Earthquake Commentary for July, 2001

Map of recent earthquake activity

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20:25 PDT, Tuesday, July 31

The Coso swarm produced a M3.6 at 8:05 pm today. The epicenter was 5 miles southeast of Coso Junction. Undoubtably this quake as felt. We just don't have the reports yet. Most swarm members over about M3.5 are being reported as felt.

07:20 PDT, Tuesday, July 31

The Coso swarm continues at a low level. We had another M3+ quake ... a M3.3 at 2:06 am, located 3 miles east-southeast of Coso Junction.

17:55 PDT, Monday, July 30

... and another: M3.5 at 5:47 pm today, located 3 miles east of Coso Junction.

16:44 PDT, Monday, July 30

There has been another M3.7 quake in the Coso swarm. It occurred at 4:34 pm today, 3 miles east of Coso Junction.

09:40 PDT, Sunday, July 29

Another M3+ Coso quake occurred at 1:19 pm Saturday. The location was 6 miles southeast of Coso Junction, and the magnitude was M3.0.

09:37 PDT, Saturday, July 28

There were two more Coso quakes above M3 overnight. The first was a M3.7 at 6:09 pm and the other was a M3.0 at 2:41 am. Both were located 3 miles east of Coso Junction.

09:45 PDT, Friday, July 27

The Coso swarm produced another M3.0 at 10:24 pm last night. Like the others, this quake was located 5 miles southeast of Coso Junction.

17:41 PDT, Thursday, July 26

The TriNet system has recorded and located 1,065 earthquakes in the last week. At this point we have reviewed more than half of them, including most above M2.0 and those with obviously bad locations. Click here to see a summary and map .

17:09 PDT, Wednesday, July 25

At 4:46 pm, there was a M3.0 quake, located somewhere other than Coso. The epicenter was 6 miles west-northwest of the town of Big Bear Lake. It probably was felt, although we do not have any reports yet.

07:05 PDT, Tuesday, July 24

There were three more small Coso quakes overnight: a M3.0 at 1:37 am, a M3.2 at 1:54 am, and a M3.3 at 6:43 am. As far as we know, they were not felt.

14:31 PDT, Monday, July 23

There has been another Coso quake. At 2:02 pm today, there was a M3.7, located 5 miles southeast of Coso Junction. Although this member of the swarm was big enough to have been felt in the area, we have no reports so far.

07:54 PDT, Monday, July 23

There were a few M3's over the weekend. On Saturday, at 12:57 pm, a M3.1 quake occurred 8 miles west-southwest of Ocotillo, near the U.S./Mexico border.

Early Sunday morning, at 1:13 am, there was a M3.0 quake located 5 miles west-northwest of Big Bear. The Big Bear quake was felt slightly in the epicentral area and in San Bernardino.

Also, of course, there were Coso quakes. The largest were a M3.2 at 4:15 am on Saturday and a M3.1 at 3:07 pm on Saturday.

08:29 PDT, Friday, July 20

The TriNet system recorded and located 1,458 earthquakes in the last week, mostly in the Cosa area. Click here to see a summary and map .

Our human analysts have only had time to review about a third of these events, so there may be some minor changes down the line in the data set for this week. In particular, the 1,458 total will probably creep higher as we include more small, but clearly recorded quakes.

14:00 PDT, Thursday, July 19

There has just been a M3.8 quake widely felt in the Inland Empire, at least between Victorville and Riverside. The epicenter is located 5 miles northwest of Devore. Although this location is very close to the San Andreas fault, the preliminary focal mechanism is thrust, which is incompatible with the San Andreas. This probably means the quake is on some minor subsidiary fault.

07:14 PDT, Thursday, July 19

The Coso swarm has slowed down considerably. However, we a M3.5 at the swarm site at 3:56 am this morning.

By the way, the TriNet system has recorded 1,239 earthquakes in the general vicinity of Coso, since late Friday afternoon.

11:53 PDT, Wednesday, July 18

Submitted by Egill Hauksson, Kate Hutton

The recent earthquake swarm near Coso Junction began last week and has so far culminated with a M4.9 event, 17 July 2001. Focal mechanisms of most of these earthquakes show right-lateral motion, striking N5 to N20W (DEG), with a small normal component. This swarm is located along the southern edge of a similar swarm that occurred in the first half of 1992 and lasted (off and on) for several months. These events are not volcanic in nature, because they have normal frequency content as expected for crustal tectonic earthquakes.

Earthquake swarms are common in the Coso Range and the erea to the south, extending into Indian Wells Valley, because of the extensional tectonics and presence of a magma chamber beneath the Coso Geothermal field. The extension in the Coso Range and to the south is driven by a releasing stepover between the right-lateral Airport Lake and Owens Valley faults along the east side of the Sierra Nevada. The footwall of this system, the Coso Range, is attached to the Sierra Nevada, and is effectively being pulled to the northwest from beneath Wildhorse Mesa, which is moving as part of the Walker Lane belt. The brittle upper crustal extension may be accommodated at depth by ductile stretching and emplacement of igneous bodies, the presence of which have been inferred in the middle to upper crust from analysis of seismic waves. Late Cenozoic intrusions are interpreted to be the source of heat for the Coso geothermal field.

North of the Coso Range, the major active fault along the eastern margin of the Sierra Nevada is the Owens Valley fault that was the source of the M7.6 1872 Owens Valley earthquake. Surface rupture associated with the 1872 earthquake extended as far south as the western margin of Owens Lake, within 10 km of the Coso Range. Coseismic surface displacement during the 1872 event was predominantly dextral. Currently, theOwens Valley fault slips about 6 mm/yr, which is comparable to the 5 mm/yr rate estimated for the Airport Lake fault south of the Coso Range. These relations thus suggest that earthquake swarms on Quaternary faults in the Coso Range transfer dextral slip along the eastern margin of the Sierra Nevada northward from Indian Wells Valley to the southern Owens Valley.

09:14 PDT, Tuesday, July 17

By the way, if you have felt any of these earthquakes and would like to share your experience, go to, select the event from the list, and fill out the questionaire. Your observations will go into a data base that will help us relate the intensity of earthquake shaking with magnitudes, soil types, and so forth.

06:34 PDT, Tuesday, July 17

The Coso swarm that began on Sunday intensified significantly this morning, with a M4.9 at 5:07 am. This quake was widely felt in southern Owens Valley, the Mojave desert, and as faw away as San Francisco. The M4.9 was followed shortly by intense swarm activity, including at least six quakes over M3. A M4.7, which was also widely felt, occurred at 5:59 am.

These epicenters are between 3 and 4 miles east-southeast of Coso Junction along Rt. 395, in Rose Valley. Rose Valley abuts the Coso Range, which is an active geothermal area, with prehistoric volcanic activity and plenty of recent earthquake swarms.

Check this web site later in the day for further updates.

17:24 PDT, Monday, July 16

The highlights of the Coso activity today were: a M3.5 at 8:55 am and a M3.0 at 1:30 pm. Both of these (and more!) were located 4 miles east-southeast of Coso Junction. The 8:55 am quake was felt in the Ridgecrest area.

07:18 PDT, Monday, July 16

At 8:21 pm last night, there was a M3.0 quake in the San Jacinto fault zone, 14 miles north of Borrego Springs. There were a couple of "felt" reports at about that time from the Redlands area.

Meanwhile, the Coso swarm continues at a somewhat lower level than yesterday. A pair of M3.0's occurred at 4:55 am and 6:05 am this morning, both at the previous location 4 miles east-southeast of Coso Junction. In case you don't know where Coso Junction is, you pass it on the 395 about half way between L.A. and Mammoth.

20:57 PDT, Saturday, July 14

It appears yet another Coso swarm has started up again. It abruptly kicked off this Saturday morning with two earthquakes occurring within the same minute, that of 10:30 am. The first earthquake, the foreshock, is a preliminary M3.6, then 11 seconds later a preliminary M3.8 quake followed. Two minutes after that, at 10:32 am, a preliminary M3.9 quake occurred. All roughly have the same location, that of 4 miles east-southeast of Coso Junction. The earthquakes were felt in the town of Little Lake; and as south as Ridgecrest and as north as Lone Pine. No, or very slight, damaged is expected from these earthquakes.

14:42 PDT, Friday, July 13

We have recorded and located 124 earthquakes in the last week. Click here to see a summary and map .

12:09 PDT, Thursday, July 12

A M3.0 quake occurred yesterday evening at 6:02 pm. The epicenter was 11 miles northeast of Lake Isabella. The quake had a M2.5 aftershock at 6:39 pm.

Earlier in the day, at 1:37 pm, there was a M2.5 quake, located 6 miles north of Banning.

Also, early this morning, at 12:11 am, there was a M2.8 Landers aftershock, 25 miles north of Yucca Valley. None of these quakes were reported felt.

07:21 PDT, Monday, July 09

At 10:04 am on Sunday, a small quake (M2.6) was felt in the Big Bear Lake area. The epicenter was located 8 miles east of Big Bear City.

18:05 PDT, Thursday, July 05

We have recorded and located 209 earthquakes in the last week. Click here to see a summary and map .

10:04 PDT, Tuesday, July 03

A M3.9 quake was fairly widely felt this morning in the Inland Empire and Big Bear areas. The epicenter was under Big Bear City, and the time was 4:40 am. Felt reports came in from Big Bear Lake, Moreno Valley, the Banning area, and the Cajon Pass area. There were a few other quakes at the same location, the largest being a M2.2 at 4:48 am and a M2.4 at 4:43 am.

07:34 PDT, Monday, July 02

At 1:44 am today, a small quake was felt in Rialto and heard in Mira Loma. The epicenter was 2 miles north-northeast of Fontana, and the magnitude was M2.5.

There was also a M2.8 quake this morning, in a sparsely populated area 11 miles west-northwest of Ocotillo. It happened at 3:44 am. As far as we know, it was not felt.

09:38 PDT, Sunday, July 01

The M3.3 quake at 10:14 pm last night was felt in the town of Lake Isabella, Weldon, Camp Nelson, Ponderosa, Porterville and Three Rivers. The epicenter was located 25 miles west-southwest of Olancha.

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Last updated 20:25 PDT July 31, 2001.