PSN-L Email List Message

Subject: RE: 1906 epicenter data
From: "Steve Hammond" shammon1@.............
Date: Fri, 16 Apr 2004 10:44:00 -0700


Hi Jim- Hi Kareem,
 I agree Jim-- The Olema area is worth going to visit.  Just don't try to do
it on a summer day when the rest of San Francisco is trying to go to the
beach like I did. Kereem, you asked about US Geological Survey Professional
Paper 1515, The San Andreas Fault System, California (1990) Library of
Congress QE606.5.U6S26. This text is actually a 283 page book I purchased
from the USGS bookstore in 1991. It details all the fault system and each of
the principal earthquakes. Below, please find the text on the 1906 event
(page 159-161). There is a discussion in the second part about the work
completed to determine the magnitude. There is also a discussion before that
on the epicenter location. You will see the USGS references two citations,
Bolt, 1968 and Boore, 1977.  Keep in mind, Bolt was a Professor at UC
Berkeley where he did the research on the location of the 1906 event. I
would expect Berkeley to back Bolt's findings. I found it odd that Bolt
never publishes his findings in his own books. For example, in Earthquake,
Bolt 1978 and in Earthquakes and Geological Discovery (1993) he simply
states, "April 18, 1906, early morning in California.... At 5:12 A.M., a few
kilometers from the Golden Gate..." Another interesting point, Charles
Richter in Elementary Seismology (1958) lists the epicenter location to be
38N 123W 8.3 13:12.0 UTC which is the location of Point Reyes west of the
fault system.
Regards, Steve Hammond
PSN San Jose  Aptos, CA

APRIL 18, 1906 (M=8V4)

The California earthquake of April 18, 1906, ranks as one of the most
significant earthquakes of all time. Today, its importance comes more from
the wealth of scientific knowledge derived from it than from its sheer size.
Rupturing the northernmost 430 km of the San Andreas fault from northwest of
San Juan Bautista to the triple junction at Cape Mendocino (fig. 6.6), the
earth-quake confounded contemporary geologists with its large, horizontal
displacements and great rupture length. Indeed, the significance of the
fault and recognition of its large cumulative offset would not be fully
appreciated until the advent of plate tectonics more than half a century
later. Analysis of the 1906 displacements and strain in the surrounding
crust led Reid (1910) to formulate his elastic-rebound theory of the
earthquake source, which remains today the principal model of the earthquake
cycle.

As a basic reference about the earthquake and the damage it caused, geologic
observations of the fault rupture and shaking effects, and other
consequences of the earthquake, Lawson's (1908) report remains the
authoritative work, as well as arguably the most impor-tant study of a
single earthquake. In the public's mind, this earthquake is perhaps
remembered most for the fire it spawned in San Francisco, giving it the
somewhat misleading appellation of the "San Francisco earthquake" (fig.
6.7). Shaking damage, however, was equally severe in many other places along
the fault rupture. The frequently quoted value of 700 deaths caused by the
earthquake and fire is now believed to underestimate the total loss of life
by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and
189 were reported else-where.

At almost precisely 5:12 a.m. local time, a foreshock occurred with
sufficient force to be felt widely through-out the San Francisco Bay area.
The great earthquake broke loose some 20 to 25 s later, with an epicenter
near San Francisco (Bolt, 1968; Boore, 1977). Violent shocks punctuated the
strong shaking, which lasted some 45 to 60 s. The earthquake was felt from
southern Oregon to south of Los Angeles and inland as far as central Nevada
(fig. 6.6). The highest MMI's of VII to IX paralleled the length of the
rupture, extending as far as 80 km inland from the fault trace. One
important characteristic of the shaking intensity noted in Lawson's (1908)
report was the clear correlation of intensity with underlying geologic
conditions. Areas situated in sediment-filled valleys sus-tained stronger
shaking than nearby bedrock sites, and the strongest shaking occurred in
areas where groundreclaimed from San Francisco Bay failed in the
earth-quake. Modern seismic-zonation practice accounts for the differences
in seismic hazard posed by varying geologic conditions (see Borcherdt, 1975,
and Ziony, 1985, for analyses of the San Francisco Bay and Los Angeles
regions, respectively).

The characteristics and amount of surface fault slip in this earthquake
varied to a remarkable degree along the length of the rupture. Peak
displacements of 6 m were measured near Olema on the Point Reyes peninsula,
where the surface trace of the rupture formed a sharp, well-defined break
(fig. 6.8). In contrast, the fault break was extremely difficult to
recognize along its southern-most 90 km, where the surface offset averaged
only about P/2 m or less (see chap. 7).

The magnitude of 8.3 commonly quoted for the 1906 earthquake comes from
Richter (1958) and, within the precision of reporting, is identical to the
8V4 listed by Gutenberg and Richter (1954). Table 6.1 also lists other
magnitudes for this earthquake, derived from recent analyses of both the
same data used by Gutenberg and Richter and new data. Strictly speaking, a
"Richter magnitude" (ML) for the earthquake cannot be deter-mined because no
appropriate seismographs were in



operation at the time. Jennings and Kanamori (1979) used related
measurements extracted from simple pendulums at Yountville, Calif., and
Carson City, Nev., to derive ML=6.9, substantially smaller than the
traditionally quoted value. ML, which is based on the single largest peak on
a seismogram at approximately 1-s period and takes into account neither the
duration of the event nor longer period motions, is saturated for this
event.

Geller and Kanamori (1977) used the unpublished worksheets of Gutenberg and
Richter to compute a body-wave magnitude of mb=7A, using the procedure of
Gutenberg and Richter (1956). Because long-period (14 s) P-waves were used
in this calculation, it cannot be directly compared to the short-period mb
values routinely reported today.

Other workers since Gutenberg and Richter have studied the long-period
surface waves of the 1906 earth-quake and computed Ms values. Bolt (1968)
confirmed an Ms of about 8V4, whereas Lienkaemper (1984) found Ms= 8.3 from
an analysis of all the records collected by Reid (1910). Lienkaemper's
magnitude combined data from both damped and undamped instruments,
correcting each for magnification at the appropriate period of motion. Abe
(1988), who analyzed only the undamped Milne seismograms, obtained Ms= 7.8,
using slightly different procedures and a systematic set of
station-magnitude corrections. Also, the four damped seismometers (all in
Europe) give Ms=8.1. Longer period (50-100 s) surface waves analyzed by
Thatcher (1975) indicate a seismic moment of 4xl027 dyne-cm, equivalent to
M=7.7, in agreement with the seismic moment of 5xl027 dyne-cm obtained from
geodetic data, thus giving M=7.8 (Thatch-er and Lisowski, 1987). Finally,
Toppozada and Parke (1982) assigned an intensity magnitude (M/) of 7.8 on
the basis of the total area (48,000 km2) undergoing shaking of MMI VII or
higher.

The "traditional" magnitude of 8V4 is retained here, except where seismic
moment is used for quantitative purposes.

  -----Original Message-----
  From: psn-l-request@.............. [mailto:psn-l-request@.................
Behalf Of Kareem Lanier
  Sent: Friday, April 16, 2004 6:15 AM
  To: psn-l@..............
  Subject: RE: 1906 epicenter data


  Yeah, I agree. I've visited the park in the Point Reyes area where you can
take the "Earthquake Trail". There, you'll see the offset fence posts and a
tree which was split by the strike-slip rupturing. I believe they still hold
the concept that the epicenter is around Olema. It seems that the USGS
mostly agrees with this but UC Berkeley tends to push for a 1906 epicenter
just offshore of Daly City. I cant seem to find any documentation that
discusses this to any detail.

  Kareem



----------------------------------------------------------------------------
--
  From: psn-l-request@.............. [mailto:psn-l-request@...............
On Behalf Of Connie and Jim Lehman
  Sent: Friday, April 16, 2004 4:57 AM
  To: psn-l@..............
  Subject: Re: 1906 epicenter data


  Quake enthusiasts--
    The interest in 1906 event never goes away, especially with the
centennial  approaching.  Olema is mentioned.  This is a "must see" site for
anyone --expecially us easterners--seeking the effects of faulting at the
surface.  The National Park Service visitor center is just off Rt. 1 about
20 miles north of Golden Gate Bridge.  The excursion getting there is a
scenic adventure in itself....
                                                 Jim Lehman







Hi=20 Jim- Hi Kareem,
 I agree Jim-- The Olema area is worth going to = visit. =20 Just don't try to do it on a summer day when the rest of San Francisco = is trying=20 to go to the beach like I did. Kereem, you asked about US Geological = Survey=20 Professional Paper 1515, The San Andreas Fault System, California (1990) = Library=20 of Congress QE606.5.U6S26. This text is actually a 283 page book I = purchased=20 from the USGS bookstore in 1991. It details all the fault = system=20 and each of the principal earthquakes. Below, please find=20 the text on the 1906 event (page 159-161). There is a=20 discussion in the second part about the work completed to = determine the magnitude. There is also a discussion before that on the = epicenter=20 location. You will see the USGS references two citations, Bolt, = 1968 and=20 Boore, 1977.  Keep in mind, Bolt was a Professor at UC Berkeley = where he=20 did the research on the location of the 1906 event. I would = expect=20 Berkeley to back Bolt's findings. I found it odd that Bolt never = publishes=20 his findings in his own books. For example, in Earthquake, Bolt = 1978 and in=20 Earthquakes and Geological Discovery (1993) he simply states, = "April=20 18, 1906, early morning in California.... At 5:12 A.M., a few kilometers = from=20 the Golden Gate..." Another interesting point, Charles Richter in=20 Elementary Seismology (1958) lists the epicenter location to=20 be 38N 123W 8.3 13:12.0 UTC which is the location of Point = Reyes west=20 of the fault system.
Regards, Steve Hammond
PSN=20 San Jose  Aptos, CA    
 

APRIL=20 18, 1906=20 (M=3D8V4)

The California earthquake of = April 18,=20 1906, ranks as one of the most = significant=20 earthquakes of all time. Today, its=20 importance comes more from the wealth of scientific knowledge = derived=20 from it than from its sheer size. = Rupturing=20 the northernmost 430 km of the San Andreas fault from northwest of San = Juan=20 Bautista to the triple junction at Cape Mendocino (fig. 6.6), the = earth­quake confounded = contemporary=20 geologists with its large,=20 horizontal displacements and great rupture length. Indeed, the significance of the fault = and=20 recognition of its large cumulative offset would not be fully = appreciated=20 until the advent of plate = tectonics more=20 than half a century later. Analysis of the 1906 displacements and = strain in the surrounding crust led = Reid (1910)=20 to formulate his = elastic-rebound=20 theory of the earthquake source,=20 which remains today the principal model of the earthquake cycle.

As a = basic=20 reference about the earthquake and the damage it caused, geologic = observations=20 of the fault rupture = and=20 shaking effects, and other consequences of the earthquake, Lawson's = (1908)=20 report remains the authoritative work, as well as = arguably=20 the most impor­tant study of = a single=20 earthquake. In the public's mind, this earthquake is perhaps remembered = most for=20 the fire it spawned in San Francisco, giving it the somewhat = misleading appellation of the "San = Francisco=20 earthquake" (fig. 6.7). = Shaking=20 damage, however, was equally severe in many other places along the fault = rupture. The=20 frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to = underestimate the total = loss of=20 life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were = reported=20 else­where.

At = almost=20 precisely 5:12 a.m. local time, a foreshock occurred with=20 sufficient force to be felt widely through­out the = San=20 Francisco Bay area. The great earthquake broke loose some 20 to 25 s = later, with an=20 epicenter near San Francisco = (Bolt, 1968;=20 Boore, 1977). Violent shocks punctuated the strong shaking, which = lasted some=20 45 to 60 s. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far = as=20 central Nevada (fig. 6.6). The highest MMI's of VII to IX paralleled the = length of the rupture, = extending as=20 far as 80 km inland from the fault trace. One important = characteristic of=20 the shaking intensity noted in = Lawson's=20 (1908) report was the clear correlation of intensity with = underlying=20 geologic conditions. Areas = situated in=20 sediment-filled valleys sus­tained stronger shaking than nearby = bedrock sites,=20 and the strongest shaking = occurred=20 in areas where groundreclaimed from=20 San Francisco Bay failed in the earth­quake. Modern seismic-zonation = practice accounts=20 for the differences in = seismic=20 hazard posed by varying geologic conditions (see Borcherdt, 1975, and = Ziony,=20 1985, for analyses of the = San=20 Francisco Bay and Los Angeles regions, respectively).

The=20 characteristics and amount of surface fault slip in this = earthquake=20 varied to a remarkable degree along the length of the rupture. Peak = displacements=20 of 6 m were measured near Olema on = the Point=20 Reyes peninsula, where the surface trace of the rupture formed a sharp,=20 well-defined break (fig. = 6.8). In=20 contrast, the fault break was extremely difficult to recognize = along its=20 southern­most 90 km, where = the surface=20 offset averaged only about P/2 m or=20 less (see chap. 7).

The = magnitude of=20 8.3 commonly quoted for the 1906 earthquake comes=20 from Richter (1958) and, within the precision of=20 reporting, is identical to the 8V4 listed by Gutenberg and=20 Richter (1954). Table 6.1 also lists other magnitudes for=20 this earthquake, derived from recent analyses of both the same data = used by=20 Gutenberg and Richter and new = data. Strictly=20 speaking, a "Richter magnitude"=20 (ML) for the earthquake cannot be = deter­mined because no appropriate = seismographs were=20 in

operation at the=20 time. Jennings and Kanamori (1979) used related measurements extracted = from simple=20 pendulums at Yountville, Calif., and Carson City, Nev., to derive ML=3D6.9, substantially = smaller than the=20 traditionally quoted value. ML, which is based = on the=20 single largest peak on a seismogram at approximately 1-s period and = takes into account neither the duration = of the=20 event nor longer period = motions, is=20 saturated for this event.

Geller = and=20 Kanamori (1977) used the unpublished worksheets of=20 Gutenberg and Richter to compute a body-wave=20 magnitude of mb=3D7A, using the procedure of = Gutenberg and=20 Richter (1956). Because long-period (14 s) P-waves = were used=20 in this calculation, it cannot be directly = compared=20 to the short-period mb values routinely reported = today.

Other = workers since=20 Gutenberg and Richter have studied = the=20 long-period surface waves of the 1906 earth­quake = and=20 computed Ms values. Bolt (1968) confirmed an = Ms=20 of = about 8V4,=20 whereas Lienkaemper (1984) found Ms=3D 8.3 = from an=20 analysis of all the records collected by Reid (1910). Lienkaemper's magnitude = combined=20 data from both damped and = undamped=20 instruments, correcting each for=20 magnification at the appropriate period of motion. Abe (1988), who analyzed only the = undamped Milne=20 seismograms, obtained Ms=3D 7.8, using slightly = different=20 procedures and a systematic set = of=20 station-magnitude corrections. Also, the four damped seismometers = (all in=20 Europe) give Ms=3D8.1. Longer period (50-100 s) surface waves analyzed by Thatcher (1975) = indicate a=20 seismic moment of 4xl027 dyne-cm, equivalent to = M=3D7.7, in=20 agreement with the seismic = moment of=20 5xl027 dyne-cm obtained from geodetic data, thus giving = M=3D7.8=20 (Thatch­er and Lisowski, 1987). Finally, Toppozada and Parke = (1982) assigned an intensity magnitude = (M/) of=20 7.8 on the basis of the total area (48,000 km2) undergoing = shaking of=20 MMI VII or = higher.

The = "traditional"=20 magnitude of 8V4 is retained here, except where seismic moment is = used for=20 quantitative purposes.

-----Original Message-----
From: = psn-l-request@................. [mailto:psn-l-request@...............On Behalf Of Kareem=20 Lanier
Sent: Friday, April 16, 2004 6:15 AM
To:=20 psn-l@..............
Subject: RE: 1906 epicenter=20 data

Yeah, I agree. = I've visited the=20 park in the Point Reyes area where you can take the "Earthquake = Trail". There,=20 you'll see the offset fence posts and a tree which was split by the=20 strike-slip rupturing. I believe they still hold the concept that the=20 epicenter is around Olema. It seems that the USGS mostly agrees with = this but=20 UC Berkeley tends to push for a 1906 epicenter just offshore of Daly = City. I=20 cant seem to find any documentation that discusses this to any=20 detail.
 
Kareem


From: psn-l-request@.............. = [mailto:psn-l-request@............... On Behalf Of Connie and = Jim=20 Lehman
Sent: Friday, April 16, 2004 4:57 AM
To:=20 psn-l@..............
Subject: Re: 1906 epicenter=20 data

Quake = enthusiasts--
  The interest in 1906 event = never goes=20 away, especially with the centennial  approaching.  Olema is = mentioned.  This is a "must see" site for anyone --expecially us=20 easterners--seeking the effects of faulting at the surface.  The = National=20 Park Service visitor center is just off Rt. 1 about 20 miles north of = Golden=20 Gate Bridge.  The excursion getting there is a scenic = adventure in=20 itself....
          =             &= nbsp;           &n= bsp;           =20 Jim Lehman
 

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