Earthquake Commentary for April, 1999

Map of recent earthquake activity

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21:30 PDT, Thursday, April 29

We recorded and analysed 174 earthquakes in the past week. Click here to see a summary and map .

20:32 PDT, Thursday, April 29

Last night, Wednesday, the people of Camarillo felt a M2.8 earthquake at 11:27 pm.

12:44 PDT, Saturday, April 24

There have been two earthquakes of possible interest today. The first was a M2.7 at 8:38 am today, located at a familiar location 14 miles southwest of Ocotillo.

The other was a M3.1 at 10:42 am, located 6 miles east-northeast of Desert Hot Springs. This is also a familiar location, being the epicenter of a M3.4 on April 13. Both the April 13 and today's quakes are Landers aftershocks.

17:22 PDT, Thursday, April 22

We recorded and analysed 189 earthquakes in the past week. Click here to see a summary and map .

11:19 PDT, Wednesday, April 21

A M3.9 quake was felt late this morning, at 10:28 am, in the San Bernardino and Riverside areas. The epicenter was located 4 miles northeast of Mt. San Gorgonio, in the San Bernardino Mtns. The focal mechanism was strike-slip. The location is not the same one as the M4.7 last October 1, which was located 6 miles west-northwest of Mt. San Gorgonio.

10:43 PDT, Monday, April 19

We have had a moderately active weekend, seismically speaking. The largest events were a M3.6 and a M4.2, both located 15 miles southwest of Ocotillo (in northern Mexico). They were felt in the San Diego area and in the Imperial Valley. The times were 7:33 am and 8:53 am on Sunday. This particular area has been active throughout February and March of this year, with the largest events being a M4.3 on March 13 and another M4.2 on February 18.

On Saturday evening, at 11:22 pm, there was a small Landers aftershock, 20 miles northeast of Lucerne Valley. The magnitude was M2.6. Because of the remote location, this quake is unlikely to have been felt.

Earlier, at 10:01 am on Saturday, the Mojave Desert 22 miles north-northeast of Boron experienced a M2.6 quake. That quake is also unlikely to be reported 'felt'.

Yesterday evening, at 8:58 pm, there was a very small, but nonetheless felt, quake 1 mile east-southeast of Baldwin Hills. The magnitude was M2.1. This location is in the complex Newport-Inglewood fault zone, which is a reasonably common source of microearthquakes. By the way, a microearthquake is, by definition, a quake with magnitude less than 3.0.

14:35 PDT, Friday, April 16

There was another Landers aftershock at 11:51 am this morning, located 6 miles east-northeast of Desert Hot Springs. This is the same location as a M3.4 on Tuesday afternoon.

09:52 PDT, Friday, April 16

A M3.2 Landers aftershock, located 26 miles north of Yucca Valley, happened this morning at 8:53 am. Considering the sparse population in this desert area, we don't really expect any 'felt' reports.

07:36 PDT, Friday, April 16

There were two small quakes in the outlying areas of the seismic network overnight. The first was a M2.5 offshore, 29 miles south-southwest of San Clemente. The time was 6:44 pm. The second event was on the eastern edge of the Coast Ranges, 25 miles north of Taft. It occurred at 1:40 am, and had a magnitude of M2.7. Neither event was, as far as we know, felt.

17:44 PDT, Thursday, April 15

We recorded and analysed 206 earthquakes in the past week. Click here to see a summary and map .

10:56 PDT, Thursday, April 15

Imperial Valley experienced a small swarm yesterday evening, at Obsidian Butte, at the southwestern end of the Salton Sea. The two largest events were a M3.3 at 8:51 pm and a M3.1 at 8:59 pm. These two earthquakes were felt in the northern Imperial Valley. There were also a M2.5 in between those two, at 8:56 pm and a M2.7 at 9:12 pm, plus a number of smaller quakes. Obsidian Butte is in an active swarm area known as the Brawley Seismic Zone, which connects the northern end of the Imperial fault with the southern end of the San Andreas fault at Bombay Beach.

Later, at 12:08 am, there was a M2.8 in the Big Bear area, 6 miles east-southeast of Big Bear City. There were no inquiries about it, but it might have been felt. This epicenter is within the Big Bear aftershock zone (of the June 28, 1992 Mw6.2 Big Bear aftershock of Landers). However, there have been almost no Big Bear aftershocks in the past year or so, so we are inclined to call this morning's quake part of the general background seismicity, as opposed to an aftershock.

Note that there was also activity overnight in the Pinnacles area, 22 miles south-southeast of Hollister, in the Northern California Seismic Network. The largest was M3.8. Click on on "Map of recent earthquake activity" above for more information.

17:51 PDT, Tuesday, April 13

There was another M3.4 late this afternoon, this time a Landers aftershock, located 6 miles east-northeast of Desert Hot Springs. The time was 4:39 pm. This epicenter is near the southern end of the Landers aftershocks, on the western edge of the aftershock zone. Like quite a few of the smaller aftershocks in that region, the focal mechanism is oblique normal. We haven't gotten any 'felt' inquiries, but this afternoon's quake could well have been felt in the Desert Hot Springs area.

14:07 PDT, Tuesday, April 13

An earthquake was felt in Orange County at 11:22 am this morning. The magnitude was 3.4, and the epicenter was 3 miles south of Fountain Valley (between Fountain Valley and Costa Mesa). Although the quake occurred not far from the Newport-Inglewood fault, the focal mechanism has a large thrust component, incompatible with the strike-slip Newport-Inglewood. The quake was felt as far away as the Long Beach area.

08:28 PDT, Tuesday, April 13

There was a small quake, M2.7, 19 miles north-northeast of the town of Lake Isabella in the Sequoia National Forest, this morning at 6:47 am. As far as we know, it was not felt.

15:53 PDT, Sunday, April 11

There was another Whiskey Springs aftershock, magnitude M2.7, at 1:08 pm today. In fact, it was an aftershock of an aftershock, namely the M3.1 at 10:21 am this morning. We got no inquiries.

12:09 PDT, Sunday, April 11

Earthquakes that happened this morning include (the largest) a M3.6 Northridge aftershock, at 2:09 am. It was located 3 miles west-southwest of Valencia, and was felt as far away as Altadena. The focal mechanism is mostly strike-slip.

The second quake of interest this morning was an aftershock of the M4.9 Whiskey Springs quake, on October 26, 1998. It occurred at 10:21 am. Like the rest of the Whiskey Springs sequence, this morning's quake was located 4 miles north of Big Bear City. It also had a mostly strike-slip focal mechanism. We've gotten no inquiries from anyone who felt this quake.

Earlier, at 1:40 pm on Friday, there was a M2.8 quake in the Santa Barbara Channel, 15 miles south-southeast of Pt. Conception. Then, at 6:18 pm on Friday, there was a M2.5 quake, located 3 miles west-northwest of Loma Linda. We have received no inquiries about either of these quakes.

18:05 PDT, Thursday, April 08

We recorded and analysed 119 earthquakes in the past week. Click here to see a summary and map .

The quarterly statistical summary follows. I've discontinued the totals for the Landers and Northridge aftershock sequences, as the seismicity rates are getting low enough that background earthquakes dominate in a search of the usual rectangular geographic box.

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 --------------------------------------------------------------------

12:48 PDT, Thursday, April 08

There was another small quake 15 miles southwest of Ocotillo, at 11:13 pm last night. The magnitude was 3.0.

09:28 PDT, Wednesday, April 07

An earthquake was felt at 11:26 pm last night, from San Diego to Imperial Valley. It was a M4.0, located 14 miles southwest of Ocotillo. This location is just south of the U.S./Mexico border near Inkopah County Park. The focal mechanism was strike-slip.

Earlier, at 9:35 pm, there was a M2.5 quake in the San Jacinto Fault zone, 6 miles east-southeast of Hemet. We got no calls about that one.

09:50 PDT, Monday, April 05

At 8:06 pm on Friday, there was a M3.1 quake (Landers aftershock), located 2 miles north of Yucca Valley. Knowing that Yucca Valley residents are very good feelers of small earthquakes, we suspect that this one did not go unnoticed.

Not long after the Yucca Valley quake, there was a M2.6 at the northern front of the San Bernardino Mtns, 8 miles southwest of Lucerne Valley. The time was 9:52 pm on Friday. If anyone felt this quake, they did not inquire with us.

17:17 PST, Thursday, April 01

We recorded and analysed 160 earthquakes in the past week. Click here to see a summary and map .

07:47 PST, Thursday, April 01

At 1:48 am this morning, a M2.6 Landers aftershock occurred 6 miles east-northeast of Desert Hot Springs. As far as we know, it was not felt.

This page is Last updated 18:15 PDT April 30, 1999.