a M2.5 at 7:53 am and a M3.2 at 7:47 pm.
In addition, there was a M3.3 Hector aftershock at 11:35 am, located 31 miles north of Twentynine Palms.
There have been several quakes over the weekend that deserve mention:
First, there was another small Coso quake Friday evening, at 8:26 pm, located 4 miles northeast of Coso Junction. Magnitude was M3.0. A M2.5 occurred closeby at 5:09 am today. We got no inquiries about either quake.
There was a M2.6 quake at 6:58 Sunday morning, also north of us, this time 4 miles northeast of Ridgecrest. It was followed at 7:15 am Sunday, by a M2.5 at the same location. A larger one, M2.9, occurred at the same site at 1:48 am today. All, as far as we know, went unnoticed.
The biggest excitement was this morning at 5:49 am, when a M4.4 quake occurred near Loma Linda, very close to if not in the San Jacinto fault zone. It was felt from Victorville to south of Temecula, but was most noticable in the Riverside and San Bernardino areas.
There were two quakes in the upper M2's overnight. The first was at 9:16 pm, in the Santa Ana Mtns., 11 miles south-southeast of Corona, magnitude M2.7. The other was at 7:06 am, located 6 miles north-northwest of Borrego Springs, magnitude M2.6. We have received no public inquiries about either event.
The week's earthquake list, which is fully based on analyst processed hypocenters, totals 385 this week. Click here to see a summary and map .
A M3.1 quake occurred this morning at 8:12 am, in the Coso Ranges 4 miles northeast of Coso Junction. Coso Junction is a very small town that you pass by on 395 about halfway between L.A. and Mammoth Lakes. The area is a common source of small quakes, with occasional swarms including M5+ events.
It is our intention to resume frequent commentary of the daily earthquakes. We plan to mention Hector aftershocks that are above M3, and quakes elsewhere in the region that are above M2.5. This should last until we get overwhelmed again by the next sequence!
At 1:57 am this morning, there was a M4.4 Hector aftershock, located in the northern part of the aftershock zone, 36 miles southwest of Baker. It was followed by a number of aftershocks of its own, including a M3.3 at 4:02:07 am, a M3.1 at 4:02:49 (in other words, two M3's in the same minute), and a M3.3 at 4:38 am. The M4.4 was probably felt, although we do not have any inquiries at this time.
The week's earthquake list, which is fully based on analyst processed hypocenters, totals 406 this week. Click here to see a summary and map .
The week's earthquake list, which is fully based on analyst processed hypocenters, totals 461 this week. Click here to see a summary and map .
Below is a preliminary statistical summary, including the 4th quarter of 1999. Note that, because the Hector aftershocks are still not completely processed, the totals for 4th quarter are minimum numbers.
-------------------------------------------------------------------- Totals listed below apply to the area within a polygon bounded by the following latitude, longitude pairs: 35.0N, 121.0W; 37.0N, 119.0W; 37.0N, 115.0W; 32.5N, 115.0W; and 32.5N, 121.0W.
1st quarter 2nd quarter 3rd quarter 4th quarter all 3.0+ all 3.0+ all 3.0+ all 3.0+ 1986 3,040 16 2,865 19 6,634 192 2,300 38 1987 1,679 24 1,887 36 2,341 38 5,748 198 1988 2,749 40 2,128 45 2,195 30 2,208 27 1989 2,528 46 2,340 21 2,323 19 2,335 26 1990 2,850 56 2,293 43 1,859 27 2,039 31 1991 1,679 10 1,843 32 2,081 21 2,337 27 1992 3,389 37 10,084 741 25,849 808 9,775 181 1993 4,799 41 5,152 58 5,039 46 4,419 34 1994 10,735 418 5,766 42 5,221 48 4,813 39 1995 3,639 28 3,788 37 8,252 88 7,733 49 1996 5,735 56 3,966 32 3,690 23 4,660 50 1997 3,699 39 3,276 54 3,193 31 2,443 27 1998 3,590 56 2,830 27 2,655 37 2,352 29 1999 2,253 35 2,546 58 2,555 36 8,147+ 414+ --------------------------------------------------------------------