Regional Location:

Preliminary Magnitude:

Moment Magnitude (Mw):

Greenwich Mean Date:
Greenwich Mean Time:
Latitude:
Longitude:
Focal Depth (km):
Analysis Quality (A-D):

CENTRAL CALIFORNIA

5.4 Ml

n/a Mw ( -- )

98/08/12

14:10:0253

36.75N

121.46W

8.9 km

A

An earthquake registering a preliminary magnitude of Ml 5.4 (NCSN - U.S. Geological Survey's Northern California Seismic Network) occurred at 07:10 a.m. PDT (14:10:25 UTC), August 12, along the San Andreas fault in Central California. The focal point was estimated at 8.9 km beneath the surface. The quake was centered about 7 miles southeast of San Juan Bautista or about 6 miles south of Hollister, in the relatively remote Cienega Valley. The quake triggered along the creeping segment of the San Andreas fault which experiences numerous smaller temblors every week.

The quake was strongly felt in the epicentral area and as far away as San Francisco and the east Bay Area. Residents in the area reported the quake began with a sharp jolt (P-wave) followed a rolling motion (S-wave) that lasted for about 10-15 seconds. Items were thrown from tables and shelves and pictures knocked from walls, but no significant damage was reported. Commercial stores in the epicentral area told of isles littered with broken bottles, canned goods and other retail items. Some people were frightened by the jolt but there were no injuries. The California Highway Patrol reported cracks across Highway 101 but it was unclear if they were fault or subsidence related. The highway remained open without any problems.

The USGS has issued an Aftershock Warning for possible larger earthquakes. Their warning said there is about a 20% chance of a strong to damaging shock to occur within the next 7 days. In addition, the warning said there was a 5 - 10% chance of a quake of equal or larger magnitude could occur within the next 7 days. About two dozen small aftershock have been recorded in the first couple hours, including six in the M2 range, and there were a few foreshocks, the largest of which registered M3.1.

This morning's event was the largest earthquake to occurred in the San Juan Bautista vicinity since April 18, 1990, when a M5.4 earthquake struck the small town of Chittenden, about 6 miles northwest of San Juan Bautista. More recently, the area experienced a M4.4 temblor on May 28 earlier this year. That quake was centered about 2 miles northwest of San Juan Bautista. On October 17, 1989, the Santa Cruz Mountains segment of the San Andreas fault ruptured with the powerful Loma Prieta M7.1 earthquake. The Chittenden earthquake was probably an aftershock to the Loma Prieta quake, but today's tremor was centered too far south to be considered an aftershock to the Loma Prieta earthquake. Moreover, the Loma Prieta aftershock died off a few years ago.

Since 1800, there have been 29 earthquakes which have measured M5.0 or stronger in the San Juan Bautista area, including three in the M6 range. The largest earthquake registered M6.2 and occurred occurred October 11, 1800. Although adobe houses were built with double walls, all were damaged and completely wrecked. In February of 1864, a M5.9 quake occurred north of Watsonville and caused widespread local damage and was felt as far away as Napa. In 1890, there was a quake that measured M6.0 that damaged brick and framed buildings from San Juan Bautista to Green Valley. The San Andreas fault was said to have had surface ruptures where it crossed the Pajaro River. This quake apparently had a robust aftershock sequence which lasted several days.

The last M6+ earthquakes to occurred in this area happened in 1926 and and was a double quake sequence that was centered just off the coast of Monterey. The M6.2 and M6.1 earthquakes triggered exactly one hour apart and caused considerable damage at Monterey and Santa Cruz and substantial damage at San Juan Bautista.

It is interesting to note the longest period between M5 quakes since the 1880's was 25 years (1926 to 1951) and the shortest period was one hour (1926). The average period between M5+ events between 1882 and present is 4.3 years.

Here is a listing of earthquakes registering M5.0 or greater near San Juan Bautista since 1800:

Year	Mth	Day	Lat	Lon	Mag
1800	10	11	36.9	-121.6	6.2
1864	2	26	37.1	-121.7	5.9
1882	3	6	36.9	-121.2	5.7
1883	3	30	36.9	-121.6	5.6
1885	3	31	36.7	-121.3	5.5
1885	4	2	36.8	-121.4	5.4
1890	4	24	36.9	-121.6	6.0
1892	11	13	36.8	-121.5	5.6
1899	4	30	36.9	-121.7	5.6
1910	3	11	36.9	-121.8	5.8
1910	12	31	36.83	-121.42	5.0
1916	8	6	36.67	-121.25	5.3
1916	8	8	37.00	-122.00	5.0
1920	10	5	36.58	-121.67	5.2
1926	10	22	36.75	-122.00	6.2
1926	10	22	36.75	-122.00	6.1
1951	7	29	36.58	-121.18	5.0
1954	4	25	36.93	-121.68	5.3
1960	1	19	36.78	-121.43	5.0
1961	4	8	36.68	-121.30	5.6
1961	4	8	36.70	-121.30	5.5
1963	9	14	36.85	-121.63	5.4
1969	10	27	36.90	-121.34	5.0
1972	2	24	36.62	-121.18	5.0
1974	11	28	36.90	-121.61	5.2
1986	1	13	36.57	-121.20	5.0
1988	2	20	36.80	-121.30	5.3
1990	4	18	36.93	-121.66	5.4
1998	8	12	36.75	-121.46	5.4