LECTURE 5 NOTES - DISTRIBUTION OF SEISMICITY (updated 10/15/97)
Instructor: Professor Barbara Romanowicz
Director of Seismological Laboratory
Office Hours: Thursday 2-4 pm , upon appointment only
475 Mc Cone Hall
DISTRIBUTION OF SEISMICITY
Global Seismicity
Review: types of faults, how orientation is defined- mechanisms:
compressions/dilatations...
Earthquake focus: point from which seismic waves first emanate.
Simplification: in reality source spread through a volume of rock, but
when observed at large distances, this looks like a point.
Focus is at some depth below the ground (except for artificial
earthquakes, very near the surface).
Point on the ground surface directly above the focus is called the
epicenter.
Most earthquakes at shaloow depth , down to a few tens of kilometers,
where crust is brittle.
seismic moment
mechanical model: fault surface suddenly slipping in response to tectonic
stresses has led toa most useful measure of overall earthquake size.
to rotate a table: effort is reduced when hands are more widely
separated: force-couple. Apply two equal and opposite forces..
The size of this couple is called the moment: value is the product of th
value of one of the two forces and the distance betweent hen.
Same idea to system of forces that produce slip on a geological fault. In
this case seismic moment is defined as product of three quantities:
elastic rigidity of the rocks
area over which force applied
fault offset that takes place, or fault slip.
Advantage of this measure, results are not distorted by any dissipation
of energy during wave propagation. When available, can measure moment
directly from length of surface rupture and depth of rupture inferred
from the depths of the aftershock foci.
Seismic moments may vary over many orders of magnitude from smallest to
largest earhtquake. M2 to M 8 : range over six orders of magnitudes.
Moment of 1906 SF earthquake (450 km length of fault) is 10 times the
moment of 1989 which ruptured over about 45 km length.
Aftershocks of shallow earthquakes - occur in "same" place
following the earthquake, with decreasing size. Delineate areas of the
fault that did not rupture during the main shock, in which rupture is not
quite uniform: asperities and barriers.
Global distribution of earthquakes
Through the end of last century, seismologists had very poor picture of
the distribution of earthquakes because their knowledge was restricted
mostly to earthquakes felt on continents.
20th century: global seismograph network (cf VOn Rebeur Paschwitz...)
grew,
methods for locating earthquakes improved and today we have very precise
maps (show) showing the positions of thousands of earthquakes.
This has depended on worldwide cooperation of scientists in exchanging
data on travel times of seismic waves (in 1990 -3300 seismographic
observatories participating in exchange of data):
every day, many stations around the world send their readings of
earthquakes and underground explosions by fax, cable, e-mail or mail to
the NEIC of the
USGS in Golden, COlorado: used to compute rapidly location and magnitude
of earthquakes around the world. Also to ISC in England , which prints
catalogs which serve to print maps of global earthquake distribution.
Studies of global earthquake distribution has provided crucial evidence
on the present geodynamics and deformation of the whole earth. Have
thrown light on long standing puzzles of mountain building and volcanic
belts, spreading of the seafloor and stresses within crustal rocks.
Geographic Patterns
Earthquakes are not randomly scattered but, generally, concentrated in
narrow zones.
Many earthquakes: along ridges in the centers of oceans, no hazard to
humans. 10% of earthquakes occur there and contribute only 5% of energy.
Predominant style of faulting: normal faults
Greatest seimsic activity concentrated along the Pacific margins and in
southern Europe and Asia, which are densely inhabited and where plates
converge: more than 90% of the world's release of seismic energy from
shallow earthquakes. Majority of the largest earthquakes, 1960 and 1085
CHile, 1964 Alaska, 1985 Mexico: orioginated in subduction regions as a
result of one plate thrusting beneath another.
Interiors of oceans, away from ridges, are very quiet.
Antarctic has quietest margins and almost no interior earthquakes.
There are between 18,000 and 22,000 shallow-focus earthquakes of M>=2.5
each year.
Depth of focus:
can range from a few km to almost 700 km.
Shallow earthquakes: occur by elastic rebound along a fault
Deep earthquakes
Earthquakes below 70km are especially intriguing. Were discovered at
beginning of the century and established by Japanese seismologist Kiyoo
Wadati in 1928.
Deep earthquakes quite restricted geographically. Most along island arcs:
aleutian, Japan, Marianas, Tonga-Kermadec-New Zealand... Caribbean,
Abtilles and Aegean Sea.
along continental margins with deep ocean trenches: South American Andes
and central AMerica.
Few concentrations under inland mountain chains such as Himalayas and
Carpathians and a few under Spain.
More than 60,000 with d>70 km in 30 years: 22% of all earthquakes of
known depth.
Can sometimes be destructive: Carpathian mountains, March4, 1977, depth
90 km, considerable damage in Bucharest, Romania.
Deepest have foci around 680 km.
These earthquakes may not be generated by simple elastic rebound along a
fault: rocks at these depths are subjected to great pressures (~20 Gpa)
and temperatures (up to about 2000oC): rock softened from brittle
conditions, would no longer brake, and should flow rather than fracture
when strain increases. Mechanism remains speculative at present: how is
the stress released in rocks that are greatly compressed by overlying
material? If a crack opened, the weight of the rock above would weld it
together. DOminant Deformation at such high temperatures is by plastic flow.
Look at difference between shallow and deep earthquakes:
- deep ones usually have few aftershocks.
Columbia, 1970 M 7.6 depth 650 km: no aftershocks at all.
Bolivia, 1994
- distribution of aftershocks for shallow earthquakes; generally
delineate the plane that has slipped. Deep aftershocks seem to be more or
less randomly distributed around the initial focus.
This suggested that cause of deep earthquakes is sudden change in volume
of rcoks, resulting in change of phase state of the mineral material (eg
water increases in volume when changes to ice): sudden dialtion of rocks
would produce seismic waves: implosion or explosion would be detected
around the world in sign of first motions: no such pattern has been
found: pattern of P onsets same as for shallow earthquakes, in quadrants.
Also, production of S waves, which would not happen.
Two processes recently proposed:
- 1) water fundamentally influences the brittle and plastic properties of
the rock at high T and P. Many minerals contain water which may become
mobilized at high T and P (eg laboratory experiments with green hydrous
mineral "serpentine" sustain brittle fracture under highT and P:
lubrication of fractures and faults in rocks, allowing slip to occur.
2) sudden change of mineral phase into different form, but not with
volume change, rather phase transition occurs between the boundaries of
rock lenses where fluid conditions are particularly favorable for sudden
transition.
To test these hypothesis: conditions recreated in the laboratory by
squeezing tiny samples of rock between two diamonds (nutcracker). Laser
beam shone through the diamonds heats the rock and allows any sudden
physical transition to be photographed. Acoustic sensors detect any
sudden release of energy (analogue of earthquakes). On going....
Below 680 km, where no earthquakes have been detected, either the plate
is altogether absorbed into the rocks of the interior or sufficiently
softened by high T that it is no longer brittle enough to rebound in
earthquake.
style of faulting: where the plate bends: at the top - extension: normal
faulting
at the bottom boundary, compression: thrust faulting.
Deep earthquakes generally not hazardous even though they can be very
large. Bolivia 1994/06/09: felt in Canada (Toronto)
Intraplate earthquakes
Plate theory accommodates both the most obvious earthquake patterns and
the systematic source mechanisms very satisfactorily.
Intraplate eq. can be very large, and theory does not provide a ready
explanation for these earthquakes. At least 15 major earthquakes occcured
since the 16th century in places that would be regarded as stable.
China: over 3000 years of historical record, devastating earthquakes away
from Himalayan collision zone and subduction zones on eastern margins of
this country.
Europe: Basel 1356 M about 7.4 estimated from the described intensity.
China January 23, 1556: Shensi Province near city of Sian: greatest loss
of life ever recorded in an earthquake. 830,000 people died (official
catalog- is it true?). People lived in caves excavated in hill sides of
soft sediments (loess): little resistance to earthquake shaking:
collapsed on sleeping people at 5 am.
1811-1812: New Madrid earthquakes in Missouri. 3 shocks magnitudes 8.2,
8.1 and 8.3, more than a 1000 miles from the closet plate boundary.
Intraplate earthquake triplets are relatively common: also 1990 Sudan , in
5 days: (M 7.3, ,6.7, 7.2),( as opposed to interplate earthquakes which
generally present a characteristic mainshock/aftershock sequence).
Each principal shock occurs on a different fault segment, and segments
form complex intersecting patterns, which reflects the complexity of the
crust in which the rupture happens.
Why would there be such earthquakes in the middle of continents? Absence
of surface faulting hindered research, no evidence of elastic rebound.
Recent work has led to the view (examining small earthquakes in the
region) that the N Am. plate is flawed by a major rift in the old coastal
rocks near the morthern part of the Mississipi embayment: flat region
with low-lying drainage patterns and abandoned meanders of the river
course. Faults deeply buried under river mud and marine sediments
deposited on the floor of an ancient sea. 3 deeply buried faults: are
probably remnants of an old geological rift system, similar to the
spreading center at a mid-ocean ridge: failed continental rift system.
Mechanisms suggest presence of compressive stresses oriented in an
east-west direction,
sometimes producing strike-slip and sometimes thrust earthquakes
depending on orientation of fault. In agreement with geodetic survey
results.Faults in the old rift, perhaps inactive for millions of years,
are reactivated because of changing tectonic forces. Why reactivation? No
solid answer so far.
Such earthquakes result from the transfer of stresses form the plate
boundary across the whole, mainly rigid plate.
All large stable continent earthquakes occur in crust that has
experienced extensional tectonics.
Seismic Gaps
Plate pattern and steady rate of plate spreading imply that along the
plate edge the slip should be on average constant over many years. If two
slips some distance apart along a trench produce earthquakes, we might
expect that a similar slip will occur between them in due course. This
idea suggests that the historical patterns of segments and time intervals
between major earthquakes along major plate boundaries provide at least a
crude indication of places at which large earthquakes might occur soon.
Eg Alasks-Aleutian arc
Many sections of the arc are covered by areas of seismic-energy release
of recent large earthquake. There are some gaps, called "seismic gaps"
which could be likely areas for sudden plate slip and thus for major
earthquakes in the future.
Shumagin gap: rupture in 1788,1847 and 1903(?).
Yakataga gap: ruptured in 1899
SUrveys indicate convergence of N. Am plate into subduction zone about
1.6 cm per year in N15oW direction, roughly perpendicular to Alaska arc.
Strain accumulation measured from variations in the distances between
ground markers in both regions since 1980. Most likely sites for the next
great thrust earthquakes along this arc. But no significant crustal
deformation in the Shumagin gap detected, raising the speculation that
subduction is sometimes episodic: intervals of slow strain accumulation
occasionally interspersed by episodes of rapid accumulation.
In California: seismic gap along San Andreas fault between 1906 (southern
most end of segment that slipped) and northernmost end of 1857 Fort Tejon
earthtquake.
Other example: 1985 Mexico earthquake.
subduction zone under Pacific Margin of Mexico ruptured.
Loma Prieta, may also fit the seismic-gap theory: see maps of seismicity.
Must be cautious about application of seismic gap theory: known exceptions.
1979 Imperial Valley earthquake in the same section of the IMperial fault
that had had a similar earthquake (same size) in 1940. Quick repetition
of earthquakes from the same fault section cannot be ruled out.
Peru earthquake of May 31, 1970, 25 km from city of Chimbote.
depth 50 km M 7.75
Most catastrophic seismological disaster in the western Hemisphere.
Not known for weeks fully as rescue hampered by landslides and rock
avalanches that disrupted communications and blocked roads in the AANdes.
75,000 sq km: 50,000 deaths, 50,000 injuries, 200,000 homs destroyed and
800,000 people homeless.
earthquake begun iwht gentle swaying, then vibrations became more
intense, lasting from 30 to 60 sec or more.
Debris avalanche from the north peak of Huascaran Mountain. More than
50,000,000 cu m of rock, snow,ce and soil traveld 15 km to the touwn of
Yungay at speed ~ 320 km/h. 18,000 people buried under the avalanche.
read page 122 new Bolt - "crest of the wave had a curl at least 80 m high".
Distribution of seismic gaps around the Ppacific suggest that as the
mechanics and speeds of lithospheric plates become better understood,
long-term prediction may be possible for plate edge earthquakes. From
history of large earthquake occurrence: map the current lag in elastic
strain release...we will discuss this further in class on earthquake
prediction.
Comparison of seismic energy release in the largest earthquakes with other
sources of energy.
Temporal distribution of earthquake energy release around the world: non
uniform. Show patterns as a function of time...
California changes in the last century
Periodicities:
Parkfield
Dating at Pallet Creek or in Dead Sea rift : recurrence intervals
Earthquake statistics:"B-value"
earthquake swarms: Mammoth lakes, volcanic regions.
CALIFORNIA SEISMICITY
Geology and plate tectonic environment
Foundation laid in the 1960's.
Tuzo Wilson 1965: showed that San Andreas fault is a transform fault
connecting two spreading ridges, later modified to account for triple
junction and global vector rates
SAF separates major crustal blocks
North Central California: SE trending boundary
Salinian block West: granitic, metamorphic
Franciscan East: Great Valley Sequence, sedimentary and volcanic
bend in transverse ranges
splices further south, cutting through precambrian and younger
metamorphic rocks, but no distinctive lithologic constrast.
largest offset postulated by Hill and Dibblee (1953): 560 km
Late mesozoic, before development of the San Andreas system, highly
controversal
some think: highly convergent plate interaction, continental margin of
Andean type
some think: dextral translation, some late mesozoic rocks of coast Ranges
translated great distances from equatorial sites during Jurassic times.
Pattern of magnetic anomalies indicate that subduction related trench lay
offshore north america during Tertiary time because of convergence of
Farallon plate.
strike-slip movement on faults of the SA system began no earlier than 30
Ma (Oligocene) when Pacific Plate first impinged on N. American Plate
Triple junctions formed and migrated to NW and SE as the subduction of
Farallon plate continued:
Mendocino
Rivera (at mouth of Baja California) TRIPLE JUNCTIONS now 2,500 km apart
Early transform boundary probably along faults that are now mostly west,
at the edge of the continent
Modern SAF in southern California started working only when Gulf of
California opend about 4Ma, since then, Baja has moved 260 km away from
mainland Mexico.
Present rate of movement, revised from 6 cm/yr (Atwater, 1970), to 5.6
cm/yr (Minster and Jordan, 1978) to 4.8 cm/yr (De Metz et al., 1987).
Rate is substantially greater than slip rates based on measured offsets
of geologic features along the SAF.
e.g. offset at Wallace Creek in central California
3.5 cm/yr for pas 3,700 yrs
3.6 cm/yr for past 13,250 yrs
geodetic survey: slip rates 2.9 cm/yr for upper 15km, 3.7 km/yr below 15 km
Part of the total slip is probably occurring in small increments along
other faults in a broad zone of interaction as far as the B&R province.
Quaternary Deformation
Punta Gorda to N. Gabilean range
120 x 500 km domain
San Andreas plus 4 major faults plus some smaller 5 to 20 km
All are active seismically and displace quaternary deposits
San Gregorio, San Andreas, Hayward (+Rodgers Creek+ MAacama)
Calaveras, (+ Concord plus Green Valley faults)
+ ill defined zone along eastern margin of Coast Ranges (Stony Creek
Fault, Greenville+Ortigalita)
North of that: offshore east west trending portion located chiefly on
seismic + bathymetric evidence
Punta Gorda to Punta Arena: SAF offshore, known from seafloor features,
coastal geomorphology, stream offsets:
indicate displacements of at least 10 km in the last ~500,000 years
(quaternary)
Fault displaced geologic units in SF peninsula provide basis for
estimating rate of strike-slip on this section : about 1.5 cm/yr for
Quaternary
slip deficit can be accounted for slip on other faults nearly parallel to
SAF.
San Gregorio: northernmost of 400 km set of coastal faults, SW of main SAF
from monterey Bay to Bolinas Bay
complex 3-5 km wide zone of near vertical strike-slip and NE dipping
reverse faults
offset stream channels: 0.6-1.1 cm/yr across the fault zone
additional slip: Hayward, Calaveras, Green Valley, quaternary slip rates
weakly constrained.
Calaveras, Hayward , Rodgers Creek plus Maacama: total 375 km series of
right stepping breaks: 0.5-1 cm/yr
further east: Greenville: 1892 Vacaville major eq
0.1-0.3 cm/yr on these system boundary faults
Central California: SAF dominates, down to transverse ranges
South of transverse ranges to Salton Sea
broad belt of NW trending strike-slip faults bounded on NE by SAF, SE by
Santa Cruz/Catalina ridge fault zome
south of Salton: SAF system merges with complex pattern of active ridge
segments and transform faults --> gulf of CA for more than 1200 km.
San Jacinto: 1-2 cm/yr
Elsinor : 0.4 cm/ur
SAF 2,5 cm/Yr
Newport-Inglewood
Seismicity
Great (M~8) earthquakes along the main branch of the SAF accommodate most
of the relative plate motion. These plate boundary earthquake rupture the
entire 15-20 km thickness of brittle crust with slip as large as 10 m,
extending several hundred km along the fault trace.
recurrence rates: several hundred years
1857 FOrt Tejon
1906 SF only the most recent such great events along SAF
Interevent seismicity contributes only marginally to relative plate
motion, but is symptomatic of processes underlying earthquake cycle. The
smaller, more frequent eq. provide clues as to the kinematics and state
of stress. Largest of these, M6-7 can caulse extensive damage when they
strike near major population centers.
Monitoring networks: NCSN, SCSN, BDSN, Terrascope
Breadth of seismicity pattern in California and W Nevada suggests lateral
extent of deformation associated with the plate boundary.
Most remarkable aspect of seismicity pattern is the nearly complete
absence of earthquake activity down to the smallest magnitudes M ~1.5
along those sections that have ruptures with the largest historical
earthquakes: 1857,1906.
Southernmost section of SAF : Indio to Salton Sea also lacks seismicity
although no alrge earthquake in last 200 yrs.
Sharp contrast to segments marked by persistent linear concentrations of
small to moderate earthquakes. These segments probably seldom if ever
rupture with great earthqaueks although they may be capable of generating
M~6.
creep: those fault segents that show persistent microearthquake activity
most: central California: match the long term displacement rates of 32-34
mm/yr
some Salton Trough, Garlock fault: an order of magnitude less than the
long term deformation rates.
Mendocino Triple Junction: Image of subduction, focal depths 25-30 km
beneath submarine Gorda plate to nearly 80 km beneath southern Cascade
volcanoes.
Apart from SCruz mountainsand SF Peninsula; rupture zone of 1906 eq.
nearly aseismic
Central Creeping part: San Juan Bautista to Cholame
Because it appears that little if any shear strain is accumulating in the
blocks on either side , most seismologists believe that this section is
unlikely to rupture in a great earthqauke
moderate eq.: Coyote Lake 1979 M 5.9
Morgan Hill, 1984 M 6.2 ruptured 20 km long segments on Calaveras,
south of the junction with the Hayward Fault.
Gilroy 1994, M 5.0
Livermore 5.5,5.8 in 1980, on the Greenville fault.
Parkfield: 5 identical M=6 events same 30 km stretch of SAF enar
Parkfield, ~22 yr interval, since 1881.
Transition from creep to locking, most recent in 1966
intensive monitoring experiment, next one should be 1988 +- 10 yrs.
Coalinga M 6.7 1983
Kettleman Hills M 5.7 1985 reverse slip on NW striking planes sub
parallel to SAF
1857 began near Parkfield, propagated southeast through Carrizo Plain,
around Big Bend near Tejob Pass --> Mojave to Cajon Pass where San
jacinto branches to the south
offsets: 9 m Carrizo Plain
6m Fort Tejon
3-4 m Mojave
Conjugate left lateral faults around the bend: large scale asperities?
the straight Carrizo plain segment is aseismic and also Mojave segment
southernmost segment Indio seems to have much in common with 1857 and
1906 ruptures: geologic evidence for at least 4 major ruptures since 1000
AD, THE LAST ONE about 1000 yrs ago, but shows minor aseismic creep and
has shown slip accompanying M~6 EQ ON IMPERIAL FAULT AND southern San
jacinto.
Associated, subparallel faults are very active: San jacinto, Elsinore
Imperial fault: M 7.1 in 1940 and 6.6 in 1979, ruptured north 2/3 of the
fault
Two largest earthquakes in recent times
7.2 Eureka 1980
6.7 Coalinga 1983
are off the faults of SA System proper
Coalinga: crustal shortening with reverse slip perpendicular to SAF -->
local deviations from simple rigid plate approximations
lat few years:
1992 Mendocino 7.2
1992 Landers 7.2
1994 Northridge 6.7
7.7 Kern County eq 1952 southeast dipping white wolf -left oblique
reverse fault
largest earthquake until now
7.1 San Fernando 1971
Earthquake History
incomplete record before seismographic instrumentation introduced around
the turn of the 20th century. Before, it depends on density and
distribution of people who left written accounts of their experiences.
Franciscan missions established 1769, secularized 1830
detailed accounts of events that damaged the missions is the primary
source material for eq during this period. After 1830 quality of record
degrades (cessation of annual reports of missions)
Discovery of gold in 1848 transformed the written record --> newspapers
throughout the Sierran Foothill goldfields and SF Bay Region
Catalog complete 1850 to M~6 in Bay Area , not as complete in S. Calif
until 1890
Statewide, complete since 1850 down to M~7
less complete for central Nevada, some questions about events as late as
1903.
Earliest Seismographs 1887 MtHamilton, UCB
useful estimates of magnitude, but very precise only after WA
seismographs deployed 1926, then instrument emasurements supplant non
instrument estimates of magnitudes and locations.
~6.0 catalog complete after 1898
Noteworthy events
June 10, 1836 (M 6.75)
SF Bay Area
comparison with 1868 event: rationale for associating it with Hayward
Fault. Probably 1836 ruptured northern half and 1868 we know ruptured
southern half.
June 1838 (M 7)
San Andreas Fault, no precise date, probably around 60 km rupture on the
San F. peninsula
1865 San Andreas fault (M 6)
Oct 21, 1868: known as the great SF earthquake until 1906: Hayward Fault.
heavy damage along fault and in SF and San Jose
many engineering lessons learned: hazards of building on "made ground"
reclaimed from SF Bay, or admonition to "build no more cornices",,,, long
forgotten by 1906.
1872 M 7.6 Owens Valley
levelled the town of Lone Pine, CA. predominantly right lateral
strike-slip, average displaceent 6 m, felt as far as Salt lake City,
comparable to 1857, 1906
all of them have seismic moments more than 100 times smaller than M=9.2
Alaska 1964 earthquake
they are among the largest known strike slip earhtquakes , must be close
to the size of the largest possible strike-slip events along the SAF system.
1892 M 7 US Mexican Border
^.5 and 6.25 Vacaville earthquakes (west side of Sacramento Valley)
1899 M 6.4 San Jacinto
Summary:
Right lateral slip on fewer than 10 major faults dominates Quaternary
record within the SAF system. Other elements:
shorter strike-slip
reverse and thrust
regional fold systems .....
all result from predominantly horizontal motion between Pac and Nam.
Big Bend: transverse ranges
W-NW trending folds, NW trending strike-slip faults abut against
compressional domain characterized by EW trending folds, active thrust
and reverse faults, accelerated rates of
vertical uplift.
Different slip rates in N and S:
Pleistocene structures (1Ma) displaced ~ 35 km on SAF in central Calif,
creating two independent structural domains. Same period of time, Newport
inglewood system less than 500 m displacement: initial structural
patterns still connected
1- Quaternary tectonism within SAF
deformed length ~1,100 km
from Punta Gorda to Salton Sea
from Great Valley and Mojave desert to faults zones offshore in pacific
Ocean ~145 km width
2- Rate of right lateral strike-slips typically exceeds geologically
determined rates for other processes by an order of magnitude: horizontal
slip dominates the quaternary period.
3- central section, much slip along the main SAF, highest within the
system ~3.5 cm/yr
4- strike-slip more broadly distributed in North and South: SAF splits
into several active branches.
5- observed rates insufficient to account for all of the relative
movement (~5 cm/yr). Cause not well constrained.
6- EW or NW trending reverse or thrust faults occur near many strike-slip
faults: the longest and best defined are in the transverse ranges.
Discussion of seismicity
1857-1906: 2/3 rupture of total SAF length, large earthq. conspicously
absent along the rest of SAF. These account for half of the seismic
strain release since 1769: most of the rest occurs on other, smaller
elements of the fault system
eg 1952 Kern County, mechanism different from right lateral strike-slip
Spatial clustering of activity at specific localities:
Imperial fault
Calaveras Fault
San Jacinto Fault
Parkfield
virtual absence of smaller events along SAF segments that ruptured in
1857 and 1906, also
between 1857 reupture and Imperial valley.
Sites of future large earthq. cannot be identified on the basis of minor
seismicity alone.
Current rates of plate motion 5 cm/yr would imply a moment rate of 2
10**26 dyne-cm for a 10 km thick brittle crust, equivalent to an M 6.8
earthquake.
Such events occur far less often, principal contribution to plate motion
comes from infrequent large events.
numerous events of M <6 occurring each year contribute less than 10% to
total seismic strain release.
Total release since 1852: 70% of MAN-PAC plate motion predicted by plate
tectonic models ---> deficit.
Explanation of paradox includes deformation within Basin and Range
province in E. California and W. Nevada (extension plus dextral shear)
Seismic Cycle
Repetitive cycle of strain accumulation and release
definite stages
1 - low level of seismicity
2 - rise of regional activity as strain accumulates
3 occurrence of large earthquakes plus foreshocks and aftershocks
such characteristincs are displayed along the rupture of 1906 eq.
high activity in 19th century, particularly after 1850. After 1906:
moderate events ceased for 50 years, and since mid 1950's the activity
increased, has begun to approach 19th century level. Is this a
premonitory increase?
Same in S. California along the 1857 rupture