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Amending and complicating Chile’s seismic catalog with the Santiago earthquakeof 7 August 1580

Marco Cisternasa,*, Fernando Torrejónb, Nicolás Gorigoitiac

a Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, Av. Altamirano 1480, Valparaíso, P.O. Box 1020, Valparaíso 1, ChilebCentro de Ciencias Ambientales EULA-Chile, Universidad de Concepción, Barrio Universitario s/n, P.O. Box 160-C, Concepción, Chilec Instituto de Historia, Pontificia Universidad Católica de Valparaíso, Paseo Valle 396, P.O. Box 1049, Viña del Mar, Chile

a r t i c l e i n f o

Article history:Received 6 June 2011Accepted 19 September 2011

Keywords:Historical earthquakesSeismic catalog of Chile16th century Chilean earthquakes1580 Santiago earthquake

a b s t r a c t

Historical earthquakes of Chile’s metropolitan region include a previously uncatalogued earthquake thatoccurred on 7 August 1580 in the Julian calendar. We found an authoritative account of this earthquakein a letter written four days later in Santiago and now archived in Spain. The letter tells of a destructiveearthquake that struck Santiago and its environs. In its reported effects it surpassed the one in the samecity in 1575, until now presumed to be the only earthquake in the first century of central Chile’s writtenhistory. It is not yet possible to identify the source of the 1580 earthquake but viable candidates includeboth the plate boundary and Andean faults at shallows depths around Santiago. By occurring just fiveyears after another large earthquake, the 1580 earthquake casts doubt on the completeness of theregion’s historical earthquake catalog and the periodicity of its large earthquakes. That catalog, based oneyewitness accounts compiled mainly by Alexander Perrey and Fernand Montessus de Ballore, tells oflarge Chile’s metropolitan region earthquakes in 1575, 1647, 1730, 1822, 1906 and 1985. The addition ofa large earthquake in 1580 implies greater variability in recurrence intervals and may also mean greatervariety in earthquake sources.

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1. Introduction

Chilean seismic history from eyewitness accounts was compiledearly on by Perrey (1854) and Montessus de Ballore (1911, 1912a,1912b, 1912c, 1915, 1916). Both French scientists benefited fromtheir era’s excellence in Chilean historiography. They searched andanalyzed primary sources, mainly collected by historians, so thor-oughly that Chilean seismologists have rarely found need to lookfurther.

One of the main results of those compilations is a 470-year-longseismic historical record formetropolitan central Chile, the country’slongest. Thanks to the early and continuous Spanish settlement ofcentral Chile (32�e35� S; Fig.1), it is known that this part of Chile hasbeen struck by destructive earthquakes in 1575, 1647, 1730, 1822,1906 and 1985 (Lomnitz, 1970, 2004). This sequence has promotedthe idea that central Chile’s large subduction (inter-plate) earth-quakes recur at regular intervals close to 80 years (Lomnitz, 1970;Kelleher, 1972; Comte et al., 1986; Beck et al., 1998; Barrientos,2007). The last of these earthquakes, in 1985, was even forecast asfilling a seismic gap on the thrust boundary between the subductingNazcaPlate and theoverriding SouthAmerica Plate (Nishenko,1985).

Following this logic, the next large earthquake in metropolitanChile won’t occur until late in the 21st century. Such a forecastwould depend on at least two doubtful assumptions:

(i) All five of the above predecessors to the 1985 earthquakeresembled it in having a thrustmechanism and in occurring onthe plate boundary. For the pre-instrumental earthquakes, thisassumption is difficult to verify because the region has threegeneralized earthquake sources: the megathrust at theboundary between the Nazca Plate and the South AmericaPlate; faults from tension and compression within the sub-ducting Nazca Plate; and faults in the Andean part of the SouthAmerica Plate (Barrientos et al., 2004; Barrientos, 2007). Thisassumptionmay be safer for the pre-instrumental earthquakesof 1730and1822because theyspawneddocumented tsunamis.However, tsunamis could also result from outer-rise earth-quakes in the Nazcaplate, seaward of the trench, as the disas-trous 1896 tsunami in northeast Japan (Satake and Tanioka,1999). Lack of categorical evidence for tsunami from a centralChile earthquake may mean that the shock originated in theSouth American Plate or deep in the subducting Nazca plate.

(ii) The historical catalog is complete. As mentioned above, mostof the modern analyses on the earthquake recurrence ofcentral Chile (Lomnitz,1970; Kelleher,1972; Comte et al., 1986;

* Corresponding author. Tel.: þ56 32 2274261.E-mail address: marco.cisternas@ucv.cl (M. Cisternas).

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Ramírez, 1988; Beck et al., 1998; Barrientos, 2007; Bilek, 2010)have relied on the pioneer compilations of Perrey and Mon-tessus de Ballore, in themiddle of the 19th century and early inthe 20th century respectively. Few recent efforts have beenaddressed to look for gaps in their catalogs.

Here we challenge this second assumption and comment on thefirst by reporting recently discovered evidence that augments thehistorical catalog of 16th-century earthquakes in central Chile. Theevidence consists of an original letter from the lieutenant governorof Chile to the King of Spain, Felipe II, telling of a destructive earth-quake in Santiago and environs on 7 August 1580.We infer that thisearthquake had greater intensity in Santiago than did its prede-cessor in 1575, which was until now presumed to be the only largeearthquake in the first century of central Chile’s written history.

2. The lieutenant governor and his letter

The letter was found in the Archivo General de Indias in Seville,Spain in the course of a search for records of 16th century Chileanearthquakes. The Archivo consists of official documents about theSpanish colonial administration of the Americas and the Philip-pines. The documents total more than 80 million pages and occupy8 linear kilometers of shelf. Most are governmental reports sentfrom the colonies to Spain.

The letter that describes the 1580 earthquake forms part of anofficial report from the administration of theKingdomof Chile to theKing of Spain, Felipe II (Fig. 2). The letter was written in Santiago, in1580, by Chile’s lieutenant governor, Luís López de Azoca. Unlikemost conquistadors, López de Azoca was not a soldier. Instead, he

held a doctorate in law from the old University of Osuna at Seville(Barros Arana, 2000). He had been appointed lieutenant governor,the colony’s second-highest post, two years earlier by Felipe II.

On the evening of 7 August 1580, while the lieutenant governorwas at work on the report to the King, an earthquake interruptedhim. López de Azoca did not return to the letter until 11 August. Thecompleted letter, six pages long, deals mainly with colonialgovernment and a war discussed below (section 4), but alsoincludes half a page on the earthquake and its effects.

There are many reasons to accept the testimony of the writer astrue and authoritative: the literacy and administrative position ofLópezdeAzoca; the clarityofhiswritingand its attention todetail; theregal statusof the recipient; thewriter’s involvementasaneyewitnessinconvenienced by the earthquake; and the probable lack of a histor-ical motive for him to invent the disaster he recounts (section 4).

López de Azoca was writing as an agent of the King and an over-seer of Chile’s Spanish governor. He was reporting directly andindependently to the King, in addition of serving as the colony’shighest judicial official. He describes the earthquake in enoughdetailto afford estimates of its size (section 5). Considering that López deAzoca had the King’s confidence and was fairly new to his position,we doubt that he would have invented any of the details. His owninvolvement in the story increases their credibility; the earthquakedelayed the completion of his letter, he said. The time sequence itselffits the dates and days of the week in the Julian calendar.

3. Date and time of the earthquake

López de Azoca assigns the earthquake to 7 August 1580. Thisdate is in the Julian calendar, which was in force in Chile until 1583.

Fig. 1. Index maps showing tectonic and historical setting. A, South America. B, Central and southern Chilean cities founded before 1580. C, North-central, Central and South-centralChile. Approximate coastwise extent of interplate rupture marked by uplift in 1906 (Steffen, 1907), and aftershocks in 1985 (Barrientos, 1988). D, Detail of a 1580 Spanish portolan ofthe Chilean coast. Red gothic letters identify (pa d la) Serena and (la) Concepción as the only noteworthy coastal towns between 30� and 36� S; Valparaíso goes unlabeled (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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By this calendar, the date accords with the day of the week that thewriter reports: Sunday. Hementions setting down the letter on thatday and resuming it in the evening three days later, which he callsa Wednesday. He dates the letter one more day later, on 11 August1580.

Indirectly, López de Azoca gives the approximate time of theearthquake as early evening. In an epoch when clocks were rare,López de Azoca did not report a numbered hour. But he did statethat the earthquake interrupted his letter-writing in “la tarde”, theafternoon or evening, and he further stated that it took place “afterVespers”. Vespers is one of the Divine offices, those daily prayers attimes denoted by the Canonical hours of the Roman Catholic

Church. The Canonical hours are called Matins (at w24:00,midnight), Lauds (w3:00), Prime (w6:00), Terce (w9:00), Sext(w12:00, noon), None (w15:00), Vespers (w18:00), and Compline(w21:00) (Fig. 3). In the absence of a clock, Vespers would likelybegin around sunset. Today, in Santiago the 7 August sunset time isat w18:08. The earthquake probably took place later than that.

The time of the earthquake can be further bracketed by allowinga half hour for Vespers and at least another quarter hour for Lópezde Azoca to resume his letter writing. A modern Vespers includesa hymn, two psalms, a New Testament canticle, a short reading ofthe Bible, the Virgin Mary Magnificat, Our Father, and a closingprayer. Its duration is commonly close to 30 min. If the lieutenant

Fig. 2. Account of the earthquake of 7 August 1580 in Santiago, Chile. The account is part of a letter, now in the Archivo General de Indias in Spain, that was addressed to King Felipe IIand written in Santiago by Chile’s lieutenant governor Doctor Luís López de Azoca.

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governor faithfully observed the Divine offices, befitting his rankand especially on Sundays, is feasible to propose he finished hisprayer around 18:30 and returned to his writing table by w18:45(Fig. 3). Because the writer does not use the next Canonical hour,Compline, as a time reference (e.g., he does not say, “an hour beforeCompline”), we infer that the earthquake occurred earlier than20:00. Hence the earthquake recorded by López de Azoca probablystruck Santiago and environs between 18:45 and 20:00 on Sunday,7 August 1580, in the Julian calendar.

4. Historical context of the earthquake

4.1. Chile circa 1580

Lasting Spanish settlement in Chile began with the foundationof Santiago in 1541. As the conquistadors pushed southward fromthere, the Indians of south-central Chile began an insurgency thatwas to persist, off and on, for more than 300 years (Bengoa, 1991,2003). This conflict was the main concern of the conquistadorsduring the last half of the 16th century. They tried to establishcontrol by building military outposts (Guarda, 1990) and byfounding fortified villages that they called “cities”.

By 1580 these so-called cities, from north to south, ranged fromLa Serena and Santiago in central Chile, through Concepción, Angol,La Imperial, Villarica, Valdivia, and Osorno on the belligerent south-central Chilean mainland, to Castro on Isla Chiloé (Fig. 1). Theephemeral Chillán, 80 km northeast of Concepción, was founded inthe middle of 1580, but because its location, in the heart of theIndian territory, suffered consecutive destructions and reestab-lishments (Barros Arana, 2000).

Among these cities, only Santiago and Valdivia hosted morethan one thousand Spaniards at the end of the 16th century (Peña,1944; Contreras et al., 1971). In total, as of 1580, colonists and theiroffspring numbered less than 7000 in all of Chile, and more thanhalf of these people were engaged in trying to secure the conquestof south-central Chile (Mellafe, 1984; Villalobos, 1995). The rest ofthe Spaniards occupied central Chile, mainly between Santiago andthe current city of Talca, where there were large estancias devotedmainly to the livestock industry.

Though frighteningly outnumbered in south-central Chile by anestimated 150,000 Indians (Bengoa, 2003), the region’s 16th-century Spaniards managed to develop an incipient economy basedon gold mining, non-native crops, and livestock (Pacheco, 1991;Torrejón and Cisternas, 2002). However, brutal working condi-tions in gold placers incited a widespread Indian uprising thatbegan in 1574 and subsided only partway by 1581 (Villalobos, 1995;Bengoa, 2003). A subsequent outburst in 1598 took more thana thousand Spanish lives and cost the colonists all of their “cities”

and military outposts south of Concepción (Villalobos, 1995;Torrejón et al., 2004).

In addition to Indian wars, the conquistadors struggled withinternal quarrels that hobbled the governance of Chile. Thiscombination persuaded Felipe II to send plenipotentiary officials,whose main task was to report directly to the crown about theKingdom’s administrative and political affairs (Barros Arana, 2000).Thus it was that in 1578, Felipe II nominated Doctor López de Azocato replace Gonzalo Calderón, who had served in that position forfive years (Medina, 1906; Barros Arana, 2000). López de Azocaarrived in Chile the following year, shortly before the death ofGovernor Rodrigo de Quiroga. The new governor, Martín Ruiz deGamboa, proposed radical legal changes to protect Indians fromabuses (Gay, 1845; Barros Arana, 2000). These measures met stiffresistance from the conquistadors, providing López de Azoca withplenty to tell the King.

4.2. Known 16th century earthquakes in central Chile

Although current catalogs mention several notable shocks in thelast half of the 16th century, only one of these, the Santiagoearthquake of 17 March 1575, occurred in central Chile (Montessusde Ballore, 1912c). Additional 16th century earthquakes, muchlarger and better documented, took place in south-central Chile on8 February 1570, with damage to Concepción, and on 16 December1575 in southern Chile, with effects similar in extent and magni-tude to that of the Mw 9.5 Chilean earthquake of 22 May 1960(Lomnitz, 1970; Ramírez, 1988; Cisternas et al., 2005). Yet anotherearthquake may have struck La Imperial in 1562, but we agree withRamírez (1988) that there is not enough evidence to confirm itsoccurrence.

The sole known report of the 17 March 1575 central Chileearthquake is the brief account of chronicler Alonso de Góngora yMarmolejo in his Historia de Chile (Fig. 4). This manuscript, whichnarrates the history of Chile from its discovery until 1575, wasfinished in Santiago only nine months after the earthquake, on 16December 1575. While Montessus de Ballore (1912c) called ita “semi-earthquake”, Lomnitz (1970) estimated a magnitudebetween 7.0 and 7.5 on the basis of details given by the chronicler incomparison with effects of modern earthquakes. Late in the 20thcentury, seismologists interpreted the 1575 Santiago earthquake asthe first in a central Chile series of large earthquakes at regularintervals (Comte et al., 1986; Beck et al., 1998; Barrientos, 1997).

5. Relative sizes of the 1575 and 1580 earthquakes

The 7 August 1580 Santiago earthquake was larger than the oneoccurred in the same city five years earlier, on 17 March 1575. Wereach this conclusion by comparing the characteristics and effectsreported by López de Azoca for the first event (Fig. 2) and byGóngora y Marmolejo for the latter (Fig. 4).

In 1580 the earth shook “greatly” during “almost a half hour”.This interval seems too long for a mainshock alone. However, thegiant 1575 earthquake in southern Chile lasted “a quarter hour” inValdivia according to three likely independent accounts (Cisternaset al., 2005, table S1, records 1 and 6; Silgado, 1985, p.14) and “morethan a half hour” according to a fourth Valdivian account (Silgado,1985, p.13). In central Chile, the duration of the devastating 13 May1647 Santiago earthquake, was similarly described by the city’sbishop: “the earthquake lasted [.] about a half quarter hour”(Ramírez, 1988, p. 41). By contrast, Góngora y Marmolejo, whowitnessed the 1575 Santiago earthquake (Vicuña Mackenna, 1877,p. 263), does notmention its duration; perhaps it was too brief to benoteworthy.

Fig. 3. Canonical hours and earthquake time. In an epoch when clocks were rare,López de Azoca does not report a numbered hour for the 7 August 1580 earthquake.However, in his letter to the King Felipe, he states that it took place soon after“Vespers,” an early evening prayer time, and he does not mention the later Compline.From this bracketing we estimate an earthquake time between 18:45 and 20:00.

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The 1580 earthquake likely also had a wide areal extent. San-tiago and “the whole shire” (toda la comarca) were seriouslydamaged. In the 15th and 16th centuries, comarca referred toa frontier region that bordered another; it meantmore than just theimmediate surroundings of a settlement (Coromias, 1976; Tort,2003). Because Santiago was the sole inhabited center forhundreds of kilometers around, it is difficult to define the comarcaof López de Azoca. However, at a minimum it referred to Santiagoand its environs, and it may have embraced all of central Chile,between the northern comarca of La Serena and the southern one ofConcepción. Such an extent would better explain the cost estimatesby the lieutenant governor (see below), which may include damageon widely scattered estancias.

Although Santiago grew little between 1575 and 1580, only the1580 earthquake reportedly caused much damage. In 1575 “nohouse fell at all [.]; opening up some of them” (Fig. 4). In contrast,the 1580 earthquake “overthrew some houses and in general allreceived very large damage because for many parts were leftasunder and tileless” (Fig. 2). In Chile, where tile roofs werecommon until the end of the 19th century, the fall of tiles from roofsserves as a gauge of seismic intensity. The 3 March 1985 earth-quake, Mw 8, brought down most of the house tiles and roof ridgesin Ñuñoa, an old Santiago neighborhood. In central Chile tilesusually fall with modified Mercalli intensities � VII (Rodolfo Sar-agoni, written commun., 2007).

Neither earthquake, 1575 nor 1580, caused reported fatalities.However, the 1580 account ends just three days after the earth-quake, likely too soon for a full assessment of losses. Accordingly,López de Azoca states “we don’t know what had been [happened]in the rest of the cities of this kingdom”. By contrast, Góngora yMarmolejo had enough time, nearly nine months, to include in hisreport the fatalities, if any, in 1575.

The relative sizes of the 1575 and 1580 earthquakes might alsobe inferred from aftershocks. Unlike Góngora y Marmolejo in 1575,López de Azoca tells of continued shaking. When the lieutenant

governor, three days after the 1580mainshock, returned to his deskon the night of Wednesday, 10 August, he wrote “since that greatquake until today[,] Wednesday night[,] there have been manyother little quakes and these have not lasted long”. Similarly, the 3March 1985 earthquake was followed by several weeks of after-shocks felt in all central Chile, and during nine days, between 8 and17 March, 91 aftershocks were instrumentally recorded at Pelde-hue, 30 km north of Santiago (Comte et al., 1986).

López de Azoca’s rough estimate of financial losses furthersuggests that the 1580 earthquake was large. He reported to theKing, “In the whole shire of this city there has been such greatdamage that it is understood it will not be repaired with onehundred thousand pesos”. By peso he means a Castilian silver coinweighing 1 Castilian ounce (RAE, 1780). In the Toledean conversionsystem, one Castilian ounce is equivalent to 1/8 mark (Burriel,1758), so that each coin contained 28.8 g of silver. Thus the lieu-tenant governor’s estimate of 100,000 pesos implies 2.88 tons ofsilver. Then valued at the rough equivalent of USD 240 per troyounce (in 1998 dollars; AGIN, 2007), such a quantity of silvercorresponds to USD w22.5 million. Perhaps more telling, and lessanachronistic, is comparison with the yearly cost of fighting Chile’sIndians in 1600. During that year, in response to the 1598 Indianrebellion, the crown created the Real situado (Royal salary) tofinance the war in Chile. It was to cover the annual salary ofw2000soldiers, their food, services, supplies, and weapons (Villalobos,1995). The allocation for the year 1600 totaled 84,000 pesos,a little less than the estimated losses from the 1580 earthquake.

What types of structures likely suffered damage from the 1580earthquake to justify the financial losses estimated by López deAzoca? At its founding in 1541, Santiago had only six adobe houseswhile the rest were constructed mainly of bahareque and straw(León, 1975). Bahareque was a framework of branches covered bymud; a technique adapted from the Indians. Although brick, stone,and lime began to appear around 1550, adobe still prevailed(Vicuña Mackenna, 1869). Most edifices had a single story; as of

Fig. 4. The 17 March 1575 Santiago earthquake as recounted later that year in a Chilean history manuscript by Alonso de Góngora y Marmolejo. It is the earliest known writingabout a central Chile earthquake. The effects described appear minor compared with those of the 1580 earthquake (Fig. 2).

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1551 only three rose to two stories (Guarda, 1978). Only by 1575were the main Santiago’s streets paved with stones. A town councilordinance of 1580 mandated use of tiles on all the adobe wallsbordering the main square (León, 1975). Around 1600, the city had160 houses inhabited by near 1400 dwellers (Peña, 1944; Guarda,1978). Though the financial losses from the 1580 earthquake gavethe lieutenant governor reason to seek increased funding from thecrown, we know of no reason to believe that he would haveexaggerated those losses as the plenipotentiary representative ofthe King. Rather, we take him at his word that there really wasextensive damage in “the whole shire”.

6. Discussion

6.1. Open questions regarding the historical account of the 1580earthquake

After our initial astonishment at finding, in Spain, the record ofa large but uncatalogued earthquake in Santiago, we were evenmore surprised to find later that Chile itself possesses two non-original copies of the document. One is in the “Claudio Gay”collection of the National Archive, in Santiago. Claudio Gay(1800e1873) was a French naturalist based in Chile between 1828and 1842. The document, entitled “Carta del teniente gobernador.Terremoto del 7 de agosto de 1580. [.]” (Vol. 16, doc. 15, pp. 52-53),transcribes only the earthquake description of López de Azoca. Gayobtained the extract from a 17th century Spanish document, datedin Madrid, 17 December 1626, and addressed to King Felipe IV. Whydoes Gay’s contemporary and compatriot, the seismologist AlexisPerrey (1807e1882), mention nothing of the 1580 earthquake in hisearthquake catalog published in 1854? And why did Gay ignore the1580 earthquake, while including the 1575 Santiago earthquake, inhis own Historia Física y Política de Chile (Gay, 1845)? Perhaps Gayfound the document after both volumes had been published.

We found a second copy in the “José T. Medina” collection at theChilean National Library. It is a full transcription of López de Azoca’sletter, probably based on the same document that we found in theArchivo, entitled “Carta del doctor Lope de Azoca a S.M. el Rey, fechaen la ciudad de Santiago a 11 de agosto de 1580 [.]” (Library José T.Medina,Manuscritos, T. 92, M.1321, pp. 82e95;MicrofilmeMs. M3).This finding is also surprising because the Chilean historian José T.Medina (1852e1930), contemporary and university colleague ofMontessus de Ballore (1851e1923), allowed him to search hisarchives: “With the major unselfishness the famed Chilean histo-rian, don José Toribio Medina, has allowed us to examine his rich-ness manuscript archive and we have found in there severalcontemporary documents still unpublished. However, most resul-ted to be complaints or solicitudes addressed to the PeruvianViceroy or to the King, with the purpose to obtain donations, reliefor tax exemptions” (Montessus de Ballore, 1912c, p. 11).

Perhaps the half page account of the 1580 earthquake, insertedbetween administrative matters, passed unnoticed by Montessusde Ballore. Alternatively, Medina might have come upon the lieu-tenant governor’s letter after Montessus de Ballore had finishedresearching the earthquake history of Chile, a task that began in1907 and ended with the publication of the last volume of thecatalog in 1916.

Although probably unaware of López de Azoca’s 1580 earth-quake report, Montessus de Ballore did see an oblique reference towhat was almost certainly the Santiago earthquake of 7 August1580. In the last volume of his Historia sísmica (1916), a supplementcontaining the latest unreported findings, the French seismologistbriefly mentions that a “great quake” occurred there on 7 August1582 (Montessus de Ballore, 1916, p. 7). He inferred this two-yearyounger date from a colonial document that had been recently

excerpted in a Franciscan journal (Lagos, 1913). The original copy ofthat colonial document still survives in the Franciscan archive ofSantiago (Fondo S.T., Asuntos Varios, Vol. 1, fjs. 162e192). It isa sworn declaration, dated in Santiago, on 1 February 1585,requesting from the King Felipe II a donation to reconstruct in stonethe legendary San Francisco church, one of the few 16th-centurySantiago buildings still standing today. The document makes clearthat the original church, built in adobe, had suffered recent earth-quake damage. It recounts questioning before the city mayor, DonLuis de las Cuebas, in which witnesses respond to Joan de Torralba,the custodian of the San Francisco’s monastery. Torralba asks thewitnesses whether they know that in Chile great quakes arecommon and that they usually destroy adobe buildings. MelchorCalderón, the Chilean commissioner of the Spanish Inquisition, thetribunal established by the Catholic Monarchs intended tomaintainCatholic orthodoxy in their kingdoms, answers that during the lastten years, in both the city and Kingdom, quakes have been common,and they have devastated adobe buildings. Calderón adds that in“this city on seven August had been three years ago a great quakecame that left no house fixed” and caused the partial collapse of thehome of “this witness himself”. Because the manuscript is dated 1February 1585, Montessus de Ballore took Calderón’s estimation(“had been three years ago”) to the letter and assigned the earth-quake to 7 August 1582. But the coincidental month and daysupport that Calderón was referring instead to the very earthquakethat had abruptly interrupted López de Azoca’s work in 1580.

There is a third account that probably also refers to the 1580earthquake and its subsequent years of aftershocks. We found it inthe “José T. Medina” collection at the Chilean National Library. It isa sworn declaration before the Chile’s governor, Alonso de Soto-mayor, asking the King to help fund repairs to earthquake-damagedbuildings (Manuscritos, t. 93, doc. 1375, pp. 281e312). The requestoriginates on 2 December 1585 when Alonso del Campo, on behalfof the Santiago’s Nuestra Señora de la Concepción convent, asks thegovernor to inform the King about the nuns’ housing. Two yearslater, in Santiago on 4 November 1587, the governor presides overan inquiry in which witnesses are asked what they know of thenuns’ facilities. Baltasar Sánchez, schoolmaster of the Santiago’scathedral, answers that “[I know] the needs the said nuns have ofbuilding a dormitory, refectory, sickbay, [.] and other necessaryoffices [.] they are in narrowness [.] and they are even in dangerand health risk because the built edifice was left asunder [and]settled down in many parts by cause of the quake there was in thiscity the past years and usually are felt such quakes, because sincelast six years until today almost no year goes by without the landshaking once or twice or three times”.

The 1580 earthquake is thus mentioned in as many as threeindependent sources: in the López de Azoca letter of 1580, theCalderón testimony of 1585, and the Sánchez testimony of 1587. Bycontrast, although we have searched no less diligently for them, wefound no account of the 1575 earthquake other than the Góngora yMarmolejo’s one of 1575.

6.2. Open questions regarding the 1580 earthquake

We know too little about the characteristics and spatial distri-bution of the 1580 earthquake’s effects to infer a sourcemechanismor even an approximate source area. As mentioned above, centralChile has three generalized earthquake sources: the megathrust atthe boundary between the subducting Nazca Plate and the over-riding South America Plate; faults within the Nazca Plate; and faultsin the South America Plate (Barrientos et al., 2004; Barrientos,2007).

Among these three, the least likely is a source within the Nazcaplate beneath the Santiago area. The catastrophic 1939 Chillán

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earthquake, which caused more deaths than any other in Chile’swritten history, had such a source, beneath the Chillán area(Campos and Kausel, 1990; Beck et al., 1993; Barrientos, 2007).Worldwide such earthquake sources, in the mafic rocks of sub-ducted oceanic crust, are known to produce strong shaking fromhigh stress drops (Kausel, 1991; Singh et al., 2000). Though the 1580earthquake was probably the first large earthquake in Santiago’swritten history, too many of the city’s buildings probably remainedstanding for the 1580 earthquake to be likened to 1939 Chillán.

As for the subduction thrust at the plate boundary, along thecoast and offshore, it provides the simplest explanation for thelargest earthquakes in central Chile (Barrientos et al., 2004;Barrientos, 2007). Whether this explanation extends to the 1580earthquake, however, is difficult to say. If it was a large interplateunderthrusting event, it should have been marked along the coastby strong shaking, land-level changes, and a tsunami. These effectswere observed at Valparaíso, on the coast 100 km northwest ofSantiago, during the central Chile underthrusting events of 1730,1822, 1906, and 1985 (Lomnitz, 2004; Comte et al., 1986). However,between 30� and 36� S, only Santiago had the population andpermanence to supply written records of the 1580 earthquake. It isdifficult to expect a report of the 1580 earthquake’s results fromValparaíso, then a humble hamlet at best. Two years before, in 1578,Valparaíso amounted to ten dwellings around a “strawy andmiserable chapel” (Vicuña Mackenna, 1936). By 1594 the outposthad been reduced to a “temporary warehouse” for maritimecommerce and a navy battery watched over by few soldiers(Guarda, 1978). A beautiful 1580 Spanish portolan, depicting thecoast of Chile, highlights din red gothic letteringdthe solesignificant coastal towns of central Chile by the earthquake’s year:(pa d) la Serena and (La) Concepción (Fig. 1D). La Serena, the nearesttown to the north from Santiago, lay 400 km distant, and hada Spanish population of 90 (Guarda, 1978). In the nearest town tothe south, Concepción, 450 km from Santiago, 150 Spaniards facedincessant Indian sieges (Campos-Harriet, 1989).

Alternatively, the 1580 Santiago earthquake had a shallowsource within the South America Plate, perhaps in the Andes ortheir foothills. An activation of one of the multiple faults aroundSantiago area (Charrier et al., 2005, 2007) could well explain the1580 earthquake’s reported characteristics. A potential sourcecould be the rupture of a shallow fault, as a northward extension ofEl Fierro fault for instance, known as located in the forearc (Comteet al., 2008; Pardo et al., 2008; Farías et al., 2010). A possible analogis the shallow 1958 earthquake near Las Melosas, 60 km southeastfrom Santiago (Barrientos, 2007). It attained an estimated magni-tude close to 7, and it produced landslides and road damage near itssource. However, Santiago went undamaged (Lomnitz, 1961). Thiscontrast suggests a further possibility about the earthquakes of1575 and 1580: both were of similar magnitude, and resulted fromshallow faulting in the South America Plate, but the 1580 sourcewas simply closer to Santiago.

7. Conclusion

On the evening of Sunday 7 August 1580 (Julian calendar),a destructive earthquake struck Santiago and its environs. Theearthquake took place between 18:45 and 20:00. In its reportedeffects it surpassed the one in the same city five years earlier, the1575 earthquake that until now has been taken as the only largeshock of the first century of written history of central Chile. It is notyet possible to identify the source of the 1580 earthquake. Viablecandidates include both the plate boundary that would laterproduce, at Santiago’s latitude, the great earthquakes of 1730, 1822,1906, and 1985, and Andean faults at shallows depths aroundSantiago that have generated moderate earthquakes like the one in

1958. Whatever its source, the 1580 earthquake finding serves asa caution against assuming that central Chile’s historical earth-quake record is complete.

Acknowledgments

This work was supported by Chile’s Fondo Nacional de Desar-rollo Científico y Tecnológico (FONDECYT N� 1110848) and byDirección de Investigación of the Pontificia Universidad Católica deValparaíso. Philologist Manuel Contreras (Universidad Austral deChile) helped us read Lopez de Azoca’s handwriting and estimatethe value of his pesos. Brian Atwater and Rob Wesson (U.S.Geological Survey), Lisa Ely (Central Washington University), Rey-naldo Charrier (Universidad de Chile) and an anonymous reviewerprovided helpful suggestions on the manuscript.

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