Earthquake Commentary for March, 2000

Map of recent earthquake activity

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12:43 PST, Friday, March 31

It was a busier week than last with more earthquakes being felt - the people of Valencia can vouch for this too! There were a trio of earthquakes occurring this week, all M2.8, and all located 3 miles south of Valencia. The first (which was presumed felt) occurred at 5:34 am Wednesday morning, the second at 11:32 am the same day (which WAS felt), and the third on Thursday at 10:12 am.

11:30 PST, Friday, March 31

The week's earthquake list totals 412 this week. Click here to see a summary and map .

15:35 PST, Tuesday, March 28

It appears a full blown Coso swarm is now in progress. It started off at 7:16 am this morning when a M4.3 earthquake rattled the people of Little Lake and Coso. It, so far, has been followed by 5 aftershocks greater than magnitude 3: a M3.5 two minutes after the mainshock, a M3.2 and a M3.3 in the same minute of 7:22 am, a M3.1 at 10:50 am and a M3.3 at 1:30 pm. We have reports that the mainshock and the M3.5 have been felt, it is unknown if the rest were also. They are all generally in the same location; 4 miles east-southeast of Coso Junction.

16:38 PST, Monday, March 27

We had some activity this weekend, seismically speaking that is. On Saturday evening (the 25th) at 11:34 pm there was a M3.0 earthquake located 12 miles north of Ridgecrest. It's unknown whether this was felt, we received no calls. But on Sunday evening (the 26th) there was a duo of earthquakes that were most definitely noticed by the people of Imperial Valley, mainly by those in Westmorland, Brawley and El Centro. At 9:07 pm a M4.0 earthquake occurred 2 miles northwest of Brawley. Ten minutes later a M3.8 aftershock followed in generally the same location. Focal mechanisms for both earthquakes agree with the strike-slip faults in the area. Actually there was pair of felt earthquakes in the same area last week (see below in Monday's commentary). Earthquakes this size in a populated area are expected to cause moderate shaking with the possibility of only very light damage if any at all.

18:38 PST, Thursday, March 23

The week's earthquake list totals 320 this week. Click here to see a summary and map .

08:42 PST, Thursday, March 23

The only quake above M2.5 overnight was a M2.7 at 6:09 am this morning, located 1 mile west-northwest of Goldstone Lake.

09:19 PST, Wednesday, March 22

A pair of M3's occurred out in the desert overnight. The first was a M3.2 Landers aftershock (of the June 28, 1992 Mw7.3 quake), at 8:35 pm last night. The second was a Hector aftershock (of the October 16, 1999 Mw7.1 quake), at 7:51 am today, magnitude M3.0. We haven't got any phone inquiries on either one, and there does not seem to be any "blip" in the web hits.

11:15 PST, Monday, March 20

We saw a few small quakes over the weekend:

For instance, there was a M2.5 at 8:45 am on Saturday, 4 miles south-southeast of Loma Linda.

At 9:40 pm on Saturday, there was a M2.6 located 7 miles east of Desert Hot Springs.

A really tiny one, M2.1, occurred at 10:38 pm on Saturday, under the Santa Monica Mtns. 3 miles south-southeast of Encino.

Another M2.6 occurred at 4:17 am on Sunday, 12 miles northeast of Arvin.

Another tiny one, M2.3, occurred at 5:23 am on Sunday, 4 miles west of Isla Vista.

There was a M2.5 at 2:14 pm on Sunday, 5 miles south-southeast of Big Bear City.

An aftershock in the Yorba Linda sequence (of a M4.0 on March 6) occurred at 10:18 pm on Sunday. The magnitude was M2.4.

At 12:52 am and 12:53 am this morning, a pair of quakes (M2.8 and M2.7) occurred 2 miles northwest of Brawley, in the Imperial Valley. As far as we know, the only events of these that were felt were the Brawley quakes. Incidently, our system manager recently showed me how to spot "felt" events by looking at the web hit statistics!

A number of Hector aftershocks occurred as well, up to magnitude M2.7. Hector aftershocks less the M3 are almost never being reported felt, however, so I am omitting them from the commentary.

16:30 PST, Thursday, March 16

The week's earthquake list totals 377 this week. Click here to see a summary and map .

09:15 PST, Thursday, March 16

A quake was felt yesterday evening, at 8:35 pm, in the Malibu area. The epicenter was 3 miles north-northwest of Pt. Dume, and the magnitude was M3.4. There was a small aftershock (M2.4) at 4:40 am this morning.

Let me also point out a northern California event of some interest: a M5.8 off the coast of Cape Mendocino (49 miles west of Punta Gorda) at 7:19 this morning. For further information, visit the Recent Earthquakes in California site.

08:47 PST, Wednesday, March 15

A small quake was felt in the Big Bear area overnight. The magnitude was M2.7, and the epicenter was located 4 miles south of Big Bear City.

17:53 PST, Tuesday, March 14

A summary of the seismicity for the last "weekend" seems in order. It was a little bit more active than usual.

At 2:55 am on Friday, there was a M2.5 located 5 miles southwest of Riverside. Did anyone feel it?

We know that the M3.4 at 7:56 am on Friday was felt. As previously mentioned, it occurred 8 miles south of Joshua Tree, making it a Landers aftershock.

At 1:46 pm on Saturday, a M3.1 quake was felt in the Fontana, Yorba Linda area. The location was 5 miles southeast of Yorba Linda.

The largest quake of the weekend was a M3.8 at 10:59 pm on Saturday, located 6 miles northwest of Pt. Sal (near Lompoc). It was reported felt. It was followed at 11:05 pm by a M3.0 aftershock, at the same locations.

A smaller quake (M2.4) occured only 3 minutes later, 1 mile east of Baldwin Hills in the L.A. area. It was also reported felt. There was another M2.4 near the sam location at 8:47 pm on Sunday.

And lastly, this afternoon, there was a M2.8 quake located 3 miles south-southeast of Loma Linda.

Aside from the usual crop of Hector afteshocks, that was about all that was of interest.

17:33 PST, Friday, March 10

At 7:56 am (PST) some of the people of Joshua Tree felt the M3.4 earthquake that occurred this morning. No damage from the quake was reported (nor really expected). Its epicenter was located 8 miles south of Joshua Tree.

16:50 PST, Thursday, March 09

The week's earthquake list totals 379 this week. Click here to see a summary and map .

10:39 PST, Thursday, March 09

At 2:45 pm yesterday, a M3.0 quake occurred 3 miles northeast of Ridgecrest.

This morning, at 4:17 am, a M3.0 Hector aftershock took place 34 miles southwest of Baker.

Then, at 9:02 am this morning, a M3.1 quake occurred 2 miles west of Fontana. Although we have gotten no felt reports, the location and magnitude suggest that it probably was felt. The focal mechanism was strike-slip.

10:42 PST, Wednesday, March 08

At 7:43 pm last night, there was a M3.1 Hector aftershock, located 28 miles north of Joshua Tree. No-one reported feeling it.

At 9:57 pm, there was a M2.6 located 6 miles northeast of Big Bear City, an aftershock of yesterday's M3.6 at the same location.

14:56 PST, Tuesday, March 07

Yesterday's Orange County quake had a M3.1 aftershock at 9:49 am today. Like its M4.0 mainshock, it was located 7 miles southeast of Yorba Linda. It has been reported felt.

09:33 PST, Tuesday, March 07

Another potentially feelable quake happened overnight, this time in the Big Bear area. It was a M3.6 at 2:35 am. The epicenter was 6 miles northeast of Big Bear City. Surprisingly, we haven't received any calls.

To answer a question that we've been getting commonly, it is difficult if not impossible to assign small quakes such as this to a particular fault. This is especially true using only the location. The reason is that quakes in the 2's, 3's and even 4's can easily be on faults that are too small to be mapped. Often such faults do not even intersect the Earth's surface. Sometimes, if the quake maps very close to a known fault AND the focal mechanism agrees in strike and dip with that fault, we may make a tentative attribution. Most of the time, however, we want to be cautious about misleading people into thinking we know what fault was the source of the quake. This is the main reason why we no longer list the nearest mapped fault in our descriptive location; in short, it was more often wrong than right, and most people were reading it as the absolute truth.

17:06 PST, Monday, March 06

Orange County and eastern Los Angeles County felt a quake this afternoon: a M4.0 located in the Santa Ana Mountains 7 miles southeast of Yorba Linda. The time was 4:20 pm. It was felt as far away as the Pasadena area. Interestingly, although today/s quake was not a member of any of our on-going aftershock sequences, it was very similar in location and magnitude to a M3.9 on December 7, 1999.

16:37 PST, Thursday, March 02

The week's earthquake list totals 386 this week. Click here to see a summary and map .

10:41 PST, Thursday, March 02

The two latest additions to the Coso swarm roster are: a M4.0 at 7:00 am today and a M3.5 at 9:09 am today. Both were located 19 miles east of Coso Junction. The M4.0 was felt in the epicentral area.

13:34 PST, Wednesday, March 01

A small quake (M2.5) was felt at 1:50 am this morning in Orange County. The epicenter was located 2 miles southwest of Fountain Valley.

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