Remembering the Cape Mendocino Quakes of 1992

USGS report

On April 25 at 11:06 a.m. PDT, a major earthquake struck northern California. This magnitude 7.2 earthquake was located about 30 miles (50 km) south-southwest of Eureka. Ninety-five people were injured and considerable damage occurred in southwestern Humboldt County. The preliminary estimate of damage from this earthquake, and two strong aftershocks described below, is 66 million dollars. The observed maximum intensity, VIII, was experienced at Ferndale, Honeydew, Petrolia, Rio Dell, and Scotia. Landslides and rockfalls occurred in the Honeydew-Petrolia area. The quake was felt throughout much of northern California and as far south as San Francisco and southeast to Carson City and Reno, Nevada. It was also felt in many areas of southern Oregon. A tsunami was generated with maximum wave heights (peak-to-trough) of 1.1 meters at Crescent City and 0.2 meters at Point Arena, California, and 0.1 meters on Hawaii.

At 12:42 a.m. PDT on April 26, a magnitude 6.5 earthquake hit the same area causing some additional damage in the Ferndale, Petrolia, and Fortuna areas. A fire caused by a broken gas main destroyed much of the business district of Scotia. The quake was felt throughout much of northern California.

Less than 4 hours after the second quake, at 4:19 a.m. PDT, the same area experienced a third strong quake. This magnitude 6.7 earthquake caused additional damage in the area of Ferndale, Fortuna, and Petrolia. It was also felt thoughout much of northern California and southern Oregon and to the south as far as Salinas, Santa Cruz, and San Francisco. These earthquakes were accompanied by numerous aftershocks. Many of the aftershocks were in the magnitude 4.0 to 4.8 range. Some of them were strongly felt, but they caused very little damage.

3.2 Earthquake Felt in Oklahoma

UPDATED (5/29/07)

A 3.2 magnitude earthquake struck near Henryetta, Oklahoma shortly after 4:00 PM local time on May 27, 2007. There were no injuries and no damage was reported.

Earthquakes of this size are not all that uncommon in this part of Oklahoma. Historical records indicate that since 1975 at least 21 earthquakes with a magnitude of 3.0 or higher struck within 125 miles of today's quake.

Here is a list of those quakes, starting with the most recent:





Here is a map showing the region and all quakes since 1975

Click on image for larger version





Here is a closer map

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Here is an even closer view showing the specific location of this most recent Oklahoma quake

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Earthquake Preparedness

The Los Angeles Fire Department published a great earthquake preparedness guide which they have made available on the Internet.

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To those of us who live and work in the Greater Los Angeles area, earthquakes are a reality. In order to deal with this situation, earthquake preparedness must become a way of life. In the event of a major earthquake, freeways and surface streets may be impassable and public services could be interrupted or taxed beyond their limits. Therefore, everyone must know how to provide for their own needs for an extended period of time, whether at work, home or on the road.

Our goal in providing this information is to encourage you to prepare for a major earthquake and to maintain that readiness. Part of becoming ready is having the necessary supplies. Earthquakes, in our area, can happen at any time. They are not totally predictable. There are long periods between episodes. This is why it is important to maintain fresh emergency supplies through rotation of older stock into daily use. The quality of life and the potential for survival are greatly increased by being prepared.

William R. Bamattre
Chief Engineer and General Manager
Los Angeles City Fire Department

THE EARTHQUAKE PREPAREDNESS HANDBOOK

Earthquake Reported Near Kīlauea volcano

Report prepared by the Hawaiian Volcano Observatory:

A magnitude-4.7 earthquake was recorded by the U.S. Geological Survey’s Hawaiian Volcano Observatory on May 24, 2007 at 9:13 a.. H.s.t. It was located beneath the upper east rift zone of Kīlauea volcano, near Puhimau crater, at a depth of 2 km (1 mile). A magnitude 4.1 aftershock occurred at 9:33 a.m. and was located 1.5 km (1 mile) farther downrift beneath Koko‘olau crater. A magnitude-3.9 aftershock occurred at 10:51 a.m. and was located 4 km (2 miles) farther downrift than the magnitude-4.7 at a depth of 1 km (1 miles). These earthquakes are the largest so far in a flurry of earthquake in the upper east and southwest rift zones that started May 12, 2007. The earthquake flurry has not been accompanied by any unusual swelling of the summit or other signs of unusual summit activity. HVO is watching the situation closely.

Since 1998, a few earthquakes with magnitudes greater than 4.0 have occurred at shallow depths beneath the upper east rift zone. Today’s event was the largest in at least the last 50 years.

Big Island Earthquakes are Normal

from: Hawaii Big Island Volcano Update

We live on volcanos!

But nonetheless, they often make the news so we feel compelled to share any updates. Many times though, earthquakes never make the news outside of the local neighborhoods surrounding Pu`u O`o. It just depends on the volumes of news the big papers are already dealing with. Here’s the scoop on the latest newsworthy earthquake.

“A magnitude-4.7 earthquake occurred at 9:13 am HST at shallow depth beneath the summit caldera and was followed by several aftershocks, the largest of which was a 4.1 at 9:33 am. The earthquake and its aftershocks had no appreciable effect on the continuing Kilauea eruption.

The long story that won’t change daily: Over the past several months, Kilauea caldera has been expanding at a rate of 1.5 cm/month indicating minimal magma storage there; most of the magma supply continues to feed the Pu`u `O`o vent. Pu`u `O`o cone is slowly collapsing as shown by cracks spreading on its south flank and about 1 cm/month subsidence of its north flank.”

Two Quakes Shake Southern California

May 23, 2007

Parts of Southern California were shaken by two mild earthquakes Wednesday night. Both quakes measured 3.9, and were centered about 2 miles South-Southeast of Devore, California. The first came in at 11:11 PM while the second struck less than five minutes later at 11:15 PM.

Although there have been no reports of injuries or damage, the quakes have been felt all the way the Los Angeles coastline, nearly 80 miles away.

Quakes are not uncommon in this area of the Southland. There have now been five quakes with a magnitude of 3.0 or greater in this general area since the start of the year.

I have created a special page showing the last 20 years of earthquake activity in the immediate Devore area. CLICK HERE to see a list of all quakes (2.5+) between 1987 and 2007.

Remembering the 1999 Bolinas Earthquake

Written in August 1999 by JCP Seismologists

BOLINAS, CA -- Foundations rocked and rolled as far south as Santa Cruz County -- and shuddered sharply in Bolinas at the epicenter of one of the first significant earthquakes to strike the Bay Area since the Loma Prieta disaster nearly a decade ago. The temblor was felt thoughout much of the North Bay region.

Fortunately, the quake's size was fairly small, at magnitude 5.0. With the exception of a few emptied store shelves and a toppled chimney in the community of Bolinas, 15 miles northwest of San Francisco, there was little damage to speak of.

The shock struck north of the Golden Gate along the San Andreas fault zone near its intersection with the San Gregorio fault, which extends offshore southward from the Bolinas area. Although the San Andreas fault in this area is visited frequently by minor seismic activity, Tuesday's quake was one of the largest to occur along on this stretch of the fault since it ruptured here in the great San Francisco earthquake in 1906.

The U.S. Geological Survey forecast a 1-in-10 chance of strong and possibly damaging aftershocks in the Bolinas area during the week following Tuesday's quake.

Although this earthquake is insignificant in comparison to the disastrous magnitude 7.4 shock in Turkey just the day before, it may prove interesting for local scientists studying the likelihood of future earthquakes in the Bay Area.

The USGS and collaborating scientific agencies are currently updating their estimates of earthquake probability for the region's fault segments, one of which is the San Francisco Peninsula segment. This is a section of the San Andreas fault that extends from Los Gatos northwestward to the Bolinas area, and is immediately north of the Santa Cruz Mountains segment that ruptured in 1989 causing the Bay Area's most recent earthquake disaster.

In its magnitude of 5.0 and its location near the end of a San Andreas fault segment, Tuesday's jolt at Bolinas is not unlike two moderate quakes that closely preceded the Loma Prieta shock. Those events, of magnitude 5.3 and 5.4, occurred near Lake Elsman just south of Los Gatos, about 16 months and 2 months before the earthquake on October 17, 1989.

The Lake Elsman events struck where the northern end of the Santa Cruz Mountains segment and the southern end of the Peninsula segment meet. Their timing and location has led some scientists to regard the Lake Elsman quakes as "preshocks" to the subsequent magnitude 7.1 Loma Prieta rupture -- useful harbingers, if so, had they been recognized as such.

Is the Bolinas quake a preshock of a larger quake to come? On this question the jury hasn't yet had time to convene.

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USGS Report

8/17/99 Magnitude 5 earthquake near Bolinas, CA

The magnitude 5 earthquake that occurred on Tuesday, August 17 at 6:06PM local time was located beneath Bolinas Lagoon in Marin County at a depth of about 4.2 miles. It was felt broadly throughout the San Francisco Bay Area and yielded instrumental records of shaking that reflect various ground conditions in the Bay Area.

The earthquake occurred along a stretch of the San Andreas fault that had more than 20 feet of slip in 1906 as indicated by offset fence lines. In contrast, Tuesday's event probably resulted in slip of a few cm on a small patch of fault at depth.

Scientists consider Tuesday's quake to be a small to moderate size event. It would take approximately 4000 such earthquakes to equal the energy released by the 1989 Loma Prieta earthquake or 30,000 such earthquakes to equal the energy released in the magnitude 7.4 earthquake in Turkey which occurred on the same day.

The Bolinas earthquake is interesting to scientists for two reasons:

• It occurred in a region that has had only one previous earthquake in the last 30 years of detailed seismic recording.

• Information obtained from the earthquake waves indicate an up-down direction of slip not the sideways (or lateral) slip expected for the San Andreas fault.

The first observation was somewhat of a surprise. Much of the 300 mile length of the San Andreas fault that ruptured in 1906 (from San Juan Bautista to Cape Mendocino) is currently aseismic (relatively free of earthquakes) and has been since 1906. Scientists consider the San Andreas fault in Northern California to be in a "locked" part of its earthquake cycle.

The second observation suggests that, in detail, this earthquake probably was not on the San Andreas fault, but rather occurred on a small fault adjacent to the main fault at depth.