Earthquake Commentary for August, 1998

Map of recent earthquake activity

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10:05 PDT, Monday, August 31

We had a few small quakes on Sunday and early this morning. The first was a M2.5 Landers aftershock at 1:23 am on Sunday, located 8 miles south of Joshua Tree.

Following that, there was a M2.9 quake at 4:01 pm on Sunday, in the Mojave Desert 20 miles east-northeast of Boron. The focal mechanism was strike-slip.

The next was at 5:08 pm Sunday, a M3.0 aftershock of the August 16 M4.8 quake near Mt. San Gorgonio in the San Bernardino Mtns. The epicenter was 6 miles west of Mt. San Gorgonio. The focal mechanism was strike-slip also.

Following that, there was a tiny little swarm in Alhambra, including three quakes: a M1.7 at 11:52 pm Sunday, a M1.4 at 1:26 am this morning, and a M2.0 at 2:25 am this morning. I mention these only because they were almost 'in our backyard'. The town of Alhambra is not on the list of towns in our computer, so the descriptive locations showed 2 miles south-southwest of South Pasadena. I wonder if anyone was insomniac enough to feel the M2.0?

16:06 PDT, Saturday, August 29

We had a M2.5 Landers aftershock last night at 6:58 pm. The location was 17 miles north of Yucca Valley.

21:11 PDT, Thursday, August 27

We recorded and analysed 227 earthquakes in the past week. Click here to see a summary and map .

10:06 PDT, Thursday, August 27

There was a M3.4 quake at 8:10 pm last night in northern Baja, which could conceivably have been felt in the Imperial Valley. It's location was 18 miles southeast of Calexico.

At 1:29 am today, another aftershock of the August 16 M4.8 San Gorgonio quake hit. It had a magnitude of M2.6. Like the others, it was located about 6 miles west of Mt. San Gorgonio.

Following that, there was a Landers aftershock: a M2.5 at 3:32 am. The epicenter was 18 miles north of Yucca Valley.

15:25 PDT, Wednesday, August 26

Ooops, another one! The latest quake was a M2.7 at 2:51 pm, located 2 miles west-northwest of Culver City. It was felt in the epicentral area.

14:38 PDT, Wednesday, August 26

A M3.3 quake at 1:46 pm today was felt in the San Fernando Valley and as far away as Pasadena. The epicenter was 1 mile northeast of the city of San Fernando, making it an aftershock of the January 17, 1994 Mw6.7 Northridge quake.

10:07 PDT, Wednesday, August 26

There was a small quake at 8:50 am this morning, located 25 miles east of Big Pine, north of the Saline Valley. Our magnitude estimate is M3.6, and the U.S.G.S. at Menlo Park's estimate is M4.1. We are looking into resolving this difference. This quake is not ideally situated for location by either of our seismic networks. Also due to its location, we do not expect to get "felt" reports.

12:51 PDT, Sunday, August 23

There have been a few small quakes so far this weekend. The first was a M2.9 at 5:58 pm on Friday. It was located 12 miles north of Trona.

The next was at 7:33 am today. It had a magnitude of M2.6 and an epicenter 4 miles north-northeast of Lake Isabella. As far as we know, neither of these last two was felt.

A quake was felt, however, in the San Bernardino/Yucaipa area, at 8:42 am today. Magnitude was M3.4, and the epicenter was 6 miles northeast of Yucaipa. Note this is a slightly different location from the Mt. San Gorgonio quake a week ago (M4.8 at 6:34 am last Sunday).

We are, by the way, still having a few aftershocks from last Sunday's quake. There was one at 8:57 am this morning, magnitude M2.8.

08:54 PDT, Friday, August 21

We recorded and analysed 215 earthquakes in the past week. Click here to see a summary and map .

The total number of aftershocks so far to Sunday's M4.8 quake was 74. Only six of them were magnitude M2.5 or larger.

21:17 PDT, Thursday, August 20

At 4:49 pm today, there was a M4.4 quake located 2 miles west northwest of Wrightwood. It was widely felt in the Inland Empire and the greater Los Angeles area. It's epicenter was close to the San Andreas fault. However, it's focal mechanism was thrust, which is inconsistent with what we know about the San Andreas.

17:16 PDT, Monday, August 17

We just experienced another aftershock of Sunday morning's M4.8. It occurred at 4:18 pm, 7 miles west-northwest of Mt. San Gorgonio, and had a magnitude of M3.2. We have gotten reports that it was felt.

12:43 PDT, Monday, August 17

The largest aftershock so far of the M4.8 Mt. San Gorgonio event has been a M3.0 at 12:05 pm yesterday.

We also had a M3.5 very early (2:44 am) this morning, but it occurred at a different location, 2 miles northwest of Lake Arrowhead. It is not, therefore an aftershock of the earlier event. The focal mechanism appears to be "normal", indicating extension in the crust. It was felt in the epicentral area.

09:45 PDT, Sunday, August 16

At 06:34 AM PDT (13:34 GMT) a moderate earthquake (Magnitude 4.8) occurred 20 miles east of San Bernardino. The location was near Mt. San Gorgonio. The earthquake had a strike-slip mechanism with nodal planes striking N30W and N60E. Preliminary location of aftershocks suggested that the northwestplane was the fault plane. This event is located several miles north of the San Andreas fault in a region that has had a moderate level of seismicity over the last 10 years. In the 2 hours following the mainshock there have been 7 small magnitude (M2) aftershocks. The event was lightly felt across a wide area of southern California, but there has been no reported damage. US Geological Survey, Pasadena and Caltech Seismologial Laboratory

17:00 PDT, Friday, August 14

At 12:32 pm today, there was a M3.0 quake offshore, located 15 miles southeast of Santa Barbara Island.

07:45 PDT, Friday, August 14

We recorded and analysed 159 earthquakes in the past week. Click here to see a summary and map .

13:35 PDT, Wednesday, August 12

Now for a different spot in the Mojave Desert! There was a M3.6 quake at 12:56 pm today, located 11 miles south of Boron. It also had a thrust focal mechanism.

12:33 PDT, Wednesday, August 12

At 11:16 am today, we had a M3.2 located 15 miles west-northwest of Victorville. The focal mechanism appear to be thrust.

10:11 PDT, Wednesday, August 12

The 'action' this morning occurred outside our coverage area, 7 miles south-southeast of San Juan Bautista or 8 miles south-southwest of Hollister. The time was 7:10 am, and the magnitude was ML5.3. Look for further info on the Menlo Park web page .

We had our own small 'quake of interest' this morning at 9:04 am. It was a M3.1 offshore, 11 miles southwest of Pt. Arguello.

10:09 PDT, Monday, August 10

At 9:13 pm last night, there was a M2.6 in the Santa Ana Mtns., 8 miles east-northeast of Tustin. We got no inquiries.

Then, at 3:41 am today, there was a M2.5 located 3 miles north-northeast of Wheeler Ridge. Wheeler Ridge is on the northern side of the Grapevine. We got no inquiries about that quake either.

14:20 PDT, Friday, August 07

At 12:33 this afternoon, there was a M2.8 offshore of Palos Verdes, 4 miles south of Palos Verdes Point. It might have been felt, although we have not heard that it was.

08:47 PDT, Friday, August 07

We recorded and analysed 183 earthquakes in the past week. Click here to see a summary and map .

Note that we had another small quake near Julian at 10:26 pm last night. It had a magnitude of M2.6. The epicenter is almost the same as that of an M2.9 earlier this week, on Tuesday morning.

08:57 PDT, Thursday, August 06

The Big Bear Lake area experienced a small earthquake sequence last night. The first and largest was a M2.8 at 8:29 pm, located 2 miles north of Big Bear City. It was followed by a handful of aftershocks, at the same location, including a M2.5 at 1:41 am. We haven't gotten any inquiries. These quakes were in the same vicinity as the Big Bear quake of June 28, 1992 (Mw6.4, itself an aftershock of the Mw7.3 Landers quake the same day). Since, however, the aftershocks from that quake seem to have died off a few years ago, we aren't calling last nights quakes aftershocks.

16:58 PDT, Wednesday, August 05

At 8:53 am today, there was another small one, M2.5, located in the Imperial Valley, 1 mile west-northwest of Brawley.

09:16 PDT, Wednesday, August 05

Quakes of interest overnight included a M2.9 at 10:50 pm, located 5 miles north-northeast of Beaumont. So far, we have not reports of it having been felt.

Another one, of approximately the same size - M2.8, occurred at 3:43 am, 25 miles southwest of Oceanside. This one is unlikely to have been felt.

13:09 PDT, Tuesday, August 04

The largest quake in our area overnight occurred at 4:05 am, 15 miles east-southeast of Julian. The magnitude was M2.9. The focal mechanism was strike-slip, consistant with the nearby Elsinore fault.

09:55 PDT, Monday, August 03

There were a few minor quakes over the weekend. For example, a M2.5 occurred on Saturday afternoon, at 5:44 pm, located 9 miles south of Salton City, on the western edge of the Imperial Valley.

On Sunday afternoon, at 5:03 pm, there was a M2.8 at one of the usual Coso Range locations, 16 miles east of Coso Junction. It was accompanied by a few smaller quakes. Just a few minutes later, at 5:45 and 6:30 pm, two earthquakes occurred 44 miles south of Calexico. The had magnitudes of M3.3 and M3.7, respectively, and are mentioned here only because it is possible that they could have been felt in the Imperial Valley.

Last updated 10:10 PDT August 31, 1998
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